Diimides stereoselectivity

A well-known example of group transfer reaction with p = 0, q = 2 is represented by the concerted reduction of alkenes and alkynes with the reactive intermediate diimide stereoselectively in the cis fashion (Scheme 6.2). The driving force of the reaction is the formation of the stable nitrogen molecule. 

The origin of stereofacial selectivity in electrophilic additions to methylene-cyclohexanes (2) and 5-methylene-l,3-dioxane (3) has been elucidated experimentally (Table 2) and theoretically. Ab initio calculations suggest that two electronic factors contribute to the experimentally observed axial stereoselectivity for polarizable electrophiles (in epoxidation and diimide reduction) the spatial anisotropy of the HOMO (common to both molecules) and the anisotropy in the electrostatic potential field (in the case of methylenedioxane). The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar p C-H a-bonds and opposing p C—O or C—C cr-bonds. In contrast, catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects. ... 

This chapter is devoted to the discussion of the reduction of carbon-carbon double and triple bonds by noncatalytic methods, These methods include reductions by diimide, by dissolving metals in the presence or absence of proton donors, by low-valent metal ions, by metal hydride-metal halide combinations and by so-called ionic hydrogenation procedures. Of these widely diverse methods of reduction of carbon-carbon double and triple bonds, the reduction by diimide appears to be the most versatile. The reduction of carbon-carbon double and triple bonds by diimide occurs with complete stereoselectivity and stereo-specificity, and can be effected in the presence of a variety of other, very chemically reactive functional... 

Reduction ofC==C and C C by Noncatalytic Chemical Methods 33.2.3.2 Stereoselectivity of diimide reductions... 

These A-sulfinyl Diels-Alder reactions are also highly stereoselective, giving products of syn addition to the 1,3-diene. The same holds true for the sulfur diimide cycloadditions . The stereoselectivity with respect to the dienophile is not very well known because the stereochemistry of sulfur in the starting A-sulfinyl dienophile and in the resulting thiazine derivatives has usually not been determined. A representative sample of the stereoselective preparation of 3,6-dihydrothiazine 1-oxides and 1-imines is shown in Scheme 34 <84JA786i, 84JA7867>. 

SCHEME 6.2 Stereoselective concerted reduction of an aUcene derivative using diimide. 

Ketene formation using l,8-bis(dimethylamino)naphthalene 406 in the presence of benzoylqijinidine (BQd) as a known nucleophilic chiral catalyst and Zn(OXf)2 as Lewis acid catalyst and ori/io-benzoquinone diimides 407 as coreactants afforded quinoxaline derivatives 408 with high stereoselectivity nearly in all cases (Scheme 127) (2006JA13370). Ketene enolates 405 are suggested as the reaction intermediates. Transformation of the product 408 (R = Et) to 409 by reduction was also performed. 

Exocyclic bis-silylated olefins have been constructed through the Pd(OAc)2-catalyzed reaction of alkynes with a tethered disi-lanyl group. The reactions are carried out in the presence of a tert-alkyl isocyanide, although the precise role of this ligand is unclear. Diimide reduction of the disilylated alkene so-formed followed by Fleming-Tamao-type oxidation of the two C-Si bonds in the saturated product then affords 1,2,4-triols in a stereoselective manner (eq 83).l ... 

A number of alternatives to catalytic hydrogenation have been developed for stereoselective reductions of olefins. They include reductions with diimide, dissolving metal reductions, ionic reductions with organosilanes, and enzymatic reductions [10-13]. 

An illustrative example of a stereoselective reduction with diimide is the conversion of the steroid derivative 82 into 83 as the only observed isomer (Equation 19) [61]. This transformation was used as a convenient stereo-and regioselective method for isotopic labeling of 83. Use of diimide avoided the formation of diastereomeric mixtures and prevented partial unwanted isotopic labeling at the allylic position, both of which had otherwise been observed with use of standard heterogeneous catalytic hydrogenations. 

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