Carbamates allylic substitutions

To acliieve diastereoselectivity in tlie course of allylic substitution, tlie cnnitoliing cliital inforniation may not only reside in tlie substtate skeleton but may also be pan of tlie allylic leaving group. Tlius, a cliital carbamate bas been developed as a... 

Scheme 6.32. Different stereochemical results with mesylate (156) and carbamate (158) leaving groups upon allylic substitution with organocuprates.

Scheme 6.33. Different stereochemical and regiochemical results with acetate ( 157) and carbamate ( 162) leaving groups on allylic substitution of 161 A/ith a higher order methylcuprate.

Scheme 6.3S. Interpretation of the chirality transfer during the course of allylic substitution of acyclic carbamate derivative (R)-163.

Scheme 6.36. Regioselective allylic substitution of Z carbamate 170 with a silylcuprate reagent.

Scheme 6.38. Influence of reagent and alkene geomet7 on allylic substitution of y-silyl-substituted allylic carbamates 181 (Ts = para-toluenesulfonyl, NMP = N-methylpyrrolidinone).

Scheme 6.43. Control of allylic substitution stereochemistry with the aid of a chiral carbamate leaving group.

Allylic substitution with free carbamate nucleophiles was not accomplished until advanced catalyst precursors were developed. However, products from substitution with carbamate nucleophiles were generated by a decarboxylative allylation. In this process, the imidodicarbonate was shown to undergo decarboxylation to form the carbamate nucleophile, and reaction of the resulting carbamate with the 7i-allyliridium intermediate formed branched allylic carbamate products (Scheme 17) [92, 95]. 

In contrast, reactions catalyzed by la were typically conducted with added [Ir (C0D)C1]2 to trap the K -phosphoramidite ligand after dissociation, and thereby, to leave the unsaturated active catalyst. Under these conductions, as much as half of the iridium in the system is present in an inactive acyclic species. In contrast, when ethylene adduct lb is used as the catalyst, all of the iridium belongs to the active metalacyclic species. Hartwig and coworkers have recently taken advantage of the increased availability of the active catalyst generated from lb to develop new allylic substitution reactions. These new processes include the reactions of carbamates, nitrogen heterocycles, and ammonia. 

Scheme 25 Allylic substitution with carbamate nucleophiles...

A wide application of Newcomb s method provides a variety of N-heterocyc-lic systems, such as perhydroindoles, pyrrolizidines and aza-brigded bicycles [59, W, 146], The mild reaction conditions are compatible with several funtional groups of the substrate and several trapping agents to functionalize the cyclized product. 2-Substituted pyrrolizidines 132 are accessible by tandem cyclization of iV-allyl-substituted PTOC carbamate 131. In this case the allyl group on the nitrogen serves as an internal trap for the intermediate carbon radical. The Af-methylcyclohept-4-enaminium radical cation, produced from the corres-... 

Table 9 Direct catalytic allylic substitution of alcohol 6a with various sulfonamides, carbamates, and carboxamides 2...

The regioselective and enantiospecific allylic substitution of alkyl-substituted allyl benzoates and carbamates with (Me2PhSi)2Zn and Cul has been shown to occur by an oxidative addition - reduction elimination mechanism rather than an SN2 mechanism.16... 

The efficient catalytic cyclization (aminocarbonylation) of A-(3-hydroxy-4-pentenyl)amides and carbamates in acetic acid gave m-fused bicyclic pyrrolidine lactone compounds54,56 (Table 2), in agreement with the observed ra-directing capability of the hydroxy group in analogous electrophile mediated additions (Section 7.2.6). In tetrahydrofuran the reaction rate is unacceptably low. In methanol a competitive allylic substitution leads to 1,2,5,6-tetrahydropyridines. Furthermore, lower yields were obtained in the cyclization of the corresponding ureas. 

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