Carbenoids Vinylcarbenoids

Section 14.2 describes the highly stereoselective cyclopropanation chemistry of the donor/acceptor-carbenoids (Fig. 14.1a) [16]. This section introduces the range of vinyl, aryl, alkynyl, and heteroaryl functionalities that have been used as donor groups in this chemistry. Also, chiral auxiliaries and chiral catalysts that achieve high asymmetric induction in this chemistry are described [25]. The next two sections cover chemistry that is unique to the vinylcarbenoid system, namely [3-t4] cycloaddition with dienes (Fig. 14.1b see also Section 14.3) [13] and [3-1-2] cycloaddition with vinyl... 

A third mechanistically distinct [3 -1- 2] cycloaddition between vinyl ethers and vinyl-carbenoids was discovered and reported in 2001 [26]. This reaction is remarkable because when Rh2(S-DOSP)4 is used as the catalyst, the cis-cyclopentenes 142 are formed in up to 99% enantiomeric excess. The reaction occurs between vinylcarbenoids unsubstituted or alkyl-substituted at the vinyl terminus and vinyl ethers substituted with an aryl or vinyl group. Some illustrative examples are shown in Tab. 14.12. The reaction is considered to be a concerted process, which would be consistent with the highly stereoselective nature of the reaction [26]. Contrary to the [3-1-2] cycloaddition derived by means of vinylogous carbenoid reactivity, this latest [3 -1- 2] cycloaddition is not influenced by solvent effects. Due to steric demands on the carbenoid, the [3-1-2] cycloaddi-tion only occurs with cis-vinyl ethers. 

In addition to insertions into polar X-H bonds by means of ylide intermediates, carbe-noids are capable of inserting into nonpolar bonds such as Si-H and C-H. The Si-H insertion by vinylcarbenoids has been developed as a novel method for the synthesis of allylsilanes 166 and 167 of defined geometry as illustrated in Eqs. (17) and (18) [28]. The alkene geometry of the vinyldiazoacetate is not altered during carbenoid formation or the subsequent Si-H insertion. 

Recently, cyclopropane derivatives were produced by a ruthenium-catalyzed cyclopropanation of alkenes using propargylic carboxylates as precursors of vinylcarbenoids [51] (Eq. 38). The key intermediate of this reaction is a vinylcarbene complex generated by nucleophilic attack of the carboxylate to an internal carbon of alkyne activated by the ruthenium complex. Then, a [2+1] cycloaddition between alkenes and carbenoid species affords vinylcyclo-propanes. 

An alternative strategy for achieving asymmetric control may be by covalent attachment of a chiral auxiliary to the carbenoid. This strategy has so far met with rather limited success in cyclopropanation reactions (see eq. (3) for a similar palladium-catalyzed reaction). However, the use of a-hydroxy esters as chiral auxiliaries with stabilized rhodium(II) vinylcarbenoids allowed entry into both series of enantiomeric vinylcyclopropanes with predictable stereochemistry. Optical yields are fair to excellent [14] and the outcome of the reaction was rationalized on the basis of interactions between the carbonyl oxygen of the chiral auxiliary and the carbenoid carbon. The strategy led to an efficient synthesis of optically active hydroxy vitamin D3 ring A [28]. 

The first examples of C-H insertion with vinylcarbenoids were reported by Davies and coworkers in 1997 and 1998 [79, 80], In these studies, it was found that vinylcarbenoids reacted with cyclohexane in higher yield and with much higher enantioselectivity than carbenoids derived from EDA or acceptor/acceptor diazo compounds. However, in the absence of suitable traps, they also reacted with themselves to form polycyclic products. 

The tandem cyclopropanation/Cope rearrangement sequence between metal-stabilized vinylcarbenoids and dienes is more widely applicable, since this method allows cyclopropanes with a wide range of substitution patterns to be synthesized, and the carbenoids are readily prepared by heating a vinyldiazomethane in the presence of a metal salt829,... 

Reaction with benzene follows a similar pathway, yielding a bicyclo[3.2.2]nonatriene structure. Vinylcarbenoids also react with pyrroles to give tropanes via a cyclopropanation-Cope rearrangement route. The direct addition of carbenes to acetylenes does not give satisfactory yields of cyclopropanes, but the rhodium carboxylate catalysed reaction of diazo compounds with acetylenes is a useful source of cyclopro-panes. Carbenoids can also attack a carbonyl oxygen atom, giving rise to a zwitterion (249). An excellent review of intramolecular carbenoid reactions has appeared. ... 

De Meijere and co-workers have reported on the reactions of the parent vinyldiazomethane and chlorinated vinyldiazomethanes with oxygenated dienes. Rhodium(II) acetate catalyzed decomposition of these vinyldiazomethanes in the presence of 2-siloxydienes proceeds in good overall yields (Scheme 15). Because the vinylcarbenoid does not contain the donor/acceptor combination of the carbenoids derived from vinyldiazoacetates, the stereoselectivity of the cyclopropanation is poor and mixtures of the [344] annulation products and the trans-divinylcyclopropanes are formed. 

Carbenoid Reactions. Other synthetically useful reactions include carbenoid addition to form 2-oxabicyclo[3.1.0]hex-3-ene systems. 1 The reaction of vinylcarbenoids with furan provides two products (eq 22). These results may be rationalized invoking a nons3tnchronous cyclopropanation mechanism that leads to two possible dipolar transition states. - ... 

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