Gas-Phase Synthesis of Organoargenate Anions and Comparisons with Their Organocuprate Analogues

Abstract

The gas-phase double decarboxylation of the silver carboxylate centers [RCO 2 AgO 2 CR] − and [MeCO 2 AgO 2 CR] − was investigated as a means of synthesizing homo- [RAgR] − (R = Me and Et) and heteroargenates [MeAgR] − (R = Et, Pr, i Pr, t Bu, Allyl, PhCH 2, Ph). The formation of these organoargenates was examined by multistage mass spectrometry experiments employing collision-induced dissociation (CID) and by density functional theory. A key side reaction in competition with the second stage of decarboxylation involves the loss of the anionic carboxylate ligand. Interpretation of the decarboxylation pathway of the heterocarboxylates [MeCO 2 AgO 2 CR] − was more complex due to the possibility of decarboxylation occurring at either of the two different carboxylate ligands, giving rise to the possible isomers [MeAgO 2 CR] − and [MeCO 2 AgR] − . This difficulty was overcome through the use of 13 C labeling experiments in which [Me 13 CO 2 AgO 2 CR] − was subjected to CID, thereby providing direct evidence for the relative population of the isomers through the losses of 13 CO 2 and CO 2 . For example, [Me 13 CO 2 AgO 2 C t Bu] − underwent exclusive loss of 13 CO 2, indicating decarboxylation from the MeCO 2 − ligand, while [Me 13 CO 2 AgO 2 CAllyl] − underwent exclusive loss of CO 2, indicating decarboxylation from the AllylCO 2 − ligand. MeCO 2 − was preferentially decarboxylated when R = Et, Pr, i Pr, and t Bu, while RCO 2 − was preferred for R = Allyl, PhCH 2, and Ph. Subsequent fragmentation was in agreement with the assigned structures, including in cases where sufficient yields of both the isomeric products [Me 13 CO 2 AgR] − and [MeAgO 2 CR] − were formed (R = PhCH 2 and Ph), each being independently isolated and subjected to CID. Detailed DFT calculations were carried out on the potential energy surfaces for the first and second decarboxylation reactions of all homo- and heteroargenates, as well as possible competing reactions. These reveal that in all cases the first decarboxylation reaction is favored over loss of the carboxylate ligand. In contrast, carboxylate ligand loss can become favored over the second decarboxylation reaction. Organoargenate species formed included [MeAgMe] − and [MeAgR] − (where R = Et, Pr, Allyl, PhCH 2, and Ph). The organoargenate [MeAgR] − is the principle product for R = Allyl, PhCH 2, and Ph but is only a minor product when R = Et and Pr. Finally, comparisons are made with previous results on the gas-phase formation of organocuprates via double decarboxylation of the related copper carboxylate centers [RCO 2 CuO 2 CR] − and [MeCO 2 CuO 2 CR] − .