Selectively converting linear alkanes to α-olefins under mild conditions is a highly desirable transformation given the abundance of alkanes as well as the use of olefins as building blocks in the chemical community. Until now, this reaction has been primarily the remit of noble-metal catalysts, despite extensive work showing that base-metal alkylidenes can mediate the reaction in a stoichiometric fashion. Here, we show how the presence of a hydrogen acceptor, such as the phosphorus ylide, when combined with the alkylidene complex (PNP)Ti=CHBu(CH) (PNP=N[2-P(CHMe)-4-methylphenyl]), catalyses the dehydrogenation of cycloalkanes to cyclic alkenes, and linear alkanes with chain lengths of C to C to terminal olefins under mild conditions. This Article represents the first example of a homogeneous and selective alkane dehydrogenation reaction using a base-metal titanium catalyst. We also propose a unique mechanism for the transfer dehydrogenation of hydrocarbons to olefins and discuss a complete cycle based on a combined experimental and computational study.

In this report, we have demonstrated proof-in-principle that simple dimethyl metal complexes could be used to generate perfluorometallacyclobutane derivatives. With the new methodology, we have more than doubled the number of known perfluorometallacyclobutanes in the literature. We have also structurally characterized three platinum perfluorometallacyclobutanes and have described a unique transformation whereby one [Pt­(C3F6)] functionality is converted to [Pt­(C3F4O2)]. The new methodology to prepare perfluorometallacyclobutanes also avoids the use of tetrafluoroethylene, which is becoming increasingly difficult to purchase due to the explosion hazards associated with its use.

The complex (PNP)Ti═CHBu(CHBu) (PNP = N[2-PPr-4-methylphenyl]) dehydrogenates cyclohexane to cyclohexene by forming a transient low-valent titanium-alkyl species, [(PNP)Ti(CHBu)], which reacts with 2 equiv of quinoline (Q) at room temperature to form HCBu and a Ti(IV) species where the less hindered C═N bond of Q is ruptured and coupled to another equivalent of Q. The product isolated from this reaction is an imide with a tethered cycloamide group, (PNP)Ti═N[CHN] (1). Under photolytic conditions, intramolecular C-H bond activation across the imide moiety in 1 occurs to form 2, and thermolysis reverses this process. The reaction of 2 equiv of isoquinoline (Iq) with intermediate [(PNP)Ti(CHBu)] results in regioselective cleavage of the C═N and C-H bonds, which eventually couple to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)Ti(CHBu)] complex can ring-open and couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)Ti═N[CHN] (4). Multinuclear and multidimensional NMR spectra confirm structures for complexes 1-4, whereas solid-state structural analysis reveals the structures of 2, 3, and 4. DFT calculations suggest an unprecedented mechanism for ring-opening of Q where the reactive intermediate in the low-spin manifold crosses over to the high-spin surface to access a low-energy transition state but returns to the low-spin surface immediately. This double spin-crossover constitutes a rare example of a two-state reactivity, which is key for enabling the reaction at room temperature. The regioselective behavior of Iq ring-opening is found to be due to electronic effects, where the aromatic resonance of the bicycle is maintained during the key C-C coupling event.

We report the synthesis and structure of a titanium(iv) benzophenone adduct bearing a terminal oxo ligand, (PNP)Ti = O(OTf)(OCPh) (PNP = N[2-PPr-4-methylphenyl]) (2). Complex 2 is readily synthesized in 71% yield from the previously reported titanium alkylidene (PNP)Ti[double bond, length as m-dash]CHBu(OTf) (1) and two equivalents of benzophenone, by extruding the olefin PhC[double bond, length as m-dash]CHBu. Treatment of benzophenone adduct 2 with a bulky aryloxide salt results in formation of (PNP)Ti[double bond, length as m-dash]O(OAr) (3) (OAr = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide), concurrent with salt elimination and displacement of the benzophenone. Complexes 2 and 3 were characterized by single-crystal X-ray diffraction and a variety of spectroscopic techniques, including NMR, UV-Vis-NIR spectroscopies, and TD-DFT calculations.