Phosphepines:  Convenient Access to Phosphinidene Complexes

Abstract

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...

Acetylene‐Substituted Phosphane Oxides: Building Blocks for Macrocycles

“…BrMg-CϵC-SiMe 3 [24] and 1,2-diethynylbenzene [22] were prepared according to literature procedures. All experiments, except for the oxidative Hay coupling reactions, were performed under dry nitrogen.…”

“…We opted for 1,2-diethynylbenzene (17) that has, like 7, an acute angle between the conjugated acetylenic groups and is readily accessible from commercially available ophthalaldehyde. [22] Intermolecular cyclization of building blocks 3 and 17 indeed proved successful (Scheme 7). Thus, addition of the diacetylide 17, along with n-BuLi, to a THF solution of 3 gave both the cis and trans isomers of cyclic bis(phosphane oxide) 18 (25 %) The sulfide and silane analogues have also been reported: They were synthesized by the reaction of 1,2-diethynylbenzene with either bis(phenylsulfonyl) sulfide (for sulfurization) or dichlorophenylsilane (for silylation).…”

A Promising Method for Phosphinidene Generation: Complexes of Phosphinidenes with N‐Donor ligands

“…Dissociation energies (DE dissoc (nR)) and Gibbs free energies (DG dissoc (nR)) at 298 K of the complexes nR (n = 1-7 see Scheme 4; in kcal mol À1 , B3LYP/6-311+G**//B3LYP/6-311+G**). analogues of the norbornadiene and norcaradiene compounds reported by Mathey [10] and Lammertsma, [12] 10 d is the ÀN=CA C H T U N G T R E N N U N G (SiH 3 ) 2 analogue of the phosphirane 11 the decomposition of which has afforded mesityl phosphinidene previously. [5] The quite negative B3LYP/6-311+G** dissociation Gibbs free energies of 8 d and 9 d (À33.6 and À48.5 kcal mol À1 , respectively) suggest that the norbornadiene and norcaradiene compounds can dissociate easily.…”

“…[11] Later on, Lammertsma and co-workers applied (benzo)phosphepine as viable phosphinidene source, which released the phosphinidene complex via a phosphanorcaradiene intermediate (a bicyclic isomer of the sevenmembered-ring phosphepine; Scheme 1B). [12] The driving force in both types of reactions is the formation of an aromatic ring. Recently, the generation of a stabilised amino-…”

“…Fragmentation of the bicyclic 1,4,7-diazaphosphanorbornadiene framework in compounds of type C is similar to the currently used synthetic approaches of phosphinidene complexes reported by Mathey [10] and Lammertsma. [12] The aim of this study is to investigate computationally the possible formation of phosphinidenes by means of decomposition of the N-heterocyclic precursors mentioned above. The influence of benzannelation pattern and substituent effects will be discussed in detail.…”