[(NHC)Ni^IIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)Ni^II]‐Assisted C−C Rearrangement

Abstract

Ac ross-hydroalkenylation/rearrangement cascade (HARC), using ac yclopropene and alkyne as substrate pairs, was achieved for the first time by using new [(NHC)Ni-(allyl)]BAr F catalysts (NHC = N-heterocyclic carbenes). By controlling the (NHC)Ni II Hr elative insertion reactivity with cyclopropene and alkyne,abroad scope of cyclopentadienes was obtained with highly selectively.The structural features of the new (NHC)Ni II catalyst were important for the success of the reaction. The mild reaction conditions employed may serve as an entry for exploring (NHC)Ni II -assisted vinylcyclopropane rearrangement reactivity.Cyclopropenes (1)are important building blocks in organic synthesis,a nd significant advances for their synthesis have been made recently by using new transition-metal catalysts. Other than the use of traditional methods, [1] catalysts like Au, [2] Rh, [3] Cu, [4] Zn, [5] Pt, [6] and Pd [7] are particularly useful in ring-opening reactions of 1. [8] Thei ntermediates,s uch as the reactive vinyl metal carbenoids and the cyclopropane ions,are often trapped by either nearby functional groups on the skeleton or external nucleophiles (Scheme 1a), providing ag eneral route to substituted furans,p yrans,a nd acyclic products.T ransition-metal hydride catalysts or their equivalents [9] could potentially be one of the most ideal approaches for cyclopropene functionalization since it could insert into the olefin and allow subsequent insertions of various reaction partners.However, their use in selective transformation of 1 is rarely reported. [7] An umber of other possible reactivities of 1 with transition-metal hydride lead to unfavorable consumption of 1,and have severely limited the progress of the crossreaction development. With either too high of an insertion activity of 1 or competing pathways from other reaction partners,such as the alkyne 2,the non-selective oligomerization or homoreaction reactivity often dominates (Scheme 1bi).M oreover,o ther possible rearrangements were reported as one of the possible pathways for allylÀNu formation, and that made the hydride approach even more complicated (Scheme 1bii).A sac onsequence,h ighly selec-tive cross-transformations of 1 by transition-metal hydride catalyst and external p systems [2b,3d, 4, 10] remains elusive. Oxidative addition followed by either rearrangement or trapping is still dominating the field in Ni-catalyzed threemembered ring opening (Scheme 1c). [11,12] Herein, across-hydroalkenylation/rearrangement cascade (HARC) of 1 and 2 is achieved for the first time by using an ew set of [(NHC)Ni(allyl)]BAr F catalysts (Scheme 1d; NHC = N-heterocyclic carbene). Unlike the cross-hydroalkenylation of two different alkenes, [9b,13, 14] it provides highly efficient access to synthetically valuable,functionalized cyclopentadienes (3)r ather than gem-olefins.I nstead of using oxidative addition conditions for three-membered ring opening by Ni 0 in Scheme 1c,t he CÀCb ond cleavage is possibly assisted by (NHC)Ni II in as ubsequent rearrangement under mild cond...