Reactive Intermediate Chemistry

Evidence for the generation of a silylene was obtained in the presence of triethylsilane, which afforded 2,3-diphenyltetrasilane (82) in 79% yield. The reactions of some 1,3-diynes, R0C CC CR0 (83), with silylenes afforded the silylene adduct. The course of this reaction strongly depends on the nature of the substituents R and R’. When R is an alkyl group, the bis(silirene) (84) is formed initially

but undergoes rearrangement to a bicyclohexadiene derivative upon longer photolysis (Eq. 9).108

R2'

Si

Dimesityland dibutylgermylenes react with acetylene in the presence of a catalytic amount of palladium complexes to yield the corresponding C-unsubstituted germoles (85) and compound (86). When acetylene was bubbled into a toluene solution of hexamesitylcyclotrigermane in the presence of Pd(PPh3)4 at 80 C, 1,1-dimesitylgermole was obtained in 85% yield (Eq. 10).109

ð10Þ

4.2.2. Addition to Olefins, Dienes, and Related Compounds. Reaction of methylphenylsilylene with cyclohexene, followed by treatment with methanol gives the methanolysis product expected from a silirane.110 In this reaction, attempts to isolate the siliranes from the reaction mixture were unsuccessful. Hexa-tert-butyl- cyclotrisilane photolyzes to yield both a silylene and a disilene.111 Stable siliranes are obtained by the reactions with alkenes, as predicted (Scheme 14.42). Stereospecific addition of bis(triisopropylsilyl)silylene (i-Pr3Si)2Si: to cis-2-butene has been observed,112 and a singlet ground state was deduced for diadamantylsilylene when it was found to undergo stereospecific addition to cisand trans-2-butenes.38 Dimethylsilylene reacted with 2,3-dimethyl-1,3-butadiene to afford a silacyclopentene via rearrangement of a vinylsilirane intermediate (87) (Eq. 11).113

(104), followed by rearrangement to (106), probably through a biradical intermediate (105) (Scheme 14.49).121 Under mild conditions, more direct evidence was found for the intermediacy of oxasiliranes. When 2-adamantanone and 7-norbor- none were allowed to react with photochemically generated dimethylsilylene, the major products could be formulated as dimers (108) and the carbonyl adducts

Scheme 14.49

(110) of oxasiliranes (107) (Scheme 14.50).122 Stable oxasilirane (111a) was isolated by the reaction of dimesitylsilylene with 1,1,3,3-tetramethyl-2-indanone (112a).123 The sulfur analogue thiasilirane (111b) was also isolated from the reaction of dimesitylsilylene with 1,1,3,3-tetramethyl-2-indanethione (112b). Although

Me2

Si

Me2Si : + O=CR2

OCR2

R2C O

Si 108

Me2

Scheme 14.50

adamantanone (113a) yielded the 2:1 adduct (114) instead of oxasilirane, adamantanethione (113b) gave the stable thiasilirane (115) (Scheme 14.51). Using matrix isolation techniques, spectroscopic evidence was obtained for the intermediacy of the silacarbonyl and silathiocarbonyl ylides (117a and 117b) in low-temperature matrices (Scheme 14.52).124 Irradiation of the oxasilirane (111a) in matrix (Ip/3-Mp) at 77 K with a low-pressure mercury lamp led to the appearance of a new

682 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

Scheme 14.52

band at 610 nm in the UV–vis spectrum and the matrix became interestingly blue. This absorption band was stable at 77 K on prolonged standing. However, it immediately disappeared on brief irradiation with a Xenon lamp (l > 460 nm) or when the matrix was allowed to melt. This colored species was independently generated by the reaction of dimesitylsilylene with 113a.122c

Dimethylsilylene reacts smoothly with a-diketones to yield 1,3-dioxa-2-sila- cylopentane-4-enes (118 and 119, Scheme 14.53).125

684 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

5. SYNTHESIS, STRUCTURES, REACTIONS, AND DIMERIZATIONS OF STABLE SILYLENES

5.1. Synthesis and Isolation of a Stable Dialkylsilylene

Divalent silicon compounds (silylenes), one of the most interesting class of lowcoordinated silicon compounds, had been known as highly reactive, short-lived

transient species that resembled the carbon analogues (carbenes), until the recent success in the synthesis of the first stable silylene by West and co-workers.128,129

Their structures had been much less explored than those of their heavier congeners such as germylenes and stannylenes. Although dodecamethylsilicocene (19), formally also a silylene, has been known since 1986,36 compound 19 is stabilized by Z5-coordination of pentamethylcyclopentadienyl ligands and is not a congener of a carbene.

Recently, introduction of amino substituents on the silicon atom resulted in an

epoch-making breakthrough in this field, that is, the synthesis of stable silylenes such as 122–125 (Scheme 14.56).128,130 Although these silylenes are all well char-

acterized by either X-ray crystallographic analysis or electron-diffraction analysis,

Scheme 14.56

the structural features of these special silylenes revealed that they are stabilized by strong interaction between the vacant p-orbital at the divalent silicon and filled p-type orbitals of the nitrogen atoms in the substituents. The structures and reactivities of these silylenes indicate that their frontier orbitals are considerably perturbed by their heteroatom substituents to such an extent that they are different from those of a true analogue of a singlet carbene. Several stable silylenes have been synthesized, and some of them in fact showed remarkable thermal stability (Scheme 14.56). West, Denk et al.128 prepared silylene 122 according to Eq. 12. Dehalogenation of the dichloride was carried out using molten potassium metal in refluxing THF, and their rather vigorous conditions have become standard for the synthesis of stable silylenes of this type. Silylene 122 is a colorless crystalline

substance having true thermal stability. Silylene 122 is not only indefinitely persistent at room temperature but also unchanged after heating at 150 C in toluene for 4

months. Solid 122 finally decomposes at its melting point of 220 C. The analogous compounds, 123,129a 124,130b and 125,130c have also been synthesized. Two nitro-

gen atoms bonded to the divalent silicon stabilizes these compounds, although 123 is only marginally stable.

The five-membered rings in 122 and 124 are planar, while the ring in 123 is necessarily puckered. The N C distances in the ring are shorter in 122 and 124 than in 123 by 10 pm. The Si N bond lengths are similar to the normal Si N single bond distances ( 172–174 pm). However, bonds to a divalent silicon atom are predicted to be longer, by 8 pm, than those to a tetravalent Si.131 The bond lengths in the silylenes mentioned above are, therefore, consistent with some multiple bond character in the Si N bonds.

Stable silylenes all react with alcohols via the expected insertion into the O H bond to afford the corresponding alkoxyhydrosilanes (126).130b,132 Reaction of

silylene 122 with ketones afforded four-membered disilaoxetane compounds 127 in high yields (Scheme 14.57).133 The mechanism for this transformation is best