Unsymmetrical disilenes

The 1/(Si=Si) values of a variety of unsymmetrically substituted disilenes were first reported in the same paper. The values are in the range 155-158 Hz, which is ca 1.8... 

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. 

Generation of various phenyl-substituted disilenes by the photolysis of the masked disilenes, 7,8-disilabicyclo[2.2.2]octadiene derivatives, is quite useful, especially for unsymmetrically substituted disilenes. Investigation of the regiochemistry as well as the diastereochemistry of alcohol addition to phenyltrimethyldisilene was made possible for the first time by using this method27. 

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 [2+2] cycloaddition reaction of the unsymmetrically substituted disilene with benzophenone proceeded with a high degree of regioselectivity to yield the 1,2,3-oxadisiletane 45 (Scheme 16) <1995CB935>. 

Indirect evidence for the validity of this assumption is provided by the structure [rf(Si-Si)=2.229(l) A, 359.9°], and especially the 1/si,Si coupling constant of the disilaoxirane 81 derived from the unsymmetrically substituted disilene 2996. With a Jsi,Si value of 123 Hz, this coupling constant is appreciably larger than those observed for other disilanes with a similar substitution pattern (85 Hz) and approaches the value of 160 Hz for the disilene 29. 

In view of the preference of the tetrasilabuta-1,3-diene 139 for the s-cis form, it seemed worthwhile to examine its behavior in [4 + 2] cycloadditions of the Diels-Alder type. Since 139, like many disilenes, should behave as an electron-rich diene, we attempted to react it with various electron-poor and also with some electron-rich olefins. No reaction was detected in any case. Only in the presence of water did 139 react with quinones to furnish the unsymmetrically substituted disilenes 36 and 37 (see Section III.A). The effective shielding of the double bonds by the bulky aryl groups and, above all, the 1, 4-separation of the terminal silicon atoms of about 5.40 A appear to be responsible for these failures. Thus, it was surprising that treatment of 139 with the heavier chalcogens afforded five-membered ring compounds in a formal [4 + 1] cycloaddition (see below). 

The size of the ring formed in each case is strongly dependent on the reaction conditions. When KC8 is used instead of sodium for the dehalogenation of the oligosilane R3SiSiBr2Cl, the unsymmetrically substituted disilene 151 is obtained. 151 is the first synthesized cyclotrisilene (equation 37>137. The mechanism of formation of 151 is also still a matter of speculation. 

The X-ray structure analyses of the unsymmetrically substituted disilenes 22 and 23 revealed the presence of somewhat stretched Si=Si and Si-Si bonds. It is not clear as to why no 1,2-additions of the hydrogen halides to the double bonds of 6 occur in these reactions or why instead the previously unobserved 1,4-additions with formation of new double bonds take place. The closest precedent involves the action of small amounts of water on 6, which is presumably initiated by a 1,2-addition and subsequent 1,3-hydrogen shift to form compound 17. 

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... 

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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