Benzylidene transfer

Equilibrium concentrations of semi-stabilized benzylidene ylides derived from optically pure arsonium salts and alkali metal ethoxides react with aromatic aldehydes to produce high yields of enantiomerically enriched truns-2,3-diaryloxiranes and optically pure tertiary arsines with retention of configuration at arsenic (equation 

Examples of the reaction are given in Table 4 for the tertiary arsines 17, 73 and 75 (equations 9-11). The absolute configuration of (/ )-( + )-15 was determined by X-ray crystal structure determination the absolute configurations of (5)-(+)-73 and (5)-(—)-75 were determined by X-ray crystal structure analyses of appropriate metal complexes (References 92 and 93, respectively). 

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Chiral arsonium ylides containing the benzylidene group react with aromatic aldehydes to produce trans-2,3-diaryloxiranes with optical purities of up to 41%. The degree of asymmetric induction depends upon the nature of the substituents on the ylide, the substrate, and upon the reaction conditions. For the variety of arsonium ylides investigated, however, the yields of recovered optically active arsines were almost quantitative with complete retention of configuration at arsenic in each case. Thus, apart from the value of stereoselective benzylidene transfer for the asymmetric synthesis of trans-2,3-diaryl oxiranes, the reaction provides the cleanest route to the recovery of optically active arsines from resolved benzylarsonium salts (see Section III.B.5). The mechanism of the reaction, which must take into account the exclusive formation of trans-diaryloxiranes, is believed to occur via the stereoselective decomposition of a pair of (R, S )-betaines (Scheme 16). 

Amalgam reduction As(n-Bu)MePh (12) AsMePh(n-Pr) (65) Hydride reduction As(n-Bu)MePh (12) AsMePh(n-Pr) (65) Cyanolysis AsBnMePh (69) AsMePh(n-Pr) (65) Benzylidene transfer AsMe(a-Naph)(p-Tol) (17) AsMePh(8-Quin) (73) l,2-C6H4(AsMePh)2 (75)... 

New methods for the generation of ylide (1) in the presence of a substrate have been introduced. Solid potassium hydroxide in nonpolar solvents containing trace amounts of water has proven a highly effective medium for the promotion of methylene and benzylidene transfer reactions (equation 1). The counterion of the sulfonium salt and the base used has a substantial effect on the success of Ae re-action. 2... 

Similar conversions were reported with 1,2,3,4,5-pentamethylcyclopenta-l,3-diene, to give stereoselectively the ent/o-6-phenyl-e.vo-4-methyl isomer of pentamethylphenylbicy-clo[3.1.0]hex-2-ene with > 85% yield.Benzylidene transfer to (Z)- and ( )-penta-l,3-diene, isoprene and 2-chlorobuta-l,3-diene regioselectively occurred at the more electron-rich C-C double bond to give trans- and cw-phenylvinylcyclopropanes. The transjcis ratio ranged from 0.52 to 2.2. 

In order to account for the origin of the enantioselectivity and diastereoselectivity of benzylidene transfer, it is necessary know whether the sulfur ylide reactions are under kinetic or thermodynamic control. From cross-over experiments it was found that the addition of benzylsulfonium ylide to aldehydes was remarkably finely balanced (Scheme 9) [28]. The trans-epoxide was derived directly from irreversible formation of the anti-betaine 4 and the cis-epoxide was derived from partial reversible formation of the syn-betaine 5. The higher transselectivity observed in reactions with aromatic aldehydes compared to aliphatic aldehydes was due to greater reversibility in the formation of the syn-betaine. 

Table 4 Imine acceptors for benzylidene transfer from phenyldiazomethane...

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