Mechanism of Phosphine-Catalyzed Novel Rearrangement of Vinylcyclopropylketone to Cycloheptenone: A DFT Study

Donor-Acceptor Cyclopropane Ring Expansion to 1,2-Dihydronaphthalenes. Access to Bridged Seven-Membered Lactones

A Lewis-acid-promoted domino ring-opening cyclization of readily available donor-acceptor cyclopropanes with a preinstalled electrophilic center, embedded in a donor group, to functionalized 1,2-dihydronaphthalenes is reported herein. The obtained compounds are transformed to pharmacologically attractive bridged tricyclic esters in a diastereospecific manner.

Organophosphorus-Catalyzed Borylative Ring-Opening of Vinylcyclopropanes: A Stereoselective Route to δ-Valerolactones

We report an operationally simple route to δ-valerolactones through an organophosphorus-catalyzed borylative ring-opening/allylation of vinylcyclopropanes providing δ-hydroxy esters stereoselectively. The δ-hydroxy esters were lactonized to obtain densely substituted δ-valerolactones. The present methodology exhibited enhanced functional group tolerance compared to the existing metal-mediated methods. A plausible mechanism for borylative ring-opening reaction has been suggested. ³¹P NMR studies indicated the involvement of a phosphonium zwitterionic species. The synthetic utility of the intermediate allyl boronates was demonstrated.

Synthesis of trans-stilbenes via phosphine-catalyzed coupling reactions of benzylic halides

An efficient and practical phosphine-catalyzed homo-coupling reaction of benzyl chlorides is described. The reactions proceed smoothly in the presence of CsF/B(OMe)₃ and NaH as the base, respectively, to provide trans-stilbenes in good yields with a broad scope. Unsymmetrical stilbenes are also generated from the reactions of benzyl chlorides with phosphonium salts. Several P-based key intermediates have been detected by NMR and HRMS analyses, which shed light on the postulated catalytic cycle. In the presence of different bases, the transformations involve two different pathways, in which phenylcarbene and phosphonium alkoxide are considered as key intermediates, respectively. The two pathways are complementary in synthesis but different in mechanisms. The synthetic utility, including gram-scale reactions and straightforward access to π-conjugated molecules, has been demonstrated as well.

Expanding Stereoelectronic Limits of endo-tet Cyclizations: Synthesis of Benz[b]azepines from Donor-Acceptor Cyclopropanes

The importance of intramolecular constraints in cyclic transition-state geometries is especially pronounced in n-endo-tet cyclizations, where the usual backside approach of a nucleophile to the breaking bond is impossible for the rings containing less than eight atoms. Herein, we expand the limits of endo-tet cyclizations and show that donor-acceptor cyclopropanes can provide a seven-membered ring via a genuine 6-endo-tet process. Substrates containing a N-alkyl-N-arylcarbamoyl moiety as an acceptor group undergo Lewis acid-induced cyclization to form tetrahydrobenz[b]azepin-2-ones in high yields. The reaction proceeds with the inversion of the configuration at the electrophilic carbon. In this process, a formally six-membered transition state yields a seven-membered ring as the pre-existing cycle is merged into the forming ring. The stereochemistry of the products can be controlled by the reaction time and by the nature of Lewis acid, opening access to both diastereomers by tuning of the reaction conditions.