[7301-95-3] · C21H20BrP · Isopropenyltriphenylphosphonium Bromide · (MW 383.28)
(Michael-Wittig combinatorial reagent for preparation of 2-methyl allylic ethers,1 3-methyl-2H-1-benzopyrans,1 3-methyl-2-cyclopentenones,2 3-methyl-2,5-dihydrothiophenes,3 and 1-methyltricyclo[3.2.1.02,7]octan-6-ones4)
Alternate Name: ITB.
Physical Data: mp 195-197 °C.
Solubility: sol alcohol, CH2Cl2, DMF, DMSO.
Preparative Method: prepared by fusion of Triphenylphosphine with anhydrous Nickel(II) Bromide followed by addition of dry benzonitrile and azeotropic removal of the residual water, addition of 2-bromopropene, and heating of the resultant mixture from 50-80 °C to 200 °C over a period of 2 d.1 Workup includes steam distillation to remove benzonitrile, filtration of a black insoluble material, and extraction of the filtrate with CH2Cl2. After drying and concentration at reduced pressure, the crude solid thus obtained is purified by recrystallization from CH2Cl2-EtOAc.
Although Michael addition of organolithiums to ITB had been known earlier,5 the synthetic utility of ITB was only realized when Schweizer1 reported his results on comparison studies of ITB with Vinyltriphenylphosphonium Bromide (VTB). As in the case of VTB, the treatment of ITB with sodium ethoxide and benzaldehyde in DMF gave the corresponding chain-extended product in 71% yield via the Michael-Wittig reaction sequence (eq 1).
When the nucleophile used in the Michael-Wittig reaction sequence with ITB contains a carbonyl moiety, cyclization of the phosphorane intermediate occurs to give rise to a variety of methyl-substituted cyclic compounds. Thus 3-methyl-2H-1-benzopyran1 (eq 2), 3-methyl-2,5-dihydrofuran1 (eq 3), 2-methyl-3H-pyrrolizines1 (eq 4), 3-methyl-2-cyclopentenones2 (eq 5), and 3-methyl-2,5-dihydrothiophenes3 (eq 6) can all be prepared.
Bicycloannulation of 2-cyclohexenones with ITB provides 1-methyltricyclo[3.2.1.02,7]octan-6-ones via Michael addition followed by intramolecular cyclopropanation (eq 7).4 In addition, 3-methylpyrazole hydrobromide has been obtained via 1,3-dipolar cycloaddition of ITB with Diazomethane followed by thermal decomposition of the 1-pyrazoline cycloadduct (eq 8).6
Kevin C. Lee
DuPont Agricultural Products, Newark, DE, USA