Tris(pentafluorophenyl)borane as an efficient catalyst in the guanylation reaction of amines

Article abstract of DOI:10.1039/c6dt01237j

Tris(pentafluorophenyl)borane, [B(C₆F₅)₃], has been used as an efficient catalyst in the guanylation reaction of amines with carbodiimide under mild conditions. A combined approach involving NMR spectroscopy and DFT calcul

Full text of DOI:10.1039/c6dt01237j

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Tris(pentafluorophenyl)borane as an Efficient
Catalyst in the Guanylation Reaction of Amines.
Cite this: DOI: 10.1039/x0xx00000x
Antonio Antiñolo,*a Fernando Carrillo-Hermosilla,*a Rafael Fernández-
Galán,a Jaime Martínez-Ferrer,a Carlos Alonso-Moreno,b Iván Bravo,c
Sonia Moreno-Blázquez,a Manuel Salgado,a Elena Villaseñora and José
Albaladejod
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Tris(pentafluorophenyl)borane, [B(C6F5)3], has been used as an efficient catalyst in the
guanylation reaction of amines with carbodiimide under mild conditions. A combined approach
involving NMR spectroscopy and DFT calculations was employed to gain a better insight into the
mechanistic features of this process. The results allowed us to propose a new Lewis acid-
assisted Brønsted acidic pathway for the guanylation reaction. The process starts with the
interaction of tris(pentafluorphenyl)borane and the amine to form the corresponding adduct,
www.rsc.org/
[(C6F5)3B-NRH2]
the carbodiimide, PrN=C=NiPr, to produce, in two consequent steps, a guanidine-borane adduct,
[(C6F5)3B-NRC(NiPrH)2]
The rupture of this adduct liberates the guanidine product
RNC(NiPrH)2
and interaction with additional amine restarts the catalytic cycle. DFT studies
1, followed by a straightforward proton transfer to one of the nitrogen atoms of
i
2
.
3
have been carried out in order to study the thermodynamic characteristics of the proposed
pathway. Significant borane adducts with amines and guanidines have been isolated and
characterized by multinuclear NMR in order to study the N–B interaction and to propose the
existence of possible Frustrated Lewis Pairs. Additionally, the molecular structures of significant
components of the catalytic cycle, namely 4-tert-butylaniline-[B(C6F5)3] adduct 1b and both free
and [B(C6F5)3]-bonded 1-(phenyl)-2,3-diisopropylguanidine, 2a and 3a respectively, have been
established by X-ray diffraction.
group 4 metallocenes.3,4 This work encouraged the renaissance
of studies focused on perfluoroarylborane derivatives.
Introduction
Carbonꢀnitrogen bonds are amongst the most important bonds
in Chemistry and life sciences. In the available methodologies
to obtain this kind of bond, palladium and copper complexes
Remarkable examples include (a) the role of [(C6F5)2BH] as a
hydroborating agent,5 (b) the use of [(C6F5)4B]– as a nonꢀ
coordinating anion,6 (c) the activity of [B(C6F5)3]/group 4
metallocene systems as catalysts in olefin polymerization,7 and
(d) the role of [B(C6F5)3] as a strong Lewis acid.8 Of particular
interest is the behavior of tris(pentafluorophenyl)borane when
interacting with bulky Lewis bases to prevent the formation of
the corresponding adduct, but also act cooperatively by
activating hydrogen or small molecules. The resulting systems
are known under the concept of the Frustrated Lewis Pair,
FLP.9
The guanidine functionality is an essential structural group in
topꢀselling drugs and natural products.10 Compounds that
contain the guanidine CN3 core possess a rich chemical
reactivity that can be modulated by appropriate substitution on
the nitrogen atoms and these compounds have a variety of
possible resonance forms. The capacity for coordination of
are
the
most
commonly
used
systems.1
The
hydrofunctionalization reaction provides 100% atom efficiency
and a wasteꢀfree process from relatively lowꢀcost and
ubiquitous starting materials. The growing cost and recognized
toxicity of many commonly used metal catalysts are the driving
forces behind the development of alternative methods.
Tris(pentafluorophenyl)borane, [B(C6F5)3], has received a great
deal of attention as a nonꢀtoxic and stable Lewis acid. Although
this compound was first prepared in the early 1960’s by
Massey, Park and Stone,2 along with some Lewis acidꢀbase
adducts, it took almost 30 years for the scientific community to
focus on this compound. This interest is mainly due to the use
independently by Marks et al. and Ewen and coꢀworkers of this
system as a cocatalyst for olefin polymerization together with
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these molecules as monodentate or bidentate chelate ligands has
been widely demonstrated in a large variety of transition or
main group metal complexes.11 Additionally, guanidines are
some of the strongest neutral organic bases known due to the
ability of their protonated forms to delocalize charge over three
nitrogen atoms.12 Continuous research on the synthesis of
guanidines led scientists to develop alternative procedures and
there is a strong demand for the development of sustainable,
more selective and efficient processes for the syntheses of such
compounds. Although a large proportion of reported guanidines
are prepared by stoichiometric reactions, catalytic
hydroamination of carbodiimides is a valuable option.13 This
chemistry is dominated by transition metal and rareꢀearth metal
complexes, while examples with mainꢀgroup based catalysts
have deserved less attention.14 As part of our ongoing research
into the chemistry of catalytic guanylation15 and guanidine
Table 1. Catalytic guanylation of various primary and secondary amines with
B(C6F5)3.a
Entry
Amine
T
(ºC)
Time
(h)
Conv.%
1
2
a
70
70
1
4
> 99
> 99
b
3
4
5
c
d
e
70
70
70
1
1
> 99
> 99
27
24
6
7
f
f
70
40
24
28
83
derivatives,16
we
present
here
the
use
of
tris(pentafluorophenyl)borane as an excellent and versatile
catalyst for the addition of amines to a representative
carbodiimide under mild conditions. The different reactivity
observed prompted us to study the interactions of this borane
with amines and carbodiimides and to gain more inꢀdepth
knowledge of the mechanism involving Lewis acid catalysts.
100
8
g
70
40
6
Results and Discussion
9
g
h
100
70
24
24
10
0
In recent years we have reported several new or commercially
available simple compounds as catalysts for the catalytic
guanylation of amines with carbodiimides in the context of an
atom economic reaction.15 With this idea in mind, we set out to
study the potential of the metalꢀfree Lewis acid
tris(pentafluorophenyl)borane in the catalytic guanylation
(Scheme 1) of some selected aryl or alkyl amines, including
heterocyclic and secondary cyclic amines (Table 1).
10
11
12
13
i
j
k
nBuNH2
iPrNH2
tBuNH2
70
70
70
2
24
24
> 99
> 99
0
14
15
l
70
70
24
24
60
60
m
aConditions: amine, 1 mmol; carbodiimide 1 mmol; catalyst, 5% mol;
solvent: tolueneꢀd8. Conversions of reagents were determined by 1H NMR
spectroscopy.
The conversion values were determined by 1H NMR
spectroscopy. Aryl amines with lower levels of hindrance led to
higher conversions to
N,N',N"ꢀtrisubstituted guanidines in 1–4
hours (Table 1, entries 1–4). It can be seen from these results
that the boraneꢀcatalyzed guanylation displayed remarkable
functional group tolerance and groups such as Br or OMe were
unaffected under the reaction conditions used. Surprisingly, the
reaction with 4ꢀCF3(C6H4)NH2 resulted in lower yields, a
finding that indicates some influence of the electronꢀ
withdrawing effect of the trifluoromethyl group (Table 1, entry
5). Bulkier amines such as 2,4,6ꢀMe3(C6H2)NH2 or 2,6ꢀ
iPr2(C6H3)NH2 were transformed into the corresponding
guanidines in low or moderateꢀtoꢀgood yields only under more
forcing conditions (Table 1, entries 6–9). The presence of an
Scheme 1. Catalytic guanylation of amines with [B(C6F5)3]
In all cases, the reactions were carried out in toluene in the
presence of 5 mol% of catalyst under mild conditions.
Diisopropylcarbodiimide, DIC, was selected as the starting
material. In the absence of catalyst, the reaction of N'
diisopropylcarbodiimide at 70 or 100 ºC with aniline in
deuterated toluene did not take place.
N,N'ꢀ
N
, ꢀ
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additional heterocyclic nitrogen atom, as in 4ꢀaminopyridine, The interaction between the amine and the boron atom in 1a
could inhibit the reaction (Table 1, entry 10). Aliphatic amines was confirmed by 19F NMR spectroscopy (see Table 2), with
were also evaluated in this transformation. Less acidic alkyl the spectra containing the expected peaks for ortho meta and
–m
,
amines RNH2 (Table 1, entries 11–13) showed a high para fluoroꢀsubstituents and indicating a tetracoordinate boron
dependence on the size of R. In fact, when R = nBu the reaction atom.22a The 11B NMR spectra showed a single broad signal
proceeded rapidly in good yield but more sterically demanding (see Table 2), which was centred at around –5 ppm for the
i
amines, e.g., R = Pr, required 24 hours to achieve full aromatic amines and –7 ppm for the alkyl amines. In the case of
conversion to the corresponding guanidine or the reaction did 4ꢀaminopyridine, the coordination of the boron atom takes
t
not proceed at all when R = Bu. Finally, secondary aromatic place almost exclusively at the Nꢀpyridinic atom as the
1
and cyclic amines (Table 1, entries 14–15) were less active and interaction with the NH2 group was hampered.23 In fact, the H
only an adequate 60% conversion was obtained after 24 hours.
NMR spectrum was consistent with the NH2 protons being less
The molecular structure of the previously described 1ꢀ(phenyl)ꢀ affected when compared with those of the other amineꢀborane
2,3ꢀdiisopropylguanidine 3a17 was established by Xꢀray adducts. The molecular structure of the amineꢀborane complex
diffraction (Figure 1S).
formed by 4ꢀtert-butylaniline and [B(C6F5)3], 1b, was
Different types of reaction mechanisms have been categorized determined in the solid state by Xꢀray diffraction analysis
for catalytic guanylation reactions of amines with (Figure 1). Single crystals were obtained from a concentrated
carbodiimides:
[2
+
2]
cycloaddition/protonation, solution of 1b in toluene/hexane. The molecule shows the
insertion/protonation, activation of carbodiimide/nucleophilic proposed boronꢀNH2 interaction with a B1–N1 distance of
addition/intramolecular
protonation,
and 1.645(3) Å, which is typical of this kind of adduct.22 One of
protonation/nucleophilic addition/disassociation.18
amino hydrogen atoms is ca. 2.15 Å from two orthoꢀF atoms
Very few studies on Lewis acidꢀbased catalytic guanylation suggesting the presence of
a bifurcated, intramolecular
have been reported but, in those that have, carbodiimide F…H…F hydrogen bonding system.22c,d
activation and consequent nucleophilic attack of an amine has
been proposed, although unequivocal evidence has yet to be
provided.14c,19 In some cases the proposed mechanism starts
with activation of the carbodiimide by the Lewis acid to yield a
guanidinate complex as the actual catalytic species.20 In order
to gain a deeper insight into the reaction pathway for the
catalytic guanylation with [B(C6F5)3], a combination of NMR
stoichiometric experiments and DFT calculations was
employed.
Despite the current importance of the amineꢀborane adducts,21
few examples using [B(C6F5)3] have been described.22
Therefore,
initially
the
adducts
formed
by
tris(pentafluorophenyl)borane and the amines
a–m (Scheme 2)
were synthesized and characterized (1a–m, see Table 2 for the
more characteristic spectroscopic data). Compounds 1a and 1k
have been described previously.22a,b Adduct formation occurred
smoothly to yield crystalline colourless solids. NMRꢀscale
experiments
indicated
that
the
reactions
proceed
Figure 1. Molecular structure of 1b. Some hydrogen atoms have been omitted
for clarity.
instantaneously after mixing the reagents. In the 1H NMR
spectra the downfield shift of the NH protons was observed
after contact with the [B(C6F5)3] Lewis acid.
With regard to the carbodiimide borane adducts, Stephan et al
.
recently carried out a thorough study on the reactivity of alkylꢀ
substituted carbodiimides towards [B(C6F5)3], where the boron
atom is bonded to a nitrogen atom to give stable adducts.24
Subsequently, on the assumption of a mechanism starting with
a molecule of carbodiimide activated by the acid catalyst, we
carried out the reaction of 1 equivalent of aniline with the
H
R
N
B(C6F5)3
R'
RR'NH
1a-g, 1i-m
B(C6F5)3
aforementioned adduct formed by [B(C6F5)3] and
N,N'ꢀ
H
H
B(C6F5)3
diisopropylcarbodiimide (DIC). The reaction was monitored by
1H and 19F NMR spectroscopy in deuterated toluene. In contrast
to the expected result, the spectrum contained new peaks
similar to those found for the amine adduct, 1a, and free
N
N
N
N
H
H
1h
1
carbodiimide (Figure 2a). In the H NMR spectra, these peaks
Scheme 2. Proposed amine-[B(C6F5)3] adduct formation.
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are slightly shifted with respect to those of the separate
compounds; downfield for the amineꢀborane adduct and upfield
for the free DIC. An identical spectrum was obtained when the
amine adduct, 1a, was reacted with DIC. In fact, the broad peak
assigned to NH protons was clearly affected by the presence of
the carbodiimide.
Table 2. Significant NMR data (Chemical shift, δ) for amineꢀborane
compounds.
Amine
Adduct
Chemical shifts, δ
19F
11B
–132.8 (d, 3JFF = 21 Hz, oꢀ
ArCF), –154.6 (t, 3JFF = 21
Hz, pꢀArCF), –161.8 (m, mꢀ
ArCF)
1a22a
–5.3 (br, s)
Theoretical calculations were carried out using the Gaussian09
(Rev. C.01) software package to aid the discussion and a
detailed computational analysis was carried out using a hybrid
density functional metaꢀGGA M06ꢀ2X method (see
Experimental Section). The initial estimates for the different
intermediates were generated from crystallographic structures
and geometries optimized at the selected level of theory. In a
first approximation, the relative free energies of both the aniline
and carbodiimide borane adducts were calculated (Figure 3)
and the former was around 8 Kcal.mol–1 more stable than the
diimide adduct. This result is in accordance with the
experimental observations and represents a major change in
proposing whether the reaction starts with the attack of the
amine to an activated carbodiimide, as stated in the literature
for other Lewis acid catalysts,18 or whether the first step
involves an interaction between the free carbodiimide and an
activated, boraneꢀbonded amine.
–132.7 (d, 3JFF = 18 Hz, oꢀ
ArCF), –155.8 (t, 3JFF = 19
Hz, pꢀArCF), –162.8 (m, mꢀ
ArCF)
1b
1c
1d
–5.6 (br, s)
–5.6 (br, s)
–5.0 (br, s)
–133.0 (d, 3JFF = 21 Hz, oꢀ
ArCF), –155.1 (t, 3JFF = 20
Hz, pꢀArCF), –162.5 (m, mꢀ
ArCF)
–132.9 (d, 3JFF = 12 Hz, oꢀ
ArCF), –154.5 (t, 3JFF = 14
Hz, pꢀArCF), –162.1 (m, mꢀ
ArCF)
–63.2 (s, CF3), –132.7 (d,
3JFF = 19 Hz, oꢀArCF),
–154.3 (t, 3JFF = 20 Hz, pꢀ
ArCF),
1e
1f
–5.6 (br, s)
–4.0 (br, s)
–162.0 (m, mꢀArCF)
Upon gentle heating of this reaction mixture the observed peaks
1
disappeared and new broad peaks were observed in the H
–135.9 (d, 3JFF = 27 Hz, oꢀ
ArCF), –154.9 (t, 3JFF = 18
Hz, pꢀArCF), –164.1 (m, mꢀ
ArCF)
NMR spectrum (Figure 2b). At the same time, the three
multiplets observed in the 19F NMR spectrum, which
correspond to the C6F5 groups of the amineꢀborane adduct
present in the mixture, disappeared and a complex spectrum
was obtained that contained fourteen coupled peaks. This
observation indicates that a new compound had been formed,
with a species bonded to the boron atom where the free B–C or
B–N rotation was blocked, thus making all of the fluorine
atoms inequivalent. This reaction takes place in even at room
temperature providing nearly 60% of conversion after 16 h. It
was noted that the broad peaks observed in the 1H NMR
spectrum could be assigned to the alkyl protons of a guanidine
derivative. On the basis of this result, the reaction between
guanidine NPhC(NiPrH)2 3a and [B(C6F5)3] was carried out in
toluene solution at room temperature to obtain the
corresponding adduct [(C6F5)3BNPhC(NiPrH)2] 2a (Scheme 3).
–127.9 (m), –154.8 (m),
–141.2 (m), –160.5 (m),
–162.9 (m), –164.1 (m)
1g
–3.7 (br, s)
–131.8 (d, 3JFF = 18 Hz, oꢀ
ArCF), –156.8 (t, 3JFF = 20
Hz, pꢀArCF), –163.3 (m, mꢀ
ArCF)
1h
1i
–4.6 (br, s)
–7.7 (br, s)
–134.5 (d, 3JFF = 21 Hz, oꢀ
ArCF), –155.5 (t, m, pꢀ
nBuNH2
iPrNH2
tBuNH2
ArCF), –162.5(m, mꢀArCF)
–133.0 (d, 3JFF = 20 Hz, oꢀ
ArCF), –155.3 (t, 3JFF = 22
Hz, pꢀArCF), –162.4 (m, mꢀ
ArCF)
1j
1k22b
1l
–7.7 (br, s)
–9.0 (br, s)
–3.5 (br, s)
–131.1 (m, oꢀArCF), –155.1
(m, pꢀArCF),
–162.2 (m, mꢀArCF)
–133.6 (d, 3JFF = 12 Hz, oꢀ
ArCF), –155.7 (t, 3JFF = 22
Hz, pꢀArCF), –162.6 (m, mꢀ
ArCF)
–135.6 (d, 3JFF = 14 Hz, oꢀ
ArCF), –159.2 (t, 3JFF = 20
Hz, pꢀArCF), –164.3 (m, mꢀ
ArCF)
1m
–5.3 (br, s)
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Figure 2. Evolution of the reaction of aniline, DIC and [B(C6F5)3], in toluene-d8, monitored by 1H NMR. (a) Mixture of DIC-borane adduct and aniline, or aniline-borane
adduct and DIC; (b) after heating for 1 h, at 70ºC; (c) after addition of a slight excess of aniline and heating for 1 h, at 70ºC.
Figure 3. Energy profile for the calculated interaction between [B(C6F5)3] and aniline and N,N'-diisopropylcarbodiimide.
ꢀ
G = 0 for the starting reagents.
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F
Finally, addition of a slight excess of aniline to the former
reaction mixture and heating at 70 ºC for 1 hour gave spectra in
which the peaks corresponding to the free guanidine and
anilineꢀborane adduct represented the major products (Figure
2c).
F
F
F
F
F
F
F
F
NH
HN
F
B
F
F
+ B(C6F5)3
HN
HN
N
F
N
F
Crystals of compound 2a suitable for single crystal Xꢀray
diffraction were grown from a concentrated toluene/hexane
solution and the molecular structure is represented in Figure 4.
To the best of our knowledge, this is the first guanidine adduct
of tris(pentafluorophenyl)borane to be characterized
crystallographically. In this compound the borane is bonded to
the imine nitrogen as a result of the greater basicity of this atom
with regard to the other two in the CN3 core. The C=N distance
was longer than the expected value of ca. 1.28 Å found in a
C=N bond and slightly longer than that found in the free
guanidine 3a (see ESI). The B–N distance of 1.607 Å is shorter
than those found in [B(C6F5)3]ꢀimine adducts,25a or similar to
those found in [HB(C6F5)2]ꢀimine adducts.25b
F
2a
Scheme 3. Proposed formation of guanidine [B(C6F5)3] adduct 2a
.
1
The H NMR spectrum was identical to that obtained in the
reaction between carbodiimide, aniline and borane (Figure 2b).
The spectrum contained three broad peaks at 0.20, 0.42 and
0.61 ppm, integrating in a ratio of 6:3:3 protons, respectively,
and the first and last signals were coupled to a peak at 2.54
ppm, integrating for one proton. This peak was coupled to
another singlet at 5.41 ppm, integrating for one proton. The first
and second peaks were coupled to a peak at 3.14 ppm,
integrating for one proton. In turn, this last peak was coupled to
an overlapped peak at 3.20 ppm, integrating for one proton. The
peaks discussed above were assigned, respectively, to methyl,
methine and NH moieties in a guanidine coordinated to the
borane (See Experimental Section). Finally, the corresponding
peaks for the aromatic protons were observed between 6.66 and
7.35 ppm. The 11B spectrum shows a peak at –7.1 ppm and this
is consistent with
a tetrasubstituted boron atom. The
coordination of the guanidine to the boron atom is also
manifested in the 19F NMR spectrum, which contains the
fourteen multiplets (two
mꢀF overlapping) mentioned above
and reflects the molecular dissymmetry of this product, thus
indicating the blocked rotation of the C6F5 moieties due to
steric factors and possible intramolecular hydrogen bonds.22c,d
A simpler spectrum was observed on heating this solution at
100 ºC. In the 1H NMR spectrum, the three broad peaks
assigned to the methyl groups of the isopropyl moieties
collapsed to give a broad doublet and the corresponding
methine protons behaved similarly to give a broad singlet. The
observed pattern was quite similar to that of the free guanidine,
where prototropy between aminic and iminic nitrogen atoms
takes place to make the two isopropyl groups equivalent.
Likewise, the 19F NMR spectrum was simplified and para- and
metaꢀfluorine atoms were observed as two broad singlets, with
the ortho-fluorine remaining as five very broad peaks. These
observations indicate that rotation of the C6F5 groups becomes
faster at the NMR time scale as the temperature increases.
Figure 4. Molecular structure of 2a. Some hydrogen atoms, except for those
bonded to the nitrogen atoms, are omitted for clarity. Selected bond lengths [Å]
and angles [°]: B1–N1 1.607(2); N1–C25 1.349(2); N2–C25 1.337(2); N3–C25
1.346(2); B1–N1–C25 121.44(13); N1–C25–N2 120.15(15); N1–C25–N3
121.41(14); N2– C25–N3 118.44(14).
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Figure 5. Free energy profile for the guanylation reaction of aniline to form PhNC(NiPrH)2 3a with [B(C6F5)3].
Theoretical calculations were again carried out in order to stable amineꢀborane complex (I1) in which the acidic behavior
gather and analyze information on the reaction mechanism of the NH protons is enhanced. Neutral adducts of [B(C6F5)3]
followed by [B(C6F5)3]. The reaction profile shown in Figure 6 and amines, including aromatic amineꢀborane complexes, have
was modelled for the reaction of DIC and aniline, which gives a enhanced Brønsted acidity and it has been shown that they
very high yield of NPhC(NiPrH)2 3a in the presence of easily protonate M–R or M–N bonds.26 The same reactivity was
[B(C6F5)3]. The thermodynamic barriers are summarized in found in the deprotonation of the Lewis acidꢀbonded amines
Table 3. It is worth noting that Gibbs free energies are used with Nꢀheterocyclic carbenes.22a In the resulting intermediate I1
here, which inherently embrace thermodynamic and kinetic a fourꢀcoordinate boron centre is formed. From I1, a protonꢀ
aspects, to give a suitable insight into the guanylation transfer to the free carbodiimide occurs and a subsequent
mechanism.
reorientation of the electrophilic central carbon of the
protonated DIC allows a nucleophilic attack via TS1 from the
negatively charged nitrogen atom bonded to the boron atom
(see selected geometrical properties of TS1 in Figure 6). This
step would be favoured for acidic and nonꢀsterically demanding
amines. Partial proton transfer in a concerted way from a
coordinated amine has been proposed in the intramolecular
hydroamination of aminoalkenes.27 Downhill from TS1, a
neutral guanidine coordinated to the boron atom by means of an
aminic nitrogen atom is generated (I2). The initially formed
isomer I2 is readily transformed into the thermodynamically
more favourable isomer I3 by an intramolecular proton
exchange. As a result of the intramolecular protonation, the B–
N bond length decreases from 1.72 to 1.61 Å when I2 is
transformed into I3 (this value agrees well with that
experimentally found for 2a). In the final step the guanidine
product detaches through the concerted transition state TS3,
which leads to the generation of [B(C6F5)3] species and
completion of the catalytic cycle. It is noteworthy that such a
Table 3. Gibbs free energies (ꢁG) calculated for the intermediates along the
studied reaction pathway. Calculated imaginary frequencies (cm–1) for the
studied transition states at 298.15 K are also shown. The level of theory used
was M06ꢀ2X/6ꢀ31G*.
Imaginary frequency /
Intermediate
ꢁG/ kcal mol–1
cm–1
R
I1
TS1
I2
TS2
I3
TS3
P
0.0
–22.4
0.9
–18.6
3.9
–42.6
–16.2
–16.3
–323.23
–1816.23
–59.87
Overall, the DFT energy profile shown in Figure 5 is indicative
of a kinetically affordable and exergonic mechanism, which is
compatible with the experimental results. As shown in Figure 5,
on starting from the reactants a molecule of aniline could be
activated by the acid catalyst to form the previously described
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strong base and a good nucleophile like guanidine can be
displaced from this kind of adduct. Recently, Stephan described
access to typical FLP reactivity in classical Lewis acidꢀbase
adducts in equilibrium with the free constituents.28 In the case
reported here, the guanidine borane adducts can release the
guanidine at mild temperatures, in a possible case of thermal
activation of the adduct,29 to trap a new amine molecule and
regenerate the activated amineꢀborane adduct to reꢀenter the
cycle.
R' N C N R'
H
RH2N-B(C6F5)3
R' N C N R'
RNH2 + [B(C6F5)3]
1
N
H
B(C6F5)3
H
H
R
N
N
R'
R'
N
RNH2
R
3
R'
R'
NH
H
NH
NH
R'
B(C6F5)3
+
(C6F5)3B N
N
N
R
R'
R
R'
NH
N
(C6F5)3B
NH
R'
R
2
TS1
Scheme 4. Proposed mechanism for the catalytic guanylation reaction using
[B(C6F5)3] as catalyst.
The different reactivity observed in the guanylation reaction
can be explained on the basis of the proposed mechanism.
Anilines bearing bulky substituents do not give rise to very
stable adducts with borane in solution. Similarly, the CF3ꢀ
substituted aniline, which is more acidic but a weaker donor,
showed the same behaviour. In fact, a competitive NMR
experiment between [B(C6F5)3], DIC and pꢀCF3C6H4NH2 gave
rise to a mixture of diimide and aniline adducts. This lack of an
adequate concentration of activated amine would result in lower
activity. Furthermore, an excessive volume of the resulting
adduct could prevent the interaction with the diimide. In this
case, a compromise between low steric hindrance, acidity and
donor capacity of the aniline is a prerequisite for a better
reactivity. The results obtained with primary alkyl amines,
which are less acidic but are better donors than anilines, are
consistent with the fact that a decrease in the size of the alkyl
substituent increases the conversion rate. The interaction of
secondary cyclic amines, e.g., piperidine, with [B(C6F5)3] could
produce a very stable adduct with F–H interactions, which in
turn would make the guanylation reaction more difficult.
Complex 1H and 19F NMR spectra were obtained for this adduct
(Table 2 and Experimental Section) and these were similar to
the spectra described for the pyrrolidine analogues, in which the
aforementioned kind of intramolecular interaction is present.22
In order to gain further insights into the nature of the interaction
between the [B(C6F5)3] and the guanidines, the reaction with
TS2
TS3
Figure 6. Optimized geometries at the M06-2X/6-31G* level for transition states
(TSn)
.
To the best of our knowledge, this is the first attempt to
characterize fully a catalytic guanylation process based on an
acid catalyst by quantum chemical calculations.
As models of possible intermediate components in the proposed
catalytic cycle (Scheme 4), isolated adducts 1a and 2a were
tested as catalysts in the guanylation reaction of aniline with
carbodiimide, under the same conditions, and the same result
was obtained in terms of time and conversion ratio.
i
bulkier guanidines, NR1C(NR2H)2 (R1 = 2,6ꢀiPr2C6H3, R2 = Pr
30
3g; R1 = R2 = 2,6ꢀiPr2C6H3, 3o
)
was carried out in a similar
manner to that described in Scheme 3.
The reaction was monitored by NMR spectroscopy and this
revealed different behavior for each combination. Thus, the 11B
8 | J. Name., 2012, 00, 1-3
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NMR spectrum of the equimolar mixture of compound 3g and weaken the BꢀN bond in a potential Lewis adduct, thus
[B(C6F5)3] showed the formation of a new species, 2g, which justifying the observed FLP behavior.
exhibited a resonance at –6.8 ppm. The 19F NMR spectrum of
2g gave rise to twelve separate peaks due to considerable
dissymmetry, as observed for 2a, and it is believed that this
1
results from restricted rotation of the C6F5 rings. The H NMR
spectrum of 2g contains very broad peaks that are shifted with
respect to those of the starting compound. On the basis of these
data, we assume the formation of a conventional Lewis adduct
[(C6F5)3BN(2,6ꢀiPr2C6H3)C(NiPrH)2]. In contrast, monitoring of
the NMR features in deuterated toluene of the mixture of 3o
and [B(C6F5)3] revealed a broad 11B NMR signal at ca. 60 ppm
and 19F NMR resonances at –128.8, –142.0 and –160.2 ppm.
1
The H NMR spectrum appeared to be identical to that of the
free guanidine. It seems that the extremely bulky guanidine 3o
and [B(C6F5)3] form an FLP that does not interact under these
conditions. It is clear that the steric demands of the three
iPr2C6H3 substituents on the guanidine core preclude
coordination to the boron atom.
The reaction of this mixture with PhC≡CH, as a benchmark of
FLP behavior, resulted in deprotonation of the alkyne in a
similar way to the cases where more basic donors were
employed.31 This reaction afforded the corresponding
guanidinium
alkynylborate
salt
[C(Nꢀ2,6ꢀ
iPr2C6H3H)3][PhCCB(C6F5)3] 4o. Surprisingly, solutions of 2g
reacted in a straightforward manner with PhC≡CH at room
temperature to give a similar guanidinium alkylnylborate salt
[NH(2,6ꢀiPr2C6H3)C(NiPrH)2][PhCCB(C6F5)3] 4g (Scheme 5).
Reaction of these bulky guanidines and alkyne, in absence of
[B(C6F5)3], does not afford any product, thus indicates the
influence of the Lewis acid in the CꢀH activation process.
Presumably, the steric bulkiness of 2g is not sufficient to
prevent formation of the dative adduct but this pair is almost as
strained as the inherent frustrated complex of 2o. Compounds
4g and 4o exhibit characteristic 11B NMR resonances at –20.8
and –21.1 ppm, respectively, and this is consistent with the
formation of a borate anion.32
Figure 7. Optimized geometries at the M06-2X/6-31G* level for the plausible
adducts of 3g (up) and 3o (down) with [B(C6F5)3].
Conclusions
The wellꢀknown Lewis acid tris(pentafluorophenyl)borane has
been successfully employed for the first time in the preparation
of guanidines in the reaction of amines and a carbodiimide. A
Lewis acidꢀenhanced protonation and nucleophilic attack of an
amineꢀborane adduct to the carbodiimide is proposed. After the
nucleophilic addition step, the resulting coordinated guanidine
could easily isomerize to form a guanidine borane adduct.
Indeed, the first guanidine borane adduct to be fully structural
characterized is described. This adduct then releases the
guanidine and it can in turn react with a new amine molecule to
restart the catalytic cycle. This mechanism might be an
alternative reaction pathway involving Lewis acid catalysts in
guanylation reactions.
R
NH
HN R
R'
Ph
B(C6F5)3
+ PhCCH
HN
R
NH
R'
(C6F5)3B
N
HN
R
R = iPr, R' = 2,6-iPr2C6H3
4g
R = R' = 2,6-iPr2C6H3
4o
Scheme 5. Reactivity of borane-guanidine mixtures and PhC≡CH.
In addition, as far as we know, guanidineꢀborane adducts have
not previously been reported and they represent a new family of
compounds where the size of the guanidine base may play an
important role in the interaction with the boron centres and
inherent or thermally induced Frustrated Lewis Pairs can be
envisaged in some cases. Efforts to explore of new FLPs based
on guanidines and their reactivity are in progress in our lab.
Geometries of plausible Lewis adducts of guanidines 3g and 3o
with [B(C6F5)3] have been optimized at the selected level of
theory (Figure 7). The data showed BꢀN bond lengths of 1.626
Å and 1.640 Å, significantly longer than that observed in
compound 2a (1.607(2) Å) or that calculated for I3 (1.61 Å),
revealing that steric demands of these bulkier guanidines could
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Anal. Calcd for C25H9BF15NO: C, 47.28; H, 1.43; N, 2.21.
Found: C, 47.32%; H, 1.55%; N, 2.25%.
Data for 1d: Isolated yield 98% (268 mg). H NMR (400 MHz,
Experimental Section
1
General Procedures
tolueneꢀd8):
8.2 Hz, 2H, mꢀArC
MHz, tolueneꢀd8):
δ
= 6.67 (d, J = 8.2 Hz, 2H, oꢀArC
H), 6.04 (d, J =
All reactions were performed using standard Schlenk and
gloveꢀbox techniques under an atmosphere of dry nitrogen.
Solvents were purified by passage through a column of
activated alumina (Innovative Tech.) and degassed under
nitrogen before use. Microanalyses were carried out with a
PerkinꢀElmer 2400 CHN analyzer. NMR spectra were recorded
on a Varian FTꢀ400 spectrometer using standard VARIANꢀFT
software. Amines and diisopropylcarbodiimide were purchased
from Aldrich. Bis(2,6ꢀdiisopropylphenyl)carbodiimide was
purchased from TCI. Tris(pentafluorophenyl)borane was
synthetized according the literature method.2b
H
), 5.76 (br s, 2H, NH2). 13C NMR (100
δ
= 149.5 (Ar
C
C
F), 147.5 (Ar
F), 141.1 (Ar
C
C
F), 142.3
F), 133.4
(Ar
(Ar
C
C
F), 139.0 (Ar
C
F), 136.9 (Ar
N), 132.9 (Ar
C
Br), 124.3 (Ar
C
H), 123.3 (Ar
C
H). 19F
NMR (376 MHz, tolueneꢀd8):
δ = –132.9 (d, JFF = 12 Hz, oꢀ
3
3
ArCF), –154.5 (t, JFF = 14 Hz, pꢀArCF), –162.1 (m, mꢀArCF).
11B NMR (128 MHz, tolueneꢀd8):
δ = –5.0 (br, s). Anal. Calcd
for C24H6BBrF15N: C, 42.14; H, 0.89; N, 2.05. Found: C,
42.20%; H, 1.01%; N, 2.15%.
1
Data for 1e: Isolated yield 96% (258 mg). H NMR (400 MHz,
tolueneꢀd8):
δ
= 6.77 (d, J = 6.5 Hz, 2H, oꢀArC
6.5 Hz, 2H, mꢀArC
), 5.86 (br s, 2H, NH2). 13C NMR (100
MHz, tolueneꢀd8):
= 149.6 (Ar F), 147.3 (Ar F), 142.4
F), 139.2 (Ar F), 137.4 (Ar
N), 127.1 (Ar H), 123.4 (Ar
H), 6.22 (d, J =
H
δ
Procedure for Guanylation Reactions at NMR Tube Scale.
Catalytic reactions were performed on a small scale in an NMR
tube fitted with a concentric Teflon valve with 1 mmol of
amine, 1 mmol of carbodiimide and 5 mol% of catalyst in the
deuterated toluene under nitrogen. Conversion of the starting
material to product was determined by integration of the
C
C
C
(Ar
(Ar
C
C
F), 140.0 (Ar
F), 131.9 (Ar
C
C
CCF3), 136.1
C
C
H). 19F NMR
(376 MHz, tolueneꢀd8):
δ
= –63.2 (s, CF3), –132.7 (d, JFF = 19
3
3
Hz, oꢀArCF), –154.3 (t, JFF = 20 Hz, pꢀArCF), –162.0 (m, mꢀ
ArCF). 11B NMR (128 MHz, tolueneꢀd8):
= –5.6 (br, s). Anal.
1
δ
product resonances relative to the substrate peaks in the H
Calcd for C25H6BF18N: C, 44.61; H, 0.90; N, 2.08. Found: C,
NMR spectrum.
44.70%; H, 0.85%; N, 2.12%.
Data for 1f: Isolated yield 93% (241 mg). H NMR (400 MHz,
1
Procedure for the preparation of amineꢀborane adducts 1a–
tolueneꢀd8):
δ
= 6.24 (br s, 2H, NH2), 6.16 (s, 2H, ArC
H), 1.83
m
.
(s, 3H, CH3), 1.54 (s, 6H, CH3). 13C NMR (100 MHz, tolueneꢀ
To a solution of amine (0.40 mmol) in toluene (5 mL) was
added a solution of [B(C6F5)3] (0.40 mmol) in toluene (5 mL).
The reaction mixture was stirred for 1 h. The solvent was
removed under vacuum and the resulting solid was washed with
cold hexane (10 mL) and dried under vacuum to obtain the
product as a white powder.
d8):
δ
= 149.8 (Ar
F), 138.9 (Ar
CH3), 130.1 (Ar
C
F), 147.8 (Ar
C
C
F), 142.2 (Ar
N), 136.7 (Ar
H3), 16.2 (
C
C
F), 140.1
F), 130.9
H3). 19F NMR
(Ar
(Ar
C
C
C
F), 138.2 (Ar
H), 17.4 (
C
C
C
(376 MHz, tolueneꢀd8):
δ
= –135.9 (d, 3JFF = 27 Hz, oꢀArCF), –
154.9 (t, 3JFF = 18 Hz, pꢀArCF), –164.1 (m, mꢀArCF). 11B NMR
(128 MHz, tolueneꢀd8):
= –4.0 (br, s). Anal. Calcd for
δ
Data for 1b: Isolated yield 90% (238 mg). Xꢀray quality
crystals were obtained from a toluene/hexane solution of the
C27H13BF15N: C, 50.11; H, 2.02; N, 2.16. Found: C, 50.20%; H,
product and cooling to –35 ºC
.
1H NMR (400 MHz, tolueneꢀd8):
), 6.36 (d, J = 9.5 Hz, 2H,
H
), 5.86 (br s, 2H, NH2), 0.96 (s, 9H, C(CH3)3). 13C
1.99%; N, 2.21%.
1
Data for 1g: Isolated yield 90% (248 mg). H NMR (400 MHz,
δ
= 6.76 (d, J = 9.5 Hz, 2H, oꢀArCH
tolueneꢀd8):
δ
= 7.14 (br s, 2H, NH2), 6.74 (m, 1H, pꢀArC
6.58 (m, 2H, mꢀArC ), 2.64 (m, 2H, C ), 0.80 (d, J = 6.8 Hz,
12H, CH3). 13C NMR (100 MHz, tolueneꢀd8):
H),
mꢀArC
NMR (100 MHz, tolueneꢀd8):
δ
= 152.5 (Ar
C
N), 149.3 (Ar
F), 139.9 (Ar
H), 121.9 (Ar
C
C
F),
F),
H
H
δ
= 150.0
146.9 (Ar
136.4 (Ar
116.4 Ar
tolueneꢀd8):
C
F), 141.9 (Ar
C
F), 139.4 (Ar
C
C
F), 131.7 (ArCtBu), 126.4 (Ar
C
H),
(Ar
(Ar
(Ar
C
C
C
F), 147.6 (Ar
F), 138.8 (Ar
C
C
F), 142.4 (Ar
F), 136.2 (Ar
C
C
F), 141.9 (Ar
C
C
N), 139.9
C), 129.6
C
Me3), 30.7 (CMe3). 19F NMR (376 MHz,
F), 129.8 (Ar
C
B), 34.4 (
C
H), 124.0 (Ar
C
H), 29.1 (
= –127.9 (m), –154.8 (m), –141.2 (m), –
160.5 (m), –162.9 (m), –164.1 (m). 11B NMR (128 MHz,
tolueneꢀd8):
= –3.7 (br, s). Anal. Calcd for C30H19BF15N: C,
CH), 25.1 (
C
H3). 19F NMR (376
3
δ
= –132.7 (d, JFF = 18 Hz, oꢀArC
F
), –155.8 (t,
). 11B NMR (128
= –5.6 (br, s). Anal. Calcd for
MHz, tolueneꢀd8):
δ
3JFF = 19 Hz, pꢀArC
MHz, tolueneꢀd8):
F), –162.8 (m, mꢀArCF
δ
δ
C28H15BF15N: C, 50.86; H, 2.29; N, 2.12. Found: C, 50.80%; H,
52.28; H, 2.78; N, 2.03. Found: C, 52.35%; H, 2.85%; N,
2.25%; N, 2.20%.
1
2.10%.
Data for 1c: Isolated yield 97% (246 mg). H NMR (400 MHz,
1
tolueneꢀd8):
δ
= 6.35 (m, 2H, oꢀArC
H), 6.22 (m, 2H, mꢀArCH),
Data for 1h: Isolated yield 97% (235 mg). H NMR (400 MHz,
tolueneꢀd8):
δ
= 7.68 (m, 2H, py), 5.33 (m, 2H, py), 3.01 (m,
2H, NH2). 13C NMR (100 MHz, tolueneꢀd8):
= 155.4 (Ar N),
149.3 (Ar F), 146.8 (Ar F), 145.7 (py), 138.4 (Ar F), 135.9
(Ar = –
F), 107.9 (py). 19F NMR (376 MHz, tolueneꢀd8):
5.86 (br s, 2H, NH2), 3.01 (s, 3H, OCH3). 13C NMR (100 MHz,
tolueneꢀd8):
141.7 (Ar
136.7 (Ar
54.9 (O
H3). 19F NMR (376 MHz, tolueneꢀd8):
δ
= 159.9 (Ar
F), 139.8 (Ar
F), 126.7 (Ar
C
C
C
O), 149.1 (Ar
F), 138.7 (Ar
H), 123.5 (Ar
C
C
C
F), 147.2 (Ar
F), 137.2 (Ar
H), 114.6 (Ar
C
C
C
F),
F),
H),
δ
C
C
C
C
C
C
C
δ
3
3
131.8 (d, JFF = 18 Hz, oꢀArCF), –156.8 (t, JFF = 20 Hz, pꢀ
ArCF), –163.3 (m, mꢀArCF). 11B NMR (128 MHz, tolueneꢀd8):
C
δ = –133.0 (d,
3JFF = 21 Hz, oꢀArCF), –155.1 (t, 3JFF = 20 Hz, pꢀArCF), –162.5
(m, mꢀArCF). 11B NMR (128 MHz, tolueneꢀd8):
δ
= –5.6 (br, s).
δ
= –4.6 (br, s). Anal. Calcd for C231H6BF15N2: C, 45.58; H,
1.00; N, 4.62. Found: C, 45.70%; H, 0.96%; N, 4.75%.
10 | J. Name., 2012, 00, 1-3
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1
Data for 1i: Isolated yield 90% (210 mg). H NMR (400 MHz,
tolueneꢀd8):
= 4.16 (br s, 2H, NH2), 2.12 (m, 2H, CH2), 0.78 158.94 (
(m, 4H, CH2), 0.60 (m, 3H, CH3). 13C NMR (100 MHz, 145.28 (Ar
tolueneꢀd8):
= 149.5 (Ar F), 147.4 (Ar F), 141.8 (Ar F), 131.1 (Ar
139.7 (Ar N), 138.8 (Ar F), 136.7 (Ar F), 115.6 (Ar B), 46.6 (br,
44.7 ( H2), 31.5 ( H2), 19.5 ( H2), 13.2 (
H3). 19F NMR (376 21.4 ( H3). 19F NMR (376 MHz, tolueneꢀd8):
MHz, tolueneꢀd8): = –134.5 (d, 3JFF = 21 Hz, oꢀArCF), –155.5 ArCF), –122.3 (m, ArCF), –124.4 (m, ArCF), –127.8 (m,
(t, m, pꢀArCF), –162.5(m, mꢀArCF). 11B NMR (128 MHz, ArCF), –128.5 (m, ArCF), –133.9 (m, ArCF), –152.3 (t, JFF
C δ =
H3), 0.20 (m, 6H, CH3). 13C NMR (100 MHz, tolueneꢀd8):
δ
C
N3) 150.8 (Ar
), 141.4 (Ar
H), 130.5 (Ar
H), 44.7 (br,
C
C
C
F), 149.6 (Ar
F), 138.9 (Ar
H), 126.7 (Ar
C
C
C
F), 147.21 (Ar
F), 136.3 (Ar
H), 126.6 (Ar
H3), 21.8 (br,
= –120.8 (m,
C
C
C
F),
F),
H)
C
δ
C
C
C
C
C
C
C
C
C
C
C
H), 22.8 (br,
C
C
H3),
C
C
δ
C
C
δ
3
=
=
3
3
tolueneꢀd8):
δ = –7.7 (br, s). Anal. Calcd for C22H11BF15N: C, 20 Hz, ArCF), –152.9 (t, JFF = 20 Hz, ArCF), –153.3 (t, JFF
45.16; H, 1.89; N, 2.39. Found: C, 45.23%; H, 1.92%; N, 20 Hz, ArCF), –157.9 (m, ArCF), –159.0 (m, ArCF), –159.5
2.35%.
(m, ArCF), –159.9 (m, ArCF), –160.2 (m, ArCF). 11B NMR
Data for 1j: Isolated yield 92% (210 mg). H NMR (400 MHz, (128 MHz, tolueneꢀd8):
= –7.3 (br, s). Anal. Calcd for
tolueneꢀd8):
= 3.99 (br s, 2H, NH2), 2.39 (m, 1H, C ), 0.31 C31H21BF15N3: C, 50.91; H, 2.89; N, 5.75. Found: C, 50.80%;
(d, J = 6.0 Hz, 6H, CH3). 13C NMR (100 MHz, tolueneꢀd8):
149.3 (Ar F), 147.5 (Ar F), 141.5 (Ar F), 139.9 (Ar
138.8 (Ar F), 115.7 (Ar B), 48.8 ( H), 21.8
= –133.0 (d, JFF
1
δ
δ
H
δ
=
H, 2.85%; N, 5.85%.
C
C
C
C
C
δ
CN),
Procedure for the study of the reactivity of bulky guanidines 3g
and 3o with B(C6F5)3 and the subsequent reactivity with
PhC≡CH
C
F), 136.4 (Ar
C
3
(
C
H3). 19F NMR (376 MHz, tolueneꢀd8):
=
3
20 Hz, oꢀArCF), –155.3 (t, JFF = 22 Hz, pꢀArCF), –162.4 (m,
mꢀArCF). 11B NMR (128 MHz, tolueneꢀd8):
Anal. Calcd for C21H9BF15N: C, 44.17; H, 1.59; N, 2.45. Young valve NMR tubes. In a typical procedure, the tube was
Found: C, 44.23%; H, 1.65%; N, 2.55%. charged in the glovebox with 0.05 mmol of 3g dissolved in
Data for 1l: Isolated yield 96% (238 mg). H NMR (400 MHz, deuterated toluene and then 0.05 mmol of
tolueneꢀd8):
= 6.74 (m, 3H, ArC ), 6.34 (m, 1H, ArC ), 6.22 tris(pentafluorphenyl)borane dissolved in deuterated toluene
(m, 1H, ArC ), 2.15 (m, 1H, N
), 1.51 (s, 3H, CH3). 13C NMR was slowly added to yield a colourless solution, which was
(100 MHz, tolueneꢀd8):
= 149.3 (Ar F), 146.9 (Ar F), 141.4 characterized by NMR spectroscopy. A similar experiment was
N), 140.2 (Ar F), 138.9 (Ar F), 135.9 carried out using 3o
δ = –7.7 (br, s). The reactions were performed under an inert atmosphere in J.
1
δ
H
H
H
H
δ
C
C
(Ar
(Ar
C
C
F), 140.5 (Ar
F), 129.6 (Ar
C
C
C
C
.
H), 121.2 (Ar
C
H), 120.1 (Ar
C
H), 39.5 To a freshly prepared mixture of 0.05 mmol of 3g, or 3o, and
= –135.6 0.05 mmol of tris(pentafluorophenyl)borane, in CD2Cl2, was
(
CH), 35.1 (
C
H3). 19F NMR (376 MHz, tolueneꢀd8):
δ
3
3
(d, JFF = 14 Hz, oꢀArCF), –159.2 (t, JFF = 20 Hz, pꢀArCF), – added 0.05 mmol of PhC≡CH. The resulting red solution was
164.3 (m, mꢀArCF). 11B NMR (128 MHz, tolueneꢀd8):
(br, s). Anal. Calcd for C25H9BF15N: C, 48.50; H, 1.47; N, 2.26. Data for 2g: NMR (400 MHz, tolueneꢀd8):
Found: C, 48.60%; H, 1.55%; N, 2.20%. ArC ), 6.77 (m, 3H, ArC ), 6.46 (br s, 1H, N
Data for 1m: Isolated yield 91% (217 mg). 1H NMR (400 MHz, 1H, N
), 4.25 (m, 2H, C ), 2.60 (m, 2H, C ), 1.45 (br m, 3H,
tolueneꢀd8):
= 5.19 (m, 1H, N ), 2.91 (m, 2H, CH2), 1.74 (m, H3), 1.17 (br m, 3H, CH3), 0.98 (br m, 6H, CH3), 0.75 (br m,
2H, CH2), 0.99 (m, 4H, CH2), 0.52 (m, 2H, CH2). 13C NMR 3H, CH3), 0.68 (br m, 3H, CH3), 0.57 (br m, 3H, CH3), 0.43 (br
(100 MHz, tolueneꢀd8): F), 147.2 m, 3H, CH3). 13C NMR not reported due to sample instability in
= 150.6 (Ar F), 148.2 (Ar
F), 144.3 (Ar F), 139.5 (Ar = –126.3 (m,
F), 138.5 solution. 19F NMR (376 MHz, tolueneꢀd8):
F), 50.1 ( H2), 44.5 ( H2), 25.5 ( H2), ArCF), –131.1 (m, ArCF), –133.9 (m, ArCF), –156.4 (t, JFF
H2). 19F NMR (376 MHz, tolueneꢀd8):
= –127.5 (m), – 20 Hz, ArCF), –157.3 (t, JFF = 20 Hz, ArCF), –162.1 (m,
142.7 (m), –154.0 (m), –156.0 (m), –160.8 (m), –162.6 (m). 11
ArCF), –162.5 (m, ArCF), –163.4 (m, ArCF), –164.2 (m,
NMR (128 MHz, tolueneꢀd8): = –5.3 (br, s). Anal. Calcd for ArCF), –164.6 (m, ArCF). 11B NMR (128 MHz, tolueneꢀd8):
C23H11BF15N: C, 46.26; H, 1.86; N, 2.35. Found: C, 46.34%; H, = –6.8 (br, s).
δ
= –3.5 allowed to stand overnight, after which it had turned yellow.
= 6.91 (m, 2H,
), 4.55 (br s,
δ
H
H
H
H
H
H
δ
H
C
δ
C
C
(Ar
(Ar
22.6 (
C
C
F), 146.1 (Ar
F), 136.1 (Ar
C
C
C
C
C
δ
3
C
C
δ
C
=
3
B
δ
δ
1.95%; N, 2.26%.
Data for 4g: NMR (400 MHz, CD2Cl2):
ArC ), 6.50 (br s, 1H, N
), 3.90 (br s, 1H, C ), 3.65 (br s, 1H, C
), 1.39 (m, 6H, CH3), 1.20 (m, 12H, CH3), 1.11 (m,
(5 mL) was added a solution of [B(C6F5)3] (239 mg, 0.45 6H, CH3). 13C NMR (100 MHz, CD2Cl2):
= 151.8 ( N3),
mmol) in toluene (5 mL). The reaction mixture was stirred for 1 149.4 (Ar F), 146.9 (Ar F), 146.5 (Ar ), 139.4 (Ar F), 137.8
F), 136.9 (Ar F), 135.3 (Ar F), 131.9 (Ar
H), 130.5 (Ar ), 128.8 (Ar ), 128.3 (Ar
H), 127.3 (Ar F), 125.8 (Ar ), 125.2 (Ar
), 83.4 ( ≡C), 76.9 ( ≡C), 45.0 (br, H), 28.6 (
H3), 23.0 (br, H3), 22.3 (br,
H3). 19F NMR (376 MHz,
), 6.66 CD2Cl2):
= –132.8 (d, 3JFF = 26 Hz, oꢀArCF), –163.8 (t, 3JFF
δ
=7.54–7.08 (m, 8H,
H), 4.35 (br s, 1H,
H
H
), 5.42 (br s, 1H, N
Synthesis of 2a
N
H
H
H
), 2.85 (sept, J = 7
To a solution of 3a in toluene (100 mg, 0.45 mmol) in toluene Hz, 2H, C
H
δ
C
C
C
C
C
h. The solvent was removed under vacuum and the resulting (Ar
white solid was washed with cold hexane. Isolated yield 89% (Ar
(293 mg). Xꢀray quality crystals were obtained from a (Ar
C
C
C
C
C
C
C
C
C
C
CH), 131.0
CH), 127.8
CH), 122.0
toluene/hexane solution of the product and cooling to –35 ºC
.
(Ar
), 6.95 (br,
C
C
C
CH3), 23.7
1H NMR (400 MHz, tolueneꢀd8):
(m, 1H, ArC ), 6.86 (m, 1H, ArC
(m, 1H, ArC ), 5.41 (m, 1H, N ), 3.20 (m, 1H, N
1H, C ), 2.49 (m, 1H, C ), 0.61 (m, 3H, CH3), 0.42 (m, 3H, CD2Cl2):
δ
H
= 7.35 (m, 1H, ArC
), 6.80 (m, 1H, ArC
H
H
C
C
C
H
H
δ
=
H
H
), 3.14 (m, 18 Hz, pꢀArCF), –167.4 (m, mꢀArCF). 11B NMR (128 MHz,
H
H
δ
= –20.9 (br, s).
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 11

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View Article Online
ARTICLE
Journal Name
DOI: 10.1039/C6DT01237J
Data for 4o: NMR (400 MHz, CD2Cl2):
δ =7.55 (t, J = 7.8 Hz, for systems with B atoms involved39 The initial estimates for
3H, ArC
ArCH), 7.37 (m, 3H, ArCH), 6.27 (s, 3H, N
6.4 Hz, 6H, C ), 1.48 (d, J = 6.4 Hz, 18H, CH3), 1.18 (d, J = selected level of theory. An initial conformational analysis was
6.4 Hz, 18H, CH3). 13C NMR (100 MHz, CD2Cl2) partial:
carried out for all the species to obtain the lowest energy
153.2 ( N3), 147.5 (Ar F), 145.3 (Ar F), 137.3 (Ar F), 136.1 conformation. The nature of the stationary points was assessed
(Ar F), 135.9 (Ar F), 27.6 ( H), 23.9 ( H3), 20.7 (
H3). 19F by means of the normal vibration frequencies calculated from
NMR (376 MHz, CD2Cl2): = –132.6 (m, oꢀArCF), –163.9 (t, the analytical second derivatives of the energy. The firstꢀorder
3JFF = 22 Hz, pꢀArCF), –167.4 (m, mꢀArCF). 11B NMR (128 saddle points, which are related to transition states, must show
H
), 7.51 (m, 2H, ArCH), 7.43 (d, J = 77.8 Hz, 6H, the different intermediates were generated from the
H
), 3.11 (sept, J = crystallographic structures. Geometries were optimized at the
H
δ
=
C
C
C
C
C
C
C
C
C
δ
MHz, CD2Cl2):
δ
= –20.9 (br, s).
an imaginary value for the frequency associated with the
eigenvector primarily describing the product formation step;
whereas the real minima of the potential energy hypersurface,
which are related to stable species, have to show positive real
Xꢀray crystal structure determination
Suitable colourless crystals of guanidine 3a and borane adducts values for all the vibrational frequencies. In addition, Intrinsic
1b and 2a were structurally characterized by singleꢀcrystal Xꢀ Reaction Coordinate, IRC, calculations at M06ꢀ2X/6ꢀ31G*
ray diffraction. Crystals were coated with mineral oil and level were carried out to assess each reaction pathway. Gibbs
mounted on a MiTeGen MicroMount. The samples were free energies used were obtained as the sum of electronic and
transferred to a Bruker D8 KAPPA series II diffractometer with thermal free energy at 298.15 K.
APEX II areaꢀdetector system equipped with graphite
monochromated Mo Kα radiation (λ = 0.71073 Å). Collection
Acknowledgements
and data details for all of the crystals can be seen in the
Supporting Information material. After data collection and
integration with the Bruker SAINT software package,33
absorption corrections (SADABS)34 were applied to the
collected data as well as corrections for Lorentz and
polarization effects. The software package Bruker SHELXTL35
was used for space group determination, structure solution and
refinement. The space group determination was based on a
check of the Laue symmetry, and systematic absences were
confirmed using the structure solution. The structures were
solved by direct methods (SHELXSꢀ97), completed with
We gratefully acknowledge financial support from the
Ministerio de Economía y Competitividad (MINECO), Spain
(Grant No. CTQ2011ꢀ22578 and Grant No. CTQ2013ꢀ48411ꢀ
P), and Junta de Comunidades de CastillaꢀLa Mancha (Grants
No. PEIIꢀ2014ꢀ041ꢀP, PEII11ꢀ0220ꢀ5740, PEII11ꢀ0279ꢀ8538,
and PEIIꢀ2014ꢀ043ꢀP). The authors thank Professor Douglas
Stephan (University of Toronto) for very helpful discussion
regarding FLP chemistry. We also would like to thank
Professor Isabel Cuadrado and Dr. Josefina Perles (Universidad
Autónoma de Madrid) for their help with the Xꢀray diffraction
studies.
different Fourier syntheses, and refined with fullꢀmatrix leastꢀ
2
squares using SHELXLꢀ97 minimizing
Weighted
F2; conventional
ω
(
Fo
–
Fc2)2.36,37
Notes and references
R
factors (Rw) and all goodness of fit S are based on
factors ( ) are based on . All nonꢀ
R
R
F
a
Centro de Innovación en Química Avanzada (ORFEOꢀCINQA),
hydrogen atoms were refined with anisotropic displacement
parameters. The hydrogen atom positions were calculated
geometrically and were allowed to ride on their parent carbon
Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad
de Ciencias y Tecnologías Químicas, Universidad de CastillaꢀLa Mancha,
Campus Universitario, Eꢀ13071 Ciudad Real, Spain.
b
Centro de Innovación en Química Avanzada (ORFEOꢀCINQA),
or nitrogen atoms with fixed isotropic
and anomalous dispersion factors are contained in the
SHELXTL 6.10 program library. The crystal structures of 3a
U. All scattering factors
Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad
de Farmacia, Universidad de CastillaꢀLa Mancha, Campus Universitario
de Albacete, 02071ꢀAlbacete, Spain.
c Departamento de QuímicaꢀFísica, Facultad de Farmacia, Universidad de
CastillaꢀLa Mancha, Campus Universitario de Albacete, 02071ꢀAlbacete,
Spain.
,
1b and 2a have been deposited at the Cambridge
Crystallographic Data Centre with deposition numbers CCDC
1451327ꢀ1451329. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
d
Departamento de QuímicaꢀFísica, Facultad de Ciencias y Tecnologías
Químicas, Universidad de CastillaꢀLa Mancha, Campus Universitario, Eꢀ
13071 Ciudad Real, Spain.Electronic
Computational methodology
Supplementary Information (ESI) available:
DFT calculations were carried out using the Gaussian09 (Rev.
C.01) set of programs.38 The geometries of all species involved
were optimized at the hybrid metaꢀGGA M06ꢀ2X functional,
along with the 6ꢀ31G* basis set. M06ꢀ2X is especially
recommended for thermodynamic and kinetic calculations, and
has been proven to give reliable results to the structural and
energetic properties of nonꢀcovalent systems and reaction
energy barriers. Besides, this approach has been widely used
Cartesian coordinates of the optimized geometries.
Crystalographic data for compounds 1b, 2a and 3a, and cif files.
See DOI: 10.1039/b000000x/
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Dalton Transactions
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14 | J. Name., 2012, 00, 1-3
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