Mechanism of the ene reaction between singlet oxygen and olefins

Abstract

except from single-crystal measurements utilizing polarized X-rays34). Furthermore, EXAFS diminishes rapidly beyond the first and second coordination shells (typically r ;$ 4 Á) except in cases where atoms are nearly collinear. In such cases, EXAFS from atoms as far as 6 A can be observed due to amplitude enhancement called focusing effect. In fact, both amplitude and phase of the EXAFS of a more distant neighbor are significantly affected by the intervening atom(s) for bond angle > 120°. For these systems, one must therefore take into account multiple scattering processes involving the intervening atom(s). Recently, a new multiple scattering formalism has been developed35 which enables bond angle determinations with an accuracy of 5% or ca. 5°. Nevertheless, the structural content of EXAFS is unparalleled by other spectroscopic techniques when (35) B. K. Teo, submitted for publication. one considers that the few most important bonds in a complex system can be probed within minutes. The future of EXAFS spectroscopy is as bright as the future synchrotron radiation sources. Dedicated synchrotron radiations with energy ranging from UV to hard X-rays are now available.36 These highly intense light sources will undoubtedly open up a new era in exciting chemical, biological, and material research.

Biomimetic Total Synthesis of Litseaverticillols A, C, D, F, and G: Singlet‐Oxygen‐Initiated Cascades

“…The regioselective oxidation of one of the three trisubstituted double bonds of 1 occurs for a combination of electronic and steric reasons: [15] the C10 = C11 double bond is more accessible to the electrophilic 1 O 2 than C6 = C7, and the C2 = C3 bond is electron deficient. The equimolar formation of the tertiary and secondary allylic hydroperoxides (19 and 20) is consistent with the formation of intermediate perepoxide 18 [16] (Scheme 5), in which the negatively charged oxygen atom is directed towards the more-substituted side of the previously present double bond (cis effect). [17] Allylic hydrogen abstraction from C9 and C15 of perepoxide 18 results in balanced formation of the ene reaction products, allylic hydroperoxides 19 and 20.…”

Diastereoselective Reactions of the Tiglic Acid Functionality Mediated by Oxazolidine Chiral Auxiliaries: A Mechanistic Comparison of DMD andm‐CPBA Epoxidations versus Singlet Oxygen and PTAD Ene Reactions

“…The mechanism for the 1 O 2 ene reaction has been intensively debated, but the current consensus favors the stepwise mechanism, with a perepoxide-like oxygen transfer. [32] Indeed, such an epoxide-type trajectory resembles the peracid epoxidation reaction and justifies a mechanistic comparison of the singlet-oxygen ene reaction with the peracid epoxidation. For the particular case of the electron-deficient alkenes investigated here, a fast reversible step operates to give the perepoxide intermediate first and, subsequently, in the second, rate-determining step, the allylic hydrogen abstraction takes place, as established by isotope effect studies.…”

Bifurcations on Potential Energy Surfaces of Organic Reactions

“…[40] Three mechanisms-concerted, stepwise, and exciplex perepoxide formation-have all been proposed on the basis of experimental and theoretical considerations (Scheme 6). Early ab initio studies favored a stepwise mechanism, [41] while measured kinetic isotope effects (KIEs) and observed stereospecific suprafacial product (20) to 3-methylene-1,5-hexadiene (22).…”