Analysis of diacyl peroxides by Ag + coordination ionspray tandem mass spectrometry: free radical pathways of complex decomposition

Abstract

Organic peroxides have significance in organic synthesis and biological processes. Characterization of these compounds with weak O–O bonds is sometimes difficult due to their thermal instability and sensitivity to acid or base. Coordination of diacyl peroxides with AgBF 4 provides a means for analysis of these compounds by coordination ionspray tandem mass spectrometry (CIS-MS/MS). Precursor ion (Q 1) scans of acetyl benzoyl peroxide give two Ag + adducts, [M + Ag + solvent] + and [M + Ag + M] +. These silver ion adducts can be selectively dissociated (CID) to give unique structural information about the analyte. Decomposition of the [M + Ag + solvent] + adduct generates fragmentation products due to apparent homolytic cleavage of the O–O bond followed by decarboxylation of the resultant radicals. The bis-diacylperoxide complex, [M + Ag + M] + gives CID pathways that involve homolysis of the O–O bond and free radical cross-coupling of the two diacyl peroxides coordinated to the silver ion, i.e. formation of dibenzoyl peroxide, phenyl benzoate, and biphenyl from acetyl benzoyl peroxide. The observation of free radical CID modes is uncommon in mass spectrometry but these pathways are consistent with well-known solution and gas phase processes for peroxide compounds. The proposed fragmentation pathways have been supported by experiments with 18O and deuterated substrates. This technique can be applied to analyze diacyl peroxides with different substituents as well.