Dehydrogenation of hydrocarbons with metal-carbon multiple bonds and trapping of a titanium(ii) intermediate

Abstract

Reacting (PNP)Ti[double bond, length as m-dash]CH(t)Bu(CH2(t)Bu) with 2,2'-bipyridine (bipy) in cyclohexane or heptane results in dehydrogenation, cleanly producing cyclohexene and 1-heptene, respectively, and a Ti(II) intermediate that is trapped by bipy to produce [(PNP)Ti(III)(CH2(t)Bu)(bipy˙(-))] (1). This titanium(ii) intermediate reduces the bipy ligand upon coordination to form a Ti(III) center, where the unpaired electron is antiferromagnetically coupled to the electron of the reduced [bipy˙(-)] π-radical moiety, giving an overall diamagnetic species. Complex 1 has been characterized by NMR and UV-vis spectroscopies as well as single crystal X-ray diffraction studies.

Room temperature olefination of methane with titanium–carbon multiple bonds

“…A UV-Vis spectrum of 3 shows a strong absorption band at 565 nm ( ε = 8370 M –1 cm –1 ), which we propose contributes to the violet color. 18 The latter property is in accord with this species possessing a π-radical bipy ligand and we assign this feature as an MLCT, consistent with 3 having a Ti( iii ) radical antiferromagnetically coupled to a bipy˙ – . 18 , 19 …”

Room-Temperature Ring-Opening of Quinoline, Isoquinoline, and Pyridine with Low-Valent Titanium

“…Unlike Py and picoline, which readily undergo [2+2]-cycloaddition across the alkylidyne ligand in A , the more hindered Q does not react, therefore allowing dehydrogenation of the solvent medium to take place instead. We propose that dissociation of cyclohexene from C produces the low-valent and unsaturated complex [(PNP)­Ti­(CH 2 t Bu)] ( D ), which then activates, ring-opens, and couples two Q molecules to ultimately produce 1 . To probe whether an intermediate such as D (or the olefin adduct C ) is activating Q , we prepared a more stable olefin complex, [(PNP)­Ti­(CH 2 t Bu)­(η 2 -H 2 CCH n Hex)], from n -octane dehydrogenation via A , as a mixture of two diastereomers (Scheme ).…”

Modern applications of low-valent early transition metals in synthesis and catalysis

“…5b]. This system takes advantage of 1,2-C-H addition of the alkane C-H bond to a Ti alkylidiyne 50 , followed by β-H abstraction of the resultant Ti-alkyl-alkylidene ( 51 ) to formally reduce Ti, making a new Ti 11 alkyl-alkene adduct 52 (REFS 40–41–203–204 ). Remarkably, this system is rendered catalytic by addition of a methylene transfer agent, phenyldibenzophosphole methylene ylide, H 2 CP(C 12 H 8 )Ph ( 48 ), which serves as an oxidant to reoxidize the Ti II fragment ( 53 ) back to a Ti IV alkylidene ( 54 ) 202 .…”