160833-63-6Relevant articles and documents
Effects of γ-Cyano substituent in allylic bridgehead solvolyses: Evidence for cyano π conjugation in carbocations
Takeuchi, Ken'ichi,Kitagawa, Toshikazu,Ohga, Yasushi,Nakakimura, Akitoshi,Munakata, Motohiro
, p. 8155 - 8164 (2007/10/03)
Slightly flexible 4-methylene-3-homoadamantyl mesylate solvolyzes 2 x 105 times faster than rigid 2-methylene-1-adamantyl mesylate. Placement of a cyano substituent on the (E) position of the vinylic carbon of these systems hardly alter the rate ratio (6 x 105). The result suggests fortuitous cancellation of the inductive destabilizing effect of the cyano substituent by its mesomeric stabilizing effect toward the partially conjugated allylic cation. PM3 calculations on simple allylic cations as models supported the experimental results.
Solvolyses of Bicyclooct-1-yl and 1-Adamantyl Systems Containing an Ethylidene Substituent on the 2-Position: Typical Examples of Rate Enhancements Ascribed to Relief of F-Strain
Ohga, Yasushi,Munakata, Motohiro,Kitagawa, Toshikazu,Kinoshita, Tomomi,Takeuchi, Ken'ichi,et al.
, p. 4056 - 4067 (2007/10/02)
The first typical examples are described on the solvolysis rate enhancements ascribed to the relief of F-strain between an alkyl group and the leaving group atom directly attached to the reaction center.The rates and products of solvolyses in ethanol were studied for 2-methylene- and (Z)- and (E)-2-ethylidenebicyclooct-1-yl triflates.Solvolyses were also conducted in ethanol and 2,2,2-trifluoroethanol (TFE) on 2-methylene- and (Z)- and (E)-2-ethylidene-1-adamantyl compounds having OMs, F, Cl, Br, or I as a leaving group.All the substrates gave the corresponding bridgehead substitution products as kinetic control products.The Z:E rate ratios at 25 deg C were 217 +/- 6 for 2-ethylidenebicyclooct-1-yl triflates (ethanol) and 109 +/- 11 (ethanol) and 117 +/- 1 (TFE) for 2-ethylidene-1-adamantyl mesylates. 18O scrambling studies on the ethanolyses of (Z)- and (E)-2-ethylidene-1-adamantyl mesylates showed that the titrimetrically determined Z:E rate ratios can be used as a measure of the rate ratios for the ionization step.The Z:E rate ratio in TFE at 25 deg C for 2-ethylidene-1-adamantyl halides varied in the sequence F (ca. 70), Cl (1020 +/- 160), Br (2230 +/- 90), and I (9500 +/- 280).The significant increases in the rate ratio with the increase in the atomic size of halogen were explained in terms of the presence of F-strain in the Z substrates and its essential absence in the E substrates.Linear correlations were found in a plot of 1.36*logZ/kE> against the MM2 steric energy difference between the Z and E isomers (slope 1.0) and against Hansch's Es, demonstrating the significance of F-strain effect in the enhanced rates of the (Z)-2-ethylidene-1-adamantyl system.These correlations showed an intercept of 0.8 kcal mol-1, which suggested the greater stability of the (Z)-2-ethylidene-1-adamantyl cation than the corresponding E cation by this amount.Ab initio calculations (RHF/6-31G**)showed that the Z cation is more stable than the E cation by 1.0 kcal mol-1, and that the large Z:E rate ratios are in part ascribed to the difference in the cation stability.
A typical example of rate acceleration by F-strain in solvolysis
Takeuchi, K.,Ohga, Y.,Kitagawa, T.
, p. 1631 - 1638 (2007/10/02)
The new bridgehead triflates or mesylates were synthesized for the Z and E forms of the 2-ethylidenebicyclooct-1-yl (3), 2-ethylidene-1-adamantyl (5), and 2-ethylidenebicyclonon-1-yl (7) systems, and their Z/E rate ratios at 25 deg C compared in the solvolysis in ethanol or 2,2,2-trifluoroethanol (TFE).The Z/E rate ratios are 217 for 3-OTf in EtOH and 126 for 5-OMs in TFE, suggesting the marked F-strain in such rigid Z systems.Contrarily, the Z/E rate ratio for rather flexible 7-OMs is O.30, demonstrating the importance of a rigid structure for exerting the F-strain.Molecular mechanics and AM1 calculations supported the rate data.
