Controllable Si−C Bond Activation Enables Stereocontrol in the Palladium‐Catalyzed [4+2] Annulation of Cyclopropenes with Benzosilacyclobutanes

Abstract

A novel and unusual palladium‐catalyzed [4+2] annulation of cyclopropenes with benzosilacyclobutanes is reported. This reaction occurred through chemoselective Si−C(sp2) bond activation in synergy with ring expansion/insertion of cyclopropenes to form new C(sp2)−C(sp3) and Si−C(sp3) bonds. An array of previously elusive bicyclic skeleton with high strain, silabicyclo[4.1.0]heptanes, were formed in good yields with excellent diastereoselectivity under mild conditions. An asymmetric version of the reaction with a chiral phosphoramidite ligand furnished a variety of chiral bicyclic silaheterocycle derivatives with good enantioselectivity (up to 95.5:4.5 er). Owing to the mild reaction conditions, the good stereoselectivity profile, and the ready availability of the functionalized precursors, this process constitutes a useful and straightforward strategy for the synthesis of densely functionalized silacycles.

Catalytic regio- and stereoselective silicon–carbon bond formations on unsymmetric gem-difluorocyclopropenes by capture of silyl metal species

“…Considering the importance of benzosilacyclobutenes as starting materials in the synthesis of medium-ring silacycles, 18 we initially checked the effect of various phosphine ligands on the palladium-catalyzed functionalization of unsymmetric gem-difluorocyclopropene 1a with benzosilacyclobutene 2a through a [4 + 2] annulation process. Among the P-ligands evaluated in this work (see Table S1 of the ESI †), triphenylphosphine (PPh 3 ) promoted the reaction with promising conversion (46% yield of product 3a), whereas tris(3,5-bis(trifluoromethyl)phenyl)phosphane exhibited opposed activity in this reaction.…”

An unusualtrans-hydrosilylation of prochiral 1,1-disubstituted cyclopropenes revealing the different nature of asymmetric palladium and rhodium catalysis

“…19 The other is the possible steric repulsion or aromatic interaction between the Pd/ligand catalyst and the substrate, in which the stereoselective induction of steric repulsion or aromatic interactions can at least be inferred from the experimental results of palladium-catalyzed hydrosilylation reactions. First, it should be noted that the unavoidable and weak coordination capacity of the Pd-L complex 24 with the 1,1-disubstituted cyclopropenes bearing both a carbonyl group and an aromatic ring at the C-1 position possibly shows conflicting diastereoselective control because it can be directed by the oxygen atom of the carbonyl group or the π-π stacking interaction between the catalyst and substrate. In general, moderate diastereoselectivity of the corresponding products must be obtained in the catalytic asymmetric hydrosilylation of diphenylsilane 2a and arylcyclopropene 1.…”

Recent advances in the reactions of silacyclobutanes and their applications

“…In 2020, Xu and co-workers realized an efficient palladium catalyzed ring expansion/cycloaddition of cyclopropenes with benzosilacyclobutanes, affording silabicyclo[4.1.0]heptanes with excellent diastereoselectivity under mild conditions (Scheme 6). 12 Unfortunately, a drawback associated with this method was relatively low yields. On the basis of their experimental results as well as DFT calculations, two possible reaction pathways were proposed, as shown in Scheme 6.…”