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Nondegenerate equilibrium in (2-propyl)cycloalkyl cations. Comparison of (2-propyl)cyclopentyl and (2-propyl)cyclohexyl cation using 13 C NMR spectroscopy, equilibrium isotope eects and quantum chemical calculations † Hans-Ullrich Siehl,* a Valerije Vrc ˘ek b and Olga Kronja * b a Division of Organic Chemistry I, University of Ulm, D-89069, Ulm, Germany. E-mail: [email protected]; Fax: 49-(0)731 50 22787; Tel: 49-(0)731-5022800 b Faculty of Pharmacy and Biochemistry, University of Zagreb, 10000, Zagreb, Croatia. E-mail: [email protected]; Fax: 385-1-485216; Tel: 385-1-4818301 Received (in Cambridge, UK) 28th August 2001, Accepted 29th October 2001 First published as an Advance Article on the web 3rd December 2001 Tertiary (2-propyl)cyclopentyl and (2-propyl)cyclohexyl carbocations were investigated by 13 C NMR spectroscopy in superacid solution. Both ions undergo fast nondegenerate 1,2-hydride shifts to the corresponding 2-cycloalkyl- 2-propyl cations. The direction of these equilibria depends on the size of the ring. The more stable isomer of the (2-propyl)cyclopentyl cation has the formal positive charge at the endocyclic carbon atom, while the more stable isomer of the (2-propyl)cyclohexyl cation has the formal charge at the exocyclic carbon atom. The dynamic NMR results were conrmed by NMR spectroscopic measurement of the equilibrium isotope eects and rationalized by quantum chemical calculations. Introduction Carbocations often show facile rearrangement reactions due to their shallow energy hypersurface. Typical examples for rapid nondegenerate rearrangements are 1,2-hydride shifts in tertiary carbocations A B, such as in 2,3-dimethyl-3-pentyl cation, 2,3-dimethyl-2-hexyl cation 1 and in 2-aryl-3-methyl-2-butyl cations. 2 Carbocations undergoing reversible rearrangements, which are fast relative to the NMR time scale, show NMR spectra with concentration weighted averaged chemical shifts for the interchanging sites. For a nondegenerate 1,2-hydride shift, such as A B, the equilibrium constant K eq = [B]/[A] 1 and two peaks are observed for all sites which are averaged by the fast hydride shift. These peaks show temperature dependent averaged chemical shifts. The peak for the carbocation center is shifted upeld and the peak for the β-carbon is shifted down- eld compared to that of static carbocations. 1 In some carbo- cations nondegenerate equilibria have been detected by 1 H- and 13 C-NMR spectroscopic methods, but often no details of those rearrangements are known. We have investigated the 1,2-hydride shift equilibrium 1A 1B of the 2-cyclopentyl-2-propyl cation 1A and the 1-(2- propyl)-cylopentyl cation 1B and the analogous rearrangement reaction 2A 2B which equilibrates the 2-cyclohexyl-2-propyl cation 2A and the 1-(2-propyl)cyclohexyl cation 2B (Scheme 1) † Electronic supplementary information (ESI) available: redundant coordinates for structures 1A, 1B, 1TS, 2A, 2B1, 2B2 and 2TS; van’t Hoplot for equilibrium 1A 1B. See http://www.rsc.org/suppdata/ p2/b1/b107756b/ using high eld 13 C NMR spectroscopy, equilibrium isotope eects and quantum chemical calculations. Earlier NMR studies have indicated that the equilibrium 1A 1B is moved toward the structure 1B in which the positive charge is located in the ring. On the contrary, for the equilibrium 2A 2B the structure 2A with the positive charge on the exocyclic carbon was found to be more stable. 3,4 The results we report in this study explain the dierent stabilities of alkyl-substituted ve- and six-membered cyclo- alkyl cations 1B and 2B compared to the isomeric cycloalkyl- substituted 2-propyl cations 1A and 2A and give additional information on the stabilization modes and the structural and conformational consequences in these type of carbocations. Results and discussion The carbocations 1A/1B and 2A/2B and their isotopomers 5 (1A-d 6 /1B-d 6 , 1A-d 4 /1B-d 4 , 2A-d 6 /2B-d 6 and 2A-d 4 /2B-d 4 ), as well as mixtures of deuterated and non-deuterated compounds were prepared from the corresponding alcohols with excess Lewis- acid SbF 5 and a 2 : 1 mixture of SO 2 F 2 –SO 2 ClF as solvent, using contemporary high vacuum matrix co-condensation techniques. 6 Scheme 1 2 PERKIN 106 J. Chem. Soc., Perkin Trans. 2, 2002, 106–113 DOI: 10.1039/b107756b This journal is © The Royal Society of Chemistry 2002

What's Keeping You From Getting Began In Real Estate Investing?

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What's Keeping You From Getting Began In Real EstateInvesting?

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