Quaternary ammonium arylspiroborate esters as organo-soluble, environmentally benign wood protectants

Article abstract of DOI:10.1071/CH05226

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. CSIRO 2005.

Full text of DOI:10.1071/CH05226

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2005, 58, 901–911
Quaternary Ammonium Arylspiroborate Esters as Organo-Soluble,
Environmentally Benign Wood Protectants
Jenny M. Carr,A Peter J. Duggan,B,C,H David G. Humphrey,A,F,H James A. Platts,D
and Edward M. TyndallE,G
A ENSIS—Wood Processing and Products, CSIRO, Clayton South VIC 3169, Australia.
B School of Chemistry, Monash University, Melbourne VIC 3800, Australia.
C CSIRO Molecular and Health Technologies, Clayton South VIC 3169, Australia.
D School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK.
E Centre for Green Chemistry, Monash University, Melbourne VIC 3800, Australia.
F Present address: Koppers Arch Wood Protection, Tullamarine VIC 3043, Australia.
G Present address: Biota Holdings Ltd, Notting Hill VIC 3168, Australia.
H Corresponding authors. Email: peter.duggan@csiro.au; david_humphrey@koppersarch.com.au
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,Oꢀ]borate 3, and tetra-n-butylammonium bis[salicylato(2−)-O,Oꢀ]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 per-
manence 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.
Manuscript received: 26 August 2005.
Final version: 9 November 2005.
Introduction
The numerous advantages of wood and wood-based products
are offset by the low natural durability of most sustainable,
commercially grown plantation timbers. For many applica-
tions, it is necessary to protect wood of this type against
biodegradation by insects, decay fungi, and marine organ-
isms. The most effective means of increasing resistance to
biodeterioration is to apply a preservative, and for the past
20–30 years chromated copper arsenate (CCA) has been the
preservative most widely used by industry. The decision to
phase out the use of CCA in Europe, and its reduced usage
in North America, has led the industry to turn to alterna-
tive preservatives, such as those that combine copper with
an organic fungicide, e.g., a triazole or quaternary ammo-
nium salt. In Australia, the use of CCA will not be permitted
beyond February 2006 in end uses where intimate human
contact is possible. Other biocides currently used globally
by the preservation industry are also under scrutiny. Cre-
osote, pentachlorophenol, and the organo-tin compounds,
bis(tributyltin)oxide and tributyltin naphthenate, are also
predicted by some to have limited market lifetimes. As a
consequence, the industry is seeking new actives that provide
a specified level of performance at an acceptable cost and
display improved environmental credentials.
The ‘green’ qualities of boron are generally acknowl-
edged: low mammalian toxicity, high activity, non-
flammability, and non-corrosiveness.[1–3] The toxicity of the
borate anion towards microorganisms and insects is thought
to result from its ability to form esters with diol- or hydroxy-
acid-containing compounds of biological importance. It has
been shown that borate binds with co-enzymes to disrupt
metabolism, effectively starving cells to death by a biostatic
rather than biocidal action.[4] Borate can also act as a direct
inhibitor of enzymes, and possibly binds polyols in cell mem-
branes to disrupt cell activity.[5,6] The primary disadvantage,
however, is that commonly used boron compounds, such as
boric acid and various oxides of boron (e.g. sodium tetra-
borate) are readily leached from timber exposed to moisture.
Numerous attempts have been made to immobilize or fix
borates in wood.[7] However, to date, none have been com-
mercially successful and as a consequence the use of boron in
wood preservation has thus far been largely limited to internal
or remedial applications.
© CSIRO 2005
10.1071/CH05226
0004-9425/05/120901

902
J. M. Carr et al.
Ϫ
Ϫ
C9H19
ϩ
O
N(Bun)4
B
O
O
O
Cu2ϩ
Cl
O
B
O
O
Cl
O
2
C9H19
Ϫ
2
1
ϩ
O
N(Bun)4
O
Fig. 1. Structure of the mixed copper-boron compound 1 developed
B
by Maynard.[9,11]
O
O
The approach adopted at the outset of this work was
to form hydrolytically stable borate esters with specifi-
cally selected binding agents, thus modifying the chemical
behaviour of the borate ion and enabling ready variation of
certain physical properties. This approach to boron fixation
is not without precedence: Previous attempts have been made
to produce leach-resistant borate systems through ester for-
mation. Anderson et al. prepared some biguanide derivatives
of boric acid that displayed levels of fungitoxicity similar to
that of boric acid, though these complexes were not resistant
to leaching.[8]
Maynard has had some success in the development of
a mixed copper–boron system[9] (1, Fig. 1) that has been
shown to perform well in field trials.[10] The borate portion
of the system is based on a tetrahedral hydroxymethylphe-
nol spiroborate core. The anionic spiroborate ester has been
shown to promote the solubilization of copper(ii) in hydro-
carbon solvents, with the species formed assumed to be an
ion pair between the spiroborate and copper(ii) in a 2:1 ratio.
The precise role of the borate ester in the performance of
this compound as a wood preservative is not, however, well
documented in the open literature,[11] in that the anti-fungal
activity of these systems could be attributed to the copper
component.
Despite recent findings which suggest that the binding
of borate ions within organic compounds can reduce the
anti-fungal activity of boron compounds,[12] it has recently
been shown that an organo-soluble, tetrahedral ammonium
borate ester had improved biological activity against wood
decay fungi relative to control compounds such as boric
acid, the parent alcohol, and the corresponding ammonium
bromide.[13] This result indicates that ester formation, as an
approach to leach-resistant borate preservatives, is worthy of
further investigation.
Reported here is an examination of the anti-fungal and
termiticidal activities, and the resistance to leaching, of three
organo-soluble tetrabutylammonium borate esters (Fig. 2),
with tests complying as closely as possible with industry
standards.[14]
3
O
Ϫ
ϩ
O
N(Bun)4
B
O
O
O
O
4
Fig. 2. Structures of the three tetra-n-butylammonium borates,
tetra-n-butylammonium bis(ortho-hydroxymethylphenolato)borate 2,
tetra-n-butylammonium bis[catecholato(2−)-O,Oꢀ]borate 3, and tetra-
n-butylammonium bis[salicylato(2−)-O,Oꢀ]borate 4, used in this study.
follows that the lipophilicity of ammonium borate esters is
also likely to affect their activity in assays that use whole
organisms, such as termiticide assays.
A quantitative measure of lipophilicity is the partition
coefficient, log P, derived from the distribution of a com-
pound in a biphasic mixture of water and an organic sol-
vent such as chloroform, cyclohexane, or, most commonly,
octanol.[16] The spiroborate salts of interest in the present
study are susceptible to decomposition in the presence of
water, which complicates the practical measurement of log P.
It is, therefore, of use to employ molecular modelling tech-
niques to predict these properties. A popular approach to the
prediction of log P is to sum the contributions of component
fragments, which can be derived from a statistical analy-
sis of a large number of experimentally determined log P
values.[16] This method is, however, limited to compounds
for which there is sufficient data for the assignment of the
necessary fragment contributions. Such data is unavailable
for the compounds studied here, as the spiroborate moiety
has scarcely been studied in this context. In this study, a pre-
viously described theoretical method for the prediction of
log P values[19] that uses calculated total, polar, and aromatic
molecular surfaces areas has been applied, and the results are
correlated with those obtained from biological assays.
Lipophilic spiroborate salts have been described in the
literature as effective, leach-resistant wood preservatives,
and although ‘hydrolytic stability’ has been suggested as
an important property in determining leach resistance, there
have been only limited attempts to correlate the two.[9,11,20,21]
As part of the present study, the solution stability of the
The lipophilicity of the test compounds is likely to influ-
ence their performance as wood preservatives. This property
has attracted much interest in the structure–activity study of
small molecule drugs and toxins because it is a key deter-
minant of a compound’s ability to be transported to its site
of action upon ingestion by an organism.[15–18] It, therefore,

Quaternary Ammonium Arylspiroborate Esters as Organo-Soluble, Environmentally Benign Wood Protectants
903
Table 1. Results of cellulose fungal assay with P. tephropora; growth
borates shown in Fig. 2 are examined in wet dimethyl sul-
foxide (DMSO), and the results correlated with measured
leach resistances, as determined by an accelerated weathering
method.
(percentage of maximum diameter) of as a function of timeA
CompoundB
Time [days]
14
7
21
4
2
14 (3)
0
31 (5)
0
68 (5)
0
Results
B(OH)3
NBu4Br
Water control
MeCN
53 (3)
87 (1)
87 (10)
94 (1)
100 (1)
100 (1)
100 (1)
97 (1)
100 (1)
100 (1)
100 (1)
100 (1)
Synthesis of Borate Esters
Of the three ammonium spiroborates employed in
this study, the synthesis of tetra-n-butylammonium bis-
(ortho-hydroxymethylphenolato)borate 2 has already been
described.[13] Below is a discussion of the preparation of
tetra-n-butylammonium bis[catecholato(2−)-O,Oꢀ]borate 3,
andtetra-n-butylammoniumbis[salicylato(2−)-O,Oꢀ]borate4.
Catechol and many other benzene-1,2-diols have been
found to react with boric acid or the borate anion to form
1:1 and 2:1 esters.[22] The results of equilibrium studies
of these types of compounds in aqueous solution show
that, in contrast to borates derived from aliphatic diols and
α-hydroxy acids, the 2:1 ester derived from catechol did
not form readily, and in some cases was not observed at
all. In order for this species to be observed, specific condi-
tions of near-neutral pH and high reagent concentration were
required.[22] Despite the apparent low solution stability of the
bis(catecholato)borate anion, stable salts of this anion and
its analogues have been isolated, with the synthesis of potas-
sium bis(catecholato)borate being the first reported example,
described in 1918.[23] Metal and ammonium salts have been
prepared by precipitation from various concentrated aqueous
solutions.[24,25] Other salts have been prepared in organic
solvents from an aminoborane,[26] and tetraalkylammonium
salts have also been reported.[27,28]
Published methods for the preparation of tetraalkyl-
ammonium salts of bis(catecholato)borate have employed
organic solvents such as methanol and dimethylformamide
(DMF).[27,28] For simplicity, the synthesis of the tetra-n-
butylammonium salt of bis(catecholato)borate 3 described
herein was performed in an aqueous system. Recrystalliza-
tion from tetrahydrofuran gave 3 as flat white needles in 75%
yield.
The first reported spiroborates derived from hydroxy
acids were the salts of bis(salicylato)borate, as described
in 1878.[29] Since that time, many related compounds have
been prepared, in which the spiroborate anion was derived
from aromatic o-hydroxy acids or aliphatic α-hydroxy acids,
and where the counter-ion was a proton, a mono- or di-
valent metal, or an ammonium or alkylammonium cation.[24]
These syntheses were usually performed under anhydrous
conditions or by precipitation from a concentrated aqueous
solution of hydroxy acid, boric acid, and base.[24,26,30] Exten-
sive solution studies, using a range of techniques including
conductivity, polarimetry, and 11B NMR spectroscopy, have
shown that in aqueous solution, the optimum pH for borate
ester formation from hydroxy acids ranges from low to
medium (pH ∼ 4–7), with stable 2:1 esters often favoured
over 1:1 esters under these conditions. This contrasts with
A Average of four replicates. Standard deviation in parentheses.
B Concentration: 5.3 µmol cm−2
.
Table 2. Results of cellulose fungal assay with G. abietinum; growth
(percentage of maximum diameter) of as a function of timeA
CompoundB
Time [days]
14
7
21
4
2
3 (1)
0
33 (3)
22 (1)
22 (1)
8 (1)
0
92 (1)
87 (1)
97 (1)
10 (1)
0
100 (1)
100 (1)
100 (1)
B(OH)3
NBu4Br
Water control
A Average of four replicates. Standard deviation in parentheses.
B
Concentration: 5.3 µmol cm−2
.
the formation of esters of polyols or o-diphenols, where 1:1
esters are usually favoured over their relatively unstable 2:1
analogues.[22,31]
In the present study, tetra-n-butylammonium bis(salicy-
lato)borate 4 was prepared using a simple method adapted
from the literature.[30] The crude product was purified by
recrystallization from a methanol/diethyl ether solution to
give pure 4 as white needles in 53% yield.
Anti-Fungal Activity
It was recently shown that the spiroborate 2 displays anti-
fungal activity on a simple cellulose substrate against two
strains of wood decay fungi, and that its activity is superior
to that of the corresponding sodium salt, parent phenol, tetra-
n-butylammonium bromide, and boric acid.[13] The current
study began with a similar screen for anti-fungal activity,
using Gloeophyllum abeietinum (brown rot) and Perennipo-
ria tephropora (white rot) as test organisms and performed
with 4 and repeated with 2. Fungal growth was monitored as a
function of time and studied at three concentrations, i.e., 5.3,
1.1, and 0.2 µmol cm−2.∗ The greatest difference in activity
was obtained with the highest concentration, 5.3 µmol cm−2
,
and the results of the fungal bioassays at this concentra-
tion are summarized in Tables 1 and 2. Separate bioassays
have shown that the organic precursors to these borates
(the hydroxylic component), e.g., salicylic acid, and other
tetra-n-butylammonium salts such as NBu4HSO4, display no
inhibitory activity at the concentrations used.
It was of interest to determine if the same compounds, as
well as the related borate 3, could prevent the fungal decay of
∗ In bioassays of this type, concentrations are commonly expressed as µmol per unit area rather than µmol per unit volume.

904
J. M. Carr et al.
Fig. 3. Example replicates from a mini-block fungal bioassay at completion of the bioassay: The P. radiata specimens
were treated with 2 and exposed to G. abietinum (left to right, 0.13, 0.05, 0.01% w/w retentions) and a water control (far
right). Corresponding average mass loss (left to right) = 2, 0.4, 33, and 32%. A = wood specimen, B = inoculum.
100
80
60
40
20
0
Table 3. Mass loss data from agar plate mini-block fungal
bioassayA
Compound
Retention [% w/w]B
Mass loss [%]C
B(OH)3
B(OH)3
B(OH)3
2
2
2
3
3
3
4
4
4
0.13
0.07
0.01
0.13
0.07
0.01
0.13
0.07
0.01
0.13
0.07
0.01
—
12 (7)
20 (2)
28 (5)
2 (0.5)
0.4 (0.4)
33 (4)
3 (0.9)
10 (6)
24 (5)
6 (3)
0
1
2
3
4
5
6
7
Time/days
16 (10)
35 (7)
32 (8)
33 (2)
2
4
Boric acid
Water
Water control
MeCN control
Fig. 4. Termite (C. acinaciformis) mortality as a function of time for
two representative compounds 2 and 4 used in a cellulose paper bioas-
say. The concentration of the compound in each treatment solution was
0.05 M, resulting in a concentration of 0.10 µmol cm−2 on the filter
paper.
—
A Test organism: G. abietinum. Data shows averages of the nine
replicates.
B Mass of elemental boron in 100 g of oven-dried P. radiata.
C Standard deviations in parentheses. Fungal decay is considered to have
been prevented if mass loss is less than 5%.
tested at three concentrations and the mortality assessed daily
to provide mortality versus time profiles for each compound.
Fig. 4 shows a plot of the termite mortality (percentage of
initial population) as a function of time for the two best per-
forming borates 2 and 4, for boric acid as a free boron control,
and for water as a solvent control. It is clear from this plot
that both of these spiroborate test compounds display better
termiticidal activity than boric acid, at least for the first 5.5
days, and the boron-free control (water). The borate derived
from o-hydroxymethylphenol 2 displays the highest activity,
with 100% termite mortality recorded after only 1 day. The
corresponding salicylic acid-derived borate 4 was somewhat
less active, showing a 50% mortality after 2.5 days to give
an LT50 of 2.5 days (60 h) at this treatment concentration.
Good variation with treatment concentration was observed
for each compound (seeAccessory Materials), an observation
that strongly validates this test. Separate bioassays have also
shown that the organic precursors to these borates and other
tetra-n-butylammonium salts display no termiticidal activity
at the concentrations used.
wood itself. In this case, theassay involvesexposureof treated
specimens to decay fungi (G. abietinum) that are actively
growing on 2% malt extract agar plates. The specimens were
supported on plastic mesh squares, so that they were not in
direct contact with the agar. This procedure is used so that
any potential leaching of the substance of interest into the
agar is minimized. The extent of fungal decay was assessed
by the mass lost from the specimen at the completion of the
bioassay (see Fig. 3).
Shown in Table 3 are the mass losses caused by brown
rot fungus for mini-blocks treated with test compounds 2–4
and boric acid, at a range of concentrations. Mass loss is
expressed as a percentage.† Boron retention is also expressed
as a percentage, it being the mass of elemental boron in 100 g
of oven-dried wood (% w/w).
Termiticidal Activity
The termiticidal activity of the borate esters was initially
assessed using a no-choice cellulose paper bioassay with
Coptotermes acinaciformis worker termites, and compared
to the results obtained with boric acid. Each compound was
The results of the cellulose termite bioassay are also listed
in Table 4 in order of increasing LT50, which is taken as the
most accurate measure of termiticidal activity from this test.
† Percentage mass loss = [(Mi – Mf )/Mi] × 100, where Mi is the initial mass of the wood block before exposure to fungus and Mf is the mass of the wood
block after exposure to fungus (fungal matter was removed before weighing).

Quaternary Ammonium Arylspiroborate Esters as Organo-Soluble, Environmentally Benign Wood Protectants
905
Table 4. Results of cellulose paper termite bioassayA
All molecular structures were optimized using the AM1
method (see Accessory Materials for further details), as
implemented within the HyperChem package.‡[32] Polar sur-
face area (PSA) properties were then calculated as described
previously:[19] PSA is defined as the surface area of a
molecule that arises from N, O, N–H, and O–H atoms, and is
simply calculated from a three-dimensional molecular struc-
ture. It has previously been shown that PSA, along with total
and aromatic surface areas, can be correlated with partition
coefficients.[19] The results of these calculations are shown
in Table 6.
B
Compound
LT50 [h]
Relative mass loss of
celluloseC [%]
2
4
12
100
115
130
3
23
40
22
Boric acid
3
A Raw data for this and two other treatment concentrations are shown in
the Accessory Materials, together with selected images of filter papers
at the completion of the bioassay.
B Time required for 50% mortality to be achieved at 0.20 µmol cm−2
C [(mass loss treated paper)/(mass loss solvent control)] × 100.
.
Boron Fixation
The leaching of a preservative from treated wood is dependent
upon many variables, e.g., exposure conditions, orientation,
and surface area of the product, preservative distribution,
species and type of wood, etc. In order to compare this aspect
of preservative performance, a standard leaching test is com-
monly used whereby all of these variables are fixed.[14] The
procedure involves saturating the preservative-treated speci-
mens of uniform dimension with water and placing them in
a jar that contains at least three times the volume of water
as that of the sample. The jars are placed in a shaking water
bath maintained at 35◦C for five days with daily changes of
water. Chemical analysis of both leached and unleached sam-
ples then provides a measure of leach resistance or fixation.
The leaching results obtained with 2–4 are shown in Table 7.
It can be seen that while all three borates provide improved
boron fixation relative to boric acid, only the one derived
from salicylic acid 4 causes a significant amount of boron to
be retained in the timber.
Table 5. Results of mini-block termite bioassay
CompoundA
Solvent
Adjusted mass loss [%]B
2
4
3
MeCN
MeCN
MeCN
4 (0.5)
5 (1)
9 (3)
MeCN controls
Untreated controls
44 (6)
52 (6)
A Loading: 0.07% w/w boron, applied in MeCN.
B Mass loss of P. radiata specimens (20 × 20 × 10 mm3) after 8 weeks
exposure to C. acinaciformis. Values are averages of three replicates,
with standard deviations shown in parentheses.
Preliminary screening studies of the kind described above
are useful for comparing the relative activity of a range of
related compounds, where their mode of action is likely to
be similar. It does not, however, quantify the amount of the
compound required to provide some specified level of per-
formance when applied to wood. The activity of 2–4 was
thus examined in wood: P. radiata sapwood specimens were
treated with 0.07% w/w boron and exposed to C. acinaci-
formis. After eight weeks the specimens were recovered and
the extent of attack determined by weight loss. The results
are summarized in Table 5 and images of the blocks after
treatment are shown in Fig. 5.
Stability of Borate Esters—Equilibrium Constants
for Formation
It was of interest to determine if the retentions observed in the
leachingexperimentscanbecorrelatedwithboratestabilities,
i.e., susceptibility to hydrolysis. As such, an attempt to deter-
mine the relative stabilities of the borate salts investigated
here was made. The extent of formation of borate esters from
component boric acid, substrate, and cation source (tetra-
n-butylammonium hydroxide) in DMSO using 11B NMR
spectroscopy was measured. DMSO was chosen as an organic
solvent due to its favorable solubilizing properties, and as a
crude model of the polar, largely anhydrous, and, by virtue
of the presence of boric acid, slightly acidic environment that
might be found in wood. Below, the structural assignments of
the species formed are discussed followed by an evaluation
of the association constants.
Lipophilicity Calculations
As mentioned above, compound lipophilicity has long been
considered a primary determinant for the bioavailability of
bioactives, with bell-shaped relationships between activity
and lipophilicity often observed. Lipophilicity is usually
determined by a partition experiment involving a two-phase
system such as water/octan-1-ol. The results are usually pre-
sented as log P values, and are derived from the equilibrium
concentrations of the compound in each phase. Given the
susceptibility to hydrolysis of many borate esters, such an
experimental method is not generally applicable here. For this
reason, a theoretical method for predicting log P values,[19]
which involves a combination of total, polar, and aromatic
surface areas, has been used.
In aqueous solution above 25◦C, free borate appears as
a singlet in its 11B NMR spectrum. This signal is the result
of the coalescence of the intrinsic signals of trigonal (B) and
tetrahedral borate (B−), with the line-shape and chemical
shift dependent on the relative concentration of these species,
which in turns depends on solution pH.[33,34] Fig. 6a shows
‡ A variety of optimization methods were tested using the X-ray crystal structure of 2 as a reference,[13] with AM1 giving the best results—see Accessory
Materials for more information.

906
J. M. Carr et al.
(a)
(c)
(b)
(d )
Fig. 5. Selected images of treated wood blocks (20 × 20 mm2 face shown) after exposure to C. acinaciformis. (a) Compound 2:
average mass loss 4%, (b) compound 4: average mass loss 5%, (c) MeCN control: average mass loss 44%, and (d) untreated
controls: average mass loss 52%.
Table 6. Surface areas calculated for the spiroborate salts, with the
predicted lipophilicity
Table 7. Results of accelerated leaching experiments
Compound
Fixation [%]A
Molecule Total surface Polar surface Aromatic surface Predicted
area [Å2]
area [Å2]
area [Å2]
log Poct
2
3
5
7
3
4
2
594.77
644.59
616.71
36.97
62.35
23.58
48.69
53.25
46.64
5.82
6.02
6.11
4
36
1
B(OH)3
A Fixation = ([BL]/[BUNL]) × 100, where [BL] and
[BUNL] are the concentrations of boron in the leached
and unleached samples, respectively. Results are averages
taken from ten specimens each.
the spectrum of a [D6]DMSO/DMSO (1/10) solution con-
taining boric acid and tetra-n-butylammonium hydroxide in
a 1:1 ratio.§ In contrast with aqueous borate solutions, sepa-
rate signals are observed for the trigonal (B) and tetrahedral
monoborate (B−) species, which indicates that under these
conditions the exchange between these species is slower than
in water and slow on the NMR timescale. The signal near δB
+20 ppm has been assigned to boric acid (B) because this
is the only signal observed in solutions containing no base
and agrees with literature values for this shift in aqueous
and DMSO solutions.[34,35] The peak observed at a higher
field (δB +2.8 ppm) was assigned to the monoborate anion
(B−) because its intensity increased with an increasing ratio
of base and was sharper, which is consistent with its more
symmetrical geometry. A literature value for this peak in
a NaOH/DMSO/B(OH)3 (1/1/1) solution was at a slightly
higher field (δB +1.6 ppm).
Before the measurement of the 11B NMR spectra, the solu-
tions were incubated at 30◦C for 24 h, after which time the
ester formation reaction was assumed to have reached equi-
librium. This assumption is based on the fact that no change
was observed in the spectra of selected samples measured
after incubation for a further 8 h. Typical spectra obtained
when o-hydroxymethylphenol, catechol, and salicylic acid
were included in DMSO solutions containing boric acid and
tetra-n-butylammonium hydroxide are shown in Figs 6b–6d.
The peaks in these spectra were assigned from high to low
field as boric acid (B), the 2:1 borate ester (BS−2 ), the 1:1
ester (BS−), and the monoborate anion (B−), according to
the following rationale:
The peaks for B and B− were identified by comparison
with the spectra shown in Fig. 6a. In this case, however,
it is important to consider that although easily distinguish-
able from tetrahedral esters, it has been shown that signals of
trigonal boric acid (B) and trigonal boric acid 1:1 esters are
difficult to distinguish. In an analogous NaOH/DMSO/boric
acid system,[35] a shoulder at a lower field to the boric
acid peak was observed for such esters. No such shoulder
was observed in the spectra of solutions containing any of
the phenols used in this study, which is good evidence for
the assignment of the broad peak near δB 20 ppm as boric
acid alone. Furthermore, trigonal borate esters are highly
susceptible to hydrolysis, and although they are likely to
In most cases, an equimolar ratio of boric acid/tetra-n-
butylammonium hydroxide/substrate was sufficient−to allow
the observation of both 1:1 (BS−) and 1:2 (BS2 ) tetra-
substituted esters. The tetra-n-butylammonium hydroxide
solution (40 wt.-% in H2O) was included in the solutions
as a base to promote tetrahedral ester formation, and as a
cation source to ensure the species in solution were tetra-n-
butylammonium salts, and hence the same as those found in
partiallyhydrolyzedmixturesofthespiroboratessynthesized.
A boron concentration of 0.05 M was used, this being suffi-
ciently high to produce significant signals in the 11B NMR
spectra but it was assumed to be sufficiently low for minimal
polyborate formation.ꢂ
§ This solution contained some water from the 40% aqueous tetra-n-butylammonium hydroxide solution used to make up the sample.
ꢂ A study of aqueous sodium borate showed that at boron concentrations of 0.05 M and lower, polyborate signals were not detectable.[34] This limit is probably
different for the largely non-aqueous [D6]DMSO/DMSO system used in this study, but is a reasonable approximation.

Quaternary Ammonium Arylspiroborate Esters as Organo-Soluble, Environmentally Benign Wood Protectants
907
(a)
(c)
B
BSϪ
BϪ
BS2Ϫ
25
20
15
10
5
0
BϪ
B
25
20
15
10
5
0
(b)
(d)
BSϪ
BϪ
BS2Ϫ
BS2Ϫ
BϪ
B
5
0
B
25
20
15
10
5
0
25
20
15
10
5
0
Fig. 6. 11B NMR spectra of [D6]DMSO solutions of: (a) 1:1 boric acid/N(Bun)4OH, (b) 1:1:1 o-hydroxymethylphenol/boric acid/N(Bun)4OH,
(c) 1:1:1 catechol/boric acid/N(Bun)4OH, (d) 1:1:1 salicylic acid/boric acid/N(Bun)4OH.
be more stable in organic solution than in aqueous solu-
tion, the amount of water present in the systems studied
(n(H2O)/n(B) ≈ 40) is thought to preclude their formation in
significant amounts. In addition, for each 11B NMR spectrum
recorded in this study, a 1H NMR spectrum was also recorded
of the same solution. In most cases, only three substrate envi-
ronments could be detected in the 1H NMR spectra, which is
consistent with only the 1:1 ester (BS−), the 2:1 ester (BS2−),
and free substrate (S or S−) being present, hence is consistent
with the absence of trigonal borate esters (BS). Although not
conclusive, this evidence has lead to the assumption that for
all the 11B NMR spectra recorded in this study, the broad
peak near δB 20 ppm corresponds to boric acid alone, and
has a very limited contribution from boric acid esters. This
assumption simplifies the possible equilibria involved, which
allows for a simpler estimation of association constants.
The peak corresponding to the 2:1 ester (BS−2 ) was identi-
fied by a comparison with the chemical shift of an authentic
sample of the spiroborate salt, and the identity of peaks due
to this and the 1:1 ester (BS−) were further determined by
the change in peak intensity with change in substrate/boric
acid stoichiometry (see Accessory Materials). In the case of
the borate esters studied here, the signal for the 2:1 esters
appeared downfield from the corresponding signals of the 1:1
esters, an observation consistent with that found for borate
esters in aqueous solution.[36]
If it is assumed that at equilibrium the only borate species
present in the solutions studied were free borate (B and B−),
together with the 2:1 ester (BS−2 ) and 1:1 ester (BS−), the
following equilibrium expressions can be applied:
β1
Bfree + S + OH− ꢀ BS− + 2H2O
free
β1 = [BS−]/[Bfree][S] · [OH−
β2
]
(1)
free
Bfree + 2S + OH− ꢀ BS2− + 2H2O
free
β2 = [BS−2 ]/[Bfree] · [S]2 · [OH−
]
(2)
(3)
free
β3
BS− + S ꢀ BS−2 + 2H2O
β3 = β2/β1 = [BS−2 ]/[S] · [BS−]
where[Bfree]istheconcentrationoffreeborate(=[B]+[B−]),
[OH−free] is the concentration of free hydroxide ions, [BS−] is

908
J. M. Carr et al.
Table 8. Equilibrium concentrations [mM] of species in phenolic DMSO borate
solutions as determined by 11B NMRA
Substrate
Catechol
[B]
[B−]
[Bfree
]
[BS−]
[BS2−
]
[S]
4.0 (2) 4.3 (2) 8.3 (4)
36 (1)
5.9 (3)
2 (1)
o-Hydroxymethyl-phenol 23 (1) 7.2 (4) 30 (2) 10.7 (5) 9.4 (5) 20.5 (9)
A The numbers in parentheses represent the standard error for the borate concentrations,
estimated using a 5% error in area measurements. The method used for the propagation
of the errors in the calculated values for [S] is included in the Accessory Materials.
Table 9. Equilibrium constants for phenolic DMSO solutions, calculated using the
concentrations shown in Table 6A
Substrate
β1 [103 M−2
]
β2 [106 M−3
]
β3 [M−1
]
Catechol
o-Hydroxymethyl-phenol
440 (230)
0.76 (7)
30 (2)
0.033 (3)
70 (60)
43 (6)
A
Errors are shown in parentheses and were propagated as indicated in the Accessory
Materials.
the concentration of the 1:1 ester, [BS−2 ] is the concentration
of the 2:1 ester, and [S] is the concentration of free substrate.
In order to calculate values for the equilibrium constants
β1, β2, and β3, the following assumptions were made: Given
the large excess of water used, [H2O] was assumed to be
constant. In addition, the relative concentration of each boron
species present in solution could be determined by integration
of the 11B NMR spectrum, and the absolute concentration of
each species was calculated using the initial concentration
of boric acid added (0.05 M). The concentrations of non-
borate species, [OH−free] and [S], could be calculated from
the equilibrium expressions and the initial concentration of
reagents (usually 0.05 M) through mass−balance. Each nega-
tivelychargedspecies, B−, BS−, andBS2 , isderivedfromthe
reaction of one hydroxide ion with one boric acid molecule.
Therefore, the amount of unreacted hydroxide was thus the
same as the amount of unreacted boric acid. It should be
stressed that the method used herein was developed for the
rapid comparison of the stability of different borates, with
equilibrium constants calculated at a single stoichiometry. A
more rigorous determination of equilibrium constants would
require the recording of spectra at many different substrate
to boron ratios in order to determine a mean value, and the
measurement or estimation of solution acidity and substrate
ionization constants.
In the spectrum of the catechol solution (Fig. 6c), a shoul-
der appears on the upfield side of the 1:1 ester (BS−) signal.
The presence of this shoulder suggests that a tetrahedral
borate species, in addition to the BS− and BS2−, species
is present in this solution. Little extra information about
the structure of this additional species could be found from
the 1H NMR spectrum of the same solution, although it
did show that there was no free fully protonated catechol
present. As observed for aqueous and DMSO solutions of
the corresponding tetra-n-butylammonium spiroborate salts,
this catechol solution turned dark purple a few hours after
preparation. The colour was attributed to the presence of
products resulting from oxidation of either the phenolic sub-
strates to form quinones or oxidation of the borate anions.
The extra borate species might also be the result of such an
oxidation process, and hence might be an ester of a quinone.
For simplicity, the shoulder is included in the estimation of
the relative peak area, hence the relative concentration of
the 1:1 ester, and then used in the calculation of equilibrium
constants.
The concentrations of species present in the o-hydroxy-
methylphenol and catechol solutions, determined using the
above-mentioned method, are shown in Table 8. The equi-
librium constants determined using these data are shown in
Table 9.
In all of the salicylic acid solutions studied, the free sub-
strate concentration [S] was found to be close to zero, so the
calculationofequilibriumconstantswasnotpossible. Despite
this, useful qualitative observations can be made. The very
significant 2:1 ester peak, together with the absence of an
obvious peak due to a 1:1 ester, indicates that in the sys-
tem studied, salicylic acid binds very strongly with borate to
form the 2:1 ester exclusively (large β2 and very large β3).
The high stability of the 2:1 salicylate ester probably arises
from the strong delocalization of electron density through the
carbonyl and aromatic π systems.The planar geometry of the
free substrate is also thought to ensure a favourable binding of
the acid and phenol groups to borate. Although the 1:1 ester
was not observed directly, the results of kinetic and stability
studies of the reaction of o-hydroxy acids with boric acid in
water described in the literature were consistent with a step-
wise formation of the 1:1 ester, and suggest that this species
is stable.[37–39] In these studies, boric acid was maintained
in large excess (substrate to boric acid < 0.045:1), which
ensures little or no interference by the 2:1 ester. Solutions
with such a large excess of boric acid were not examined in
the present study.

Quaternary Ammonium Arylspiroborate Esters as Organo-Soluble, Environmentally Benign Wood Protectants
909
Discussion
not as marked as those found with the cellulose assay, 2 was
still seen to be the most active compound.
Anti-Fungal Activity
Lipophilicity Calculations
It is clear from Tables 1 and 2 that in the cellulose fungal
assays, the borate esters 2 and 4 inhibit fungal growth more
effectively than the same nominal concentration of boric
acid and tetra-n-butylammonium bromide. It can also be
seen that the identity of the hydroxylic component of the
borate is a critical determinant of fungicical activity, in that
at 5.3 µmol cm−2, complete inhibition of both test organ-
isms was achieved with 2, whereas 4 provided only moderate
inhibition. This is particularly evident for the results with
P. tephropora.
It should be noted that the theoretical relations used in the
log P calculations were established from data for neutral uni-
molecular species, including simple organics and drugs, and
may not be suitable for the prediction of absolute log P val-
ues of the borate salts considered here. However, the methods
used should capture the changes in physical properties that
result from changing the structure of the borate esters, and
thus should accurately predict trends in their lipophilicities.
The calculated lipophilicities are much higher than would
be appropriate for orally available drugs, but it is known
that many effective insecticides possess high log P values,[40]
especially those that operate by penetration of the cuticle.The
calculated lipophilicities show a trend of 2 > 4 > 3, which is
similar to the trend in termiticide activity found in both ter-
miticide assays.This tends to suggest that borates with higher
lipophilicities than those used here may be even more effec-
tive termiticides, and could point to a mode of action that
involves inefficient elimination by the termite’s metabolic
processes that leads to build-up within the organism’s more
hydrophobic tissues or possibly penetration of the termite
cuticle.
In the fungal mini-block assays, extensive mass losses
were observed for the solvent and untreated controls, and for
the blocks treated at low retention (0.01% w/w). This result
demonstrates that the organism was viable, so the reduced
mass loss observed for the specimens treated at higher reten-
tions were the result of the compounds inhibiting decay, and
not the result of inherently poor fungal growth. A strong
concentration effect was observed for each test compound,
which provides further evidence that differences in mass loss
were due to differences in the concentration or activity of the
compounds, and not other factors. It is worth noting that the
biological activity of 2 and 4 against the test fungus, G. abi-
etinum, is retained from the simpler, cellulose paper assay, to
the wood-block assay. The mass losses observed at the higher
boron retentions (0.07% w/w and 0.13% w/w) suggest that
the order of biological activity is 2 > 3 > 4 > boric acid.
For ease of comparison, it is useful to define a mass loss
value below which the compounds are deemed effective at
inhibiting fungal growth. A ‘toxic threshold’ value of 5%
mass loss is typically used for the mini-block fungal assay, so
at the highest test concentration, 2 and 3 can be considered
to be effective inhibitors of fungal growth, while 4 is border-
line. At a concentration of 0.07% w/w, only 2 can be deemed
effective, and at the lowest concentration (0.01% w/w), none
can be considered effective.
Boron Fixation and Borate Stability
All of the borates studied showed improved boron fixation
relative to boric acid, and the fixation found with 4 is particu-
larly impressive, considering the severity of the leaching test.
These results strongly support the notion that improved boron
retention in timber can be achieved with alkylammonium
borates.
Given the low aqueous solubility of the ammonium borate
esters examined in this study, and the susceptibility of borate
esters to hydrolysis, a plausible mechanism for the leaching
of these compounds from wood may involve hydrolysis of
the esters (NBu4[B(S)2]) to water-soluble species, i.e., boric
acid and borate salts. High resistance to hydrolysis may thus
be a cause of an improved fixation of borate esters in timber,
and the thermodynamic stability of the borate ester anions
under conditions of hydrolysis is one possible measure of
such stability. Ester formation in aqueous solution has been
studied extensively, but reports are scarce for the study of
analogousbehaviourinorganicorwetorganicsolutions. Such
conditions allow the study of borates with marginal aqueous
solubility, and might more closely resemble the environment
found in timber. A high stability translates to a high tendency
to form 1:1 and 2:1 borate ester anions, which, when com-
bined with a lipophilic cation, are believed to be less likely to
leach from timber than the more water-soluble components,
boric acid and the dihydric substrate. In the present study,
the borate derived from salicylic acid 4 was found to be the
most resistant to leaching and the most stable of the borates
studied, which suggests that a correlation between fixation
and borate stability may exist. A much larger set of borates
needs to be examined, however, before this relationship can
be fully established.
Termiticidal Activity
The observation of good biocidal activity in the termite
bioassays is a very significant result as it demonstrates that
boronintheformofanorganicspiroboratesalt, NBu4[B(S)2],
not only has activity against fungi but also against termites,
a very different organism. In addition, two test compounds,
2 and 4, show better activity against termites than boric acid
in the cellulose LT50 test, and all three borates show better
performance than boric acid in the cellulose mass loss assay.
This has very positive implications for the use of this general
compound class for new wood preservatives.
The order of potency is generally concordant between the
LT50 and relative mass loss cellulose tests. In the case of 3,
however, where the mass loss appears low for a relatively
slow termite mortality, the compound might be acting as a
deterrent (‘termitistat’) as well as a termiticide.
When tested in wood, all three test compounds showed
good activity relative to solvent and untreated controls, and
although the observed differences in activity in this assay are

910
J. M. Carr et al.
Conclusions
of the borate anion, a product of hydrolysis. It is likely that
such important preservative properties as permanence, good
delivery to a point of action, and activity at the point of action
have conflicting physicochemical requirements, and the best
preservative meets a balance between several such properties.
The synthesis and wood preservation properties of a wider
range of ammonium spiroborate esters will be the subject of
future publications.
The work described here is the first systematic examina-
tion of the proposal that the fixation in timber of metal-free
boron compounds with fungicidal and termiticidal properties
can be achieved through the application of hydrophobic and
hydrolytically stable ammonium spiroborates of the general
formula NR4[B(S)2]. Three tetra-n-butylammonium borates
derived from three related phenols, o-hydroxymethylphenol,
catechol, and salicylic acid, were prepared and their activ-
ity against wood decay fungi and termites determined, in
both cellulose and a wood substrate (radiata pine). Their per-
manence in wood when subjected to leaching by water was
also examined.The results obtained were correlated with cal-
culated hydrophobicities and stability measurements. Since
the borates examined vary in their susceptibility to hydroly-
sis, the usual organic–aqueous partition experiments (log P
determinations) could not be used to measure lipophilicities.
Instead, a theoretical method for the prediction of log P val-
ues was used that employs calculated polar, aromatic, and
total molecular surface areas. In order to assess the stabil-
ity of the organo-soluble borates in an aqueous environment,
a technique that measures equilibrium constants in aqueous
DMSO by application of 11B NMR was developed.
It was very pleasing to find that, in line with previous
results,[13] the anti-fungal properties of all three borates
examined was enhanced over that of boric acid, with the
following order of activity: 2 > 3 > 4 > boric acid. Impor-
tantly, it has been found that these ammonium spiroborate
esters are also active against subterranean termites. On cel-
lulose, all three of the compounds examined here were more
active than boric acid and had the following order of activity:
2 > 3 ≈ 4 > boric acid. Interestingly, a comparison of LT50
and cellulose mass loss data suggests that the borate ester
derived from catechol 3 may have anti-feedant properties. As
expected, the calculated log P values for these compounds
were quite high, with the most active termiticide, 2, also
having the highest calculated lipophilicity. This result might
suggest a mode of action that involves penetration of the ter-
mite cuticle or build-up of the active compounds in the tissues
of the termite due to inefficient elimination processes.
All three borates were found to be more resistant to aque-
ous leaching from timber than boric acid, with the following
order of leach resistance being found: 4 ꢁ 3 ≈ 2 > boric acid.
This order does not correlate with calculated lipophilicities,
but the most leach-resistant compound 4 was also found
to be by far the most stable ester, as determined using the
11B NMR method.
Experimental
General
The general experimental methods used in this study were the same
as those described previously.[13] Fungal bioassays, termite bioassays,
and accelerated leaching tests were conducted according to industry
standards,[14] and are described in detail in the Accessory Materials.
Tetra-n-butylammonium Bis[catecholato(2−)-O,Oꢀ]borate 3
Boric acid (0.56 g, 9.1 mmol) was dissolved in an aqueous solution
of tetra-n-butylammonium hydroxide (9.1 mmol, 6.0 mL of a 40% w/v
solution in H2O). Catechol (2.0 g, 18 mmol) was added in portions to
the stirred solution over 10 min. The slurry was warmed to ∼60◦C and
stirred for 30 min. The crude product was recovered as an off-white
solid and washed with hexane (3 × 30 mL). Recrystallization from THF
yielded the pure spiroborate (3.2 g, 75%) as white needles, mp 196–
197◦C (Found: C 71.7, H 9.4, N 2.9, O 13.6. C28H44BNO4 requires C
71.7, H 9.5, N 3.0, O 13.6%). νmax (KBr)/cm−1 2966m, 1487vs, 1235s,
1095s, 1059vs, 909m, 731m. δH (300 MHz, CDCl3) 0.90 (t, 3J 7.5, 12H,
N(CH2CH2CH2CH3)4), 1.25 (sextet, 6J 7, 8H, N(CH2CH2CH2CH3)4),
2.95 (t, 3J 8, 8H, N(CH2CH2CH2CH3)4), 6.57 (s, 8H,ArH). δC (75MHz,
CDCl3) 13.9, 19.7, 23.9, 57.8, 108.5, 117.9, 151.8. δB (160 MHz,
CDCl3) 14.3. m/z (−ve) 227.0510 (M−, C14H8BO4− requires 227.0516).
Tetra-n-butylammonium Bis[salicylato(2−)-O,Oꢀ]borate 4
Boric acid (3.1 g, 50 mmol) was dissolved with stirring and warming
in an aqueous solution of tetra-n-butylammonium hydroxide (50 mmol,
33 mL of a 40% w/v solution in H2O). Salicylic acid (6.9 g, 50 mmol)
was added to the solution and the resulting mixture was stirred
with heating (oil bath ∼60◦C). After 10 min, an off-white oily gum
had formed. A second amount of salicylic acid (6.9 g, 50 mmol)
was then added and the reaction mixture was stirred and heated
for 1 h. After this time, a viscous oil had formed, which solidified
on cooling. The excess water was decanted from the mixture and
the crude product was air-dried. This white solid was collected by
filtration and washed with ether (23.2 g, 87%) mp 97–105◦C. The
crude product was recrystallized from diethyl ether/methanol (9/1,
100 mL), to yield the pure spiroborate (14.0 g, 53%) as white nee-
dles, mp 110–112◦C (lit. 111–113◦C[41] and 104–106◦C[30]). νmax
(KBr)/cm−1 2965m, 1700vs, 1685vs, 1611s, 1466s, 1319vs, 1267m,
1244m, 1132s, 1074s, 973m, 768s. δH (300 MHz, CDCl3) 0.91
(t, 3J 7.2, 12H, N(CH2CH2CH2CH3)4), 1.30 (sextet, 6J 7.2, 8H,
N(CH2CH2CH2CH3)4), 1.44–1.60 (m, 8H, N(CH2CH2CH2CH3)4),
3.10 (t, 3J 8.5, 8H, N(CH2CH2CH2CH3)4), 6.85 (ddd, J5,4 7.2, J5,6
7.8, J5,3 <1, 2H, H5), 6.91 (d, J3,4 8.4, 2H, H3), 7.37 (ddd, J4,3 8.4, J4,5
7.2, J4,6 1.8, 2H, H4), 7.88 (dd, J6,5 7.8, J6,4 1.8, 2H, H6). δC (75 MHz,
CDCl3) 13.6, 19.6, 23.7, 58.5, 115.7, 118.5, 118.9, 129.7, 134.6, 159.6,
165.3. δB (160 MHz, CDCl3) 3.8.
It is interesting to note that the least stable ester and
apparently most lipophilic, 2, was found to have the highest
biological activity. This could indicate that the penetration of
biological membranes is assisted by the lipophilicity of the
borate, but ready hydrolysis to the B(OH)−4 is required for
good toxicity. Conversely, the most stable ester, 4, showed
the highest retention in timber.Thus, good fixation and deliv-
ery to a biological point of action are probably favored by
high stability to hydrolysis and lipophilicity, while activity
at the point of action probably requires significant amounts
Lipophilicity Calculations
Following published procedures,[19] the octanol/water partition coeffi-
cient, log Poct, was predicted from a linear combination of total, polar,
and aromatic surface areas. See Accessory Materials for further details.
General Method for the Measurement of Association Constants
NMR spectra were recorded using a Bruker DRX400 Fourier-transform
spectrometer operating at 128.4 MHz for 11B. A field-frequency lock

Quaternary Ammonium Arylspiroborate Esters as Organo-Soluble, Environmentally Benign Wood Protectants
911
was obtained independently for each sample and the temperature was
maintained at 30 1◦C. A typical acquisition time was 26 min. Spectra
were processed using a typical Fourier-transform program with a line
broadening of 2.00 Hz.A polynomial-based baseline correction function
was used to correct for the broad background signals due to boron-
containing glass in the probe and NMR tubes. 11B chemical shifts were
measured relative to external BF3·Et2O in CDCl3 with positive values
for signals with a higher shift than this reference. For both resolved
and unresolved signals, peak areas were estimated using a computer
program that fits a series of Lorentzian functions to experimental line
shapes.
In a typical sample preparation, boric acid (15.5 mg, 0.25 mmol) and
dihydricsubstrate (0.25 mmol)were dissolvedin [D6]DMSO (2 mL) in a
5 mL volumetric flask. An aqueous solution of tetra-n-butylammonium
hydroxide (0.25 mmol, 164 µL of a 40% w/v solution in H2O) was dis-
solved separately in [D6]DMSO (1 mL) and this solution was added
slowly to a volumetric flask containing the boric acid and the substrate.
[D6]DMSO (0.5 mL) was then added to the flask and the solution made
up volumetrically to 5 mL ([B] = [TBA+] = [S] = 0.05 M). This solu-
tion was incubated at 30◦C for >18 h after which time its NMR spectrum
was recorded.
[12] B. J. Parker, R. G. Veness, C. S. Evans, Enz. Microbial. Technol.
1999, 24, 402. doi:10.1016/S0141-0229(98)00144-6
[13] J. M. Carr, P. J. Duggan, D. G. Humphrey, E. M. Tyndall, Aust. J.
Chem. 2005, 58, 21. doi:10.1071/CH04247
[14] Protocols for the Assessment of Wood Preservatives 1997 (Aus-
tralasian Wood Preservation Committee: Melbourne).
[15] J. Sangster, Octanol–Water Partition Coefficients: Fundamentals
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[16] R. Mannhold, R. Rekker, Perspect. Drug Discov. Des. 2000, 18, 1.
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Accessory Materials
Accessory materials (including data for lipophilicity calcula-
tions) is available from the author or, until December 2010,
the Australian Journal of Chemistry.
[24] H. Steinberg, Organoboron Chemistry 1964, Vol. 1, pp. 1–816
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This work was funded by the ARC, Monash University, and
CSIRO Forestry and Forest Products. E.M.T. is a recipient of
an Australian Postgraduate Award. Noni Johnson is thanked
for helpful suggestions during the drafting of this manuscript.
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