Norbornyl nucleophilic substitution

As discussed by Zollinger, 1995 (Sec. 7.5) this hypothesis of a detour around intermediates of very low stability is also useful for the differentiation of classical and nonclassical ion intermediates in nucleophilic substitutions of 2-norbornyl and related compounds. 

Certain nucleophilic substitution reactions that normally involve carbocations can take place at norbornyl bridgeheads32 (though it is not certain that carbocations are actually involved in all cases) if the leaving group used is of the type that cannot function as a nucleophile (and thus come back) once it has gone, e.g.,... 

Despeyroux, D. Cole, R. B. Tabet, J. C. Ion-molecule reactions in the gas phase. XVIII. Nucleophilic substitution of diastereomeric norborneols, norbornyl acetates and benzoates under ammonia chemical ionization, Org. Mass Spectrom. 1992, 27, 300-308. 

The low-temperature stability of bicyclo[2.2.1]heptane derivatives is well documented by the huge literature on liquid-phase norbornyl carbonium chemistry under such conditions, no ring expansion is observed, but instead, multiple Wagner-Meerwein rearrangements take place, preserving the bicyclo[2.2.1]heptane structure in the course of nucleophilic substitution or elimination reactions. This remarkable behavior is at the origin of the nonclassic carbonium ion concept (28). 

After this general discussion of the nucleophilic substitution of norbornane derivatives, we will concentrate on the deamination of exo- and eAzrfo-2-norbornyl-amine. The acetolysis of norbornanediazonium ions was first studied by Corey et al. (1963) and by Berson and Remanick (1964). Their results could not be interpreted easily either by Winstein s or by Brown s hypothesis, as seen, for example, in Bartlett s annotated reprint collection (1965), in which he emphasized the need for fully resolved reagents and for better separation methods. 

Irradiation of bridgehead bromides and iodides (RX) in a variety of solvents (SH) results in the formation predominantly of the nucleophilic substitution products (RS) as well as some hydrocarbon (RH), particularly with the bromides. Both radical and heterolytic cleavage of the photoexcited RX molecules are encompassed within the mechanisms discussed. The systems studied include 1-norbornyl, 1-norbornylmethyl, and 1- and 2-adamantyl bromides and iodides. Electrochemical and metal-ammonia reduction of 1,4-dihalogenonorbornanes yields mainly nor-bornane (and not the required [2,2,l]propellane) and some l,r-bisnorbornane. ... 

In most cases chiral carbonyl compounds also afford low stereoselectivity. As for the related Passerini reaction, even the use of aldehydes that are known to give excellent asymmetric induction in the reaction with other kinds of C-nucleophiles, results in low or moderate diastereoisomeric ratios. For example, both norbornyl aldehyde 39 [47] and a-alkoxyaldehyde 40 [3, 48] gave drs lower than 2 1 (Scheme 1.16). The same happens with ortho-substituted chromium complex 41 [49], which usually leads to very high asymmetric induction in other nucleophilic additions. Finally, //-substituted aldehyde 42 [50] gave poor results as well. 

---