Silacyclopropenes photolysis

The reaction generally proceeds in good yield and could involve an addition-elimination, though the regiochemistry appears to be incorrect for this. An alternative would be an SRN1 process involving (86). The reaction has been applied to the preparation of (87) from the silacyclopropene photolysis then leads to the novel tetracycle (88) through an intramolecular [2+ 2]-cyclo-addition 87>. 

When alkynyldisilanes 13a and b were photolyzed in the presence of freshly generated dimesitylsilylene (Mes2Si ), the silylene added to the Si=C double bond of 1-silaallenes 14a and b to form disilacyclopropanes 15a and b (Scheme 5). Even without the independently generated silylene, photolysis of 13b produced 15b in 8% yield, but compound 13a gave only traces of 15a. In the case of 15b, the dimesitylsilylene most likely originated from silacyclopropene 16. 

Kumada and colleagues found that the 1,3-silyl rearrangements from silicon to carbon occurred during the photolysis of disilylalkynes, giving mixtures of silaallenes and silacyclopropenes,102 as illustrated by Eq. (12) ... 

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

The photolysis of tris(trimethylsilyl)phenylsilane in the presence of a series of alkynes alforded the silacyclopropene through silylene addition to the triple bond. Those obtained from monosubstituted alkynes underwent photochemical isomerization to the disilanyl-alkyne through a 1,2-hydrogen shift (Scheme 48) (80JOM(190)117). Disubstituted alkynes form silirenes that can be isolated by preparative GLC. 

Following this reaction sequence established by Kumada and Ishikawa, Leigh and coworkers detected the transient 1-silaallene 597 in a laser flash photolysis of the ethynyl-disilane 619. 597 was identified by its characteristic UV absorptions at 275 nm and 325 nm278 and by the trapping reaction with methanol. It is formed, however, in a mixture with silacyclopropene 620, dimethylsilylene 621 and acetylene 622. Based on the quantitative analysis of the products of methanolysis (623-626) a chemical yield of 597 of 12-15% was deduced278 (equation 205). 

Tetramethylsilacyclopropene, the first silacyclopropene derivative, was prepared in 1976145. With the help of the matrix-isolation techniques, attempts were made to synthesize the parent silacyclopropene (114). Experiments with this goal culminated in the isolation of 1, 1 -dimethylsilacyclopropene (110) in solid argon146. Sander and coworkers showed that 110 is accessible by photolysis of the corresponding bis(diazomethyl)silane (109) (equation 29). After subsequent irradiation with shorter wavelengths, isomerization into the photostable ethynyldimethylsilane (111) takes place. 

Silacyclopropenes are commonly formed from the addition of a silylene to an alkyne, or in some cases as the result of photolysis of an alkynyldisilane (see Section III.C). Substituted silacyclopropenes have been shown to undergo both 1,2- or 1,3-shifts when photolyzed, yielding silyl-substituted allenes or alkynes, respectively2. More complex behavior was observed with methylenesilacyclopropenes such as 2323 which ring-opened to a diene, as shown in Scheme 4. 

Relatively simple alkynyldisilanes, 93, have been observed to undergo both 1,2- and 1,3-silyl migrations during photolysis leading to silacyclopropenes 94 and silaallenes 95, respectively2 (equation 13). 

Recently, detailed mechanistic studies have been made of the photolysis of l-aryl-4-(pentamethyldisilanyl)-l,3-butadiynes 96 in various solvents. Photolyses in the presence of methanol led to the isomeric products 97 and 98, derived from solvolytic ring opening of the initially formed silacyclopropene, 99, resulting in turn from 1,2-silyl migration48, while photolyses in acetone led to the products 100 and 101 arising from two-atom insertion into the three-membered ring49 (Scheme 14). 

Photolysis of a hexane solution of 20 in the presence of 3-hexyne gives l-trimethylsilyl-l-phenyl-2,3-diethyl-l-silacyclopropene which can be isolated in pure form by distillation followed by preparative GLC. 

Similar photolysis of 20 in the presence of 1-trimethylsilylpropyne produces a silacyclopropene in 33% yield. In this case, small amounts of two other compounds, l-trimethylsilyl-l-(r-phenyl-2, 2, 2 -trimethyldi-silanyl)propadiene (2% yield) arising from a 1,3-hydrogen shift of the initially formed silacyclopropene and l-bis(trimethylsilyl)phenylsilylpro-pyne (2% yield), are also obtained. Formation of the latter compound can be best explained in terms of another type of photochemical migration in-... 

Irradiation of 1-ethynyl-l-phenyltetramethyldisilane (55) leads to the formation of the silacyclopropene and silapropadiene which can be trapped by methanol to give 1-trimethylsilyl-l-methoxymethylphenylsil-ylethene (29% yield) and cis- and trans-1 -trimethylsily 1-2-methoxymethy 1-phenylsilylethene (19 and 15% yield). In contrast to 52, the photolysis of... 

The photolysis of phenylethynylpentamethyldisilane (56) takes place simultaneously by at least two different processes. The main route proceeds through a silacyclopropene and the other involves transient formation of a silapropadiene. Irradiation of a benzene solution of 56 in the presence of acetone gives four products, 2,2,5,5-tetrameth-yl-3-trimethylsilyl-4-phenyl-l-oxa-2-silacyclo-3-pentene (57), 2,2,5,5-tetra-methyl-3-phenyl-4-trimethylsilyl- l-oxa-2-silacyclo-3-pentene (58), phen-yltrimethylsilylacetylene, and 1-pheny 1-1 -trimethylsily 1-3-methyl-1,2-butadiene (59), in 51,2,10, and 5% yield, respectively, with 81% conversion, of the starting disilane. The formation of 57 and 58 can be explained by insertion of acetone into the silacyclopropene. Liberation of dimethylsilyl-ene species from either direct photolysis of 56 or decomposition of the silacyclopropene results in the formation of PhC=CSiMe3. Product 59... 

The photolysis of a benzene solution of 56 in the absence of a trapping agent with a low-pressure mercury lamp and distillation of the resulting mixture under reduced pressure affords l,l-dimethyl-2-phenyl-3-trimeth-ylsilyl-l-silacyclopropene in 41% yield as a colorless liquid, along with 10% yield of trimethylsilylphenylacetylene (103). 

It has been shown that the photolysis of phenylethynyl-substituted disilanes affords a convenient route to the synthesis of the silacyclopropene derivatives (104). For instance, irradiation of l-phenylethynyl-2-phenyl-tetramethyldisilane with a low-pressure mercury lamp gives 1,1-dimethyl-2-phenyl-3-dimethylphenylsilyl-l-silacyclopropene and 1,1,2,2-tetra-... 

The photolysis of l-trimethylsilylethynyl-l,l-dimesityltrimethylsilane gives a stable crystalline silacyclopropene (66) which can be recrystallized from ethanol. Like 62, this compound is never affected by atmospheric oxygen and moisture (106). 

Silacyclopropenes were reviewed as intermediates in the photolysis of several alkynyl-substituted disilane derivatives <1996CC2609>. Photolysis of the disilane derivatives gives highly reactive silacyclopropene intermediates. 

Photolysis of cyclotrisilane 72, in the presence of five-membered ring compounds, produces l,l-di- /t-butyl-2,2-dimethyl-l-silacyclopropene 37 when reacted with cyclopentadiene (Equation 16), o o-3,3,6,6,7,7-hexa-/i t/-butyl-3,6,7-trisilacyclo-8-oxatricyclo[3.2.1.0 ]octane 38 and l,l-di-fem-butyl-2,2-dimethyl-l-silacyclopropane 39 when reacted with furan (Equation 17), and 2,2,6,6-tetra-/i m-butyl-2,6-disilabicyclo[3.1.01]hex-3-ene 40 when reacted with thiophene (Equation 18), among other products <19950M5695>. These products were characterized by H, and Si NMR, and for the furan derivative 38, also by X-ray diffraction studies. 

A mixture of reactive intermediates, including l,l-dimethyl-3,3-bis(trimethylsilyl)-Tsilaallene and dimethylsilylene, along with l,l-dimethyl-2,3-bis(trimethylsilyl)-l-silacyclopropene 86 were formed and detected from the direct irradiation of [(trimethylsilyl)ethynyl]pentamethyldisilane in hydrocarbon solution (Equation 21). These species were detected and identified using laser flash photolysis. They were trapped as their methanol adducts in steady-state irradiation experiments. Steady-state irradiation in the presence of methanol affords MeOH-addition products which are consistent with the formation of the silaallene, silacyclopropene, and silylene along with bis(trimethylsilyl) acetylene as the major product <1997JA466>. 

In a similar work, now carrying out the laser flash photolysis of pentamethyl(phenylethynyl)-disilane, the three intermediate products were determined to be stable on a millisecond time scale. The silacyclopropene 87 was found to be stable for longer than 100 ms, based on its ultraviolet (UV) spectra (it presented a characteristic absorption band at 320 nm) and its reactivity toward various reagents, exposure to methanol affords two isomeric adducts (Scheme 26) <1997OM5804>. 

Further photolysis of silacyclopropene 40 with 72 produces a colorless crystalline solid the bis(silirene) 99 (Scheme 33). The molecule is unstable in air and suffers a rearrangement to generate 2,5-disilobicyclo[2.2.8]hexa-l(6)-3-diene 100, as confirmed by X-ray crystallography <19980M1237>. 

Dimesitylsilacyclopropenes are stable to atmospheric oxygen, moisture, or alcohol at room temperature, and can be prepared by photolysis of 1,1 -dimesitylalkynyldisilanes. Previously reported silacyclopropenes were extremely unstable under these conditions (Equation (47)) <80JOM(194)147>. 

Experiments with various quenchers were carried out to quench the triplet excited state of acetone selectively [95]. From these experiments it is confirmed that the silacyclopropene formed from the photolysis of l-aryl-4-(pentamethyl-disilanyl)-l,3-butadiynes reacts with acetone via the triplet excited state of the silacyclopropene, and the triplet energy of the silacyclopropene lies around 62-68 kcal/mol. 

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