Fragmentation of Protonated 2-(2-Phenylethyl)Chromones from Agarwood: The Diagnostic Role of Ion/Neutral Complexes as Reactive Intermediates

A positive-ion electrospray ionisation collision-induced dissociation mass spectrometric study on the fragmentation of the [M + H]+ ions of 2-(2-phenylethyl)chromone and a set of nine hydroxyl- and/or methoxy-substituted derivatives has revealed a highly prominent fragmentation channel, the loss of benzoquinomethanes or a benzaldehyde, respectively, as a diagnostic feature for 2-(2-phenylethyl) chromones that bear a hydroxyl group at the para- (4′-), ortho- (2′-) and/or benzylic (α-) position of the phenylethyl residue. Derivatives that bear only a meta- (3′-) hydroxyl group do not undergo this elimination. The intermediacy of ion/neutral complexes (INCs) is invoked to explain this fragmentation, which involves the remarkable intra-complex proton or hydrogen atom transfer from the remote 4′-OH (or the 2′- or α-OH) functionalities. Density functional theory (B3LYP/6-31G(d)) calculations confirm the energetic preference for these elimination channels and agree with the limited thermochemical data known for para- and ortho-benzoquinomethanes. The INC-mediated losses of the benzaldehydes from the [M + H]⁺ ions of the α-hydroxy-substituted 2-(2-phenylethyl)chromones correspond to a particularly facile (vinylogous) Grob fragmentation. The study may be viewed as a telling example of the diagnostic role of ion/neutral complexes as intermediates for the structural assignment of constitutional isomers by mass spectrometry.