Silenes photolysis

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. 

Research has continued on the photolysis and thermolysis of such systems in recent years, particularly by Ishikawa. The silenes formed, most of which were trapped as confirmation of their formation, are referenced in Table I. 

A few other routes to new silenes involving photochemical processes have been reported, most of which have the structure RR Si=CH2110-112 and are listed in Table I. Photolysis of l,l-dimethyl-2-phenyl-l-silacyclo-but-2-ene 22 in a glass at 77 K led to the siladiene 23, which absorbed at 338 nm,113 as shown in Eq. (17) ... 

The trapping of silacyclopentadienes has also been reported recently.115 Using the pyrolysis of 27, or photolysis or pyrolysis of 28, the formation of the silylene 29 was inferred. Further photolysis or thermolysis converted the silylene into silene 30, which could be photochemically isomer-... 

Using nanosecond laser flash photolysis techniques, Leigh80 observed transient absorption spectra which he attributed to the silenes derived from photolysis of various methylphenyldisilylbenzenes. Thus the silenes 52,53, and 54 were found to absorb at 425,460, and 490 nm, respectively, in isooctane, and 55 was also found to absorb at 490 nm.75 In other studies, the silene Ph2Si=CH2 derived by laser flash photolysis was found to absorb at 323 nm.111... 

In the past decade, Ishikawa et al. have investigated the photochemistry of aryldisilanes.66 73 76 84 Lately, dimers derived from these unusual silenes have been observed from the first time.78 Thus, photolysis of 1,4-bis(penta-methyldisilyl)benzene 76 in hexane presumably gave rise to the silene 77 but, on workup employing crystallization, two head-to-head stereoiso-meric dimers 78 and 79 were obtained in about 45% yield in a 1 1 ratio. These were said to be formed from the silenecyclohexadienes 77 by the two alternative pathways shown in Scheme 12. Related dimers were also... 

Finally, Eq. (28) documents a unique situation in which a bis-silene was observed to undergo both head-to-tail and head-to-head [2 + 2] cyclodimerizations. Photolysis of the bisdiazoalkane 80 in benzene apparently... 

In view of the evident reactivity of the Brook-type silenes toward carbonyl compounds and the fact that these silenes were prepared by the photolysis of acylsilanes, it is natural to ask why the silenes apparently did not react with their acylsilane precursors. This question has been answered recently. On the one hand, as shown in Scheme 19, the silene Ph2Si=C(OSiMe3)Ad apparently did add in a [2 + 2] manner to its acylsi-... 

Brook et al. 5X1 observed such reactions during the formation of siienes by photolysis. Using radiation with A > 360 nm, they photolyzed acylsi-lanes such as 127, which bears a mesityl group attached to the carbonyl carbon. On prolonged photolysis of the initially formed silene 128, the C—H bond of the ortho methyl group of the mesityl group added to the silicon-carbon double bond to form the benzocyclobutane 129. Alternatively a 1,5-H shift would lead to the species 130, which would also yield the benzocyclobutane on electrocyclic rearrangement. 

Jones218 has described an unusual photochemically initiated rearrangement of a silene-anthracene adduct to a silene which is part of an eight-membered ring (Eq. 60). Photolysis of the adduct 190 was believed to form the silaallylic diradical 191, whose canonical form 192 affords the... 

Unlike pentamethylphenyldisilane, the photolysis of compound 5 in the presence of methanol in hexane gave no products arising from the reaction of a silene with methanol, but polymeric substances were obtained as main products, in addition to small amounts of bis-(trimethylsiloxy)phenylsilane (6) (4%) and bis(trimethylsiloxy)-phenylmethoxysilane (7) (5%). 

On the basis of the methoxy contents in the photoproducts, these silenes seem to be formed as main products in the polymeric systems, although this type of silene is produced only as a minor product in the photolysis of aryldisilanes. 

When similar photolysis of 11 in the presence of MeOD was carried out, again the product whose NMR reveals the resonance due to the Si-H proton was observed. The relative ratio of the Si-H and CH3-0 protons was identical with those of the products obtained in the presence of non-deuterated methanol. The formation of the methoxysilyl group can be understood by the addition of methanol across the silicon-carbon double bonds. H NMR spectra of all photoproducts obtained from the photolyses of 11 in the presence of methanol reveal no resonances attributed to the cyclohexadienyl ring protons. This indicates that the photochemical degradation of the polymer 11 gives no rearranged silene intermediates, but produces... 

Two families of silenes warrant special comment. The first arises from the photolysis of a number of aryldisilanes as investigated by Kumada et al. (97-102), who reported that a rearrangement occurs to give products formulated as silenes, as shown in Eq. (16). 

Another family of silenes, those derived from the photolysis of acyldi- or polysilanes at A > 360 nm, also show somewhat unusual behavior compared to simpler silenes. These silenes exhibit great stability which in some cases has allowed isolation of solid silenes, and which has allowed acquisition of much physical data relating to silicon-carbon double bonds as mentioned earlier. 

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

Photolysis of a mesitylsilene (24, R=mesityl) led to insertion into one of the methyl C—H bonds yielding a benzocyclobutene (209) insertion of silenes into C—H bonds is not a very common reaction of silenes. 

Treatment of the photolysate with methanol gave two diastereomeric methoxysilanes in about 3 1 proportions, consistent with methanol addition being a nonstereospecific process. During extended photolysis (8 hours) to convert all of 25 to 26 it was observed that the concentration of 26 decreased, and a set of signals characteristic of a different silene grew in (no change of relative proportions occurred in the dark). The chemical... 

In contrast to the observed photochemistry of adamantyltris(trimethylsilyl)silane which efficiently yields the appropriate silene compound, similar photolysis of the germanium analogue provided no evidence for the production of germene13. However, photolysis of the germanium compound in CCI4 did result in a Norrish type 1 cleavage of... 

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