Silacyclobutanes formation

The first evidence for the formation of silenes came from the thermolysis of silacyclobutanes, which resulted in a retro-[2+2] process leading to the silene Me2Si=CH2 and ethylene1 ... 

While the decomposition of silacyclobutanes as a source of silenes has continued to be studied in the last two decades, the interest has largely focused on mechanisms and kinetic parameters. However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon. Two cases of particular interest involve the apparent formation of bis-silenes. Very low-pressure pyrolysis of l,4-bis(l-methyl-l-silacyclobutyl)ben-zene94 apparently formed the bis-silene 1, as shown in Eq. (2), which formed a high-molecular-weight polymer under conditions of chemical vapor deposition. 

Conlin and co-workers have also studied the fragmentation of a siletane (silacyclobutane). In this case, both the E- and Z-isomers of 1,1,2,3-tetra-methylsilane 45 were prepared and thermolyzed (Scheme 8).144 Both E-and Z-isomers of 45 led to the same products in slightly different ratios the major products were propene with silene 46, and E- and Z-2-butenes with silene 47. Silene formation was inferred from detection of the disila-cyclobutane products. During these processes, the stereochemical integrity of the compounds was largely preserved. 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

With l,3>5-cycloheptatriene 2 can be trapped to yield four isomeric [2+2] adducts and the exo/endo isomeric [6+2] compound 16. Heating this mixture to 110°C leads to the complete transformation of the silacyclobutanes into 16 via a dipolar intermediate. The attempted synthesis of the diphenyl derivative of the [2+2] products leads to the stereospecific formation of endo-Yl which could be characterized by X-ray diffraction analysis [4]. 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

Very low pressure thermolysis of (2-silapropen-2-yl)benzene (CH2 Si(Me)C6H5) causes a 1,3-hydrogen shift from the aryl carbon to the sp2 silicon with the formation of (53) (940M4661). In a related reaction, thermolysis of the silacyclobutane (54) yields (55) by a 1,4-hydrogen shift after the four-membered ring has decomposed by loss of ethene (95JOMC4). 

Another way to activate l-(l-iodoalkyl)-l-silacyclobutanes toward ring expansion is to use silver acetate in acetic acid. In this case, the reaction is believed to proceed via formation of a carbocation a to the silicon. The acetate counterion acts as a nucleophile, attacking the activated SCB with C-Si bond migration (Scheme 35) <1991TL6383>. Silver tetrafluoroborate in dichloromethane induces ring enlargement as well, but shows much lower efficiency (30% yield upon treatment with MeLi) <1994BCJ1694>. 

The resulting 5-methylene-2-oxa-l-silacyclohexanes are insufficiently Lewis acidic to react with a second equivalent of the carbonyl compound. However, the incipient allylsilane does react with dimethyl acetals in decent yields in the presence of external Lewis acids including BF3-Et20 or AICI3. Based on these results, double allylation of dicarbonyl compounds with 3-methylene-l,l-diphenyl-l-silacyclobutane was examined, leading to the formation of 3-methylene-oxabicyclo[3.2.1]octanes. This transformation proceeded in one pot and in the presence of BF3-Et20 (Scheme 42). 

The formation of this species cannot be explained on the basis of any other obvious mechanism. Neither does the initially considered alternative, a C—C bond cleavage of the silacyclobutane instead of a Si—C bond cleavage (77), account for the ethylation of silicon that occurs in this crucial test experiment [Eq. (26)]. 

The strained silicon-carbon bonds of silacyclobutanes are subject to activation by Pd and Pt complexes. This reactivity has been used for a catalytic carbon-carbon bond formation.294,295... 

The addition of 1 equiv of the dilithio salt of rac-, -bi-2-naphthol to an equivalent amount of 1,1-dichloro-l-silacyclobutane in ethyl ether at —78 °C led to the formation oi l, 1 - rac-, 1 -bi-2-naphthoxy)-l -silacyclobutanc 220 as a white solid in 71% yield (Figure 7). The structure of 220 was confirmed by 111, 13C, and 29Si NMR spectroscopy and X-ray crystallographic studies <2005JOM2272>. 

The product formation from reactions of 9 and 10 with MeLi and LiAlH4 yields the stereo-and regioisomeric substituted derivatives (MeLi 11, 12 LiAlH4 14, 15), whereas PhMgBr reacts selectively with 10 to give the silacyclobutane 13. The reactions of silene 3 with other pentafulvenes (e.g., 16,17 and 18) lead to similar results. 

Here we report about 2+2 thermocycloreversions of 1-o-tolyl-l-methyl-1-silacycIobutane (la), 1-m-tolyl-l-methyl-1-silacyclobutane (lb), and 1-p-tolyl-l-methyl-1-silacyclobutane (Ic) resulting in ethylene and corresponding transient 1-o-tolyl-l-methylsilene (2a), l-/w-tolyl-l-methylsilene (2b), and 1-p-tolyl-l-methylsilene (2c) formations, respectively. Silenes 2a-c rearrange thermally yielding appropriate 3,4-benzo-1 -silacyclobut-3-enes 3a, 3b, 3b, and 3c (Scheme 2). 

With S-limonene 1 yields 5 and the silacyclobutanes E/Z-6 in a ratio of 73 12 15. Compound 5 is formed by an ene reaction, which is well known for reactions of 1 with alkines [11], strained ring systems, like norbomene or norbomadiene [12], and butadienes [8]. In contrast to the reaction of 1 with isoprene (ratio E/Z-C E/Z-D E = 38 38 14 8 2) the formation of ene product 5 is favored 2D-NMR investigations prove these results [13]. 

Using an aliphatic monoterpene like myrcene as reaction partner for 1 the formation of silacylobutanes E/Z-1 and E/Z-S in a ratio of 34 30 22 14 is observed an ene product could not be detected. In this case myrcene behaves like an isoprene derivative, which forms only [2+2] cycloadducts with 1 [8]. The reaction with 2, however, yields a product mixture containing the silacyclobutanes E/Z-9 and E/Z-10, the Diels-Alder compounds 11 and 12, the ene product 13, and the silene dimer 7Z-14 [14, 15] (Fig. 2). 

A useful method to prove the formation of silacyclobutanes and ene products is their derivatization by organo groups [1], Thus, the reactions of 5/6 and 7/8 with two equivalents of methyllithium yield compounds 9/10 and 15/16 (Fig, 3). 

The pioneering discovery by Mukaiyama in 1974 of the Lewis acid mediated aldol addition reaction of enol silanes and aldehydes paved the way for subsequent explosive development of this innovative method for C-C bond formation. One of the central features of the Mukaiyama aldol process is that the typical enol silane is un-reactive at ambient temperatures with typical aldehydes. This reactivity profile allows exquisite control of the reaction stereoselectivity by various Lewis acids additionally, it has led to the advances in catalytic, enantioselective aldol methodology. Recent observations involving novel enol silanes, such as enoxy silacyclobutanes and O-si-lyl M(9-ketene acetals have expanded the scope of this process and provided additional insight into the mechanistic manifolds available to this versatile reaction. 

Leigh and his co-workers have studied the photochemical decomposition of the silacyclobutanes (224). In hydrocarbon solution with added methanol, (224) undergoes decomposition and the formation of the alkoxysilanes (225). These are formed via the intermediacy of the silenes (226) formed by photochemical elimination of ethene. 

Denmark et al. have found that in the presence of excess TBAF the Pd-catalyzed coupling of vinylsilacyclobutanes with aryl iodides and vinyl halides proceeds efficiently and highly stereospecifically at room temperature (Scheme 10.209) [543]. Although aryl(methyl)silacyclobutanes are insensitive to aryl and vinyl hahdes under a variety of conditions, aryl(chloro)silacyclobutanes have enough reactivity for coupling with aryl iodides [544]. In this biaryl couphng the use of t-BusP serves to suppress the formation of homo-couphng products. 

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