Reactive Intermediate Chemistry

material. Irradiation of 38 in solid argon results in the growing of an absorption with lmax ¼ 480 nm, whereas lmax ¼ 330 nm is found if nitrogen is used in the matrix instead.51

Rearrangement of a silene to a silylene (40) via migration of a Me3Si group has been suggested as a step in the gas-phase silylene to silylene rearrangement. Labeling experiments, however, have indicated an alternative mechanism (Scheme 14.23).81

Si

Me3Si CD3

Scheme 14.23

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.80b Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1- silacyclopropenylidene (43).82 Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24).

Scheme 14.24

668 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

Azidomethylsilane (46) eliminates nitrogen upon irradiation with the light of l ¼ 254 nm in an argon matrix to give silanimine (47) as the initially observable

product, based on the comparison of the experimental and calculated IR wavenumbers (Scheme 14.25).82,83b,c On further irradiation, a second hydrogen-shift occurs

yielding aminosilylene (48) (lmax ¼ 330 nm). Additional isomerization can be achieved after an exposure of 48 to light of longer wavelength to afford aminosilene (49) (lmax ¼ 256 nm), which regenerates 48 upon irradiation with light of shorter wavelengths.

254 nm

Scheme 14.25

4. REACTIONS OF SILYLENES

4.1. Insertion into Single Bonds

It has been well established that silylene is a ground-state singlet species with an electron pair in a hybrid orbital and a vacant orbital of p symmetry. A Lewis base can thus coordinate to silicon through the vacant orbital to form a silylene-base adduct. All the interactions involve the initial formation of a donor–acceptor complex, followed by a high-energy transition state leading to an insertion product.84 The complex may also be represented as a zwitterionic species, that is, a silaylide, through the formation of the base-to-silicon s bond, in which the silicon center would no longer be an electrophile, but instead have nucleophile character.

Insertion reactions of silylene into a number of single bonds have been observed. The bonds include Si O, Si N, Si H, Si halogen, strained C O, Si Si, O H, N H, and C H (intramolecular only). Insertion into an X H bond can be initiated by the formation of silaylide (50) with the donation of a pair of electrons from a heteroatom X to form a bond to a divalent silicon atom (Scheme 14.26).

RR'Si : + X-H H SiRR' X

50

Scheme 14.26

n = 3–6

(Eq. 5).96 The formation of the series MeO(SiMe2)nOMe could be explained by the insertion of dimethylsilylene into either the Si Si or Si O bond. The results show that the reaction proceeds via the insertion into the Si O bond, and alkoxypolysilanes are much more reactive than alkoxymonosilanes toward the silylene insertion. Photochemically generated dimethylsilylene inserts into Si O single bonds of hex-

amethylcyclohexasiloxane (67) to yield 3,5,7-trioxa-1,2,4,6-tetrasilacycloheptane (68, Eq. 6).97

4.1.3. Insertion into Si H and Si Si Bonds. Silylenes, generated by thermolysis of cyclotrisilanes, inserted into the Si Cl or Si H bonds of monosilane to yield a variety of disilanes, which could be further functionalized. In contrast to carbenes, the insertion of silylenes into C H bonds has not been observed. However, the insertion into Si H bonds has been studied extensively. The occurrence of

direct insertion has been indicated by formation of nongeminate homocoupling products.98,99

Theoretical calculations suggested three steps for the insertion of silylene into a Si H bond, that is (a) the formation of the complex with the interaction of the hydrogen atom of a silane and the empty orbital of a silylene; (b) the formation

ClAr(Ar')Si-SiMe2Cl

ClArAr'Si-SiMe3

Ar = 2-(Me2NCH2)C6H4; R' = 2,4,6-Me3C6H2

Scheme 14.35

674 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

of the complex between the silyl cation and silyl anion, from which follows the migration of a hydride ion from a silane to a silylene; and (c) a disilane formation through the inversion of the silyl anion.100 The reaction of cyclotrisilane with diphenylsubstituted silanes (70) proceeded analogously; again the insertion of silylene (70) into the Si H bond of Ph2SiHCl being preferred to that into the Si Cl bond (Scheme 14.35). The observation that photolysis of a precursor, silirene (71), to silylene (t-Bu)3Si Si Si(i-Pr)3 (72) leads to the formation of a product of the

formal intramolecular C H insertion, in addition to products from intermolecular Si H insertion (Scheme 14.36).37,38 Photolysis of t-Bu2Si(N3)2 (73) was studied in

Scheme 14.36

argon and 3-MP matrices at low temperature and found to give di-tert-butylsilylene (74) through an intermediate assigned as diazosilane (75). Silylene 74 has an electronic absorption band at 480 nm; irradiation at 500 nm led to a formation of silirane (Eq. 7).59 On the other hand, thermally generated dimethylsilylene or

methylphenylsilylene reacts with 1,2-disilacyclobutenes to give trisilacyclopentenes (76, Scheme 14.37),101 which demonstrated the insertion of a silylene into

Scheme 14.37

the Si Si bond. However, no insertion is observed with hexamethyldisilane. The difference between the reactivities is the result of the strain of the Si Si bond.

4.2. Additions of Silylenes and Germylenes to Multiple Bonds

4.2.1. Addition to Acetylenes. The nature of the reactions of silylenes with acetylenes is a most perplexing problem. In 1976, Gaspar and Conlin102 isolated tetramethylsilirene (77) from the flash pyrolysis of 1,2-dimethoxy-1,1,2,2-tetra- methyldisilane in the presence of dimethylacetylene (Scheme 14.38). The thermolysis of 77 at 105 C gave only a polymer, and no disilacyclohexadiene was detected. However, thermolysis of 1,1-dimethyl-2-phenyl-3-trimethylsilyl-1-silacy- clopropene at 250 C gave 1,1,4,4-tetramethyl-2,5-diphenyl-3,6-bis(trimethylsilyl)- 1,4-disilacyclohexa-2,5-diene and its isomer in 80% yields in the ratio of 3:1. In the presence of diphenylacetylene, disilacyclohexadiene was not detected.103 The formation of disilacyclohexadiene might take place by direct dimerization of the silacyclopropene even in the presence of diphenylacetylene.

Scheme 14.38

Diadamantylsilylene Ad2Si: formed from Ad2SiI2 upon treatment with lithium under ultrasonic irradiation was compared with that formed upon pyrolysis of a silirane (Scheme 14.39).35

Germirenes were also isolated by the reaction of dialkylgermylenes with a cycloheptyne derivative.104 The addition of Me2Si: (and Me2Ge:) has been explained as the result of stepwise addition of triplet divalent species (Scheme 14.40).105

Et

54%

Scheme 14.39

676 SILYLENES (AND GERMYLENES, STANNYLENES, PLUMBYLENES)

h ν

PhH 20 oC

Scheme 14.40

Several polysilabicyclosilirenes (79) have been obtained in the reactions of polysilacyclooctynes (78) with dimesitylsilylene generated from the photolysis of 2,2- dimesityl-1,1,1,3,3,3-hexamethyltrisilane (Eq. 8).106 Thermolysis of bis(silacyclopropane) (80) in the presence of bis(trimethylsilyl)acetylene at 60 C affords bis- (silacyclopropene) (81) in 61% yield (Scheme 14.41).107