The cycloheptatriene-norcaradiene (CHT-NCD) equilibrium exemplifies a 6-el electrocyclic reaction with a modest barrier separating the valence tautomers 1C and 2C. 1 Less is known about the heterocyclic analogues (X ) O, S, NR, PR) (Scheme 1). For those with an oxygen atom, benzene oxide 2O is the more stable form, but entropy factors shift the equilibrium toward oxepin 1O at room temperature. 2 Only the monocyclic form is observed for thiepins 3 1S and 1H-azepines 4 1N, and both require bulky groups and/or extended conjugation for stability. Of the phosphorus analogue, phosphepine 1P, only its oxide 5 and 2,7-dialkylsubstituted 6 and annelated 7,8 derivatives are known, but without structural details. Here we describe a computational analysis of the 1P-2P equilibrium and present new stable phosphepine derivatives and a novel application.Phosphepines equilibrate with phosphanorcaradienes (benzene phosphines) such as CHT with NCD. DFT calculations for the parent system give an energetic preference for NCD 2P over CHT 1P (∆E 2-1 -4.2 kcal/mol, Table 1) with a modest 1P f 2P barrier (10.6 kcal/mol), reflecting the same behavior as the thia analogues (∆E 2-1 -7.8 (S) kcal/mol). A 1,5-sigmatropic shift relates NCD 2P with the 15.5 kcal/mol less stable 7-phosphanorbornadiene 3P, neither of which is known experimentally, except for a strained derivative of 3P, 10 presumably due to release of the phosphorus group to give pentamers, (PR) 5 .Transition metal coordination at phosphorus stabilizes 1P over 2P because the distal C-C bond is weakened by σ,π-interactions (Scheme 2), reversing the order for W(CO) 5 (∆E 2-1 1.5 kcal/mol). Due to extended conjugation, benzannelation has an even stronger influence, reversing the CHT-NCD equilibrium in favor of 1P by 11.0 kcal/mol. These cumulative electronic effects give a 16.8 kcal/mol energetic preference of benzophosphepine W(CO) 5 complex over its valence isomer 2P with a 24.7 kcal/mol barrier for electrocyclization. 11 On the basis of these calculations benzophosphepine complexes 7 are expected to be stable at room temperature. Indeed, 7 could be synthesized from the complexed phosphine and 1,2-diethynylbenzene (5) by a modified Märkl's procedure 7 (Scheme 3). Dialkyne 5 was obtained in 91% yield from commercially available o-phthaldialdehyde (4) by a one-carbon homologation-oxidation sequence. The base-promoted double hydrophosphination of 5 with PH 2 Ph-W(CO) 5 (6a) gave 3H-3-benzophosphepine-W(CO) 5 7a in 74% yield as yellow crystals. W(CO) 5 coordination causes shielding of the 31 P NMR resonance (δ -15 vs -33 ppm), shielding of the C1,C5 13 C NMR resonances (by 6 ppm), and reduces the 2 J CP coupling constants (1.6 vs 21.1 Hz). 7 Distinctive are the coupling constants for the olefinic protons with a sizable 3 J HH (12.4 Hz) and with a 3 J HP (33.4 Hz) being much larger than the 2 J HP (21.3 Hz).The crystal structure (C s -symmetry) shows alternating CdC bonds (no homoaromaticity) for the boat-shaped phosphepine ring (Figure 1) that has the phosphorus atom shifted out of t...

The synthesis of a variety of benzophosphepine complexes [R = Ph, t-Bu, Me; ML(n )()= W(CO)(5), Mo(CO)(5), Cr(CO)(5), Mn(CO)(2)Cp] by two successive hydrophosphinations of 1,2-diethynylbenzene is discussed in detail. The first hydrophosphination step proceeds at ambient temperature without additional promoters, and subsequent addition of base allows full conversion to benzophosphepines. Novel benzeno-1,4-diphosphinanes were isolated as side products. The benzophosphepine complexes themselves serve as convenient phosphinidene precursors at elevated, substituent-dependent temperatures (>55 degrees C). Kinetic and computational analyses support the proposal that the phosphepine-phosphanorcaradiene isomerization is the rate-determining step. In the absence of substrate, addition of the transient phosphinidene to another benzophosphepine molecule is observed, and addition to 1,2-diethynylbenzene furnishes a delicate bidentate diphosphirene complex.