Quaternary Ammonium Arylspiroborate Esters as Organo-Soluble, Environmentally Benign Wood Protectants

Abstract

As part of a larger project aimed at the development of leach resistant boron-based wood preservatives, the anti-fungal and termiticidal activities, and the resistance to leaching from timber, of three related tetra-n-butylammonium spiroborates, tetra-n-butylammonium bis(ortho-hydroxymethylphenolato)borate 2, tetra-n-butylammonium bis[catecholato(2–)-O,O′]borate 3, and tetra-n-butylammonium bis[salicylato(2−)-O,O']borate 4, have been examined. All three borates are found to be active against test organisms, with the following orders of activity being observed: 2 > 3 > 4 > boric acid against wood decay fungi, and 2 > 3 ≈ 4 > boric acid against termites. The most active compound in both assays 2 also has the highest calculated lipophilicity. In a test for permanence in wood, the following order of leach resistance is observed: 4 >> 3 ≈ 2 > boric acid. This order appears to correlate more closely with the stability constants of the borate esters, as determined using 11B NMR spectroscopy, rather than calculated lipophilicities.

Wood Protection Properties of Quaternary Ammonium Arylspiroborate Esters Derived from Naphthalene 2,3-Diol, 2,2'-Biphenol and 3-Hydroxy-2-naphthoic Acid

“…1. [7,8] All three of these esters were found to be more resistant to aqueous leaching from timber than boric acid, with the salicylate, 3, performing best in this regard. The complexation of borate in this way was expected to lead to a drop in biocidal activity, [5] as it should restrict the release of the biologically active free borate.…”

“…[8] correlated with lipophilicity rather than susceptibility to hydrolysis, whereas leach resistance correlated with hydrolytic stability rather than lipophilicity. It was concluded that important timber preservative properties such as permanence, delivery to the biological site of action in the wood decaying organism, and activity at this biological site have conflicting physiochemical requirements, and the best preservative will meet a balance between these properties.…”

“…[8] After 8 weeks, the specimens were recovered and the extent of attack determined by weight loss. The results are summarized in Table 1 and selected images of the blocks after treatment are shown in Fig.…”

“…The three spiroborate esters used in this investigation 4-6, were prepared following methods similar to those previously described; [8] two equivalents of the dihydric compound (naphthalene-2,3-diol, 2,2 -biphenol and 3-hydroxy-2-naphthenoic acid, respectively) were added to an aqueous solution of tetra-n-butylammonium borate and the product collected and recrystallized from an organic solvent. Following were obtained as tetra-n-butylammonium salts in 35%, 64% and 50% yield, respectively.…”

Chemistry in Pasteur's Quadrant

“…This is not the case for wood treatments where the CSIRO-New Zealand joint venture ENSIS has its own testing facilities. Carr, Duggan, Humphrey, Platts, and Tyndall [15] have used these facilities to show that environmentally benign boron compounds are both effective fungicides and can be made leach-resistant.…”