Inductive and Polarizability Effects of an Exocyclic Diene-Iron Tricarbonyl Group. The acetolyses of exo- and endo-irontricarbonyl complexes of 5,6-dimethylidene-2-exo-norbornyl and 2,3-dimethylidene-7-anti-norbornyl parabromobenzenesulfonates

The exo- and endo-irontricarbonyl complexes of 5,6-dimethylidene-2-exo-norbornyl alcohols 10x, 10n , p-bromobenzenesulfonates 11x, 11n , acetate 12x and of the 2,3-dimethylidene-7-anti-norbornyl alcohols 17x, 17n , p-bromobenzenesulfonates 19x, 19n and acetates 20x, 20n have been prepared. The S_N1 buffered acetolyses of 11x, 19x and 19n gave 12x, 20x and 20n , respectively (retention of configuration). The first-order rate constants of the acetolyses have been evaluated and compared with those of the acetolyses of the uncomplexed 5,6-dimethylidene-2-exo-norbornyl ( 14 ) and 2,3-dimethylidene-7-anti-norbornyl p-bromobenzenesulfonates ( 18 ). A rate retardation effect of ca. 1.5 · 10⁵ was measured for 11x → 12x (65°) compared with the acetolysis of 14 . The retardation effect is larger (> 5 · 10⁷) with 11n . Contrastingly, the acetolysis 19x → 20x was slightly accelerated with respect to that of the uncomplexed p-bromobenzenesulfonate 18 . An unsignificant rate-retardation effect was measured for the acetolysis 19n → 20n . The results are interpreted in terms of competitive inductive destabilization and charge-induced dipole stabilizing interaction by the exocyclic diene-iron tricarbonyl fragment. PMO. arguments give a rationale for the difference in polarizability between the diene-Fe(CO)₃ group in 19 and that in the endo-7-norbornadienyl-iron tricarbonyl system.     

Regioselectivity of the Diels-Alder Additions of 2-Substituted 5,6 Dimethylidenenorbornanes and-bicyclo[2.2.2]octanes

The cycloadditions of methyl propynoate and methyl vinyl ketone to 5,6-dimethylidene-2-norbornanone ( 6 ) are para′-regioselective

1 “Para” (p) designs in this paper the 4, 9-disubstituted tricclo[6.2.1.0^{2, 7}] undecane- and 4, 9-disubsstituted tricycle[6.2.20^2,7] dodecane derivatives, “meta” (m) design the corresponding 4, 10-disubstituted compounds.

. A smaller para -regioselectivity is observed for the addition of methyl propynoate to 5,6-dimethylidene-2bicyclo[2.2.2]octanone ( 10 ). No regioselectivity is observed with 5,6-dimethylidene-2exo-norbornyl alcohol ( 3 ), acetate ( 5 ) and 5,6-dimethylidene-2exo-bicyclo[2.2.2]octanol ( 9 ). PMO arguments based on the shape of the HOMO's and subHOMO's of the dienes allow to rationalize these observations. Unpredictable para′- or ‘meta’regioselectivities are found for the Diels,-Alder additions of 5,6-dimethylidene-2endo-norbornyl alcohol ( 2 ), acetate ( 4 ) and 5,6-dimethylidene-2endo-bicyclo[2.2.2]octanol ( 8 ). The carbonyl group of β,γ unsaturated ketones such as 6 and 10 can act as an electron donating homoconjugated substituent. The n(CO) ? σ[C(1), C(2)] ? π[C(5), C(6)] hyperconjugative interaction can override the usual electron-withdrawing effect of this function.     

The Homoallyl-Cyclopropylcarbinyl Cation Rearrangement in the Solvolyses of exo- and endo-5-Norbornen-2-yl p-Bromobenzenesulfonates. Application of 2H-NMR. Spectroscopy

The buffered trifluoroethanolyses and acetolyses of exo-(2-D)- (6) and endo-(2-D)-5-norbornen-2-yl brosylates (7) yielded exo-5-norbornen-2-yl and 3-nortricyclyl derivatives. The deuterium distribution in these products was determined unambiguously by ²H-NMR. and MS. In contrast to previous reports, each hydrogen and, consequently, each deuterium atom could be identified. Product ratio and label distribution in the solvolysis of 6 make unnecessary the intervention of asymmetrical homoallylic cation intermediates. The results are most economically rationalized by invoking symmetrical 3-nortricyclyl ion-pair intermediates.     

The Circular Dichroism of 5,6-Dimethylidene-2-bicyclo[2.2.n]alky Esters. Chiral Exciton Coupling between Benzoate and Exocyclic s-cis-Butadiene Chromophores

The optically pure aryl-substituted 5,6-dimethylidene-2-bicyclo[2.2.1]heptyl benzoates 12–21 were prepared; their UV absorption and CD spectra are reported. The (?)-(1S,2S)-esters 17–21 with carbonyl groups in endo-position exhibit typical excitonsplit Cotton effects whereas the corresponding (?)-(1S,2R)-esters 12 - 16 with carbonyl groups in exo-position do not present such effects. The chiral exciton coupling between the exocyclic diene and a remote p-substituted benzoate chromophore can be used for unambiguous assignment of the absolute configuration of 5,6-dimethylidene-2-endo-bicyclo[2.2.1]heptyl derivatives. The method is applied to establish the absolute configuration of 5,6-dimethylidene-2-exo and -2-endo-bicyclo[2.2.2.]octyl p-bromobenzoates (?)- 24 and (?)- 25 .     

Synthesis and Diels-Alder Reactivity of 2,3,5,6-Tetramethylidenenorbornane

Pd-catalyzed double carbomethoxylation of the Diels-Alder adduct of cyclo-pentadiene and maleic anhydride yielded the methyl norbornane-2,3-endo-5, 6-exo-tetracarboxylate ( 4 ) which was transformed in three steps into 2,3,5,6-tetramethyl-idenenorbornane ( 1 ). The cycloaddition of tetracyanoethylene (TCNE) to 1 giving the corresponding monoadduct 7 was 364 times faster (toluene, 25°) than the addition of TCNE to 7 yielding the bis-adduct 9 . Similar reactivity trends were observed for the additions of TCNE to the less reactive 2,3,5,6-tetramethylidene-7-oxanorbornane ( 2 ). The following second order rate constants (toluene, 25°) and activation parameters were obtained for: 1 + TCNE → 7 : k₁ = (255 + 5) 10^?4 mol^?1 · s^?1, ΔH≠ = (12.2 ± 0.5) kcal/mol, ΔS≠ = (?24.8 ± 1.6) eu.; 7 + TCNE → 9 , k₂ = (0.7 ± 0.02) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.1 ± 1.0) kcal/mol, ΔS≠ = ( ?30 ± 3.5) eu.; 2 + TCNE → 8 : k₁ = (1.5 ± 0.03) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.8 ± 0.7) kcal/mol, ΔS≠ = (?26.4 ± 2.3) eu.; 8 + TCNE → 10 ; k₂ = (0.004 ± 0.0002) 10^?4 mol^?1 · s^?1, ΔH≠ = (17 ± 1.5) kcal/mol, ΔS≠ = (?30 ± 4) eu. The possible origins of the relatively large rate ratios k₁/k₂ are discussed briefly.     

Interaction between Exocyclic s-cis-Butadiene and Homoconjugated Functions. Preparation and Diels-Alder Reactivity of Remotely Substituted 2,3-Dimethylidenebicyclo[2.2.2]octanes

The preparations of 5,6-dimethylidene-2exo-bicyclo[2.2.2]octanol ( 8 ), its endo isomer 9 , 5,6-dimethylidene-2-bicyclo[2.2.2]octanone ( 10 ) and 2 exo, 3 exo-epoxy-5,6dimethylidenebicyclo[2.2.2]octane ( 11 ) are described. The kinetics of their cycloaddition to tetracyanoethylene has been measured in toluene at 25° together with those of 2,3-dimethylidenebicyclo[2.2.2]octane ( 7 ) and 5,6-dimethylidenebicyclo[2.2.2]oct-2-ene (12). The effects of remote substitution on the Diels-Alder reactivity of 2,3-dimethyl idenebicyclo[2.2.2]octanes are compared with those observed in the 2,3-dimethylidenenorbornane series ( 1–6 ).     

‘Lone Pair’ and ‘CI Bond’ Ionization Energies of exo- and endo-2-Norbornyl Iodides

A PE-spectroscopic study of exo- and endo-2-norbornyl iodides suggests that the relative ability of the 2-norbornyl group to stabilize an electron deficiency on a substituent X (e.g. I) in exo- or endo-position depends on the location of the positive charge. There is no difference if the positive hole is strongly localized on on the substituent X (e.g. the 5p^?1 state of the title compounds). On the other hand, our results indicate that teh positive hole semi-localized in an exo-C? X bond is better stabilized by the 2-norbornyl group than a semi-localized, positive hole in an endo-C? X bond.     

Stereoselective Synthesis of 2,4,6-Trimethylcyclohex-4-ene-1,3-diol Derivatives and of polypropionate fragments with four contiguous stereogenic centers

The Diels-Alder adduct (±)- 3 of 2,4-dimethylfuran and 1-cyanovinyl acetate was converted stereoselectively into benzyl 6-(4-chlorophenylsulfonyl)-1,3-exo,5-trimethyl-7-oxabicyclo[2.2.1]hept-5-en-2-exo-yl ( 26 ) and -2-endo-yl ether ( 36 ). Addition of LiAlH₄ to the latter led to the 3-O-benzyl derivatives 28 and 37 of (1RS,2SR,3SR,6SR)- and (1RS,2SR,3RS,6SR)-5-(4-chlorophenylsulfonyl)-2,4,6-trimethylcyclohex-4-ene-1,3-diol, respectively. Methylenation of 6-exo-(4-chlorophenylthio)-1-methyl-5-methylidene-7-oxabicyclo[2.2.1]heptan-2-one ( 16 ), obtained by reaction of (±)- 3 with 4-Cl-C₆H₄SCl and saponification gave, 6-exo-(4-chlorophenylthio)-1-methyl-3,5-dimethylidene-7-oxabicyclo [2.2.1]heptan-2-one ( 43 ), the reduction of which with K-Selectride afforded 6-exo-(4-chlorophenylthio)-1,3-endo-dimethyl-5-methylidene-7-oxabicyclo[2.2.1]heptan-2-endo-ol ( 44 ). The 3-O-benzyl derivative 48 of (1RS,2RS,3RS,6SR)-5-(4-chlorophenylsulfonyl)- 2,4,6-trimethylcyclohex-4-ene-1,3-diol was derived from 44 via based-induced oxa-ring opening of benzyl 6-endo-(4-chlorophenylsulfonyl)-1,3-endo-5-endo-trimethyl-7-oxabicyclo[2.2.1]hept-2-endo-yl ether ( 49 ). Benzylation of 28 , followed by reductive desulfonylation and oxidative cleavage of the cyclohexene moiety afforded (2RS,3SR,4RS,5RS)-3,5-bis(benzyloxy)-2,4-dimethyl-6-oxoheptanal ( 32 ).     

Reactions of mer(exo)‐ and mer(endo)‐[Co(dien)(dapo)X]2+ Isomers (X=OH,Cl, ONO2, N3)

Properties indirectly determined, or alluded to, in previous publications on the titled isomers have been measured, and the results generally support the earlier conclusions. Thus, the common five‐coordinate intermediate generated in the OH^?‐catalyzed hydrolysis of exo‐ and endo‐[Co(dien)(dapo)X]²⁺ (X=Cl, ONO₂) has the same properties as that generated in the rapid spontaneous loss of OH^? from exo‐ and endo‐[Co(dien)(dapo)OH]²⁺ (40±2% endo‐OH, 60±2% exo‐OH) and an unusually large capacity for capturing (R=[CoN₃]/[CoOH][]=1.3; exo‐[CoN₃]/endo‐[CoN₃]=2.1±0.1). Solvent exchange for spontaneous loss of OH^? from exo‐[Co(dien)(dapo)OH]²⁺ has been measured at 0.04 s^?1 (k₁, 0.50M NaClO₄, 25°) from which similar loss from the endo‐OH isomer may be calculated as 0.24 s^?1 (k₂). The OH^?‐catalyzed reactions of exo‐ and endo‐[Co(dien)(dapo)N₃]²⁺ result in both hydrolysis of coordinated via an OH^?‐limiting process =153 M ^?1 s^?1; =295 M ^?1 s^?1; K_H=1.3±0.1 M ^?1; 0.50M NaClO₄, 25.0°) and direct epimerization between the two reactants =33 M ^?1 s^?1; =110 M ^?1 s^?1; 1.0M NaClO₄, 25.0°). Comparisons are made with other rapidly reacting Co^III‐acido systems.     

Cyclodimerization of 1-(Dimethoxyniethyl)-5,6-dimethylidene-7-oxa-bicyclo[2.2.1]hept-2-ene Induced by Nonacarbonyldiiron. Crystal Structure of (1RS, 2SR, 3RS, 4RS, 4aRS, 9aSR)- Tricarbonyl[C, 2,3, C-η-(1,4-epoxy-1,5-bis(dimethoxymethyl)-2,3-dimethylidene-1,2,3,4,4a,9,9a,10-octahydroanthracene)]iron

The transition-metal-carbonyl-induced cyclodimerization of 5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene is strongly affected by substitution at C(1) While 5,6-dimethylidene-7-oxabicyclo[2.2.1]hept–2-ene-l-methanol ( 7 ) refused to undergo [4 + 2]-cyclodimerization in the presence of [Fe₂(CO)⁹] in MeOH, 1-(dimethoxymethyl)-5,6-di-methylidene-7-oxabicyclo[2.2.1]hept-2-ene ( 8 ) led to the formation of a 1.7:1 mixture of ‘trans’ ( 19, 21, 22 ) vs. ‘cis’ ( 20, 23, 24 ) products of cyclodimerization together with tricarbonyl[C, 5,6, C-η-(l-(dimethoxymethyl)-5,6-di-methylidenecyclohexa-1,3-diene)]iron ( 25 ) and tricarbonyl[C,3,4, C-η-(methyl 5-(dimethoxymethyl)-3,4-di-methylidenecyclohexa-1,5-diene-l-carboxylate)]iron ( 26 ). The structures of products 19 and of its exo ( 21 ) and endo ( 22 ) [Fe(CO)₃(1,3-diene)]complexes) and 20 (and of its exo ( 23 ) and endo (24) (Fe(CO)₃(1,3-diene)complexes) were confirmed by X-ray diffraction studies of crystalline (1RS, 2SR, 3RS, 4RS, 4aRS, 9aSR)-tricarbonyl[C, 2,3, C-η-(1,4-epoxy-1,5-bis(dimethoxymethyl])-2,3-dimethylidene-1,2,3,4,4a,9,9a,10-octahydroanthracene)iron ( 21 ). In the latter, the Fe(CO)₃(1,3-diene) moiety deviates significantly from the usual local C_s symmetry. Complex 21 corresponds to a ‘frozen equilibrium’ of rotamers with η-alkyl, η³-allyl bonding mode due to the acetal unit at the bridgehead centre C(1).     

Deux types de participation π lors de l'hydrolyse des diazocétones γ, δ-insaturées par des acides aqueux et des superacides. L'ion oxo-5-norbornyle-2

γ, δ-Unsaturated diazoketones undergo acid catalysed hydrolysis accompanied by cyclisation; the latter is favoured by suitable geometry (cyclopentenes) and by substitution by methyl groups. If both are present, rate enhancement by anchimeric assistance has been observed. Hydrolysis of 4-diazoacetyl-cyclopentene ( 1 ) yields a product mixture similar to that formed during solvolysis of 5-oxo-norbornyl-2-endo brosylate (23) and quite different from that of the exo isomer. The results are interpreted in terms of a common intermediate, the 5-oxo-2-norbornyl carbonium ion. Solvent participation in the transition state, i. e. partial S_N 2 character, is implied by the entropies of activation and by the action of an added nucleophile (Br^?). In superstrong acids, a different type of cyclisation takes place, involving the carbonyl oxygen and the protonated C? C double bond and forming tetrahydropyrane derivatives.     

Remote inductive effects on secondary and tertiary 2-norbornyl solvolysis rates

A 5-cyano substituent decelerates the solvolysis rate of exo-2-norbornyl brosylate $\text{(1-H)}$ by a factor of 1790. A much smaller deceleration (24–90 fold) is observed for the secondary $\text{endo}$ and for both $\text{exo}$ and $\text{endo}$ tertiary 2-norbornyl derivatives. These results support the occurrence of σ-participation in the solvolysis of $\text{1-H}$.     

A mass-spectral study of exo- and endo-2-norbornyl bromide and exo and endo-8-bromobicyclo[3.2.1]octane by photoionization

The photoionization, as well as the electron-impact, mass spectra of exo-and endo-norbornyl bromide and of exo-and endo-8-bromobicyclo[3.2.1]octane show that exo-Br loss is more facile than endo-Br loss in formation of C₇H₁₁^⊕ and C₈H₁₃^⊕, respectively. The relative intensity values are compared with solvolysis data from the same systems.     

Synthese und Hydrolyse von 6-exo-substituierten 2-Methyl-2-exo-norbornyl und 2-Methyl-2-endo-norbornyl-(2,4-dinitrophenyl)äthern Norbornane. 16. Mitteilung

The Synthesis and Hydrolysis of 6-exo-Substituted 2-Methyl-2-exo-norbornyl and 2-Methyl-2-endo-norbornyl 2,4-Dinitrophenyl Ethers The synthesis of the title compounds and their hydrolysis products in aqueous dioxane are described. Upon hydrolysis, the 2-exo-ethers 1 (X=N₂phO) as well as the 2-endo-ethers 2 (X=N₂phO) yield the corresponding 2-methyl-2-exo-norbornanols 3 only. Therefore, the 2-exo-ethers react with retention of configuration at C(2), the 2-endo-ethers 2 with inversion at C(2).     

Secondary 2-Norbornyl Cation Intermediates Substituted at C(5) and C(7) by Electron-withdrawing Groups. Addition of fluorosulfuric acid to unsaturated norbornane derivatives

Low temperature (?130° to ?110°) addition of exo-norborn-5-en-2-ol ( 7 ) to excess HSO₃F in SO₂CIF yielded a mixture of exo-5-(fluorosulfonyloxy)-exo-2- and endo-2-norbornylhydroxonium ions ( 9+10 ) under kinetic control that was different from the mixture of 9+10 obtained by addition of endo-norborn-5-en-2-ol ( 8 ) to HSO₃F under kinetic control. These mixtures differed from the mixture of 9+10 observed at higher temperature (?80° to ?60°) (thermodynamic control). Addition of 3-nortricyclanol ( 23 ) or exo-2, 3-epoxynorbornane ( 24 ) to HSO₃F at -?120° ± 10° yielded a mixture containing the exo-2-(fluorosulfonyloxy)-anti-7- and syn-7-norbornylhydroxonium ions ( 26+27 ) as major adducts. Qualitative rates of the isomerization of 26+27 to the more stable ions 9+10 and of the isomerization 9 ? 10 were evaluated. The solvolysis of 9+10 in HSO₃F yielded the exo-2, exo-5- and exo-2, endo-5-norbornanediyl bis (fluorosulfates) ( 21+22 ). Norbornadiene and quadricyclane added 2 equivalents of HSO₃F and furnished kinetically a mixture of exo-2, anti-7- and exo-2, syn-7-norbornanediyl bis (fluorosulfates) ( 36+37 ) as major adducts. The latter 36+37 were isomerized into a kinetic mixture of the more stable isomers 21+22 . The rates of these isomerizations were compared. The use of DSO₃F and (exo-2-D)-norborn-5-en-2-ol ( 15 ) confirmed that heterolyses of the fluorosulfates were responsible for the observed isomerization; elimination-addition processes occurred but much more slowly. The results are interpreted in terms of substituted classical and σ-bridged secondary 2-norbornyl cation intermediates. It appears that the electron withdrawing substituents FSO₃ and H₂O⁺ (HO) destabilize the σ-bridged 2-norbornyl cation more at C(5) than C(7). If the σ-bridged ions 5-Z substituted at C(5) by Z = FSO₃ or H₂O⁺ (HO) are transition states in the isomerization of the corresponding classical ions 3-Z, 4-Z , the free enthalpy difference between the ‘non-classical’ σ-bridged ion and the classical ions is not higher than the energy barrier to the quenching of the latter intermediates by FSO in HSO₃F/SO₂CIF.     

The Reaction of Norbornene with Lead Tetraacetate and Thallium Trinitrate. Preliminary communication

The reaction of norbornene with lead tetraacetate is found to be much more complex than previously reported. In acetic acid and in benzene, the syn-7-norbornenyl, 3-nortricyclyl, and syn and anti-7-acetoxy-exo-2-norbornyl acetates were characterized. In methanol, the isolated products represented most of those expected from the competition of methanol and acetate in the neutralization of the intermediate carbocations. The reaction of norbornene with thallium trinitrate in the above solvents yielded very complex mixtures besides the above mentioned products which were formed in about 50% yield.     

Norbornanes. Part 18. Inductive Charge Dispersal in the Solvolyses of 4- and 5-Substituted 2-Norbornyl p-Toluenesulfonates

The solvolysis rates and products of 4- and 5-exo-substituted 2-exo- and 2-endo-norbornyl tosylates 9 and 10 , respectively, are reported. The logarithms of the rate constants (log k) correlate linearly with the inductive constants σ for the substituents. A comparison of the reaction constants p₁ for the 4-, 5-, 6-, and 7-substituted 2-exo- and 2-endo-tosylates 9 , 10 , 1 , and 2 respectively, indicates that inductivity is higher for 2-exo-ionization than for 2-endo-ionization in all series. This observation is attributed to the more favorable alignment of neighbouring C-atoms for dorsal participation in exo-ionization, especially, in the case of C(6).     

Exo- und endo-Tricarbonyleisenkomplexe von bicyclischen 2,3-Dimethylidenverbindungen

Exo- and endo-Tricarbonyliron Complexes of Bicyclic 2,3-Dimethylidene Compounds. The preparation of exo- and endo-tricarbonyliron complexes (exo- and endo- 5 , -6 , -8 , and 9 ) of 2,3-dimethylidene-5-bicyclo[2.2.1]heptene( 1 ), -bicyclo[2.2.1]-heptane ( 2 ), -5-bicyclo[2.2.2]octene ( 3 ) and -bicyclo[2.2.2]octane ( 4 ) is described. The complexes are obtained by thermal reaction of the bicyclic butadienes with di-ironenneacarbonyl in hexane solution. exo- and endo- 5 are also formed photochemically from ironpentacarbonyl and 1 in pentane solution at ?35°. The structural assignment of exo- and endo -5 and -6 is based on their mass-spectra and on coordination shifts in ¹H- and ¹³C-NMR.-spectra exo- and endo -6 are correlated with exo- and endo -5 , respectively, by hydrogenation. Hydrogenation of the uncomplexed double bond in exo- and endo -5 occurs in both complexes from the exo side as shown by deuteration experiments. The free ligand 1 reacts in the same stereospecific manner.     

Carbon Participation in the Solvolysis of 6- and 7-Substituted 2-Norbornyl p-Toluenesulfonates. Norbornanes,Part 11

The solvolysis rates and products of several 7-anti-substituted 2-endo-norbornyl p-toluenesulfonates 11 have been determined and compared with those of the previously reported 6-exo-substituted 2-exo-norbornyl p-toluenesulfonates 1. Although the number of bonds between the substituent and the reaction site is the same in the two series, the inductive effect of the substitutents is transmitted far more strongly in the 6-exo-2-exo-series 1 than in the 7-anti-2-endo-series 11 ; i.e. their inductivities differ widely. It is concluded that through space induction involves graded bridging of the substituent-bearing C-atom to the incipient cationic center at C(2) and that this involves differential bridging strain. The different reactivities of unsubstituted 2-exo- and 2-endo-norbornyl derivatives can then be ascribed to a stereoelectronic effect.     

Norbornanes. Part 20. Inductivity and bridging in 2-norbornyl cations

The solvolysis rates and products of several 1-substituted 2exo- and 2-endo-norbornyl p-toluenesulfonates 7 and 8 , respectively, have been determined. Hydrolyses of these epimeric tosylates yielded rearranged products in varying amounts, except when the substituent was COOCH₃ or CN. The logarithms of the rate constants (log k) for the endo-series 8 correlated linearly with the corresponding inductive constants σ with a reaction constant ρ_I of ?1.24. On the other hand, log k values for the exo-series 7 appear to fit two regression lines, the first line (ρ_I = ?1.90) defined by the tosylates that ionize, with rearrangement, to the tertiary cations 11 , the second (ρ_I = ?1.86) by the tosylates 7 (R = H, COOCH³, and CN) that ionize to an asymmetrically bridged secondary cation 19 . These results confirm the unique participation of C(6) with a ρ_I of ?2.00 in the ionization of 2-exo-nor-bornyl tosylate.