Dihalides elimination

Depending on the mode of generation, a carbene may be initially formed in either the singlet or triplet state, irrespective of its stability. Common methods used for the generation of carbenes include photolytic, thermal, or metal catalyzed decomposition of diazocompounds, elimination of halogenfrom gem-dihalides, elimination of Hx from CHX3, decomposition of ketenes, thermolysis of a-halo-mercury compounds and cycloelimination of shelf stable substrates such as cyclopropanes, epoxides, aziridines and diazirines. 

However, a-halolithiums can be prepared in high yield at low T by alternate procedures, such as deprotonation (see 5.5.2.3.2). Vicinal dihalides eliminate to give alkenes after forming one C—Li bond, as illustrated for 1,2-dibromoethane ... 

Organic dihalides, aliphatic or aromatic, with >4 carbon atoms between the two halogens do not present any difficulties other than those encountered with their monohalo analogues. To prevent coupling, which in this case may be favored by intramolecular ring closure, the addition of the dihalide should be slow Et20 is used but THE is preferable . With several cyclic dihalides, elimination or fragmentation occurs, e.g. ° ... 

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal 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 gemmal dihalides... 

A large variety of methods is applicable to the formation of isolated double bonds. This permits selection of reagents compatible with other functionality present. Alcohol dehydration, ester elimination and other nonreductive p eliminations are the most common methods. Reductive elimination of halo-hydrins, vic-dihalides, etc., and of a variety of ketone derivatives has also been used. 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

The malonic ester required for synthesis of cyclopal (107) can be obtained by alkylation of diethyl allylmalonate (115) with 1,2-dibromocyclopentane in the presence of excess base. It is probable that the reaction proceeds by elimination of hydrogen bromide from the dihalide as the first step. The resulting allilic halide (116) would be the most reactive electrophile in the reaction mixture and thus would quickly alkylate the anion of the malonate to afford 117. 

Alkynes can be prepared by the elimination of HX from alkyl halides in much the same manner as alkenes (Section 7.1). Treatment of a 1,2-dihaloaJkane (a vicinal dihalide) with excess strong base such as KOH or NaNH2 results in a twofold elimination of HX and formation of an alkyne. As with the elimination of HX to form an alkene, we ll defer a discussion of the mechanism until Chapter 11. 

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Alkyne carbon atoms are sp-hybridized, and the triple bond consists of one sp-sp a bond and two p-p tt bonds. There are relatively few general methods of alkyne synthesis. Two good ones are the alkylation of an acetylide anion with a primary-alkyl halide and the twofold elimination of HX from a vicinal dihalide. 

The generation of PPV and corresponding derivatives via the dihalide approach is possible not only in solution reaction, but also - via the gas phase -in a so-called chemical vapor deposition (CVD) process. In this process, the vapor of a dichlorinated para-xylene (a,a or a,a) is pyrolyzed at moderately low pressures (0,1-0,2 torr) to form a chlorinated para-xylylene intermediate, which then condenses and polymerizes on a suitable, cooled substrate. The coating of the chlorinated precursor polymer can be heated to eliminate HCl, to form PPV 60 (or a PPV derivative) [88]... 

The reductive elimination of vicinal dihalides has been accomplished by using many reagents, including the use of aqueous media.16 An interesting method is the reductive elimination of vicinal dihalides by an electrochemical method using vitamin Bi2 in a water-in-oil microemulsion (Eq. 6.8).17... 

Eliminations have also been carried out on a number of compounds of the form HalCH2CH2Y, where Y = OH, OR, OCOR, NH2, etc. these eliminations normally require conditions more drastic than for 1,2-dihalides, and metals or metal cations are found to be more effective than Ie. These eliminations are often found to be somewhat indiscriminate in their stereochemistry. The elimination of CO Br6 from the diastereoisomer (72) of 2,3-dibromo-3-phenylpropanoate in Me2CO is, however, found to proceed 100% ANTI, and under extremely mild conditions ... 

Fig. 2 Reductive elimination of halogens from chalcogenopyrylium dyes 6-11 containing tellurium(IV) dihalide groups. 

Cathodic elimination can remove a variety of vicinal nucleophiles (X = Br [140], Cl [141], S-C6H5 [142], or oxalate [143]) to form a double bond. Controlled potential cathodic reduction allows the selective elimination of vic-dihalides depending on the degree of alkylation [144]. Chemically, these reductions are more limited in scope they can be conducted with 1 in DMF, with Zn, Mg, or Cr + [145]. 

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