Norbomyl

Phenyl norhornane is benzoylated faster than isopropylbenzene or toluene despite the bulkiness of the norbomyl group probably because of hyperconjugation (87). Hyperconjugation of the C—C bond is at least as or more important as that of the C—H bond since 1-phenylnorhornane has no a-hydrogen atom. 

The reactions of trialkylboranes with bromine and iodine are gready accelerated by bases. The use of sodium methoxide in methanol gives good yields of the corresponding alkyl bromides or iodides. AH three primary alkyl groups are utilized in the bromination reaction and only two in the iodination reaction. Secondary groups are less reactive and the yields are lower. Both Br and I reactions proceed with predominant inversion of configuration thus, for example, tri( X(9-2-norbomyl)borane yields >75% endo product (237,238). In contrast, the dark reaction of bromine with tri( X(9-2-norbomyl)borane yields cleanly X(9-2-norbomyl bromide (239). Consequentiy, the dark bromination complements the base-induced bromination. 

Me3SiOCH3, Me3SiOTf, CH2CI2, —78°, 86% yield. A norbomyl ketone was not ketalized under these conditions. 

The acetolyses of both ero-2-norbomyl brosylate and e do-2-norbomyl brosylate produce exclusively exo-2-norbomyl acetate. The exo-brosylate is more reactive than the endo isomer by a factor of 350. Furthermore, enantiomerically enriched exo-brosylate gave completely racemic ero-acetate, and the endo-brosylate gave acetate that was at least 93% racemic. 

The description of the nonclassical norbomyl cation developed by Wnstein implies that the nonclassical ion is stabilized, relative to a secondary ion, by C—C a bond delocalization. H. C. Brown of Purdue University put forward an alternative interpreta-tioiL He argued that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical ion. The 1,2-shift that interconverts the two ions was presumed to be rapid relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown proposed that die rapid migration would lead to preferential approach of the nucleophile fiom the exo direction. 

Fig. 5.11. Contrasting potential energy diagrams for stable and unstable bridged norbomyl cation. (A) Bridged ion is a transition state for rearrangement between classical structures. (B) Bridged ion is an intermediate in rearrangement of one classical structure to the other. (C) Bridged nonclassical ion is the only stable structure.

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

Fig. 5.12. Crystal structures of substituted norbomyl cations. (A) 1,2,4,7-Tetramethylnorbomyl cation (reproduced from Ref. 154 by permission of Wiley-VCH). (B) 2-Methoxy-l,7,7-trimethyl-norbornyl cation (reproduced from Ref 155 by permission of the American Chemical Society).

Another line of evidence that bridging is important in the transition state for solvolysis has to do with substituent effects for groups placed at C-4, C-5, C-6, and C-7 on the norbomyl system. The solvolysis rate is most strongly affected by C-6 substituents, and the exo isomer is more sensitive to these substituents than is the endo isomer. This implies that the transition state for solvolysis is especially sensitive to C-6 substituents, as would be ejqiected if the C(l)—C(6) bond participates in solvolysis. ... 

Many other cations besides the norbomyl cation have nonclassical structures. Scheme 5.5 shows some examples which have been characterized by structural studies or by evidence derived from solvolysis reactions. To assist in interpretation of the nonclassical stmctures, the bond representing the bridging electron pair is darkened in a corresponding classical stmcture. Not surprisingly, the borderline between classical stmctures and nonclassical stmctures is blurred. There are two fundamental factors... 

In the discussion of the syn- and anft-norbomenyl tosylates (p. 312), it was pointed out that, relative to 7-norbomyl tosylate, the reactivities of the syn and anti isomers were 10 and 10, respectively. The high reactivity of the anti isomer was attributed to participation of die carbon-carbon double bond. What is the source of the 10 factor of acceleration in the syn isomer relative to the saturated model ... 

Figure 7-2. Hammett plot for solvolysis of substituted norbomyl esters (7) and norbomenyl esters (8) in 70 30 dioxaneiwater at 25°C. ...

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... 

Corrole is a tetrapyrrolic macrocycle 1-Norbomyl is a bicyclo[2.2.1]hept-l-yl... 

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... 

The situation grows more complex if, concurrent with racemization, chemical products are formed. The case of the optically active norbomyl cation comes to mind, since both exo and endo products can be formed in essentially irreversible reactions. This is diagrammed as follows ... 

D. Kinetic and Thermodynamic Control in the Reversible Carbonylation the 2-Norbomyl Cation. ... 

The stable 2-norbornyl cation has recently been shown to be a non-classical, unusually stabilized species. Olah et al. (1970) proved spectroscopically that this ion is a comer-protonated nortricyclene with a pentavalent carbon atom. The value for the carbonylation-decarbonyla-tion equilibrium constant K (= of the 2-norbomyl ion illustrates... 

The value for K was determined to be 10 litre mole at 20°C, which is of the same order of magnitude as those for tertiary alkyl cations ((0-07 — 2) X 10 litre mole . Section II) and dramatically different from those for secondary alkyl cations (about 10 ° litre mole , calculated from Figs. 2 and 3). These data show that the 2-norbomyl ion is only 1-6 kcal mole less stabilized than, for example, the tertiary butyl cation and about 8 kcal mole" more stabilized than secondary alkyl cations. Another thermodynamic argument for the high stability of the 2-norbornyl ion in solution is found in the work of Amett and Larsen (1968)... 

Although carbonylation of the 2-norbomyl ion at or below room temperature leads to exclusive formation of the 2-ea o-norbomyloxo-carbonium ion, reactions at higher temperatures have shown that the 2-cwdo-norbornyloxocarbonium ion is just as stable as the exo-isomer (Hogeveen and Roobeek, 1969). This means that at low temperatures the carbonylation is kineticaUy controlled, and at high temperatures thermodynatnically controlled. The detailed free-enthalpy diagram in... 

Further evidence for the SnI mechanism is that reactions run under SnI conditions fail or proceed very slowly at the bridgehead positions of [2.2.1] (norbomyl) systems (e.g. 1-chloroapocamphane, 8). 

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