Carbenoid reactions

An intermolecular carbenoid reaction followed by intramolecular displacement of acetate gives the clavulanic acid derivative (112) in one step from 4-acetoxyazetidin-2-one (91) (80CC1257). Carbene-induced reactions of penicillins and cephalosporins have been reviewed (75S547, 78T1731). 

Diazoalkanes can also be converted to ethers by thermal or photochemical cleavage in the presence of an alcohol. These are carbene or carbenoid reactions. Similar intermediates are involved when diazoalkanes react with alcohols in the presence of /-BuOCl to give acetals. ... 

Muller et al. have also examined the enantioselectivity and the stereochemical course of copper-catalyzed intramolecular CH insertions of phenyl-iodonium ylides [34]. The decomposition of diazo compounds in the presence of transition metals leads to typical reactions for metal-carbenoid intermediates, such as cyclopropanations, insertions into X - H bonds, and formation of ylides with heteroatoms that have available lone pairs. Since diazo compounds are potentially explosive, toxic, and carcinogenic, the number of industrial applications is limited. Phenyliodonium ylides are potential substitutes for diazo compounds in metal-carbenoid reactions. Their photochemical, thermal, and transition-metal-catalyzed decompositions exhibit some similarities to those of diazo compounds. 

In recent years, much attention has been focused on rhodium-mediated carbenoid reactions. One goal has been to understand how the rhodium ligands control reactivity and selectivity, especially in cases in which both addition and insertion reactions are possible. These catalysts contain Rh—Rh bonds but function by mechanisms similar to other transition metal catalysts. 

Fig. 10.8. Basic catalytic cycle and energy profile for rhodium-catalyzed carbenoid reactions. Reproduced from J. Am. Chem. Soc., 124, 1014 (2002), by permission of the American Chemical Society.

A sequence of reactions for conversion of acyclic and cyclic ketones into a,(3-unsaturated ketones with insertion of a =CHCH3 unit has been developed. The method uses l-lithio-l,l-dichloroethane as a key carbenoid reagent. The overall sequence involves three steps, one of them before and one after the carbenoid reaction. By analysis of the bonding changes and application of your knowledge of carbene reactions, devise a reaction sequence that would accomplish the transformation. 

Despite the volume of work concerned with metal-catalyzed decomposition of diazo compounds and carbenoid reactions 28>, relatively little work has been reported on the metal-catalyzed decomposition of sulphonyl azides. Some metal-aryl nitrene complexes have recently been isolated 29 31>. Nitro compounds have also been reduced to nitrene metal complexes with transition metal oxalates 32K... 

Diverging results have been reported for the carbenoid reaction between alkyl diazoacetates and silyl enol ethers 49a-c. Whereas Reissig and coworkers 60) observed successful cyclopropanation with methyl diazoacetate/Cu(acac)2, Le Goaller and Pierre, in a note without experimental details u8), reported the isolation of 4-oxo-carboxylic esters for the copper-catalyzed decomposition of ethyl diazoacetate. According to this communication, both cyclopropane and ring-opened y-keto ester are obtained from 49 c but the cyclopropane suffers ring-opening under the reaction conditions. 

Allylic C/H insertion accompanied by an allylic rearrangement has been observed for carbenoid reactions of ethyl diazoacetate with allylamines (Scheme 23)1S1). Apparently, metal-catalyzed isomerization 117 118 proceeds the C/H insertion process. Although mechanistic details have not yet been unraveled, T)3-allyl complexes... 

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. 

Few efforts have been devoted to carbenoid reactions with C=N and C=0 groups since the last two review 14,260) covering this field were written. 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

The electrophilic reactivity of lithium carbenoids (reaction b) becomes evident from their reaction with alkyl lithium compounds. A, probably metal-supported, nucleophilic substitution occurs, and the leaving group X is replaced by the alkyl group R with inversion of the configuration . This reaction, typical of metal carbenoids, is not restricted to the vinylidene substitution pattern, but occurs in alkyl and cycloalkyl lithium carbenoids as well ". With respect to the a-heteroatom X, the carbenoid character is... 

SCHEME 11. Electrophilic character of cyclopropyl carbenoids Reaction of 2,2-diphenylcyclo-propane with -butylhthium (2 equivalents)... 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

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