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652 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

1. INTRODUCTION

Much attention has been directed toward the heavier carbene analogues, that is, silylenes (:SiR2),1,2 germylenes (:GeR2),1,3 and stannylenes (:SnR2). In the early

stages of the investigation of silylenes, most of the results were concerned with dihalosilylenes (:SiX2).4 During the last two decades, however, the chemistry of organosilylenes (:SiR2)5 has progressed rapidly, accompanied by the synthesis of suitable precursors leading to silylenes and germylenes under various conditions. There has been special progress in the synthesis and isolation of ‘‘electronically (thermodynamically) stabilized’’ silylenes. Recently, some ‘‘kinetically stabilized’’ silylenes have also been isolated by the introduction of bulky substituents on the silicon atom. The synthesis of electronically stabilized silylenes has been performed by introduction of heteroatom substituents or conjugated systems on the silicon atom, while that of kinetically stabilized silylenes has been achieved by full substitution with bulky ligands, which prevent reactive divalent species from further oligomerization and/or intermolecular reactions.

2. GENERATION OF SILYLENES BY THERMALLY INDUCED a-ELIMINATION AND PHOTOEXTRUSION FROM OLIGOSILANES

2.1. Thermolysis of Polysilanes and Oligosilanes

Pyrolysis of polysilanes played an important role in the discovery of silylene reactions. Through the pyrolysis of alkoxydisilanes in the presence of diphenylacetylene, Atwell and Weynberg6 obtained a product regarded as a dimer of dimethylsilylene adduct. 1,2-Shift of a methoxy group in disilanes takes place under relatively mild conditions (Scheme 14.1).6

The reactions are interpreted in terms of concerted silylene extrusions in which a substituent migrates from the incipient divalent silicon atom during Si Si bond cleavage. The migrating group X can be hydrogen, halogen, alkoxy, amino groups,

Scheme 14.1

and so on.7 In 1964, Gilman et al.8 studied the thermal elimination of dimethyland diphenylsilylenes from the corresponding 7-sila- and 7-germanorbornadienes at relatively high temperature. Many investigators used this method later with a variety of substituents on the basal carbon atoms (Scheme 14.2).

Scheme 14.2

The larger the number of substituents on the basal carbon atoms, the more stable the 7-silanorbornadiene derivatives are. The reaction proceeds very slowly when the substituents are bulky and electron withdrawing.2 On the other hand, the germanium analogues generate the corresponding germylenes both by photolysis with

light of l ¼ 254 nm and thermolysis under relatively mild conditions (70– 150 C).9,10 Pyrolysis of siliranes also has been employed as a mild route to generate silylenes since the pioneering work of Seyferth et al.11,12 on hexamethyl-

silirane in 1976 (Scheme 14.3).

However, other siliranes are much more thermally stable than hexamethylsilirane, and do not serve as sources of :SiMe2 at such low temperatures. Boudjouk et al.13 made an important step when they found that bulky substituents such as the tert-butyl group on the silicon atom increased the stability of the silirane without preventing its thermolysis.14,15

654 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

Scheme 14.3

The reversibility of most silylene addition reactions allows the cycloadduct of a silylene to 1,3-diene to be employed as a silylene source. Extrusion of a silylene from 1-silacyclopent-3-ene (3) has been achieved by thermolysis in the gas phase16 and also by photolysis in solution (Scheme 14.4).

Scheme 14.4

In contrast to the somewhat complicated thermal behavior of siliranes, photolysis of silirene (4) readily leads to the loss of dimethylsilylene (Scheme 14.5 ).17,18

R=R'=Me;

R=Ph, R'=SiMe3

Scheme 14.5

2.2. Generation of Dimethylsilylene from Polysilanes and Oligosilanes

In spite of its long-assumed intermediacy in several reactions, no carbon-substituted silylene was directly observed for many years. In 1979, however, Drahnak et al.19 detected a broad ultraviolet (UV) absorption band (lmax ¼ 450 nm) after the photolysis of dodecamethylcyclohexasilane (6) in 3-methylpentane. This band was assigned to dimethylsilylene (5). Many different approaches to this intermediate, either photochemically or thermally, were examined (Scheme 14.6).20

Scheme 14.6

2.3. Photolysis of Linear Polysilanes

There have been a number of routes to silylenes that are related to the simple photolysis of the precursors. The principal photoprocesses of alkylpolysilanes in solution are (a) chain abridgement through elimination of silylene and (b) chain scission by Si Si bond homolysis. The photolysis of trisilanes [RR0Si(SiMe3)2] or cyclopolysilanes [(RR0Si)n] is a well-established method for the generation of silylene (Scheme 14.7).5,21

Scheme 14.7

In cyclohexane–Et3SiH (1:1), the exhaustive irradiation of poly(di-n-hexasilane) [poly-(Hx2Si)] (Hx ¼ hexa) or poly(di-n-butylsilane) [poly(Bu2Si)] with light of l ¼ 248 nm (pulsed) or 254 nm produced the silylene trapping product of the silylene [Et3SiSiR2H] and the homolytic cleavage product H(SiR2)nH (R ¼ Hx or Bu), respectively.22 The compound (Me3Si)2SiMes2 is the precursor of choice for the generation of Mes2Si:, which readily adds to alkenes to afford stable siliranes.2

Photolysis of 2,2-diaryl-substituted trigermanes generated the corresponding digermanes and diarylgermylenes. A similar reaction of bis(trimethylsilyl)diaryl-

germane gives hexamethyldisilane (HMD) and the expected diarylgermylene (Scheme 14.8).10,23,24

656 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

Scheme 14.8

2.4. Photolysis and Thermolysis of Cyclotrisilanes

and Cyclotrigermanes

Strained cyclotrisilane derivatives, which are silicon analogues of cyclopropane, are prepared by the reductive coupling of overcrowded dichlorodiarylsilanes using lithium naphthalenide (Scheme 14.9).

M=Si, Ge

M=Si, Ge

Np =

Scheme 14.9

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment.22,25 Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes from photolysis of stable cyclotrisilanes (Ar2Si)3.26 Cyclotrigermane derivatives