Ring‐opening of Epoxides Mediated by Frustrated Lewis Pairs

Abstract

Treatment of the preorganized frustrated Lewis pairs (FLPs) tBu PCH BPh (1) and o-Ph P(C H )BCat (Cat=catechol) (4) with 2-methyloxirane, 2-phenyloxirane and 2-(trifluoromethyl)oxirane resulted in epoxide ring-opening to yield the six- and seven-membered heterocycles 2 a-c and 5 a-c, respectively. These zwitterionic products were characterized spectroscopically, and compounds 2 a, 2 b, 5 a and 5 c were structurally characterized by single-crystal X-ray structure analyses. Based on computational and kinetic studies, the mechanism of these reactions was found to proceed via activation of the epoxide by the Lewis acidic borane moiety followed by nucleophilic attack of the phosphine of a second FLP molecule. The resulting chain-like intermediates afford the final cyclic products by ring-closure and concurrent release of the second equivalent of FLP that behaves as catalyst in this reaction.

Trapping of Difluorocarbene by Frustrated Lewis Pairs

“…Then, we switched to intramolecular version of FLP. Thus, a series of phosphines 2 bearing a dioxyboryl fragment in the ortho-position were prepared [13,14] and evaluated in reaction with potassium bromodifluoroacetate (Scheme 3). The reactions were performed in dimethylformamide at room temperature, and a series of compounds 3 a-c containing fivemembered phospha-bora-cycle were isolated in individual state after column chromatography.…”

Catalyst Free, Nitromethane Assisted Facile Ring Opening of Epoxide with Less Reactive Aromatic Amines

“…The similar reaction was performed at room temperature (Table 1, Entry 20 and 22) that led to the poor yield of the product, which further supported the efficiency of microwave assisted ring opening reaction. To explore the recyclability of the solvent, reactions were performed in recovered nitromethane for three consecutive cycles that afforded 85% (II recycle), 81% (III recycle) and 76% (IV recycle) yield of 3a (Table 1, entry [23][24][25], indicating the reuse and recyclability of the solvent.…”

“…[13] The epoxide ring opening with less reactive aromatic amines is reported in the presence of catalysts such as zinc tetrafluoroborate hydrate in solvent free condition [14] , Sc(OSO3C12H25)3 with chiral bipyridine ligand at room temperature in water [15] , zinc(II) perchlorate hexahydrate in solvent free condition [16] , aluminium triflate [17] , chiral zinc (II) and copper (II) [18] , lanthanide iodo binaphtholates, [19] bismuth trichloride [20] , tetrathiomolybdate [21] , antimony (III) chloride in dichloromethane at room temperature [22] , Cobalt(III) tetraphenylporphyrin chloride [23] , and lewis pair in toluene as a solvent at room temperature. [24] The obstacles associated with the ring opening of complex epoxides have been tackled with the use of heterogenous catalysts and metal triflates under microwave irradiation. [3,[25][26] However, the use of moisture and airsensitive catalysts, recovery of catalysts, requirement of stoichiometric amount of catalysts collectively limits the efficiency of these procedures.…”

Trapping of Difluorocarbene by Frustrated Lewis Pairs

“…Then, we switched to intramolecular version of FLP. Thus, a series of phosphines 2 bearing a dioxyboryl fragment in the ortho ‐position were prepared and evaluated in reaction with potassium bromodifluoroacetate (Scheme ). The reactions were performed in dimethylformamide at room temperature, and a series of compounds 3 a – c containing five‐membered phospha‐bora‐cycle were isolated in individual state after column chromatography.…”