Carbene catalysts cobalt

Cobalt-phosphine and cobalt-carbene catalysts have been developed for the hydroarylation of styrene with (91) via a chelation-assisted C-H bond activation. The regioselectivity can be controlled in favour of the branched (92) or linear product (93) at will by the ligand. Deuterium-labelling studies demonstrated a reversible C-H bond cleavage and alkene insertion steps and reductive elimination as the rate-and regioselectivity-determining step. ... 

Deng and coworkers recently developed the first silyl-donor-functionalized Af-heterocyclic carbene (NHC) cobalt complexes for use as catalysts in hydrosilylation reactions. Of the catalysts screened, 144 proved superior in rapidly producing the desired silylated products 145 and 146 in reactions between 1-octene 143 and PhSiHj while minimizing isomerization side products 147 and 148. The... 

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

The preparation of cyclopropanes by intermolecular cyclopropanation with acceptor-substituted carbene complexes is one of the most important C-C-bond-forming reactions. Several reviews [995,1072-1074,1076,1077,1081] and monographs have appeared. In recent decades chemists have focused on stereoselective intermolecular cyclopropanations, and several useful catalyst have been developed for this purpose. Complexes which catalyze intermolecular cyclopropanations with high enantiose-lectivity include copper complexes [1025,1026,1028,1029,1031,1373,1398-1400], cobalt complexes [1033-1035], ruthenium porphyrin complexes [1041,1042,1230], C2-symmetric ruthenium complexes [948,1044,1045], and different types of rhodium complexes [955,998,999,1002-1004,1010,1062,1353,1401-1405], Particularly efficient catalysts for intermolecular cyclopropanation are C2-symmetric cop-per(I) complexes, as those shown in Figure 4.20. These complexes enable the formation of enantiomerically enriched cyclopropanes with enantiomeric excesses greater than 99%. Illustrative examples of intermolecular cyclopropanations are listed in Table 4.24. 

Alcohols and jlkenes are also primary products and are not shown in the simplified Eq. 15.182. The overall reaction is complicated and, as a result, its mechanism has been the subject of considerable debate.188 The reaction may be viewed as the reductive polymerization of carbon monoxide, with molecular hydrogen as the reducing agent. A variety of heterogeneous catalysts, such as metallic iron and cobalt on alumina, have been used. It is believed that carbon monoxide dissociates on the catalytic surface to give carbides and that these are in turn hydrogenated to give surface carbenes 1 " n ... 

The conversion of methanol to ethanol with carbon monoxide and hydrogen has attracted considerable attention. Further carbonylation to higher alcohols occurs much more slowly, but acetic acid formation is a competing reaction and this leads to ester formation. Using CoI2 in presence of PBu 3 as catalyst, the selectivity to ethanol was improved by addition of the borate ion B4072. 399 This was attributed to an enhanced carbene-like nature of an intermediate cobalt-acyl complex by formation of a borate ester (equation 76). This would favour hydrogenolysis to... 

Dioximato-cobalt(II) catalysts are unusual in their ability to catalyze cyclopropanation reactions that occur with conjugated olefins (e.g., styrene, 1,3-butadiene, and 1-phenyl-1,3-butadiene) and, also, certain a, 3-unsaturated esters (e.g., methyl a-phenylacrylate, Eq. 5.13), but not with simple olefins and vinyl ethers. In this regard they do not behave like metal carbenes formed with Cu or Rh catalysts that are characteristically electrophilic in their reactions towards alkenes (vinyl ethers > dienes > simple olefins a,p-unsaturated esters) [7], and this divergence has not been adequately explained. However, despite their ability to attain high enantioselectivities in cyclopropanation reactions with ethyl diazoacetate and other diazo esters, no additional details concerning these Co(II) catalysts have been published since the initial reports by Nakamura and Otsuka. 

In 2003, Cenini and coworkers reported (tetraarylporphyrin)cobalt(II) complexes 326 as efficient catalysts (1 mol%) for cyclopropanations. In the absence of air, styrenes 321 underwent an efficient cyclopropanation with ethyl diazoacetate 322 giving cyclopropanes 324 in 65-99% yield with 3-5 1 trans/cis ratios (Fig. 77) [348]. Simple olefins and more hindered diazoesters did not react. With diazoacetate and hydrocarbons, such as cyclohexane or benzene, C-H insertion took place furnishing cyclohexyl- or phenylacetate. In line with Ikeno s proposal the cyclopropanation reaction was considerably slowed down in the presence of TEMPO, though not completely inhibited. Based on a kinetic analysis a two-electron catalytic cycle with a bridged carbene unit was formulated, however. 

In a recent study, R. Pettit et at. examined the validity of tire Fischer-Tropsch carbide mechanism, the Anderson-Emmett hydroxy carbene mechanism and the Pichlcr-Schulz mediaiiism [174. In a first experiment, the Schulz Flory distribution obtained by CO/H conversion over a cobalt catalyst in the absence and in the presence of CH N] was studied. It was found that addition of CHjN resulted in a signillcant increase of the propagation rate which is in favour of the assumption of methylene as a building block, as predicted by the carbide mechanism. Furthermore, the reaction was carried out using labeled CO (90% CO and 10% CO), H2. and CHjNj in variable ratios. The number of atoms in the propenc fraction was calculated according to the three... 

Bis(camphorquinone-a-dioximato)cobalt(II) (10) has been developed as a catalyst for enan-tioselective cyclopropanation reactions. It allows selective carbene transfer from diazoacetic esters to terminal C-C double bonds which are in conjugation with vinyl, aryl, alkoxycarbonyl or cyano groups, but not to alkyl-substituted alkenes, cycloalkenes, 1,3-dienes and al-lenes. The unusual chemoselectivity and some other experimental observations make the two mechanistic pathways proposed vide supra) questionable for these special carbene-transfer reactions. In contrast, the cobalt(II) complex 11 allows not only the cyclopropanation of styrene but also of oct-l-ene, a nonactivated alkene (ethyl diazoacetate, 35 °C, 3mol% of catalyst yield 50-60%). ... 

The ability of chiral bis(camphorquinone-a-dioximato)cobalt(Il) complexes (Section 1.2.1.2.4.2.6.3.1.) to catalyze carbene transfer from diazocarbonyl compounds (diazoacetic esters, 2-diazo-l-phenylethan-l-one) to terminal alkenes conjugated with vinyl, aryl, carbonyl, and cyano groups, has already been mentioned. The ee-values are 75-88 /o at best, often lower. The highest values are again obtained with bulky diazoacetic esters. The significance of these catalysts, however, is their ability to promote cyclopropanation of electron-deficient alkenes, such as acrylates and acrylonitriles, in contrast to the rhodium and copper catalysts discussed above. 

Interestingly, cobalt porphyrin catalysts tend to prevent carbene dimerization reactions, and allow cyclopropanation reactions with electron-deficient alkenes. This feature illustrates the more nucleophilic behavior of the carbenoid species formed as compared to typical electrophilic Fischer carbenes. The enhanced nucleophilic character of the carbene reduces its tendency to dimerize and allows reactions with more electron-deficient olefins. 

In contrast, the cyclotrimerization catalyzed by the Grubbs catalyst involves consecutive intramolecular ring metathesis through carbene complexes or formation of a ruthenabicy-clo[3.2.0]heptatriene intermediate similar to that formed in the cobalt complex-catalyzed reaction. This mechanism is illustrated in Scheme 2 using acetylene as the alkyne source and CpRuCl[COD] as catalyst. 

The heptanuclear iron carbonyl cluster [Fe3(CO)u(/u-H)]2-Fe(DMF)4 (178) acted as an efficient catalyst in the reduction of carboxamides by l,2-bis(dimethylsilyl)benzene in toluene to the corresponding amines in high yields. Several tertiary and secondary amides including a sterically crowded amide were also reduced smoothly A review of the development of optically active cobalt complex catalysts for enan-tioselective synthetic reactions has addressed the applications of ketoiminatocobalt(II) complexes such as (5)-MPAC (179) and (5)-AMAC (180), transition-state models for borohydride reduction, halogen-free reduction by cobalt-carbene complexes. 

Transition metal salts or complexes are known to catalyze effectively the cyclopropanation of olefins with diazoalkanes. Asymmetric synthesis with chiral copper catalysts (Nozaki et ai, 1966, 1968 Noyori et al., 1969 Moser, 1969), as well as a detailed kinetic study (Salomon and Kochi, 1973), has suggested the intervention of copper-carbene complexes as reactive intermediates. Recently synthesis of crysanthemic acid (CCXXXIV) (R = H) with high optical yield (60-70 %) has been achieved by applying this asymmetric catalysis (Aratani et al., 1975). The camphorglyoxime-cobalt(I) complex is also effective for the enantioselective reaction (Tatsuno et al., 1974). 

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