Reaction of Singlet Oxygen with 2‐Methylnorborn‐2‐ene, 2‐Methylidenenorbornane, and their 7,7‐Dimethyl Derivatives. The transition state geometry for hydroperoxidation

Abstract

The dye-sensitized photo-oxygenation of 2-meth ylnorborn-2-ene (3), 2-niethylidenenorbornane (4) and their 7,7-dimethyl derivatives (5 and 6) has been studied. In all cases allylically rearranged hydroperaxides WCTe formed, cxcept that 4 also gave a little norbornanone (presumably from the dioxetane) and 5 gave some endo-3,7,7-trinicthylnorbornan-2-one as a secondary photoproduct. It was found that the exolendo attack ratios by singlet oxygen on 3 and 5 are 66 and 0.19. By exploiting the C (3) monodeuteriated derivativcs, 4 and 6 showed ratios of 28 and 0.67.Rates of reactivity of the olefins 3 and 4 were compared t o methylidenecyclopentane, I-methycyclopentene and 1-rnethylcyclohexene as monocyclic standards. Additionally, comparative rates between the 7,7-dimethyl olefins and their parents were measured. When further comparison was made of the rate ratios partitioned for exo and endo attack, it was seen that oxygen expericnced a 500-to 1000-fold rate retardation on approach to the endcs side of 3 compared to that for its monocyclic analogue. Exo rates between the parent norbornene 3 and its 7,7-dimethyl derivative 5 showed a 250-fold decrease. Although four times snialler than the difference reported for epoxidation, the evidence clearly pointed t o a one-step cyclic process as the rate determining step for photo-oxygenation. The steric cvidence, taken with the low valucs found for the intermolecular isotope effects of 1.14 & 0.01 and 1.02 & 0.01 observed for exo and endo-3-deutcrio-2-methylidenenorbornanes, permits the deduction that the transition state is largely dipolar. In the early stagcs of the addition bonding between one end of the oxygen molecule and the terminal vinyl carbon is advanced. At the same time positive charge is dispersed by hyperconjugation between the central carbon atom and the allylic carbon-hydrogen bond. A t a latcr stage the anionic oxygen atom abstracts the loosened allylic hydrogen atom to create the hydroperoxide. No evidence for the formation of a discrete perepoxide intermediate was obtained

The Hydroperoxidation of Olefins by Singlet Oxygen. Validity of the zwitterionic peroxide model

“…Bond formation at the end vinyl atom C (1) is ratedetermining and precedes hydrogen-abstraction at C (3). The resulting transition state is polarized to an extent which depends on the nature of the substituents and the medium [21] [22]. A zwitterion would be favored by a n-donor substituent at reaction course give the corresponding alkoxy derivative 4 (Scheme 1).…”

Synthesis and Photo‐oxygenation of Some Substituted 1‐Benzyl‐3,4‐dihydroisoquinolines. Mechanism of Enamine Photo‐oxygenation

“…Photo-oxygenations were performed in one of two ways: a) In an apparatus previously described [26], using a CHC13 solution 1 . 0~ l O W 1~ in each of the two substrates being compared, and containing 2 .…”

Stereoselectivity in Ene Reactions with 1O2: Matrix Effects in Polymer Supports, Photo‐oxygenation of Organic Salts and Asymmetric Synthesis

“…The regiochemistry of the ene reaction with substrates with multiple sites for allylic hydrogen transfer was extensively studied and several general effects can predict the regioselective introduction of the hydroperoxy group: (a) the cis ‐effect ( syn ‐effect) in the reaction of 1 O 2 with trisubstituted alkenes (35–37) (b) the gem effect that leads to highly selective abstraction of an allylic hydrogen atom from a CHX‐substituent in α position of an α,β‐unsaturated carbonyl compound (38,39), and (c) the large‐group effect (40,41) that leads to selective (moderate) abstraction of an allylic hydrogen from the substituent geminal to a large group. Several factors that control the π facial selectivity of singlet‐oxygen ene reaction are known: (1) steric factors, mostly in rigid cyclic and polycyclic substrates (42,43); (2) conformational effects, based on the optimal position of reactive allylic hydrogens perpendicular to the alkene plane (44); and (3) hydrogen bonding effects, which have been used for control of the diastereoselectivity in the ene reaction of allylic alcohols and other substrates (45).…”