Coupling and Dearomatization of Pyridines at a Transient η²‐Cyclopropene/Bicyclobutane Zirconocene Complex

Abstract

This paper reports on stereospecific coupling reactions between an η -cyclopropene ligand and pyridine derivatives, which are preferred to alternative C-H bond activation reactions. The dicyclopropylzirconocene complex [Cp Zr(c-C H ) ] (1) eliminates cyclopropane to generate the η -cyclopropene/bicyclobutane intermediate [Cp Zr(η -c-C H )] (A). A does not activate any pyridine C-H bonds, but rather pyridine inserts into a Zr-C bond of A, yielding an azazirconacycle with a dearomatized pyridyl group [Cp Zr{κ -N,C -(2-c-C H )-C H N}] (2). Kinetic data, isotopelabelling experiments, and DFT calculations indicate that the rate-determining step of this stereospecific reaction is cyclopropane elimination, and that the stability of the intermediate [Cp Zr(η -c-C H )(NC H )] (A-py) governs the selectivity of the reaction. Complex 2 tautomerizes to [Cp Zr{κ -N,C -(2-C H )-C H N}] (6) through a base-catalyzed proton migration accompanied by cyclopropyl opening and restoration of conjugation within the zirconacycle.

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

“…Pentafluoropyridine is a weak Brønsted base and, if not included in a bidentate system, does not coordinate readily to transition metals, although a few examples, sometimes very labile, are known. , In connection with the involvement of the pyridine adduct of A [Cp 2 Zr­(η 2 - c -C 3 H 4 )­(C 5 H 4 N)] in the insertion of pyridine into the Zr–C bond of A , we surmised that a similar, perhaps more fleeting, intermediate might induce preference for C–F activation at the 2-position. Figure summarizes the two pathways that we have computed starting from 1 .…”

“…C–F bond activation occurs at position 2 of the fluorinated pyridines: a remarkable selectivity, as functionalization at position 4 is normally preferred. The C–F bond cleavage itself is not probed from these experiments, but DFT computations (see below) suggest that insertion of the fluoropyridine in the Zr–C bond of A (or oxidative coupling), not direct C–F bond activation, may be the preferred pathway.…”

“…The elimination of cyclopropane through the transition state β-H_TS to give A has been computed previously . From intermediate A , the pathways involve either direct C–F bond activation by A just presented above (pictured in black) or, as observed for pyridine, insertion of the fluorinated pyridine into a Zr–C bond prior to C–F activation and ring opening (pictured in red). Computations suggest the latter would be preferred kinetically by 9 kJ/mol (ΔΔ G 298 = 7 kJ/mol) despite the formation of the intermediate Int .…”

“…22 DFT computations (optimizations: PBE0, Zr SDD/F, N, C, and H TZVP; energies at 1 atm and 298 K: single point with dispersion and solvent corrections PBE0-GD3BJ; def2-QZVP; SMD cyclohexane) were undertaken in order to (i) probe the selectivity for C−F activation at the 4-vs the 2-position and (ii) propose a reasonable mechanism accounting for the selective C−C bond formation. As in related bond activation reactions, 31,37 experimental results indicate that cyclopropane is generated in a rate-determining step from 1, giving the η 2cyclopropene/zirconabicyclobutane intermediate A, which then reacts with the fluoropyridine to yield the product. We Pentafluoropyridine is a weak Brønsted base and, if not included in a bidentate system, does not coordinate readily to transition metals, although a few examples, sometimes very labile, 48 are known.…”

Allyl Complexes of Tungsten from the Rearrangement of Transient Cyclopropyl Precursors

“…The reaction is actually the intramolecular abstraction of a β‐H by a hydrocarbyl group R cis to the cyclopropyl group in the initial complex (Scheme ). The η 2 ‐cyclopropene/metallabicyclobutane intermediate can be trapped or undergo functionalization reactions such as oxidative coupling/insertion of unsaturated organic substrates . Zirconium and niobium η 2 ‐cyclopropene/metallabicyclobutane intermediates thermally generated from well‐defined cyclopropyl derivatives such as [Cp 2 Zr( c ‐C 3 H 5 ) 2 ] and [Tp Me2 NbR( c ‐C 3 H 5 )(MeCCMe)] [Tp Me2 = hydrotris(3,5‐dimethylpyrazolyl)borate] can cleave a variety of CH bonds of unsaturated hydrocarbons R'H.…”