Geminal halides

Castro CE, WC Kray (1966) Carbenoid intermediates from polyhalomethanes and chromium (II). The homogeneous reduction of geminal halides by chromous sulfate. J Am Chem Soc 88 4447-4458. 

Chromium(II) sulfate is a versatile reagent for the mild reduction of a variety of bonds. Thus aqueous dimethylformamide solutions of this reagent at room temperature couple benzylic halides, reduce aliphatic monohalides to alkanes, convert vicinal dihalides to olefins, convert geminal halides to carben-oids, reduce acetylenes to /raw5-olefins, and reduce a,j3-unsatu-rated esters, acids, and nitriles to the corresponding saturated derivatives. These conditions also reduce aldehydes to alcohols. The reduction of diethyl fumarate described in this preparation illustrates the mildness of the reaction conditions for the reduction of acetylenes and o ,j8-unsaturated esters, acids, and nitriles. 

The dihaloalkanes having the same type of halogen atoms are named as alkyUdene or alkylene dihalldes. The dihalo-compounds having same type of halogen atoms are further classified as geminal halides (halogen atoms are present on the same carbon atom) and vicinal halides (halogen atoms are present on the adjacent carbon atoms). In common name system, pem-dihalides are named as aUq Udene halides and uic-dihalides... 

A special use of sodium arsenite (applied in aqueous alkaline solutions) is partial reduction of trigeminal halides to geminal halides [220] and reduction of aromatic nitro compounds to azoxy compounds [221]. 

TABLE 3. Effect of structure, solvent and supporting electrolyte on exolendo product ratio of [n. 1.0]bicyclic geminal halides... 

In the case of X = Cl, Br, and I, the iminium salts exist predominantly in the polar form (I), as evidenced by spectral data, whereas in the case of X = F, the compounds are in the geminate halide form (II). 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Hydrogen halides add to alkynes in accordance with Markovnikov s rule to give alkenyl halides In the presence of 2 moles of hydrogen halide a second addition occurs to give a geminal dihalide... 

Addition of hydrogen halides to simple allenes initially gives the vinyl halide, and if the second double bond reacts, a geminal dihalide is formed. " ... 

In the presence of excess hydrogen halide, geminal dihalides aie fonned by sequential addition of two molecules of hydrogen halide to the caibon-caibon triple bond. 

Double dehydrohalogenation of geminal dihalides (Section 9.7) An E2 elimination reaction of a geminal dihalide yields an alkenyl halide. If a strong enough base is used, sodium amide, for example, a second elimination step follows the first and the alkenyl halide is converted to an alkyne. 

Further experimental details, n.m.r., and i.r. data on the compounds, N P F2 (NMe2)a (n = 3—6, x = 1—3) and their reaction products with hydrogen halides, already reported in preliminary form, have been given. Dimethylamine and MesSiNMe2 form bisdimethylamino-deriva-tives, predominantly with non-geminal structures. In the tetramer series, the bisdimethylamino-derivative has structure (40), rather than (41). The... 

Alkynes react with HC1 and HBr to form haloalkenes or geminal dihalides depending on whether one or two molar equivalents of the hydrogen halide are used. 

If the stannane bears substituents at the position geminal to tin, the reaction requires activation by Cu1 (Section 9.6.3.2.1). The cross-coupling of aryl (or heteroaryl) stannanes with aryl (or heteroaryl) halides or triflates is a general route to various biaryls and their analogues, particularly useful in those cases when the Suzuki-Miyaura reaction is less effective for... 

Magnesium carbenoids (5 and 6) were generated from a geminal diiodoalkane by diastereoselective iodine-magnesium exchange reaction with isopropylmagnesium halide in THF at —78°C for 45 min (Scheme 1). Subsequent reaction of the magnesium... 

Electrophilic addition to terminal alkynes (unsymmetrical) is regioselective and follows Markovnikov s rule. Hydrogen halides can be added to alkynes just like alkenes, to form first the vinyl halide, and then the geminal alkyl dihalide. The addition of HX to an alkyne can be stopped after the first... 

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