Silylenes alkynes

Disubstituted silole derivatives are synthesized by the palladium-catalyzed reaction of (trialkylstannyl)di-methylsilane with terminal alkynes (Equation (107)).266 The mechanism is supposed to involve a palladium silylene complex, which is generated via /3-hydride elimination from LJ3d(SiMe2H)(SnBu3) (Scheme 62). Successive incorporation of two alkyne molecules into the complex followed by reductive elimination gives rise to the silole products. 

Photochemical irradiation of (i-Pr3Si)3SiH (14) with light of 254 nm in either 2,2,4-trimethylpentane or pentane leads to the elimination of f-Pr3SiH and the generation of bis(triisopropylsilyl)silylene (/-Pr3Si)2Si (15). Silylene 15 can also be generated by the thermolysis of the same precursor 14 at 225 °C in 2,2,4-trimethyl-pentane (Scheme 14.11). Reactions of 15 include the precedented insertion into an Si H bond, and additions to the ti bonds of olefins, alkynes, and dienes. 

However, Bu3SnSiMe2H (154) reacts with terminal alkynes to give the 3,4-disubstituted silacyclopenta-2,4-diene 156 at room temperature. The dimethyl-silylene 155 is an intermediate[80]. (Me3Sn)2 undergoes facile addition to alkynes to give the 1,2-distannylalkene 157[81-83]. 

Linear C02, CS2, alkynes, RN C NR, NCO Si02,a SOS8 Silylenes, SiX2b c GeOj Sn02a PbO/... 

Silylene extrusion from siliranes in the presence of alkynes, notably bis(trimethyl-siiyl)acetylene, gives the silirene (35) in good yield (Scheme 41) (76JA6382). Compound (35) is more stable thermally than hexamethylsilirane and shows 2 Si NMR absorptions for the ring atom at 5 = 106.2 p.p.m., some 50 p.p.m. downfieid from those of silacyclopropanes, and about 100 p.p.m. downfieid from normal cyclic and acyclic tetraalkylsilanes. Notable reactions include alcoholysis and the insertion of aldehydes and ketones, dimethylsilylene... 

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. 

The first 1,2-disilacyclobutene (82) was prepared in 1973 by the gas phase reaction of dimethylsilylene and 2-butyne (73JOM(52)C21). It probably results through silylene insertion into the intermediate silacyclopropene (Section 1.20.3.4), but silylene dimerization followed by addition to the alkyne is also suggested (76JA7746), since (82) is formed in good yield if the disilene is generated directly (Scheme 127) (78JOM(162)C43). 

Disilacyclobutenes, variously substituted, can be readily oxidized, notably with bromine, to yield the isomeric mixture of hindered tetrasilylethylenes through rotation about the carbon-carbon double bond. They also give a variety of derivatives with silylenes, metal carbonyls and alkynes (Schemes 128 to 132) (80AG(E)620, 78JOM(162)C43, 74CC1013, 81IC3456). 

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. 

From (trialkylstannyl)dimethylsilane and terminal alkynes this method gives 3,4-disubstituted siloles (28, R = H, R = alkyl, aryl or alkoxy groups) in moderate to good yields44. Catalysis by Pd is better than by Ni or Rh. The proposed mechanism involves a palladium-silylene species 30 as an intermediate. This reacts with an alkyne giving successively a palladasilacyclobutene (31) and a palladacyclohexadiene (32). In the final... 

The reaction of a silirene with an alkyne in the presence of a palladium catalyst allows cyclization of two molecules of the alkyne with the silylene, as in equation 9 above. For example, Seyferth and coworkers have prepared the silole 33 in 80% yield from 1,1-dimethyl-2,3-bis (trimethylsilyl) silirene and phenylacetylene (equation 10)45. Without catalyst, this reaction yielded the silole 34 and the ene-yne 35, resulting respectively from ring expansion and cleavage by PhC=CH of the silirene. Under UV irradiation, 35 alone was formed. 

Extrusion of a silylene from a silirane or silirene is of course the inverse of silylene addition to alkenes or alkynes, respectively. The reversibility of most silylene reactions allows the inverse of addition to 1,3-dienes to also be employed as a silylene source. The first such reaction was reported by Chernyshev and coworkers, who found that transfer of SiCl2 units from l,l-dichlorosilacyclopent-3-enes was a unimolecular process and hence was likely to consist of silylene extrusion and readdition (equation 38)82. Dimethylsilylene extrusion has been found in the pyrolysis of silacyclopentenes and other products of... 

To date, chemical properties have been reported mainly for 59 and 61. Occupancy of the silicon 3p orbital by electrons from nitrogen greatly reduces the electrophilicity of these silylenes. This, together with probable aromatic stabilization, significantly mutes the behavior of 59 and 61 as silylenes. For example, 59 does not insert into Si—H bonds, or react with alkynes such as PhC=CPh367, Moreover, 59 shows no Lewis acidic behavior, even toward bases as strong as pyridine. 

The stable silylenes 83-85 do not react with conventional C=C double bonds however, diazasilole 83 is an efficient catalyst for the polymerization of alkenes, terminal alkynes, and 1,3-butadienes <2000ACR704, 2002USP028920, 2004JOM4165>. The stable bisaminosilylene 85 reacts with the activated double bond in 177-phosphirenes 134. The heterobicyclobutane 135 is however only a transient species and after addition of a second silylene 85 phosphasiletes 136 were isolated. Use of more sterically demanding substituted phosphirenes hampered the attack of the second silylene and the phosphasiletes 137 and 138, which are valence isomers of bicyclobutane 135, were obtained (Scheme 14) <2004AGE3474>. 

Several of the reactions described in Section 6.16.2.4.6.1 are two-step reactions. After the initial [2+1] addition of the silylene to the multiple bond, a second insertion reaction of a silylene into a reactive Si-X bond of the cyclopropane derivative takes place yielding four-membered ring compounds. Examples are the reactions with alkynes, nitriles, imines, and ketones. 

Tetrasubstituted silanes are also sources of silylene. Suginome and coworkers reported that palladium catalyzed the transfer of dimethylsilylene, formed from silylborane 44, to alkynes [equation (7.8)], 60 Exposure of silylborane 48 and alkyne 49 to substoichiometric amounts of palladium and P(7-Bu)2(2-biphenyl) afforded 2,4-disubstituted silole 50. This process tolerates a variety of functionality including silyl ether-, dimethylamino-, and trifluoromethyl groups. In addition to aliphatic terminal acetylenes, arylacetylenes were also competent substrates. For... 

Palladium(O) complexes also catalyze the transfer of di-terf-butylsilylene from m-dimethylsilacyclopropane 48 to alkynes (Scheme 7.6).61 In the presence of 5 mol % of (Ph3P)2PdCl2, the formation of dimethylsilacyclopropene 51 could be achieved at 110°C. In the absence of the palladium catalyst, silylene transfer from silacyclo-propane 48 to 3-hexyne occurred over 3 days at 130°C. While a reasonable mechanism was proposed involving palladacycle 49 as an intermediate, an alternative mechanism could involve palladium silylenoid intermediate 50. 

Woerpel and Clark identified silver phosphate as the optimal catalyst to promote di-ferf-butylsilylene transfer from cyclohexene silacyclopropane to a variety of substituted alkynes (Scheme 7.25).95 While this silver salt exhibited attenuated reactivity as compared to silver triflate or silver trifluoroacetate, it exhibited greater functional group tolerance. Both di- and monosubstituted silacyclopropenes were easily accessed. Terminal alkynes are traditionally difficult substrates for silylene transfer and typically insert a second molecule of the starting acetylene.61,90 93 Consequently, the discovery of silver-mediated silylene transfer represents a significant advance as it enables further manipulation of monosubstituted silacyclopropenes. For enyne substrates, silylene transfer the alkynyl group was solely observed. The chemoselectivity of the formation of 99f was attributed to ring strain as theoretical calculations suggest that silacyclopropenes are less strained than silacyclopropanes.96 97... 

Isolation of the air-sensitive silacyclopropene was avoided by development of a two-step, one-flask procedure, which transformed alkynes into the desired azasilacyclopentadienes (Scheme 7.29)." For terminal alkynes, silver phosphate was employed for di-terf-butylsilylene transfer and copper(I) triflate was used to promote nitrile insertion. These conditions successfully transformed phenylacetylene into azasilacyclopentadiene 106b. For internal alkynes, copper(I) triflate catalyzed both silylene transfer to 3-hexyne as well as nitrile insertion to produce enamine 106k. 

When silylene transfer was attempted to alkynes substituted with halides or sulfones, however, silacyclopropene formation was not observed.101 Instead, acetylenic silanes 113 were observed (Scheme 7.32). Treatment of silacyclopropene 112c (or 112d) with acetophenone and substoichiometric amounts of Cul also induced alkyne formation. 

The photolysis of tris(trimethylsilyl)phenylsilane results in formation of trimethylsilylphenylsilylene in high yield, together with a small amount of a silicon-carbon double-bonded intermediate, which will be described in detail later. This silylene has a high reactivity toward unsaturated organic substrates such as alkenes and alkynes (44). 

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