Palladium(II) carboxylates and palladium(I) carbonyl carboxylate complexesas catalysts for olefin cyclopropanation with ethyl diazoacetate

摘要

Palladium(I) carbonyl carboxylate complexes [Pd(μ-CO)(μ-RCO_2)]_n (R = Me,n =4; R = CMe,,n =6)and the corresponding palladium(II) carboxylates (acetate and pivalate) catalyze the cyclopropanationof olefins with ethyl diazoacetate. The performance of these catalysts is similar in terms of selectivityand cyclopropane yields, regardless of the oxidation state of the metal center. However the rates of thecyclopropanation reactions are significantly higher for the acetate based catalysts than for the pivalatederivatives, which suggests that the main catalytic species are carboxylate containing palladiumcomplexes. Kinetic measurements show that reaction rates are independent of the olefin concentrationwhen these are 1-hexene or styrene, but norbornene exerts an inhibitory effect. In spite of this,competition experiments indicate that the cyclopropanation of styrene is 2.2 times as favorable as thatof 1-hexene for any of the four catalysts. These observations indicate that while the rate-determiningformation of the intermediate palladium carbenoid species is controlled by the catalyst structure, this isfollowed by a rapid and less specific cyclopropanation step that is not affected by the nature of thecarboxylate groups present in the catalyst. An independent test using a 1:1 benzene/cyclohexanemixture of solvents showed that the transfer of ethoxycarbonylcarbene (:C(CO,E0H) to thesemolecules is unselective (relative rate of benzene/cyclohexane functionalization –1.8, independent ofthe catalyst). This result can be interpreted as an indication of the involvement of freeethoxycarbonylcarbene in the carbene transfer step.