Understanding intermolecular C–F bond activation by a transient titanium neopentylidyne: experimental and theoretical studies on the competition between 1,2-CF bond addition and [2 + 2]-cycloaddition/β-fluoride elimination

Abstract

Complex (PNP)TivCHBu) is treated with meta-fluoropyridine, the ring-opened product (PNP)Ti(C( In all cases, C-H bond addition of ortho-fluoropyridines or meta-fluoropyridine is discouraged because such substrate must bind to titanium via its C-H bond, which is rather weak compared to the titanium-pyridine binding.

Regioselective C–F Bond Activation/C–C Bond Formation between Fluoropyridines and Cyclopropyl Groups at Zirconium

“…At early transition metals, both selectivities have been observed depending on the mechanism of the C–F cleavage. 1,2-C–F addition is observed at unsaturated titanium alkylidyne, TiCR, and zirconium imido, ZrNR, complexes. DFT computations account for the selectivity for the 2-position in the titanium case: the first high-energy intermediate involving N coordination of 2-fluoropyridine leads by insertion in the TiCR bond (alternatively [2 + 2]-cycloaddition or oxidative coupling) to an azatitanacyclobutene, which undergoes β-C–F activation to give the alkylidene fluoride product [LTiF­(κ 2 -CR-2-C 5 H 4 N)] .…”

“…1,2-C–F addition is observed at unsaturated titanium alkylidyne, TiCR, and zirconium imido, ZrNR, complexes. DFT computations account for the selectivity for the 2-position in the titanium case: the first high-energy intermediate involving N coordination of 2-fluoropyridine leads by insertion in the TiCR bond (alternatively [2 + 2]-cycloaddition or oxidative coupling) to an azatitanacyclobutene, which undergoes β-C–F activation to give the alkylidene fluoride product [LTiF­(κ 2 -CR-2-C 5 H 4 N)] . Dinuclear oxidative addition of pentafluoropyridine at [Cp 2 Ti­(Me 3 SiCCSiMe 3 )] occurs at position 2, giving [Cp 2 Ti­(μ-F)­(μ-2-C 5 F 4 N)­TiCp 2 ], possibly via a putative [Cp 2 TiF­(2-C 5 F 4 N)] intermediate, whereas oxidative addition to the analogous zirconium species [Cp 2 Zr­(Me 3 SiCCSiMe 3 )­(NC 5 H 5 )] affords [Cp 2 ZrF­(4-C 5 F 4 N)] .…”

“…This computed pathway (Figure ) has strong bearings with that computed for C–F bond activation of 2-fluoropyridine with the titanium alkylidyne discussed above. In the titanium case (Scheme d), a four-membered azametallacyclic intermediate accounts for the regioselectivity and the C–F bond cleavage step, whereas a five-membered ring is involved in the zirconium case. The alkylidene group in the former complex is replaced by the redox equivalent cyclopropadiyl group in the latter.…”

“…Reactions c and d represent ligand coupling reactions at unsaturated highoxidation-state complexes. A zirconium imido 27 (reaction c) or a titanium alkylidyne 28 We have established that an intramolecular β-H abstraction reaction from the niobium and zirconium alkyl cyclopropyl complexes [L n MR(c-C 3 H 5 )] generates the unsaturated reactive η 2 -cyclopropene/metallabicyclobutane intermediates [L n M(η 2c-C 3 H 4 )]. 29−32 Those intermediates are able to cleave a variety of hydrocarbon C−H bonds, including that of methane, by the mechanistic reverse, 1,3-CH bond addition.…”

“…Reactions c and d represent ligand coupling reactions at unsaturated high-oxidation-state complexes. A zirconium imido (reaction c) or a titanium alkylidyne (reaction d), both generated by intramolecular α-H abstraction by an alkyl group, are selectively coupled at position 2 of the fluoropyridine with C–N or C–C bond formation. DFT computations in the titanium case indicated [2 + 2] cycloaddition (or oxidative coupling) occurred preferentially to direct 1,2-C–F bond addition across the TiC bond, accounting for the observed regioselectivity.…”