Mono and double polar [4 + 2+] Diels–Alder cycloaddition of acylium ions with O-heterodienes

✍ Scribed by Eduardo C. Meurer; Marcos N. Eberlin

• Publisher: John Wiley and Sons
• Year: 2002
• Tongue: English
• Weight: 173 KB
• Volume: 37
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.267

✦ Synopsis

Abstract

Gas‐phase reactions of acylium ions with α,β‐unsaturated carbonyl compounds were investigated using pentaquadrupole multiple‐stage mass spectrometry. With acrolein and metacrolein, CH~3~—C^+^O, CH~2~CH—C^+^O, C~6~H~5~—C^+^O, and (CH~3~)~2~N—C^+^O react to variable extents by mono and double polar [4 + 2^+^] Diels–Alder cycloaddition. With ethyl vinyl ketone, CH~3~—C^+^O reacts exclusively by proton transfer and C~6~H~5~—C^+^O forms only the mono cycloadduct whereas CH~2~CH—C^+^O and (CH~3~)~2~N—C^+^O reacts to great extents by mono and double cycloaddition. The positively charged acylium ions are activated O‐heterodienophiles, and mono cycloaddition occurs readily across their C^+^O bonds to form resonance‐stabilized 1,3‐dioxinylium ions which, upon collisional activation, dissociate predominantly by retro‐addition. The mono cycloadducts are also dienophiles activated by resonance‐stabilized and chemically inert 1,3‐dioxonium ion groups, hence they undergo a second cycloaddition across their polarized CC ring double bonds. ^18^O labeling and characteristic dissociations displayed by the double cycloadducts indicate the site and regioselectivity of double cycloaddition, which are corroborated by Becke3LYP/6–311++G(d,p) calculations. Most double cycloadducts dissociate by the loss of a RCO~2~COR^1^ molecule and by a pathway that reforms the acylium ion directly. The double cycloadduct of the thioacylium ion (CH~3~)~2~N—C^+^S with acrolein dissociates to (CH~3~)~2~N—C^+^O in a sulfur‐by‐oxygen replacement process intermediated by the cyclic monoadduct. The double cycloaddition can be viewed as a charge‐remote type of polar [4 + 2^+^] Diels–Alder cycloaddition reaction. Copyright © 2001 John Wiley & Sons, Ltd.