[53282-30-7] · C5H7F3O3S · 2-Methyl-1-propenyl Trifluoromethanesulfonate · (MW 204.19)
(precursor to dimethylvinylidene carbene,1 2-methyl-1-propenyl ethers,1 isopropylidenecyclopropanes,2 isopropylidenecyclopropenes,2 3-isopropylindazoles,3 and 2-methyl-1-propenyltriphenylphosphonium triflates4)
Physical Data: bp 65-66 °C/70 mmHg.
Solubility: sol most organic solvents.
Preparative Method: prepared in a single step from 2-methylpropionaldehyde, Trifluoromethanesulfonic Anhydride, and a hindered pyridine in 1,2-dichloroethane at reflux (eq 1). Distillation gives the product in 45% yield.5
This reagent is quantitatively converted to the electrophilic carbene (2) (eq 2) on treatment with Potassium t-Butoxide.1 This carbene readily inserts into an oxygen-hydrogen bond, and thus reacts with t-butanol to give ether (3) (eq 3).1
Carbene (2) reacts normally with a variety of substrates. When an alkene is used as solvent (eqs 4 and 5), the corresponding isopropylidenecyclopropanes are generated in good to excellent yield.1 The silicon-hydrogen bond is an efficient trap (eq 6); the reaction proceeds with retention of configuration at silicon.6
Alkynes react with (2) to yield isopropylidenecyclopropenes (4) (eq 7).2 These products cannot be isolated, but will undergo a facile Diels-Alder reaction. Alternatively, protonation of the product gives isopropylcyclopropenium ions.
Reaction of (1) in the presence of azoarenes yields the unusual 3-isopropylindazoles (eq 8), presumably via an isopropylidenediaziridine.3 In the presence of Triphenylphosphine, a palladium catalyst converts (1) into the corresponding phosphonium salt in good yield (eq 9).4
Scott McN. Sieburth & Louis Fensterbank
State University of New York at Stony Brook, NY, USA