Carbenoids palladium 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. 

Transition metal catalysts that are effective for carbenoid transformations include those of copper , palladium(II) or platinum(II), eobalt(II), and rhodium(II) (7-3, 6-3), but only copper and rhodium catalysts have been routinely employed. 

Abstract This review gives an insight into the growing field of transition metal-catalyzed cascades. More particularly, we have focused on the construction of complex molecules from acyclic precursors. Several approaches have been devised. We have not covered palladium-mediated cyclizations, multiple Heck reactions, or ruthenium-catalyzed metathesis reactions because they are discussed in others chapters of this book. This manuscript is composed of two main parts. In the first part, we emphasize cascade sequences involving cycloaddition, cycloisomerization, or ene-type reactions. Most of these reaction sequences involve a transition metal-catalyzed step that is either followed by another reaction promoted by the same catalyst or by a purely thermal reaction. A simple change in the temperature of the reaction mixture is often the only technical requirement to go from one step to another. The second part covers the cascades relying on transition metalo carbenoid intermediates, which have recently undergone tremendous... 

In contrast to the wealth of chemistry reported for catalyzed reactions of diazocarbonyl compounds, there are fewer applications of diazomethane as a carbenoid precursor. Catalytic decomposition of diazomethane, CH2N2, has been reported as a general method for the methylenation of chemical compounds [12]. The efficacy of rhodium catalysts for mediating carbene transfer from diazoalkanes is poor. The preparative use of diazomethane in the synthesis of cyclopropane derivatives from olefins is mostly associated with the employment of palladium cat-... 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

The existence of a palladium carbenoid has been shown by trapping the vinyl species 4 with an alkene. It is believed that treatment of an -acetylenic compound 3 with potassium tert-butoxide and a palladium(O) catalyst generated the carbenoid 4, which can either react intra-molecularly, yielding the cyclopropane 5 as only a minor component of the product mixture, or intermolecularly with norbornadiene to form two isomeric tricycles 8 and 9 as major products. 

The results in the diazomethane reactions involving zinc(II) chloride catalysis have been explained by invoking a carbenoid intermediate. The properties of such a species will, of course, be sensitive to the nature of the metal and this might explain the different regioselectivity observed when diphenyldiazomethane is decomposed with rhodium and palladium salts in the presence of 5-methylenebicyclo[2.2.1]hept-2-ene (9). With rhodium(II) acetate as catalyst the exocyclic double bond is attacked exclusively, whereas palladium(II) chloride catalysis directs cyclopropanation to the endocyclic double bond. ... 

Kumada coupling. A report of biaryl synthesis from ArMgBr and Ar Cl highlights the use of a Ni carbenoid (1). Both bis(Ti -allyl)nickel and palladium complexes are also useful catalysts for the cross-coupling." ... 

Beside carbenes and carbenoids, metal-catalyzed cyclopropanation using other types of intermediate is also feasible. Treatment of 5-methyl-5-phenyl-4-methylene-l,3-dioxolan-2-one with palladium(O) catalyst in the presence of norbomene produced the cyclopropyl ketone in excellent yield (Scheme 8) (60). The active intermediate was proposed to be a Zwitterion... 

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