Synthesis of Silenes

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

A somewhat milder route which appears to be devoid of the complications of isomerization is the retro-Diels-Alder reaction of bicyclo [2.2.2] octadienes, frequently substituted with aryl groups (5,30,53,65), [Eq. (2)], and recently Wiberg (88,90) described a very mild route involving both [2 + 2] and [2 + 4] cycloreversions which occur at 60°C to generate Me2Si=C(SiMe3)2. However, the generality of this latter source of silenes has not been established yet [Eq. (3)]. 

Another thermal route which has been particularly useful for the generation of silaaromatics (silabenzene, silatoluene, 4-substituted silaaromatics, etc.) and silafulvenes is the ene reaction of allylsilanes, as illustrated [Eq. (4)]. 

Two indirect routes to silenes, one derived from silylenes and the other from silylcarbenes, are of some generality and importance. Silylenes (e.g., Me3Si—Si— ]) (53) have been derived from the thermolysis of either methoxy or chloro polysilyl compounds. Thermolysis resulted in the elimination of trimethylmethoxy- or trimethylchlorosilane and yielded the silylene, which, based on products of trapping, clearly had rearranged in part to the isomeric silene [Eq. (5)]. Alternatively the silylene Me2Si has 

Several di- or polysilyl systems have been found to be useful precursors for the photochemical generation of silenes. Vinyldisilanes cleanly yield silylmethylsilenes (133), while alkynyldisilanes yield mixtures of silylated silaallenes and silacyclopropenes [Eq. (8)] (119,136). Aryldisilanes when photolyzed form species presumed to be silenes, but showing unusual chemical behavior (see below) [Eq. (9)] (97-102). 

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Intramolecularly coordinated silenes have been prepared by Oehme et al.868-871 Adopting their general procedure for the synthesis of silenes (Scheme 33), 8-dimethylaminonaphth-l-yl lithium has been reacted with (Me3Si)3SiCHCl2 yielding the disilene (Me3Si)RSiC(SiMe3)2 929 via the intermediates 930-933 (R = 8-dimethyl-aminonaphth-l-yl) (Scheme 131). 

But till now only methods involving an in situ formation of deprotonated 1 have been described. In this paper we present a procedure leading to pure, isolated 1-hydroxyalkyl-polysilanes 1. In presence of base fliey are easily converted into transient silenes, which were characterized by various dimerization and addition reactions. The availability of isolated 1 in the synthesis of silenes according to the Peterson concept offers the possibility of a free choice of the solvent and the base used to initiate the silanolate elimination. With respect to the significance of the reaction medium for the silene generation and its subsequent reactions, this is of particular importance. 

Scheme 2.157. Synthesis of silenes by the sila-Peterson reaction.

It may well be that activity in the area of silene chemistry has peaked. Certainly, the number of papers currently published is lower than it was a few years ago. This may be due, at least in part, to the fact that many of the easier and more obvious experiments have been attempted. However, only the discovery of important applications is required for the field to be revitalized. In particular, the successful synthesis of useful polymers... 

After decades of unsuccessful attempted syntheses, Gusel nikov and Flowers in 1967 reported the first compelling evidence for the existence of silenes, compounds containing a double bond between silicon and carbon. This initiated a renewed interest in the synthesis and behavior of stable silenes,8 disilenes,b iminosilanes,c phosphasilenesd and their heavier homologs. 

Summary The formation, reactivity, and cycloaddition behavior of neopentylsilenes towards suitable reaction partners is described. Especially l,l-dichloro-2-neopentylsilene. Cl2Si=CHCH2Bu (2) - easily obtained from vinyltrichlorosilane and LiBu - is a useful building block for the synthesis of SiC four membered ring compounds. These can be converted into the isomeric Diels-Alder and retro ene products upon thermolysis reactions. The mode of the silenes cycloaddition reactions ([4+2] vs [2+2] addition) can be directed by either the substitution pattern at the Si=C moiety, the choice of reaction partners or the conditions. Furthermore the products resulting from cycloaddition reactions open up a wide variety of following reactions, which possibly will lead to new organosilicon materials or pharmaceutical compounds. 

In summary, it is clear that in the relatively few years since the synthesis of the first silene was reported, a wide variety of reliable routes has been developed which can lead cleanly to silenes that vary widely in structure and stability. 

The attempted synthesis of 104 from its LiF adduct by salt elimination leads exclusively to the silene 104a65,66. 104a can be reacted with benzophenone to give the [2 + 4] and [2 + 2] cycloadducts 105 and 10667. The [2 + 4] cycloadduct of silene 104 cannot be obtained directly. The adducts 105 and 106, however, rearrange to the thermodynamically more stable 107, probably via the donor adducts 108, 109 and the free silenes 104 and 104a (equation 24). 

The most straightforward synthesis of unsaturated Si=C compounds is the formation of the double bond by 1,2-elimination of a salt. This method has been widely used by N. Wiberg s and N. Auner s groups in recent years to produce a variety of different silenes. 

A photochemical approach via a silylene-to-silene rearrangement was followed by Fink and coworkers in their synthesis of silacyclobutadienes in a 3-methylpentane matrix at low temperatures161. Irradiation of the cyclopropenyltrisilane 294 gives the relatively stable cyclopropenylsilylene 295. 295 can be efficiently converted to silacyclobutadiene 296 by irradiation into the visible absorption band of the silylene (equation 72)161 162. 

The synthesis of siladihydrotriazoles 412a is easily achieved by the [2+3] cycloaddition reaction of azides RN3 with the silenes Me2Si=C(SiMe3)2 92 (equation 226). 

This chapter reviews the body of literature (to August 2000) that deals specifically with kinetic studies of the reactions of silenes and disilenes and the mechanistic information that has been derived from them. The spectroscopic properties, structures, methods of synthesis and qualitative aspects of the reactivity of silenes and disilenes have been covered comprehensively in the preceding two volumes in this series and elsewhere, and so will not be treated extensively here. 

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