About the participation of σ, π and n orbitals in the solvolysis of 2- and 3-oxo-1-norbornyl triflates

Martı́nez, Antonio Garcı́a; Barcina, José Osío; Vilar, Enrique Teso

doi:10.1016/0040-4020(96)00845-9

Cited by 5 publications

(2 citation statements)

References 21 publications

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“…The plot approached linear lines when the initial concentrations of the triflates were decreased from 0.02 mol L -1 to 10 -3 −10 -4 mol L -1 . Obviously, the formed triflic acid catalyzed the reaction even in the presence of an excess amount of 2,6-lutidine, and the catalysis decreased with the decrease in the triflic acid concentration . Because of these ambiguities, the two triflates were excluded as substrates for kinetic studies.…”

Section: Resultsmentioning

confidence: 99%

Solvolyses of 2-Oxo Bridgehead Compounds: A Critical Examination of π-Conjugative Stabilization of α-Carbonyl Carbocations

et al. 1997

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The methodology of changing ring flexibility to detect the π-conjugative stabilization of bridgehead carbocations has been applied to eight 2-oxo (X = O) bridgehead carbocations. On the basis of the solvolytic behavior observed in kinetics and product analyses, the eight 2-oxo bridgehead substrates were classified into three categories: three substrates solvolyzing without ion-pair return that leads to primary isomers (class A), three substrates that form primary isomers by ion-pair return during solvolysis (class B), and two substrates that undergo solvent addition to the carbonyl group to form hemiacetals during solvolysis (class C). It was concluded that the substrates of class C could not be used for the present purpose. Essentially constant ethanolysis rate ratios, k(X = O)/k(X = H2), of 10-8.2−10-8.7 at 25 °C were obtained between four 2-oxo substrates in classes A and B and the corresponding parent unsubstituted ones. The result was interpreted to suggest that the π-conjugative stabilization of tertiary α-carbonyl carbocations is negligibly small, if present. Slightly more negative k(X = O)/k(X = H2) values of 10-9.7 and 10-9.2 for highly flexible bicyclo[4.2.2]dec-1-yl and bicyclo[4.3.1]dec-1-yl systems, respectively, were attributed to complex conformations in the ground and incipient carbocations. PM3 calculations on some 2-methylene and 2-oxo bridgehead carbocations supported the experimental results. Comparison of the solvolysis rates of 1,1,3,3-tetramethyl-2-oxobutyl mesylate with those of 1,1,3,3-tetramethylbutyl mesylate estimated from the rates of the corresponding chloride also failed to support the π-conjugative stabilization of α-carbonyl carbocations.

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Section: Resultsmentioning

confidence: 99%

Solvolyses of 2-Oxo Bridgehead Compounds: A Critical Examination of π-Conjugative Stabilization of α-Carbonyl Carbocations

et al. 1997

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“…The semiempirical molecular-orbital method MINDO/3 has been used to give an account of electronic factors in strained bridgehead carbocations . On the other hand, the AM1 method is used for the study of the solvolysis of bridgehead oxonorbornyl triflates . Hence, we have calculated the differences in binding energy (Δ E b ) between the hydrocarbons 9(H) , 13(H) − 17(H) and the cations 9(+) , 13(+) − 17(+) by the AM1 and the MINDO/3 methods implemented in HyperChem .…”

Section: Resultsmentioning

confidence: 99%

An Experimental and Computational Study about the Effect of a Spirocyclopropane Group on the Solvolysis Rates of Bridgehead Triflates

Cerero

et al. 1999

J. Org. Chem.

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7',7'-Dimethylspiro[cyclopropane-1,2'-norbornan]-1'-yl triflate (9) was obtained and its solvolysis rates in buffered 60% aqueous ethanol were determined at different temperatures. The solvolytic behavior of 9 and other bridgehead derivatives (13-17, see Table 1) was studied by force-field, semiempirical and ab initio [B3LYP/6-31g(d)] methods. Cation 9(+) is a slightly pyramidal cyclopropylcarbinyl cation in a nearly perpendicular conformation, showing an sp(2)-like hybridization. Its high electron demand provokes an enhancement of the sigma-participation of the C(5)-C(6) and C(4)-C(7) bonds. The introduction of a cyclopropyl group adjacent to the bridgehead cation leads to an increase of the strain energy. This fact has two opposite effects on the solvolysis rate: (a) the strain energy hinders the flattening of the cation and, hence, originates a rate depression, and (b) the strained C-C bonds are prone to stabilize the cation at C(1) by sigma-delocalization, which provokes a rise of the solvolysis rate. Both effects are accounted for only by the B3LYP/6-31g method. The claimed electron-withdrawal effect of the cyclopropyl group as a basis for the small k(17)/k(16) rate ratio is not confirmed by our calculations.

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Water-trapping of unstable carbocations taking place into the inverted region of the Marcus equation. First experimental and computational evidence

et al. 2012

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About the participation of σ, π and n orbitals in the solvolysis of 2- and 3-oxo-1-norbornyl triflates

Cited by 5 publications

References 21 publications

Solvolyses of 2-Oxo Bridgehead Compounds: A Critical Examination of π-Conjugative Stabilization of α-Carbonyl Carbocations

Solvolyses of 2-Oxo Bridgehead Compounds: A Critical Examination of π-Conjugative Stabilization of α-Carbonyl Carbocations

An Experimental and Computational Study about the Effect of a Spirocyclopropane Group on the Solvolysis Rates of Bridgehead Triflates

Water-trapping of unstable carbocations taking place into the inverted region of the Marcus equation. First experimental and computational evidence

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