Hyperdistorted Tungsten Allyl Complexes and Their Stereoselective Deprotonation to Form Dihapto-Coordinated Dienes

Abstract

Several π-allyl complexes of {TpW(NO)(PMe 3 )} are investigated as possible sources of η 2 -diene complexes. In order to prepare the diene complex as a single diastereomer, the allyl complex must undergo deprotonation stereoselectively. Allyl complexes of this tungsten system are highly distorted, with the difference between the WÀC bond lengths for the two allyl termini being as much as 0.69 Å. DFT calculations and several crystal structures are presented that collectively suggest that one terminus, C1, distal to the PMe 3 group, tends toward an sp 2 carbocation. Consistent with this interpretation, deprotonation preferentially occurs at a carbon adjacent to this allyl terminus for six-membered rings. However, in the presence of base the five-membered cyclic analogue [TpW(NO)(PMe 3 )(C 5 H 7 )] þ fails to form either isomer of the corresponding η 2 -diene complex.

Hydroamination of Dihapto‐Coordinated Benzene and Diene Complexes of Tungsten: Fundamental Studies and the Synthesis of γ‐Lycorane

“…Our synthesis ( Scheme 9) commenced with the diene 16 prepared from benzene complex 1 and the TBS‐protected enolate of methyl acetate. Treatment of the η 2 ‐diene 16 with an acetonitrile solution of triflic acid results in conversion to the η 2 ‐allyl complex 30P , which as described above, exists in equilibrium with its thermodynamically favored conformational isomer 30D [20] . In line with our earlier results, benzylamine 29 (prepared from piperonal) [40] purportedly reacts selectively with 30D to form the 3‐aminocyclohexene complex 31 .…”

“…The benzene ligand is competitively protonated at two different carbons, [14] resulting in two different arenium species, 2 and 3 . Each of these exists as two different conformational isomers in which two carbons are tightly bound by the tungsten but a third is loosely associated [20] . Herein, we indicate this weakly bound carbon as a carbocation in order to differentiate the two conformations.…”

“…While we speculate that subjecting this compound to an excess of strong acid first results in the protonation of the amino group, a second protonation can occur to form the allyl dication ( I , Scheme 4). [20] Gratifyingly, the addition of acid followed by the addition of the methoxide source Bu 4 NOH/MeOH at low temperature (−30 °C) provided the trisubstituted cyclohexene 13 .…”

Highly Functionalized Cyclohexenes Derived from Benzene: Sequential Tandem Addition Reactions Promoted by Tungsten

“…Such a system is best described as an η 2 -alkene complex with an adjacent carbocation stabilized by conjugation to the coordinated alkene (vide infra). 17 In both allyl isomers, this electrophilic carbon, represented herein as a carbocation (+), is localized distal to the PMe 3 .…”

“…Protonation was expected to generate a single π-allyl complex with two possible conformational isomers, as is characteristic of these asymmetric allyl complexes (Scheme 3). 17 The regioselectivity of the subsequent nucleophilic addition to this species could depend on the relative amounts of the two conformational isomers if outside of Curtin−Hammett conditions. 18 The diene complexes 6A, 9A, and 10A were treated with triflic acid (HOTf) in acetonitrile, and the resulting π-allyl complexes 11, 12, and 13 were precipitated from solution with Et 2 O.…”