Norbornyl derivatives

Three-bond carbon-phosphorus couplings JCP depend on configuration, as expected, transoid couplings being larger than cisoid ones, as demonstrated for cyclohexyl- and 2-norbornylphosphonates in Table 4.52. The Karplus relation (4.13) can be derived from the data for 2-norbornyl derivatives [376]. 

Figure 6.14 (a) Probable reaction coordinate diagram for the solvolysis of exo-2-norbornyl derivatives, (b) Probable reaction coordinate diagram for solvolysis of endo-2-norbornyl derivatives. 

If the norbornyl cation is formed on solvolysis of < xo-norbornyl derivatives, C2 should become equivalent to C and C7 to C3. In a most elegant tracer experiment, Roberts and Lee synthesized w-2-norbornyl-[2,3-14C] brosylate (85), solvolyzed it in acetic acid, and degraded the product. Equation 6.39 shows the product distribution expected if the symmetrical carbonium ion (82) were formed. The label was found not only at Cx, C2, C3, and C7, but also at C5 and... 

Enholm [13] has also described the synthesis of soluble designer supports by the ring-opening metathesis polymerization (ROMP) of norbornyl derivatives. Reduction of norbornene-l-carboxaldehyde 88 to the corresponding alcohol 89, followed by treatment with either 2-bromopropionic acid or 2-bromo-2-phenylacetic acid in the presence of DCC, provided the esters 90 or 91 respectively (Scheme 19). Polymerization of 90 and 91 was carried out with Grubbs catalyst and halted after 25 s by capping with excess ethyl vinyl ether to give polymers 92 and 93 respectively. 

Fig. 5 Selectivity-reactivity relationship, p vs. log A plots, for solvolyses of a,a-dialkylbenzyl OPNB in 80% aqueous acetone at 25°C plots of a,a-dialkylbenzyl-OPNB carrying dialkyl groups structurally unspecified, otherwise specified O, benzyl-OPNBs squares denote 2-norbornyl derivatives, exo- and endo-isomors , benzyl-OPNBs having conjugative a-substituents.

In a study of 3-spirocyclopropyl substituted 2-norbornyl p-nitrobenzoates in 70% aqueous acetone, Wilcox and Jesaitis found rate enhancements of about 10 and 10 for the exo- and ewiio-derivatives 41 and 42, respectively, over the corresponding non-spiro systems. The relative rates of the exo- and en o-3-spirocyclopropyl norbornyl derivatives... 

Sodium amalgam reduction of organomercurials in alkaline deuterium oxide gives products with stereospecific retention during the replacement of mercury by deuterium [71]. This was shown in the reduction of cw-8-acetoxymercuri- (Ila) and c75-8-chloromer-curidibenzobicylo[2.2.2]octadien-7-ol acetates (Ilb), /ra/75-8-chloromercuridibenzobicy-clo[2.2.2]octadien-7-ol, and some norbornyl derivatives. Reduction of these compounds with sodium borodeuteride gave products without stereospecific deuterium incorporation [71]. 

The racemization of optically active 2-norbornyl derivatives in the course of solvolysis can be accounted for, as Winstein himself appreciated8, either in terms of a rapidly equilibrating pair of classical carbocations (18) or in terms of the formation of a symmetrical nonclassical species (19-25). Consequently, the problem cannot be resolved solely on the basis of such racemization. 

Proposal No. 1. It was originally proposed back in 1966 that steric effects in the tertiary 2-methyl-2-norbornyl derivatives would be very large, with the steric requirements of the methyl substituent far exceeding those for the acyloxy group. 

The isomerization of thioncarboxylates to thiolcarboxylates has been less extensively studied, but also served to demonstrate the formation of a symmetrical (or rapidly equilibrating) intermediate from the norbornyl derivative (122). Acetolysis of optically active (122) gave ka = kt, i.e., there is no ion pair return to (122). The products were racemic and a deuterium label at C—2 of (122) was equally distributed among C-l and C—2 of the products122. ... 

These observations clearly demonstrate the dominance of steric effects in tertiary norbornyl derivatives but do not necessarily apply to secondary systems. The different response to em-dimethyl substitution in (701)90 and (702)494 as compared with (699) and (700) is a disturbing feature. 

These conclusions are supported by data on the tertiary 2-norbornyl esters (723) 510 and (724)S11) in which the C(3)-D2 effect is nearly independent of configuration. The kH/kD of (724) is much smaller than that of (723), due to charge delocalization into the phenyl ring. 6,6-Dideuteration has a negligible effect on the solvolysis rate of 1,2-dimethyl-2-norbornyl p-nitrobenzoate (725)S12 Although the interpretation of secondary deuterium isotope effects is often difficult, comparison of the various norbornyl derivatives discussed here has uniformly been claimed to support alkyl participation. 

Resume. Alkyl participation in the solvolysis of exo-2-norbornyl derivatives is not unambiguously detected by rate measurements. Neither monocyclic analogs nor the corresponding enr/o-2-norbornyl compounds appear to be good models for comparison. Secondary deuterium kinetic isotope effects, however, point to distinct ionization mechanisms of exo- and -norbornyl derivatives. 

The boron adds stereoselectively to the less sterically hindered face of the alkene. Reaction with norbornene, for example, gave 99.6% of the exo-borane (5) with only 0.4% of the endo- borane. l This is consistent with the normal addition of reagents to the exo face of norbornyl derivatives (sec. 1.5.E). 

In 1987 more than one hundred substituted 2-norbornyl derivatives were known (Laube, 1987). 

A similar statement was also made by Manuilov and Barkhash (1990), who mentioned two extreme cases of yields (0.039 and 0.114 <7o) in a paper of Collins and Benjamin (1972). These examples are, in our opinion, not completely appropriate in this context, as Collins and Benjamin used these figures only for numbering exo/endo ratios of norbornyl derivatives where the other stereoisomer was formed in yields of and 19 7o, respectively, i. e., where the large dominance of one isomer... 

Adapting the above-mentioned precedents in norbornene polymerization chemistry, the University of Texas research team was able to synthesize cycloaliphatic polymers with pendant-free carboxylic acid groups and carboxylic acids protected with acid-cleaveable groups, such as tert-hvAy esters tailored for 193-nm resist use. They polymerized norbornyl derivatives such as CBN and... 

This conclusion seems to contradict the data on the 1-alkyl-substituted norbomanes. Sargent however, considers the 1-phenyl group to interact with the developing cation centre of C only through the inductive mechanism, but without any meso-meric effect. The phenyl group fails to exert a stabilizing effect on the transition state of ionization of 2-exo-norbornyl derivatives A similar weak effect of phenyl... 

Lately Brown investigated the exo endo solvolysis rate ratio of 2-norbomyl chlorides in aqueous acetone with widely varied water content This work included linear correlations of log k, on log k , as well as those of log k, and log k, , on log k (tert-BuCl) the latter is a model k process. The data obtained agree with the assumptions that the solvolysis of exo and endo epimers are either k and k processes, respectively, or they are both processes of type k the results were found to be independent of the leaving group and hence of F-strain (cf. According to Brown introducing the a-CHj group to epimeric 2-norbomyl chlorides results in about the same kinetic effect the solvolysis of exo- and endo-norbornyl derivatives, both secondary and tertiary, proceeds by route k. ... 

Anchimeric assistance to ionization is only observed when the adjacent group participates by formation of an intermediate more stable than the one without such I rticipation. Consequently, the fact of anchimeric assistance in the solvolysis of 2-exo-norbornyl derivatives is a convincing evidence for the formation of the nonclassical ion 5 as an intermediate in this reaction. 

The PMR spectra give no possibility to chose between the nonclassical ion 5 and the rapid equilibrium of a pair of enantiomeric classical ions 6. The ion structure was therefore studied by Raman laser spectra, because the rates of vibrational transitions are far higher than those of Wagner-Meerwein rearrangements. The spectrum of the norbomyl cation proved to be very similar to that of nortricyclejie and quite different from that of norbomane. For rapidly equilibrated classical ions 6, the RS spectrum would be similar to that of norbornyl derivatives. Besides, the rapid equilibration, according to Olah, would lead to the doublet splitting of the bands of some skeletal deformational vibrations which is not observed. 

Participation of a Jt bond to generate nonclassical carbocation has been shown during the acetolysis of tosylates 4-6. Among the norbornyl derivatives the anti-tosylate (6) on acetolysis reacts 10 times faster than its saturated analog (4) while 5 has 10 times greater reactivity than 4 (Figure 2.16). 

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