1.2- Disilacyclobutanes, photolysis

The laser flash photolysis of gaseous silacyclobutanes 2224 and 2325 and 1,3-disilacyclobutane 24 produced the transient silenes 25 (from 22 and 24) and 26 (from 23) as the major primary product. The silenes 25 and 26 were identified by their UV spectra with Xmax s=ss 260 nm. Rate constants for the decay processes of the transient silenes were also measured. 

Ando and coworkers have described unusual photochemical behavior of a bis-diazo compound 263 which on photolysis gave rise to a bis-silene 264135. In inert solvents this spontaneously underwent two modes of reaction, criss-cross (head-to-tail) addition and parallel (head-to-head) addition, leading to bridged disilacyclobutanes 265 and 266 (Scheme 47). 

Using /ra .r-l,l,3,3-tetraphenyl-2,4-dineopentyl-l,3-disilacyclobutane 35a as the precursor, the reactivity of the transient l,l-diphenyl-2-neopentylsilene has been studied in various solvents by laser flash photolysis methods. These experiments verified that the isomeric compounds m-35b and ( )-36 and the expected adduct of MeOH to silene 37 are the only primary products of photolysis of 35a at low (ca. 10%) conversion (Equation 9). Similar experiments with m-35b showed it to be ca. 4 times more photoreactive than the trans-isomer, producing tram-35a, 37, and -36 with initial yields of 69%, 11%, and 19%, respectively <20050M2309>. 

Stereospecific formation of 1,3-disilacyclobutanes has been observed in photochemical treatment of bimetallic disilane precursors <2000JA8327, 2002OM5859>. Photolysis of the meso bimetallic complex affords only the // / -1,3-disilacyclobutanc, while similar photolysis of the dl-ioim gives only the m-isomer (Scheme 28) <2000JA8327>. 

Head-to-tail dimerization of 1,1-diphenylsilene (19a), produced by laser flash photolysis of 1,1-diphenylsilacyclobutane (17a), yields the 1,3-disilacyclobutane 2761,62 with a rate constant fcdim = (1-3 0.3) x 1010 M 1 s 1 in hexane solution at 25 °C (equation 17)46. This value is within a factor of two of the diffusional rate constant in hexane at this temperature, indicating that dimerization of this silene is faster than reaction with even the most potent of nucleophilic trapping reagents (see Table 3). More recently, the temperature dependence of the rate constant for dimerization of 19a has been studied63. The results of these experiments are shown in Figure 1, and lead to Arrhenius activation parameters of a = -4 2 Id moD1 and log(A/M 1 s"1) = 9.2 0.4. 

Only four transient disilenes have been studied to date by fast time-resolved spectroscopic techniques l,l,2-trimethyl-2-phenyldisilene (103), ( )- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (104) and tetrakis(trimethylsilyl)disilene (35). The first three compounds were generated by photolysis of the 7,8-disilabicyclo[2.2.2]octa-2,5-diene derivatives 101 and 102 (equation 76)148 while 35 was generated, together with 106, by photolysis of the 1,2-disilacyclobutane derivative 33 (equation 77)68. 

Direct photolysis of l,l,3,3-tetramethyl-l,3-disilacyclopentene (23) at 185 nm and 254 nm in deoxygenated pentane produces a ring contraction product l,l,3,3-tetramethyl-2-methylene-l,3-disilacyclobutane (24) and a cleavage product, 2,4,4-trimethyl-2,4-disilahex-5-yne (25) in absolute... 

Even poorer selectivity was observed with carboalkoxycarbenes. Thus, treatment of (62) with the carbene (63 R = H, R = C02Me) (derived by photolysis of methyl diazoacetate) gave the corresponding disilacyclopentane (M R = H, R = C02Me) in 14% yield and the disilacyclobutanes derived from insertion into primary and secondary CH bonds in 53% and 33% yield, respectively... 

The products of photolysis of substituted silacyclobutanes in methanol solution are either those derived from initial formation of the silaolefin [e.g. equation (9)] or by cleavage of the Si—C bond [e.g. equation (10)]. A similar sensitivity to the nature of the substituent is observed with 1,3-disilacyclobutanes, where it is observed that l,l,3,3-tetramethyl-2,4-diphenyl-l,3-disilacyclobutane undergoes photocleavage of a Si—C bond, whereas l,l,3,3-tetraphenyl-l,3-disilacyclobutane is stable. 

Sander and coworkers studied intermolecular reactions of methylsilenes in argon matrices. 1,1-Dimethylsilene (7) was generated by photolysis of precursor 6 in quantitative yield (equation 3). Warming to 35 K resulted in dimerization of silene 7 to the corresponding disilacyclobutane 8. This kind of silene dimerization leading to the formation of 1,3-disilacyclobutane was originally one of the structural proofs for the parent silene. ... 

Apeloig and coworkers have trapped (Me3Si)2Si from the photoinduced [2 + 2] cycloreversion to (Me3Si)2Si=Si(SiMe3)2 and 2,2 -biadamantylidene of the corresponding 1,2-disilacyclobutane . Since laser flash photolysis experiments indicated that the disilene was the sole primary photoproduct, the silylene was believed to be formed by photodissociation of the disilene. 

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