Acyclic disilenes

Among a number of review articles on stable disilenes published until now,6 9 the most comprehensive would be those written by Okazaki and West in 1996, which is hereafter abbreviated to review OW ,7 and by Weidenbruch in 2001.9 At the time when review OW was written, the number of known stable disilenes was 27 and the type was limited only to acyclic disilenes with rather simple substitution modes. During less than 10 years since then, the number of stable disilenes increased dramatically up to more than 70, and the types include not only acyclic disilenes but also endo- and exocyclic disilenes, bicyclic disilenes, and tetrasiladienes. Very recently, the syntheses of the first stable trisilaallene and disilyne were achieved. All these recent experimental achievements have stimulated renewed theoretical studies, and the interplay between theory and experiments has contributed to the innovation of the understanding of the bonding, structure, and reactivity of disilenes and to the development of the potentials of silicon chemistry. Since review OW appeared in a volume of Adv. Organomet. Chem. series, the present article is intended to be a supplemental review covering the studies performed during 1996-2004. 

Using these methods, 27 acyclic disilenes with the substitution types of A2Si = SiA2, A2Si = SiB2, ( )- and (Z)-ABSi SiAB, and A2Si = SiAB as shown in Chart 1 had been synthesized, at the time when review OW was written. In this article, the numbers for labeling these disilenes are kept to be identical with those used in review OW for convenience. The properties of 47 stable disilenes and two disilynes newly synthesized since then are summarized in Tables I IV. 

In the last decade, several new synthetic routes have been developed to allow the synthesis of various stable acyclic disilenes including ( )- and (Z)-ABSi = Si AC-type disilenes, but disilenes with the types of A2Si = SiBC and ABSi = SiCD are still unknown. 

The color of stable disilenes in the solid state ranges from pale yellow to blue. The longest absorption maxima of acyclic disilenes in solution are assignable to the it -mi transitions and are strongly dependent on the electronic and steric effects of substituents as shown in Tables I-IV. The Amax values are 393-493 nm for tetraalkyldisilenes, 394-452 nm for dialkyldiaryldisilenes, 400-460 nm for tetraaryldisilenes, 468 183 nm... 

Acyclic disilenes show the relatively low-field shifted resonances of unsaturated 29Si nuclei depending on the substituents. The 29Si resonances for A2Si = SiA2-type disilenes range from 53-66 ppm for tetraaryldisilenes, to 90-103 ppm for tetra-alkyldisilenes, and 131-156 ppm for tetrasilyldisilenes. 

Of the more specialized methods for the formation of acyclic disilenes, two new methods deserve mention. Kira and coworkers reported that photolysis of the silirene 1 afforded the diaminosilylene 223. In contrast to theoretical predictions24-27, the silylene 2 apparently does not undergo dimerization to afford the bridged dimer 3 as is usually preferred in the presence of electronegative substituents but rather, according to the results of variable-temperature electronic spectroscopy, furnishes the disilene 4 (equation 4)23. 

Table 1 summarizes the currently known, thermally stable acyclic disilenes. 

TABLE 1. Thermally stable acyclic disilenes R R2Si—SiR3R4... 

The molecular structures of eleven acyclic disilenes are described in detail in two recent review articles covering the literature up to about the middle of 19956,49a. The number of known compounds has doubled in the subsequent few years49b. These new, structurally characterized disilenes include not only a tetrasilabutadiene and the first molecules with an endocyclic Si=Si double bond, described in separate sections, but also some acyclic disilenes and, in particular, the unusual compound 8. Thus, another brief survey of all the known molecular structures of disilenes reported through the end of 1999 seems to be justified. 

TABLE 2. Bond lengths and angles in acyclic disilenes... 

The results described above clearly reflect the diversity of the possible reactions of acyclic disilenes. The 1,2-aryl migrations in tetraaryldisilenes have not been mentioned as they have already been reviewed in depth6. The same is true for the thermally induced decomposition of the disilene 35 to furnish the silylene molecule Tbt(Mes)Si , the unusual chemistry of which is reviewed elsewhere in this series129. 

Molecules with endocyclic Si=Si double bonds, like the tetrasilabutadiene 139, have also only recently become accessible and their chemistry has not yet been reviewed. In the less than four years since 1996 two three-membered, two four-membered and three five-membered ring compounds containing an Si=Si double bond have been prepared and, in most cases, their structures have been characterized by X-ray crystallography. Although two synthetic routes dominate in the synthesis of acyclic disilenes, the cyclic disilenes have mostly been prepared by the special methods summarized below. 

In analogy to the reactions of acyclic disilenes with the heavier chalcogens78,79 it can be assumed that an initial [2 + 1] cycloaddition of the respective chalcogen to one of the Si=Si double bonds of 139 takes place, followed by a rearrangement of the resultant three-membered ring to a less strained five-membered ring. 

In contrast to the acyclic disilenes, very little is yet known about the reactivity of the cyclic members of this class of compounds. The photochemically induced isomerization of the cyclotetrasilene 141c to the bicyclo[1.1.0]butane derivative 140b (Section V.A) has already been mentioned. Similarly to the tetrasilyldisilenes69 the cyclotrisilene 151 also reacts spontaneously with tetrachloromethane even at —70°C to furnish the trans-l, 2-dichlorocyclotrisilane 157 (equation 41)137. 

The situation is different for the molecules containing endocyclic Si=Si double bonds which just recently became accessible and for the as yet sole known tetrasilabutadiene, the chemistry of which still remains mostly unknown. Not only are further representatives of these classes of compounds to be expected but also novel modes of reactions that have as yet not been observed for the acyclic disilenes. Another interesting question is whether it will be possible to prepare molecules with an extended system of conjugated double bonds. 

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