Synthesis of a Bench-Stable Manganese(III) Chloride Compound: Coordination Chemistry and Alkene Dichlorination.

The complex [MnCl3(OPPh3)2] (1) is a bench-stable and easily prepared source of MnCl3. It is prepared by treating acetonitrile solvated MnCl3 (2) with Ph3PO and collecting the resulting blue precipitate. 1 is useful in coordination reactions by virtue of the labile Ph3PO ligands, and this is demonstrated through the synthesis of {Tpm*}MnCl3 (3). In addition, methodologies in synthesis that rely on difficult or cumbersome to prepare solutions of reactive MnCl3 can be accomplished using 1 instead. This is demonstrated through alkene dichlorinations in a wide range of solvents, open to air, and with good substrate scope. Light-accelerated halogenation and radical sensitive experiments support a radical mechanism involving stepwise Cl-atom transfer(s) from 1.

From Resting State to the Steady State: Mechanistic Studies of Ene-Yne Metathesis Promoted by the Hoveyda Complex.

The kinetics of intermolecular ene-yne metathesis (EYM) with the Hoveyda precatalyst (Ru1) has been studied. For 1-hexene metathesis with 2-benzoyloxy-3-butyne, the experimental rate law was determined to be first-order in 1-hexene (0.3-4 M), first-order in initial catalyst concentration, and zero-order for the terminal alkyne. At low catalyst concentrations (0.1 mM), the rate of precatalyst initiation was observed by UV-vis and the alkyne disappearance was observed by in situ FT-IR. Comparison of the rate of precatalyst initiation and the rate of EYM shows that a low, steady-state concentration of active catalyst is rapidly produced. Application of steady-state conditions to the carbene intermediates provided a rate treatment that fit the experimental rate law. Starting from a ruthenium alkylidene complex, competition between 2-isopropoxystyrene and 1-hexene gave a mixture of 2-isopropoxyarylidene and pentylidene species, which were trappable by the Buchner reaction. By varying the relative concentration of these alkenes, 2-isopropoxystyrene was found to be 80 times more effective than 1-hexene in production of their respective Ru complexes. Buchner-trapping of the initiation of Ru1 with excess 1-hexene after 50% loss of Ru1 gave 99% of the Buchner-trapping product derived from precatalyst Ru1. For the initiation process, this shows that there is an alkene-dependent loss of precatalyst Ru1, but this does not directly produce the active catalyst. A faster initiating precatalyst for alkene metathesis gave similar rates of EYM. Buchner-trapping of ene-yne metathesis failed to deliver any products derived from Buchner insertion, consistent with rapid decomposition of carbene intermediates under ene-yne conditions. An internal alkyne, 1,4-diacetoxy-2-butyne, was found to obey a different rate law. Finally, the second-order rate constant for ene-yne metathesis was compared to that previously determined by the Grubbs second-generation carbene complex: Ru1 was found to promote ene-yne metathesis 62 times faster at the same initial precatalyst concentration.

Ruthenium hydride-promoted dienyl isomerization: access to highly substituted 1,3-dienes.

Ruthenium hydrides were found to promote the positional isomerization of 1,3-dienes into more highly substituted 1,3-dienes in a stereoconvergent manner. The reaction can be conducted in one pot starting with terminal alkynes and alkenes by triggering decomposition of the Grubbs catalyst into a ruthenium hydride, which promotes the dienyl isomerization. The presence of an alcohol additive plays a helpful role in the reaction, significantly increasing the chemical yields. Mechanistic studies are consistent with hydrometalation of the geminally substituted alkene of the 1,3-diene and transit of the ruthenium atom across the diene framework via a π-allylruthenium intermediate.