Gem-dihalides

Seyferth prepared a-halogenoalkyl-lithium and -magnesium compounds by treating the appropriate gem-dihalides with butyllithium or with Grignard reagents at low temperature, and then used the products to prepare acyclic and cyclic a-halogenoalkyltin compounds (57-60). Typical examples are shown in the following equations. 

Reaction of carbonyl compounds with gem-dihalides and Li or BuLi... 

From a mechanistic point of view, the titanocene(II)-promoted intramolecular cyclopro-panation of gem-dihalides possessing a terminal double bond is interesting. Although the products of ring-closing metathesis, i. e. cycloalkenes, are produced in certain cases, the treatment of 6,6- and 7,7-dihalo-l-alkenes (e. g. 39 and 40) with titanocene(II) species affords bicyclo[3.1. OJhexane and bicyclo[4.1.0]heptane derivatives 41 and 42, respectively (Scheme 14.19) [34],... 

Scheme 14.19. Titanocene(l l)-promoted intramolecular reaction of alkenyl gem-dihalides.

Titanium-based reagents generated by the reduction of gem-dihalides with low-valent metal species are widely used for the alkylidenation of carbonyl compounds (Scheme 14.28). As in the case of methylidenation, the system gem-dibromide/TiCl4/Zn/TMEDA... 

Scheme 14.28. Carbonyl alkylidenation utilizing gem-dihalides and low-valent titanium reagents.

Cleavage Gem-dihalides and monohalides have been dehalogenated to chiral monohalides in the presence of alkaloids [397, 398]. l,l-Diphenyl-2-bromo-2-carboxyl (bromo or methyl carboxylate) cyclopropanes are cathodically debromi-nated in the presence of alkaloid cations with enantioselectivities up to 45% ee. A mechanism is proposed whereby the alkaloid is adsorbed at the Hg cathode, which protonates face selectively the carbanion generated by 2e reduction from the bromide [399]. 

LABORATORY METHODS OF PREPARATION 1. Dehydrohalogenation of wc-Dihalides or gem-Dihalides... 

B) is a carbonyl compound because it forms an oxime, and therefore (A) is a gem-dihalide. 

A geminal (gem)-dihalide has two halogen atoms on the same carbon, and a vicinal (vic)-dihalide has halogen atoms on adjacent carbon atoms. 

Markovnikov s rule ensures that gem-dihalides and not Wc-dihalides are the products of the addition of two moles. 

The Formation of gem-Dihalides from Aldehydes and Ketones Dihalo-de-oxo-bisubstitution... 

For a list of reagents that convert aldehydes and ketones to gem-dihalides or vinylic halides, with references, see Ref. 64, pp. 372-375. 

In a different type of procedure, primary alkyl primary amines can be converted to gem-dihalides [RCH2NH2 — RCHX2 (X = Br or Cl)] by treatment with an alkyl nitrite and the anhydrous copper(I) halide.356... 

Hydrolysis of gem-dihalides 0-4 Hydrolysis of enol esters of inorganic acids... 

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