Stereochemistry of the reaction of Si-phenyl silenes with butadienes: elaboration of the silacycloadducts to provide a novel route to substituted lactones

Silenes generated through a silyl-modified Peterson olefination procedure can be trapped with a range of alkyl butadienes via a [4 + 2] cycloaddition pathway to afford silacycles accompanied by variable amounts of competing ene, [2 + 2] and silene dimer by-products. The silacycles are formed with good chemo- and stereo-selectivity and provide access to diols and lactones via a phenyl-triggered Fleming-Tamao oxidation.     

硅杂四元环化合物的合成和反应

硅杂四元环化合物在有机硅化学中是一类非常重要的小分子环系化合物, 广泛应用于有机化学、金属有机化学以及材料化学. 环上只含有一个硅原子的硅杂环丁烷可以通过γ-卤代丙基硅烷的Grignard反应、Si＝C键与烯烃的 [2＋2]环加成反应以及硅杂环丙烷的扩环反应合成, 环上只含有一个硅原子的硅杂环丁烯可以通过格氏试剂或锂试剂参与的Si—C键的关环反应、硅杂环丁烷的转化反应、硅卡宾对C—H键的插入反应、Si＝C键与炔烃的[2＋2]环加成反应以及二炔基硅烷的分子内成环反应等途径合成. 硅杂环丁烷和硅杂环丁烯由于存在环张力和具有一定的Lewis酸性, 能够通过扩环反应生成五元和六元含硅杂环化合物, 也能够通过开环反应生成不同结构的有机硅分子和聚合物, 抑或实现有机反应在温和条件下的转化.     

Oxidative cleavage of C-Si bonds in polyhydroxylated silacyclopentanes

Oxidative cleavage of C-Si bonds of polyhydroxylated silacyclopentanes under various conditions have led to both the desired polyols and to Peterson elimination products. Further studies on the reactivity of these silacycles, under acidic and basic conditions have been carried out, leading to unexpected results. Treatment of these silacycles under basic conditions thus provided various diols after the cleavage of C_sp³-Si bonds. A mechanistic rationale has been proposed for each case.     

Synthesis of various silacycles via the lewis acid-catalyzed intramolecular trans-hydrosilylation of unactivated alkynes

Various silacycles with vinylsilane framework are synthesized via the Lewis acid-catalyzed intramolecular hydrosilylation of alkynes. The cyclization proceeds in an endo-trans or/and in an exo-trans manner, depending on the substrate structure. This methodology is applicable to the synthesis of five-, six-, seven-, and eight-membered medium-sized silacycles. Furthermore, it is possible to obtain a silole derivative via the intramolecular hydrosilylation of the ortho-alkynyl-substituted phenylsilane 10.     

Palladium-catalyzed enantioselective desymmetrization of silacyclobutanes: construction of silacycles possessing a tetraorganosilicon stereocenter

A palladium-catalyzed desymmetrization of alkyne-tethered silacyclobutanes to give silacycles possessing a tetraorganosilicon stereocenter has been developed, and high chemo- and enantioselectivities have been achieved by the use of a newly synthesized chiral phosphoramidite ligand.     

Highly Enantioselective Construction of Multifunctional Silicon-Stereogenic Silacycles by Asymmetric Enamine Catalysis

We report the first highly enantioselective construction of silicon-stereocenters by asymmetric enamine catalysis. An unprecedented desymmetric intramolecular aldolization of prochiral siladials was thus developed for the facile access of multifunctional silicon-stereogenic silacycles in high to excellent enantioselectivity. With an enal moiety, these adducts could be readily elaborated for the diverse synthesis of silicon-stereogenic compounds, and for late-stage modification.     

Construction of Benzosiloles, Six- and Eight-Membered Silacyclic Skeletons, via a Pd-Catalyzed Intramolecular Mizoroki-Heck Reaction of Vinylsilanes

A variety of silacycles including benzosiloles, six- and eight-membered silacyclic skeletons, were efficiently synthesized via a Pd-catalyzed intramolecular Mizoroki-Heck reaction of vinylsilanes.     

Ene cyclisations of alpha-(prenyl)dialkylsilyloxy aldehydes: formation and oxidative cleavage of oxasilacyclohexanols

A variety of routes are described for the synthesis of a-silyloxy aldehydes in which the silicon atom bears a prenyl side chain. These compounds are shown to undergo stereoselective carbonyl ene cyclisation under mildly Lewis acidic conditions and the derived silacycles are cleaved to afford single diastereomers of functionalised triols.     

New Organosiloxanes Containing Silacycles

Abstract

The synthesis of carbon substituted 1, 1-dichloro- and l, l-bis(diethylamino)silacyclobutanes and butenes and their polycondensation reactions with bisphenol A, 1,2-ethanediol and 1,6-hexanediol is described. The monomer silacycles and the organosiloxane polymers are characterized by NMR (¹H-, ¹³C-, ²⁹Si-), GPC, DSC and elemental analysis.     

Me3SiSiMe2(OnBu): a disilane reagent for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement

Herein, a readily available disilane Me₃SiSiMe₂(OⁿBu) has been developed for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement. This protocol enables the incorporation of a silylene into different starting materials, including acrylamides, alkene-tethered 2-(2-iodophenyl)-1H-indoles, and 2-iodobiaryls, via the cleavage of Si–Si, Si–C, and Si–O bonds, leading to the formation of spirobenzosiloles, fused benzosiloles, and π-conjugated dibenzosiloles in moderate to good yields. Preliminary mechanistic studies indicate that this transformation is realized by successive palladium-catalyzed bis-silylation and Brook- and retro-Brook-type rearrangement of silane-tethered silanols.

A readily available disilane Me₃SiSiMe₂(OⁿBu) as a silylene source has been developed for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement.     

Ring Expansion to 6-, 7-, and 8-Membered Benzosilacycles through Strain-Release Silicon-Based Cross-Coupling

The synthesis of silacycles is highly appealing due to their important applications in organic synthesis, medicinal chemistry, and materials chemistry. However, sila-tetralins and sila-benzosuberanes are surprisingly under-represented due to a lack of general methods to access these compounds. We successfully developed a Pd-catalyzed strain-release silicon-based cross-coupling as an unprecedented ring-expansion method, which constitutes a general route for preparing diverse sila-tetralins and sila-benzosuberanes.     

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.     

Temporary Silicon‐Tethered Ring‐Closing Metathesis: Recent Advances in Methodology Development and Natural Product Synthesis

Temporary silicon‐tethered ring‐closing metathesis represents an important cross‐coupling strategy for the formation of medium‐sized silacycles. These intermediates are valuable synthons in organic synthesis due to their propensity to undergo a facile refunctionalization through protodesilylation, oxidation, silane‐group transfer or transmetallation. A particularly attractive utility of this methodology is an application in the synthesis of biologically important natural products. The purpose of this review article is to highlight the recent progress in methodology development and its strategic application toward the target‐directed synthesis.     

Lewis acid catalyzed stereoselective carbosilylation. Intramolecular trans-vinylsilylation and trans-arylsilylation of unactivated alkynes.

Intramolecular trans-vinylsilylation using silicon-tethered alkynylvinylsilanes 3b-c was catalyzed dramatically by Lewis acids such as EtAlCl(2) to give the corresponding six-membered silacycles 4b-c in high yields. The reaction proceeded via an exo-mode cyclization. In addition to the vinylsilylation, the intramolecular trans-arylsilylations using carbon-tethered alkynylarylsilanes 8a-b were also catalyzed by Lewis acids such as HfCl(4) to give six- and seven-membered cyclic (E)-vinylsilanes 9a-b, respectively. The cyclization of silicon-tethered substrates 13a-d afforded five- and six-membered silacycle products 14a-d in low to high yields. All arylsilylation reactions proceeded via an exo-mode fashion exclusively.     

Temporary silicon-tethered ring-closing metathesis: recent advances in methodology development and natural product synthesis

Temporary silicon-tethered ring-closing metathesis represents an important cross-coupling strategy for the formation of medium-sized silacycles. These intermediates are valuable synthons in organic synthesis due to their propensity to undergo a facile refunctionalization through protodesilylation, oxidation, silane-group transfer or transmetallation. A particularly attractive utility of this methodology is an application in the synthesis of biologically important natural products. The purpose of this review article is to highlight the recent progress in methodology development and its strategic application toward the target-directed synthesis.     

Ring Expansion to 6‐, 7‐, and 8‐Membered Benzosilacycles through Strain‐Release Silicon‐Based Cross‐Coupling

The synthesis of silacycles is highly appealing due to their important applications in organic synthesis, medicinal chemistry, and materials chemistry. However, sila‐tetralins and sila‐benzosuberanes are surprisingly under‐represented due to a lack of general methods to access these compounds. We successfully developed a Pd‐catalyzed strain‐release silicon‐based cross‐coupling as an unprecedented ring‐expansion method, which constitutes a general route for preparing diverse sila‐tetralins and sila‐benzosuberanes.     

Palladium-catalyzed silyl C(sp3)-H bond activation

The first transition-metal-catalyzed activation of silyl C(sp(3))-H bond was realized and synthetically applied. A variety of organic skeletons substituted with SiMe(3) groups could undergo the Pd-catalyzed intramolecular coupling reaction, resulting in an unprecedented synthetic method for yielding six-membered silacycles. It was found that the adjacent Si atom played an essential role for the activation of the C(sp(3))-H bond of the SiMe(3) group; no activation reaction of the C(sp(3))-H bond of the CMe(3) group took place under the same reaction conditions.     

Lewis acid–base interactions enhance explosives sensing in silacycle polymers

The high sensitivity of silole- and silafluorene-containing polymers for detecting organic nitro, nitrate, and nitramine explosives cannot be solely attributed to favorable analyte–polymer hydrophobic interactions and amplified fluorescence quenching due to delocalization along the polymer chain. The Lewis acidity of silicon in conjugated poly(silafluorene-vinylene)s is shown to be important. This was established by examining the ²⁹Si NMR chemical shifts (Δ) for the model trimer fragment of the polymer CH₃–silafluorene–(trans-C₂H₂)–silafluorene–(trans-C₂H₂)–silafluorene–CH₃. The peripheral and central silicon resonances are up-field from a TMS reference at −9.50 and −18.9 ppm, respectively. Both resonances shift down-field in the presence of donor analytes and the observed shifts (0 to 1 ppm) correlate with the basicity of a variety of added Lewis bases, including TNT. The most basic analyte studied was acetonitrile and an association constant (K _a) of 0.12 M⁻¹ was calculated its binding to the peripheral silicon centers using the Scatchard method. Spin-lattice relaxation times (T ₁) of 5.86(3) and 4.83(4) s were measured for the methyl protons of acetonitrile in benzene-d ₆ at 20 °C in the absence and presence of the silafluorene trimer, respectively. The significant change in T ₁ values further supports a binding event between acetonitrile and the silafluorene trimer. These studies as well as significant changes and shifts observed in the characteristic UV–Vis absorption of the silafluorene group support an important role for the Lewis acid character of Si in polymer sensors that incorporate strained silacycles. The nitro groups of high explosives may act as weak Lewis-base donors to silacycles. This provides a donor–acceptor interaction that may be crucial for orienting the explosive analyte in the polymer film to provide an efficient pathway for inner-sphere electron transfer during the electron-transfer quenching process. Figure   Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Controllable Si−C Bond Activation Enables Stereocontrol in the Palladium‐Catalyzed [4+2] Annulation of Cyclopropenes with Benzosilacyclobutanes

A novel and unusual palladium‐catalyzed [4+2] annulation of cyclopropenes with benzosilacyclobutanes is reported. This reaction occurred through chemoselective Si?C(sp²) bond activation in synergy with ring expansion/insertion of cyclopropenes to form new C(sp²)?C(sp³) and Si?C(sp³) bonds. An array of previously elusive bicyclic skeleton with high strain, silabicyclo[4.1.0]heptanes, were formed in good yields with excellent diastereoselectivity under mild conditions. An asymmetric version of the reaction with a chiral phosphoramidite ligand furnished a variety of chiral bicyclic silaheterocycle derivatives with good enantioselectivity (up to 95.5:4.5 er). Owing to the mild reaction conditions, the good stereoselectivity profile, and the ready availability of the functionalized precursors, this process constitutes a useful and straightforward strategy for the synthesis of densely functionalized silacycles.     

Controllable Si−C Bond Activation Enables Stereocontrol in the Palladium-Catalyzed [4+2] Annulation of Cyclopropenes with Benzosilacyclobutanes

A novel and unusual palladium-catalyzed [4+2] annulation of cyclopropenes with benzosilacyclobutanes is reported. This reaction occurred through chemoselective Si−C(sp²) bond activation in synergy with ring expansion/insertion of cyclopropenes to form new C(sp²)−C(sp³) and Si−C(sp³) bonds. An array of previously elusive bicyclic skeleton with high strain, silabicyclo[4.1.0]heptanes, were formed in good yields with excellent diastereoselectivity under mild conditions. An asymmetric version of the reaction with a chiral phosphoramidite ligand furnished a variety of chiral bicyclic silaheterocycle derivatives with good enantioselectivity (up to 95.5:4.5 er). Owing to the mild reaction conditions, the good stereoselectivity profile, and the ready availability of the functionalized precursors, this process constitutes a useful and straightforward strategy for the synthesis of densely functionalized silacycles.