Dichlorides vicinal

It is not possible to use zinc for reductive debromination in the presence of (x-halo ketones and for transformations involving these intermediates, sodium iodide has been used. ° In some instances, e.g. 5,6-dihalo-3-ketones, iodide does not always give a completely halogen-free product, and zinc does not give clean debromination. The use of chromous chloride has proved advantageous in such cases and is the reagent of choice for vicinal dichlorides, which are inert to iodide ... 

POCI3, in spite of the 35% excess of PCI5 however, it is not possible to avoid the formation of the vicinal dichloride completely in benzene, even with an equimolar amount of PCI5. 

It is known that cloroethynylpyrazoles can enter different conversions with sodium amide (see Section TTT.C). Vasilevsky and co-workers (76TZV2288) supposed that the rate of dehydrochlorination of vicinal dichlorides by sodium amide is higher than the rates of the subsequent processes. Indeed, by using a stoichiometric amount of the base, chloroacetylenes 18a-d were obtained from dichlorides 17a-d in 80-90% yields (Table VIIT). 

From the results obtained by thermal decomposition of both low-molecular weight vicinal dichlorides in the gas phase [74,75] and of the copolymers of vinyl chloride and /rthermal instability of PVC to the individual head-to-head structures. Crawley and McNeill [76] chlorinated m-1,4-polybutadiene in methylene chloride, leading to a head-to-head, and a tail-to-tail PVC. They found, for powder samples under programmed heating conditions, that head-to-head polymers had a lower threshold temperature of degradation than normal PVC, but reached its maximum rate of degradation at higher temperatures. 

Experiments have been carried out to compare nanoscale catalysts composed of Fe-, Ni-, and Co-complexes of several porphyrins or cyanocobalamin (Dror et al. 2005). A cobalt-porphyrin complex and cyanocobalamin in the presence of Ti(III)citrate reduced the initial concentrations of tetrachloromethane and tetrachloroethene by —99.5%, and the porphyrin was equally effective with trichloroethene. The advantage of using heterogeneous catalysts was shown by experiments in repetitive cycling of tetrachloromethane. Zero-valent metals degrade vicinal dichlorides such as tetrachloroethene by a-elimination to produce dichloroacetylene and hnally acetylene (Roberts et al. 1996). 

Chromylchloride, Cr02Cl2, the main subject of the publication which led to the original discussion about the mechanism [12], shows a very different reactivity compared to the other transition metal oxides discussed above. Even in the absence of peroxides, it yields epoxides rather than diols in a complex mixture of products, which also contains cis-chlorohydrine and vicinal dichlorides. Many different mechanisms have been proposed to explain the great variety of products observed, but none of the proposed intermediates could be identified. Stairs et al. have proposed a direct interaction of the alkene with one oxygen atom of chromylchloride [63-65], while Sharpless proposed a chromaoxetane [12] formed via a [2+2] pathway. 

Chlorination of a variety of vinyl compounds RCH=CFi2 (R = Me, Bu1, Bu1, CF Cl, CF Br, CH2OH, Br, COMe, CO2H, CN) in alcohols gives a mixture of 2-alkoxy-l-chloro compounds, l-alkoxy-2-chloro compounds and vicinal dichlorides, the relative proportions of which are dependent on the substituent R, reaction temperature, molar ratio and elapse time93. Similarly, chlorination, alkoxychlorination and/or acetoxychlorination was observed with 2-methyl-but-2-ene94 and 4-substituted cyclopentenes95. 

Dimethyl sulfoxide and chlorine form highly reactive intermediates which are of some limited use as oxidants for alcohols. These intermediates are related to those derived from the reaction of the halogens with dimethyl sulfide and probably have a structure such as (27). When formed at -4S C they allow the oxidation of primary and secondary alcdiols to aldehydes and ketones when used in a two-fold excess. For very simple alcdiols the reaction proceeds in yields of greater than 90%, but there are considerable drawbacks if some types of additional functionality are present in the molecule, e.g. alkenes react very rapidly to form vicinal dichlorides. 

Vicinal dichlorides react with 2 equiv. of Bu"3SnH to give the corre.sponding saturated hydrocarbons in essentially quantitative yields, whereas vicinal dibromides lead to alkenes in high yields. ... 

Ketones and 1,2-diketones are readily converted into alkynes, which can subsequently be reduced to alkenes. Since there are a number of methods to reduce alkynes stereoselectively, this route offers a method of controlling the stereochemistry of acyclic alkenes derived from ketone starting materials. The most common approach for the ketone to alkyne transformation has been conversion of the starting ketone into a vicinal dichloride and subsequent bis elimination induced by strong base. ... 

The conversion of epoxides to vicinal dichlorides has been done with SOCh and pyridine, PhsP-CCU or PhsPCh. Symmetric a j-epoxides give mefo-dichlorides (equation 8), and Ae corresponding rrans-epoxides give D,L-dichlorides. This is a two-step mechanism involving an intermediate chlorohydrin. Two Sn2 reactions give inversion at both carbon atoms. 

When CI2 adds to an alkene, a cyclic chloronium ion intermediate is formed. The final product of the reaction is a vicinal dichloride. 

Dichlorination of cyclopentane under free-radical conditions is not a realistic approach to the introduction of two chlorines in a trans-1,2 relationship without contamination by isomeric dichlorides. Vicinal dichlorides are prepared by electrophilic addition of chlorine to alkenes. The stereochemistry of addition is anti. 

Dry chlorine and wet chlorine (hypochlorous acid) will react with organic substances containing double or triple bonds to yield vicinal dichlorides and chlorohydrins respectively. For example, hypochlorous acid will react with fatty acids in lipids such as oleic acid at pH 2 to 10, to produce 9-chloro-lO-hydroxystearic acid, while dry chlorine will produce a mixture of chlorinated products. 

Draw all isomers of the formula CeHi2Cl2 that have six-linear carbon atoms. Indicate which of these isomers have geminal dichloride and which have vicinal dichloride structures. 

The addition of iodine to aikenes is not as straightforward, and vicinal diiodides are less eommonly encountered than vicinal dichlorides and dibromides. The reaction of fluorine... 

When CI2 (or Br2) adds to an alkene, the final product of the reaction is a vicinal dichloride (or dibromide). Vicinal means that the two bromines are on adjacent carbons (vicinus is the Latin word for near ). Reactions of alkenes with Br2 or CI2 are generally carried out by mixing the alkene and the halogen in an inert solvent, such as dichloromethane (CH2CI2), which readily dissolves both reactants, but does not participate in the reaction. 

Other Applications. Thionyl chloride has been used to convert epoxides to vicinal dichlorides and for the preparation of dialkyl sulfides from Grignard reagents. Phenols react with SOCI2 to produce aryl chlorosulfites and diaryl sulfites or nuclear substitution products. As shown in eq 14, Aluminum Chloride catalysis yields symmetric sulfoxides, while in the absence of Lewis acids, aromatic thiosulfonates are the principal products. Primary amines, especially aromatic ones, react with SOCI2 to produce N-sulfiny lamines, which are potent enophiles and useful precursors to some heterocyclic compounds. ... 

The starting material can either be a geminal dichloride or a vicinal dichloride ... 

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