Disilenes with alcohols

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

Recently, however, Sekiguchi et al. reported that the transient disilene ( )- and (Z)-PhMeSi=SiMePh reacted with alcohols in a syn fashion with high diastereoselectivity, the extent of which depended on the concentration and steric bulk of alcohols used.31 These facts suggest that in the reaction with a sterically hindered disilene like (Z)-3, the addition of alcohols might also proceed diastereoselectively under appropriate reaction conditions. 

The high diastereoselectivity in the addition of i-PrOH, t-BuOH and EtOH (at low concentration) suggests that E Z photoisomerization of (E)- or (Z)-16 does not occur in solution at room temperature or that the trapping of (E)- or (Z)-16 by alcohols proceeds faster than the E Z isomerization. In addition, the results show that proton transfer in the intermediate adduct formed by the disilenes and alcohols occurs much faster than rotation around the Si—Si bond. However, in the reaction with ethanol, an appreciable amount of the anti addition product was formed. Thus, the diastereoselectivity remarkably depended on the concentration of ethanol. 

Most of the work done so far on the mechanism of alcohol additions to disilenes has been carried out with the kinetically stabilized derivatives ( )-92a151-153, ( )- and (Z)-92b152 and tetramesityldisilene (110)151,153 154. These compounds react with alcohols some 109 —1012 times more slowly than do the transient disilenes 103 and 104 and, as was shown specifically for 110, the rates vary more dramatically as a function of alcohol structure153. 

Kinetic data for other characteristic bimolecular reactions of disilenes are much more limited than is the case with alcohol and phenol additions, and hence contribute little to the understanding that product studies have already provided in regards to reaction mechansims. The only absolute kinetic data known at the present time, for reaction of disilenes 103, 104 and 35 with 2,3-dimethyl-1,3-butadiene, oxygen and a few symmetric n-alkanones in hydrocarbon solution at room temperature, are listed in Table 19. Unfortunately, none of these reactions has been specifically characterized with product studies, as far as we know. The data indicate that the reactivity of relatively nonpolar disilenes toward these reagents decreases in the order ko2 > A r2c=o 1 EtOH >... 

The data of Table 19 indicate there to be very little difference between the absolute rate constants for trapping of 103, ( )-104 and (Z)-104 by 2,3-dimethyl-l,3-butadiene158. As with alcohol and 02-additions, the tetrakis(trialkylsilyl)disilene 35 exhibits substantially lower reactivity than 103 and 10468. None of the products of these reactions has been isolated68,158. 

As stated before, there is little knowledge on the unsymmetrically substituted stable disilenes because stable disilenes are usually prepared by dimerization of silylenes, thus leading to symmetrical disilenes. Unsymmetrically substituted disilenes are produced mostly as transient species (see the preceding section), and it was found that ( )- and (Z)-l,2-dimethyl-l,2-diphenyldisilenes undergo the addition reaction with alcohols very rapidly (k2 = 10 -10 s ). The rates are only 1 to 2 orders of magnitude smaller... 

Disilenes readily add halogens14,66 and active hydrogen compounds (HX), such as hydrogen halides,63,66 alcohols, and water,27 63 as well as hydride reagents, such as tin hydride and lithium aluminum hydride.66 These reactions are summarized in Scheme 9. The reaction of the stereo-isomeric disilene (E)-3 with hydrogen chloride and alcohols led to a mixture of E- and Z-isomers, but the reaction with chlorine gave only one of the two possible stereoisomers, thus indicating that the former two reactions proceed stepwise while the latter occurs without Si—Si rotation. 

Phenyltrimethyldisilene 15, produced by irradiation of the precursor 13 (X > 280 nm) in the presence of several alcohols, gives rise to the formation of 1 -alkoxy-2-hydrido-l,l,2-trimethyl-2-phenyldisilane (26) as the major product along with a small amount of the isomeric l-alkoxy-2-hydrido-l,2,2-trimethyl-l-phenyldisilane (27) (see Scheme 3). As shown in Table 3, very high regioselectivity was observed. This is the first example demonstrating a regioselective addition reaction to the unsymmetrically substituted disilenes. 

Methoxy- and amino-substituted disilenes behave differently from 1529. Irradiation of a hexane solution of 20 in the presence of various alcohols at room temperature afforded 1,1-dialkoxyhydrodisilanes 28a-32a together with a small amount of the regioisomers 28b-32b (Scheme 5). Thus alkoxy groups direct the alcohol addition to the alkoxy-substituted silicon atom. The ratios of regioisomers (a/b) were 100/0 (EtOH, 28), 96/4 (i-PrOH, 29) and 93/7 (t-BuOH, 30). Steric bulkiness is not the only factor that determines the regioselectivity, since bulky but acidic alcohols, such as 2,6-dimethylphenol... 

Addition of an alcohol to the amino-substituted disilene 18 proceeds with an even higher regioselectivity than that to the methoxy-substituted disilene 17. Thus, the addition of t-BuOH to 18 is completely regioselective, where only l-diethylamino-l-t-butoxy-2-hydro-l,2,2-trimethyldisilane 33a is obtained. 

Irradiation (A, > 280 nm) of a hexane/isopropyl alcohol solution of the disilene precursor ( )-14 (Scheme 8) in a quartz tube at room temperature produced threo-1-isopropoxy-1,2-dimethyl-1,2-diphenyldisilane 37a, which is the product of syn addition of isopropyl alcohol to ( )-16 together with a small amount of the erythro-isomer 38a (37a/38a => 99/1, 62% yield). High syw-addition diastereoselectivity was also found... 

A mechanism involving coordination of the alcoholic oxygen to a silicon atom, followed by a fast intramolecular proton transfer explains the predominant syn addition of alcohol to disilene. However, intermolecular proton transfer leading to the awft-product competes with intramolecular proton transfer at high alcohol concentrations. The proposed mechanism is presented in Scheme 9. 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

Absolute rate constants have been reported for the reaction of aliphatic alcohols with the transient disilenes 103, 104 and 35 in hydrocarbon solvents and are collected in Table 1768, l48. In all cases, linear dependences of fcdecay on alcohol concentration were observed, indicative of a mechanism that is first order in alcohol over the range of concentrations examined. Product studies carried out with 103 (equation 78) indicate that the reaction is highly regioselective, with the alkoxy group affixing itself to the less hindered... 

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