Gas-phase polar [4+ + 2] cycloaddition with ethyl vinyl ether: a structurally diagnostic ion–molecule reaction for 2-azabutadienyl cations

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

• Publisher: John Wiley and Sons
• Year: 2003
• Tongue: English
• Weight: 135 KB
• Volume: 38
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.523

✦ Synopsis

Abstract

The intrinsic reactivity of eight gaseous, mass‐selected 2‐azabutadienyl cations toward polar [4^+^ + 2] cycloaddition with ethyl vinyl ether has been investigated by pentaquadrupole mass spectrometric experiments. Cycloaddition occurs readily for all the ions and, with the only exception of those from the N‐acyl 2‐azabutadienyl cations (N‐acyliminium ions), the cycloadducts are found to dissociate readily upon collision activation (CID) both by retro‐Diels‐Alder reaction and by a characteristic loss of an ethanol (46u) neutral molecule. Ethanol loss from the intact polar [4^+^ + 2] cycloadduct functions therefore as a structurally diagnostic test: 72 u neutral gain followed by 46 u neutral loss, i.e., as a combined ion–molecule reaction plus CID ‘signature’ for N‐H, N‐alkyl and N‐aryl 2‐azabutadienyl cations. The two N‐acyliminium ions tested are exceptional as they form intact cycloadducts with ethyl vinyl ether which dissociate exclusively by the retro‐Diels‐Alder pathway. Copyright © 2003 John Wiley & Sons, Ltd.