Bornyl cation

The chemical networks around a- and / -pinene are versatile. The four-membered ring is strained upon H+-catalysis an alkyl shift takes place and the bornyl cation is formed, which can enter various reactions. In contrast, the four-membered ring remains intact upon epoxidation or hydrogenation. 

Protons and carbenium ions always add to C=C double bonds via carbenium ion intermediates simply because no energetically favorable onium ions are available from a reaction with these electrophiles. An onium intermediate formed by the reaction of a proton would contain a divalent, positively charged H atom. An onium intermediate produced by the reaction of a carbenium ion would be a carbonium ion and would thus contain a pentavalent, positively charged C atom. Species of this type are generally rare, but an excellent example is the nor-bornyl cation (Figure 2.29). 

The related intramolecular monoalkene complexes of R3E + cations, the nor-bornyl cations of group 14 elements (Si —> Pb), have been synthesized with TPFPB as counteranion and characterized by NMR spectroscopy in solution supported by quantum mechanical calculations.40 Only the acetonitrile complex, 73, of the plumbanorbornyl cation 13 could be characterized by X-ray crystallography. Similar to the structure of the plumbyl cation 74, the Pb atom in the acetonitrile complex 73 adopts a trigonal-bipyramidal coordination sphere with the C = C double bond and the acetonitrile in the apical positions and the Pb atom in the center of the trigonal plane (see Fig. 11 and Table XIV, for details).40... 

In the present review only some of the more recent results on the 2-nor-bornyl cation will be treated and examples of the different types of degenerate reactions will be given. 

H shifts compete effectively with capture by solvent. For the parent 2-nor-bornyl cation the kc/ku ratio is ca. 1 in formic acid, ca. 2 in acetic acid, and ca. 6 in aqueous acetone50, 516. The decomposition of 2-norbornanediazonium ions in aqueous sodium hydroxide, however, proceeds with less than 4% 6,2-H shift528). Tertiary, secondary shifts, e.g. (749) - (756)505), occur more readily than secondary, secondary shifts do. Steric effects also play some role there is more 6,2—H... 

A specialized cation rearrangement first observed with bornyl systems is the acid catalyzed rearrangement of camphene hydrochloride (73) to isobornyl chloride (76).32 The reaction, which proceeds by a 1,2-alkyl shift called the Wagner-Meerwein rearrangement, initially generates bornyl cation 74, but rearrangement to... 

For 2,3- and 1,2-dimethylnorbomyl ions the values of A5 are 287 and 274 ppm, respectively, lower than for classical 1,2-dimethylcyclopentyl and 1,2-dimethylcyclo-hexyl ions ( 350 ppm) being in a rapid degenerate equilibrium. These facts agree with the stmcture earlier ascribed by Olah to the l,2-dimethyl-2-norbomyl ion — a structure of partially c-delocalized, rapidly equilibrated ions (Wagner-Meerwein rearrangement) The same applies to Saunders s data on the 1,2-dimethylnor-bornyl cation in which one CHj is replaced by CDj (see below)... 

Gream, G. E., D. Wege, and M. Mular The Camphenehydro (Methylcamphenilyl) and Isobornyl (Bornyl) Cations. I. Generation of the Enantiomeric Cations by the n- and a-Routes of Solvolysis. Austral. J. Chem. 27, 567 (1974). 

Winstein proposed that the much faster rate for the solvolysis of the exo-nor-bornyl brosylate (11) was the result of participation by the 1,6-bonding pair in the transition state, leading to the formation of a a-bridged (nonclassical) cation intermediate (15) (7). The nonclassical cation 15 possesses a plane of symmetry. Consequently, the acetate product, 12, must be optically inactive. 

Curiously, certain cyclases, notably (+)-bornyl pyrophosphate cyclase and (-)-endo-fenchol cyclase, are capable of cyclizing, at relatively slow rates, the 3S-linalyl pyrophosphate enantiomer to the respective antipodal products, (-)-bornyl pyrophosphate and (+)-endo-fenchol (74,75). Since both (+)-bornyl pyrophosphate cyclase and (-)-endo-fenchol cyclase produce the designated products in optically pure form from geranyl, neryl and 3R-linalyl pyrophosphate, the antipodal cyclizations of the 3S-linalyl enantiomer are clearly abnormal and indicate the inability to completely discriminate between the similar overall hydrophobic/hydrophilic profiles presented by the linalyl enantiomers in their approach from solution. The anomalous cyclization of the 3S-enantiomer by fenchol cyclase is accompanied by some loss of normal regiochemical control, since aberrant terminations at the acyclic, monocyclic and bicyclic stages of the cationic cyclization cascade are also observed (74). The absolute configurations of these abnormal co-products have yet to be examined. 

Irradiation of electron deficient arenes in the presence of cis-l,2-diphenylcyclopropane leads to formation of the trans isomer by an electron transfer mechanism. The reaction occurs by way of the radical cation of the cyclopropane which isomerises prior to back electron transfer. It has now been examined using menthyl and bornyl esters of benzene tetracarboxylic acid as chiral electron transfer sensitisers. °° Slight excesses of one of the enantiomers of the trans-1,2-diphenylcyclopropane were observed. The dicyanoanthracene sensitised reactions of 1,1,2,3-tetra-arylcyclopropanes have been studied.Depending on the substituents present on the arene rings these compounds rearrange to 1,1,3,3-tetra-arylpropenes. The rearrangement occurs in a ring opened radical cation intermediate. 

Complexes of unsymmetrical dienes are chiraF, and the reaction stereospedficity results in formation of a new asymmetric center, fully resolved and of known absolute configuration relative to the precursor . This is useful in organic synthesis, since cations can be resolved through related neutral complexes such as bornyl ethers or carboxylic acids". 

The structures, stabilities, and Si NMR chemical shifts for pairs of silabicyclo[2.2.2]octyl and a pair of silanor-bornyl derivatives, 50/51, 52/53, and 54/55, all show that introduction of the /3-SiMe group stabilizes the cation and decreases pyramidalization at the cation center, consistent with the well-known stability of /3-silyl versus /3-alkyl carbenium ions <2003JOC1827>. 

The bicyclo[2,2,2]octyl cation, as shown by calculations, is less stable than the 2-nor-bornyl one both in the gas phase (by about 6 kcal/mole) and in solutions At the same time the angle strain in the 2-norbomyl ion is higher, and the torsional and long-range nonbonded effects are comparable with those for the bicyclo[2,2,2]-octyl ion the difference seems to be due to the norbomyl cation geometry more favourable for a-delocalization as compared with the bicyclo[2,2,2]octyl ion. 

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