[133834-87-4] · C3H4BrI · (Z)-3-Bromo-1-iodopropene · (MW 246.87)
Physical Data: colorless liquid, distillation temperature (bulb-to-bulb) 25-45 °C/0.2 mmHg.
Solubility: insol H2O; sol pentane, Et2O, THF, CH2Cl2, CHCl3.
Preparative Methods: (Z)-3-bromo-1-iodopropene (3) is readily prepared1 by treatment of the corresponding alcohol (2) with Triphenylphosphine Dibromide in CH2Cl2 (eq 1).3 Although a number of useful procedures for the synthesis of (2) have been reported,4 probably the most convenient involves reduction of methyl (Z)-3-iodopropenoate (1)5 with Diisobutylaluminum Hydride (eq 1).1
Handling, Storage, and Precautions: when stored in a freezer over copper wire under an inert atmosphere, this reagent is stable for months. It decomposes upon exposure to heat and/or light. A slight pink coloration does not affect its reactivity, but the reagent should be distilled from dried basic alumina and copper wire just prior to use. This substance is a lachrymator and should be prepared and handled only in a fume hood.
The use of reagent (3) in performing cyclopentenone annulations involves three steps: alkylation of a carbonyl compound with (3), n-Butyllithium-mediated ring closure6 of the resultant keto vinyl iodide to a tertiary allylic alcohol, and oxidative rearrangement of the latter species with a CrVI reagent to afford the 2-cyclopenten-1-one. The method is illustrated by the conversion of keto ester (4) into the bicyclic enone (7) (eq 2).2 In those cases in which direct alkylation of a ketone substrate is problematic, nitrogen-containing analogs (e.g. N,N-dimethylhydrazones)7 may be employed effectively (eq 3).2
This annulation method played a key role in a recent total synthesis of the tetraquinane diterpenoid (±)-crinipellin B (8).1 Attempts to effect conversion of the intermediate (9) into the enone (10) via the application of a number of known cyclopentenone annulation methods8 were unsuccessful.1 However, alkylation of (9) with (3) and subsequent treatment of the resultant product with n-BuLi produced, efficiently, the allylic alcohol (11). Oxidation of (11) afforded the enone (12) as the major product (eq 4).1 Interestingly, the latter reaction, in addition to effecting the expected transformation (enone formation), caused oxidative conversion of the (hindered) silyl ether group into the corresponding ketone function. The tetracyclic enone (12) served as a suitable intermediate for the total synthesis of (8).1
Edward Piers & Christine Rogers
University of British Columbia, Vancouver, BC, Canada