Induktivität und Verbrückung in Ammonium‐ und Carbenium‐Ionen

087ChemInform Abstract While the Hammett ρ values of the amines (I)-(III) (from pKa's of the HClO4 salts) are nearly constant (1.03 ± 0.02) and therefore independent of direction and structure of the substituted ammonium ions, the ρ values of the corresponding carbon analogues such as (IV) and (V) (from kinetic data of the solvolysis of the tosylates) vary from -0.72 (V) to -2.00 (IVa). This is explained by "gradual bridging" in the carbenium ions formed by ionization of the tosylates.

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ChemInform Abstract: Inductivity and Bridging in Ammonium and Carbenium Ions.

Grob

Dratva

Gruendel

et al. 1988

ChemInform

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Polar Effects. Part 15. Twin γ‐substituent effects in the formation of adamantyl‐1‐cations

Grob

Gründel

Sawlewicz

1988

Helvetica Chimica Acta

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~ ~~~~Solvolysis rates of several adamant-I-yl ( = tri~yclo[3.3.1.1~~~]dec-l-yl) p-toluenesulfonates and bromides 2 identically substituted at C(3) and C(5) (twin substitution) have been determined and compared with the rates of the corresponding 3-monosubstituted compounds 1. As expected, the rate factors for twin substitution are much larger, but less than the square of the rate factors for single substitution. Also the rate factors per substitutent in 2 are considerably smaller than those for one substituent in 1. This attenuation is attributed to electron repulsion which limits the convergence of electron density to the cationic center. On the other hand, the inductivity for twin substitution @I = -2.55) is ea. twice as high as for mono-substitution @I = -1.26).Four kinds of polar substituent effects are discernible in solvolysis reactions of 3-substituted adamant-1-yl ( = tricyclo[3.3.1.13~7]dec-l-yl) bromides and tosylates ( = ptoluenesuljonates) 1 (X = Br and OTs, resp.), namely inductive, hyperconjugative, frangomeric, and homohyperconjugative effects [ 1] [2]. This follows from the reaction products and from the relationship between the rate constants k (in 80% EtOH) and the inductive constants o; [3] of the substituents. Alkyl groups and common electron-attracting substituents like COOCH,, OAc, Br, CN, and NO, as well as H-C(3) control rates of forination of the adamant-1-yl cations 5 by their inductive ( I ) effects only, as shown by the linear correlation of log k with the corresponding 07 values '). The latter were derived from the pK, values of 4-substituted quinuclidinium perchlorates 4 [3] which are also controlled by 1 effects.On the other hand, rates were 3 to 44 times higher than calculated from the regression line for log k us. 07, when R was a + M substituent or n electron donor such as OH, OCH,, or SCH,. These exalted polar substituent effects were attributed to C,C hyperconjugative relay of positive charge from the cationic center C( 1) to the substituents in the incipient cations 6 (the dotted lines imply weak bonding interactions). However, if the + M effect of the substituents were stronger, as in 1 (R = NH,, 0-, or S-), C,C hyperconjugation merged with concerted fragmentation [4] and led to accelerated formation of the corresponding bicyclic compounds 8 (R = NH:, 0, or S, resp.). In these cases, formula 6 represents the transition state for fragmentation to 8.Finally, the rates of 3-substituted adamant-1-yl bromides and tosylates 1 were 4 to 26 times higher than predicted by their O: values for R = CH,OH, COOQ, or (CH,),Sn [ 1][2].') According to the equation log k = pl.rri' + log k,.

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Physikalische Organische Chemie in Regensburg

1988

Nachr. Chem. Tech. Lab.

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Induktivität und Verbrückung in Ammonium‐ und Carbenium‐Ionen

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References 12 publications

ChemInform Abstract: Inductivity and Bridging in Ammonium and Carbenium Ions.

ChemInform Abstract: Inductivity and Bridging in Ammonium and Carbenium Ions.

Polar Effects. Part 15. Twin γ‐substituent effects in the formation of adamantyl‐1‐cations

Physikalische Organische Chemie in Regensburg

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