1.3- Silyl migrations photochemical

Disilanyl substituted naphthalenes exhibit unusual photochemical reactivity131. 1,4-Bis(pentamethyldisilanyl)naphthalene 219 yields compound 220 in both the absence and presence of methanol, possibly via a biradical 221. Noteworthy is the 1,8-silyl migration from position 1 to 8 of the naphthalene ring. In the presence of methanol, compound 222 is formed via an initial silene 223, which then rearranges via 224 (equation 55). In a homogenous solution of methanol/benzene (1 1.5) only 225 is formed, probably by direct reaction of the photoexcited disilane with methanol, before migration of a trimethylsilyl... 

Silyl-substituted cyclopropenes have been observed to rearrange photochemically giving high yields of allenes. A typical example is shown in equation 2 where the silylcyclo-propene 3 apparently ring-opened to the carbene 4 which, following 1,2-silyl migration, led to the allene 513. 

FIGURE 7. Kinetic profile for photophysical and photochemical processes of a phenylpentamethyld-isilane in alcohol-hexane mixtures kf, k f = fluorescence rates ka, k = radiationless decay rates k = rate for CT quenching kt, k t = reaction rates for 1,3-silyl migration and reaction with alcohol, respectively kc = formation rates of ICT state from LE. Modified from Reference 105... 

Whereas 2,2-diphenylhexamethyltrisilane (8) is a well-known photochemical precursor of diphenyl si ly Icnc72,155 156, a 1,3-silyl migration is usually a major side reaction in solution at room temperature. In the presence of excess ethanol, the irradiation of 8 in hexane gives 9 via diphenylsilylene extrusion and 10 (an isomeric mixture) via 1,3-silyl migration in 50 and 37%, respectively (Scheme 3)157. Since the product ratio does not depend on the solvent polarity, both reactions, silylene extrusion and 1,3-silyl migration, do not occur via the ICT state but via the nonpolar excited state158. However, the excited... 

As illustrated in Eq. (62), the photochemical isomerization of cyclotrisilene 48 to 88 is rationalized also by a mechanism including 1,2-silyl migration. Whereas three 1,2-silyl migration pathways are possible in this system (paths a-c), only path a leads to 88 via biradical 139 path b is an identity reaction and path c may lead to a minor product 138. 

A number of interesting photochemical l,n-silyl migrations12-14 as well as silyl migrations to transition metal centers15-19 have not been covered in the present review, since they have been surveyed in excellent review articles. 

Photochemical 1,2-silyl migration from l-tris(trialkylsilyl)cyclotrisilene 72 occurred to give the corresponding bicyclo[1.1.0]tetrasilane 70 (equation 54)132. During the photochemical and thermal interconversion among the SLiRg isomers 69, 70 and 72, the corresponding 1,3-tetrasiladiene derivative was never detected. 

Sekiguchi and coworkers observed the thermal and photochemical 1,2-silyl migrations in persilyl-l-disilagermirene 73 (equation 55)133. 

Photochemical 1,3-silyl migrations from Si to O in disilanylketones were applied to the syntheses of the first isolable silaethenes by Brook and coworkers (equation 136)343. 

Thermal or photochemical activation of the complexes results in the formation of siloxycarbenes 13 via diradicals 12. Carbenes 13 lie in a very shallow minimum and are stabilized by a [1.2]H shift, if an a-hydrogen atom is available, or by [l,2]silyl migration Vinyloxysilanol 8 or formylsilanol 11 are the final oxydation products in argon matrices. 

The results of irradiation ( i > 290 nm) of a series of aldehydes and ketones (91) in the presence of the silyl acetals (92) have been reported. The reactions are both solvent and silyl group dependent and the best results are obtained when the solvents used are /z-hexane, THF, diethyl ether or benzene and with the silyl group TBDMS. The products are the oxetanes (93) and the silyl-migrated product (94) in a ratio greater than 95 5 respectively. There is no evidence for the formation of the isomeric oxetane. Other studies from this research group" have examined the photochemical addition of a series of aryl aldehydes (95) to the cyclic silyl alkenes (96) brought about by irradiation at X,>290 nm in methylene chloride solution. The additions encountered take place with regio and exo selectivity as shown by the yields and ratios of the products (97). 

Ando and coworkers122 described an even more complex system in which the initially formed silylcarbene suffered 1,2-silyl migration to yield a silene, which then further rearranged by migration of an ethoxy group to silicon to yield a ketene (equation 79). Studies in a matrix demonstrated that this latter step was a photochemical process. 

The migratory insertion (silyl migration) of alkene into the M—Si bond is a key step for the dehydrogenative silylation catalyzed by late transition metal complexes. The first convincing results for mechanistic pathways involving this step were presented by Seitz and Wrighton (45) and obtained in a photochemical study of the reaction with [(Me3Si)Co(CO)4] complex. The insertion of ethylene into the Co—SiMes bond was spectroscopically confirmed. In addition, as already mentioned, a theoretical study of the hydrosilylation of ethylene explained the preference of rhodium over platinum complexes as catalysts in these reactions (41,43). 

The kinetic profile for the photophysical and photochemical processes of p-(trifluoromethylphenyl)pentamethyldisilane (lb) in alcohol-hexane mixtures is represented schematically in Figure 7. Whereas the nature of the excited states of aryidisilanes varies significantly depending on the electronic nature of the aromatic 7t system, the mechanistic profile shown in Fignre 7 would be generally adopted to the photoreactions of aryidisilanes. In contrast to this, Shizuka and coworkers have proposed that 1,3-silyl migration occurs from the jrd-type ICT state of phenyldisilanes, on the basis of the time-resolved emission studies of aryidisilanes. 

Silenes are formed by rearrangement of silylcarbenes. If polysilylated diazomethanes 298-300 are employed, a selective migration of a silyl group to the carbene centre occurs and silenes 301, 92 and 302 are formed (equations 74-76)164. The outcome of trapping reactions is independent of the mode of silene generation photochemical and pyrolytic methods give the same results. 

The authors propose that N-silanimines result from a migration of a silyl group with simultaneous loss of N2 in the excited state of the azidosilane. This is in accordance with evidence published earlier by Kyba and Abramovich318,319 that the photochemical decomposition of alkyl azides does not proceed via a nitrene intermediate (see also Reference 320). 

The most reasonable explanation for this is that the photochemical [l,2]silyl shift in 14e yields carbene 13e in its s-E conformation. This carbene either rearranges back to I4e or produces the conformational isomer s-Z-13e (Scheme 7). Since the migrating trimethylsilyl group is located syn to the methyl group, the silyl shift in i-Z-13e is expected to be less feasible than in the s-E conformer, and thus the [1,2]H shift to s-Z-16 can compete. Since the vinyloxysilanes 16 and 8 are both formed in the s-Z conformation it is tempting to assume that a similar mechanism is operating. 

In a similar maimer, hydride (43) [169,170], silyl groups (44) [17 l],allyl groups (45) [169,172], as well as alkyl (47a-c) and aryl groups (47d) [173-176] can migrate to imine carbon atoms of the ligand, either during the metathesis reaction or upon thermal or photochemical activation of an intermediate hypercoordinated silicon complex (46a-d). 

---