Carbonyl dibromide mercury

The dianion (62 Scheme 35), prepared from alkenethiols, gives good y-selectivity toward both alkylation and carbonyl addition. The y-products (enol sulfides) may be converted to dimethylacetals by treatment with mercury(II) chloride in methanol. When (62) is associated with magnesium dibromide, the addition of carbonyl compounds produces the a-adducts with regioselectivity greater than 90%. The reaction with methyl vinyl ketone, followed by a Cope rearrangement, results in reattachment at the original y-position. 

General procedure A solution of the carbonyl compound (0.25 mole) and methylene dibromide (or diiodide) (0.26 mole) in ether (100 ml) is added during 2 h to a stirred suspension of magnesium amalgam (from 0.5 mole of magnesium in 400 g of mercury) in ether (100 ml) under dry argon. The whole is boiled for 30 min under reflux and then worked up. 

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