Geminal dihalide

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... 

Geminal dihalide Sodium amide Alkyne Ammonia... 

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... 

Geminal dihalide (Section 9 7) A dihahde of the form R2CX2 in which the two halogen substituents are located on the same carbon... 

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. 

Double dehydrohalogenation of vicinal dihalides (Section 9.7) Dihalides in which the halogens are on adjacent carbons undergo two elimination processes analogous to those of geminal dihalides. 

Double dehydrohalogenation (Section 9.7) Reaction in which a geminal dihalide or vicinal dihalide, on being treated with a very strong base such as sodium amide, is converted to an alkyne by loss of two protons and the two halogen substituents. 

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. 

CHEC-II(1996) <1996CHEC-II(8)345> covered a range of reactions of substituents, including hydrolyses of esters to carboxylic acids and geminal dihalides to aldehydes, reduction of aldehydes and halogens directly attached to the ring, O-methylation with diazomethane, and reduction of nitroso compounds. A selection of reactions which have... 

Direct treatment of organic geminal dihalides or trihalides with strongly nucleophilic transition metal complexes can also lead to the formation of carbene complexes, presumably via intermediate a-haloalkyl complexes [484-489]. Examples of such reactions are sketched in Figure 3.17. 

A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes. 

The reducing properties of organic compounds of sulfur, such as methyl mercaptan, show up in partial reduction of trigeminal to geminal dihalides [243]. Dimethyl sulfide reduces hydroperoxides to alcohols and ozonides to aldehydes while being converted to dimethyl sulfoxide [244]. 

Geminal dihalides undergo partial or total reduction. The latter can be achieved by catalytic hydrogenation over platinum oxide [512], palladium [512] or Raney nickel [63, 512], Both partial and total reduction can be accomplished with lithium aluminum hydride [513], with sodium bis(2-meth-oxyethoxy)aluminum hydride [514], with tributylstannane [503, 514], electro-lytically [515], with sodium in alcohol [516] and with chromous sulfate [193, 197]. For partial reduction only, sodium arsenite [220] or sodium sulfite [254] are used. 

Geminal dihalides have also been applied for the construction of 1,3-dioxanes and congeners. For example, bromochloromethane readily reacted with tetrahydroxynaphthalenes to afford the tetracycle 218 in good yield (Equation 82). Bisdioxane 218 was subsequently used for the synthesis of alkannin and shikonin <1998AGE839, 2000SC1023>. 

Doyle, M. P., Siegfried, B., and Hammond, J. J. (1976). Oxidative deamination of primary amines by copper halide nitrosyls. The formation of geminal dihalides. J. Am. Chem. Soc. 98, 1627-1629. 

Alkynes. Because of their less nucleophilic character, alkynes react less readily with hydrogen halides than do alkenes and often require the use a metal halide catalyst. Vinyl halides are formed in the reaction with one equivalent of HHlg. They may react further in an excess of the reagent to yield geminal dihalides. High yields of these compounds can be achieved. The addition of HC1 to acetylene was studied in detail because of the practical importance of the product vinyl chloride (see Section 6.2.4). 

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