Rhodium II hexanoate

Alternatively, a solution of 17 (1 equiv) in CHjClj (10 mL) was added over 0.5 h to a stirred mixture of rhodium(II) hexanoate dimer (0.01 equiv) and diene 16 (quantity given in Table 1) in CH2CI2 (10-30 mL) at 10°C in an argon atmosphere. After stirring for a further 0.5 h at 10°C, the mixture was heated under reflux for 10 min, then the solvent was evaporated under reduced pressure. 

Higher seleetivities in the cyelopropanation step are observed with ethyl (/7)-2-dia7o-6-phenyl-3,5-hexadienoate (21)824. Only cw-l,2-divinylcyclopropanes are formed, which immediately rearrange to give cyeloheptadienes. The yields can be improved by using rhodium(II) hexanoate instead of rhodium(II) acetate and by lowering the temperature to 0°C. However, the use of 21 is also accompanied by a drop in the regioselectivity of the cyelopropanation step. 

In the reaction of A -acylated pyrrole 170 with vinyldiazomethanes with a single electron-with-drawing group a major side reaction occurs under the usual reaction conditions. However, the formation of byproduct 176 can be suppressed by use of rhodium(II) hexanoate in hexane899. 

In the case of vinyldiazomethanes with a single electron-withdrawing group, such as 9, the unsaturated tropane system 91 is not formed cleanly under the traditional reaction conditions of rhodium(II) ace-tate/dichloromethane (Scheme 33). A major side product is the alkylation product 92, derived from reaction at the vinyl terminus of the vinylcarbenoid. The side reaction, however, is readily eliminated by using rhodium(II) hexanoate/hexane, and under these conditions the tropane system 91 is formed in 75% isolated yield. 

The [3+4] annulation approach to the hydroazulenes is achieved with high asymmetric induction (greater than 90% de) by using (/ )-pantolactone as a chiral auxiliary (Table 7). The nature of the catalyst has a considerable effect on the level of asymmetric induction. A sterically crowded catalyst, such as rhodium pivalate, results in much lower diastereoselectivity than rhodium(II) acetate or rho-dium(II) hexanoate. Consequently, even though the enantiomers of rhodium(II) mandelate exhibit double stereodifferentiation with the (/ )-pantolactone auxiliary (entries 5,6), both catalysts are bulky and result iinferior asynunetric induction compared to that obtained with an uncrowded achiral catalyst (entries 1-3). 

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