Disilacyclobutanes formation

When starting (chloromethyl)chlorosilane contained more than one Si-Cl bond, lower yields of the 1,3-disilacyclobutane were obtained due to side reactions resulting from the availability of more Si-Cl functions (in the case of CH3Si(CH2Cl)Cl2, 2 was produced in addition to cyclo-[CH3(Cl)SiCH2]2) and to formation of higher yields of polysilmethylene [13, 14]. 

Another procedure for the synthesis of 1,3-disilacyclobutanes is the pyrolysis of monosilacyclobutanes (eq. 5), [9, 14, 16], but this method has difficulties and disadvantages [14]. One of these is that polysilmethylene formation is a side-reaction when it is carried out in the gas-phase. 

In accord with the proposed mechanism, copyrolyses of la or lb with 2,3-dimethyl-1,3-butadiene (DMB) or isoprene lead to silacyclopentene derivatives via a formal [4+1] cycloaddition of the silylenes (Scheme 2). The simultaneous existence of the silaethenes 2a/2b and the resulting silylenes 4a/4b in the gas phase is proven by the formation of the corresponding 1,3-disilacyclobutanes (5) and - in case of isoprene as the quenching partner - of the two isomeric silacyclohexenes 7 (Scheme 2) [2]. 

The intermediate formation of betaines with the carbanionic center is also postulated in the reactions of permethylsilirane, sila- and disilacyclobutanes with phosphorus ylides. For data on these betaines isomerized in situ to silylated phosphorus ylides, see Section 5.4. 

Interestingly, reaction of alkynyl disilane 14 with trityl cation did not result in the formation of stable vinyl cations. Obviously, the formation of the four-membered disilacyclobutane ring is unfavorable. Similarly, treatment of alkyne 15 with the pre-formed triethylsilylarenium ion 1 derived from toluene did not give the expected intramolecular transfer of the silylium ion to the triple bond. Instead, only a complex product mixture was obtained. 

The silene 124 is probably formed as its THF adduct and can be trapped by, e.g., 1,3-dimethyl 2,3-butadiene to give a [4+2] cycloadduct. The attempt to liberate the silene 124 from its donor adduct results in the formation of a disilacyclobutane 125. This is ascribed to the prolonged life-time of the intermediate 359 formed by the methyl migration in the silene (equation 96), which allows for a hydrogen migration to take place. 

The dimerization of silenes is probably the prominent type of reaction of silenes since the structures of most of the relatively stable and transient silenes have been established by isolation and identification of their dimers. The formation of disilacyclobutanes has... 

Relatively little is known experimentally about the mechanism of the reaction. A widely accepted mechanism originally suggested by Brook and coworkers starts with the formation of a Si—Si bond, giving a carbon centred 1,4-biradical86. This 1,4-biradical then combines in a second step to the 1,2-disilacyclobutane. This mechanism is favoured by the calculations and is also corroborated by experiments the relatively stable silene 149... 

An interesting addition reaction to silenes was recently described by Oehme and coworkers. They found that at high concentrations of LiBr the dimerization of the transient silene 371 yields 33% of the head-to-head dimer 373 and the head-to-tail dimer 407 in a 1 5.6 relative ratio108. The formation of the unexpected dimer 407 was rationalized by the addition of LiBr to the Si=C bond and intermolecular cyclization of the w-lithiosilyl bromide 408 or its reaction with the transient silene 371 with subsequent cyclization to the 1,3-disilacyclobutane 407 (equation 126)108. 

The photochemical reaction of bis(alkylidene)disilacyclobutane (24a and 24b) with C60 by a high-pressure mercury lamp (cutoff <300 nm) proceeds to afford the adducts 26a (61%) and 26b (52%) instead of 2521. The formation of these products is a result of an unexpected rearrangement of the disilacyclobutane moiety (equation 9). The FAB mass spectrum of 26a exhibits one peak at m/z 1056-1059 (CsoFUoSia, M++l cluster), as well as one for Q,o at m/z 720-723. 

Styrene and substituted styrenes react with tetramesityldisilene 1, tetra-tert-butyl-disilene 21, and tetrakis(tert-butyldimethylsilyl)disilene 22 to afford the corresponding disilacyclobutane derivatives.127,134 Similarly, [2 + 2] additions occur between the disilenes with a C = C double bond in an aromatic ring135 and acrylonitrile.136 Bains et al. have found that the reaction of disilene 1 with trans-styrene- provides a 7 3 diastereomeric mixture of [2 + 2] adducts, 201 and 202 [Eq. (95)] the ratio is changed, when czs-styrene-Ji is used.137 The formation of the two diastereomeric cyclic adducts is taken as the evidence for a stepwise mechanism via a diradical or dipolar intermediate for the addition, similar to the [2 + 2] cycloaddition of phenylacetylene to disilene ( )-3, which gives a 1 1 mixture of stereoiso-meric products.116,137... 

The authors postulate an intermediate containing a silicon-silicon double bond. Stable products include various disilacyclobutanes. A possible mechanism of formation from the postulated me2Si=Sime2 is suggested by the authors. 

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>. 

In the absence of anything else to react with, silenes dimerize in either head-to-tail or head-to-head fashion depending on the identity of the substituents at silicon and carbon, and their effect on the natural (<5+Si=C<5 ) polarity of the silenic double bond. The head-to-tail regiochemistry leading to the formation of the corresponding 1,3-disilacyclobutane is most common, while head-to-head dimerization leading to either the corresponding... 

Hawrelak EJ, Ladipo FT, Sata D, Braddock-Wilking J (1999) Synthesis, Characterization, and Reactivity of [LiC(SiMejH)3] 2THF Formation of 1,1,3,3-Tetramethyl-2,2,4,4-tetrakis(dimethylsilyl)-l,3-disilacyclobutane, [MejSiC(SiMejH)Jj. Organometallics 18 1804-1807... 

Again, the application of isoprene as the quenching substrate is less effective and only low yields of the expected silacyclohexenes were obtained. Slight variations of reaction conditions lead to the formation of additional products. For example, a small amount of the 1,3,5-trisilacyclohexane 3 can be detected within 12 h in addition to tetramethyl-l,3-disilacyclobutane as the main product if the reaction of Me2SiCl2 is carried out in THF at -78 C using lithium powder with a higher content of sodium (Eq.4). 

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