The lowering of CBH stretching frequencies in carbocations 1a-d and 2a-c induced by hyperconjugation was tested as a possible probe for estimating the electron donating ability of a-substituents. Conclusions are based on the results of high level quantum chemical calculations confirmed with experimental FT-IR spectra. Because the decrease in the CBH stretching frequency is comparable in Ib and in Ic, and in 2b and 2c respectively, it follows that a-substitution by a methyl group or by chlorine stabilizes a carbocation with almost the same effectiveness.

Addition of dibromomaleic anhydride to tetrachlorocyclopentadienone dimethyl acetal gives the pseudo-lactone 11(X-ray crystal structure analysis) and not the mixture of endo- and exo-adducts 2 previously claimed; reduction of this product with Zn/HOAc gives 13 and 14 rather than the putative isolable norbornadienone acetyl 3.

The first direct observation by IR spectroscopy in the cryogenic SbF5 matrix was made of the rearrangement of 2- and 3-chlorobicyclo[3.2.0]heptanes (3) and (4), respectively, to the 7-norbornyl cation 1 and its subsequent transformation into the 2-norbornyl cation 5.

A systematic investigation was undertaken of the effect of bridgehead substitution on the solvolytic reactivity of 7-norbornyl triflates in aqueous TFE. Methyl substitution increases the rate 60-fold and the effect of multiple substitution is additive. Chlorine, on the other hand, exhibits a combined inductive and resonance effect and decreases the reactivity 1700 times. The observed normal secondary P-deuterium isotope effect of the 1,4-d(2) derivative can be rationalized by assuming a tilted C-s geometry for the cationic transition structure which is only slightly higher in energy than the nonclassical intermediate cation of C-1 symmetry. Bridgehead methyl-d(3) kinetic isotope effects are similar to those associated with the nonmigrating methyl group in neopentyl ester solvolyses. These results are in agreement with the recently published structure of the 7-norbornyl cation calculated at the highest ab initio level which has confirmed its nonclassical nature as originally proposed by Winstein.

The cryochemical matrix isolation method has been used to measure the down frequency shift of C=O stretching vibrations (DELTAnu) which originate from ketones complexing with SbF5. Such ketone-SbF5 complexes can serve as good models of analogous carbocations. The weakening of the C=O bond in the complex is a consequence of its increased polarization. The influence of the hydrocarbon skeleton on the stabilization of the incipient positively charged carbon atom is qualitatively similar to that in carbocations. Examined ketones 1-23 can be sorted into two categories. Complexes of ketones similar to classical carbocations belong to the first category. Because in this group the inductive donation of electrons to the positively charged carbon atom is the dominant effect, the DELTAnu values correlate well with the number of C(alpha)-C(beta) bonds. Ketone complexes in which the DELTAnu values substantially deviate from the linear correlation belong to the second category where the structures of the hydrocarbon skeleton are the same as in carbocations in which other stabilizing effects are operative. The observed deviation from linearity can be rationalized by other effects such as hyperconjugation, bridging, participation and homoaromaticity. This view has been supported by semiempirical and ab initio calculations.

AbstractThe IR spectra of isomeric bicyclobutonium (1), delocalized cyclopropylcarbinyl (2) and 1‐methylally (3) cations were recorded at 180 K in SbF5 matrices. Cations 1 and 2 generated from cyclopropylcarbinyl and cyclobutyl chloride, respectively, rearrange to 3 at temperatures above 230 K. The structures 1, 2 and 3 were confirmed by comparison of the recorded frequencies with the MP2/6–31G*‐calculated values. These results are in accord with prediction that ions 1 and 2 are rapidly equilibrating non‐classical structures.

The solvolysis of 2-(ω-methylthioalkyl)- and 2-(ω-benzylthioalkyl)-3-methyl-2-cyclohexenyl p-nitrobenzoates 5 and 6 in 80 vol % ethanol includes several competitive reactions. Path k1 is a stepwise process (kH/kD = 1.18–1.20) which includes allylic cation 9 as a reaction intermediate, while path k2 involves a neighboring sulfur participation (kH/kD = 1.01–1.03) and formation of an intermediate cyclic sulfonium cation 10. Esters 5 and 6 solvolyze in 80% EtOH with a greater neighboring group participation than in 97% TFE. It has been concluded that in solvolytic reactions of allylic substrates a pronounced neighboring group participation (k2/k1 = 900) may occur only with very strong internal nucleophiles, and in solvents which can not form strong hydrogen bonds with these nucleophiles.

Direct observation of the 7-norbornyl cation was achieved for the first time by spectroscopy in an SbF5 matrix at low temperatures. Ab initio calculations at the highest level and the comparison of calculated and experimental IR spectra suggest that the unsymmetrically bridged structure 1 is the most probable for this cation.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Reaction of MeCHCl2, CH2ClCH2Cl and CH2BrCH2Br respectively with SbF5 in a solid matrix at 77-200 K affords the most stable cations 2 and 3 as predicted by previously published high-level ab initio calculations.

The IR spectrum of the 2-butyl cation, measured at –125 °C in an SbF5 matrix and calculated ab initio(MP2(FULL)/6-31G*), is characterized by a C(H [graphic omitted] ) C peak at 2175 cm–1 assigned to the nonclassical H-bridged (C2-symmetry) structure 1.

The symmetrically H-bridged structure of the cyclooctyl cation is confirmed definitively by the characteristic IR vibration at 1845 cm-1 and by the agreement of the IGLO chemical shifts calculated with MP2/6-31G* geometry of the chair-boat (C(s)) conformation with the experimental NMR values.

ChemInformVolume 21, Issue 18 Physical Organic Chemistry ChemInform Abstract: Experimental and Theoretical IR Spectra of the 2-Norbornyl Cation W. KOCH, W. KOCH Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this authorB. LIU, B. LIU Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this authorD. J. DEFREES, D. J. DEFREES Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this authorD. E. SUNKO, D. E. SUNKO Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this authorH. VANCIK, H. VANCIK Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this author W. KOCH, W. KOCH Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this authorB. LIU, B. LIU Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this authorD. J. DEFREES, D. J. DEFREES Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this authorD. E. SUNKO, D. E. SUNKO Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this authorH. VANCIK, H. VANCIK Wiss. Zent. Heidelberg, IBM Deutschland GmbH, D-6900 HeidelbergSearch for more papers by this author First published: May 1, 1990 https://doi.org/10.1002/chin.199018036Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume21, Issue18May 1, 1990 RelatedInformation

ChemInformVolume 20, Issue 35 Physical Organic Chemistry ChemInform Abstract: Solid-State Chemistry in Antimony Pentafluoride Matrices. IR Spectra of Reactive Intermediates H. VANCIK, H. VANCIK Dep. Chem., Fac. Sci., Univ. Zagreb, 41000 Zagreb, Croatia, Yugosl.Search for more papers by this authorD. E. SUNKO, D. E. SUNKO Dep. Chem., Fac. Sci., Univ. Zagreb, 41000 Zagreb, Croatia, Yugosl.Search for more papers by this author H. VANCIK, H. VANCIK Dep. Chem., Fac. Sci., Univ. Zagreb, 41000 Zagreb, Croatia, Yugosl.Search for more papers by this authorD. E. SUNKO, D. E. SUNKO Dep. Chem., Fac. Sci., Univ. Zagreb, 41000 Zagreb, Croatia, Yugosl.Search for more papers by this author First published: August 29, 1989 https://doi.org/10.1002/chin.198935038Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume20, Issue35August 29, 1989 RelatedInformation

In an attempt to study possible π -participation in allyl derivatives, 3-alkenyl-5,5-dimethyl-2-cyclohexenyl p-nitrobenzoates 4 and 2-alkenyl-3-methyl-2-cyclohexenyl p-nitrobenzoates 5 were solvolyzed in 97 wt % trifluoroethanol and 80 vol % ethanol. Water soluble 1-methyl-3-(3-alkenyl-5,5-dimethyl-2-cyclohexenyl) pyridinium iodides 6 were solvolyzed in water and in aqueous solvents, as well as under micellar conditions. All esters show in each of the solvents normal values of secondary α-deuterium isotope effects (kHkD = 1.17–1.23). Also in comparison to saturated analogues the investigated esters show a lower solvolytic reactivity. On the basis of these results it was concluded that the solvolysis proceeds via a stepwise mechanism involving a resonance-stabilized cyclohexenyl cation as the reaction intermediate.

A systematic investigation of solvolytic reactivities of allylic and benzylic nicotinates and their N-methylated derivatives in 80% EtOH, 97% TFE and 97% HFIP was undertaken. The nicotinates in 97% HFIP are slightly more reactive than the corresponding p-nitrobenzoates, whereas the ratio of these reactivities is inverse in 80% EtOH and 97% TFE. This observation was explained by the stronger hydrogen bonding of HFIP with nicotinate leaving group than with the p-nitrobenzoate group, which is in keeping with the larger basicity of the former comparing with the latter group. N-Methylnicotinyl esters under the same conditions show approximately 50 times greater solvolytic reactivities than the corresponding p-nitrobenzoates. These relative reactivities are insensitive to the hydrogen bonding ability of the solvent. The UV and IR spectroscopic parameters of ethyl nicotinate and ethyl p-nitro-benzoate in fluorinated alcohols are fully consistent with this observation.