A Catalytic Carbon–Phosphorus Ylide Reaction: Phosphane‐Catalyzed Annulation of Allylic Compounds with Electron‐Deficient Alkenes

Abstract

To ensure high efficiency, turning a stoichiometric reaction into a catalytic one, and developing new catalytic reactions have been long-standing goals in modern synthesis. Ylide reactions are among the most powerful tools for constructing olefins [1] and small rings, [2] but few are catalytic, in particular those involving phosphorus ylides. [3] The first example of a catalytic Wittig-type reaction of arsenic ylides, in which a stoichiometric amount of reducing agent was used to regenerate the arsane, was described by Huang's group in 1989. [4] Afterwards, catalytic olefinations, epoxidations, aziridinations, and cyclopropanations of tellurium or sulfur ylides were accomplished. [2,5] Aggarwal's group developed catalytic, asymmetric ylide reactions that afforded three-membered rings by a carbene approach, in which the ylide was formed directly from aryl diazomethane in the presence of a catalytic amount of a chiral sulfide and a transition-metal complex. [6] To our knowledge, there are no phosphane-catalyzed versions of the carbon-phosphorus ylide reaction. [7] This might be due to the fact that, compared to other elemento-organic com-pounds of Groups 15 and 16, phosphanes exhibit higher oxyphilicity and the phosphorus-oxygen bond in phosphine oxides have a higher bond energy. To develop a catalytic reaction with carbon-phosphorus ylides, it was thus necessary to explore a new type of reaction.Two types of reactions of allylic phosphorus ylides A with electron-deficient alkenes have been reported (Scheme 1). The initial nucleophilic addition of the ylide to the olefin is followed by an intramolecular Wittig reaction to afford 1,3cycloalkadienes [8a-f] or by intramolecular nucleophilic substitution to give cyclopropanes. [8f,g] However, both reactions are stoichiometric with regard to the phosphane and the products are obtained in low yields.The success of the phosphane-catalyzed isomerizations, aand g-additions, and [3þ2] cycloadditions of electron-deficient allenes or alkynes was ascribed to the presence of electron-withdrawing groups, which facilitated the nucleophilic additions and the elimination of phosphanes to complete the catalytic cycles. [9] With this in mind, we thought that phosphane-catalyzed reactions might be realized with a modified allylic phosphorus ylide B (see Scheme 1). Here we report a novel phosphane-catalyzed annulation reaction of modified allylic compounds with electron-deficient alkenes.To initiate our study, phosphonium salt 1, compound 2, and potassium carbonate were stirred in toluene at 90 8C to afford the adduct 3 a and triphenylphosphane [Eq. (1); E = CO 2 Et]. Since 1 was prepared from ethyl 2-bromomethyl-2propenoate (4 a) and PPh 3 , it was expected that this reaction might take place starting from 4 a by utilizing a catalytic amount of PPh 3 . Indeed, the phosphane-catalyzed annulation Scheme 1. Two types of reactions of allylic phosphorus ylides.

Lewis Base Catalysis in Organic Synthesis

“…The same zwitterionic intermediates discussed in the preceding section undergo formal [3+2] cycloadditions with imines, [133] a,b-unsaturated ketones, [134] a,b-unsaturated nitriles, [135] and thioamides [130] (Scheme 32). These intermediates can be accessed either through reaction of an alkynoate or allenoate.…”

Lewis‐Base‐Katalyse in der organischen Synthese

“…Die im vorherigen Abschnitt besprochenen zwitterionischen Zwischenstufen, die aus Alkinoaten oder Allenoaten erhalten werden können, gehen mit Iminen, [133] a,b-ungesättigten Ketonen, [134] a,b-ungesättigten Nitrilen [135] und Thioamiden [130] formale [3+2]-Cycloadditionen ein (Schema 32). Zhang und Fu haben mit chiralen Phosphanen hoch regio-und enantioselektive Cycloadditionen induziert.…”

Baylis–Hillman Bromides as a Source of 1,3‐Dipoles: Sterically Directed Synthesis of Oxindole‐Fused Spirooxirane and Spirodihydrofuran Frameworks

“…4 These Baylis–Hillman adducts and their derivatives have also been successfully employed as dipolarophiles3c,h (with benzonitrile oxide,5a azomethine ylides,5b,c etc. as dipoles) in [3+2] cycloaddition reactions and also as a source for generating dipoles (with methyleneindolinones,4c isatylidene malononitriles,4d N ‐phenylmaleimide,4a, 6a enones,4g diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD),6b propargyl sulfones,4e etc. as dipolarophiles), thereby producing a variety of heterocyclic and carbocylic compounds of medicinal importance 3c,d…”