Solvent-silene complexes

Again, the same general trends as were found for MeOH, AcOH, n-BuNII2 and MeOTMS are observed in the temperature dependences for reaction of the two silenes, 19a and 19e, with acetone. In both solvents, the differences are consistent with the efficiency of product formation from the intermediate complex, being greater for 19e than for 19a. Interestingly, the curvature observed in the temperature dependence for silene 19e in hexane solution indicates a turnover temperature of ca —15 °C, and the calculated complexation rate constant at this temperature, kc 1.5 x 1010 M-1 s 1, is identical to the diffusional rate constant within experimental error (fcdiff = 1.3 x 1010 M s-1). 

Equation 58 defines the equilibrium between free silene and its Lewis acid-base complex with a nucleophilic solvent. Since the complexed form of the silene can clearly be expected to be relatively unreactive toward nucleophiles compared to the free silene, then the result will be a reduction in the overall rate constant for reaction with a nucleophilic reagent (Nu-H) in a complexing solvent relative to a non-complexing one like hexane. This rate reduction is described quantitatively in equation 59. 

THF can also have an accelerating effect on reactivity, in cases where the weakly basic solvent can get directly involved in the reaction via a catalytic pathway and complexation with the free silene is weak. Such an effect has been observed for the reaction of alcohols with the aryldisilane-derived l,3,5-(l-sila)hexatriene derivatives 21a-c, as shown by the second-order rate constants for reaction of the three silenes by MeOH and TFE in isooctane, MeCN and THF solution (Table 11 note that the third-order rate constants for reaction of 21a-c with MeOH have been omitted)48. Table 11 also includes data for the reactions with acetone in the same three solvents, as an example of a reaction which has no catalytic component47. The rate constants for all three reactions decrease in the order isooctane > MeCN > THF for 21a, which complexes relatively strongly with the ether solvent, as demonstrated by the distinctive red shift in its UV absorption spectrum in THF (kmax = 460 nm) compared to isooctane and MeCN (kmax = 425 nm)48. Compound 21b exhibits a ca 10 nm shift of its absorption band in THF solution while none is detectable in the case of 21c, indicating that the equilibrium constant for THF complexation within this series of silenes decreases with increasing phenyl substitution at... 

Silene Insertion into CH Bonds By thermolyzing complexes of silaneimines with ethyldimethyl amine in solvents, one surprisingly observes the formation of insertion products of the silenes into a-C-H bonds of the donor [3]. For example, the silaneimine tBu2Si=NSiChBu2 slowly transforms in benzene even at room temperature into the mentioned insertion product (Scheme 9) [12]. 

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