Silacyclobutane

Silacyclobutane (SCB) or siletane is a four-membered heterocylic ring (silacycle) consisting of one silicon atom and three carbon atoms with the general formula (CH2)3SiH2. Silacyclobutane is one of the simplest molecules in the family of organosilicon compounds. The four-membered ring framework of silacyclobutane is analogous to that of cyclobutane, but one carbon atom is replaced by silicon. The four atoms of silacyclobutane form a nonplanar, puckered ring. Derivatives of silacyclobutane are called silacyclobutanes with the general formula (CH2)3SiR2, though substituion at the carbon atom is common. Since the first synthetic report of silacyclobutane in the 1950s, silacyclobutane and its analogues have garnered considerable attention owing to their high ring strain, Lewis acidity, and tunable silicon-carbon bond activation, which enable diverse ring-opening and ring-expansion pathways. Other notable applications include using silacyclobutanes as reagents to achieve enantioselective intermolecular C–H silylation and to synthesize Si-stereogenic vinylsilanes.

Structure The four carbon atoms in cyclobutane are not coplanar but rather adopt a "puckered" or "butterfly" conformation resulting from the large barrier to internal rotation due to the repulsions of the three adjacent methylene groups. The dihedral angle of the silacyclobutane puckered ring has been calculated as 35.9 ± 2°. For silacyclobutane, the C-Si-C and C-C-C bond angles have been calculated as 77.2-78.8° and 97.0-100.5°, respectively utilizing multi-configurational self-consistent field (MCSCF) and gas electron diffraction.

Like cyclobutane, the small heterocyclic ring of silacyclobutane is highly strained, resulting in lower bond energies when compared to related linear or unstrained, silicon-containing or hydrocarbon rings, such as hexane, cyclohexane, or silacyclohexane. Interestingly, the energetic barrier to puckering of silacyclobutane (440 cm−1) is lower than that of cyclobutane (498 cm−1), perhaps owing to the greater flexibility of C-Si-C, longer Si–C bonds, and greater bond angle flexibility. Electronic structure calculations of silacyclobutane reveal that the LUMO is lower in energy compared to cyclobutane. The highly strained nature of silacyclobutanes can be investigated through thermolysis, which can provide insights into ring stability and decomposition. Interestingly, there is a considerable difference in liquid- vs. gas-phase thermal decomposition for these compounds. The liquid-phase pyrolysis of 1,1-dimethyl-1-silacyclobutane with the formula (CH2)3SiMe2 results in ring-opening polymerization at 150-200 °C. In comparison, gas-phase pyrolysis of has been demonstrated to primarily undergo unimolecular decomposition to ethene and dimethylsilene at 400-460 °C. It was later shown that there are two minor decomposition pathways: one that forms methyl radicals via the Si–CH3 bond cleavage and another propene–dimethylsilylene species.

Synthesis and history Fredrick Stanley Kipping, a pioneer of silicon chemistry in the 1920s was the first to conceptualize silacyclobutane in his series of manuscripts titled "Organic Derivatives of Silicon" and is often credited in its history. However, products formed Kipping's experiments were complex mixtures and he could not conclusively isolate or characterize the silacycles. Kipping's work was one of the earliest demonstrations that silicon could form stable cyclic structures analogous to carbon rings, giving rise to modern organosilicon chemistry.

Grignard reagent addition In 1954, Sommer and Baum reported the first successful synthesis and isolation of a silacyclobutane, 1,1-dimethyl-1-silacyclobutane. Starting from (3-bromopropyl)trimethylsilane, they formed a disiloxane intermediate and subsequently generated a terminal dihalide that was treated with magnesium metal and dilute diethyl ether to form the respective in situ Grignard reagents (see also Barbier reaction), which perform Wurtz-type coupling intramolecular ring closure by preferentially attacking the bromine site first, followed by the chloride site to form a four-membered ring.

Classically, silacyclobutanes can be formed using Grignard addition. Notably, this method is employed in the formation of benzosilacyclobutanes from a 1-bromo-2-(bromomethyl)benzene starting material by Gilman and Atwell in the 1960s, followed by de Boer et al. and Kang et al. in the 1980s.

Halogenated silacyclobutanes and nucleophilic substitution Vdovin et al. and Laane et al. in the 1960's synthesized the first halogenated silacyclobutanes, 1,1-dichloro-silacyclobutane and 1,1-difloro-silacyclobutane. The former SiCl2-containing silacyclobutane can be prepared from either (3-bromopropyl)trichlorosilane, which forms more readily, or (3-chloropropyl)trichlorosilane reagents in similar yield. The Si-Cl bond lability stems two-fold from the electrophilicity of Si and the weak base chloride ion having propensity to be good a leaving group. Thus, 1,1-dichloro-silacyclobutane proves to be a useful intermediate for accessing further structurally modified silacyclobutanes through nucleophilic substitution. Indeed, as proof of concept, Laane synthesized silacyclobutane-1,1-d2 species using LiAlD4, a deuterated, strong reducing agent isotopologue to LiAlH4. In 1980, Auner and Grobe further expanded the collection of known silacyclobutanes through substitution of the SiCl2-containing silacyclobutane using dimethylamine, Grignard reagents, and sodium cyclopentadienide.

[2+2] cycloaddition with alkenes Alternatively, silacyclobutanes can be prepared through [2+2] cycloaddition reaction with alkenes. Jones et al. demonstrate this using in situ formed vinyldimethylchlorosilane, t-BuLi, and 1,3-butadiene to form a mixture of disilacyclobutanes, monosilacyclobutanes, and silicon-containing products (1). When 1,3-butadiene is utilized as a trapping reagent, the reaction is high yielding and E/Z stereochemical products are formed in moderate ratio (2).

Reactivity

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