C sp2 —Br bonds

TABLE 37. C(sp2)—Br bond distances (in pm) in ethylenes and other compounds 

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The C(sp2)—Br bond in bromoethylene [188.2(2) pm] (Table 37) is ca 6 pm shorter than the C(sp3)—Br bond in bromomethane [193.88(5) pm]. This shortening corresponds to about twice the difference between the covalent radii of sp3- and sp2-hybridized carbon. The same C—Br distances have been determined for 2-bromopropenal and tetrabromoethyl-ene. Thus, the bond shortening observed between mono- and tetrasubstituted... 

Crystal structures of some bromine-substituted compounds were selected from the CSD according to the criteria (b) to (e) used for the chlorinated compounds. The number of minimum bromine atoms per molecule [criterion (a)] was reduced to 3 for C(sp3)—Br bonds and to 2 for C(sp2)—Br bonds. With these restrictions 43 observations were listed for C(sp3)—Br bonds with a mean value of 195.6 pm and a large standard deviation of 6.5 pm. The individual values range from 179.4 pm to 224.8 pm. The extreme values are listed in Table 41. The mean distance of 47 C(sp2)—Br bonds is 189.0 pm with a standard deviation of 2.2 pm. The individual values vary from 185.1 to 195.9 pm. Some examples of these recent crystal studies are given in Tables 41 and 42. 

The alkylidenecyclobutene synthesis is restricted to propargyl alcohols bearing a terminal triple bond. The scope of the reaction has been established for a variety of carboxylic acids, for example acetic, benzoic, methoxyacetic, and pent-4-enoic acid. It can also be applied to N-protected amino acids, and to phenol derivatives containing reactive sp2-C-Br bonds (Scheme 14) [24, 25]. 

In Section 16.5, a few other C,C coupling reactions of alkenes and of aromatic compounds, which contain an sp2—OTf, an sp2—Br, or an sp2—Cl bond, will be discussed because these C,C couplings and the preceding ones are closely related mechanistically. These substrates, however, react with metal-free alkenes. Palladium complexes again serve as the catalysts. 

Bromine is more electronegative than carbon and so the C-Br bond is polarized towards the bromine. If this bond were to break completely, the bromine would keep both electrons from the C-Br bond to become bromide ion, Br, leaving behind an organic cation. The end carbon would now only have three groups attached and so it becomes trigonal (sp2 hybridized). This leaves a vacant p orbital that we can combine with the n bond to give a new molecular orbital for the allyl system. 

Why should this proton be removed rather than any other The bromine atom is electronegative and the C-Br bond is in the plane of the sp2 orbital and removes electrons from it. The stabilization is nonetheless weak and only strong bases will do this reaction. 

The carbanion is in an sp orbital in the plane of the ring. Indeed, this intermediate is very similar to the aryl cation intermediate in the SnI mechanism from diazonium salts. That had no electrons in the sp orbitai the carbanion has two. Why should this proton be removed rather than any other The bromine atom is electronegative and the C—Br bond is in the plane of the sp2 orbital and removes electrons from it. The stabilization is nonetheless weak and only exceptionally strong bases will do this reaction. 

What is the difference between the two secondary carbocations Compare CC bond distances in the reactant to those in the two carbocations. What changes does Br loss cause in each of the carbocations How do you explain these changes (Hint Changes in C hybridization, such as sp3 —> sp2, may be responsible for some changes in distance.)... 

A number of different polar and nonpolar covalent bonds are capable of undergoing the oxidative addition to M( ). The widely known substrates are C—X (X = halogen and pseudohalogen). Most frequently observed is the oxidative addition of organic halides of sp2 carbons, and the rate of addition decreases in the order C—I > C—Br >> C—Cl >>> C—F. Alkenyl halides, aryl halides, pseudohalides, acyl halides and sulfonyl halides undergo oxidative addition (eq. 2.1). 

When steric hindrance in substrates is increased, and when the leaving anion group in substrates is iodide, SET reaction is much induced (Cl < Br < I). This reason comes from the fact that steric hindrance retards the direct nucleophilic reduction of substrates by a hydride species, and the a energy level of C-I bond in substrates is lower than that of C-Br or C-Cl bond. Therefore, metal hydride reduction of alkyl chlorides, bromides, and tosylates generally proceeds mainly via a polar pathway, i.e. SN2. Since LUMO energy level in aromatic halides is lower than that of aliphatic halides, SET reaction in aromatic halides is induced not only in aromatic iodides but also in aromatic bromides. Eq. 9.2 shows reductive cyclization of o-bromophenyl allyl ether (4) via an sp2 carbon-centered radical with LiAlH4. 

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