[formula: see text] The enantioselective intramolecular C-H insertion reaction of aryldiazoacetates has been explored with use of dirhodium(II) carboxylate catalysts, which incorporate N-phthaloyl- or N-benzene-fused-phthaloyl-(S)-amino acids as chiral bridging ligands. Dirhodium tetrakis[N-phthaloyl-(S)-tert-leucinate], Rh2(S-PTTL)4, has proven to be the catalyst of choice for this process, providing exclusively cis-2-aryl-3-methoxycarbonyl-2,3-dihydobenzofurans in up to 94% ee.

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As a result of enormous ring strain, cyclopropene compounds display a range of diverse reactivities in both noncatalytic and transition-metal-catalyzed transformations, thus presenting unique opportunities for organic synthesis. [1,2] To further enhance the synthetic potential of cyclopropenes, the development of expeditious methods for the synthesis of enantioenriched cyclopropenes is highly desirable.[3] In this context, a cyclopropenation reaction of alkynes with diazo compounds that is catalyzed by chiral dirhodium(II) complexes represents one of the most powerful means for the construction of this class of optically active building blocks. Doyle, Müller, and co-workers were the first to demonstrate asymmetric induction (up to ! 98 % ee) in cyclopropenation reactions of terminal alkynes including propargyl alcohol or propargylamine derivatives with diazoacetates using [Rh 2 (5S-mepy) 4 ] (2 a; Scheme 1) as a chiral catalyst.[4] Doyle et al. also reported an enantioselective intramolecular cyclopropenation of diazoacetates, in which [Rh 2 (4S-ibaz) 4 ] (2 b) provided macrocyclic cyclopropenes in up to ! 99 % ee.[5] Corey and coworkers demonstrated that a new mixed carboxylate/carboxamidate catalyst [Rh 2 (OAc)(dpti) 3 ] (3) is highly exceptional for cyclopropenation of a broad range of terminal alkynes with ethyl diazoacetate.[6] The extension of this methodology to include a-substituted a-diazoacetates is particularly attractive because it has the capability to form cyclopropenes with a quaternary stereogenic carbon center. [7][8][9] Although high levels of enantioselectivity (up to 99 % ee) in cyclopropenations of terminal alkynes with aryldiazoacetates [7a] or arylvinyldiazoacetates [7b] under catalysis by [Rh 2 (S-dosp) 4 ] (4) have been reported by Davies and co-workers, the goal for the reaction with a-alkyl-a-diazoesters remains elusive because of the propensity to form a,b-unsaturated esters through a 1,2-hydride shift.[10] Panne and Fox recently disclosed that dirhodium(II) tetrapivalate exhibits high selectivity for cyclopropenation over alkene formation in the reaction of terminal alkynes with a-alkyl-a-diazoesters. [11,12] However, to the best of our knowledge, an enantioselective version of this reaction has not been reported.Our research group has previously demonstrated the first examples of highly enantio-, diastereo-, and chemoselective intramolecular C À H insertion reactions of a-alkyl-a-diazoesters by using dirhodium(II) tetrakis[N-phthaloyl-(S)-tertleucinate] ([Rh 2 (S-pttl) 4 ], 1 a) in which high levels of asymmetric induction (up to 95 % ee) were achieved. [13][14][15] Herein, we report that [Rh 2 (S-tbpttl) 4 ] (1 f), a new dirhodium(II) carboxylate complex that incorporates N-tetrabromophthaloyl-(S)-tert-leucinate as chiral bridging ligands, catalyzes the cyclopropenation reaction of terminal alkynes with 2,4-dimethyl-3-pentyl a-alkyl-a-diazoacetates to give 1,2-disubstituted 2-cyclopropenecarboxylates in good to high yields and with up to 99 % ee. Scheme 1. Chiral dirhodium(II) com...

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Over the last decade, the exceptional power of chiral dirhodium(ii) carboxylate and carboxamidate catalysts has been demonstrated in a diverse array of enantioselective metal carbene transformations of diazocarbonyl compounds.[1] Aside from the superiority in diazo decomposition, a dirhodium(ii) complex with vacant coordination sites at the [*] Dr.