Facile Cleavage of the P=P Double Bond in Vinyl‐Substituted Diphosphenes

Abstract

The reactions of the cyclic alkyl amino carbene (CAAC) 1 with phosphaalkynes generate the kinetically unstable CAAC‐derived phosphirenes 4 and 5, which undergo rearrangement/dimerization reactions to give the vinyl‐substituted diphosphenes 2, 3, and 6. The P=P double bond scission of 2 or 3 is unprecedentedly effected by S8, [AuCl(tht)], or MeOTf at room temperature, which affords a dithiophosphorane 7, a phosphepine Au complex 8, or phosphepinium cations 9 and 10, respectively. The cationic species feature little homoaromaticity while representing the first examples of the phosphorus‐containing analogue of the tropylium ion.

A Modular Access to Divinyldiphosphenes with a Strikingly Small HOMO–LUMO Energy Gap

“…Notably,t he C4ÀP1 bond lengths of 5 (1.785(1)) and 6 (1.797(1) )a re considerably shorter compared with those of the Stephan's compounds VI (1.854(2), R = tBu;1 .846(3) ,R= Ad). [10] The latter are consistent with C sp 2ÀPs ingle-bond lengths (1.85 ), [20] indicating aw eak or negligible p-conjugation between C=Ca nd P=Pm otifs in VI.T his may also be due to the presence of as terically demanding substituent (R = tBu or Ad) at the vinyl carbon atom, pushing the C=Cm oiety out of the P=Pp lane andt hus precluding the effective orbital overlap. This is not the case with planarp henyl substituents in 5 and 6.I nterestingly,t he impact of effective p-conjugation is also manifestedi nt he frontiersm olecular orbital( FMO) energies of 5, 6,and VI (see below).…”

“…Electron-rich vinyl substituents, such as N-heterocyclic vinyl (NHV) groups [9] in IV and V (Figure 1), can be ar ational choice as ar eactivity enhancement tool by lowering the HOMO-LUMO energy gap of ad iphosphene through p-conjugation.I nf act, during the preparationo ft his manuscript for submission, an article by Stephan and co-workers describing the synthesis of diphosphenes VI (Figure 1; R = tBu or adamantyl (Ad)) supported by Bertrand's cyclic alkyl amino carbene (CAAC)w as published. [10] Compounds VI were prepared by reacting CAAC (VII)w ith RCP. In this context,e arly reports by Hahn and co-workers on the reactivity studies of NHCs and RCP( R= tBu or iPr 2 N) to prepare phosphorus heterocyclesare noteworthy.…”

Crystalline Divinyldiarsenes and Cleavage of the As=As Bond

“…bis-pnictinidenes), structural andN MR spectroscopic data favor the latter as the main formalism. [15] ClassicalN HCs are weaker p-acceptors compared with CAACs, [11] and hence NHVs derived from the former should be better p-donors compared with those derived from the latter. [12] More recently,T amm and co-workers employed an anionic NHC to prepare variousd ipnictenes III.…”

“…This situation is however different forc ompounds derived from more paccepting carbenes,s uch as CAACsd evelopedb yB ertrand and co-workers [9,11] For CAAC derivatives, the dipnictobutadiene (IIa)r esonance form is more prevalent compared with the bispnictinidene form IIb. Synthesis of NHC-based divinyldiarsenes by adopting as imilarm ethodology [15] seems challenging due to limited accessibility of appropriate arsaalkynes [16] as well as the weaker p-accepting property of NHCs than CAACs. [13] In recent years, N-heterocyclic vinyl (NHV)s ubstituents (IV) have been recognized as very efficient p-donors to stabilize numerousc ompounds containing al ow-valent main-group element.…”

“…Theb ond energy (in kcal mol À1 )o fN 2 (226) is nearly twice that of P 2 (116) [22] and thrice that of As 2 (83), [23] thus suggesting ad iminished importance of p-p bonding among third and fourth-period elements. [15,17] Among pnictinidenes, an in situ generated Bi I compound supported by aN CN pincer-type ligand was shown to cleave the EÀEb ond of diphenydichalcogenides PhEEPh (E = S, Se, Te ), producing Bi III phenylchalcogenolate derivatives. [15,17] Among pnictinidenes, an in situ generated Bi I compound supported by aN CN pincer-type ligand was shown to cleave the EÀEb ond of diphenydichalcogenides PhEEPh (E = S, Se, Te ), producing Bi III phenylchalcogenolate derivatives.…”