3/2-Order Chain Kinetics Involving a Postulated Dicationic Intermediate in the Isomerization of a P−Isocyano to a P−Cyano Azaphosphatrane Monocation

Abstract

The isomerization of organic isocyanides to cyanides has sporadically piqued interest over the past 45 years. 1 In the gas phase, evidence has been put forth for a unimolecular sigmatropic pathway, 2 whereas in solution, sigmatropic, free radical, and cationic chain mechanisms can occur, depending on the conditions. 1a,3 Silyl cyanides have been shown to exist in equilibrium with the isocyanide form, exchanging intermolecularly in the liquid phase and intramolecularly in dilute solution. 4 Because of severe instability, the few compounds known in which isocyanide is bonded to a heteroatom 5 (O, S, N, P) have not been candidates for isomerization studies.We recently reported the stable isomers 1 and 2. 6 Here we report a detailed kinetic study of the isomerization of 1 in which we postulate an unusual dicationic intermediate in a chain mechanism.Isomerization of 1 to its more stable isomer 2 takes place slowly in acetonitrile (90% conversion in 96 h at 100°C), but not in benzene (<2% conversion under the same conditions). 6 Failure of the reaction in benzene is consistent with the inability of this low dielectric constant solvent to support ionic intermediates. The bromo analogue 3, when present, also plays an important role in the isomerization (see below).The kinetics of the transformation of 1 to 2 was studied by 31 P NMR spectroscopy in CD 3 CN. The reaction clearly does not follow first-order kinetics, in that plots of ln([1] t ) against time are not linear; further, and even more telling, the approximate first-order rate constants increase with [1] 0 . These data rule out a mechanism in which 1 dissociates into its component ions which then recombine to form 2. After reflection on the nature of the variation of [1] t and other possible kinetic schemes, we decided to attempt to fit the kinetic data to the mathematical form for the reaction sequence presented in eqs 1-4, Scheme 1. Note that the reverse of eq 1 is not included because eq 4 takes precedence owing to the greater thermodynamic stability of 2. Although this scheme appears to resemble that of a standard chain reaction, with eqs 1 and 4 being the respective initiation and termination steps and eqs 2 and 3 the propagation steps with the intermediates P 2+ and CN -, respectively, there is a major difference. One intermediate does not undergo a reaction to regenerate the other; rather, each regenerates itself (eq 2). Such a reaction step has been previously postulated for the isomerization of tritylisocyanide. 3b The postulate of a bicyclic P 2+ intermediate in Scheme 1 suggests structure 4, which is isoelectronic/isostructural with boratrane 5 whose structure has been determined. 7With the steady-state approximation for the concentrations of the P 2+ and CN -intermediates, these expressions result:The expression for the reaction rate, with the further assumption that the kinetic chains are long (i.e., that the rate of propagation is much larger than initiation), is the following:According to this analysis, the kinetic data should follow 3/2-order kinet...

On Transannulation in Azaphosphatranes: Synthesis and Theoretical Analysis

“…24 The dication is also a proposed intermediate in azaphosphatrane isomerization but has not been studied experimentally or theoretically to our knowledge. 39 Oxidation of P(i-BuNCH 2 CH 2 ) 3 N at phosphorus leads to increasing transannulation according to our calculations. Conversion of 1 to [P(i-BuNCH 2 CH 2 ) 3 N] + (1 + ) results in a transannular contraction of 0.171 Å to a distance of 3.232 Å 40 while the oxidation to [P(i-BuNCH 2 CH 2 ) 3 N] 2+ (1 2+ ) leads to further shortening by an additional 1.369 Å to a distance of 1.863 Å.…”

“…The radical cation has been invoked in hydrogen atom abstraction, but such a species has only been examined computationally when N eq bears a hydrogen . The dication is also a proposed intermediate in azaphosphatrane isomerization but has not been studied experimentally or theoretically to our knowledge …”

Frustrated Lewis Pair Oxidation Permits Synthesis of a Fluoroazaphosphatrane, [FP(MeNCH2CH2)3N]+

“…Degradation pathways are evidently operative that result in removal of a portion of the P atoms from the azaphosphatrane scaffold preventing an efficient synthetic route from being established. The Verkade group previously proposed that a small nucleophilic ion might be able to attack the P center of a five-coordinate azaphosphatrane and generate a six-coordinate intermediate or transition state. , The fluoroazaphosphatrane cations formed in the reaction with XeF 2 result from the formal addition of F + to the P center with release of Xe gas and an equivalent of F – . Repeated nucleophilic attack of these released fluoride ions on the fluoroazaphosphatrane cation is thought to extrude the P atom from the ligand framework, generating PF 6 – .…”

Synthesis, Characterizations and Applications of Fluoroazaphosphatranes

“…The mass spectrum also supports the formation of this species, with the expected exact mass of 463.5 for the 3 a‐F + cation (figure S60). Indeed, a small nucleophilic ion can attack the electrophilic P centre of the five‐coordinate azaphosphatrane as previously proposed by J. G. Verkade, D. Stephan and our group [7,14,15,16] . The resulting six‐coordinate transition state or intermediate can then give rise to the fluoroazaphosphatrane derivative.…”

“…Indeed, a small nucleophilic ion can attack the electrophilic P centre of the five-coordinate azaphosphatrane as previously proposed by J. G. Verkade, D. Stephan and our group. [7,14,15,16] The resulting six-coordinate transition state or intermediate can then give rise to the fluoroazaphosphatrane derivative. These results demonstrate the formation of the 3 a-F + .PF 6 À salt during the titration experiment.…”