An organocatalytic Cloke-Wilson rearrangement of cyclopropyl ketones to 2,3-dihydrofurans is exploited utilizing the homoconjugate addition process. With 1,4-diazabicyclo[2.2.2]octane as the catalyst, the rearrangement in DMSO at 120 °C proceeded in generally high yields, exclusive regioselectivity, and a broad substrate scope. An examination of the mechanism including stereochemical analysis and intermediate isolation supports an S1-type ring opening of the mechanism.

Ylide is the key: The phosphane‐catalyzed [3+2] annulation of 2 a with aromatic aldehydes 1 to form tetrahydrofurans 3 in moderate to excellent yields is described (see scheme). This reaction represents an efficient and convergent one‐step route toward the construction of tetrahydrofurans and demonstrates a distinctive reactivity pattern of γ‐substituted allenoates with aldehydes.

Silong Xu a and Zhengjie He* b Organic synthetic reactions mediated by tertiary phosphines have attracted much attention in the organic chemistry community in the past two decades. These reactions can be divided into two categories: phosphine-catalyzed and stoichiometric phosphine-mediated transformations. While the phosphinecatalyzed reactions mechanistically rely on the unique properties of tertiary phosphines such as excellent nucleophilicity and good leaving group ability, the stoichiometric transformations are usually driven by nucleophilicity and strong oxyphilicity of tertiary phosphines. Since tertiary phosphines represent an important class of versatile chemical reagents in organic synthesis, stoichiometric phosphine-mediated reactions have recently demonstrated their uniqueness and high efficiency in organic synthesis, particularly with respect to the construction of carbon-carbon and carbon-heteroatom bonds, and therefore have stimulated much research interest. In this review, recent advances in stoichiometric phosphine-mediated reactions primarily including olefinations and annulations are summarized.

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We report ap hosphine-catalyzed ring opening of electron-deficient alkylidenecyclopropanes (ACPs) to generate allylic phosphonium zwitterions that resemble the well-studied phosphine-allene adducts but exhibit distinct properties.T he potent reactivity of these intermediates has been demonstrated in three types of substrate-controlled phosphine-catalyzed rearrangements of alkylidenecyclopropylketones,w hichc hemoselectively affordt ri-and tetrasubstituted furans,a nd trisubstituted dienones in good yields.Alkylidenecyclopropanes (ACPs) [1] are at ype of extremely versatile yet readily available [2] building blocks in organic synthesis.B yv irtue of significant ring strain, [3] ap lethora of ring-opening reactions of ACPs have been developed for the construction of various molecular architectures. [4] To date,the reactivity of ACPs has been predominantly unlocked by transition-metal catalysis, [1d,p] Lewis acid catalysis, [1b,i,n] radical-induced reactions, [5] and thermally induced cyclizations. [6] However,t othe best of our knowledge,t he organocatalytic activation of ACPs has not been reported.Phosphine catalysis [7] has emerged as afertile land for new reaction discovery.I np articular, electron-deficient allenes 1 exhibit remarkable reactivity in phosphine-catalyzed annulation reactions through apivotal zwitterionic intermediate A (Scheme 1). [7d,e,g,j] [3+ +2], [8] [4+ +2], [9] and [4+ +1] [10] annulations from the Lu, Kwon, and Tong groups,respectively,represent seminal advances in this area and have inspired am yriad of annulations, [11] including asymmetric variants. [12] In an effort to expand the field of phosphine catalysis,w es urmised that electron-deficient alkylidenecyclopropanes (ACPs) 2 might be av iable substrate for nucleophilic phosphine catalysis, leading to an intriguing homologue intermediate B with unexplored reactivity and potential for new reaction discovery (Scheme 1). In continuation of our research in Lewis base catalysis, [13] we report herein the phosphine-catalyzed ringopening of electron-deficient ACPs in three types of substrate-controlled rearrangements of alkylidenecyclopropylketones,w hich chemoselectively afford tri-and tetrasubstituted furans,and trisubstituted dienones in good yields.Inspired by the elegant palladium-catalyzed rearrangement reactions of alkylidenecyclopropylketones reported by Ma and co-workers, [14] we initially examined the ring-opening reactivity of alkylidenecyclopropylketone 2-1 a under aset of phosphine-catalyzed conditions (Table 1). Gratifyingly,w ith acatalytic amount of PPh 3 (20 mol %) in DMSO at 120 8 8C, 2-1a underwent af acile rearrangement to afford trisubstituted furan 3a in 78 %yield (Entry 1). [15] Arange of alkyl and aryl phosphines were examined (Entries 2-8), in which P(4-MeOC 6 H 4 ) 3 outperformed all others tested, providing 3a in 81 %yield. Anucleophilic amine DABCO also promoted the ring opening of 2-1 a but generated 3-alkylidene-2,3-dihydrofuran 3a' ' [14b] in 54 %yield (Entry 9). Among several common solvents tested (Entrie...

Efficient construction of quaternary carbon centers represents one of the most challenging goals in organic synthesis, and it has drawn tremendous research interest from organic chemist community. The past decade has witnessed a significant advancement of direct C-H functionalization, which delivers various synthetic tools for assembling structurally diversified architectures with the advantage of step- and atom-economy, operational simplicity, and readily available substrates. In this Minireview, synthetic methods based on transition-metal-catalyzed C-H functionalization as efficient tools for the construction of quaternary carbon centers are summarized.

An amine-catalyzed [4 + 2] annulation of Morita-Baylis-Hillman allylic acetates 2 with electron-deficient alkenes or diazenes has been developed for efficient syntheses of highly functionalized cyclohexenes, tetrahydropyridazines, and important spirocycles. This reaction unveils a new reactivity pattern of the intensely studied allylic compounds 2 acting as a C(4) synthon in Lewis base catalyzed annulation reactions and also showcases divergent catalysis between tertiary amines and phosphines.

Herein we describe a phosphine-catalyzed internal redox [4 + 2] annulation of 1,4-enynoates with electron-deficient alkenes, in which the γand φ-C(sp 3 )−H of the enynoates are formally oxidized for the annulation while the alkynyl moiety is converted to an alkene. The reaction offers an efficient synthesis of highly functionalized cyclohexenes in moderate to good yields with exclusive regioselectivity and high diastereoselectivity under mild conditions.