Geminal dihaloalkanes

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... 

Alkynes have been prepared from 1,1- and 1,2-dihaloalkanes and from haloalkenes under the influence of a quaternary ammonium salt geminal- and vic-dibromoalkanes are converted into alkynes under liquid liquid [10, 11] and soliddiquid [12-14] conditions with Aliquat or tetra-n-octylammonium bromide... 

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... 

Due to the acidifying effect of a second halogen atom, dihalo-snbstituted lithium carbenoids 30 can be obtained by deprotonation of geminal dihaloalkanes, the most frequently applied method for the generation of lithium carbenoids with this snbstimtion... 

Geminal dihaloalkanes are very commonly used reagents for the addition of alkylidene groups to alkenes. The simpler dihalides, especially the dihalomethanes, are readily available and are certainly much less hazardous to use than the corresponding diazoalkanes. Nevertheless, any reactive alkylating agent should be regarded as a toxic substance. 

A variation of this reaction was developed in 1986 by Takai and Utimoto, in which geminal dihaloalkanes were added to aldehydes in a reaction mediated by chromium dichloride. This led to the stereoselective formation of the corresponding tran5-olefins 10 [12]. The major drawback of this method is the rather cumbersome access to the corresponding substituted dihalomethane compounds, which prevents a broad application of this reaction for synthesis. 

We encounter reductive bond cleavage of a acceptors when there are no readily reducible n acceptors, such as aryl, carbonyl, and analogous functionalities present. Hence, this reaction pattern is commonly realized in aliphatic systems containing the electrophores depicted in Scheme 12, such as in simple reductions R-Hal R H [19]. Equally of importance is the dehalogenation of geminal [20] and vicinal dihaloalkanes [21], which has also been described for SO2R, SR, OAc, OMe, OH,... 

Base-induced dehydrohalogention of vicinal and geminal dihaloalkanes proceeds via the intermediacy of the vinyl halides and requires three equivalents of the base to generate the triple bond. 

C-C double bonds are elTiciently cyclopropanated electrochemically in a one-compartment cell fitted with a sacrificial zinc anode which allowed the formation of organozinc species from geminal dihaloalkanes. Electrolysis of dibromomethane in dichloromethane/dimethyl-formamide as solvent mixture is recommended as the standard condition for electrolysis. The best chemical yields were obtained with allylic alcohols and unfunctionalized alkenes. For example electrolysis of allyl alcohol 23 gave cyclopropane 24 in 70% yield. 

Addition of HX (Section 7.6B) Addition of HX is regioselective. Reaction by way of a vinylic carbocation intermediate follows MarkovnikoVs rule. Addition of 2 HX gives a geminal dihaloalkane. 

Gandelman and co-workers have reported the synthesis of secondary alkyl fluorides using a Suzuki-Miyaura cross-coupling of 1-halo-l-fluoroal-kanes (Scheme 13.19). Geminal dihaloalkanes were used as starting materials and the report demonstrated that these simple 1-fluoro-l-haloalkanes... 

Addition of hydrogen halides forms haloalkenes and geminal dihaloalkanes... 

The stereochemistry of this type of addition is typically anti, particularly when excess halide ion is used. A second molecule of hydrogen bromide may also add, with regioselec-tivity that follows Markovnikov s rule, giving the product with both bromine atoms bound to the same carbon, a geminal dihaloalkane. 

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