Chiral Cyclopentadienyl Iridium(III) Complexes Promote Enantioselective Cycloisomerizations Giving Fused Cyclopropanes

Abstract

The cyclopentadienyl (Cp) group is avery important ligand for many transition-metal complexes which have been applied in catalysis.The availability of chiral cyclopentadienyl ligands (Cp x )lags behind other ligand classes,thus hampering the investigation of enantioselective processes.W er eport al ibrary of chiral Cp x Ir III complexes equipped with an atropchiral Cp scaffold. Ar obust complexation procedure reliably provides Cp x Ir III complexes with tunable counterions. In aproof-of-concept application, the iodide-bearing members are shown to be highly selective for enyne cycloisomerization reactions.The dehydropiperidine-fused cyclopropane products are formed in good yields and enantioselectivities.

Enantioselective Synthesis of Spiroindenes by Enol‐Directed Rhodium(III)‐Catalyzed CH Functionalization and Spiroannulation

“…Within the accepted model for enantioinduction using complexes 3, [2b, 5] cyclorhodation can generate up to four species,w hich differ in which directing group participates in cyclometallation, and/or the orientation of the rhodacycle within the chiral pocket (Scheme 2). This situation contrasts with existing processes, [2][3][4][5] including the dearomatizing oxidative spiroannulations of Youand co-workers, [4d] in which only two conformations of one rhodacycle need to be considered. Given the possibility of other reaction pathways with potentially different stereochemical outcomes,t he development of ah ighly enantioselective process was far from certain.…”

“…However,i nc ontrast to existing enantioselective Rh III -catalyzed CÀHf unctionalizations, which all rely upon aryl C(sp 2 )-H activation of substrates containing as ingle directing group (Scheme 1a), [2][3][4][5] the substrates 1 required for our proposed study contain two potential directing groups (Scheme 1b). Within the accepted model for enantioinduction using complexes 3, [2b, 5] cyclorhodation can generate up to four species,w hich differ in which directing group participates in cyclometallation, and/or the orientation of the rhodacycle within the chiral pocket (Scheme 2).…”

“…Benzoyl peroxide,w hich was employed as an additive in previous enantioselective Rhcatalyzed CÀHfunctionalizations, [2,4] was unnecessary, [13] and in all cases,o nly one regioisomer of spiroindene 4a was detected. Thep arent complex 3a (R = H) gave 4a in 93 % NMR yield, but the enantioselectivity was moderate (entry 1).…”

“…High selectivity between two possible directing groups,aswell as control of the direction of rotation in the isomerization of an O-bound rhodium enolate into the C-bound isomer,appear to be critical for high enantiomeric excesses.Cyclopentadienyl rhodium(III) complexes are well-established as highly active and versatile precatalysts in ad iverse array of CÀHf unctionalization reactions.[1] However, enantioselective variants of these reactions only became possible with the development of chiral C 2 -symmetric cyclopentadienyl ligands by Ye and Cramer, [2] and an artificial Rh III -containing metalloenzyme by Ward, Rovis,and co-workers. [3] To date,ahandful of catalytic enantioselective Rh III -catalyzed CÀHf unctionalizations have been described, [2][3][4][5] but there is ac ompelling need to develop new processes to access novel classes of enantioenriched products.…”

“…[1] However, enantioselective variants of these reactions only became possible with the development of chiral C 2 -symmetric cyclopentadienyl ligands by Ye and Cramer, [2] and an artificial Rh III -containing metalloenzyme by Ward, Rovis,and co-workers. [3] To date,ahandful of catalytic enantioselective Rh III -catalyzed CÀHf unctionalizations have been described, [2][3][4][5] but there is ac ompelling need to develop new processes to access novel classes of enantioenriched products. [6] We recently reported Ru-and Pd-catalyzed oxidative annulations of a-aryl cyclic 1,3-dicarbonyl compounds (or their enol tautomers) with alkynes that provide achiral or racemic spiroindenes.…”