Substituted disilenes

An almost 60-nm red shift on going from f-butyl-substituted disilene... 

Estimated from the experimental cis-trans isomerization barrier in a substituted disilene (84MI6). 

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... 

Figure 56)155. Compounds investigated included three forms of tetramesityldisilene the solvent-free form 69156, the toluene adduct 69-C7Hs157 and the tetrahydrofuran solvate 69 THF158. Also studied were a second tetraaryldisilene (70)159 dialkyldiaryl-substituted disilene, 71160, and the only tetraalkyldisilene known to be stable as a solid, 72161. Three silyl-substituted disilenes, 73, 74 and 75, were also investigated162,163. To assist in the interpretation of the experimental results, ab initio molecular orbital calculations of the 29Si chemical shift tensors were carried out for model disilene molecules. 

Various silyl-substituted disilenes undergo reaction with potassium in DME or with Li in THF to yield the corresponding vicinal dianions (equation 30)76. 

B. Generation of Phenyl-substituted Disilenes. Spectra and Kinetics of... 

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. 

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. 

TABLE 1. Rate constants for quenching of phenyl-substituted disilenes by alcohols28... 

Alkoxy- and amino-substituted disilenes (17, 18 and 19) are produced by photolysis of the corresponding masked disilenes (20, 21 and 22)29. Thus, when a degassed 3-methylpentane (3-MP) matrix of 20 was irradiated at 77 K, the corresponding alkoxy-substituted disilene 17 was produced (equation 5) as indicated by its broad band at 373 nm. 

The formation of amino-substituted disilenes 18 and 19 was also confirmed in similar experiments using 21 and 22 (equations 6 and 7, respectively). Broad bands growing at 395 and 408 nm, respectively, were observed. Absorption maxima of several transient disilenes are summarized in Table 2, in which the absorption spectral data of the recently prepared (trimethylsilylmethyl)trimethyldisilene30 is also included. The electronic spectral data of stable disilenes are well documented8. 

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 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. 

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... 

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

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. 

As a functional disilene, Wiberg synthesized a halogen-substituted disilene. Treatment of trihalosilane 85 in pentane with a THF solution of tri(7< r/-butyl) silylsodium gives (E)-1,2-dichloro-1,2-disilyldisilene 42 in about 12% yield 22... 

As a typical unimolecular reaction of disilenes, the /i,Z-isomcrization is discussed first. In contrast to the isomerization of an alkene that occurs via the rotation around the C = C double bond with an activation energy of ca. 60kcalmol-1 the E,Z-isomerization of disilenes is known to occur more easily. As shown in review OW, the E,Z-isomerization in aryl-substituted disilenes 3,4,20,26, and 27 proceeds under mild conditions to allow the kinetic studies at 40-80 °C by NMR spectroscopy. Recently, the T,Z-isomerization between tetrakis(trialkylsilyl)disilenes ( )- and (Z)-33 was found to occur more rapidly with the rates of the NMR time scale at 30 °C 63... 

Stable aryl- and alkyl-substituted disilenes undergo the [2 + 4] cycloadditions with benzyl, acylamines, and 1,4-diazabutadienes but not with 1,3-butadienes.77 131 On the other hand, stable tetrasilyldisilene 22 reacts with 2,3-dimethyl-1,3-butadiene at rt giving the corresponding 4,5-disilacyclohexene 218 [Eq. (104)].127 The reason for... 

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>. 

Thus, in 1997 Weidenbruch and coworkers have reported the synthesis and isolation of the first stable conjugated Si-Si double-bond compound, i.e. hexatipyltetrasilabuta-1,3-diene 9479. Tetrasilabutadiene 94 was prepared through a rather unique synthetic route starting from the corresponding tetraaryl-substituted disilene 95 via the mono-lithiated disilene 96 as shown in Scheme 35. 

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