C sp2 —Cl bonds

Various chlorine-substituted benzenes have been studied extensively by ED and MW and the most accurate results were obtained by joint analyses of ED intensities, rotational constants (from MW or high-resolution IR spectra) and dipolar coupling constants from liquid-crystal NMR spectra. Ab initio calculations have also been performed for some of these derivatives246 (Table 27). The primary interest in these studies is the degree of distortion of the benzene ring caused by chlorine substitution but in this context we will discuss only the variation of the C—Cl bond distances. In chlorobenzene this distance [173.90(23) pm] is very slightly longer than that in chloroethylene [173.0(4) pm], similar to the observation for C—F bonds. In the three disubstituted derivatives the C—Cl bond distances shorten only by a few tenths of a pm and are almost independent of the relative position of the two chlorines. These experimental results are reproduced correctly by the ab 

TABLE 27. C(sp2)—Cl bond distances (in pm) in benzenes and other cyclic compounds 

C(sp)—Cl bond distances in various acetylenes (Table 28) have a remarkably constant value of ca 163.5 pm and variations due to electron-donating (Me, t-Bu, SiH3) or electron-withdrawing substituents (F, Cl, CN) at the opposite carbon are smaller than the experimental uncertainties. ED and MW for chlorobromoacetylene result in rather different ra and r0 values for the C—Cl bond length and this discrepancy may be due to large-amplitude bending vibration of this linear molecule. A similar, but smaller difference between ra and rs values occurs for chlorocyanoacetylene. The rs value for chlorine cyanide is also in line with the results for the acetylenes. 

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Crystal structures of some highly chlorinated compounds were selected from the CSD according to the following criteria (a) number of chlorine atoms > 5, (b) compound contains only H, N and/or O beside C and Cl, (c) R factor < 0.05, (d) standard deviation for C—C bonds < 0.5 pm, (e) publication between 1983 and 1991. With these restrictions 135 C(sp3)—Cl bonds were listed with a mean value of 176.8 pm (o - 1.8 pm). The individual values range from 173.2 to 184.4 pm. The above criteria result in 81 C(sp2)—Cl bonds with a mean value of 172.0 pm (cr = 1.6 pm), and individual bond lengths ranging from 168.7 to 178.0 pm. Some examples of highly chlorinated compounds of relatively small size are presented in Table 30 [C(sp3)—Cl distances] and Table 31 [C(sp2)—Cl distances]. 

The typical C(sp2)—Cl bond distances in Table 29 are split into bonds to terminal carbon (substructures VII and IX) and into bonds to aromatic rings (substructures IX and X). Since the values are nearly equal in both cases (173.4 and 173.9 pm) and shorter by about... 

Two crystal structures with highly unusual C—Cl bond distances have been reported very recently. An extremely long C(sp2)—Cl bond of 185.5(7) pm occurs in the ethylene part of 1 -chloro-2,2-bis(4-chlorophenyl)-1 -lithioethylene (43), more than 10 pm longer than the typical value for vinyl C—Cl bonds (173.4 pm)292. A very short C—Cl bond of 166.8 pm has been determined for 44, which is the first crystal structure analysis for a chlorinated carbocation293. The shortening of this bond is rationalized by chlorine back-donation and partial double-bond character of the C—Cl bond. The aromatic C—Cl bond has a normal length of 172.6(9) pm. 

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. 

Vinylboronates are generally less reactive than vinylzirconocenes towards various electrophiles and hence selective reactions of the latter should be possible. It was found that selective cleavage of the carbon—zirconium bond in 45 by N-halosuccinimides provides (a-haloalkenyl)boronic esters 53 in excellent chemical yields and with complete re-gioselectivity (Scheme 7.17) [54], An X-ray crystal structure determination of 45 confirmed the configuration of the four-coordinate Zr complex, with two cyclopentadienyl rings, Cl, and C(sp2) as the four ligands (Fig. 7.5) [54,126]. 

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. 

The C—Cl bond in chloroethylene [173.0(4) pm] is by about 4-5 pm shorter than the C—Cl bond in chloromethane. This difference is larger than that between the covalent radii of sp2 and sp3-hybridized carbon (74 and 77 pm, respectively). The C—Cl bond distances... 

The authors point out that, unlike vinyl halides, the vinyl chloronium ion 23a could hardly show double bond character in the C-Cl bond because of unfavourable charge repulsion in 23b. The unreactivity of vinyl halides may therefore primarily be a consequence of the strength of the a bond to the sp2, carbon. However if opening of 23a is assumed to be due to nucleophilic attack of the solvent (or any other nucleophile) to either C5 or C2, the greater amount of shifted products obtained from alkynes than from alkenes is a direct consequence of the relative facility of nucleophilic attack at the sp3 carbon and sp% carbon (Rappoport, 1969 Modena, 1971). 

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