Olefins diazoalkane cycloaddition reactions

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

The transition metal-catalyzed reaction of diazoalkanes with acceptor-substituted alkenes is far more intricate than reaction with simple alkenes. With acceptor-substituted alkenes the diazoalkane can undergo (transition metal-catalyzed) 1,3-dipolar cycloaddition to the olefin [651-654]. The resulting 3//-pyrazolines can either be stable or can isomerize to l//-pyrazolines. 3//-Pyrazolines can also eliminate nitrogen and collapse to cyclopropanes, even at low temperatures. Despite these potential side-reactions, several examples of catalyzed cyclopropanations of acceptor-substituted alkenes with diazoalkanes have been reported [648,655]. Substituted 2-cyclohexenones or cinnamates [642,656] have been cyclopropanated in excellent yields by treatment with diazomethane/palladium(II) acetate. Maleates, fumarates, or acrylates [642,657], on the other hand, cannot, however, be cyclopropanated under these conditions. 

The reactions of diazoalkanes 9.21 with alkenes lead to pyrazolines 9.22, which are thermally transformed into cyclopropanes. Similar transformations occur during thermal reactions of diazoesters. The use of diazoesters of chiral alcohols did not give useful results, so chiral residues have been introduced on the olefin dipolarophile. Meyers and coworkers [327] carried out the reaction of diazomethane 9.21 (R = R = H) and diazopropane 9.21 (R = R = Me) with chiral lactams 1.92 (R = i-Pr or ferf-Bu, R = Me). These 1,3-dipolar cycloadditions are regioselective, but only CH2N2 leads to an interesting stereoselectivity (Figure 9.9). Morever, when the RM substituent of lactam 1.92 is H, the reaction is no longer stereoselective. 

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