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Computational investigation of the potential energy surfaces of dehydro[10]- and dehydro[14]annulenes revealed that mechanisms involving Hückel and Möbius π-bond shifting can explain the observed or proposed configuration change reactions. Unlike the case of annulenes, in which bond-shift midpoints correspond to transition states, for transformations of dehydroannulenes with Δtrans = 0, "hidden" Hückel bond shifts occur on the side of an energy hill, on the way to a cumulenic, purely conformational transition state. For example, interconversion between CTCCTC-dehydro[14]annulene (1a) and CCTCTC-dehydro[14]annulene (2a) has a CCSD(T)/cc-pVDZ//BHLYP/6-31G* barrier of 18.7 kcal/mol, consistent with experimental observations, and proceeds via a conformational transition state, with Hückel π-bond shifts occurring both before and after the transition state. However, when Δtrans = 1, a true Möbius π-bond shift transition state was located. The isomerization of CCTC-dehydro[10]annulene (10) to CCCC-dehydro[10]annulene (11) occurs by an initial "hidden" Hückel bond shift, followed by passage through a Möbius bond-shift transition state to 11, with an overall barrier of 29.8 kcal/mol at the CASPT2(12,12)/cc-pVDZ//(U)BHLYP/6-31G* level of theory. This is the lowest energy pathway between 10 and 11, in contrast to a cyclization/ring-opening route via a bicyclic allene described in previous reports.

Mechanisms of benzene ring contractions in phenylenes were studied using density functional and coupled cluster methods. Rearrangement of biphenylene to benzopentalene can proceed via a carbene route, by initial 1,2-carbon shift followed by a 1,2-hydrogen shift, with a CCSD(T)/cc-pVDZ//B3LYP/6-31G* barrier of~77 kcal/mol. An alternative carbene pathway consisting of an initial 1,2-hydrogen shift followed by a 1,2-carbon shift has a slightly higher computed barrier of 79 kcal/mol. The preferred carbene mechanism is computed to have a barrier at least 25 kcal/mol lower than competing diradical mechanisms at the BD(T)/cc-pVDZ level. The various possible benzene ring contractions in angular [3]phenylene are predicted to have barriers of 79-82 kcal/mol, with little preference for one pathway over the others. Thus, mechanistic proposals to explain pyrolysis products of angular [3]phenylene can reasonably invoke any of the four possible initial reaction modes via carbene intermediates.

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a b s t r a c t Reactions of 2-mesitylmagnesium bromide with N,N 0 -diarylformamidines afforded five Mg compounds [(DPhF)Mg(THF) 2 ] 2 (l-Br) 2 (1), [D(3,5-Xyl)F] 2 Mg(THF) 2 (2), [D(2,6-Xyl)F] 2 Mg(THF) (3), [D(2-i PrPh)F]-MgBr(THF) 3 (4), and [D(2-t BuPh)F] 2 Mg(THF) ( 5). Complexes 1, 2 and 4 displayed monomeric octahedral metal centers supported by formamidinates, bromide counter anions, and coordinating THF solvent molecules, while the metal cores in 3 and 5 were five-coordinated and in distorted square-pyramidal geometries. Detailed structural analysis indicated that only dimagnesium or mononuclear complexes were obtained through the use of formamidinate ligands. Ligands of increased steric demands resulted in the formation of monomeric complexes. Solvent molecules and counter anions that can coordinate to the metal cores further regulated the product conformation. Monoanionic formamidinates in the complexes, mostly featuring two nearly identical N-C bonds on the N-C-N backbone upon complexation, exhibited a symmetric bidentate chelating (g 2 ) coordination mode.