Cycloaddition of Alkenes and Alkynes to the P‐centered Singlet Biradical [P(μ‐NTer)]₂

Abstract

The reaction of biradical [P(μ‐NTer)]2 (1, Ter = 2,6‐bis(2,4,6‐trimethylphenyl)phenyl) towards different alkenes (R = 2,3‐dimethyl–butadiene, 2,5‐dimethyl‐2,4‐hexadiene, 1,7‐octadiene, 1,4‐cyclohexadiene) and alkynes (R = 1,4‐diphenyl‐1,3‐butadiyne) was studied experimentally. Although these olefins can react in different ways, only [2+2] cycloaddition products (1R) were observed. The reaction with 2,3‐dimethylbutadiene also led to the [2+2] product (1dmb). Thermal treatment of 1dmb above 140 °C resulted in the recovery of biradical 1 upon homolytic bond cleavage of the two P–C bonds and the release of 2,3‐dimethylbutadiene. In contrast to this reaction, all other [2+2] additions products (1R, R = 1,7‐octadiene, 1,4‐cyclohexadiene, 1,4‐diphenyl‐1,3‐butadiyne) began to decompose at temperatures between 200 °C and 300 °C. Only unidentified products were obtained but no temperature‐controlled equilibrium reactions were observed. Computations were carried out to shed light into the formal [2+2] as well as the possible [4+2] addition reaction.

A cyclic thioketone as biradical heterocyclopentane-1,3-diyl: synthesis, structure and activation chemistry

“…Nevertheless, the reaction solution was left overnight at 5 °C to crystallize, and two different types of crystals were isolated: brown cuboidal crystals and yellow needles. While the yellow needles were identified as Ter-NC, which we had often observed as a decomposition product of 5S (vide supra), the reddish-brown cuboids were an unexpected product (8). The product is a dimeric (but not centrosymmetric) magnesium salt in which both magnesium(II) ions are coordinated by three sulfur atoms and one imino-nitrogen atom in a distorted tetrahedral coordination environment.…”

“…The biradical character of 5S was computed utilizing CASSCF (8,6) calculations (CASSCF = complete active space self-consisting field; cf. p. S50 ‡).…”

“…[1][2][3][4] The diphosphadiazanediyl, [(μ-NR)P • ] 2 (R = Ter = 2,6-dimesitylphenyl, 1), 5 is a fourmembered heterocycle with a significant biradical character (25%) and is known to readily activate small molecules with single and multiple bonds (Scheme 1). [6][7][8][9][10] Here, the activation of molecules with a triple bond, e.g. CO or isonitrile R′-NC (R′ = tbutyl, diisopropylphenyl), proved to be particularly interesting, 11,12 since in the first step the activation leads to a 1,1-or 1,2-bridged species (2R vs. 4X; X = O, NR′; R, R′ = aryl, alkyl), which further reacts under insertion into the four-mem-bered P 2 N 2 ring of 1 (Scheme 1).…”

Radical Reactivity of the Biradical [⋅P(μ‐NTer)2P⋅] and Isolation of a Persistent Phosphorus‐Cantered Monoradical [⋅P(μ‐NTer)2P‐Et]

“…[13,[31][32][33] Mechanistically, these reactions mostly represent concerted [2 + 2] additions or insertion reactions (i. e., typical "closed-shell" reactivity). [34] Radical reactivity of 1 (i. e., stepwise addition reactions), which would close the gap between P-centered biradicals and Pcentered monoradicals, has not been described so far.…”

“…For example, we could show that 1 is capable of activating small molecules bearing single (H 2 , chalkogenes) and multiple bonds (alkenes, alkynes, isonitriles) [13,31–33] . Mechanistically, these reactions mostly represent concerted [2+2] additions or insertion reactions (i. e., typical “closed‐shell” reactivity) [34] . Radical reactivity of 1 (i. e., stepwise addition reactions), which would close the gap between P‐centered biradicals and P‐centered monoradicals, has not been described so far.…”