SYNTHESIS OF 2,6-DIMETHYLHEPTA-1,5-DIENE

STEP 1: To a stirred solution of trimethylsilylmagnesium chloride, made from chloro­methyltrimethylsilane (2.5 g, 0.02 mol) and magnesium (0.5 g, 0.02 mol) in sodium-dried ether (25 ml), is added a solution of 6-methylhept-5-en-2-one (2.5 g, 0.02 mol) (Aldrich) in sodium-dried ether (5 ml).

STEP 2: The addition is drop-wise at such a rate that a gentle reflux of the mixture is maintained.

STEP 3: The reac­tion mixture is refluxed with continued stirring.

STEP 4: After 3 hours, the reaction mixture is cooled in an ice bath and thionyl chloride (1.8 ml, 0.025 mol) is added.

STEP 5: The ice bath is removed and stirring continued at room temperature.

STEP 6: After 1 hour, the reaction mixture is hydrolysed by the dropwise addition of a saturated solution of ammonium chloride.

STEP 7: The coagulated solid is filtered off and washed with ether.

STEP 8: Distillation of the combined filtrate-washings after drying gives 1.4 g (57%) of 2,6-dimethylhepta-1,5-diene as a clear colourless liquid, b.p. 135-136 °C; /113²³ 1.4385; p.m.r. (neat) 6 1.52 (s, 3H), 1.62 (s. 6H), 2.06 (m, 4H), 4.64 (s, 2H), and 5.06 (m, 1H).