Norbomyl compounds

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbomyl cation has also been extensively studied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers prepared the 2-norbomyl cation in... 

Brown, 1972). On the other hand, there are data which some authors have interpreted as indicating varied degrees of participation or bridging of the bond between G-1 and C-6 during the solvolysis of exo-2 norbomyl compounds. 

Three major arguments have been presented as evidence of bridging or a-participation during solvolysis of norbomyl compounds. 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation... 

Let us now consider Scheppele s discussion of the solvolysis of 2-d-exo- and -endo-bicyclo[2.2.1] hept-2-yl p-nitrobenzoates and p-toluenesulfonates. A maximum a isotope effect of 1 22 is suggested for kH/kD for the limiting solvolysis of simple secondary sulphonates (solvolyses where the rate is not affected by the nucleophilicity of the medium). The endo-esters yield a similar value which might be taken to imply the absence of steric hindrance in their reaction. However, whereas the maximum isotope effect for the simple sulphonates is believed to arise partly from an activated complex in the conversion of the internal ion pair into a solvent-separated ion pair, B - C, the effect in the endo-norbomyl compounds is suggested to arise during the transition state involved in the formation of the intimate ion pair, A - B. The latter is deduced from the fact that polarimetric and titrimetric rate constants are the same in acetic acid, aqueous acetone and ethanol (Winstein and Trifan, 1949 and 1952). If this is the case, the endo-effect of 1-20 should be... 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation has also been extensively smdied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers have prepared the 2-norbomyl cation in stable solutions at temperamres below 150°C in SbFs—SO2 and FSO3H SbF5 S02, where the stmcmre is static and hydride shifts are absent Studies by proton and NMR, as well as by laser Raman spectra and X-ray electron spectroscopy, led to the conclusion that under these conditions the ion is nonclassical. A similar result has been reported for the 2-norbomyl cation in the sohd state where at 77 and even 5 K, NMR spectra gave no evidence of the freezing out of a single classical ion. ... 

ProUem 28.19 (a) Show how a nonclassical ion intermediate could account for both the stereospecificity and the unusually fast rate (if it is unusually fast) of rearrangement of camphene hydrochloride into isobomyl chloride, (b) How do you account for the fact that optically active product is formed here, in contrast to what is obtained from solvolysis of norbomyl compounds ... 

Carbon is known with all coordination numbers from 0 to 8 though compounds in which it is 3- or 4-coordinate are the most numerous. Some typical examples are summarized in the Panel (p. 291). Particular mention should also be made of hypercoordinate non-classical carbo-nium ions such as 5-coordinate CHj", square pyramidal CsHs (cf. the isoelectronic cluster B3H9, p. 154), pentagonal pyramidal C6Me6 " (cf. iso-electronic Bf,Hio, p. 154) and the bicyclic cation 2-norbomyl, C7H] 1... 

Cobalt provides only a few examples of this oxidation state, namely some fluoro compounds and mixed metal oxides, whose purity is questionable and, most notably, the thermally stable, brown, tetraalkyl, [Co(l-norbomyl)4]. Prepared by the reaction of C0CI2 and Li(l-norbomyl), it is the only one of a series of such compounds obtained for the first row transition... 

Table 7 contains shift information for a number of other compounds that have been used to analyse the norbomyl spectra. Included among these are data for the isopropyl ion which has been used as the principal model for the secondary ion, and for the 1,2-dimethylnorbomyl cation which has been reported to be an example of an equilibrating classical cation, the Wagner-Meerwein shift being too fast to be stopped at —140°. In the table it may also be noticed that the t-butyl cationic centre experiences a shift of about 11 p.p.m. on being transferred from neat SbFs to SbFs—SO2. 

Next, one may consider the implications of laser-Raman studies of the norbomyl ion. Raman spectra of the norbomyl cation at —70° to —80° were reported in the FSO3H—SbFs—SOj system (Olah et al., 1968). The spectra of it and related compounds are given in Table 10. 

The solvolysis of 252 is one of the rare examples for a norbornyl-norpinyl rearrangement. While the ew-trimethylsilyl brosylate 251 yields mainly substitution and elimination products 253-255 with an intact norbomyl framework, 252 gives nearly 86% of norpinene 256 (equation 39). The bis-(trimethylsilyl)substituted compound 257 gives almost exclusively the norpinene derivative 258 (equation 40). While the trimethylsi-lyl group(s) in 252 and 257 exert no kinetic effect on the reaction rate, the /3-effect on the intermediate carbocations 252A and 259, respectively, determines the product distribution99. 

Comparison of the cyclic systems in Table 17 leads to the opposite conclusion, however the destabilization of the 1-norbornyl radical relative to the 1-adamantyl is less for the azo decompositions. Perhaps the mechanism of the azo decompositions of the more unreactive systems is different from that of, for example, the f-butyl azo compound (i.e. the rate determining step of the 1-norbomyl azo compound may be a one bond homolysis rather than the synchronous two bond fission of the f-butyl system312, 315)). Also, the smaller 1-norbornyl/1-adamantyl rate ratio for the f-butyl perester decompositions may be due to a greater influence of polar effects in these reactions 309a). This problem is under active investigation 309a). 

The stability of the tetrahedral purple alkyl Fe(l-norbomyl)4 is doubtless due to the steric bulk of the ligand. The (Me5C5)2Fe2+ ion is also reasonably stable in AlCl3-l-butylpyridinium chloride melts the yellow compound (7j6-toluene)Fe(H)2-(SiCl3)2 can tolerate short air exposures.60... 

A well-characterized CoIV compound is the remarkable alkyl complex tetrakis-(l-norbomyl)cobalt, which is made from CoCl2 and lithium norbomyl (nor). The... 

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