A Carbon-chlorine bonds

Resonance of a Carbon-Chlorine Bond and an Adjacent Double Bond.—We expect the phosgene molecule to resonate among the structures... 

The use of 1,6-diene systems usually does not result in cyclization reactions with palladium ) salts. For example, with 1,6-heptadiene a /i-elimination takes place from the cqjr-intermediate to give diene 22 as the major product (equation 10)27. However, more recently Trost and Burgess21 have shown that with a 4,4-bis(phenylsulfonyl) derivative of 1,6-heptadiene (23) an insertion takes place to give a 5-membered ring product (24, equation 11). The final step of the latter reaction is oxidative cleavage of the palladium-carbon bond by CuCl2 to produce a carbon-chlorine bond. 

A double bond has formed between oxygen and carbon, and a carbon-chlorine bond has broken. Electrons move from oxygen to form the double bond and from carbon to chlorine. 

Farwell and coworkers [209,210] investigated the electrochemistry of mono- and poly-chlorobiphenyls at mercury in DMSO containing TEABr. These compounds exhibit sequential two-electron removal of chlorine, but there is a tendency (due to the varied pattern of chlorine substitution) for more than one product to be formed sometimes two chlorines are expelled simultaneously. In another study, Maruyama and Murakami [211] reported that chlorinated biphenyls are reduced in DMF via one-electron cleavage of a carbon-chlorine bond to yield chloride and an aryl radical, which subsequently abstracts a hydrogen atom from the solvent. Rusling and Arena [212] probed the reduction of 4-bromo-, 4,4 -dibromo-, 3,4-dichloro-, and 2,2, 5,5 -tetrachlorobiphenyl at mercury in... 

Organic molecules often have polar covalent bonds as a result of unsym-metrical electron sharing caused by difterences in the electronegativity of atoms. For example, a carbon-chlorine bond is polar because chlorine attracts the shared electrons more strongly than carbon does. Carbon-hydrogen bonds are relatively nonpolar. Many molecules as a whole are also polar owing to the cumulative effects of individual polar bonds and electron lone pairs. The polarity of a molecule is measured by its dipole moment, p. 

The NQR frequencies of halogenated compounds are related in a very simple manner to the valency p orbital populations on the halogen atom by the Townes and Dailey formula (4 ). For a carbon-chlorine bond, the Cl nucleus NQR frequency can be set equal to ... 

The reaction proceeds stereospecifically with inversion of configuration at the carbon-oxygen bond that is converted into a carbon-chlorine bond. Thus treatment of d-( —)-2-carboxy-2,4,5-trimethyl-l,3-dioxolane (10) yields i.-( t )-eryt/iro-3-chloro-2-butyl acetate (11). Treatment of (11) with KOH in ethylene glycol yields d-( + )-2,3-epoxybutane (12), a reaction known to proceed with inversion at the carbon-halogen bond. 

A substituent on a carboxylic acid may also assist in stabilizing the carboxylate anion. For example, a carbon-chlorine bond is a polar bond because chlorine is more... 

Electron sharing between dissimilar atoms is not always equal. When one atom in the pair has a higher affinity for the shared electrons than the other atom, the bond is known as a polar covalent bond. For example, in a carbon-chlorine bond such as in polyvinyl chloride, the chlorine has a higher electron affinity than carbon, so it gets a disproportionate share of the electrons, resulting in the C-Cl bond being a polar covalent bond. The importance of this polarity will be discussed in Section 2.3.2. Electron sharing between identical atoms, such as C-C or 0-0, is nonpolar. 

An alternative method for the formation of enantioenriched a-chloroesters, using A-heterocyclic carbene catalysts, was reported by Reynolds and Rovis (Scheme 13.14) [34]. In a similar mechanism to that presented in Scheme 13.12, initial attack of the carbene to the aldehyde and loss of HCl generated a chiral enolate. Asymmetric protonation of this enolate followed by displacement of the azolium species by a phenol produced enantiopure a-chloroesters. In contrast to the approach to chiral a-chloroesters presented in Scheme 13.13, a variety of aryl esters can be incorporated into the product by using different aryl alcohols (ArOH). Additionally, a carbon-chlorine bond is not formed in this reaction. Rather the introduction of a stereocenter in the chlorinated products is achieved via asymmetric protonation. This method was elaborated to use water as the proton/alcohol source to produce chiral a-chloro carboxylic acids (i.e., as in Scheme 13.12) [28]. Moreover, the use of D2O generated chiral a-chloro-a-deutero carboxylic acids. 

Sladek MI, Braun T, Neumann B, Stammler H-G (2002) Aromatic C-F activation at Ni in the presence of a carbon-chlorine bond the nickel mediated synthesis of new pyrimidines. J Chem Soc Dalton Trans 3 297-299... 

The transformation of the intermediate, I, to the final product, N-allyl-2,4-dinitroanilme, involves the breaking of both a carbon-chlorine bond and a nitrogen-hydrogen bond. Whether this takes place in two consecutive steps or in a single, concerted process, a valid mechanism must be consistent with both the absence of a measurable deuterium isotope effect and the fact that the observed rates are affected by the process in which the nitrogen-hydrogen bond is broken. 

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