Dicationic phosphonium salts: Lewis acid initiators for the Mukaiyama-aldol reaction

Abstract

Two strategies were used to prepare dicationic phosphonium cations. The first method consists of the reaction of 1-chlorocyclopropenium salts with phosphines to obtain cyclopropenium-substituted phosphonium salts 10a-f[BF4]. Anion exchange was performed to access the corresponding [B(CF)] analogues 10a-f[B(C6F5)4], which showed much higher solubility in organic solvents. In addition, we developed a synthesis of dicationic phosphonium salts containing 2-, 3-, or 4-methylpyridinium substituents 11a-c[TfO], which were converted as well to their [B(CF)] analogues 11a-c[B(C6F5)4]. Finally, the phenoxy-substituted phosphonium salt 12[B(C6F5)4] was also prepared. All salts demonstrated remarkable stability in air as compared with their fluorinated analogues. The Lewis acidity of these salts was evaluated by means of theoretical calculations and finally, they were shown to be effective in initiating the Mukaiyama-aldol reaction.

Air- and water-stable Lewis acids: synthesis and reactivity of P-trifluoromethyl electrophilic phosphonium cations

“…Diagnostic peaks: 1 H NMR (400 MHz, CHCl 2 ): δ 5.18 (s, 1H, -C(H)-), 3.70 (s, 3H, -OMe), 1.21 (s, 3H, Me), 1.07 (s, 3H, Me'), 0.00 (s, 9H, -OTMS) ppm. The results are in agreements with those reported in the literature 16.…”

Reactivity vs. Stability of Cyclopropenium Substituted Phosphonium Salts

“…As observed for the tetrafluoroborate analogue, the 31 P NMR spectrum of [ 2 ](OTf) 2 displayed a single signal shifted downfield relative to PPh 3 ( δ P = –5.9 ppm) at δ P = 9.3 ppm. In the 13 C NMR spectrum, the C– + PPh 3 carbon atom was found to be slightly deshielded ( δ C = 140.9 ppm, brs) with respect to the C–Cl carbon atom of precursor [ 1 ](OTf) ( δ C = 133.4 ppm, s) in agreement with a sp 2 ‐type carbon atom .…”

“…According to the recent report of Alcarazo and co‐workers, 3‐triphenylphosphonio‐1,2‐bis(diisopropylamino)cyclopropenium [ 2 ](OTf) 2 was smoothly prepared in good yield from related 3‐chlorocyclopropenium salt [ 1 ](OTf) and PPh 3 in the presence of NaOTf in THF at 60 °C (Schemeà, method a) . The dicationic phosphonium [ 2 ](OTf) 2 was also readily obtained by reacting cyclopropenium precursor [ 1 ](OTf) with the PPh 3 /Me 3 SiOTf system in CH 2 Cl 2 at room temperature (Schemeà, method b) …”

“…Among cyclopropenylidene‐phosphorus derivatives, stabilized phosphenium cations of type C were recently prepared for the purpose of accessing novel α‐cationic phosphane ligands . Cyclopropenium substituted phosphonium salts of type D were briefly investigated with the main objective of designing highly electrophilic phosphorus cations that can serve as Lewis acid initiators . Thanks to the critical effect of π‐donating amino substituents, a large gamme of unsaturated three membered cycles A – D , neutral, mono‐ or dicationic in nature, were thus isolated (Figureà).…”

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

“…[2][3][4] Among the differently substituted cyclopropenium cations,t he aminocyclopropenium ions first described by Yoshida in 1971, [5] have received special attention. [6] Most recent work has focused on their use as phase transfer, Lewis acid or organocatalysts, [7][8][9][10][11][12][13][14][15][16] electrophotocatalysts, [17] ligands for catalytic metal complexes, [18][19][20] ionic liquids, [21][22][23][24][25] persistent radical cations, [26] redox active polymers for redox flow batteries, [27][28][29][30][31][32] fluorescent materials, [33][34][35] aromatic cations in hybrid halide perovskites, [36] biologically active compounds such as transfection agents, [37][38][39] and nanoparticles. [40,41] Surprisingly, the self-assembly of cyclopropenium compounds into liquid crystalline phases has not been reported.…”