Using the phospha-Michael reaction for making phosphonium phenolate zwitterions

Abstract

The reactions of 2,4-di-tert-butyl-6-(diphenylphosphino)phenol and various Michael acceptors (acrylonitrile, acrylamide, methyl vinyl ketone, several acrylates, methyl vinyl sulfone) yield the respective phosphonium phenolate zwitterions at room temperature. Nine different zwitterions were synthesized and fully characterized. Zwitterions with the poor Michael acceptors methyl methacrylate and methyl crotonate formed, but could not be isolated in pure form. The solid-state structures of two phosphonium phenolate molecules were determined by single-crystal X-ray crystallography. The bonding situation in the solid state together with NMR data suggests an important contribution of an ylidic resonance structure in these molecules. The phosphonium phenolates are characterized by UV–vis absorptions peaking around 360 nm and exhibit a negative solvatochromism. An analysis of the kinetics of the zwitterion formation was performed for three Michael acceptors (acrylonitrile, methyl acrylate, and acrylamide) in two different solvents (chloroform and methanol). The results revealed the proton transfer step necessary to stabilize the initially formed carbanion as the rate-determining step. A preorganization of the carbonyl bearing Michael acceptors allowed for reasonable fast direct proton transfer from the phenol in aprotic solvents. In contrast, acrylonitrile, not capable of forming a similar preorganization, is hardly reactive in chloroform solution, while in methanol the corresponding phosphonium phenolate is formed.

Synthesis, characterization and stability of phosphonium phenolate zwitterions derived from a (diphenylphosphino)phenol derivative and oxiranes

“…As a next step, we sought to isolate and identify the unknown main product, which we hypothesized to be the corresponding phosphonium phenolate zwitterion resulting from the ring opening of PGE by 1 followed by hydrogen transfer. Similar zwitterionic species have been prepared by reaction of 1 with Michael acceptors [28], and we anticipated a similar reactivity with epoxides. In case of using epoxides, the reaction is additionally driven by the release of the ring strain (about 27 kcal/mol) [29,30].…”

“…In 2d, the conformer with the intramolecular hydrogen bond was found with a O1-O2 distance of 2.519(4) Å (Fig, 2 gray box). The P1-O1 distances of 2.813(4) Å (2a, intramolecular hydrogen bonding) and 2.708(4) Å (2a, intermolecular hydrogen bonding) as well as 2.826(3) Å in 2d suggest a weak binding interaction between the phenolate and the phosphonium center as observed in similar zwitterions with P-O distances in the range of 2.60-2.95 Å [28,35,36] (Fig. 3, ochre box).…”

“…However, when 1 was reacted with PGE at ambient conditions in dichloromethane (1.3 mmol/dm 3 ) no conversion to the above detected species was observed over one week. Drawing parallels to the formation of zwitterions with Michael acceptors [28], the lack of reactivity is likely due to hindered hydrogen transfer from the phenol derivative 1 to the alkoxide in I (see Scheme 1). Accordingly, 5 equivalents of methanol were added to the reaction mixture and slow formation of the desired product was observed at room temperature.…”

“…For comparison, the corresponding carbon atom in similar alkyltriphenylphosphonium halides resonates at 27-18 ppm ( 1 J PC ≈ 54 Hz) [33,34]. The 31 P NMR shift (against H 3 PO 4 , 85%) of the adducts is in the range of 18.9-18.3 ppm, which is slightly shifted upfield compared to the corresponding values for the Michael acceptor derived zwitterions of 1 [28]. For comparison, the phosphine 1 has a 31 P NMR shift of −29.7 ppm [31].…”

“…For comparison, the same proton in the parent phosphine 1 shows a resonance at 6.88 ppm [31]. In the Michael acceptorderived zwitterions, this proton is similarly shifted upfield as in 2a-2e and is found in the range of 6.09 to 6.21 ppm [28].…”