Carbenoids cobalt catalysts

Cyclopropanation of C=C bonds by carbenoids derived from diazoesters usually occurs stereospeciflcally with respect to the configuration of the olefin. This has been confirmed for cyclopropanation with copper 2S,S7,60 85), palladium 86), and rhodium catalysts S9,87>. However, cyclopropanation of c -D2-styrene with ethyl diazoacetate in the presence of a (l,2-dioximato)cobalt(II) complex occurs with considerable geometrical isomerization88). Furthermore, CuCl-catalyzed cyclopropanation of cis-2-butene with co-diazoacetophenone gives a mixture of the cis- and trans-1,2-dimethylcyclopropanes 89). 

Enantioselective carbenoid cyclopropanation can be expected to occur when either an olefin bearing a chiral substituent, or such a diazo compound or a chiral catalyst is present. Only the latter alternative has been widely applied in practice. All efficient chiral catalysts which are known at present are copper or cobalt(II) chelates, whereas palladium complexes 86) proved to be uneflective. The carbenoid reactions between alkyl diazoacetates and styrene or 1,1 -diphenylethylene (Scheme 27) are usually chosen to test the efficiency of a chiral catalyst. As will be seen in the following, the extent to which optical induction is brought about by enantioselection either at a prochiral olefin or at a prochiral carbenoid center, varies widely with the chiral catalyst used. 

The rhodium(II) catalysts and the chelated copper catalysts are considered to coordinate only to the carbenoid, while copper triflate and tetrafluoioborate coordinate to both the carbenoid and alkene and thus enhance cyclopropanation reactions through a template effect.14 Palladium-based catalysts, such as palladium(II) acetate and bis(benzonitrile)palladium(II) chloride,l6e are also believed to be able to coordinate with the alkene. Some chiral complexes based on cobalt have also been developed,21 but these have not been extensively used. 

The diastereoselectivity of the cydopropanation reaction of alkenes with diazoacetates has two facets. The configuration of the alkene double bond is normally fully retained ( stereospecific cydopropanation ), but may be lost partially when a bis(camphorquinonedioxima-to)cobalt(II) catalyst is employed. As far as the stereochemical relationship between the alkene substituents and the carbenoid building block is concerned, the sterically less encumbered diastereomer is usually formed preferentially, but the diastereomeric excess is normally not very impressive (for examples, see Tables 7 and 9). Within certain limits, the diastereomeric ratio depends on the catalyst as well as on the nature of the diazoacetic ester and the alkene s substituents. The thermodynamically more stable trans- or anti-) diastereomer is increasingly favored with increasing steric bulk of the substituents at the C-C double bond (e.g. 1-R-sub-stituted-l-trimethylsiloxyethene ) and of the ester residuc, furthermore in the following sequence of the metal catalyst Pd < Rh, Ru < In contrast, comparisons of... 

Interestingly, cobalt porphyrin catalysts tend to prevent carbene dimerization reactions, and allow cyclopropanation reactions with electron-deficient alkenes. This feature illustrates the more nucleophilic behavior of the carbenoid species formed as compared to typical electrophilic Fischer carbenes. The enhanced nucleophilic character of the carbene reduces its tendency to dimerize and allows reactions with more electron-deficient olefins. 

Uemura and Katsuki have independently reported the enantioselective [2,3] -sigmatropic rearrangement of allylic sulfur ylides using a catalytic amount of chiral rhodium(II) and cobalt(III) complexes, respectively. The carbenoids derived from diazoacetate were enantioselectively added to the sulfur atom of cin-namyl phenyl sulfide to afford the product with low to good enantioselectivity [53,54] [Eq. (11)]. McMillen and coworkers have also reported a similar enantioselective [2,3] -sigmatropic rearrangement of allylic sulfur ylides using a Cu(I)-bis(oxazoline) catalyst [55]. 

C-H alkylation and amination reactions involving metal-carbenoid and metal-nitrenoid species have been developed for many years, most extensively with (chiral) dirhodium(ll) carboxylate and carboxamidate complexes as catalysts [45]. When performed in intramolecular settings, such reactions offer versatile methods for the (enantioselective) synthesis of hetero- and carbocy-cles. In the past decade, Zhang and coworkers had explored the catalysis of cobalt(II)-porphyrin complexes for carbene- and nitrene-transfer reactions [46] and revealed a radical nature of such processes as a distinct mechanistic feature compared with typical metal (e.g., rhodium)-catalyzed carbenoid and nitrenoid reactions [47]. Described below are examples of heterocycle synthesis via cobalt(II)-porphyrin-catalyzed intramolecular C-H amination or C-H alkylation. 

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