Topic
Annulation
About: Annulation is a research topic. Over the lifetime, 10152 publications have been published within this topic receiving 189701 citations.
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TL;DR: The corresponding triarylphosphine-catalyzed annulations of α-methylallenoate with arylidenemalononitriles manifested a polarity inversion of the 1,4-dipole synthon, providing 4, 4-dicyano-5-substituted cyclohex-1-enecarboxylates in excellent yields.
Abstract: Phosphine-catalyzed [4 + 2] annulations of α-alkylallenoates with activated olefins allow the efficient syntheses of cyclohexenes. Hexamethylphosphorous triamide (HMPT)-catalyzed [4 + 2] annulations of α-alkylallenoates with arylidenemalononitriles provided highly functionalized 5,5-dicyano-4,6-disubstituted cyclohex-1-enecarboxylates in excellent yields (77−98%) and moderate to high diastereoselectivities (1:2−12:1). Remarkably, the corresponding triarylphosphine-catalyzed [4 + 2] annulations of α-methylallenoate with arylidenemalononitriles manifested a polarity inversion of the 1,4-dipole synthon, providing 4,4-dicyano-5-substituted cyclohex-1-enecarboxylates in excellent yields (80−93%). The polarity inversion of α-alkylallenoates from one 1,4-dipole to another under phosphine catalysis presumably resulted from a change in the balance of the equilibrium between the phosphonium dienolate and the vinylogous phosphonium ylide intermediate.
291 citations
TL;DR: Mechanistic studies revealed the five-membered manganacycle and manganese hydride species as key reaction intermediates in the catalytic cycle, and the redox neutral reaction produces H2 as the major byproduct and eliminates the need for any oxidants, external ligands, or additives.
Abstract: Described herein is a manganese-catalyzed dehydrogenative [4+2] annulation of NH imines and alkynes, a reaction providing highly atom-economical access to diverse isoquinolines. This transformation represents the first example of manganese-catalyzed CH activation of imines; the stoichiometric variant of the cyclomanganation was reported in 1971. The redox neutral reaction produces H2 as the major byproduct and eliminates the need for any oxidants, external ligands, or additives, thus standing out from known isoquinoline synthesis by transition-metal-catalyzed CH activation. Mechanistic studies revealed the five-membered manganacycle and manganese hydride species as key reaction intermediates in the catalytic cycle.
288 citations
TL;DR: Investigations demonstrate that the counterion of the azolium salt plays a key role in the formation of the catalytically active species in the synthesis of enantioenriched kojic acid derivatives.
Abstract: In the presence of a chiral azolium salt (10 mol %), enols and ynals undergo a highly enantioselective annulation reaction to form enantiomerically enriched dihydropyranones via an N-heterocyclic carbene catalyzed variant of the Claisen rearrangement. Unlike other azolium-catalyzed reactions, this process requires no added base to generate the putative NHC-catalyst, and our investigations demonstrate that the counterion of the azolium salt plays a key role in the formation of the catalytically active species. Detailed kinetic studies eliminate a potential 1,4-addition as the mechanistic pathway; the observed rate law and activation parameters are consistent with a Claisen rearrangement as the rate-limiting step. This catalytic system was applied to the synthesis of enantioenriched kojic acid derivatives, a reaction of demonstrated synthetic utility for which other methods for catalytic enantioselective Claisen rearrangements have not provided a satisfactory solution.
280 citations
TL;DR: Fischer-type carbene complexes have received increasing interest as selective reagents in organic synthesis Apart from its electron-acceptor properties exploited in carbene ligand centered reactions, the metal carbonyl fragment provides a template for non-classical cycloaddition reactions as discussed by the authors.
Abstract: Over the past three decades Fischer-type carbene complexes have received increasing interest as selective reagents in organic synthesis Apart from its electron-acceptor properties exploited in carbene ligand centered reactions, the metal carbonyl fragment provides a template for non-classical cycloaddition reactions The most useful among them is the chromium-mediated benzannulation which allows a one-pot access to densely functionalized oxygenated arenes coordinated to a Cr(CO)3 fragment It is compatible with a variety of functional groups, occurs under mild conditions with remarkable chemo-, regio- and diastereoselectivity, and thus has considerable potential in the synthesis of complex targets and natural products
278 citations
TL;DR: This novel reaction provides a facile route to synthesize indenones from readily available benzimides and internal alkynes.
Abstract: Doubled up: a rhodium(III)/copper(II) system co-catalyzes the annulation of benzimides with internal alkynes for the synthesis of indenones (see scheme; Cp*=C(5) Me(5)). The reaction involves an uncommon nucleophilic addition of a transition-metal-carbon bond to an imide moiety. This novel reaction provides a facile route to synthesize indenones from readily available benzimides and internal alkynes.
278 citations