Siliranes: Formation, isonitrile insertions, and thermal rearrangements

Strain-Driven, Non-Catalysed Ring Expansion of Silicon Heterocycles

Silacycles are ubiquitous building blocks. Small silacycles can typically be expanded catalytically. A silirane, silirene and phosphasilirene as well as a siletane and a silolene were prepared starting from the base-free bromosilylene [(dtbp Cbz)SiBr] (dtbp Cbz=1,8-bis(3,5-ditertbutylphenyl)-3,6-ditertbutylcarbazolyl). As these heterocycles were derived from a dicoordinated silylene, they are susceptible to reactions with an external base. The three-membered silacycles readily undergo non-catalysed ring expansion reactions with isonitriles yielding the related four-membered silacycles. Surprisingly, the ring-expanded derivatives of the silirane undergo up to two further isomerisation reactions, first by enamine formation and then by another ring expansion. DFT computations were utilised to gauge the scope of this reactivity pattern. Three-membered silacycles should essentially universally undergo a ring expansion with isonitriles, while for four-membered silacycles, only very few instances are predicted to accommodate more challenging kinetic requirements of this ring expansion. Larger silacycles lack the ring strain energy required for this ring expansion reaction and are not expected to be expanded.

Isocyanides in the synthesis of nitrogen heterocycles

Isocyanides have long proved themselves to be irreplaceable building blocks in modern organic chemistry. The unique features of the isocyano group make isocyanides particularly useful for the synthesis of a number of important classes of nitrogen heterocycles, such as pyrroles, indoles, and quinolines. Several cocyclizations of isocyanides via zwitterions and radical intermediates as well as transition-metal-catalyzed syntheses of different types of heterocycles have recently been developed. Methods starting from isocyanides often have distinct advantages over alternative approaches to the same heterocycles because of their enhanced convergence, the great simplicity of most of the operations with them, and the great variety of isocyanides readily available for use. Isocyanides have also been used in some enantioselective syntheses of chiral heterocyclic compounds, including natural products as well as precursors thereof.

Silylenes, Silenes, and Disilenes: Novel Silicon-Based Reagents for Organic Synthesis?

Silylenes, silenes, and disilenes are silicon analogues of carbenes and alkenes. Since the first detection and isolation of these species a few decades ago, focus has been given to their fundamental structure and reactivity properties. Recent developments show that the time is set to exploit their unique chemistry in applied areas. Emerging applications in catalysis and stereoselective synthesis point to a new field within synthetic organosilicon chemistry.

Mechanism of Silver-Mediated Di-tert-butylsilylene Transfer from a Silacyclopropane to an Alkene

Kinetic studies of the reactions of cyclohexene silacyclopropane 1 and monosubstituted alkenes in the presence of 5 mol % of (Ph3P)2AgOTf suggested a possible mechanism for silver-mediated di-tert-butylsilylene transfer. The kinetic order in cyclohexene silacyclopropane 1 was determined to be one. Inverse kinetic saturation behavior (rate inhibition) was observed in monosubstituted alkene and cyclohexene concentrations. Saturation kinetic behavior in catalyst concentration was observed. A reactive intermediate, a silylsilver complex, was observed using low temperature 29Si NMR spectroscopy. Competition experiments between substituted styrenes and a deficient amount of 1 correlated well with the Hammett equation and provided a rho value of -0.62 +/- 0.02 using sigmap constants. These data support a mechanism involving reversible silver-promoted di-tert-butylsilylene extrusion from 1 followed by irreversible concerted electrophilic attack of the silylsilver intermediate on the alkene.

Metal-Catalyzed Di-tert-butylsilylene Transfer: Synthesis and Reactivity of Silacyclopropanes

Metal-catalyzed di-tert-butylsilylene transfer was developed as a mild, operationally simple, functional-group-tolerant method for silacyclopropane formation. Di-tert-butylsilylene was transferred from cyclohexene silacyclopropane 1 to an alkene through the use of a metal salt. Silacyclopropanation occurred at temperatures as low as -27 degrees C when AgOTf or AgOC(O)CF(3) were used as catalysts. Complex silacyclopropanes were formed stereospecifically and diastereoselectively from functionalized alkenes. Silacyclopropanes reacted with various carbonyl compounds, including aldehydes, ketones, formate esters, and formamides, in an overall process that efficiently converts alkenes into oxasilacyclopentanes with defined stereochemistry.

Diastereoselective silacyclopropanations of functionalized chiral alkenes

Lithium reduction of di-tert-butyldichlorosilane and thermal silylene transfer (105-125 degrees C) are complementary methods for the highly diastereoselective silacyclopropanations of a range of functionalized chiral olefins to afford complex silacycles. We have shown that functionalized cyclohexenes, cyclopentenes, norbornenes, and 1,1-disubstituted alkenes undergo silacyclopropanation with excellent diastereoselectivity (92:8 to >99:1). Our results demonstrate that steric interactions, rather than oxygen-directing effects, control the approach of the silylene or silylenoid intermediate to the olefin. We believe that the sterically demanding nature of the di-tert-butylsilylene species prevents coordination to the oxygen functionality. Thermal silylene transfer conditions exhibit broad functional group tolerance; the elevated temperatures for silylene transfer, however, cannot be employed for the silacyclopropanation of substituted cyclohexenes and 1,1-disubstituted alkenes. Elaboration of the resulting functionalized silacyclopropanes provides an efficient route to polyoxygenated products.

Development of reactions of silacyclopropanes as new methods for stereoselective organic synthesis

In this Account, we describe the development of stereospecific, stereoselective, regioselective, and chemoselective carbon-carbon bond-forming reactions of silacyclopropanes that occur under mild conditions. By appropriate choice of metal salt catalyst, the regiochemistry of these reactions may be tuned to give the desired product. Stereoselective nucleophilic substitution reactions and stereospecific oxidation of the C-Si bond to a carbon-oxygen bond demonstrate that the products of these reactions serve as useful intermediates for the synthesis of polyoxygenated organic molecules.

Stereospecific and Regioselective Isocyanide Insertions into Siliranes and Reactions of the Resulting Iminosilacyclobutanes

The insertions of p-tolyl and tert-butyl isocyanide into siliranes yielded iminosilacyclobutanes with stereospecific retention of configuration. Monosubstituted siliranes underwent insertion into the more substituted Si-C bond of the ring, although this regioselectivity was eroded as substitution increased on the silirane ring. The iminosilacyclobutane products tautomerized thermally or in the presence of a palladium catalyst to yield the thermodynamically more stable aminosilacyclobutenes. Ring-expansion reactions of iminosilacyclobutanes were promoted by acids: treatment with aqueous copper sulfate produced an oxasilacyclopentane in high yield, whereas with trifluoroacetic acid, oxasilacyclohexanes were formed.