Gem-Dihaloalkanes

In an extension of the procedure, thiols react with gem-dihaloalkanes (Table 4.4) to produce thioacetals [ 10,20-23] and the reaction can be employed in the Corey-Seebach synthesis of aldehydes and ketones (see ref. 24 and references cited therein), gem-Dichlorocyclopiopanes having an electron-withdrawing group at the 2-position react with thiols to produce the thioacetals [25]. In the corresponding reaction of the thiols with biomochloromethane exclusive nucleophilic substitution of the bromo group by the thiolate anion occurs to yield the chloromethyl thioethers [13, 14] (Table 4.5). 

Method A The gem-dihaloalkane (0.02 mol) and the thiol (0.04 mol) in PhMe (25 ml) containing Aliquat (20 mg, 0.05 mmol) are added to dry K2CO, (10 g). The mixture is stirred for 8 h at room temperature and then filtered. The filtrate is fractionally distilled under reduced pressure to yield the thioacetal. 

In general, the preparation of gem-dizincioalkane via the treatment of 1,1-dihaloalkane with zinc is more difficult than the preparation of gewt-dizinciomethane. The reduction of gem-dihaloalkanes carrying fi-hydrogens may suffer from /J-hydride elimination and result in the formation of alkenes. In addition, depending on the substrate, the intermediary a-haloalkylzinc compound is less stable than a-halomethylzinc towards the a-elimination (Scheme 2). 

Of greater interest, gem-dimethylation of ketones is possible with reactions conducted in the temperature range — 30 to 22° by a two-step sequence involving addition followed by methylation of the intermediate titanium alcoholate. The method is widely applicable, although subject to steric effects, and yields are generally in the range 60 85%. In addition, tertiary alcohols and tertiary alkyl chlorides are methylated by reactions conducted at room temperature in yields of about 75%.2 gem-Dihaloalkanes can also be converted into gcm-dimcthylalkanes.1... 

Scheme 5.15. Reduction of a gem-dihaloalkane and possible decomposition routes of the product.

On the other hand, it is not necessary to prepare such unstable gem-dihaloalkanes in advance. Esters can be alkylidenated by treatment with Zn-CH3CHBr2-TiCl4-TMEDA (Scheme 5.18) [15]. This reaction requires a large excess of the reagent. It should be noted that the zinc powder used in these procedures was also pyrometallurgy grade zinc, containing about 0.04-0.07% of lead. 

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