Reaction mechanisms, Ether cleavage, Density functional calculations
Life Sciences | Medicine and Health Sciences
Boron tribromide is a versatile reagent utilized in diverse areas ranging from polymer chemistry to natural product synthesis. Owing its high reactivity to the Lewis acidic boron center, BBr3 reactions include haloborylation, boron–silicon exchange, and rearrangement of 7,7-diphenylhydromorphone derivatives. While there is no shortage in the diversity of BBr3-mediated reactions, many of the mechanisms for these transformations have not been fully elucidated. In this report we investigate the mechanism of ether cleavage by BBr3 [5–10] in anisole. Conceptually, demethylation of anisole is initiated by the formation of an ether adduct 1 followed by the loss of bromide. Free bromide nucleophilically attacks the methyl group of the cationic intermediate (2) cleaving the C–O bond and producing PhOBBr2, which undergoes hydrolysis upon aqueous work-up. While this pathway (Scheme 1) at first appears to be viable, we calculated that the formation of 2 and bromide in dichloromethane is thermodynamically inaccessible (ΔG = +38.9 kcal/mol). Recently, alternative mechanisms for ether cleavage were proposed by Sousa and Silva that involve unimolecular or bimolecular rate-determining steps that circumvent formation of bromide in solution (Scheme 2). While a unimolecular process is kinetically favored for ethers containing one or more substituents (e.g. branched alkyl) that stabilize carbocation character in an SN1-like transition state, this barrier for demethylation of primary C atoms, like in the
methyl group of anisole, lies too high on the potential energy surface to be accessible under reported reaction conditions. They found that a bimolecular process (Scheme 2, bottom) decreases the kinetic barrier for anisole demethylation significantly. During this reaction pathway, one of the bromides of the first ether adduct nucleophilically attacks the methyl group of the second ether adduct. This is analogous to an SN2 reaction with 180o attack of the methyl group by a bromide in the nucleophilic ether adduct. However, this bimolecular pathway produces two highly charged intermediates 2 and 3 that Sousa and Silva did not investigate. Their computational investigation stopped with the calculation of the initial kinetic barrier. We speculate that these charged intermediates may undergo a similar bimolecular reaction to yield two equivalents of PhOBBr2 and MeBr. Moreover, if charged intermediates are formed then we believe an important set of mechanistic pathways may have been overlooked, namely, those where Lewis acidic BBr3 abstracts bromide from the ether complex to form BBr4 – in a mechanism related to the pathway introduced in Scheme 1.
Lord, Richard L.; Korich, Andrew L.; Kosak, Talon M.; and Conrad, Heidi A., "Ether Cleavage Re-Investigated: Elucidating the Mechanism of BBr3- Facilitated Demethylation of Aryl Methyl Ethers" (2015). Funded Articles. 37.