Lithium-silicon bonds

Organic groups are bound to the silica surface after grinding silica in organic liquids (277). A more controlled substitution of surface silanol groups was reported by Wartmann and Deuel (194). Silica gel which had been treated with thionyl chloride was allowed to react with phenyl lithium. Silicon-phenyl bonds could be detected by infrared spectroscopy. The phenyl content of Aerosil treated in this way as estimated from carbon analysis corresponded to 85% of the silanol groups (188). However, it is not certain whether the reaction... 

The authors proposed the following picture of the silylene anion-radical formation. Treatment of the starting material by the naphthalene anion-radical salt with lithium or sodium (the metals are denoted here as M) results in two-electron reduction of >Si=Si< bond with the formation of >SiM—MSi< intermediate. The existence of this intermediate was experimentally proven. The crown ether removes the alkali cation, leaving behind the >Si - Si< counterpart. This sharply increases electrostatic repulsion within the silicon-silicon bond and generates the driving force for its dissociation. In a control experiment, with the alkali cation inserted into the crown ether, >Si — Si< species does dissociate into two [>Si ] particles. 

Phosphorus-silicon bonds in trimethylsilyl phosphanes can be cleaved by lithium alkyls 11). Such reactions occur in most cases even below 20°C in the presence of a solvating ether like THF or DME. 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

When the counterion is varied from lithium to sodium to potassium, the proportion of inversion increases. The relatively covalent lithium-oxygen bond favours a retentive mechanism. However, as the metal-oxygen bond becomes more ionic, the components may function more independently, allowing attack of RO on the back face of the silicon tetrahedron while electrophilic assistance by M+ of the leaving group aids inversion of configuration. 

An interesting silacycloalkane containing silicon-silicon bonds in the ring (XVII) has been obtained by introducing isobutylene into a reaction mixture of dimethyldichlorosilane and lithium metal in tetrahydrofuran at 0°-10° C (137). 

Aldimines can be trifluoromethylated at the imine carbon using Me3SiCF3 in dimethyl formamide at —20 °C, using a lithium carboxylate as catalyst.71 It is proposed that the carbon-silicon bond of the reagent is activated via formation of a lithium silicate bearing carboxylate and DMF ligands on silicon. A similar process has been used for diastereoselective addition to sulftnylimims.12... 

Wurtz coupling reactions of chlorosilanes are the main route to the silicon-silicon bonded compounds. For example, hexamethyldisilane can be prepared by refluxing trimethylchlorosilane with lithium sand in THF (97%). Lithium may be substituted by sodium by using a mixture of HMPA-THF as the solvent. Linear and branched oligosilanes can be prepared by the same method (equations 55-57). 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

The silyl halides can now" be prepared in high purity and high yield by the facile hydrogen halide cleavage of the carbon-silicon bond in arylsilanes. " No catalyst is required, and the use of the hazardous intermediate reagent, silane, is avoided. Bromosilane is prepared by the reaction of hydrogen bromide and phenylsilane. The latter is obtained by lithium hydro-aluminate reduction of the commercially available phenyltri-chlorosilane. lodosilane can be prepared in a similar fashion however, mixtures of iodosilane and benzene are difficult to separate. The preferred alternative procedure described below utilizes an isomeric mixture of 2-, 3-, and 4-chlorophenylsilanes as the intermediate. This intermediate is obtained by the chlorination of phenyltrichlorosilane, and is then reduced to the hydride. 

It was observed that ammonolysis of B(C2H,Si(R)H2)3 (Scheme 2, route A) requires basic catalysts such as n-butyl lithium. The reaction is performed in analogy to the potassium hydride-catalyzed cross-linking of cyclic silazanes described by Seyferth et al. [8]. Most probably, n-BuLi initially deprotonates the weak nucleophile ammonia with the formation of lithiiun amide and evaporation of n-butane. The stronger nucleophilic amide then replaces a silicon-bonded hydride, which subsequently deprotonates ammonia, leading to the evolution of molecular hydrogen. The silylamines that arise are not stable under the reaction conditions applied (refluxing solvent), and by fast condensation of ammonia the polymeric precursors form [6]. 

Recently, we were able to show that lithium organyls are also able to interact with CO [6] in a newly developed LXe cell constructed from one piece of single crystal silicon [7]. In a first step carbon monoxide is complexed by back-bonding to BuLi (n(CO) 2047 cm ) and inserts in a second step at higher temperature into the lithium-carbon bond (n(CO) 1635 cm ). Further... 

The formation of an asymmetric silyllithium reagent by lithium cleavage of the silicon-silicon bond of an optically active disilane 10 (eq. [7]) has been reported (26). Hydrolysis of the silyllithium reagent 11 yielded an optically active silicon hydride. This result demonstrates that silyllithium has considerably enhanced optical stability relative to acyclic alkyllithium compounds. 

The iron-silicon bond in these complexes appeared to be very stable. Nucleophilic cleavage was observed only with water and with lithium aluminum hydride (Scheme 52). Although not specific, nucleophilic cleavage occurs with predominant retention of configuration at silicon. 

The observed stereoselective lithium cleavage of a silicon-silicon bond and stereoselective protonolysis demonstrate the configurational stability of the silyllithium reagent. 

After considerable experimentation, a similar hydrosilylation protocol was used as a key step for the syntheses of jatrophatrione and citlalitrione by Paquette and co-workers.32 Following the stereoselective reduction of a tricyclic ketone with lithium aluminum hydride to provide alochol 28, silylation and platinum catalyzed hydrosilylation were effected to produce 29. Finally, the carbon-silicon bond was successfully cleaved to generate diol 30 in an impressive 93% yield. 

The low ionic character of the aluminium-silicon bond has been cleverly utilized to develop a very mild, general and effective synthesis of acyl silanes, successful for aliphatic, aromatic, heteroaromatic, a-aUcoxy, a-amino and even a-chiral and a-cyclopropyl acyl sUanes. Acyl chlorides are treated with lithium tetrakis(trimethylsilyl)aluminium or lithium methyl tris(trimethylsilyl) aluminium in the presence of copper(I) cyanide as catalyst to give the acyl silanes in excellent yields after work-up. Later improvements include the use of 2-pyridinethiolesters in place of acyl halides, allowing preparation of acyl silanes in just a few minutes in very high yields indeed (Scheme 9) °, and the use of bis(dimethylphenylsilyl) copper lithium and a dimethylphenylsilyl zinc cuprate species as nucleophiles. 

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