Vinylsilanes cyclic

Thereacdonof acetyl nitrate with cyclic vinylsilanes gives l-nitrQcyclo ilkenes or 1,1-dinitro 3-nitrated cyclo ilkanes, depending on the ring size, as shown in Eqs 3 37 and 3 38... 

Entry 5 is an example of use of an a-trimethylsilylallyl group to prepare a vinylsilane. The stereochemistry is consistent with a cyclic TS having the trimethylsilyl substituent in a quasi-axial position to avoid interaction with the bridgehead hydrogen of the bicyclic ring. 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

Denmark pursued intramolecular alkyne hydrosilylation in the context of generating stereodefined vinylsilanes for cross-coupling chemistry (Scheme 21). Cyclic siloxanes from platinum-catalyzed hydrosilylation were used in a coupling reaction, affording good yields with a variety of aryl iodides.84 The three steps are mutually compatible and can be carried out as a one-pot hydro-arylation of propargylic alcohols. The isomeric trans-exo-dig addition was also achieved. Despite the fact that many catalysts for terminal alkyne hydrosilylation react poorly with internal alkynes, the group found that ruthenium(n) chloride arene complexes—which provide complete selectivity for trans-... 

This reaction was successfully applied to various hydrazones derived from acyclic or cyclic ketones and extended to a variety of unsubstituted vinylsilanes as acceptors (trialkyl, triphenyl and alkoxy vinylsilanes). In all cases, the regioselectivity of the addition reaction was consistent with the formation of a silicon-stabilized organozinc reagent. 

The chiral Mo-alkylidene complex derived from AROM of a cyclic olefin may also participate in an intermolecular cross metathesis reaction. As depicted in Scheme 16, treatment of meso-72a with a solution of 5 mol % 4a and 2 equivalents of styrene leads to the formation of optically pure 73 in 57% isolated yield and >98% trans olefin selectivity [26]. The Mo-catalyzed AROM/CM reaction can be carried out in the presence of vinylsilanes the derived optically pure 74 (Scheme 16) may subsequently be subjected to Pd-catalyzed cross-coupling reactions, allowing access to a wider range of optically pure cyclopentanes. 

With halogen electrophiles both retention and inversion of stereochemistry have been observed. In this case the addition of the electrophile may lead to the -silicon cation, or a cyclic halonium ion. Scheme 5 shows a generalized mechanism for the reaction of vinylsilanes with electrophilic reagents120. 

The photolysis of the rigid l-silabicyclo[2.2.1]heptene 250, which may be regarded as a cyclic vinylsilane, induces a 1,3-carbon shift to produce a cyclic silene intermediate 251, which is trapped by alcohols to give 252 (equation 62)141 142. 

Dimethyldioxirane oxidizes acyclic vinylsilanes at room temperature to the corresponding epoxides 166 in excellent yield (equation 141)255. Allylic oxidation is found in appreciable amounts when cyclic vinylsilanes are used. It is interesting to note that simple alkenes react faster with dioxirane than vinylsilanes. The trend appears to be reversed when MCPBA is employed as the oxidant. 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

Under these optimized conditions, di-tert-butylsilylene could be transferred to a range of acyclic and cyclic olefins (Schemes 7.9 and 7.10).11,74 The method was not sensitive to the steric nature of the R substituent nearly quantitative silylene transfer to olefins bearing ra-butyl, isopropyl, or tert-butyl groups was observed. Vinylsilanes were also tolerated as substrates. Olefins containing silyl ether, benzyl ether, and pivolate substituents were all effective traps of di-tm-butylsilylene. 

The synthesis of L-daunosamine began with the condensation of trans-crotonaldehyde (56) with dibenzylhydrazine (Scheme 17). Sharpless asymmetric dihydroxylation of the resulting ( )-a, (3-unsaturated hydrazone 57 afforded the syn-diol 58 (70% yield, 89% ee by HPLC), and silylation with chlorodimethyl-vinylsilane then provided the radical cyclization precursor 59 in 98% yield. In the key step, exposure to thiyl radicals generated from PhSH and AIBN led to radical cyclization of dibenzylhydrazone 59. The unstable cyclic intermediate was then directly treated with fluoride to afford vinyl adduct 60 in 77% yield (dr 91 9, H NMR). In control experiments with corresponding monosilyl derivatives, the (3-O-silyl... 

Intramolecular hydrosilylation. The hydrodimethylsilyl ether (2) of a homo-propargyl alcohol (1) undergoes intramolecular hydrosilylation catalyzed by H2PtCl6 to give a cyclic (E)-vinylsilane (3). This can be converted to a 0-hydroxy ketone (4) by hydrogen peroxide oxidation or into (Z)-3-bromo-3-decene-l-ol (5) by brom-ination/bromodesilylation with inversion of geometry. 

Intramolecular allylsilylation of alkynes is a convenient route to cyclic vinylsilanes.211 Propargylsilanes are available for Lewis acid-promoted carbosilylation of alkynes.212 Unlike allylsilanes, both cr-substituted and unsubstituted propargylsilanes react at the a-position, which can be rationalized by in situ isomerization of these reagents to allenylsilanes (Equation (55)). 

Acyclic a,/3-unsaturated acid chlorides do not undergo this acylation. However, cyclic vinylsilanes also undergo Friedel-Crafts acylation with a,/S-unsaturated acid chlorides to give dienones of type a, which are cyciized by SnCl to cyclopentenones. A typical example is formulated for the reaction of 5-methyl-l-tri-methylsilylcyclopentene (3) with j3,iS-dimethylacryloyl chloride. The first step is conducted with AICI3 and gives 4, which after isolation is cyciized with SnCU to a mixture of 5 and 6. The cyclopentenone 6 is obtained in 55% overall yield after isomerization with RhCU. 

The efficient conversion of a ketone to a fused cyclopentenone of the general type outlined in Scheme 5 has been demonstrated by Paquette and coworkers.The overall strategy is illustrated in Scheme 6 and begins with Freidel-Crafts acylation of the cyclic vinylsilane (8) to give the enone (9), which undergoes... 

The vinylsilane-terminated cyclization strategy has been extended to the preparation of eight-membered cyclic ethers. Oxocenes (Scheme 17) with AAunsaturation (3,6,7,8-tetrahydro-2//-oxocins) were prepared efficiently by the SnCU-catalyzed cyclization of the mixed acetals (39) with complete re-giochemical control. Electrophilic addition on the 2-(trimethylsilyl)-l-alkene occurs predominanfiy at the terminal position of the alkene to form a tertiary a-silyl, rather than a primary 3-silyl, carbocation. The... 

Through the efforts of the Overman research group, the acetal-vinylsilane cyclization reaction has been shown to be a useful strategy for the stereoselective preparation of a variety of medium-sized oxygen heterocycles. This cyclization strategy has been used successfully in the asymmetric synthesis of the marine natural product (-)-laurenyne (40a), an eight-membered cyclic ether. These molecules are members of an unusual class of C-15 nonisoprenoid metabolites. 

Vinylsilanes as nucleophilic terminators offer several additional advantages over ordinary nonactivated alkenes (c/. Section 4.2.2.1). The silyl group is readily substituted stereo- and regio-selectively by the electrophilic carbon atom of the oxocarbenium ion, as demonstrated in Overman s synthesis of alkyl-idenetetrahydropyran (109 Scheme 53). This strategy was also applied to the preparation of five- and seven-membered cyclic ethers. ... 

Mallya, M. N., Nagendrappa, G. cis-Hydroxylation of cyclic vinylsilanes using cetyltrimethylammonium permanganate. Synthesis 1999, 37-39. 

Stereoselective syntheses of substituted cyclic amines by acid-catalyzed cyclization of vinylsilanes bearing an amino group 03SL143. 

Intramolecular allylsilylation of alkynes is a convenient route to cyclic vinylsilanes. The HfCU-MesSiCl catalyst system effectively induces endo cyclization of al-kynylated allylsilanes to l-silyl-3-vinylcycloalkenes (Scheme 10.193) [515]. The endo selectivity is probably because the exo cyclization pathway via zwitterionic intermediate 144 is inhibited by a severe steric repulsion between the substituent R and the allylsilane moiety. 

This can be illustrated by the acylation of linear and cyclic vinylsilanes. It is quite simple to prepare methylvinylketone (59) or l-methyl-6-TMS-hexa-2,5-diene-4-one (70) In the latter case the starting material is bis(trimethylsilyl)-ethene (62) which can either be achieved via hydrosilylation (vide supra) or by the more classical approaches " Furthermore, because of the hi stereospecifi-ty, compounds like E- cinnamaldehyde (77) are obtainable in good yields and with superb optical purity ... 

Eight- and nine-membered cyclic ethers are also generated via type-III reactions by treating the 5-hexen-l-ol acetals with Lewis acids. 8-endo Cyclization of 5-hexenyl methoxyethoxymethyl ether in the presence of two equivalents of tin(IV) chloride forms a 2 1 mixture of chlorinated and unsaturated 1 -oxycyclooctanes 11 and 1232. Vinylsilane undergoes an 8-endo Prins cyclization. After O-desilylation, oxocene 13, which is used as a precursor in a laurenyne synthesis, is obtained in 37% yield33. 

Overman has also shown that eight- and nine-membered cyclic ethers can be prepared by type 111 cy-clizations.136 Treatment of (257) with 2 equiv. of SnCL for 13 h at -20 C gives (258), which cyclizes to give a 2 1 mixture of (259) and (260) in 83% yield.137 Cleaner reaction mixtures have been obtained using vinylsilanes, as in the synthesis of laurenyne.138 SnCU-mediated cyclization of acetal (261) in CH2CI2 at 0 C, followed by 0-desilylation produces oxocene (262) in 37% yield as the sole cyclic ether (Scheme 38). 

A number of intramolecular acylations of vinylsilanes have been reported164,165 (equations 52 and 53). Trost has cyclised the thioketal 30 using dimethyl (methylthio)sulphonium fluoroborate which leads to the rearranged product 31166 (see equation 54). Overmann has used iminium salts to achieve both endocyclic and exo-cyclic ring closures167 - 169 (equations 55 and 56). In this latter case it is proposed that 32... 

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