Rates of Hydroxide Decomposition of Cyclic Phosphonium Salts

Abstract

The rates of hydroxide decomposition of 1-benzyl-l-phenylphospholanium bromide (8), 1-benzyl-1-phenylphosphorinanium bromide (9), benzylethylmethylphenylphosphonium iodide (10) and seven substituted phosphetanium salts 1-7 have been measured in 1:1 ethanol-water at various temperatures. A marked rate enhancement was observed in going from the larger to the smaller rings; the rate of the acyclic salt 10 at 25°was comparable to the six-membered ring compound. Changes in both AH * and ASwere responsible for the rate variations; expected trends as well as apparent anomalies reflect the complexity of these overall, third-order reactions. Ring strain, the inductive effect of substituents on the ring and at phosphorus, and the degree of carbanion character adopted by the leaving group partially account for the observed activation parameters.

Synthesis and Properties of Phosphetanes

“…Thus, for example, Pbenzylphosphetanium bromides are hydrolyzed to the corresponding oxides 10 4 times faster than analogous phospholanium bromides. 150 This reflects relief of ring strain in the intermediate phosphorane. Ring strain also accelerates the alkaline hydrolysis of phosphinate esters (10 2 times faster than acyclic analogues).…”

“…It is noteworthy that alkaline hydrolysis rates for cyclic phosphonium salts increase significantly upon moving from large ring or acyclic compounds to smaller ring compounds. Thus, for example, P -benzylphosphetanium bromides are hydrolyzed to the corresponding oxides 10 4 times faster than analogous phospholanium bromides . This reflects relief of ring strain in the intermediate phosphorane.…”

Biphilic Organophosphorus Catalysis: Regioselective Reductive Transposition of Allylic Bromides via PIII/PV Redox Cycling

“…Evidently, direct uncatalyzed reduction of 3 with LiAlH­(O t Bu) 3 outcompetes notional biphilic catalysis by triphenylphosphine. Following on Gallagher’s hypothesis and abundant precedent demonstrating enhanced electrophilic reactivity of cyclic phosphorus-based compounds compared to their acyclic congeners, the conversion σ 4 -P + → σ 5 -P was targeted for acceleration. While neither dibenzophosphole 8 nor phospholane 9 derivatives (proven catalyst motifs in several catalytic P III /P V O redox methods) satisfactorily alter the reduction outcome, catalytic loading of phosphonium salt derived from a four-membered phosphetane 10 , delivers reduction products with excellent γ/α-selectivity.…”

Hybrid P,E Ligands: Synthesis, Coordination Chemistry and Catalysis

“…As mentioned previously, P−C bond cleavage of five-membered cyclic phosphonium salts with various hydroxides, 78,79 producing the corresponding phosphine oxides, is a known reaction type. This suggests that compound 1 did not react with sodium methoxide, but rather slow formation of sodium hydroxide occurred in solution due to the presence of water, which in turn reacted with compound 1, cleaving one of the benzylic P−C bonds and producing compound 2.…”

“…This compound is similar to known cyclic dimethyl-and diphenyl-substituted phosphonium bromides. 76 Other than the treatment of these compounds with methylenetrimethylphosphorane 76 or potassium t-butoxide 77 to form cyclic phosphorus ylides, and P−C bond cleavage with various hydroxides, 78,79 the reactivity of these five-membered cyclic phosphonium salts has not been widely studied. The direct reaction of cyclic phosphonium bromide 1 with lithium metal in THF followed by quenching with water resulted largely in the recovery of unchanged starting material (Scheme 2.5).…”