Gas-phase ion-molecule chemistry of borate and boronate esters

Abstract

Borate esters B(OR)3 and boronate esters RB(OR)2 undergo ion-molecule reactions to yield both addition products (by an implied radiative emission mechanism), ligand exchange, and proton transfer products, in both positive and negative ion modes. Although an acidity for CH3B(OR)2 could not be determined, HOB(OR)2 has an acidity between acetaldehyde and nitromethane. In light of the negligible polar electron acceptor properties of the -B(OR)2 group, that functionality must therefore be one of the best resonance electron acceptor groups known, almost half again as effective as the nitro group.

Gas phase reactions of trimethyl borate with phosphates and their non-covalent complexes

“…When subjected to CAD, the reaction product ion (m/z ϭ 322) loses CH 3 OH to give an ion at m/z ϭ 290. MS 4 and MS 5 experiments indicate the sequential loss of 2 H 2 O molecules to produce an ion at m/z ϭ 254. This ion then loses CO to give m/z ϭ 226 in an MS 6 experiment indicating a breakdown of the phosphoserine backbone.…”

“…Energies reported in the text are at the MP2/6-31ϩG(d,p) level corrected for zero-point vibrational energies (ZPE, scaled by 0.9135) [11]. The mass spectrum from the reaction of H 2 PO 4 Ϫ with TMB is shown in Figure 1a and data for all the reactions are compiled in Table 1. The predominant feature of the spectrum is the formation of an ion at m/z ϭ 169 which corresponds to the addition of TMB with the loss of CH 3 OH via an addition/elimination mechanism (Scheme 1).…”

“…If the initial product were simply a cluster of I complexed to CH 3 CO 2 H, one would expect that loss of CH 3 CO 2 H would be an important CAD channel, but it is not. For example, CAD of the reactant ion, H 2 PO 4 Ϫ (CH 3 CO 2 H), cleanly leads to H 2 PO 4 Ϫ by loss of CH 3 CO 2 H. When VI is subjected to CAD (Figure 4c), CH 3 CO 2 H is lost to give an ion at m/z ϭ 137 that appears to be equivalent to II A curious aspect of the reaction pathway is that there is a distinct preference in each step with regards to the ligand that is lost from boron, but it is not always the same ligand that is preferentially lost. During the addition and the first CAD process, CH 3 OH is lost almost exclusively, but in the final CAD, CH 3 CO 2 H is lost.…”

“…When the M Ϫ H anion from phosphoserine is allowed to react with TMB, products at m/z ϭ 256 and m/z ϭ 224 dominate, and correspond to addition followed by the loss of one or two CH 3 OH molecules (Figure 5a). When the ion at m/z ϭ 224 is subjected to CAD (Figure 5b), several products are formed corresponding to the loss of H 2 O (m/z ϭ 206), the loss of CH 3 OH (m/z ϭ 192), the loss of C 4 Ϫ ϩ 2 TMB Ϫ 2 CH 3 OH), and 264 (m/z ϭ 224 ϩ TMB Ϫ 2CH 3 OH). When subjected to CAD, the ion at m/z ϭ 192 yields PO 3 Ϫ .…”

“…This compound and related boron reagents have previously been shown by Bowie and co-workers [1,2] to react readily with hydrogen-bonded alcohol/alkoxide complexes to give species where the boron is linked to both components of the original complex. TMB has also been used as a reagent for identifying structural motifs in poly-functional molecules via positive ion mass spectrometry [3,4]. Despite the inherent low reactivity of phosphates in the gas phase [5], we have found that TMB gives rapid reactions that appear to involve the formation of covalent bonds between the partners in the phosphate complexes.…”