Protonolysis and thermolysis reactions of functionalised NHC-carbene boranes and borates

Article abstract of DOI:10.1039/c4dt01464b

A set of β-ketoimidazolium and β-ketoimidazolinium salts of the general formula [R¹C(O)CH₂{CH[NCR³CR³N(R²)]}]X (R¹ = ^tBu, naphth; R² = ⁱPr, Mes, ^tBu; R³ = H, Me, (H)₂; X = Cl, Br) show contrasting reactivity with superhydride bases MHBEt₃; two are reduced to chiral β-alcohol carbene-boranes R¹CH(OH)CH₂{C(BEt₃)[NCR³CR³N(R²)]} 2 (R¹ = ^tBu; R² = ⁱPr, Mes; R³ = H), two with bulky R² substituents are reduced to chiral β-borate imidazolium salts [R¹CH(OBEt₃)CH₂{CH[NCR³CR³N(R²)]}]X 3 (R¹ = ^tBu, naphth; R² = Mes, ^tBu; R³ = H, Me; X = Cl, Br), and the two saturated heterocycle derivatives remain unreduced but form carbene-borane adducts R¹C(O)CH₂{C(BEt₃)[NCR³CR³N(R²)]} 4 (R¹ = ^tBu, naphth; R² = Mes; R³ = (H)₂). Heating solutions of the imidazolium borates 3 results in the elimination of ethane, in the first example of organic borates functioning as Bronsted bases and forming carbene boranes R¹CH(OBEt₂)CH₂{C[NCR³CR³N(R²)]} 5 (R¹ = naphth; R² = Mes; R³ = Me). The 'abnormal' carbene borane of the form 2 R¹CH(OH)CH₂{CH[NC(BEt₃)CR³N(R²)]} (R¹ = ^tBu; R² = ^tBu; R³ = H), is also accessible by thermolysis of 3, suggesting that the carbene-borane alcohol is a more thermodynamically stable combination than the zwitterionic imidazolium borate. Higherature thermolysis also can result in complete cleavage of the alcohol arm, eliminating tert-butyloxirane and forming the B-N bound imidazolium borate 7. The strong dependence of reaction products on the steric and electronic properties of each imidazole precursor molecule is discussed. This journal is

Full text of DOI:10.1039/c4dt01464b

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Protonolysis and thermolysis reactions of
functionalised NHC–carbene boranes and borates†
Cite this: Dalton Trans., 2014, 43,
15419
Polly L. Arnold,* Nicola L. Bell, Isobel H. Marr, Siyi She, Jonathan Hamilton,
Craig Fraser and Kai Wang
A
set of β-ketoimidazolium and β-ketoimidazolinium salts of the general formula [R1C(O)CH2-
{CH[NCR3CR3N(R2)]}]X (R1 = tBu, naphth; R2 = iPr, Mes, tBu; R3 = H, Me, (H)2; X = Cl, Br) show contrasting
reactivity with superhydride bases MHBEt3; two are reduced to chiral β-alcohol carbene–boranes
R1CH(OH)CH2{C(BEt3)[NCR3CR3N(R2)]} 2 (R1 = Bu; R2 = Pr, Mes; R3 = H), two with bulky R2 substituents
t
i
are reduced to chiral β-borate imidazolium salts [R1CH(OBEt3)CH2{CH[NCR3CR3N(R2)]}]X 3 (R1
=
tBu,
naphth; R2 = Mes, Bu; R3 = H, Me; X = Cl, Br), and the two saturated heterocycle derivatives remain
t
t
unreduced but form carbene–borane adducts R1C(O)CH2{C(BEt3)[NCR3CR3N(R2)]} 4 (R1 = Bu, naphth;
R2 = Mes; R3 = (H)2). Heating solutions of the imidazolium borates 3 results in the elimination of ethane,
in the first example of organic borates functioning as Brønsted bases and forming carbene boranes
R1CH(OBEt2)CH2{C[NCR3CR3N(R2)]} 5 (R1 = naphth; R2 = Mes; R3 = Me). The ‘abnormal’ carbene borane
of the form 2 R1CH(OH)CH2{CH[NC(BEt3)CR3N(R2)]} (R1 = Bu; R2 = Bu; R3 = H), is also accessible by
thermolysis of 3, suggesting that the carbene–borane alcohol is a more thermodynamically stable
combination than the zwitterionic imidazolium borate. High-temperature thermolysis also can result in
complete cleavage of the alcohol arm, eliminating tert-butyloxirane and forming the B–N bound
imidazolium borate 7. The strong dependence of reaction products on the steric and electronic properties
of each imidazole precursor molecule is discussed.
t
t
Received 19th May 2014,
Accepted 18th August 2014
DOI: 10.1039/c4dt01464b
www.rsc.org/dalton
Introduction
Carbene–boranes derived from N-heterocyclic carbenes (NHCs)
and possessing the general formula [HCN(R)]2C:BR′3 (R,R′ =
hydrocarbyl), such as a in Chart 1, are receiving increasing
levels of attention due to recent displays of some interesting
reactivity.1–4 For example, the B–H bond in NHC–BH3 com-
pounds is significantly weakened (compared to those in R3NBH3)
and can be removed as H+ by a base5 or as H− by treatment
with phosphine–boranes PhH2PB(C6F5)3 (b in Chart 1).4
Frustrated Lewis Pair (FLP) combinations of NHCs and per-
fluorinated aryl boranes (c in Chart 1) have been shown to acti-
vate and cleave dihydrogen, THF, and amines,6–9 although the
rearrangement to the ‘abnormal’ carbene–borane adduct (d in
Chart 1) provides a route to shut down further small molecule
reactivity.6,8 Despite this Tamm has noted reactivity between a
bulky NHC–borane FLP and white phosphorus which yields
the ‘abnormal’ NHC·P4·B(C6F5)3 adduct (e in Chart 1).10
Chart 1 Carbene–borane adducts, frustrated pairs, and selected reac-
tion products.
EaStCHEM School of Chemistry, University of Edinburgh, The King’s Buildings,
Edinburgh, EH9 3JJ, UK. E-mail: polly.arnold@ed.ac.uk
†Electronic supplementary information (ESI) available: Additional synthetic and
Yamaguchi has previously described the reactivity of imida-
zolium salts towards LiHBEt3 yielding triethylborohydride-
NHC adducts.11,12 Notably the triethylborane group was shown
crystallographic details for the complexes. CCDC 1003569–1003576. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt01464b
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to exchange with BH3, BF3 and with [M(η3-allyl){η2-(NPh)2CH}-
(CO)2(py)] (M = W, Mo) reflecting the similarities between the
Lewis acidic boron and early transition metal cations.
Carbenes have also previously been shown to be catalysts
for the reduction of ketones with borane, for which only a few
examples of organocatalysts exist.13–15 Other NHC–borane cata-
lysed reactions include radical hydrogenation reactions, hydro-
boration of alkynes,16 and the reduction of xanthates.17
We have studied a range of alkoxy tethered carbenes for
Lewis acidic metals and have previously demonstrated the
reactivity of the labile carbene–metal bonds towards
boranes.18,19 More recently we became interested in further
functionalisation of the pendant alkoxide arm of these ligands
with sterically demanding and potentially reactive BR3 group.
However, in the development of these new carbene proligands
some unexpected reactivity was observed.
Herein we report the different chemistry of a range of
β-ketoimidazolium and imidazolinium (for saturated hetero-
cyclic derivatives) salts towards superhydride bases and seek to
rationalise their relative susceptibility towards either ketone
reduction or imidazolium deprotonation. We also describe pre-
viously unseen intramolecular acid–base alkane elimination
chemistry of the imidazolium-borate species.
Fig. 1 Solid-state structure of 1d. For clarity, all hydrogen atoms except
the imidazolium CH, and all solvent molecules are omitted (displace-
ment ellipsoids are drawn at 50% probability). Selected distances (Å) and
angles (°): O1–C16 1.216(3), C1–N1 1.334(3), C1–N2 1.328(3), C1–Br1
3.456(2), N1–C1–N2 108.6(2).
In the 1H NMR spectra of 1a–c the imidazolium protons reso-
nate at 10.53, 10.01 and 10.79 ppm respectively whereas in 1d
the same proton appears at 9.78 ppm and at 9.68/9.27 ppm in
1e/f. The FTIR CvO stretching frequencies in solutions of 1b
and 1e are 1724 and 1719 cm−1; un-shifted from the solid state
values of 1721 and 1715 cm−1 respectively, suggesting minimal
hydrogen bonding between the ketone oxygen and acidic imi-
dazolium proton. Accordingly, the value of the high-frequency
imidazolium 1H NMR resonance may be taken as a measure of
the different acidity of the imidazolium protons in these com-
pounds. From these data and comparisons with the literature,25
we estimate the order of acidity to be 1a ∼1b ∼1c >1d >1e ∼1f.
X-ray quality crystals of 1d were grown by slow cooling of a
methanol solution at −30 °C, Fig. 1. The molecular structure
shows a CvO distance of 1.216(3) Å and in the solid state a
long C1–Br distance of 3.456(2) Å suggests a weak hydrogen
bonding interaction between the bromide anion and the acidic
imidazolium CH.
Results and discussion
Proligand synthesis
In a modification of a literature preparation, a range of alkyl-
ketone N-functionalised imidazolium salts can be accessed
from mono-N-substituted alkyl or aryl imidazoles by treatment
with an equimolar quantity of a 2-haloketone in refluxing
toluene solution, eqn (1).20–24 The resultant β-ketoimidazolium
(or imidazolinium for saturated backbone derivatives) salt 1
precipitates as a colourless solid and can be isolated by fil-
tration. Salts 1a–f ([R1C(O)CH2{CH[NCR3CR3N(R2)]}]X (R1
=
tBu, naphth; R2 = iPr, Mes, tBu; R3 = H, Me, (H)2; X = Cl, Br) see
eqn (1)) containing a range of different substituents were iso-
lated in excellent yield.
Reduction with group 1 metal borohydrides
ð2Þ
ð1Þ
Treatment of 1a with either KHBEt3 or NaHBEt3 affords a
colourless precipitate, and after workup of the filtrate, an
orange oil, characterised as the alcohol-functionalised carbene–
borane 2a ((tBu)CH(OH)CH2{C(BEt3)[NC(H)C(H)N(iPr)]}),
eqn (2). The 1H NMR spectrum of 2a shows no imidazolium
CH proton confirming carbene formation. In addition an AB2
coupling system is evident at 4.80, 3.41, and 3.39 ppm
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(1H each) assigned as the C(H)OCH2 protons in the N-alkyl
arm resulting from reduction of the ketone group. The 13C
NMR spectrum contains a four line resonance at 175.3 ppm
indicative of a carbene borane. Regardless of the number of
equivalents of borohydride used or reaction temperature 2a is
the only organic product formed.
The reaction between equimolar amounts of 1b and
KHBEt3 proceeded similarly, yielding a pale yellow solid after
workup which was characterised as the analogue, 2b ((tBu)CH-
(OH)CH2{C(BEt3)[NC(H)C(H)N(Mes)]}), eqn (2). Similarly, a
doublet of doublets at 4.95 ppm demonstrates reduction of the
ketone bond has occurred. Additionally, desymmetrisation of
the mesityl methyl groups is now evident in the NMR spectra
implying hindered rotation about the C–N bond as a result of
the coordination of the carbene to the bulky BEt3 moiety.
Fig. 2 Solid-state structure of 3c. For clarity, all hydrogen atoms except
the imidazolium CH, and all solvent molecules are omitted (displace-
ment ellipsoids are drawn at 50% probability). Selected distances (Å) and
angles (°): B1–O1 1.544(3), O1–C9 1.385(3), C1–N1 1.329(3), C1–N2
1.328(3), C9–O1–B1 123.64(16), N1–C1–N2 109.0(2).
Crystals of 3c suitable for X-ray diffraction were grown by
slow cooling a toluene solution to −30 °C, Fig. 2. The structure
confirms the assignment as an O-functionalised imidazolium
borate with a C–O distance of 1.385(3) Å, within the expected
range for a single bond (1.33–1.54 Å) and significantly longer
than the CvO bond in 1d (1.216(3) Å). The angles around the
C9 carbon are also indicative of a sp3 hybridised carbon
centre, confirming reduction of this bond. The identity of the
imidazolium proton H1 can also be inferred in the structure
from the absence of a counter-ion.
X-ray quality crystals of 3d were grown by slow cooling of a
toluene solution (−30 °C). The molecular structure is shown in
Fig. 3. The C–O distance of 1.386(6) Å is significantly longer
than that in 1d (1.216 Å) confirming the reduction of the
ketone bond. The B–O bond 1.536(7) Å is within the expected
range (1.324–1.590 Å).
ð3Þ
In contrast, treatment of 1c or 1d with KHBEt3 yielded a col-
ourless or pale yellow solid respectively after workup, which
were characterised as the imidazolium borates 3c (tBu)CH-
(OBEt3)CH2{CH[NC(H)C(H)N(tBu)]} and 3d ((tBu)CH(OBEt3)-
1
CH2{CH[NC(CH3)C(CH3)N(Mes)]}), eqn (3). The H NMR spec-
trum of 3c contains a resonance at 10.07 ppm which is shifted
to only a slightly lower frequency than the imidazolium CH of
1c (10.79 ppm). Similarly, the 13C NMR spectrum shows a res-
onance at 137.2 ppm corresponding to the imidazolium CH
carbon which contrasts with the carbene carbon resonance in
2a of 175.3 ppm. However, three multiplets at 3.52, 3.88 and
4.10 ppm (1H each) confirm reduction of the ketone group. In
the 1H NMR spectrum of 3d the imidazolium H1 proton
appears at 9.24 ppm. The presence of three sets of resonances
at 5.63, 4.14 and 3.65 (1H each) confirms this reduction and
the formation of a chiral centre at the ketone carbon. A 13C
NMR spectral resonance at 145.3 ppm is also indicative of an
imidazolium CH, similar to that in 3c (137.2 ppm).
The presence of a CH imidazolium proton in 3c, rather
than the carbene species observed in 2a and 2b is likely due to
the steric bulk of the tert-butyl imidazolium substituent (R2)
which is able to effectively block coordination of the BEt3 frag-
ment to the adjacent carbene carbon.
The difference between b and d is electronic and not steric:
the pKa of the acidic proton in 1d is higher than in 1b due to
the 4,5-dimethylimidazolium ring substituents in d.
Fig. 3 Solid-state structure of 3d. For clarity, all hydrogen atoms except
the imidazolium CH, and all solvent molecules are omitted (displace-
ment ellipsoids are drawn at 50% probability). Selected distances (Å) and
angles (°): B1–O1 1.536(7), O1–C16 1.386(6), C1–N1 1.327(6), C1–N2
1.324(6), C16–O1–B1 120.7(4), N1–C1–N2 108.7(5).
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Formation of NHC–BR3 instead of ketone reduction
Thermally induced alkane elimination
The pKas of the saturated imidazolinium C–H group (the conju- Attempts to deprotonate the imidazolium CH group of 3d
gate acid of the strongly basic 4,5-dihydroimidazol-2-ylidene) using a variety of strong bases including KN(SiMe3)2, KCH2Ph
are higher than those of unsaturated imidazolium salts (conju- and KH proved unsuccessful (see ESI†). However, heating a
gate acids of imidazole-2-ylidenes), so 1e–f should be harder to toluene solution of 3d to reflux for 48 hours in a teflon-valved
deprotonate, but no significant electronic effect on the ketone NMR tube resulted in a loss of the characteristic imidazolium
1
group was anticipated. Surprisingly, treatment of either 1e or 1f proton, formerly at 9.21 ppm in the H NMR spectrum, and
with one equivalent of KHBEt3 in THF yielded a colourless the formation of a new species which has been assigned as
powder after workup, characterised as the ketone-functionalised the carbene borane 5d ((tBu)CH(OBEt2)CH2{C[NC(CH3)-
carbene–borane 4e ((tBu)C(O)CH2{C(BEt3)[NCH2CH2N(Mes)]}) C(CH3)N(Mes)]}) resulting from the intramolecular de-
and 4f ((naphth)C(O)CH2{C(BEt3)[NCH2CH2N(Mes)]}) respect- protonation of the imidazolium moiety by a BEt3 group,
ively, eqn (4).
Scheme 1.
A resonance at 0.81 ppm in the 1H NMR spectrum was
assigned as ethane and is no longer visible in spectra recorded
after freeze–pump–thaw degassing of the solution. Two triplets
1
in the H spectrum of 5d at 1.44 and 1.39 ppm (3H each) are
ð4Þ
assigned to the ethyl CH3 groups and two doublets of doublets
at 1.11 and 0.47 ppm, the ethyl CH2 protons. The 11B NMR
spectrum contains a singlet at 1.2 ppm which, compared with
the chemical shift in 3d of 2.2 ppm, suggests a more electron
rich boron centre in 5d.
1
The H NMR spectrum of 4e contains no resonances above
7 ppm indicating loss of the imidazolinium proton (9.68 ppm
in 1e). In addition the α-keto methylene group appears as a
sharp singlet at 4.56 ppm (2H) demonstrating that no
reduction of the ketone has occurred. In spite of this, a triplet
at 1.19 ppm (9H) and a quartet at 0.51 ppm (6H) demonstrate
the presence of a BEt3 group. The 13C NMR spectrum also
shows a four-line resonance at 201.6 ppm indicative of
carbene–borane formation. The 11B NMR spectrum contains a
singlet at −14.1 ppm, close to that exhibited by 2a
(−12.1 ppm). The spectral data for 4f are comparable to those
of 4e.
X-ray quality crystals of 4e were grown from slow cooling of
a toluene solution, Fig. 4. The refined structure contains a
short CvO bond of 1.207(7) Å indicative of the unreacted
ketone and a C–B bond length (1.664(8) Å) within the expected
range (ca. 1.55–1.68 Å).26 X-ray quality crystals of 4f were also
grown from slow diffusion of a solution of 4f in toluene and
the refined structure is shown in the ESI.†
Fig. 4 Solid-state structure of 4e. For clarity, all hydrogen atoms, are
omitted (displacement ellipsoids are drawn at 50% probability). Selected
distances (Å) and angles (°): B1–C1 1.664(8), O1–C5 1.207(7), C1–N1
1.344(7), C1–N2 1.328(5), O1–C5–C4 120.2(5), N1–C1–N2 108.0(4).
Scheme 1 Intramolecular ring closure of 3c to form the carbene
borane 5c which at higher temperatures forms the ‘abnormal’ carbene 6
and the degradation product 7.
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Fig. 7 Solid-state structure of 7. For clarity, all hydrogen atoms except
the imidazolium CH are omitted (displacement ellipsoids are drawn at
50% probability). Selected distances (Å) and angles (°): N1–C4 1.345(2),
N2–C4 1.325(2), B1–N2 1.635(2), N1–C1 1.501(2), N2–C4–N1 110.88(12).
Fig. 5 Solid-state structure of 5d. For clarity, all hydrogen atoms except
the imidazolium CH, and all solvent molecules are omitted (displace-
ment ellipsoids are drawn at 50% probability). Selected distances (Å) and
angles (°): B1–C1 1.651(4), B1–O1 1.513(3), O1–C16 1.393(3), C1–N1
1.344(3), C1–N2 1.353(3), O1–B1–C1 105.62(19), C16–O1–B1 117.32(18),
N1–C1–N2 104.6(2).
1.60 (3H) and 1.45 (3H) and complex overlapping multiplets
between 1.22 and 0.89 ppm.
The major product 7 (Et3B{CH[NCHCHN(tBu)]}, ca. 90%) is
the N–B bonded imidazolium borate, identified initially by X-
ray diffraction since X-ray quality single crystals were isolated
X-Ray quality crystals of 5d were grown by slow cooling
of a toluene–THF solution to −30 °C, Fig. 5. The molecular
structure confirms the presence of a carbene–borane bond
which is significantly longer (1.651(4) Å) than the mean C–B
NHC–borane distance in the CCDC (1.609 Å) although shorter
than in Yamaguchi’s diisopropylimidazolyltriethylborane,
(1.683 Å).11
Heating a solution of 3c in d8-toluene to reflux in a teflon-
valved NMR tube for 72 h affords two imidazolyl-containing
compounds according to 1H NMR spectroscopy, Scheme 1.
The minor product (ca. 10%) of this reaction was crystallised
and shown to be the abnormal carbene 6 ((tBu)CH(OBEt2)
CH2{CH[NCCHN(tBu)]}), Fig. 6. The 1H NMR spectra of crystals
of 6 showed two resonances in the aromatic region at 6.44 (1H)
and 6.33 (1H) ppm as well as two multiplets at 3.76 (1H) and
3.38 (2H). The BEt2 groups are characterised by two triplets at
1
by slow cooling of a hot hexane solution (Fig. 7). The H NMR
spectra of 7 show three high-frequency ring protons at 7.52,
7.02 and 6.08 ppm indicative of a protonated imidazolium
rather than a carbene functionality. In addition a triplet and
quartet at 1.26 (9H) and 1.01 (6H) ppm confirmed the presence
1
of a BEt3 moiety. Careful analysis of the H NMR spectrum of
the reaction mixture shows the presence of two resonances at
2.46 (1H) and 2.27 (2H) ppm representing the known epoxide
2-tert-butyloxirane (see ESI†). The 13C NMR spectrum also
showed resonances representing the free epoxide. This
suggests that degradation of 3c occurs by cleavage of the imi-
dazole tether to eliminate the epoxide.
To avoid extensive epoxide loss, heating solutions of 3c at
only 50 °C for 14 days shows the formation of an alcohol-func-
tionalised carbene borane complex, 2c ((tBu)CH(OH)CH2{CH-
[NC(BEt3)C(H)N(tBu)]}) identified by NMR spectroscopy as con-
taining the ‘abnormal’ rather than ‘normal’ carbene group.
This mirrors the chemistry seen for the systems a and b, but
the bulky c substituents allow this to be isolated, and favour
the ‘abnormal’ carbene formation. This is also reasonable
since the ultimate alkane elimination product is ‘abnormal’ in
this system.
Discussion
The reduction of this set of six similar β-ketoimidazolium (or
imidazolinium) salts described above yield a range of different
products depending on the alkyl and N-atom substituents, and
on the degree of unsaturation of the carbene-precursor, sum-
marised in Chart 2. Further heating of the neutral complexes
in some cases shows alkane elimination from the BEt3 group
in what is the first example of a borate group acting as a
Brønsted base. Comparisons of the reactivity allow us to make
some tentative conclusions.
Fig. 6 Solid-state structure of 6. For clarity, all hydrogen atoms are
omitted (displacement ellipsoids are drawn at 50% probability). Selected
distances (Å) and angles (°): C13–N1 1.3872(19), N1–C7 1.3353(19), C13–
C8 1.362(2), C13–B1 1.643(2), B1–O1 1.519(2), O1–B1–C13 106.24(12),
C8–C13–B1 135.56(14).
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Chart 2 Comparison of the products isolated from the reaction of 1
with superhydride base.
Firstly, the reaction with the superhydride reagent MBHEt3
in the cases 1a–d results in a reduction of the ketone and
addition of the BEt3 group, although in 1a–b the product is an
alcohol–carbene–borane, and in 1c–d the product is a boronate
imidazolium, whilst the ketone in 1e–f is untouched, forming
the ketone–carbene–borane. The pattern of these reactions
does not simply follow a trend according to the relative pKas of
the imidazol(in)ium; we estimate this order is 1a ∼1b ∼1c >1d
>1e ∼1f from comparisons with the literature and analysis of
spectral data. It depends also on the steric demands of the per-
ipheral groups and the degree of carbene saturation.
Compounds a–d have the most acidic imidazolium CH
group, arguing against the deprotonation of the imidazol(in)
ium CH as the first step. Compounds a–b have lower steric
crowding around the C1 position. These yield carbene borane
species of type 2. However if either the sterics at the N1 posi-
tion or the pKa of the C1 proton are increased then the imida-
zolium borate species 3 is favoured. With compounds e–f
(β-ketoimidazolinium), reduction of the ketone bond was not
Scheme 2 Proposed routes for the different reactivities of 1a–f with
MHBEt3.
2. In the solid state, the structure of 3c (Fig. 2) shows a
observed suggesting an alternative mechanism for non-aro- short O⋯H distance of 2.286 Å (d(O⋯C1) = 2.774(2) Å)
matic heterocycles to yield the β-ketocarbene borane species 4. suggesting a weak hydrogen bonding interaction may stabilise
Assuming the initial kinetic products do not rearrange once this intermediate. The X-ray structure of 3d (Fig. 3) shows a
formed, two different reactions with the [HBEt3]− anion are longer interaction (d(O⋯H)/d(O⋯C1) = 2.467 Å/2.825(5) Å
proposed:
respectively) reflecting the lower acidity of the 4,5-dimethylimi-
1. The unsaturated β-ketoimidazolium salts 1a–d likely dazolium C1 proton. The C1 proton in 1e–f is significantly less
follow the pathway (a) shown in Scheme 2. First, reduction of acidic than in 1a–d which may result in the destabilisation of
the ketone occurs forming the zwitterionic imidazolium borate pathway (a, Scheme 2) for imidazolinium based species relative
3a–d, for which 3a–b were not observed directly but instead to the deprotonation pathway (b, Scheme 2) which yields 4e–f
are presumed to be intermediates in the formation of 2a–b. If in a process rendered irreversible due to the evolution of di-
the N-R2 substituent is small or the pKa of the C1 proton is hydrogen in this pathway.
low, rearrangement occurs to afford the alcohol and carbene–
The reactions are summarised in Scheme 3. The thermo-
borane groups in 2. Thereafter, a 1,5 sigmatropic shift of the lysis of imidazolium borates 3 results in the highly unusual
highly acidic imidazolium proton allows a strong carbene– extrusion of ethane, in which the imidazolium group functions
borane bond to form, unless the steric bulk of the R2 position as a Brønsted acid. To our knowledge, this is the first example
is sufficiently high to prevent coordination of the BEt3 frag- of this type of acid–base reactivity with a simple borane
ment, see 3c. If the imidazolium CH has a higher pKa (as for although Köster has noted that elimination of ethane occurs
the dimethylimidazolium in 1d) the rearrangement is also dis- on heating the salt Na[HOBEt3] to yield NaOBEt2.27 The
favoured, see 3d.
authors proposed that this was catalysed by NaHBEt3 but
By these arguments, proligands 1e–f would be expected to control reactions of 3 with additional NaHBEt3 do not result in
form the imidazolinium analogues 3e–f based on their high ethane elimination, arguing against the involvement of any
CH pKa. Therefore the isolation of 4e–f suggests an alternate metal ions in the ethane elimination process in this system
mechanism for these saturated backbone-derivatives:
(see ESI†). In particularly sterically crowded compounds
15424 | Dalton Trans., 2014, 43, 15419–15428
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position or the pKa of the C1 proton are increased then the
imidazolium borate species 3 is favoured. With β-ketodihydro-
imidazolium salts reduction of the ketone bond was not
observed suggesting an alternative mechanism for non-
aromatic imidazoles to yield the β-ketocarbene borane species
4. Heating of imidazolium borates of type 3 can result in the
formation of closed ring carbene boranes such as 5 which con-
tains a relatively long C–B bond, suggesting reactions in which
it behaves as a carbene–borane Lewis acid–base pair may be
possible. In the case of 3c where there is significant steric hin-
drance around the C1 carbon the ‘normal’ carbene species 5c
is not observed with the ‘abnormal’ species 6 being isolated
instead, albeit in low yield. The unexpected degradation to
the imidazole borane 7, via extrusion of a molecule of
epoxide, occurs at very high temperatures in a rare example
of a process that cleaves a B–O bond to form a B–N bond.
This suggests that the variety of resonance forms that can be
drawn for the bonding in 7 are able to provide a significant
additional stabilisation to this molecule. Work is in progress
to identify the coordination chemistry of the complexes and
the ability of small molecules and organometallic fragments
to re-open the boracycles 5 and 6 to afford new carbene
compounds.
Experimental details
Scheme 3 Summary of reactions that thermally eliminate ethane or
tert-butyloxirane.
All reactions were carried out under an atmosphere of dry,
oxygen-free dinitrogen, using standard Schlenk techniques or
in a MBraun glovebox. Solvents were distilled and dried by
standard methods or used directly from a Glass Contour
solvent purification system. Mass spectra were recorded on a
MAT 900 XP (EI/FAB), Thermo-Fisher LCQ Classic (ESI). NMR
spectra were recorded either on 400 MHz, 500 MHz or
600 MHz Bruker Advance III spectrometers. 1H and 13C chemi-
cal shifts are reported in ppm relative to SiMe4(δ = 0) and were
referenced internally with respect to the protio solvent impur-
ity or the 13C resonances respectively. 11B chemical shifts are
reported in ppm relative to BF3·OEt2(δ = 0). Infra–red spectra
were recorded from solution or a nujol mull using cells with
NaBr windows or as neat solids using a Perkin-Elmer Spectrum
65. Elemental analyses were carried out by Mr Stephen Boyer
at London Metropolitan University.
(for example where R2
borane 6 is formed from ethane elimination.
=
tBu 3c) the ‘abnormal’ carbene–
Although the C–B bond is relatively long in both 5d
(1.651 Å) and 6 (1.643 Å) compared with the mean NHC–B dis-
tance in the CSD26 of 1.609 Å we have been unable to elicit
further B–C insertion chemistry from either 5 or 6 to date.
Higher-temperature treatments of the most electron-rich
and bulkiest complexes (3c) results in an equally unusual
thermal degradation involving elimination of the alcohol arm,
replacing a B–O bond with a B–N bond in the formation of
the imidazolium borate 7 in preference to 6. It may be that
elimination of the epoxide is facilitated since it can readily
polymerise to the polyether at these reaction temperatures,
which would drive any equilibrium process towards the for-
mation of 7.
The compounds N-mesitylimidazole,28 N-tert-butylimida-
zole,29 N-isopropylimidazole,30 4,5-dimethyl-N-mesitylimida-
zole31 and N-mesitylimidazoline32 were synthesised according
to literature procedures. All other chemicals were obtained
from Sigma-Aldrich and used as received.
Conclusions
The reaction between β-ketoimidazolium salts and an alkali
metal triethylborohydride yields a range of products which
General synthesis of β-ketoimidazol(in)ium salts 1
depend strongly upon both the steric and electronic properties The imidazole CH[NCR3CR3N(R2)] (R2 = Pr, Mes, Bu; R3 = H,
of the imidazole. Highly acidic imidazoles with low steric Me, (H)2) (4 mmol) was dissolved in toluene and the 2-halo-
crowding around the C1 position yield carbene borane species ketone [R1C(O)CH2X] (R1 = tBu, naphth; X = Cl, Br) (one equiv.)
of type 2, probably via the zwitterionic imidazolium borate 3, was added, affording a cloudy solution. The mixture was
which can be isolated when the imidazolium N-substituent is heated to reflux for 8 hours during which time a colourless
sufficiently large. However if either the sterics at the N1 precipitate formed. After allowing the suspension to cool to
i
t
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room temperature, the precipitate was isolated by filtration 123.3 (CH), 54.8 (NCH2), 51.2 (NCH2CH2N), 50.5 (NCH2CH2N),
and washed with hot hexane to yield [R1C(O)CH2{CH- 21.0 (C(CH3)3), 18.0 (Ph(CH3)2); ESI (+ve) M+ = 357.2 m/z
t
i
t
[NCR3CR3N(R2)]}]X (R1 = Bu, naphth; R2 = Pr, Mes, Bu; R3 = [M+ − Cl]; Anal. Calcd C24H25BrN2O·CHCl3 (556.75): %C =
H, Me, (H)2; X = Cl, Br) 1 as a colourless solid.
53.93; %H, 4.71; %N, 5.03; found: %C = 54.14; %H, 5.29;
1a: [(tBu)C(O)CH2{CH[NC(H)C(H)N(iPr)]}]Cl. Yield: 80%; %N, 5.02.
1H NMR (CDCl3, 500 MHz) δ 10.53 (s, 1H, NCHN); 7.48 (s, 1H,
General procedure for reaction of 1 with KHBEt3
NCHCHN); 7.39 (s, 1H, NCHCHN); 5.91 (s, 2H, NCH2CO); 4.66
(sept., J = 6.7 Hz, 1H, CH(CH3)2); 1.56 (d, J = 6.7 Hz, 6H, A solution of compound 1 in THF was treated with a 1 M
CH(CH3)2); 1.24 (s, 9H, C(CH3)3); 13C NMR (500 MHz, CDCl3): solution of KHBEt3 in THF. If a precipitate formed then the
206.9 (NCH2CO), 137.3 (NCN), 124.2 (NCHCHN), 118.9 solution was filtered and the solvent removed from the filtrate
(NCHCHN), 54.2 (NCH2), 53.3 (NC(CH3)2), 43.5 (C(CH3)3), 26.2 in vacuo.
(C(CH3)3), 23.0 (C(CH3)2); ESI (+ve) M+ 209.1 m/z [M+ − Cl];
2a:
Anal. Calcd C12H21ClN2O (244.76), %C = 58.89; %H = 8.65; isolated as a viscous orange oil. Yield: 72%; 1H NMR (C6D6,
%N = 11.45; found, %C = 58.62, %H = 8.73, %N = 11.34. 400 MHz) δ 6.72 (s, 1H, NCHCHN); 6.25 (s, 1H, NCHCHN);
(tBu)CH(OH)CH2{C(BEt3)[NC(H)C(H)N(iPr)]}. Product
1b: [(tBu)C(O)CH2{CH[NC(H)C(H)N(Mes)]}]Cl. Yield: 86%; 5.48 (sept., J = 6.7 Hz, 1H, CH(CH3)2); 4.82 (m, 1H, CH2CH(tBu)
1H NMR (CDCl3, 600 MHz) δ 10.01 (s, 1H, NCHN); 7.60 (s, 1H, OH); 3.41 (m, 2H, NCHHC); 3.39 (s, 1H, NCHHC) 1.12 (t, J =
NCHCHN); 7.13 (s, 1H, NCHCHN); 7.02 (s, 2H, PhH); 6.29 7.7 Hz, 9H, B(CH2CH3)3); 1.05 (dd, J = 6.7 Hz, 6H, CH(CH3)2);
(s, 2H, NCH2CO); 2.36 (s, 3H, Ph(CH3)); 2.12 (s, 6H, Ph(CH3)2); 0.93 (m, 6H, B(CH2CH3)3); 0.86 (s, 9H, C(CH3)3); 13C NMR
1.35 (s, 9H, C(CH3)3); 13C NMR (500 MHz, CDCl3) δ 207.0 (CO), (500 MHz, C6D6) δ 175.3 (NC(B)N), 123.5 (NCCN), 115.0
141.5 (NCN), 139.1 (NCCN), 134.6 (NCCN), 130.9 (Ph), 125.0 (NCCN), 74.4 (CH(tBu)OH), 51.0 (NCH2CO), 49.2 (NCH(CH3)2),
(Ph), 122.1 (Ph), 55.6 (NCCO), 43.6 (Ph(CH3), 26.5 (C(CH3)3), 35.2 (C(CH3)3), 26.2 (B(CH2CH3)3), 23.6 (C(CH3)2), 23.3
21.1 (C(CH3)3), 17.7 (Ph(CH3)2); ESI (+ve) M+ = 285.1 m/z (C(CH3)2), 15.3 (B(CH2CH3)3), 12.6 (C(CH3)3); 11B (C6D6,
[M+ − Cl]. IR (CH2Cl2) νmax/cm−1 1724 (s); IR (neat) νmax/cm−1 160.5 MHz)
δ
−12.1; EI M+
= 308.3 m/z; Anal. Calcd
1721 (s).
C18H37BN2O (308.32), %C = 70.12; %H = 12.10; %N = 9.09;
1c: [(tBu)C(O)CH2{CH[NC(H)C(H)N(tBu)]}]Cl. Yield: 93%; found, %C = 69.85, %H = 12.27, %N = 8.86; IR (neat) νmax/cm−1
1H NMR (CDCl3, 400 MHz) δ 10.79 (s, 1H, NCHN), 7.37 (s, 1H, 3426 (br, O–H).
NCHCHN), 7.35 (s, 1H, NCHCHN), 5.98 (s, 2H, CH2), 1.67
2b: (tBu)CH(OH)CH2{C(BEt3)[NC(H)C(H)N(Mes)]}. Tritura-
(s, 9H, C(CH3)3), 1.28 (s, 9H, C(CH3)3); 13C NMR (CDCl3, tion with Et2O yielded a colourless sticky foam which could
126 MHz) δ 207.1 (CO), 137.2 (CH), 124.2 (CH), 118.4 (CH), not be crystallised. Yield: 69%; 1H NMR (C6D6, 400 MHz):
60.2 (Cq), 54.2 (CH2), 43.5 (Cq), 30.0 (CH3), 26.3 (CH3).
δ 6.96 (s, 1H, NCHCHN); 6.69 (s, 2H, PhH); 5.80 (s, 1H,
1d: [(naphth)C(O)CH2{CH[NC(CH3)C(CH3)N(Mes)]}]Br. Yield: NCHCHN); 4.95 (dd, 1H, CH(OH)); 3.66 (dd, 1H, NCHHCO);
0.3707 g (80%); 1H NMR (CDCl3, 400 MHz): δ 9.78 (s, 1H, 3.42(dd, 1H, NCHHCO); 2.08 (s, 3H, Ph(CH3)); 2.07 (s, 3H,
NCHN), 8.86–7.55 (m, 7H, CH), 7.02 (s, 2H, CH), 6.74 (s, 2H, Ph(CH3)); 2.04 (s, 3H, Ph(CH3)); 1.20 (t, 9H, B(CH2CH3)3); 0.83
CH2), 2.35 (s, 3H, CH3), 2.26 (s, 3H, CH3), 2.07 (s, 6H, CH3), (s, 9H, C(CH3)3); 0.73 (q, 6H, B(CH2CH3)3); 13C NMR (500 MHz,
2.00 (s, 3H, CH3).
C6D6) δ 179.0 (NC(B)N), 138.6 (Cq), 137.6 (Cq), 135.3 (Cq), 135.1
1e: [(tBu)C(O)CH2{CH[NCH2CH2N(Mes)]}]Cl. Yield: 83%; (Cq), 129.1 (CH), 129.0 (CH), 122.4 (NCCN), 120.9 (NCCN), 78.5
1H NMR (CDCl3, 500 MHz) δ 9.68 (s, 1H), 6.92 (s, 2H, CH), 5.44 (CH(tBu)OH); 51.0 (C(CH3)3), 35.1 (NCH2), 25.8 (C(CH3)3), 21.0
(s, 2H, CH2), 4.16 (s, 4H, CH2), 2.32 (s, 6H, CH3), 2.27 (s, 3H, (CH3), 18.2 (CH3), 18.1 (CH3), 15.3 (B(CH2CH3)3), 12.6 (C(CH3)3);
CH3), 1.25 (s, 9H, C(CH3)3).13C NMR (126 MHz, CDCl3) δ 208.6 11B (C6D6, 160.5 MHz) δ −13.2; HRMS (EI) Calcd for C24H41ON2B
(CvO), 161.5 (CH), 140.1 (Cq), 135.1 (Cq), 130.5 (Cq), 129.8 [M+] requires m/z 384.32770, found m/z = 384.33065.
(CH), 52.9 (NCH2), 51.0 (NCH2CH2N), 49.9 (NCH2CH2N), 43.1
3c: (tBu)CH(OBEt3)CH2{CH[NC(H)C(H)N(tBu)]}. Trituration
(C(CH3)3), 26.1 (C(CH3)3), 20.9 (C(CH3)3), 17.7 (Ph(CH3)2); ESI with Et2O yielded a colourless foam. Yield: 61%; 1H NMR
(+ve) M+ = 287.21 m/z [M+ − Cl]; Anal. Calcd C18H27ClN2O (C6D6, 500 MHz) δ 10.07 (s, 1H), 6.03 (d, J = 1.5 Hz, 1H), 5.98
(322.88): %C = 66.96; %H, 8.43; %N, 8.68; found: %C = 67.10; (d, J = 1.5 Hz, 1H), 4.10 (dd, J = 13.6, 4.0 Hz, 1H), 3.83 (d, J =
%H, 8.36; %N, 8.61; IR (CH2Cl2) νmax/cm−1 1719 (s); IR (neat) 4.0 Hz, 1H), 3.62 (d, J = 13.6 Hz, 1H), 1.49 (t, J = 7.7 Hz, 9H),
νmax/cm−1 1715(s).
1.00–0.75 (m, 6H) 0.93 (s, 9H), 0.83 (s, 9H); 13C NMR (C6D6,
1f [(naphth)C(O)CH2{CH[NCH2CH2N(Mes)]}]Cl. Analytically 126 MHz) δ 137.2 (CH), 122.6 (CH), 116.2 (CH), 75.1 (CH(tBu)-
pure material was obtained by recrystallisation from chloro- OBEt3), 58.8 (Cq), 53.2 (NCH2), 36.2 (Cq), 29.2 (CH3), 27.6
form. Yield: 90%; 1H NMR (CDCl3, 500 MHz) δ 9.27 (s, 1H), (CH3), 17.0 (BCH2), 12.6 (C(CH3)3); 11B (C6D6, 160.5 MHz,)
8.52 (s, 1H), 7.93 (d, J = 8.4 Hz, 1H), 7.90 (d, J = 8.4, 1H), 7.65 δ 1.1; EI M+ = 307.2 m/z [M+ − H]; Anal. Calcd C19H39BN2O
(dd, J = 8.4, 8.4 Hz, 2H), 7.43 (t, J = 7.5 Hz, 1H), 7.34 (t, J = (322.34), %C = 70.80; %H = 12.20; %N = 8.69; found, %C =
7.5 Hz, 1H), 6.75 (s, 2H), 5.84 (s, 2H), 4.27 (t, J = 10.6 Hz, 2H), 70.71, %H = 12.10, %N = 8.67.
4.13–4.00 (t, J = 10.6 Hz, 2H), 2.19 (s, 6H), 2.12 (s, 3H);
3d: (tBu)CH(OBEt3)CH2{CH[NC(CH3)C(CH3)N(Mes)]}. The
13C NMR (126 MHz, CDCl3) δ 192.7 (CO), 161.1 (CH), 140.3 pure product was obtained after recrystallisation from toluene
(Cq), 136.0 (Cq), 135.3 (Cq), 132.4 (Cq), 131.1 (CH), 130.5 (Cq), at −30 °C. Yield: 68%; 1H NMR (C6D6, 400 MHz): δ 9.24 (s, 1H,
130.0 (CH), 129.2 (CH), 128.9 (CH), 127.7 (CH), 127.0 (CH), Himidazole-2), 7.98–7.21 (m, 7H, naphthyl-CH), 6.58 (s, 1H, PhH),
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6.46 (s, 1H, PhH), 5.63 (dd, 1H, naphthyl-CHO), 4.16–4.11 (dd, B–CH2), 0.44–0.35 (m, 1H, B–CH2). 13C{1H} NMR (500 MHz,
1H, naphthyl-CHO–CH2), 3.67–3.63 (dd, 1H, naphthyl-CHO– C6D6): δ 142.9 (NCN), 139.6 (Cq), 136.7 (Cq), 136.6 (Cq), 134.6
CH2), 1.97 (s, 3H, imidazole-CH3), 1.93 (s, 3H, imidazole-CH3), (Cq), 134.0 (Cq), 133.2 (Cq), 124.0 (Cq), 123.6 (Cq), 129.7 (CH),
1.58–1.54 (t, 9H, CH2CH3), 1.40 (s, 3H, para-PhCH3), 1.11–1.04 129.7 (CH), 128.8 (CH), 128.7 (CH), 126.4 (CH), 126.0 (CH),
(m, 6H, B–CH2), 0.76 (s, 3H, ortho-PhCH3), 0.74 (s, 3H, ortho- 125.8 (CH), 125.8 (CH), 70.8 (naphthyl-CHO), 53.2 (HCO–CH2),
PhCH3). 13C{1H} NMR (C6D6, 400 MHz): δ 145.3 (NCHN), 141.3 21.4 (B–CH2), 18.5, 18.3 (imidazole-CH3), 12.6 (B–CH2–CH3),
(Cq), 135.9 (Cq), 134.7 (Cq), 134.3 (Cq), 133.5 (Cq), 129.7 (Cq), 11.0, 8.1 (phenyl-CH3). 11B NMR (C6D6, 400 MHz): δ 1.2.
127.5 (Cq), 124.5 (Cq), 136.3 (CH), 130.4 (CH), 130.0 (CH),
6: (tBu)CH(OBEt2)CH2{CH[NCCHN(tBu)]} and 7: Et3B{CH-
127.7 (CH), 126.3 (CH), 126.1 (CH), 126.0 (CH), 125.7 (CH), [NCHCHN(tBu)]}. 3c (0.1925 g, 0.60 mmol) was dissolved in
70.3 (naphthyl-CHO), 56.6 (NCH2), 21.3 (BCH2CH3), 17.9 dry toluene (4 mL) in an ampoule and heated in an oil bath at
(CH3), 17.2 (CH3), 12.9 (B–CH2CH3), 7.9 (CH3), 7.2 (CH3). 120 °C under reflux for 72 h. The reaction mixture was cooled
11B NMR (C6D6, 400 MHz): δ 2.2. Anal. Calcd for C32H43BN2O: to room temperature and the solvent removed in vacuo. The
C, 79.66; H, 8.98; N, 5.81. Found: C, 79.80; H, 9.06; N, 5.74. IR residue was redissolved in hexane and crystals of 7 formed at
(neat): νmax/cm−1 1124 (C–O).
room temperature. Filtration and cooling to −30 °C yielded a
4e: (tBu)C(O)CH2{CH[NCH2CH2N(Mes)]}. 1H NMR (C6D6, few crystals of 6.
1
500 MHz) δ 6.66 (s, 2H), 4.56 (s, 2H), 3.06–2.97 (m, 2H),
6 H NMR (C6D6, 400 MHz) δ 6.44 (d, J = 1.8 Hz, 1H), 6.33
2.97–2.86 (m, 2H), 2.22 (s, 6H), 2.10–1.99 (m, 3H), 1.19 (t, J = (d, J = 1.8 Hz, 1H), 3.76 (t, J = 6.5 Hz, 1H), 3.37 (d, J = 6.5 Hz,
7.7 Hz, 9H), 1.01 (s, 9H), 0.51 (q, J = 7.7 Hz, 6H); 13C NMR 2H), 1.61 (d, J = 7.5 Hz, 3H), 1.48 (dd, J = 7.5 Hz, 3H), 1.26
(126 MHz, C6D6) δ 208.8 (CvO), 202.0 (C–B), 137.8 (Cq), 137.6 (s, 9H), 1.22–0.89 (m, 6H), 0.65 (t, J = 6.5 Hz, 9H); 13C (126 MHz,
(Cq), 135.6 (Cq), 129.5 (CH), 53.4 (CH2), 51.9 (CH2), 48.2 (CH2), C6D6) δ 116.5 (CH), 115.4 (CH), 75.8 (CH), 59.2 (Cq), 48.6 (Cq),
43.1 (Cq), 26.5 (CH3), 21.0 (CH3), 18.21 (CH3), 14.98 (CH2), 34.5 (Cq), 29.0 (CH3), 26.8 (CH3), 11.6 (CH3), 11.5 (CH3); 11B NMR
12.46 (CH3); 11B NMR (C6D6, 400 MHz): δ −14.1; Anal. Calcd (C6D6, 400 MHz): δ 0.4; HRMS (EI) Calcd for C17H33O1N2B [M+]
C24H41BN2O (384.42), %C = 74.99; %H = 10.75; %N = 7.29; requires m/z 292.26805, found m/z = 292.26795.
1
found, %C = 75.13, %H = 10.72, %N = 7.40.
7 H NMR (C6D6, 400 MHz) δ 7.53 (t, J = 1.5 Hz, 2H), 7.02
4f: (naphth)C(O)CH2{C(BEt3)[NCH2CH2N(Mes)]}. 1H NMR (t, J = 1.5 Hz, 2H), 6.15 (t, J = 1.5 Hz, 2H), 1.22 (t, J = 7.7 Hz,
(C6D6, 500 MHz) δ 8.45 (s, 1H), 8.11–8.00 (dd, J = 8.6, 1.7 Hz, 19H), 0.97 (q, J = 7.7 Hz, 13H), 0.69 (s, 18H); 13C (126 MHz,
1H), 7.55 (d, J = 8.1 Hz, 1H), 7.52 (d, J = 8.6 Hz, 1H), 7.49 (d, J = C6D6) δ 131.9 (CH), 125.7 (CH), 116.7 (CH), 59.5 (Cq), 29.4
8.1 Hz, 1H), 7.23 (ddd, J = 8.2, 6.9, 1.3 Hz, 1H), 7.18 (ddd, J = (CH3), 15.8 (BCH2), 11.5 (CH3); 11B NMR (C6D6, 400 MHz):
8.2, 6.9, 1.3 Hz, 1H), 6.68 (s, 2H), 3.25 (ddd, J = 13.5, 9.9, δ 2.1; Anal. Calcd C13H27BN2 (222.18), %C = 70.28; %H =
1.7 Hz, 2H), 3.12 (ddd, J = 13.5, 9.9, 1.7 Hz, 2H), 2.28 (s, 6H), 12.25; %N = 12.61; found, %C = 70.06, %H = 12.25, %N = 12.45.
2.06 (s, 3H), 1.30–1.09 (t, J = 7.7 Hz, 9H), 0.60 (q, J = 7.7 Hz,
2c: (tBu)CH(OH)CH2{CH[NC(BEt3)C(H)N(tBu)]}. 3c was dis-
6H); 13C NMR (126 MHz, C6D6) δ 193.1 (CO), 137.5 (Cq), 137.2 solved in C6D6 and heated to 50 °C for 21 days yielding ∼60%
(Cq), 135.8 (Cq), 135.3 (CH), 132.6 (Cq), 132.4 (Cq), 129.6 (CH), conversion to 2c along with ca. 25% 3c and ca. 15% 7.
129.4 (CH), 129.2 (CH), 128.8 (CH), 128.4 (CH), 126.7 (CH),
1H NMR (500 MHz, C6D6) δ 7.63 (d, J = 2.0 Hz, 1H), 6.76
123.4 (CH), 54.3 (CH2), 51.7 (CH2), 48.7 (CH2), 20.6 (CH3), 17.9 (d, J = 2.0 Hz, 1H), 5.00–4.89 (m, 1H), 3.47–3.37 (m, 2H), 1.33
(CH3), 12.1 (CH3); 11B NMR (C6D6, 400 MHz): δ −13.9; Anal. (t, J = 7.7 Hz, 9H), 1.03–0.95 (m, 6H), 0.88 (s, 9H), 0.84 (s, 9H);
Calcd C30H39BN2O (440.44), %C = 79.08; %H = 8.47; %N = 13C NMR (126 MHz, C6D6) δ 162.7 (C–B), 130.9 (CH), 119.1
6.36; found, %C = 79.33, %H = 8.78, %N = 6.05.
(CH), 77.8 (CH(tBu)OBEt3), 55.8 (Cq), 49.3 (NCH2), 34.9 (Cq),
29.3 (CH3), 25.9 (CH3), 17.4 (BCH2), 16.0 (BCH2) 12.0
(C(CH3)3); 11B NMR (C6D6, 400 MHz): δ −15.2; Anal. Calcd
Thermolysis of imidazolium borates
5d:
(tBu)CH(OBEt2)CH2{C[NC(CH3)C(CH3)N(Mes)]}. 3d C19H39BN2O (322.34), %C = 70.80; %H = 12.20; %N = 8.69;
(9.6 mg, 0.02 mmol) was dissolved in dry toluene in a found, %C = 70.68, %H = 12.04, %N = 8.78.
J. Young’s tap NMR tube. The NMR tube was placed in an oil
bath and heated to 120 °C for 2 days. After cooling to the room
temperature, the colour of the solution had changed from
white to pale yellow. The solvent was removed in vacuo to
Acknowledgements
The authors are grateful for the support of the Technische Uni-
afford a yellow solid. The pure product 5 was obtained after
recrystallisation from toluene–THF at −30 °C. Yield: 9 mg (100%).
1H NMR (C6D6, 400 MHz): δ 8.29–7.29 (m, 7H, naphthyl-
CH), 6.74 (s, 1H, PhH), 6.73 (s, 1H, PhH), 5.38–5.35 (dd, 1H,
naphthyl-CHO), 3.65–3.62 (dd, 1H, naphthyl-CHO–CH2),
3.52–3.46 (dd, 1H, naphthyl-CHO–CH2), 2.05 (s, 3H, imidazole-
CH3), 2.03 (s, 3H, imidazole-CH3), 1.93 (s, 3H, para-PhCH3),
1.46–1.43 (t, 3H, B–CH2–CH3), 1.41–1.37 (t, 3H, B–CH2–CH3),
1.35 (s, 3H, ortho-PhCH3), 1.29 (s, 3H, ortho-PhCH3), 1.23–1.14
(m, 1H, B–CH2), 1.09–1.00 (m, 1H, B–CH2), 0.58–0.49 (m, 1H,
versität München – Institute for Advanced Study, funded by
the German Excellence Initiative, Sasol Technology, the Univer-
sity of Edinburgh, and the EPSRC.
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