Hydrocarboxylation of allene

The third reaction was related to the hydrocarboxylation of allenes with C02, catalyzed by a tridendate silyl pincer-type palladium complex [108]. In this reaction, a a-allyl palladium species (via hydropallation of allene) was formed, and its trigonal bipyramidal geometry allowed the facile coordination of C02 (presumably in a side-on fashion) and a following nucleophilic addition to realize the carboxylation of allene (Figure 4.16). This reaction proved to be very attractive not only as a C02 fixation reaction, but also as a general method for the synthesis of Py-unsaturated carboxylic acids. 

In the case of Au(I)-catalysed asymmetric hydrocarboxylation of allenes a high ee of 82% was observed when (5)-BINAP(AuCl)2 was combined with the counter ion of opposite chirality, that is /f-3.60. In the presence of the 5 -3.60 counter ion the reaction produced nearly racemic product. These examples clearly illustrate the powerful effect of the chiral counter ion on the enantioselectivity. 

Scheme 5.18 Hydrocarboxylation of allenes with CO2 catalyzed by silyl pincer-type palladium complex...

The q -(Si-H)Pd(0) 43 was found to catalyze the hydrocarboxylation reaction of allenes, indicating that 43 worked as the palladium hydride complex 13 in solution via reversible oxidative addition/reductive elimination of the Si-H bond after dissociation of PPhj (Scheme 9.12) [21]. This result prompted us to investigate a... 

Toste and coworkers reported the first asymmetric intramolecular hydrocarboxylation of an allene by combining a chiral ligand with a chiral counterion on a Au(I) catalyst (Scheme 2.27). Chiral ion pair formation was crucial to the high selectivity [69[. In another account, a related carboxyallene species was proposed to be an intermediate in a Pd-catalyzed cyclization of alkynoic adds [70]. 

Table 37. Allenic acids by hydrocarboxylation of oL-halogenafed alkynes...

The selective hydrocarboxylation of one double bond in e.g. cyclo-octadiene-1,5 or cyclododecatriene-1,5,9, succeeds with the highly reactive complex Pd-catalysts whereas the other double bonds in the molecule are unchanged [469]. In the presence of Ru-catalysts allene reacts to give metha-crylic acid [497, 498]. 

For alkynes (and in part, allenes), synthetically useful protocols for Markovnikov and anti-Markovnikov selective hydrations, hydroalkoxylations (mainly intramolecular), and hydrocarboxylations are available and find increasing applications in organic synthesis. In the past decade, the research focus on cationic gold(l) complexes has led to new additions to the catalysis toolbox. It can be predicted that a further refining of such tools for alkyne functionalization with respect to catalytic activity and functional group tolerance will take place. 

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