Disilenes 1,2-addition reactions

Finally, the chemical properties of the new structures available from disilene addition reactions have hardly been touched. In future developments, theoretical and experimental studies are likely to proceed together and complement one another, as they have from the beginning days of this research. 

Mechanism and structures in alcohol addition reactions of disilenes and silenes... 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

Nevertheless, mechanistic investigations on the addition reaction to disilenes and silenes advanced considerably in recent years. In this chapter, the author tries to survey the progress achieved recently. The author will not try to review all aspects of the chemistry of silicon unsaturated species since many extensive reviews on this topic are already available6-12. 

These E-Z isomerization studies of disilenes indicate that the n overlap between two 3p orbitals of silicon is sufficiently effective to retain the configuration around the double bond, although the 7r bonding of disilenes is significantly weaker than that of the C=C double bond. Therefore, it is expected that if appropriately substituted disilenes can be generated, regiochemistry as well as diastereochemistry of addition reactions to disilenes can be investigated even with transient reactive disilenes. 

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. 

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. 

TABLE 4. Addition reaction of alcohols to silicon-substituted disilenes XMeSi=SiMe229... 

Apeloig and Nakash have reported recently a Hammett-type study for the addition reactions of seven para- and meto-substituted phenols to tetramesityldisilene 39 (equation 10)54. They used a large excess of the phenol to enforce pseudo-first-order kinetics. The addition reactions are indeed firstorder in both the disilene and the phenol. 

Keywords silylenes, disilenes, tetrasilabutadiene, multiple bonds, addition reactions, cycloadditions, germanium compounds... 

It is well known from organic chemistry that butadienes undergo not only 1,2-additions but also 1,4-addition reactions. Very recently we have found that treatment of 24 with hydrogen halides, which were slowly generated from trichlorosilane or lithium bromide and trifluoroacetic acid, respectively, furnished unsymmetrically substituted disilenes as formal 1,4-addition products... 

Although the chemistry of disilenes has been developed considerably after the isolation of stable disilenes by West, Finle and Michl, rather little has been known about the mechanism, especially on the stereochemistry for the addition reactions to Si=Si bonds of nucleophiles or electrophiles. West and coworkers reported that the addition reaction of ethanol to ( )-l,2-di-t-butyl-l,2-dimesityldisilene in THE gave a 1 1 mixture of two diastereomers of alkoxysilanes (equation 9), suggesting a stepwise mechanism. However, the bulky substituents necessary to stabilize disilenes sometimes complicate the stereochemistry. In fact, for the addition reaction of water to the parent disilene (H2Si=SiH2), theoretical calculation predicted a concerted-type four-center-like transition state, leading to a yw-addition product . 

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... 

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