Studies on Electrophilic Reaction of Tertiary 2,3-Allenols with NBS in H2O or Aqueous MeCN: An Efficient Selective Synthesis of 2-Bromoallylic Ketones, 1,2-Allenyl Ketones, or 3-Bromo-2,5-dihydrofurans

Previously, we observed that the electrophilic reaction of 2,3-allenols with X+ (X = Br, I) affords 3-halo-2-alkenals or 2-halo-2-alkenyl ketones in aqueous MeCN (MeCN/H2O = 15: 1). However, the reaction of tertiary 2,3-allenols with NBS under these reaction conditions affords a mixture of rearrangement products (aldehydes or ketones) together with 1-bromovinyl epoxides in a low selectivity. Due to the synthetic potential of 2-haloallylic ketones, we decided to explore this reaction further. After screening, we observed that the electrophilic reaction of terminal tertiary 2,3-allenols with NBS in water affords 2-bromoallylic ketones highly selectively in up to 97% yields via a sequential electrophilic interaction of Br+ with the allene moiety to form a possible three-membered bromonium intermediate with the more substituted C=C bond, which would be followed by 1,2-aryl or 1,2-alkyl shift to open the strained three-membered ring and proton elimination to form the carbonyl functionality, When both R-1 and R-2 (the substituents on the carbon atom connected to the hydroxyl groups) are alkyl groups, one of these two groups is migrated; with 1,2-propadienyl cycloalkanols, a ring expansion reaction was observed; with R-1 being an aryl group, R-1 would be transferred exclusively to form the 2-bromoallylic ketones. Interestingly, these 2-bromo-1-aryl-2-propenyl ketones may easily be converted into 1,2-allenyl ketones after column chromatography on silica gel pre-eluented with 10 drops of Et3N; when there is at least an alkyl substituent on the 4-position of the tertiary 2,3-allenols, their electrophilic reaction with NBS in CH3CN/H2O = 15/1 or H2O, under the same reaction conditions as above, affords 3-bromo-2,5-dihydrofurans in 61-84% yields, indicating that the electronic effect and the steric effect of the two C=C bonds determine the reaction pathway, i.e., the Br+ interacts with the C=C bond at the 3-position.