Silyl cobalt complexes

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

Diketimininates, for cobalt(II) complexes, 7, 16 /3-Diketonates, liquid crystals, 12, 238 Diketones, asymmetric hydrogenation, 10, 48 a-Diketones, bis-silylation, 10, 746 l,l -Dilithioruthenocene, preparation, 6, 630 Dimanganese decacarbonyl complexes, characteristics, 5, 761... 

While platinum and rhodium are predominantly used as efficient catalysts in the hydrosilylation and cobalt group complexes are used in the reactions of silicon compounds with carbon monooxide, in the last couple of years the chemistry of ruthenium complexes has progressed significantly and plays a crucial role in catalysis of these types of processes (e.g., dehydrogenative silylation, hydrosilylation and silylformylation of alkynes, carbonylation and carbocyclisation of silicon substrates). 

Transition metal carbonyls such as Co2(CO)8 and CoH(CO)4, formed in the reaction of R3SiH with dimer (but also Fe(CO)5 and M3(CO)i2 (M = Fe, Ru, Os)) have been found to be active catalysts for the hydrosilylation of olefins, dienes, unsaturated nitriles, and esters as well as for hydrosilylation C=0 and C=N bonds [56]. Hydrosilylation of phenylthioacetylenes in the presence of this catalyst is extremely regioselective [57]. Cobalt(I) complexes, e. g., CoH(X)2L3 (X = H, N), could be prospective candidates for investigation of the effectiveness of alkene hydrosilylation by trialkoxysilanes as well as dehydro-genative silylation [58]. Direct evidence for the silyl migration mechanism operative in a catalytic hydrosilylation pathway was presented by Brookhart and Grant [59] using the electrophilic Co cationic complex. 

FIGURE 19 First example of the Nicholas reaction [434, 173] [313] on a solid support. Various propa-rgylic alcohols were immobilized on a Merrifield resin. After forming the cobalt-alkyne complex, Nicholas reaction was performed as a diversifying cleavage step. Trimethylallylsilane, anisole, and silyl enol ethers were used as the carbon nucleophiles. 

H, 13C and 29SiNMR spectroscopy have been used extensively to assign structures and study fluxional processes in transition-metal silyl complexes. For example, cobalt silyls (CO)4CoSiR3 (R = Ph, F, Cl) are fluxional in solution as determined by variable-... 

The catalytic addition of organic and inorganic silicon hydrides to alkenes, ary-lalkenes, and cycloalkenes as well as their derivatives with functional groups leads to their respective alkyl derivatives of silicon and occurs according to the anti-Markovnikov rule. However, under some conditions (e.g., in the presence of Pd catalysts), this product is accompanied by a-adduct (i.e., the one containing an internal silyl group). Moreover, dehydrogenative silylation of alkenes with hydrosilanes, which proceeds particularly in the presence of iron- and cobalt-triad complexes as related to hydrosilylation (and very often its side reaction), is discussed. 

The investigations described so far indicate that reactions of metal carbonylates in general lead only to the coordination of two metal carbonyl groups to the 1,3,5-trisilacyclohexanes. From the chemistry of silylcobalt compounds it is known that silanes of the type R SiH (n = 1-3) cleave dinuclear Co2(CO)g, producing silyl-cobalt-carbonyl complexes [166, 167, 171]. 

Konno reported a detailed study of the reaction of fluorine-containing propargyl acetates under Nicholas conditions with a variety of nucleophiles. Allylstannanes and allylsilanes provided moderate yields of the desired products, while enamines, silyl enol ethers, and silyl ketene acetals furnished the target compounds with excellent efficiency. In one example, cobalt-alkyne complex 25 reacts with silyl ketene acetal 26 in the presence of trimethylsilyl triflate to yield, after cobalt decomplexation, ester 27. ... 

As part of his research into the synthesis of oxygen-bridged nine- and ten-membered cycloalkanes, Montana introduced a key carbon fragment via the Nicholas reaction. Boron trifluoride promoted reaction between silyl enol ether 28 and cobalt-alkyne complexed propargyl alcohol 29 provides substitution product 30 in excellent yield. ... 

Tyrrell demonstrated a three step tandem sequence involving an intermolecular Nicholas reaction, intramolecular Nicholas reaction, and a cationic cyclization. Treatment of silyl enol ether 55 with hexacarbonyl(propiolaldehyde diethyl acetal) dicobalt and boron trifluoride provides cobalt-alkyne complex 56. Exposure of this material to tetrafluoroboric acid promotes an intramolecular Nicholas reaction to form the second six-membered ring. Alkyne decomplexation with ceric ammonium nitrate enables the final cyclization step to yield the target tricycle 57." ... 

The mechanistic and synthetic puzzle of alkyne hydrosilylation opened more fully with the discovery that rhodium will catalyze the /r.mr-hydrosilylation of terminal alkynes.22 There is much work extant in this area, and good summaries of the various catalytic systems exist.11 A trans-addition process to give (Z)-j3-silane products G is well precedented with trialkylsilanes (Table 3), for both rhodium and mixed rhodium-cobalt complexes (entry 4).22,26 However, the selectivity erodes significantly upon switching to Me2PhSiH (entry 5), and, due to the mechanistic requirements for equilibration of the /3-silyl vinylrhodium intermediate, electron-poor silanes react exclusively to give CE)-/3-silane products B (see entries 6 and 7). 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

Silicon and germanium hydrides react with cobalt, manganese and rhenium carbonyls affording complexes having a silicon (or germanium)-metal bond. These reactions, described previously for inactive compounds have been used in the synthesis of optically active silyl and germyl-transition metals ... 

Unlike Pt catalysts, the triad complexes of iron and cobalt catalyze competihvely both dehydrogenative silylation and hydrosilylation [11]. The reaction can proceed via a complex containing the o-alkyl and o-silylalkyl ligands (Scheme 14.1). 

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