Cycloaddition with carbonyl ylids

The synthetic applications of 1,3-dipolar cycloaddition of carbonyl ylids generated from diazo ketones have been reviewed with particular focus on pseudolaric acids.12... 

The final class of dipoles discussed here are the carbonyl ylids. Padwa et al. generated these reactive intermediates from a rhodium (Il)-catalyzed reaction with l-diazopentanediones.388 a synthetic example of this reaction is taken from Padwa s synthesis of ribasine,389 where diazo compound 497 was treated with Rh2(tfa)4 [tfa = trifluoroacetic acid ligand] to give carbonyl ylid 498. In the presence of dimethylacetylene dicarboxylate (DMAD), cycloadduct 499 was isolated in 82% yield. Interestingly, formation of the carbonyl ylid and cycloaddition was very dependent on the rhodium catalyst used, and when Rh2(OAc)4 was used, 482 was not produced. 

E) alkenes. One explanation for this is that the reaction of the ylid with the carbonyl compound is a 2-1-2 cycloaddition, which in order to be concerted must adopt the [rt2s+n2al pathway. As we have seen earlier (p. 1079), this pathway leads to the formation of the more sterically crowded product, in this case the (Z) alkene. If this explanation is correct, it is not easy to explain the predominant formation of ( ) products from stable ylids, but (E) compounds are of course generally thermodynamically more stable than the (Z) isomers, and the stereochemistry seems to depend on many factors. 

Fig. 2.3 shows the core structures of the most important 1,3-dipoles, and what they are all called. As with dienes, they can have electron-donating or withdrawing substituents attached at any of the atoms with a hydrogen atom in the core structure, and these modify the reactivity and selectivity that the dipoles show for different dipolarophiles. Some of the dipoles are stable compounds like ozone and diazomethane, or, suitably substituted, like azides, nitrones, and nitrile oxides. Others, like the ylids, imines, and carbonyl oxides, are reactive intermediates that have to be made in situ. Fig. 2.4 shows some examples of some common 1,3-dipolar cycloadditions, and Fig. 2.5 illustrates two of the many ways in which unstable dipoles can be prepared. 

The carbenes may also be trapped by nucleophilic groups that lack a hydrogen atom. The use of carbonyl groups, in an intramolecular fashion, in this way generates unstable, reactive oxonium ylids (Scheme 8.138). These ylids 8.511 are 1,3-dipoles and can participate in a cycloaddition reaction with an added dipolarophile. Many tandem processes can be designed around this concept. ... 

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