Cyclopropane, stannyl

Several catalytic systems have been reported for the enantioselective Simmons Smith cyclopropanation reaction and, among these, only a few could be used in catalytic amounts. Chiral bis(sulfonamides) derived from cyclo-hexanediamine have been successfully employed as promoters of the enantioselective Simmons-Smith cyclopropanation of a series of allylic alcohols. Excellent results in terms of both yield and stereoselectivity were obtained even with disubstituted allylic alcohols, as shown in Scheme 6.20. Moreover, this methodology could be applied to the cyclopropanation of stannyl and silyl-substituted allylic alcohols, providing an entry to the enantioselective route to stannyl- and silyl-substituted cyclopropanes of potential synthetic intermediates. On the other hand, it must be noted that the presence of a methyl substituent at the 2-position of the allylic alcohol was not well tolerated and led to slow reactions and poor enantioselectivities (ee<50% ee). ... 

Upon addition of cyclopropane 1 to one equivalent of SnCl in methylene chloride at 0 °C, an exothermic reaction occurs to give 3-stannyl propionate 11 in approx. 70% yield [9, 11]. Addition of a second equiv of the cyclopropane slowly converts 11 to the dialkylated tin compounds, 12 Eq. (15). No further alkylation was observed. Chelate structures of such homoenolates implied by their spectral properties are supported by single crystal X-ray analysis [30]. 

Simple 3-acylfurans are not accessible by direct Friedel-Crafts-type substitution of furan, but a most unusual route to these compounds has been discovered. Thus, reaction of the stannylated cyclopropane derivative 1 with acid chlorides (RCOC1) in refluxing toluene for 5 hours gave the dihydrofurans 2 in 64-83% yield. Treatment of 2 with 1.2 equivalents of BF3,OEt2 in dichloromethane at -78°C followed by slow wanning of the reaction mixture to room temperature gave the 3-acylfurans 3 in 30-62% yield. 

Cyclopropan 2,2(bzw. 2,3)-Dimethyl-1 -ethoxycarbonyl-1 -trimethyl-stannyl- El9b, 1457 (Carben + En)... 

The chiral bis(sulfonamide) catalyzed cyclopropanation was extended successfully to y-silyl-and -stannyl-substituted ally lie alcohols, for example, giving cyclopropanes 90 and... 

Table 23. 2,2-Dichloro-1-stannyl(or stannylalkyl)- or -2-germanyl(or ger-manylmethyl)cyclopropanes ...

Although a great number of cyclopropanes bearing a trialkylsilicon, -germanium, -tin, or -lead substituent are known, few are prepared by an addition reaction of a metal-substituted carbene to a C-C double bond. The syntheses and reaction pathways of silyl-, germyl-, stannyl-, and plumbylcarbenes have been described in detail in Houben-Weyl, Vol. E19b, p 1410. 

Cyclopropanes 11 and 13, respectively, are obtained when y-stannyl alcohols 10 or 12 are treated with thionyl chloride or with acids (cf. Section 1.1.). 

Ring-opening reactions of (silylmethyl)cyclopropanes by halogenating reagents are also known (Table 11, entries 7-9). The reaction of (trimethylsilylmethyl)cyclopropane with iodine gives 4-iodobut-l-ene. A similar transformation can be achieved by the reaction of (trimethyl-stannylmethyl)cyclopropane with iodine in chloroform. In these iodination reactions, the direction of electrophilic attack is determined by the silyl or the stannyl group. 

The regiochemistry of ring opening has been examined with 2-methyl-l-(stannyl-methyl)cyclopropane. Thus, the reaction of 2-methyl-l-(stannylmethyl)cyclopropane in chloroform with sulfur dioxide, trifluoroacetic acid, and iodine proceeds preferentially with ring cleavage and addition at the cyclopropane methylene. On the other hand, iodination and acidolysis in methanol proceeds exclusively with Sn —Me bond cleavage. 

The effect of solvent was also studied and complexing solvents such as THF or Et20 inhibited the cyclopropanation reaction. Furthermore, the presence of an unprotected allylic alcohol was found to be essential, since the methyl or benzyl ether derived from cinnamyl alcohol afforded almost racemic cyclopropanes. This method has also been extended to the enantioselective cyclopropanation of vinylsilanes and -stannanes (Scheme 4) [13]. The corresponding optically active silyl- and stannyl-substituted cyclopropyhnethanols were obtained in the presence of the chiral N,iV-bis(p-nitrobenzenesulfonyl)-l,2-cyclohexane-diamine 9. 

Cyclopropanation of vinylorganometallic compounds. Molandcr cyclopropana-tion of allylic alcohols substituted by silyl or stannyl groups can show high diastereo-sclcctivity particularly when carried out with a large excess of the samarium reagent. 

One of the more commonly used organotin reagents in both free-radical reactions and Lewis acid-mediated reactions is allyltributyltin [3, 7]. This reagent permits the construction of new carbon-carbon bonds from free radical precursors such as alkyl halides however, reactions with a-ketocydopropanes were poorly understood. Tin(IV) enolates generated from a-ketocyclopropane 44 and allyltributyltin undergo both radical allylation and electrophilic quench as shown in Scheme 11, forming O-stannyl ketyl 45 with allyltributyltin and subsequent scission of the cyclopropane... 

Cyclopropanation. P-Stannyl carbocations collapse by formation of a three-membered ring. Methods for generating such active intermediates, and hence compounds containing a cyclopropane unit, are available. ... 

Recently, Lewis acidic hydrostannane was shown to be valuable for highly stereoselective homolytic hydrostannylation of allyl and homoallyl alcohols. Using EtsB-dry air as radical initiator, the hydrostannylation of Morita-Baylis-Hillman (MBH) adduct 1 took place smoothly, and subsequent buty-lation of the reaction mixture with BuLi gave y-stannylated alcohol 2 in 64% yield with good syn diastereoselectivity (Scheme 4.1). Following treatment with pyridine and thionyl chloride, syn-2 could be easily transformed into trans-, l-disubstituted cyclopropane 3 in 85% yield." ... 

Cyclopropanated allylboration reagents have been used to homoallylate and homocrotylate aldehydes. A zinc-mediated a-prenylation of aldehydes and ketones has been reported conventional conditions have yielded mainly the y-adduct to date. " A racemic allene has been hydroborated in an enantioconvergent manner the enantioenriched crotylborane then reacts with aldehydes in high ee to give ( j-stannyl-homoallylic alcohols. A copper-catalysed y-selective and stereospecific allylic alkylation of ketene silyl acetals has been reported. ... 

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