Skeletal ring enlargement is gaining renewed interest in synthetic chemistry and has recently focused on insertion of one or two atoms. Strategies for heterocyclic expansion through small-ring insertion remain elusive, although they would lead to the efficient formation of bicyclic products. Here, we report a photoinduced dearomative ring enlargement of thiophenes by insertion of bicyclo[1.1.0]butanes to produce eight-membered bicyclic rings under mild conditions. The synthetic value, broad functional-group compatibility, and excellent chemo- and regioselectivity were demonstrated by scope evaluation and product derivatization. Experimental and computational studies point toward a photoredox-induced radical pathway.

Heavier main-group element-catalyzed reactions provide an increasingly attractive tool to perform transformations mimicking the behaviors of transition metal catalysts. Recently, Magre and Cornella reported a Bicatalyzed synthesis of aryl sulfonyl fluorides, which involves a fundamental organometallic step of SO 2 insertion into the BiÀ Ph bond. Our theoretical studies reveal that i) the ability of hypervalent coordination of the Bi(III) center allows facile coordination sphere expansion for the SO 2 coordination via one oxygen atom; and ii) the high polarity of the BiÀ Ph bond makes the Ph migration from the Bi(III) center feasible. These features enable the heavier main group element to resemble the transition metal having flexibility for ligand association and dissociation. Furthermore, iii) the available π electron pair of the migrating Ph group stabilizes the SO 2 insertion transition state by maintaining interaction with the Bi(III) center during migration. The insight helps us better understand the heavier main-group catalysis.

It is a common practice to associate a d 8 electron configuration with square planar, 4-coordinated complexes. A 5-coordinated complex with a d 8 configuration, although following the 18electron rule perfectly, is not as common as it might seem. On the other hand, a considerable number of 5-coordinated d 8 Co(I) complexes has been reported, and some of them serve as active intermediates in catalysis. Understanding the preference of 4-versus 5-coordination for d 8 complexes is therefore of practical concern in catalysis design. In this work, we have presented an exploration of the possible factors that govern the preference between the two. Based on DFT calculations, we have investigated the free energy change of ligand binding to several Co(I) pincer complexes and have revealed the role of σ and π interaction of ligands.