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Allenyl azide cycloaddition chemistry. 2,3-cyclopentennelated indole synthesis through indolidene intermediates.

ABSTRACT: The thermal, photochemical, and photochemical/CuI-mediated cascade cyclizations of a range of substituted 1-(2-azidophenyl)-3-alkenylallenes are described. These reactions provide both 1,2- and 2,3-cyclopentennelated indole products in varying ratios. In most cases, high regioselectivity for the 2,3-annelated isomer can be achieved under the hnu/CuI conditions. Computational studies of this multistep reaction support the intermediacy of indolidene intermediates whose electrocyclizations (with or without copper present) define the regioselectivity branch point in the sequence.

SUBMITTER: Feldman KS

PROVIDER: S-EPMC2794409 | biostudies-literature | 2009 Jul

REPOSITORIES: biostudies-literature

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Allenyl azide cycloaddition chemistry. 2,3-cyclopentennelated indole synthesis through indolidene intermediates.

Feldman Ken S KS Hester D Keith DK Iyer Malliga R MR Munson Paul J PJ Silva López Carlos C Faza Olalla Nieto ON

The Journal of organic chemistry 20090701 14

The thermal, photochemical, and photochemical/CuI-mediated cascade cyclizations of a range of substituted 1-(2-azidophenyl)-3-alkenylallenes are described. These reactions provide both 1,2- and 2,3-cyclopentennelated indole products in varying ratios. In most cases, high regioselectivity for the 2,3-annelated isomer can be achieved under the hnu/CuI conditions. Computational studies of this multistep reaction support the intermediacy of indolidene intermediates whose electrocyclizations (with or ...[more]

PMID: 19472993

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Project description:Catalytic enantioselective boron-hydride additions to 1,3-enynes, which afford allenyl-B(pin) (pin = pinacolato) products, are disclosed. Transformations are promoted by a readily accessible bis-phosphine-Cu complex and involve commercially available HB(pin). The method is applicable to aryl- and alkyl-substituted 1,3-enynes. Trisubstituted allenyl-B(pin) products were generated in 52-80% yield and, in most cases, in >98:2 allenyl:propargyl and 92:8-99:1 enantiomeric ratio. Utility is highlighted through a highly diastereoselective addition to an aldehyde, and a stereospecific catalytic cross-coupling process that delivers an enantiomerically enriched allene with three carbon-based substituents. The following key mechanistic attributes are elucidated: (1) Spectroscopic and computational investigations indicate that low enantioselectivity can arise from loss of kinetic stereoselectivity, which, as suggested by experimental evidence, may occur by formation of a propargylic anion generated by heterolytic Cu-C cleavage. This is particularly a problem when trapping of the Cu-allenyl intermediate is slow, namely, when an electron deficient 1,3-enyne or a less reactive boron-hydride reagent (e.g., HB(dan) (dan = naphthalene-1,8-diaminato)) is used or under non-optimal conditions (e.g., lower boron-hydride concentration causing slower trapping). (2) With enynes that contain a sterically demanding o-aryl substituent considerable amounts of the propargyl-B(pin) isomer may be generated (25-96%) because a less sterically demanding transition state for Cu/B exchange becomes favorable. (3) The phosphine ligand can promote isomerization of the enantiomerically enriched allenyl-B(pin) product; accordingly, lower ligand loading might at times be optimal. (4) Catalytic cross-coupling with an enantiomerically enriched allenyl-B(pin) compound might proceed with high stereospecificity (e.g., phosphine-Pd-catalyzed cross-coupling) or lead to considerable racemization (e.g., phosphine-Cu-catalyzed allylic substitution).

Dataset Information

Allenyl azide cycloaddition chemistry. 2,3-cyclopentennelated indole synthesis through indolidene intermediates.

Publications

Allenyl azide cycloaddition chemistry. 2,3-cyclopentennelated indole synthesis through indolidene intermediates.

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