P-oxidation of gem-dicationic phosphines

Abstract

In spite of the general oxophilic character and l 5 valence state accessibility of phosphorus centers, the oxidation of phosphanes bearing proximal positive charges remains challenging, in particular for dicationic representatives. Previously described diimidazolo-and diimidazolio-phosphane substrates were first resumed in the P-phenylated series, where the reported absence of coordinating ability of the N,N-dimethylated dication towards a neutral Rh(I) center is here confirmed towards a promoted oxygen atom. By contrast, the P-tert-butylated homologue is shown to undergo P-oxidation to give a stable gem-dicationic phosphine oxide (in 91% yield with m-CPBA). The latter could also be obtained by N,Ndimethylation of the neutral diimidazolo-phosphine oxide precursor. The strict covalent vs. dative character of the N 2 C-P bond in dicationic systems (N 2 C + ) 2 P(X)R (R ¼ Ph, tBu; X ¼ lone pair, :O) has been investigated by DFT calculations on a model series where (N 2 C + ) ¼ 1,3-dimethyl-1H-imidazolio-2-yl. The relative magnitude of Gibbs free energies of the homolytic and heterolytic dissociation modes shows that diamidinio-phosphines and corresponding P-oxides are not N 2 C / P donor-acceptor adducts (as mono(amidinio)phosphines are), but rather true gem-dicationic phosphines and phosphine oxides in the strict sense, respectively.

Asymmetric Cationic Phosphines: Synthesis, Coordination Chemistry, and Reactivity

“…This has not been possible with previous imidazolium phosphines as the NCN carbon is bonded to the phosphorus. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The in situ 31 P{ 1 H} NMR spectrum of a solution of [Ag(α-CgPAmHMe)(OTf)]BF4 in THF after treatment with 1.2 equivalents of KHMDS showed an ex-tremely broad peak that was barely discernible. After isolation of a solid complex and redissolution, the resultant 31 P{ 1 H} NMR spectrum showed a sharp set of doublets at 98.5 ppm ( 1 JP-Ag = 481, 416 Hz) and two broad doublets at 64.8 ppm (average 1 JP-Ag = 358 Hz) and 62.2 ppm (average 1 JP-Ag = 359 Hz).…”

“…Cationic phosphines containing positively-charged imidazolinium, 1-6 pyridininium 7 and cyclopropenium 8 substituents attached directly to phosphorus by a C-P bond have emerged as user-friendly phosphite mimics with the potential for improved catalytic activity and/or longevity. Alcarazo and co-workers have been at the forefront of this cationic phosphine revolution through the introduction and application of various mono-, 9,10 di- 10,[11][12][13] and tri-cationic 14 ligands including very recent examples of chiral forms. 15,16 Inspired by this elegant work and that of others, 17,18 we sought to combine our interest in polycyclic phosphines and amidinium salts (as precursors to NHCs) to generate cationic phosphines where both constituent units are asymmetric and connected by an N-P bond (figure 1).…”

Bis(cyclopropenium)phosphines: Synthesis, Reactivity, and Applications

“…), which are comparable to that of the bis(imidazolium)‐substituted sister compound 9 ( δ =−50.8 ppm.) . This could be then ascertained by single‐crystal X‐ray diffraction analysis of 1 d , 1 f , and 8 a(SbF 6 ) (Figure ).…”

“…), which are comparable to that of the bis(imidazolium)-substituted sister compound 9 (d = À50.8 ppm.). [8] This could be then ascertained by single-crystal X-ray diffraction analysiso f1d, 1f,a nd 8a(SbF 6 ) ( Figure 3). The geometry aroundt he phosphorus atom in these three compounds remains pyramidal, although slightly flattened compared to 5 (sum of angles around P1:3 14.48,3 08.08,a nd 306.08 in 1d, 1f, and 8a(SbF 6 ), respectively).…”

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

“…Coordinatively inert species were only observed in the case of diimidazoliophosphines, in which the P atom is substituted by two imidazolium moieties. For instance, N , N′ ‐(dimethyl)diimidazoliodiphenylphosphine ( 20 ; see Scheme ) was found not only to be beyond the coordinating limit towards a Rh I center, but also inert with respect to oxidation . The change of the P ‐phenyl substituent for a more electron‐donating tert ‐butyl or diisopropylamino group was shown, however, to restore the coordinating ability of the P atom.…”