Multi-configuration spin-coupled description of organometallic reactions: a comparative study of the addition of RMBr (M = Mg and Zn) to acetone

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

“…In all cases, the carbonmagnesium σ-bond of an allylmagnesium 88,[90][91][92] or dienylmagnesium 91 species is oriented such that overlap with the carbon-carbon double bond can occur (as illustrated for 41). This feature, which is reproduced by density functional calculations of allylmagnesium bromide, 93 is illustrated by the structures of solvated species 43 and 45 ( Figure 3). The allyl groups of these complexes adopt orientations where the magnesium atom is positioned above the allyl group (as suggested by the spectroscopic studies).…”

“…Computational studies provide more insight into the structure of allylmagnesium reagents. Calculations of allylmagnesium hydride 99 and allylmagnesium bromide 93 confirm that the η 1 -allyl structure is favored, but isomerization should be rapid. 99 On the other hand, diallylmagnesium has been calculated to include two η 3 -allyl units.…”

“…120 More recently, higher-level computations evaluated the mechanism for addition, but, again, transition states were examined for monomeric species in the absence of solvent, whereas solvent and aggregates are likely to be important to the addition pathway. 93 Nevertheless, these studies indicate that the concerted, six-membered transition state 52 (Figure 4), involving allylic transposition, is responsible for the allylation reaction. In this sense, the mechanism of the addition of allylmagnesium halides to carbonyl compounds shares similarities with reactions involving allylic boranes and some allylic silanes.…”

Allylmagnesium Halides Do Not React Chemoselectively Because Reaction Rates Approach the Diffusion Limit

“…54 Experimental results have been used to support an open, S E 2′like transition state (B), 44 although computational studies suggest that a closed six-membered ring transition state, such as C, is favored. 55 Regardless of the mechanism, which could depend upon the electrophile, addition must be particularly rapid to be consistent with the reactivity-selectivity relationships reported here.…”

“…Single-electron transfer may occur in reactions with aromatic carbonyl compounds, forming species such as A . ,, By contrast, single-electron pathways are unlikely to be involved in additions to aliphatic carbonyl compounds . Experimental results have been used to support an open, S E 2′-like transition state ( B ), although computational studies suggest that a closed six-membered ring transition state, such as C , is favored . Regardless of the mechanism, which could depend upon the electrophile, addition must be particularly rapid to be consistent with the reactivity-selectivity relationships reported here.…”