Radical clocks as probes of 1,4-biradical intermediates in the photochemical cycloaddition reactions of 2-cyclopentenone with alkenes

Abstract

ANDREAS RUDOLPH and ALAN C. WEEDON. Can. J. Chem. 68, 1590 (1990). The products of the photochemical reaction of 2-cyclopentenone with 1,6-heptadiene and with vinylcyclopropane have been examined. With 1,6-heptadiene the products were cyclobutanes, which arise from 2 + 2 photocycloaddition between the cyclopentenone carbon-carbon double bond and one of the two terminal double bonds of the heptadiene. The 1,4-biradical that is an intermediate in this reaction contains a derivative of a 1-hexenyl radical; no products derived from cyclization of this intermediate to a cyclopentylmethyl radical were observed. With vinylcyclopropane some of the products isolated also arose from 2 + 2 photocycloaddition; in addition, comparable quantities of products resulting from rearrangement of the intermediate 1,4-biradical were obtained. In this case the initially produced intermediate 1,4-biradical contains a cyclopropylmethyl radical that can rearrange to a homoallylic radical; the products isolated were derived from intramolecular disproportionation in the new 1,7-biradical containing the homoallylic radical. The rate constant for the rearrangement of the cyclopropylmethyl radical to the homoallylic radical is known and was used as a clock to estimate the lifetime of the initially produced 1,4-biradical. The value estimated is of the order of 50 ns. The structures of the products indicate that the initial bonding between the excited cyclopentenone and vinylcyclopropane takes place at both the a-position and the P-position of the enone and not exclusively at the a-position as a recent report claims.Key words: enone photocycloaddition, 1,4-biradicals, radical clocks.ANDREAS RUDOLPH et ALAN C. WEEDON. Can. J. Chem. 68, 1590 (1990) On a examink la nature des produits de la rCaction photochimique he la cyclopent-2-&none avec I'hepta-1,6-dikne et avec le vinylcyclopropane. Avec l'hepta-I ,6-dikne, les produits sont des cyclobutanes qui proviennent d'une photocycloaddition 2 + 2 entre la double liaison carbone-carbone de la cyclopentknone et l'une des deux doubles liaisons terminales de I'heptadikne. Le 1,4-diradical intermkdiaire dans cette rkaction contient un dCrivt d'un radical hex-1-knyle; on n'a pas observt la formation de produits provenant de la cyclisation de cet intermediaire en un radical cyclopentylmtthyle. Avec le vinylcyclopropane, quelques produits isolks provenaient aussi d'une photocycloaddition 2 + 2; de plus, on a obtenu des quantitks comparables de produits provenant d'une transposition du 1,4-biradical intermediaire. Dans ce cas, I'intermCdiaire 1,4-biradical qui se forme initialement contient un radical cyclopropylmCthyle qui se transpose en un radical homoallylique; les produits isolCs proviennent d'une disproportionation intramolCculaire en un nouveau 1,7-biradical qui contient le radical homoallylique. La constante de vitesse pour la transposition du radical cyclopropylmCthyle en radical homoallylique est connue et on l'a utilisCe comme horloge pour Cvaluer le temps de vie du 1,4-biradical produit initialeme...

Nanosecond Kinetic Absorption and Calorimetric Studies of Cyclopentenone: The Triplet, Self‐quenching, and the Predimerization Biradicals

“…Note added in proof Rudolph and Weedon (1990) have estimated a 50 ns lifetime for the l,Cbiradical(s) resulting from addition of cyclopentenone triplet to vinylcyclopropane, close to that for the biradical(s) of the present study.…”

Norrish’ type I and II reactions and their role in the building of photochemical science

“…In a study using potential radical clocks as probes in the addition to cyclopentanone, both rearranged and cyclopropane ring conserving adducts were obtained, thus supporting that 1,4-diradicals live less than 10 μs, and evidencing that photochemistry enjoys from the control on still another (beside the irradiation wavelength) further dimension, time [55]. On the other hand, the topochemical principle has been largely applied for the photochemistry in the crystals, offering a range of examples with an exquisite control of competing path, e.g.…”

“…In a study (see Scemes 15,16,17) based on the use of potential radical clocks, it was found that vinylcyclopropane gave an adduct with a lifetime shorter than 10 μs, while ω-pentenyl adducts essentially did not rearrange [55].…”

“…Gas evolved during irradiation was removed by manipulation of the Toepler pumps without interrupting the experiment and stored in either of the vessel E or F. At the Scheme 15 Expected products from the addition of vinylcyclopropane and 2-cyclopentenone [55] Scheme 16 Expected adducts from cyclopentanone and hexadiene55 conclusion of each experiment the liquid nitrogen surrounding the cooling coil was replaced by a mixture of dry ice and ethyl alcohol, and any substance volatile at this temperature was transferred to the storage vessel. Finally, the small amount of solvent, which was invariably carried over by the gas into the coil was distilled back into the reaction vessel.…”

Μελέτη Του Μηχανισμού Της Φωτοχημικής Κυκλοπροσθήκης Του Φουλερενίου C60 Με Αλκένια

“…Χαρακτηριστικό παράδειγµα αποτελεί η περίπτωση της ανίχνευσης και παγίδευσης 1,4δίριζων ενδιαµέσων κατά τη φωτοχηµική κυκλοπροσθήκη αλκενίου στη 2-κυκλοπεντενόνη από τους Rudolph και Weedon 105. Ειδικότερα, η φωτοχηµική κυκλοπροσθήκη του βινυλοκυκλοπροπανίου στη 2-κυκλοπεντενόνη οδήγησε στ σχηµατισµό τόσο των[2+2] …”