Carbon-13 N.M.R. studies of carbocations. II. An investigation of the correlation between carbon chemical shifts and σ+ constants for 1-phenylethyl cations

Abstract

Eighteen 1-methyl-1-phenylethyl (cumyl) (1) and 1-phenylethyl (styryl) (2) carbocations have been prepared in known concentration in FSO3H/SbF5 solution from their corresponding carbinols and their 1H and 13C spectra have been measured. The correlations of both the methyl 1H shifts [δH(Me)] and methyl 13C shifts [δC(Me)] with the cationic carbon shifts [δ(C+)] have been determined. In the case of δC(Me) curves are obtained indicating non-linearity of chemical shifts with charge density. Correlations of δ(C+) with Brown's σ+ constants are only fair, the deviation from linearity being interpreted in terms of non-linear dependence of either δ(C+) or σ+, or both, on charge density at this level of charge. The use of an enhanced substituent constant (σ++) and/or the dual substituent parameter equation improves the correlation. Non-linearity in the plot of δ(C 1) against σ+ reported for 2-arylnorborn-5-en-2-yl(4) and 2-arylnorbornan-2-yl cations (5) has been shown to occur also in the case of δC(Me) of the cumyl cations (1). The implications of this finding with respect to the structures of (4) and (5) are discussed.

Structural effects in solvolytic reactions; carbon-13 NMR studies of carbocations † : Effect of increasing electron demand on the carbon-13 NMR shifts in substituted tert -cumyl and 1-aryl-1-cyclopentyl carbocations—correlation of the data by a new set of substituent constants, σ C+

“…The meta-and para-substituted tert-cumyl alcohols and I-phenyl-l-cyclopentanols were ionized in "magic acid," FSO3H/SbF5 (1:1, mol/mol), diluted with SO2CIF at -78°C, taking care to ensure the presence of a four-fold excess of acid (11). The 13C NMR shifts of the resulting solutions of 1 and 2 are summarized in Tables 1 and 2, along with some previously recorded data (9,10 (Fig. 1). A plot of the data against Chemistry: Brown et al the Brown a. constants reveals a much better correlation with r = 0.990 and the slope p+ = -18.18 (Fig.…”

13 C NMR chemical shift correlations in application of “tool of increasing electron demand” to stable long-lived carbocations: Comprehensive evaluation

“…The method was successful in demonstrating the clearly different nature of certain carbocationic systems such as the 7-norbornenyl (9), 5-norbornene-2-yl (2) (10), 2-norbornyl (3) (10, 11) and 8-tricyclo[5.2. 1.02 '6]decyl (4) (10) (18) and by Kelly and Spear (19). In this paper we demonstrate that such comparisons are of general validity.…”

“…However, these conclusions have been disputed by Kelly and Spear (19) substituents. This indicates that, as the aryl ring becomes less effective in delocalizing the positive charge at the C(2) position, the neighboring C(1)-C(6) o bond begins to interact with the empty p orbital at the cationic center-i.e., the onset of odelocalization.…”

“…The analysis of several other norbornyl-type systems shows Kelly and Spear (19) further argued that breaks in plots ofthe C(1) chemical shifts of the 2-aryl-2-norbornyl cations does not imply the onset of o-delocalization because they found that, when the C(1) chemical shift of the 2-aryl-2-norbornyl cation is plotted against the chemical shift of the methyl carbons of the t-cumyl cations 11 in which no or or Xr bridging is possible, an excellent straight line is obtained. They then concluded "that breaks in plots of C(1) chemical shifts of2-norbornyl derivatives cannot be used as evidence for the presence of Abridging in these cations."…”

“…For example, the methyl carbon chemical shifts of the 1-aryl-l-ethyl cations 22 and 1-methyl-l-arylethyl cations 11 can be correlated to a limited degree by a+ constants (r = 0.989 and 0.990, respectively) (19). Likewise, the C(l) chemical shifts of the 2-aryl-2-norbornyl cation can be correlated quite well with c + constants (19) (r = 0.997) and the para carbon of the unsubstituted ring of a series of 4-substituted benzhydryl cations 18 shows a good correlation with c (22).…”