Mise en evidence du mecanisme “s2 (intermediaire)” dans des solvolyses de tosylates secondaires encombres. absence de correspondance vitesses-produits
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Reactions of the epimeric 4-tert-butylcyclohexyl tosylates in hexafluoroisopropyl alcohol (HFIP) (le, l a ) allow for the first time to observe SN1-type non-stereospedic substitution, whereas conventional solvents including even trifluoroethanol yield SN2-type inversion by solvent assisted pathway (ak,). Large differences are found between the epimers in the solvent participation, measured kinetically by the Schleyer-Bentley equation. This, as well as the even enhanced elimination with the equatorial isomer 1 e as compared to 1 a (98% vs. 96% in HFIP) points to the occurrence of non-chair intermediates from le-derivatives, and to more E2-than E,-type reactions. Kinetic measurements, including those of cis-3,s-dimethylcyclohexane esters (2% 2e) and 3~/3&(tosyloxy)androstanes (3a, 3e) show little differences between the equatorial esters in agreement with MM2 calculations, which establish small strain energy variations between the differently substituted twist-boat intermediates. Large differences, however, of up to 500% are measured between the axial esters (la, 2a, 3a), although the alkyl substituents are remote from the leaving group and do not alter the chair geometry. These variations, which demonstrate the severe limitations of the Winstein-Holness equation for solvolysis reactions, are explained by MM 2 calculated significant strain differences between the educts and the corresponding substituted cyclohexenes. Solvolysereaktionen und Kraftfeld-Recbnungen rnit epimeren Cyclohexanderivaten ' )Reaktionen der epimeren 4-tert-Butylcyclohexyltosylate (1 e, 1 a) Hexafluorisopropylalkohol (HFIP) ermoglichen erstmals die Beobachtung von SN1 -artigen, nicht stereospezifischen Substitutionen, wahrend konventionelle Solventien, unter EinschluD sogar von Trifluorethanol, SN2-artige Inversionen durch einen solvensgestutzten Mechanismus (ak,)ergeben. Die Epimeren zeigen hier groDe Unterschiede in den nach Schleyer-Bentley et al. erhaltenen k&-Werten. Dies sowie die sogar verstarkte Eliminierung bei der Reaktion aquatorialer Epimerer (1 e: 98%, 1 a: 96%, in HFIP) laBt auf das Auftreten von Nicht-SesselFormen aus 1 e schlieDen sowie auf eher E2-als El-artige ubergangszustande. Kinetische Messungen, einschlieBlich solcher rnit cis-3,5-Dimethylcyclohexanestern (2 a, 2e) und 3u/38-(Tosyloxy)androstanen (3a, 3e) zeigen kleine Unterschiede zwischen den aquatorialen Estern, in ubereinstimmung rnit MM 2-Rechungen, welche ebenfalls kleine Variationen der Spannungsenergie zwischen den verschieden substituierten Twist-Boot-Intermediaten ergeben. Dagegen zeigen die axialen Epimeren (la, Za, 3a) erhebliche Unterschiede von bis zu 500%, obwohl die verschiedenen zusatzlichen Alkylgruppen entfernt von der Abgangsgruppe stehen und die Sesselgeometrie nicht storen. Die mit la, 2a, 3 a beobachteten Variationen, welche die nahezu prohibitive Limitierung der Winstein-Holness-Gleichung bei Solvolysen demonstrieren, lassen sich rnit M M 2-berechneten erheblichen Spannungsdifferenzen zwischen den Edukten und den entsprechenden substituierten Cyclo...
Reactions of the epimeric 4-tert-butylcyclohexyl tosylates in hexafluoroisopropyl alcohol (HFIP) (le, l a ) allow for the first time to observe SN1-type non-stereospedic substitution, whereas conventional solvents including even trifluoroethanol yield SN2-type inversion by solvent assisted pathway (ak,). Large differences are found between the epimers in the solvent participation, measured kinetically by the Schleyer-Bentley equation. This, as well as the even enhanced elimination with the equatorial isomer 1 e as compared to 1 a (98% vs. 96% in HFIP) points to the occurrence of non-chair intermediates from le-derivatives, and to more E2-than E,-type reactions. Kinetic measurements, including those of cis-3,s-dimethylcyclohexane esters (2% 2e) and 3~/3&(tosyloxy)androstanes (3a, 3e) show little differences between the equatorial esters in agreement with MM2 calculations, which establish small strain energy variations between the differently substituted twist-boat intermediates. Large differences, however, of up to 500% are measured between the axial esters (la, 2a, 3a), although the alkyl substituents are remote from the leaving group and do not alter the chair geometry. These variations, which demonstrate the severe limitations of the Winstein-Holness equation for solvolysis reactions, are explained by MM 2 calculated significant strain differences between the educts and the corresponding substituted cyclohexenes. Solvolysereaktionen und Kraftfeld-Recbnungen rnit epimeren Cyclohexanderivaten ' )Reaktionen der epimeren 4-tert-Butylcyclohexyltosylate (1 e, 1 a) Hexafluorisopropylalkohol (HFIP) ermoglichen erstmals die Beobachtung von SN1 -artigen, nicht stereospezifischen Substitutionen, wahrend konventionelle Solventien, unter EinschluD sogar von Trifluorethanol, SN2-artige Inversionen durch einen solvensgestutzten Mechanismus (ak,)ergeben. Die Epimeren zeigen hier groDe Unterschiede in den nach Schleyer-Bentley et al. erhaltenen k&-Werten. Dies sowie die sogar verstarkte Eliminierung bei der Reaktion aquatorialer Epimerer (1 e: 98%, 1 a: 96%, in HFIP) laBt auf das Auftreten von Nicht-SesselFormen aus 1 e schlieDen sowie auf eher E2-als El-artige ubergangszustande. Kinetische Messungen, einschlieBlich solcher rnit cis-3,5-Dimethylcyclohexanestern (2 a, 2e) und 3u/38-(Tosyloxy)androstanen (3a, 3e) zeigen kleine Unterschiede zwischen den aquatorialen Estern, in ubereinstimmung rnit MM 2-Rechungen, welche ebenfalls kleine Variationen der Spannungsenergie zwischen den verschieden substituierten Twist-Boot-Intermediaten ergeben. Dagegen zeigen die axialen Epimeren (la, Za, 3a) erhebliche Unterschiede von bis zu 500%, obwohl die verschiedenen zusatzlichen Alkylgruppen entfernt von der Abgangsgruppe stehen und die Sesselgeometrie nicht storen. Die mit la, 2a, 3 a beobachteten Variationen, welche die nahezu prohibitive Limitierung der Winstein-Holness-Gleichung bei Solvolysen demonstrieren, lassen sich rnit M M 2-berechneten erheblichen Spannungsdifferenzen zwischen den Edukten und den entsprechenden substituierten Cyclo...
Photochemical Formation and Chemistry of Long-Lived Adamantylidene-Quinone Methides and 2-Adamantyl Cations
Hydroxymethylphenols strategically substituted with the 2-hydroxy-2-adamantyl moiety, AdPh 8-10, were synthesized, and their photochemical reactivity was investigated. On excitation to the singlet excited state, AdPh 8 undergoes intramolecular proton transfer coupled with a loss of H(2)O giving quinone methide 8QM. The presence of 8QM has been detected by laser flash photolysis (CH(3)CN-H(2)O 1:1, tau = 0.55 s) and UV-vis spectroscopy. Singlet excited states of AdPh 9 and 10 in the presence of H(2)O dehydrate giving 9QM and 10QM. Photochemically formed QMs are trapped by nucleophiles giving the addition products (e.g., Phi for methanolysis of 8 is 0.55). In addition, the zwitterionic 9QM undergoes an unexpected rearrangement involving transformation of the 2-phenyl-2-adamantyl cation into the 4-phenyl-2-adamantyl cation (Phi approximately 0.03). An analogous rearrangement was observed with methoxy derivatives 9a and 10a. Zwitterionic 9QM was characterized by LFP in 2,2,2-trifluoroethanol (tau = 1 mus). In TFE, in the ground state, AdPh 10 is in equilibrium with 10QM, which allowed for recording the (1)H and (13)C NMR spectra of the QM. Introduction of the adamantyl substituent into the o-hydroxymethylphenol moiety increased the quantum yield of the associated QM formation by up to 3-fold and significantly prolonged their lifetimes. Furthermore, adamantyl substituent made the study of the alkyl-substituted quinone methides easier by LFP by prolonging their lifetimes and increasing the quantum yields of formation.
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