Influence of Functional Groups on the Ene Reaction of Singlet Oxygen with 1,4‐Cyclohexadienes^†

Abstract

The photooxygenation of 1,4-cyclohexadienes has been studied with a special focus on regio-and stereoselectivities. In all examples, only the methyl-substituted double bond undergoes an ene reaction with singlet oxygen, to afford hydroperoxides in moderate to good yields. We explain the high regioselectivities by a "large-group effect" of the adjacent quaternary stereocenter. Nitriles decrease the reactivity of singlet oxygen, presumably by quenching, but can stabilize proposed perepoxide intermediates by polar interactions resulting in different stereoselectivities. Spiro lactams and lactones show an interesting effect on regio-and stereoselectivities of the ene reactions. Thus, singlet oxygen attacks the double bond preferentially anti to the carbonyl group, affording only one regioisomeric hydroperoxide. If the reaction occurs from the opposite face, the other regioisomer is exclusively formed by severe electrostatic repulsion in a perepoxide intermediate. We explain this unusual behavior by the fixed geometry of spiro compounds and call it a "spiro effect" in singlet oxygen ene reactions.

To DISP or Not? The Far‐Reaching Reaction Mechanisms Underpinning Lithium‐Air Batteries

“…From concomitant H 2 O 2 (Figure 5a) formation with activated ethylene (À CH 2 À ) 2 groups, [29b, [30][31] [2+2] cycloadditions with alkenes [32,33] (generating dioxetanes that upon breakdown release either carbonyl compounds or diols), [48] insertion into activated CÀ H bonds (Figure 5b, e.g. adjacent to ether groups), [33] to hydrogen abstraction (Figure 5c), 1 O 2 has been proposed and shown to be capable of reacting in a variety of ways.…”

“…[53] However, NMAc, DMAc, N-ethylacetamide (NEAc), DMIA, N,N-dimethylpivalamide (TMDMA), NMP, N,N'tetramethylurea (TMU), 1,3-dimethyl-2-imidazolidinone (DMI) and N,N'-dimethylpropyleneurea (DMPU) were also Examples of solvent motifs that have been explored for lithium-oxygen batteries and proposed reactivities of antagonistic oxygenated species: (a) concerted hydrogen peroxide formation [28] followed by cycloaddition [29] and dioxetane breakdown. [29b,30] (b) CÀ H insertion [31] followed by proton abstraction [32] or Hock rearrangement. [31] (c) Hydrogen abstraction by 1 O 2 [5b,33] followed by addition of O 2 .…”

“… Examples of solvent motifs that have been explored for lithium‐oxygen batteries and proposed reactivities of antagonistic oxygenated species: (a) concerted hydrogen peroxide formation [28] followed by cycloaddition [29] and dioxetane breakdown [29b,30] . (b) C−H insertion [31] followed by proton abstraction [32] or Hock rearrangement [31] . (c) Hydrogen abstraction by 1 O 2 [5b,33] followed by addition of O 2 [1a] .…”

Birch Reduction of Arenes as an Easy Entry to γ‐Spirolactones

“…The syntheses and analytical data of lactones 5 b, 5 c, and 5 e have been published by our group very recently. [25] General procedure for Birch reductions Arene carboxylic acid 1 (10 mmol) was introduced into a threenecked flask (100 mL), equipped with a dry-ice condenser and cooled to À 78 °C by a dry-ice acetone bath. Ammonia (50 mL) was condensed into the three-necked flask, and lithium (175 mg, 25 mmol) was added in small pieces to the solution at À 78 °C, until it remained blue and the solution was stirred at À 78 °C for 1 h. Ethylene oxide (2 mL, 40 mmol) was condensed into a dropping funnel, which was cooled by a dry ice acetone bath, and added within 5 min.…”

“…Indeed, to the best of our knowledge, Birch reduction in the presence of ethylene oxide was unknown until our very recent studies on photooxygenations of spiro compounds, where we investigated only three simple examples. [25] Herein, we describe that the sequence Birch reduction of aromatic carboxylic acids -trapping with ethylene oxide is a general method for the synthesis of γ-spirolactones 5 via hydroxy acids 4. Additionally, subsequent catalytic hydrogenation would afford saturated or partly unsaturated γ-spirolactones 6 or 7 as well (Scheme 1b).…”