SYNTHESIS, STRUCTURE AND CATALYTIC APPLICATION OF NOVEL CARBENE COMPLEXES WITH BENZOTHIAZOLIN 2 YLIDENE LIGANDS 1

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SYNTHESIS, STRUCTURE AND CATALYTIC APPLICATION OF NOVEL CARBENE COMPLEXES WITH BENZOTHIAZOLIN 2 YLIDENE LIGANDS 1

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Chapter One: Introduction Introduction Chapter One Introduction Carbenes, the simplest being methylene CH , are defined as neutral divalent carbon species bearing two non-bonding electrons.1 The existence of CH was first postulated in the 1930s.1 However, definitive evidence for its existence did not come until 1959. Carbene chemistry has fascinated and challenged chemists for decades, and in recent years this field has experienced tremendous growth. The first stable nucleophilic N, N-heterocyclic carbene (NNHC) was isolated by Arduengo in 1991.2 Conceptually, NHCs may be considered as phosphine mimics.3 NHC ligands are regarded as strong -donors with some degree of back-donation possible.3 This type of ligand can stabilize metals in various oxidation states in catalytic reactions.4 R R R R N N 1.1 R R N N 1.2 R R N N R R 1.3 R N S 1.4 R N S 1.5 Fig 1.1 N-heterocyclic carbenes.5 Most of the carbene work in literature is focused on N,N-heterocyclic carbenes (NNHC). The NNHC (1.1-1.3) derived from imidazole, imidazoline and benzimidazole have been successfully employed as ancillary ligands in catalysis.5 One way to “enrich” a NNHC is to attach heteroatoms to the parent N,N-heterocycle Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Introduction carbene moiety. Such a strategy is being used to tune the electronic properties and the donating abilities of NHC ligands.6 Carbene complexes with thiazolin-2-ylidene (1.4) and benzothiazolin-2-ylidene (1.5) ligands are more acidic compared to their imidazolin-2-ylidene (1.1) and benzimidazolin-2-ylidene (1.3) analogues. The replacement of N-R by S in the 1-position makes 1.4 and 1.5 better -acceptors due to the availability of empty d-orbitals.7 This larger sulfur atom bears no exocyclic substituent and might be expected to diminish p-p interactions between the carbene center and the neighboring heteroatoms (nitrogen) in this case.8 Prior to this project, there were only a few reports of N,S-heterocyclic carbene (NSHC) complexes and their use as catalysts.8-14 As the electronic configuration of NSHC and the chemical properties of its metal carbene bond, as well as the catalytic activity are less known than NNHC,3 it would be in our interest to explore this type of compound. In this chapter, the development of NSHC ligands and their complexes will be reviewed and its application in catalysis is discussed. 1.1 The Development of N,S-Heterocyclic Carbene Complexes Early work by Breslow described the application of N,S-heterocyclic carbene or thiazolin-2-ylidene (1.6) as organocatalysts such as the vitamin B1 catalyzed benzoin condensation reactions (Fig 1.2). 15-16 Transition metal complexes of NSHC were explored quite early. Lappert et al. and Stone et al. are among the pioneers in synthesizing NNHC and NSHC (Table 1.1).9-10 Rh(I)-NSHC complex (1.7) was reported in 1974 by Lappert.9 Stone and his co-workers managed to synthesize some cationic NSHC carbene complexes of Ir(III), Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction The Development of N,S-Heterocyclic Carbene Complexes Ni(II), Pd(II) and Pt(II) (1.8-1.15).10(a) Later, they also reported Ir(I), Rh(I), Mn(I), Cr(0) and Fe(0)-NSHC complexes (1.16-1.23).10(b) N CH3 N N NH2 S Cl HO 1.6 Fig. 1.2 Thiamine (vitamin B1), a coenzyme.15-16 Raubenheimer et al. reported the synthesis of complexes 1.24 and 1.25 in 1985, ca 10 years after the work of Stone and Lappert.11(a) They explored the Au(I)NSHC complexes (1.26-1.32) a few years later.11(b) Some of the thiazolyl and thiazolinylidene complexes of cyclopentadienyliron(II) (1.33-1.40),11(c) and molybdenum and tungsten, (1.41-1.44) were prepared and characterized by them.11(d) A Pd(II)-NSHC (1.45) was reported by Calò et al. at 2000. This complex was studied in the Mizoroki-Heck reaction.12 Hahn and Huynh et al. reported Ir(I) complexes (1.46-1.49) with coordinated and pendant allyl substituent.13 Grubbs et al. also reported Ru(II)-NSHC (1.50-1.56) with applications in olefin metathesis.14 The transition metal complexes of NSHC are less well known than the complexes with NNHC ligands. Table 1.1 summarizes the transition metal complexes with NSHC ligands that are currently known in the literature. As seen in Table 1.1, only a limited numbers of complexes have been prepared. These complexes cover the Group and 8-11 transition metals. It clearly suggests that there is much room for the future development of the chemistry of NSHC transition metal complexes. Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction The Development of N,S-Heterocyclic Carbene Complexes In the following section a comparison between some related NSHC and NNHC complexes is discussed. In the literature, most of the syntheses of these NSHC-complexes were done under inert condition and judicious handling was needed. In section 1.4, the literature methods for preparation of NSHC- and selected NNHC metal complexes are reviewed. These complexes are the currently known complexes that are closely related or to a certain extent they are analogous. The synthesis of NNHC forms the background of the synthetic route of NSHC complexes described in this thesis. Table 1.1 Complexes with thiazolin-2-ylidene ligands. Complexes S N CO Rh Cl Ref. N S 1.7 Cl L Ir L CO S NR Cl 1.8: R = H, L = PMePh2 1.9: R = Me, L = PMePh2 Cl Cl L Ir L CO S NH 10(a) 1.10: L = PMePh2 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction The Development of N,S-Heterocyclic Carbene Complexes R PPh3N OC M PPh3S N Cl(L)2M S 1.11: M = Pt, L = PEt3 1.12: M = Pt, L = PMePh2 1.13: M = Pd, L = PPh3 1.14: M = Pd, L = PMePh2 1.15: M = Ni, L = PPh3 PPh3N OC M PPh3S 10(a) BX4 1.16: M = Ir, R = Me, X=F 1.17: M = Rh, R = Me, X = F 1.18: M = Ir, R = Et, X = Ph BF4 10(b) OC I CO Mn OC CO S N 1.19: M = Ir 1.20: M = Rh 1.21 CO CO Cr OC CO S N CO Fe CO OC S N 1.22 1.23 OC OC 10(b) Cr(CO)5 11(a) N N R S 10(b) H AuX N S AuPPh3 PF6 11(b) S 1.24: R = H 1.25: R = Et H 1.26: X = 2,4-dimethylthiazole 1.27: X = Cl 1.28: X = (C6F5) H N S 1.30 N PF6 Au 11(b) H N S 1.29 N Au S S 1.31 NBu4 N N Au S S 1.32 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction The Development of N,S-Heterocyclic Carbene Complexes L-L S S L NH S L dppe 1.34: CNC6H4S-o dppm 1.35: CNC(Me)CHS dppm 1.36: CNC(Me)CHS dppe 11(c) S OC S Fe 11(c) N N Fe 1.33: CNC6H4S-o S CF3SO N CO H Fe N CO OC S 1.37: CNC6H4S-o N 1.39: CNC6H4S-o N 1.38: CNC(Me)CHS 1.40: CNC(Me)CHS S S M(CO)5 W(CO)5 N 11(d) N 1.41: M = Mo 1.42: M = W 1.43: M = Cr 1.44 S N I Pd I 12(a) N S 1.45 H N S Ir Br 1.46 S 13 N Ir Br 1.47 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction The Development of N,S-Heterocyclic Carbene Complexes 13 N N S Ir OC OC Br S Ir Br 1.49 1.48 14 Ar N S Cl Ru Ar = Cl O , 1.50 , 1.52 , 1.51 , 1.53 , 1.54 14 Ar N Ar = S Cl Ru Cl Ph , 1.55 , 1.56 PCy3 1.2 The Comparison of NSHC- and NNHC- Metal Complexes Some of the NSHC- and NNHC- metal complexes are selected where they are closely related. In this section, the similarity and differences of NSHC- and NNHCmetal complexes are discussed. Caló et al. managed to isolate and characterize Pd(II)NSHC trans-1.45 in 2000.12(a) The analogous Pd(II) complex with 1,3di(benzyl)benzimidazolin-2-ylidene had been isolated by the Huynh group in 2005.17 Both X-ray crystal structures of the Pd(II) complexes were characterized as the transisomer (Table 1.2). Huynh et al reported that the yield of trans-1.46 could be increased by using DMSO as solvent. Cis-1.45 readily converts to trans-1.45 within 1.5 h upon heating in DMA solvent at 100 C, as observed previously by Caló. Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction The Comparison of NSHC- and NNHC- Metal Complexes However, there is no report on such trans- to cis- isomerism for the related Pd(II)NNHC case with X = I. The 13 C carbene NMR signals of trans-1.45 were more deshielded ( = 210.5 ppm) than trans-1.46 ( = 181.0 ppm). This is due to the better -acceptor behaviour of NSHC resulting in a more acidic Pd metal center. However, the bond lengths and angles of both complexes were not significantly different in solid state. Although both complexes were studied in the Mizoroki-Heck reaction, their results could not be directly compared as the conditions were different. However, both were active in catalysis. The details of the catalytic application of Calo’s NSHC will be reviewed in Section 1.5. Table 1.2 A comparison of Pd(II)-NSHC and Pd(II)-NNHC metal complexes. NSHCStructure I Pd I S N NNHC- N N S N S H + N N I Pd I H + N N N trans-1.45 Pd(OAc)2 I (i) or (ii) - AcOH N S + S N N Pd(OAc)2 I THF, Reflux - AcOH S N transi-1.46 transi-1.45 Synthesis I Pd I Pd I I cis-1.45 N S N I Pd I N N N + N N I Pd N I cis-1.46 trans-1.46 (i) THF, RT: trans-, 40% - yield of trans-: 80% - cis- : 12% cis-, 54% (ii) DMSO, 80 C: trans-, 77% cis-, 20%  (13C carbene ) trans-,  = 210.5 ppm trans-,  = 181.0 ppm ppm Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction The Comparison of NSHC- and NNHC- Metal Complexes Another relevant comparison is of Ir(I)-NSHC13 and Ir(I)-NNHC18 complexes. Reaction of [Ir(-OMe)(cod)] with 3-(2-propenyl)benzothiazolium bromide or 1,3di(2-propenyl)benzimidazolium bromide under similar conditions gave different complexes . O Ir Ir O N +2 H Br N acetone N Ir Br AgBF4 N Ir N BF4 N 1.46 1.47 Scheme 1.1 Formation of Ir(I)-NNHC complexes 1.46-1.47.20 For the Ir(I)-NNHC, the synthetic pathway was straightforward (Scheme 1.1) where the five-coordinated and n2-C coordinated mode of Ir(I)-NNHC (1.46) was obtained.18 The cationic derivative [Ir(cod)(n2:n2-C-NNHC)]BF , (1.47) was obtained by treatment of 1.46 with AgBF .18 Instead of the target complex of the fivecoordinated Ir(I) for NSHC complex (1.48) (Scheme 1.2),13 similar reaction gives the unexpected complex, N-coordinated unsubstituted benzothiazole ligand complex, 1.49. The difference can be explained by the radical [1,3]-sigmatropic rearrangement of Nallyl dibenzotetraazafalvalene. In order to obtain complex 1.48, a different synthetic pathway was designed. [Ir(-Cl)(cod)] is used as the substrate to react with AgBF in CH CN to help replace chloride by CH CN. The benzothiazolium salt was added followed by KOtBu as an external base. Using this procedure, the five-coordinated Ir(I)-NSHC (1.48) was obtained. The reaction between [Ir(-Cl)(cod)] and 1,3-di(2-propenyl)- benzimidazolium bromide in ethanol with excess of NaOEt gives the four-coordinate complex 1.52 (Scheme 1.3).13 The excess presence of NaOEt leads to slow Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction The Comparison of NSHC- and NNHC- Metal Complexes hydrogenation of the carbene ligand allyl substituents. Ir(I)-NSHC 1.50 with the Npropylbenzothiazolin-2-ylidene ligand was obtained by using the excess of NaH. Bubbling CO into the solution of 1.50 affords complex 1.51.13 S H AgBF4 CH3CN Cl N Ir Ir Br Ir Cl NCCH3 BF4 BF4 N Ir Br NCCH3 S NaH KOtBu N N S Ir Br S Ir Br 1.48 O Ir Ir O S Br +2 1.50 acetone CO CH2Cl2 H N H S N N OC Ir Br S Ir OC Br 1.51 1.49 Scheme 1.2 Formation of Ir(I)-NSHC complexes 1.48-1.51.13 N Cl Ir Ir Cl + N Br H Ir Br N NaOEt EtOH N N N Ir Br 1.52 Scheme 1.3 Formation of Ir(I)-NNHC complex 1.52.13 The two examples given above suggest that there are times when different formation pathways could be needed for the analogous NSHC and NNHC complexes. Their catalytic properties have not been studied yet. Table 1.3 shows the comparison of 13 C carbene NMR signals and selected Ir-C bonds for analogous NSHC and NNHC complexes. All the chemical shifts for the 10 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes Other metal precursors such as [(cod)Ir(-OR) Ir(cod)] can be used for the synthesis of carbene complexes.69 This method based on azolium salts with basic metal has been used to synthesize a variety of carbene complexes of Group to 11 metals from benzimidazolium, benzothiazolium, pyrazolium, triazolium and tetrazolium salts.70 Moreover, this method can be modified by using bases such as NaH, NaOAc, KOtBu or MHMDS (M = Li, Na, K). Many NHC complexes bearing three-,71-72 four-,51 six-73 or seven74-membered cycles have been synthesized by in situ deprotonation of azolium salts. 1.3.5 Preparative methods based on transmetallation from lithiated heterocycles N-heterocyclic carbene complexes can be obtained from lithiated heterocycles (azoles) by transmetallation. Raubenheimer and co-workers have reported the preparation of carbene complexes from lithiated heterocycles. The reaction involved the initial deprotonation of an azole compounds followed by an alkylation reaction.11(e) HCl / Et2O H N (OC)5Cr S N OEt (CO)5Cr C + Ph 1.24 Li S Et [Et3O][BF4] N (OC)5Cr S 1.25 Scheme 1.9 Reaction of 2-lithiobenzothiazole with [Cr(CO) {C(OEt)Ph}].11(a) Scheme 1.9 shows the reaction of 2-lithiobenzothiazole (LiBtz) with [Cr(CO) {C(OEt)Ph}].11(a) In this reaction, the alkoxyphenylcarbene ligand is 17 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes replaced and complexes 1.24 and 1.25 are formed upon acidification or alkylation. Both of these complexes were isolated in extremely low yields (1% and 5% respectively). Another limitation of this method is the use of highly sensitive lithiocompounds. Several N,S-heterocyclic carbene complexes containing Cr, Mo, W,11(a), 11(d) Fe11(c),75 and Au11(b),76 have been prepared from a lithiated thiazole. Hahn and Waldvogel et al. obtained W-NNHC complexes from the reaction of lithiated Nethylbenzimidazole and [W(CO) (thf)]. Subsequently, this compound reacts with HCl yielding the tungsten complex with a NH,NR-stabilized benzimidazolin-2-ylidene ligand.77 1.3.6. Preparative methods based on carbene transfer to other metalNNHC/NSHC complexes Besides the above synthetic routes, carbene transfer is another useful way of obtaining transition metal carbene complexes. Complexes of Ni(II),78-79 Pd(II),80 Pt(II),81 Au(I),82-91 Rh(I),92-101 Ir(I),96,100,102 Ru(II),103-108 Ru(III),106 Cu(I), 85,108-113 Cu(II),108,109,111,113 have been prepared. This method is particularly useful for preparing carbene complexes with NNHC ligands bearing base-sensitive functional groups or an activated -hydrogen. Pd(II)-NNHC complexes were obtained by transferring of the NNHC ligand from a Ag(I)-NNHC complex to various Pd(II) metal precursors: PdCl ,114-116 [Pd(allyl)Cl] ,125-126 PdCl (CH CN) , 83-84,117-124 Pd(cod)CH Cl,41,118-119,127-128 PdCl (PhCN) ,95,119 Pd(cod)Cl , 41,118,119,128-130 Pd(cod)Br ,41 Pd(cod)CH Br.41 Ag(I)-NNHC complexes can be prepared from Ag O and carbene precursors and they have become widely used carbene transfer 18 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes reagents.131 Carbene transfer reactions can be carried out under aerobic conditions and in the presence of water.83 It offers great advantages to those reactions which were previously carried out under an inert atmosphere.83 Et N Et N Br H Ag2O Et N Pd(MeCN)2Cl2 Ag N Et N Et AgBr2 -AgBr N Et Et N N Et Cl Pd Cl Et N N Et 1.59 1.58 Scheme 1.10 Transfer of carbene ligand from Ag(I)-NNHC complex 1.58 to Pd(MeCN) Cl 84,124-133 to give Pd(II) carbene complex 1.59. This method has become popular since it was discovered by Wan and Lin in 1998.84,132-133 They have shown that Ag(I)-NHC complex 1.58 can be used as a convenient reagent to transfer the NNHC ligand to a Pd(II) precursor, giving Pd(II)NNHC complex 1.59 (Scheme 1.10). The study of the Ag(I) carbene intermediates have been discussed in the literature 80(b),134 and notably AgX(NNHC) and [Ag(NNHC) ][AgX ], in their dinuclear forms.135 Other more in-depth reviews on Ag(I)-NNHC carbenes are also available.130-131 Garrison and Youngs have reviewed the properties of silver NNHC complexes.136 The coordination chemistry of Ag(I)-NNHC complexes and their antibiotic properties have also been described.137 R N M(CO)5 N R PdCl2(PhCN)2 DCM, r.t. M = Cr (1.60), Mo (1.61), W (1.62) R N N R R Cl Pd Cl N + M(CO)3(PhCN)2 + 2(CO)2 N R 1.63 Scheme 1.11 Tramsmetallation of imidazolin-2-ylidene ligands in Cr, Mo and W complexes 80(a),121 to a Pd(II) complex. 19 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes Cr, Mo and W complexes (1.60-1.63) with saturated imidazolin-2-ylidene ligands have been used as reagents to transfer the corresponding carbene ligands to Pd(II) (Scheme 1.11).80(a),121 There are some rare cases where the pyrazolin-3-ylidene ligands of chromium complexes were transferred to Au(I), Pd(II) and Pt(II) complexes.138 1) Ag2O, CH2Cl2, r. t. Ar N S Cl Ar N PCy3 Cl Ru 2) Cl S Cl Ru Cl Ph Ar = Ph , 1.55 , 1.56 PCy3 PCy3 Scheme 1.12 Ruthenium carbene complexes with thiazole-2-ylidene ligands prepared by the 14 carbene transfer method. Grubbs and co-workers used the carbene transfer method to synthesize ruthenium carbene complexes. A new series of ruthenium-based olefin metathesis catalysts bearing thiazole-2-ylidene ligands, 1.55 and 1.56 were prepared in this way (Scheme 1.12).14 1.3.7. Preparative methods based on the oxidative addition of M0 by imidazolium salts The oxidative addition of 2-chloro derivatives of azolium salts (5methylthiazole, benzothiazole, benzoxazole and N-methyl-2-chloro-5- methylthiazolium cation) to complexes of iridium, palladium, platinum and nickel were first described by Stone et al.10 and Roper et al.139 For example, oxidative addition of 2-chlorobenzothiazole to IrCl(CO)(PMe Ph) afforded IrCl (CO)(NSHC)(PMe Ph) 1.64 (Scheme 1.13).10(a) 20 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes Various NNHC complexes were pepared by oxidative addition of azolium salts to low-valent metal precursors (Ni0, Pd0, Pt0, RhI, IrI, Fe0, Mn0, CrI). 10(b),140 . This requires C2-X bond activation (C2 = carbon at 2-position between S and N; X = halogen) Cl S IrCl(CO)(PMe2Ph)2 N Cl PMe Ph Cl Ir PhMe2P CO S NH 1.64 Scheme 1.13 Preparation of IrCl (CO)(NSHC)(PMe Ph) complex by oxidative addition of a 2-chlorobenzothiazole to a IrCl(CO)(PMe Ph) precursor.10(a) Mes Mes N N M N N Mes Mes M = Ni, 1.65 Pd, 1.66 N + N BF4 r. t. THF Mes N N M N H Mes N Mes N BF4 N Mes M = Ni, 1.67 Pd, 1.68 Scheme 1.14 The oxidative addition reaction of imidazolium salts with low-valent (M = Pd, Ni) to give carbene-M-hyrido complexes.141-142 Cavell and co-workers showed that the oxidative addition of imidazolium salts to Ni(0)-NNHC 1.65 at ambient temperature or Pd(0)-NNHC 1.66 at 55 C led to tris(NNHC) hydrido complexes 1.67-1.68 (Scheme 1.14). 141-142 They also reported the density functional calculations suggesting that the MII(hydrido)(NNHC) complexes could be generated by oxidative addition of 2-H-imidazolium salts to zerovalent Group 10 metal precursors.140(b),143 The aforementioned oxidative addition reactions may be relevant to the application of imidazolium salts (typically ionic liquids) in catalysis with low-valent M0 (M = Pd, Ni) complexes bearing strong -donor ligands 21 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes as precatalysts.144 Some NNHC-metal complexes were synthesized by oxidative addition, for example the Pt(II),145-147 Pd(II),148-150 Ir(III)151 Ru(III),151 Os(III)152 and Ga(III)153 NNHC-complexes. N N BF4 H N BF4 + Pd H3C COD + Pd(0) N 1.69 1.70 Scheme 1.15 Reductive elimination of a Pd(II)-NHC complex.3(e), 47, 155-156 The ‘reverse’ reaction, namely the reductive elimination of 2- organylimidazolium salts from PdII(hydrocarbyl)(NNHC) complexes, was also found to be facile under certain circumstances.154 Cavell et al. synthesized a variety of methylpalladium carbene complexes of the type [Pd(Me)(NNHC)(chelate)] (chelate = cod) from 1,3-dimethylimidazolium-2-ylidene and PdClMe(cod) where a methyl ligand is cis to the NNHC ligand.37,47,127 The cationic complex {[Pd(Me)(dmiy)(cod)][BF ] (dimy = dimethylimidazolin-2-ylidene) 1.69 has three different type of Pd-carbon bonds and it decomposes upon heating to Pd0, cod and the 1,2,3-trimethylimidazolium salt 1.70 (Scheme 1.15).3(e), 47,155 The reductive elimination process could be considered as an important pathway in the deactivation of catalytically active NNHC complexes as suggested by theoretical and experimental studies.117, 156-157 Cavell et al. combined the principle of oxidative addition and reductive elimination reactions (Scheme 1.16).158-159 The nickel catalyst [Ni(PPh ) ] which was generated in situ underwent oxidative addition to the C2-H bond of an imidazolium salt to form a nickel(II) hydrido complex. The alkene is then inserted into the Ni-H 22 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes bond in this complex. This was followed by reductive elimination to give the 2alkylimidazolium salt 1.73. N X + H R N cat. [Ni(cod)2/PPh3] (1: 2.1) RT or 55 oC 48 h N X R N (5 equiv) 1.71(a), X = BF4 1.72(b), X = Br 35 - 100 % 1.73(a-b) R = Bu, Ph, H Scheme 1.16 Imidazolium C-H/Alkene Coupling Reaction.158-159 1.3.8 Preparative methods based on template synthesis of metal carbene complexes Template synthesis is another method used to prepare metal carbene complexes. Pd(II)-NHC and Pt(II)-NHC complexes can be obtained by reacting the aziridine, thirane, oxirane or epoxides with isocyanide ligands or isocyanic acid bound to Pd(II) or Pt(II).160-168 Scheme 1.17 shows the reaction of isocyanide ligands in Pd(II) with thiirane affording the corresponding NSHC derivatives, 1.74.160 Ruiz et al. synthesized Mn(I) NHC complexes by metal-mediated coupling of propargylamines or propargylic alcohols and isocyanide ligands.169 Hahn et al. also reported the Pt(II) tetracarbene complexes from the template synthesis of [Pt(PMe ) ](CF SO ) with 2-azidophenyl ligands.170 There has been recent interest in NH,O- and NR,O-benzoxazolin-2-ylidene ligands171-175 and Cr,176 W,176-177 Re,178 Fe,179 Pd,180 Pt180 and B181 complexes. Scheme 1.18 shows the synthesis of Pd(II) carbene complex 1.76 by template synthesis. Pd(II). 23 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes Pt(II) complexes with benzo[1,3]oxazin-2-ylidene heterocycles were prepared by using the cyclization of a -functionalized phenyl isocyanide.182 R Cl Cl Pd C N R L THF, 25 oC + Cl Cl Pd L S N S 1.74 L = PPh3, PMe2Ph R = C6H4OMe-4, C6H11NC, C6H11 Scheme 1.17 Preparation of Pd(II) carbene complexes 1.74 from the reaction of coordinated isocyanide ligand with thiirane.160 Me3SiO PdI2 + CH3CN OSiMe3 N C N C Bu4NF/MeOH I Pd I - SiMe3 C N OSiMe3 HN O I Pd I O NH 1.76 1.75 Scheme 1.18 Preparation of Pd(II) carbene complexes by template synthesis.180 OH Ph3P Cl N C Cl Re Ph3P Cl Re(III), 1.77 Cl O O Cl Re O N Cl N H H O PPh3 Re(V), 1.78 Fig. 1.3 Re(III) complex 1.77 and Re(V) carbene complex 1.78.178(a) The intramolecular nucleophilic attack is influenced by the MCNR backbonding or d* back-bonding from the metal center to the isocyanide. For 24 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes example, the complex with a Re(III) center 1.77, which is a more electron-rich metal center, is more stable upon cyclization compared to Re(V) 1.78 (Fig. 1.3).178(a) 1.4 Application of Metal NSHC Complexes in Catalysis Many metal complexes with NNHC ligands are known to be active in homogeneous catalysis. Such complexes are supported by the strong -donor properties of the ligand.183 As NSHC ligands have also similar -donor properties, they should also be able to support many catalytically active complexes. The mechanism of Pd-NNHC-mediated cross-coupling reactions is similar to those catalyzed by the Pd-phosphine complexes.3(f) The established mechanism is believed to involve three steps: oxidative addition, transmetallation and reductive elimination in Mizoroki-Heck and Suzuki-Miyaura reactions.3(f) The choice of solvent, temperature and transmetallation promoters (e.g. bases and additives) could influence the catalytic efficiency.3(f) The Mizoroki-Heck184-195 and the SuzukiMiyaura3(h), 196-208 reactions catalyzed by Pd-NNHC complexes 4, 186(a),209-211 have been extensively reported. Although NSHC-metal complexes have been known since the 1970’s,7,8 the catalytic activity of Pd(II)-NSHC complexes was not explored until early 2000’s. A catalytically active Pd(II) complex with 3-methylbenzothiazolin-2-ylidene 1.45 (Fig. 1.4) was synthesized and characterized by Caló et. al 12 Complex 1.45 is a useful precatalyst, showing good catalytic activity in the coupling of aryl iodides and bromides with styrene or butyl acrylate with TONs of up to one million using 10-4 mol% of 1.45 in DMA or DMF (Scheme 1.19).12(a), 12(g)-(h) 25 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Application of Metal NSHC Complexes in Catalysis Upon addition of formic acid, sodium acetate or sodium formate to the reaction mixture, an enhancement in the catalytic activity was observed. Heck Reaction in Ionic Liquids Heck Reaction in DMA or DMF Arylation of Allylic Alcohols in Ionic Liquids N S I Pd I 1.45 S N Arylation of -substituted Acrylates in Ionic Liquids Carbonylation of Aryl Halides Heck Reaction of -substituted Acrylates in Ionic Liquids 12 Fig. 1.4 The Pd(II)-NSHC complex 1.45 was applied in different catalytic systems. [Pd] (1mol%) R X + CO2R X = I, Br R = H, Electron withdrawing group CO2R DMA or DMF base R Scheme 1.19 A typical Mizoroki-Heck reaction in DMA or DMF.12(a), 12(g)-(h) [Pd] (1mol%) Ar-Br + CO2Bu Ar CO2Bu TBAB, NaHCO3 Ar = Ph, p-MeC6H4, p-MeOC6H4, 2-naphthyl, 6-MeO-2-Naphthyl Scheme 1.20 A standard Mizoroki-Heck reaction in an ionic liquid.12(b), 12(g)-(h) 26 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Application of Metal NSHC Complexes in Catalysis This complex 1.45 proved to be an excellent precatalyst in the phosphine-free Mizoroki-Heck reaction of different aryl bromides (Scheme 1.20).12(b), 12(g)-(h) The reaction time was fast ( 3h) when compared to other conventional methods. NaHCO was used in the reaction and high yields of the coupling products were achieved. Tetrabutylammonium bromide (TBAB) was used and lead to an easy separation of the reaction products. TBAB was immiscible with the solvent, so that both the ionic liquid (IQ) and the catalyst could be recycled. -substituted R2 R1 R2 R1 R3 Ar [Pd] R3 R2 + R1 O R3 Ar Ar Ar R2 ArX/Base OH R1 -substituted OH R3 + R1 O R3 O Ar = Ph, p-MeC6H4, C6H5, 4-MeC6H4, 4-MeOC6H4 X = Br, Cl R1 = H R2 = H, Me R3 = H, Me, C2H5, nC5H11 Scheme 1.21 Arylation of allylic alcohols in an ionic liquid.12(f), 12(g)-(h) Complex 1.45 was found to efficiently couple aryl bromides and activated chlorides with allylic alcohols in TBAB to afford -aryl ketones or aldehydes regioselectively (Scheme 1.21).12(f), 12(g)-(h) The regioselectivity of the coupling products decreased when -alkyl or aryl-substituted allylic alcohols is used. Caló et al. reported that complex 1.45 was stable in molten TBAB and efficiently catalyzed the reaction of various p-substituted bromoaromatics with 27 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Application of Metal NSHC Complexes in Catalysis hydroxymethylenealkanoates to give -arylketones without the formation of oxoalkanoates (end product of a competitive retro-Baylis-Hillman reaction) (Scheme 1.22).12(c), 12(g)-(h) OH + Ar-Br R CO2Me TBAB, NaHCO3 O O [Pd] (2 mol%) R Ar + Ar R CO2Me Ar = p-MeC6H4, p-MeOC6H4, p-MeCOC6H4, 2-Naphtyl R = Ph, Me, i-propyl, n-propyl, n-octyl Scheme 1.22 Arylation of -substituted acrylates in ionic liquid.12(c), 12(g)-(h) Ar1-Br + Ar2 CO2Et TBAB, NaHCO3 Ar2 Ar1 [Pd] (1.5 mol%) Ar2 CO2Et + Ar1 CO2Et Ar1 = p-MeC6H4, p-MeOC6H4, p-MeCOC6H4, 2-Naphtyl, p-CN Ar2 = Ph Scheme 1.23 Heck reaction of -substituted acrylates in ionic liquid.12(d), 12(g)-(h) The same complex 1.45 is used in the reaction of various p-substituted bromoaromatics and p-chloronitrobenzene with (E)-ethyl cinnamate in molten TBAB as solvent (Scheme 1.23).12(d), 12(g)-(h) The end products, ,-diaryl acrylates are useful intermediates for the synthesis of the angiotension II antagonist,212 the platelet activating factors antagonist213 and the SRS-A antagonists (slow-reacting substances of anaphylaxis).214 28 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Application of Metal NSHC Complexes in Catalysis [Pd] ArX + CO + ArCONu Nu X = Cl, Br, I Nu = OH, OR, NR2 Scheme 1.24 Carbonylation of aryl halides.12(e), 12(g)-(h) The catalytic activity of complex 1.45 in the carbonylation of aryl halides was studied by Caló et al.(Scheme 1.24).12(e), 12(g)-(h) Complex 1.45 is employed in this reaction at an atmospheric pressure of CO in DMA in the presence of bases such as sodium acetate or amines. When TBAB is used under slightly increased of CO pressure, better results were obtained. TBAB is better than more conventional ionic liquids and its use allows for recycling of the catalyst. The application of NSHC-complexes in catalysis is not particularly wellspread. Grubbs et. al. reported the synthesis and catalytic study of olefin metathesis.14 They managed to synthesize a series of seven new ruthenium-based olefin metathesis catalysts bearing NSHC ligands with only one exocyclic substituent adjacent to the carbenic center, [Ru] 1.50-1.56. The catalytic performance of these complexes has been evaluated in (i) ring-closing metathesis of diethyldiallyl and diethylallylmethallyl malonate (Scheme 1.25(a)-(b)), (ii) the ring-opening metathesis polymerization of 1,5-cyclooctadiene and norbornene (Scheme 1.25(c)-(d)), (iii) the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene (Scheme 1.25(e)) and the macrocyclic ring-closing of a 14-membered lactone (Scheme 1.25(f)). These catalysts exhibit unexpectedly high stability. 29 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction EtO2C Application of Metal NSHC Complexes in Catalysis CO2Et mol% [Ru] EtO2C CO2Et (a) C6D6, 50 oC 0.1M substrate EtO2C CO2Et 2.5 mol% [Ru] EtO2C CO2Et (b) C6D6, 50 oC 0.1M substrate 0.1 mol% [Ru] C6D6, 30 oC 0.5M substrate * * n (c) 0.2 mol% [Ru] CH2Cl2, -20 oC mol% [Ru] OAc + AcO Ph O * Ph CHCl3, 60 oC 0.2 M substrate * n (d) OAc (e) mol% [Ru] O CHCl3, 50 oC mM substrate O (f) O Scheme 1.25 Ruthenium-based olefin metathesis.14 1.5 Project Aims and Thesis Overview The synthesis of the N,S-heterocyclic carbene complexes is less developed compared with the corresponding N,N-heterocyclic carbene complexes. The catalytic studies of NSHC-metal complexes are also less common. The stereochemistry of NSHC and NNHC may have different and it is interesting to compare them. This thesis will deal with binary Pd(II)-NSHC complexes, Pd(II) complexes with NSHC 30 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Project Aims and Thesis Overview and other auxiliary ligands (phosphine, aromatic N-heterocycle, pyridyl- and azole ligands) and their use in catalysts in Mirozoki-Heck and Suzuki-Miyaura reactions. This work also includes the catalytic annulation of heterocycles involving the N,Nheteroyclic and N,S-heterocyclic azolium salts. Cyclometallated Pt(II)-NSHC complexes will also be discussed. The aims of this study are: (i) Preparation of new transition metal (Pd(II) and Pt(II)) complexes containing the NSHC ligand. (ii) Investigation of the catalytic activities of Pd(II)-NSHC complexes in Mizoroki-Heck and Suzuki-Miyaura coupling reaction. (iii) Study of the catalytic annulation of heterocycles involving the N,Nheteroyclic and N,S-heterocyclic azolium salts. This thesis comprises of chapters that focus on the preparation of a broad range of NSHC- and NNHC-complexes and a study of their catalytic activity, and includes a comprehensive introduction (Chapter 1) as well as the reviews of the relevant literature published up to early 2009. The results are presented and discussed in Chapter 2. This Chapter is divided into parts. Part I (Section 2.1-2.4) is focused on the NSHC-precursors, novel Pd(II)NSHC complexes and the catalytic studies of these complexes in the Mizoroki-Heck and the Suzuki-Miyaura coupling reactions. Part II (Section 2.5-2.9) is devoted to the synthesis and characterization of a range of mixed-ligand carbene complexes with phosphine, aromatic N-heterocycle, pyridyl and azole ligands. Some of these complexes have been applied in the sp3-sp2 Suzuki-Miyaura coupling reaction. Five Pd(II) complexes with N, N-benzimidazolin- 31 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands Chapter One: Introduction Project Aims and Thesis Overview 2-ylidene ligand have also been synthesized. The coordination chemistry and catalytic activities between N,S- and N,N-NHC Pd(II) complexes are compared (Section 2.9). Besides Pd(II)-NSHC chemistry, Part III-Section 2.10 presents the ringopening formation of benzothiazolium salts from the reaction of Ag O and N,Sheterocyclic carbene precursors. Three Pt(II)-NSHC-(C^N) complexes (C^N = 2phenyl-pyridine) are described in Section 2.11. Section 2.12 presents the study of the catalytic process. This is relevant to the oxidative addition and reductive elimination via a metal-hydride carbene complex intermediate giving 5-membered and 6membered fused ring imidazolium salts. The experimental details are given in Chapter 3. The conclusion and summary of this thesis is presented in Chapter 4. This study may provide a clearer picture on the N,S-heterocyclic carbene complexes of Pd(II) and Pt(II) transition metals. The design of the 5-membered and 6membered fused ring imidazolium salts could lead to a better understanding of the synthesis route to fused-rings ionic liquid. The results of this study may be useful for researchers who wish to embark on further investigation of NSHC-metal complexes. 32 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin-2-ylidene Ligands [...]... transferring of the NNHC ligand from a Ag(I)-NNHC complex to various Pd(II) metal precursors: PdCl 2 ,11 4 -11 6 [Pd(allyl)Cl] 2 , 12 5 - 12 6 PdCl 2 (CH 3 CN) 2 , 83-84 ,11 7- 12 4 Pd(cod)CH 3 Cl, 41, 118 -11 9, 12 7 - 12 8 PdCl 2 (PhCN) 2 ,95 ,11 9 Pd(cod)Cl 2 , 41, 118 ,11 9, 12 8 -13 0 Pd(cod)Br 2 , 41 Pd(cod)CH 3 Br. 41 Ag(I)-NNHC complexes can be prepared from Ag 2 O and carbene precursors and they have become widely used carbene. .. properties .13 , 18 Table 1. 3 Comparison of the 13 C carbene NMR signals and selected Ir-C carbene bond data of analogous NSHC and NNHC complexes Complexes L = NSHC  ( C carbene )(ppm)a 20 2 .1 (1. 46) 21 8.9 (1. 52) 20 9 .2 (1. 48) L = NNHC  ( C carbene )(ppm)b 1 72. 3 (1. 49) 17 3.3 (1. 50) 19 1 .2 (1. 51) - Ir-C carbene Length (Å) 1. 98(6) (1. 46) 1. 98(4) (1. 52) a: Measured in CD 2 Cl 2 b: Measssured in CDCl 3 Ir-C carbene. .. synthesis of [Pt(PMe 3 ) 4 ](CF 3 SO 3 ) 2 with 2- azidophenyl ligands. 17 0 There has been recent interest in NH,O- and NR,O-benzoxazolin -2- ylidene ligands1 71- 175 and Cr ,17 6 W ,17 6 -17 7 Re ,17 8 Fe ,17 9 Pd ,18 0 Pt180 and B1 81 complexes Scheme 1. 18 shows the synthesis of Pd(II) carbene complex 1. 76 by template synthesis Pd(II) 23 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene. .. chemistry of Ag(I)-NNHC complexes and their antibiotic properties have also been described .13 7 R N M(CO)5 N R PdCl2(PhCN )2 DCM, r.t M = Cr (1. 60), Mo (1. 61) , W (1. 62) R N N R R Cl Pd Cl N + M(CO)3(PhCN )2 + 2( CO )2 N R 1. 63 Scheme 1. 11 Tramsmetallation of imidazolin -2- ylidene ligands in Cr, Mo and W complexes 80(a), 12 1 to a Pd(II) complex 19 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes. .. in DMA or DMF. 12 ( a), 12 ( g)-(h) [Pd] (1mol%) Ar-Br + CO2Bu Ar CO2Bu TBAB, NaHCO3 Ar = Ph, p-MeC6H4, p-MeOC6H4, 2- naphthyl, 6-MeO -2- Naphthyl Scheme 1 .20 A standard Mizoroki-Heck reaction in an ionic liquid. 12 ( b), 12 ( g)-(h) 26 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter One: Introduction Application of Metal NSHC Complexes in Catalysis... substances of anaphylaxis) . 21 4 28 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter One: Introduction Application of Metal NSHC Complexes in Catalysis [Pd] ArX + CO + ArCONu Nu X = Cl, Br, I Nu = OH, OR, NR2 Scheme 1 .24 Carbonylation of aryl halides. 12 ( e), 12 ( g)-(h) The catalytic activity of complex 1. 45 in the carbonylation of aryl... useful way of obtaining transition metal carbene complexes Complexes of Ni(II),78-79 Pd(II),80 Pt(II), 81 Au(I), 82- 91 Rh(I), 92- 10 1 Ir(I),96 ,10 0 ,1 02 Ru(II) ,10 3 -10 8 Ru(III) ,10 6 Cu(I), 85 ,10 8 -11 3 Cu(II) ,10 8 ,10 9 ,11 1 ,11 3 have been prepared This method is particularly useful for preparing carbene complexes with NNHC ligands bearing base-sensitive functional groups or an activated -hydrogen Pd(II)-NNHC complexes. .. Stone et al .10 and Roper et al .13 9 For example, oxidative addition of 2- chlorobenzothiazole to IrCl(CO)(PMe 2 Ph) 2 afforded IrCl 2 (CO)(NSHC)(PMe 2 Ph) 1. 64 (Scheme 1. 13) .10 (a) 20 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes Various NNHC complexes were... polymerization of 1, 5-cyclooctadiene and norbornene (Scheme 1 .25 (c)-(d)), (iii) the cross metathesis of allyl benzene with cis -1, 4-diacetoxy -2- butene (Scheme 1 .25 (e)) and the macrocyclic ring-closing of a 14 -membered lactone (Scheme 1 .25 (f)) These catalysts exhibit unexpectedly high stability 29 Synthesis, Structure and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands Chapter... Complexes with Benzothiazolin- 2- ylidene Ligands Chapter One: Introduction Preparative methods for NSHC- and NNHC- Metal Complexes Cr, Mo and W complexes (1. 60 -1. 63) with saturated imidazolin -2- ylidene ligands have been used as reagents to transfer the corresponding carbene ligands to Pd(II) (Scheme 1. 11) .80(a), 12 1 There are some rare cases where the pyrazolin-3 -ylidene ligands of chromium complexes . PdCl 2 (PhCN) 2 , 95 ,11 9 [Pd(allyl)Cl] 2 , 12 5 - 12 6 Pd(cod)CH 3 Cl, 41, 118 -11 9, 12 7 - 12 8 Pd(cod)Cl 2 , 41, 118 ,11 9, 12 8 -13 0 Pd(cod)Br 2 , 41 Pd(cod)CH 3 Br. 41 Ag(I)-NNHC complexes can be. of ligand can stabilize metals in various oxidation states in catalytic reactions. 4 NS NSNN NN R R R RR R RR NN R R R R 1. 1 1 .2 1. 3 1. 4 1. 5 11 1 1 1 2 2 2 22 Fig 1. 1 N-heterocyclic carbenes. 5 . and Catalytic Application of Novel Carbene Complexes with Benzothiazolin- 2- ylidene Ligands 7 Ir Br S N 1. 48 Ir Br OC OC S N 1. 49 13 NSAr Ru Cl Cl O Ar = , 1. 50 , 1. 51 , 1. 52 , 1. 53 , 1. 54

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