Structure of allylic Grignard reagents

Abstract

A series of 7-substituted allylic Grignard reagents (Me, Et, z'-Pr, Z-Bu) and a,7-dimethylallylmagnesium chloride were studied by proton nmr spectroscopy. These Grignards exist as a rapidly equilibrating mixture of Z and E primary stereoisomers (structure 1). Temperatures as low as -80°were not sufficient to stop this rapid interconversion. Protonation and coupling constant studies show that the percentage of Z isomer decreases as the size of the alkyl substituent increases: methyl, 57%; ethyl, 42%; isopropyl, 27%; tert-butyl, ~1 %. At low temperatures, the Schlenk equilibrium was detected. The stereochemical behavior of the diallyl-

Effects of hexamethylphosphoric triamide (hempa) upon allylic grignard reagents: Synthesis of long chain alkenol acetates

“…We also prepared the Grignard reagent of (E)-l-chloro-2undecene in THF and, after adding Hempa to aliquots of this solution, quenched them with I% hydrochloric acid (HC1). The ratio of (Z)-to (E)-2-undecene we obtained in THF alone was similar to that reported by Grutzner from 2-penten-2-yl magnesium halide (7). (Table II).…”

“…The most rigorous investigation of the nature of allylic Grignard reagents (7) revealed them to be rapidly equilibrating Z-E mixtures with some evidence that magnesium is bonded directly to the primary carbon atom. In that study, the Z-E ratios were determined by NMR spectroscopy directly with the equilibrated allylic Grignard isomers and indirectly by protonation of the mixture and determination of the resulting A2-alkenes.…”

Reactions of Allylmagnesium Halides with Carbonyl Compounds: Reactivity, Structure, and Mechanism

“…83 Substituted allylmagnesium reagents show a much lower preference for the RMgX form in Et 2 O. 84 These studies indicate that the presence of diallylmagnesium species 85 in reaction mixtures needs to be considered.…”

“…84 Due to the electronic nature of the allylic moiety, these reagents could be represented as either of two rapidly equilibrating η 1 -allyl isomers (39a,b) or a single η 3 -allyl structure (40, Scheme 10). Early 1 H NMR spectroscopic studies 84 of substituted allylmagnesium halides revealed vicinal coupling constants (resulting from an averaging of J 1,2a and J 1,2b , with one anti relationship and one gauche relationship) of about 9 Hz, which would be expected if the magnesium atom were localized to one carbon atom ( Figure 2). The conformation expected as a result of these coupling constants, 41, would be stabilized by hyperconjugation between the carbonmagnesium bond and the π* orbital of the alkene.…”

Genarris: Random generation of molecular crystal structures and fast screening with a Harris approximation

“…This has been demonstrated as a reasonable approximation for the treatment dimers of weakly interacting molecules with dispersion-inclusive DFT in the van der Waals regime, where there is no significant density overlap or polarization. 38,100,101 Genarris uses the HA to construct the density of a molecular crystal by replicating, translating, and rotating the self-consistent density of a single molecule, which is calculated only once.…”