Kinetics and mechanism of rhenium-catalyzed oxygen atom transfer from pyridine N-oxides to phosphines.

Abstract

The oxygen atom transfer (OAT) reaction cited does not occur on its own in >10 h. Oxorhenium(V) compounds having the formula MeReO(dithiolate)PZ(3) catalyze the reaction; the catalyst most studied was MeReO(mtp)PPh(3), 1, where mtpH(2) = 2-(mercaptomethyl)thiophenol. The mechanism was studied by multiple techniques. Kinetics (initial-rate and full-time-course methods) established this rate law: v = k(c)[1][PyO](2)[PPh(3)](-1). Here and elsewhere PyO symbolizes the general case XC(5)H(4)NO and PicO that with X = 4-Me. For 4-picoline, k(c) = (1.50 +/- 0.05) x 10(4) L mol(-1) s(-1) in benzene at 25.0 degrees C; the inverse phosphine dependence signals the need for the removal of phosphine from the coordination sphere of rhenium prior to the rate-controlling step (RCS). The actual entry of PPh(3) into the cycle occurs in a fast step later in the catalytic cycle, after the RCS; its relative rate constants (k(4)) were evaluated with pairwise combinations of phosphines. Substituent effects were studied in three ways: for (YC(6)H(4))(3)P, a Hammett correlation of k(c) against 3sigma gives the reaction constant rho(c)(P) = +1.03, consistent with phosphine predissociation; for PyO rho(c)(N) = -3.84. It is so highly negative because PyO enters in three steps, each of which is improved by a better Lewis base or nucleophile, and again for (YC(6)H(4))(3)P as regards the k(4) step, rho(4) = -0.70, reflecting its role as a nucleophile in attacking a postulated dioxorhenium(VII) intermediate. The RCS is represented by the breaking of the covalent N-O bond within another intermediate inferred from the kinetics, [MeReO(mtp)(OPy)(2)], to yield the dioxorhenium(VII) species [MeRe(O)(2)(mtp)(OPy)]. A close analogue, [MeRe(O)(2)(mtp)Pic], was identified by (1)H NMR spectroscopy at 240 K in toluene-d(8). The role of the "second" PyO in the rate law and reaction scheme is attributed to its providing nucleophilic assistance to the RCS. Addition of an exogenous nucleophile (tetrabutylammonium bromide, Py, or Pic) caused an accelerating effect. When Pic was used, the rate law took on the new form v = k(NA)[1][PicO][Pic][PPh(3)](-1); k(NA) = 2.6 x 10(2) L mol(-1) s(-1) at 25.0 degrees C in benzene. The ratio k(c)/k(NA) is 58, consistent with the Lewis basicities and nucleophilicities of PicO and Pic.