Halosilanes, racemization

The tendency of halosilanes to be racemized by nucleophiles (HMPA, etc.) is parallel to the tendency of the Si-X bonds to be displaced... 

In conclusion, all these observations suggest the presence of a five-coordinate silicon intermediate both in nucleophilic displacements at silicon and in the racemization of halosilanes as previously suggested (5, 59). The ability to form a pentacoordinate silicon adduct is not controlled by the electronegativity of the substituents at silicon, but by the tendency of the Si—X bond to be stretched under the influence of a nucleophile ... 

Nucleophilic activited racemizations have been observed in the case of chlorophosphonates 93) and halosilanes 39). They show similar rate laws ... 

A reaction related to the nucleophile-catalysed hydrolysis of silanes is the nucleophile-catalysed racemization of silanes. The racemization of a range of halosilanes was found to have an order with respect to nucleophile varying from 1 to 346,47. The reactions have entropies of activation which are large and negative, and enthalpies of activation which are small and sometimes negative. Both racemization and hydrolysis are slowed by increasing steric crowding at silicon.48... 

Recently Bassindale et al. (288) discovered a new mechanism of halo-silane racemization in the presence of nucleoophiles. They performed dynamic NMR studies of the inversion at silicon catalyzed by HMPA and N-trimethylsilylimidazole, using as a model a halosilane having di-astereotopic methyl groups. Diastereotopic separation of the H- and 13C-NMR signals is apparent in both the substrate and the silylonium complex. Thus, inversion at the Si center leading to coalescence may be... 

Activation parameters at coalescence temperature show that the coordinate interaction in these compounds is not a function of the electronegativity of X but is controlled by the ability of the nitrogen atom to stretch the Si—X bond. The tendency of the silicon atom to increase its valency decreases in the order X = OCOR, Br, Cl > SR F > OR, H This sequence corresponds directly to the rate of racemization of halosilanes and to the substitution of R3SiX with inversion of configuration Although no intramolecular coordination was observed in solutions of acetoxysilanes (CH3) Si(OCOR)4 by the Si NMR method the shape of the H NMR spectra of these compounds with chiral silicon atom points to Si -0 interaction... 

The above data, together with previous reports (10, 11, 13), suggest a close relationship between the rate of racemization of halosilanes, the substitution of R3Si-X with inversion of configuration, and the ease of pentacoordination at silicon atom ... 

In the past few years, numerous experimental results have illustrated the fundamental importance of penta- and hexacoordinate silicon species in reactions at silicon. The implication of pentacoordinate intermediates in substitution reactions at silicon is now well accepted (10, 11) the nucleophilically induced racemization (13,268) and hydrolysis (or alcoholysis) of halosilanes (268, 269), both controlled by entropy factors, take place through expansion of coordination at silicon. Pentacoordinate species with two or three carbon atoms attached to silicon have been isolated (129, 155) and finally, five-coordinate anions with five carbon atoms around silicon have been identified in the gas phase (107). This shows how much the expansion of coordination at silicon is an energetically favorable process. 

The efficiency of the racemizing reagents is HMPT > DMSO > DMF. The ability of the halosilanes to be racemized is Si-Br > Si-Cl Si-F. The process is controlled by entropic factors (AS = -60 e.u.), suggesting a highly organized tran-... 

TABLE 9. Kinetic orders for the nucleophile-catalysed racemization of halosilanes ... 

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