Experimental and computational studies of anti-bredt amidinium salts

Article abstract of DOI:10.1002/chem.201302474

Experimental and computational investigations of anti-Bredt amidinium salts are presented. Calculations show that the pyramidalization of an amino group can significantly destabilize the formal carbocation center of amidiniums, due to the decreased π donation. In some cases, the unfavorable -I effect of nitrogen surpasses its beneficial +M effect, and amidiniums become less stable than iminiums. It is shown that although 1-aza-3-azonia[3.3.1]bicyclo-non-2-enes can be isolated, they feature a nonclassical reactivity, which is more typical for iminium than amidinium salts, such as pronounced electrophilicity and azomethineylide instead of carbene formation. Copyright

Full text of DOI:10.1002/chem.201302474

FULL PAPER
DOI: 10.1002/chem.201302474
Experimental and Computational Studies of Anti-Bredt Amidinium Salts
David Martin, Nicolas Lassauque, Florian Steinmann, Gerald Manuel, and
Guy Bertrand*[a]
Abstract: Experimental and computa-
tional investigations of anti-Bredt ami-
dinium salts are presented. Calcula-
tions show that the pyramidalization of
an amino group can significantly desta-
bilize the formal carbocation center of
amidiniums, due to the decreased p
donation. In some cases, the unfavora-
ble -I effect of nitrogen surpasses its
beneficial +M effect, and amidiniums
become less stable than iminiums. It is
shown that although 1-aza-3-azonia-
AHCTUNGTRENNG[UN 3.3.1]bicyclo-non-2-enes can be isolat-
ed, they feature a nonclassical reactivi-
ty, which is more typical for iminium
than amidinium salts, such as pro-
nounced electrophilicity and azomethi-
neylide instead of carbene formation.
Keywords: azomethineylides ·
carbenes · carbocations · strained
molecules · substituent effects
Introduction
center, it was predicted that the remaining inductive effect
of nitrogen atoms would result in a functional group featur-
ing both a “superelectrophilic” carbon center and basic
amino groups, which would allow dihydrogen activation.[7]
Recently, we described the synthesis of 1a, the first amidi-
nium salt with a strained bridgehead amino group and dem-
onstrated that the corresponding carbene 2a retains the
strong nucleophilicity of classical diaminocarbenes,[8] but
with a significantly enhanced electrophilicity.[9] Herein, we
report detailed experimental and computational studies of
anti-Bredt amidinium salts. We showcase the fundamental
differences with their nonstrained counterparts and their
consequences, in particular for the extension of the concept
of stable anti-Bredt N-heterocyclic carbenes (NHCs).
The general reluctance, and sometimes the impossibility, for
bridgehead carbon=carbon double-bond formation in a
strained polycyclic structure is known as the Bredtꢀs rule.[1]
BicycloACHTUNGTRENNUNG[2.2.2] oct-1-enes I, which feature a pyramidalized
double-bonded carbon are classical examples (Scheme 1).
Scheme 1.
They undergo immediate retro Diels–Alder ring opening[2]
and have only been observed as encapsulated species.[3] The
implications of Bredtꢀs rule are not limited to carbon=
carbon double bonds. The N=C partial double bonds of
amides and ureas, which are usually very robust, become
very sensitive towards hydrolysis and polymerization when
featuring a nitrogen in a bridgehead position.[4,5] For amidi-
nium salts, Berkessel and Thauer[6] showed computationally
that derivatives II with nonplanar amino groups would sig-
nificantly be more electrophilic than their known planar
counterparts III. In the extreme case, when the nitrogen
lone pairs are totally prevented to interact with the carbon
Results and Discussion
We synthesized 1-aza-3-azoniaACTHNUTRGENUGN[3.3.1]bicyclonon-2-enes 1b–d
following the procedure reported for 1a (Scheme 2). In the
first step, the corresponding neat ethyl formamidinate was
heated with commercially available 3-piperidinemethanol.
Then, the resulting amidines were reacted with trifluorome-
thanesulfonic anhydride in the presence of a base, which
gave triflate salts 1a–d in overall good yields. Purification of
[a] Dr. D. Martin, Dr. N. Lassauque, F. Steinmann, G. Manuel,
Prof. G. Bertrand
UCSD-CNRS Joint Research Laboratory (UMI 3555)
Department of Chemistry and Biochemistry
University of California San Diego
La Jolla, CA 92093 0343 (USA)
E-mail: guybertrand@ucsd.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302474.
Scheme 2. Synthesis of 1-aza-3-azoniaACTHNUTRGENUGN[3.3.1]bicyclonon-2-enes salts 1a–d.
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amidinium salts 1a–d involved washing with water, and
therefore they are stable toward air and moisture.
the bridgehead nitrogen is sufficient to alter the electronic
properties of the carbenium center of 1a–d, as shown by the
reaction of 1a with ammonia borane, a mild hydride
donor.[14] The reaction occurred at low temperature in di-
chloromethane affording aminal 3 almost quantitatively
(Scheme 3). In marked contrast, nonstrained amidi-
As shown in Table 1, amidiniums1a–d feature similar key
spectroscopic and structural parameters. The 13C (d=163–
1
166 ppm) and H NMR (d=8.35–8.52 ppm) chemical shifts
nium 4 is inert towards H3N·BH3, even after a night
at 408C.
The “nonclassical” reactivity of amidinium 1a
Table 1. Solid-state structure of 1a (left), 1c (middle); and 1d (right) with hydrogen
atoms and triflate anions omitted for clarity. Selected NMR data for 1a–d and select-
ed bond lengths [pm] and angles [8] for 1a, c, and d.
also came to the fore when attempting to generate
carbene 2a. A strong base, such as n-butyllithium,
does not deprotonate 1a, but instead directly adds
to the carbenium center giving adduct 5. Even
though potassium hexamethyldisilazide deproto-
nates 1a at À788C, the resulting hexamethyldisila-
zane and carbene 2a reacts together when warming
the reaction mixture to room temperature to form
adduct 6a.[9a] Interestingly, this reaction is reversi-
ble, because according to NMR spectroscopy data,
dissolving crystals of 6a in benzene gives back a
small amount of 2a and hexamethyldisilylazane.[9b]
Compound NMR spectroscopy data[a]
XRD data
À
d 1H [ppm]
d
13C [ppm] N1 C1 N2 C1 N1 CN2 f [8] t [8]
À
À
122.7
–
121.6
123.5
1a
1b
1c
1d
8.46
8.35
8.35
8.52
166.2
165.9
166.0
163.0
131.0
–
134.1
–
25.6 33.8
–
–
130.7
127.0
134.1
134.2
24.7 36.2
24.5 29.0
[a] Chemical shifts of the N2CH+ moieties in CDCl3.
of the carbenium moiety are intermediate between those of
nonbridged cyclic formiminiums (d=180–190 and 9–10 ppm,
respectively)[10] and formamidiniums (d=150–162 and 7–
9 ppm, respectively).[11] X-ray diffraction analyses of 1a, 1c,
and 1d showed the distortion around the bridgehead nitro-
gen. By analogy with the Winkler–Dunitz parameters of
amides,[12] we consider two descriptors for the geometry of
amidiniums featuring one nonplanar amino group: 1) the di-
hedral angle t between the nitrogen lone pair and the
formal empty p orbital of the carbenium center; 2) the bend
angle f (Figure 1). There is a significant twist around the
Scheme 3. Reactivity of 1a with nucleophiles and KHMDS.
We then investigated the deprotonation of salts 1b and c,
but the 13C NMR spectra of the crude reactions with
KHMDS revealed the formation of complex mixtures of
products. In the case of the 1-adamantyl-substituted amidini-
um 1d, the reaction with KHMDS led to the formation of
adduct 6d (Scheme 4). The corresponding carbene could not
be observed, even when monitoring the reaction by
13C NMR spectroscopy at low temperature (À658C). This
result suggests a direct addition of the disilylamide to the
electrophilic center of 1d rather than a two-step deprotona-
Figure 1. Cram representation (left) and Newmann projection (center
and right) of an amidinium with a nonplanar amino group. The twist
angle t and bend angle f are deduced from dihedral angles w1 and w2:
f={(180Àw1Àw2)/2} and t=(w1 +f).
À
N2 C1 bond (t=29–368), and the bridgehead nitrogen of
1a, c–d is clearly not in a planar environment with f=24–
268, in the range of the values found for related bicyclic N-
bridgehead amines.[13] As a result, the N2 C1 bonds (d=
tion/N H insertion mechanism, which was evidenced for 1a.
À
À
À
134.1–134.2 pm) are longer than the N1 C1 bonds (d=127–
However, similarly to 6a, adduct 6d could be used as a car-
bene source. Indeed, it cleanly reacts after a night at room
temperature with 0.5 equivalent of cyclo-octadiene–iridiu-
m(I) chloride dimer to give the iridium complex 7.[15]
131 pm), but remain short. The latter observation indicates
that the p donation of the distorted nitrogen is still signifi-
cant.
As a consequence of the remaining p donation of the lone
pair of the bridgehead nitrogen, salt 1a is inert towards elec-
trophiles, such as trifluoromethanesulfonic acid or methyl
trifluoromethanesulfonate. However, the distortion around
When LiHMDS is used instead of KHMDS, a totally dif-
ferent reaction was observed. After work-up, amidine 10
was isolated in 78% yield and fully characterized, including
an X-ray crystallographic study.[16] The formation of 10 can
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Studies of Anti-Bredt Amidinium Salts
FULL PAPER
Figure 3. Energy of parent amidinium (H2N)2CH+ as a function of t and
f relative to the all-planar form (t=f=08).
be explained by a three-step mechanism: a) deprotonation
at a NCH2 carbon leading azomethineylide 8; b) addition of
the ylide to the carbenium center of a second equivalent of
1a giving 9; c) ring opening of 9 resulting from the attack of
a second equivalent of LiHMDS (Scheme 4). The difference
in the deprotonation site seems to be correlated with the
softness of the countercations of hexamethyldisilazide.
Indeed, when NaHMDS was used, a 2:3 mixture of 6a and
10 was obtained.
Figure 2. Energy of azomethineylides IV’, V’, and VI’, related to their
carbene isomers IV, V, and VI, respectively; optimized geometry and rep-
resentation of the highest occupied molecular orbital of azomethineylides
IV’–VI’.
The formation of azomethineylides by deprotonation of
iminium salts is well documented.[17] But to our knowledge,
the existence of amino-substituted azomethineylides, not
stabilized by electron-withdrawing substituents, has never
been evidenced. This is not surprising, because such com-
pounds are significantly destabilized by the strong p donor
amino substituent.[18] Calculations at the B3LYP/6-31G*
level of theory confirmed this intuitive statement
(Figure 1).[19] Indeed, six-membered ring diaminocarbene IV
was found to be significantly more stable than its isomeric
ylide IV’ (+18.9 kcalmolÀ1), whereas anti-Bredt carbene V
and cyclic alkyl aminocarbene VI are energetically close to
the corresponding ylides V’ and VIꢀ’ (+1.7 and À2.9 kcal
molÀ1, respectively). The destabilization of the ylide by +M
groups is evident when considering the LUMO of IV’,
which can be seen as an anti-bonding perturbation of the
LUMO of VI’ by the p orbital of an amino substituent.
Anti-Bredt ylide V’ is clearly intermediate between these
two cases. Note that, as expected, the enhanced +M dona-
Scheme 4. Formation of 6d, 7, and 10.
donation of the twisted amino group. Indeed, deformation
around the nitrogen also results in a change of the steric
interactions between the
N substituents. For example,
when considering the N-methyl-substituted compounds
(Me2N)2CH+ (red dashed line in Figure 4), the minimal
energy does not correspond to the all planar geometry (t=
f=08), which is optimal for p conjugation, but to a slightly
twisted structure (t=158), which allows moving apart two
methyl substituents. Similarly, the increase of energy when
increasing f is more significant with the larger methyl N
substituents.
Isodesmic reactions of Scheme 5 give a better insight into
the contribution of the amino groups to the p system of ami-
dinium salts. We chose nonstrained cyclic formamidinium
IX as a standard. We inferred from Equation (1) that the
presence of the first amino group stabilizes the carbocation
center by E1ref =À54.8 kcalmolÀ1. According to Equa-
À
tion to the ylide, from VI’ to IV’, results in longer C N
bonds and a pyramidalization of the formal carbanionic cen-
ters.
Then, we studied the impact of the distortion of the
bridgehead nitrogen on its p donation. The energy of the
parent amidinium (H2N)2CH+ as a function of the twist
angle t and the bend angle f is shown in Figures 2 and 3. As
expected, t has a significant influence on the energy, which
rises up to 30 kcalmolÀ1 upon complete twisting (t=908).
Increasing the bend angle f has also an energetic cost, but
to a lesser extent. Note that, although informative, this ap-
proach does not allow the quantitative assessment of the p
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G. Bertrand et al.
Table 2. Calculations on anti-Bredt amidiniums at the B3LYP/6-31G*
level of theory.
Entry
Amidinium[a]
t
f
[8]
E3
Relative
[8]
[kcalmolÀ1
]
stabilization[b]
1
2
24
33
24
33
À10.4
À2.8
44%
12%
3
4
5
6
23
32
31
26
23
32
31
26
+1.2
+5.5
+0.3
+5.6
À5%
À23%
À1%
À24%
[a] See data in the Supplementary Information. [b] Nonstrained amidini-
um IX was chosen as a reference, the relative stabilization was thus cal-
culated as: (E3/E2ref)ꢂ100; a negative value corresponds to a destabilizing
effect on the carbenium center, compared with an alkyl substituent.
Figure 4. Energetic cost for the deformation of one amino group in a
planar amidinium salt with hydrogen (plain line) and methyl (dashed
line) N substituents. Top: increasing twist angle t with a planar environ-
ment (bend angle f=08); bottom: increasing f with t=08.
IX (À23.7 kcalmolÀ1). As was expected, entries 1–4
(Table 2) show that the p donation of the bridgehead amino
group decreases when the twist angle t increases. The effect
is even more pronounced when both nitrogens are part of a
more strained five-membered ring (compare entries 1–4 and
entries 5–6, Table 2). Importantly, in most of the examples,
E3 is positive. This means that the replacement of an alkyl
group by a bridgehead amino destabilizes the formal carbo-
cation center because the p donation is too weak to counter-
balance the ÀI effect of nitrogen.
Conclusion
Scheme 5. Isodesmic reactions for the evaluation of the stabilization of
carbocations by amino groups.
The introduction of a twisted amino group can significantly
destabilize the formal carbocation center of amidiniums due
to the decreased p donation. In the extreme case, the unfav-
orable -I effect of nitrogen surpasses the beneficial +M
effect, and amidinium salts become less thermodynamically
tion (2), the presence of the second amino group only con-
tributes to an additional E2ref =À23.7 kcalmolÀ1. Such results
are in line with previous calculations on acyclic amidini-
ref
ums.[20] Note that E2 is in the same order of magnitude as
stable than iminium salts. The 1-aza-3-azonia
ACHUTGTNREN[UNG 3.3.1]biACHTUNGTRENNUNGcyclo-
the energetic costs for the bending and twisting of an NH2
group in the parent (H2N)2CH+. This is coherent with the
expected critical impact of such deformation on the p dona-
tion of the nitrogen. The first isodesmic reaction [Eq. (1)] is
not relevant for anti-Bredt amidiniums, because most bicy-
clic carbocations, which would correspond to VII, exist as
nonclassical bridged species.[21] However, comparing Equa-
tions 2 and 3 allows evaluating the stabilization resulting
from p donation of the bridgehead nitrogen.[22] As shown in
Table 2, this value is equal to À10.4 kcalmolÀ1 for 1-aza-3-
ACHTUNGTRENnNUGN on-2-enes 1 are borderline cases, in which the bridgehead
nitrogen still contributes moderately to the stabilization. Al-
though these amidinium salts can be isolated, they feature a
nonclassical reactivity, which is more typical for iminium
than amidinium salts, such as pronounced electrophilicity
and azomethineylide formation. From these results, it is
clear that the isolation of amidinium salts featuring a more
pyramidalized amino group than 1 will be challenging.[23]
However, the synthesis of transition-metal complexes with
the corresponding carbenes as ligand is conceivable, provid-
ing to use nonamidinium precursors, which is under active
investigation in our laboratory.
azonia
ACHTUNGTRENNUNG[3.3.1]bicyclonon-2-enes (Table 2, entry 1). This is
only 44% of the calculated value for nonstrained amidinium
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Studies of Anti-Bredt Amidinium Salts
FULL PAPER
(sept, J=7 Hz, 1H), 2.55–2.40 (m, 3H), 2.15 (m, 1H), 1.94 (m, 1H), 1.70
(m, 2H), 1.26 (t, J=7 Hz, 3H) 1.15 ppm (t, J=7 Hz, 3H); 13C NMR
(CDCl3, 75 MHz): d=165.9 (CH), 140.6 (C), 138.3 (C), 135.3 (C), 130.6
(CH), 127.1 (CH), 126.9 (CH), 58.4 (CH2), 51.6 (CH2), 48.5 (CH2), 29.5
(CH), 24.1 (CH2), 23.4 (CH2), 22.8 (CH2), 14.6 (CH3), 14.1 ppm (CH3);
MS (FAB): m/z calcd for C17H25N2: 257.2018 [M]+; found: 257.2036.
Experimental Section
General: Experiments were performed under an atmosphere of dry
argon by using standard Schlenk techniques, unless otherwise stated. Sol-
vents were dried by standard methods and distilled under argon. 1H and
13C NMR spectra were recorded on Varian Inova 400 and 500 and
Bruker 300, 400, and 500 MHz spectrometers. NMR multiplicities are ab-
breviated as follows: s=singlet, d=doublet, t=triplet, sept=septet, m=
multiplet, br=broad signal. Melting points (uncorrected) were measured
with an Electrothermal MEL-TEMP apparatus. Ethyl N-arylformamidi-
nates were synthesized from the corresponding aniline and triethylortho-
formate, according to published procedures.[24]
Synthesis of N-1-adamantylformamide:[25] The preparation was improved
by adapting the formylation procedure of Huffman.[26] A mixture of
formic acid (4.5 mL, 120 mmol) and acetic anhydride (5.66 mL, 60 mmol)
was stirred one hour at room temperature, and then added dropwise to a
stirred solution of 1-adamantamine (6.86 g, 45.4 mmol) in dichlorome-
thane at À788C. After one night at room temperature, the solution was
poured into an oversized flask (1 L), and a saturated aqueous solution of
K2CO3 was slowly added under vigorous stirring, until the aqueous layer
became basic again (pH >9). The aqueous layer was extracted with di-
chloromethane (two times). The organic layers were combined, dried
over magnesium sulfate, and evaporated under vacuum. N-1-adamantyl-
formamide was obtained as a white powder (yield: 8 g, 99%).
Synthesis of N-2,4,6-trimethylphenyl-substituted amidinium salt 1c: The
general procedure was followed with ethyl N-2,4,6-trimethylphenylforma-
midinate (2.3 g, 19.4 mmol). The intermediate N-2,4,6-trimethylphenyl
substituted amidine was obtained as a yellow oil. 1H NMR (CDCl3,
300 MHz): d=7.14 (s, 1H), 6.83 (s, 2H), 4.5 (broad, 1H), 3.61 (m, 2H),
3.46 (m, 2H), 3.2–3.0 (m, 2H), 2.24 (s, 3H), 2.12 (s, 6H), 1.79 (m, 2H),
1.66 (m, 1H), 1.52 (m, 1H), 1.33 (m, 1H) ppm; 13C NMR (CDCl3,
75 MHz): d 153.5 (CH), 146.9 (C), 131.5 (C), 129.8 (C), 128.6 (CH), 63.7
(CH2), 48 (br, CH2), 37.8 (br, CH2), 27.1 (CH2), 24.4 (CH2), 20.7 (CH3),
18.6 ppm (CH3); MS (FAB): m/z calcd for C16H25N2O: 261.1967 [M+H]+
; found: 261.1967. Amidinium salt 1c was purified as described for 1b,
giving colorless crystals (yield: 5.3 g; 70%). M.p. 97–998C; 1H NMR
(CDCl3, 300 MHz): d=8.40 (s, 1H), 6.95 (s, 1H), 6.93 (s, 1H), 4.26 (br,
1H), 3.79 (dd, J=15 and 6 Hz, 1H), 3.7–3.3 (m, 3H), 3.21 (d, J=12 Hz,
1H), 3.04 (m, 1H), 2.28 (s, 6H), 2.16 (s, 3H), 1.94 (m, 1H), 1.67 (m, 2H),
1.37 ppm (m, 1H); 13C NMR (CDCl3, 75 MHz): d=166.2 (CH), 140.7
(C), 134.8 (C), 134.4 (C), 132.8 (C), 130.7 (CH), 130.0 (CH), 57.8 (CH2),
51.9 (CH2), 48.8 (CH2), 29.8 (CH), 24.4 (CH2), 24.0 (CH2), 21.0 (CH3),
17.4 (CH3), 17.2 ppm (CH3); MS (FAB): m/z calcd for C16H23N2 [M]+;
243.1861; found: 243.1866.
Synthesis of ethyl N-1-adamantylformamidinate: A solution of N-1-ada-
mantylformamide (4.9 g, 27 mmol) in dichloromethane (25 mL) was
added to triethyloxoniumtetrafluoroborate[27] (7.6 g, 32.7 mmol) at
À788C. After stirring overnight at room temperature, the solution was
poured into a saturated aqueous solution of K2CO3 (pH >9). The aque-
ous layer was extracted with dichloromethane (two times). The organic
layers were combined, dried over magnesium sulfate, and evaporated
under vacuum. The product was obtained as a yellow powder (yield:
4.8 g, 86%). 1H NMR (C6D6, 300 MHz): d=7.42 (s, 1H), 3.97 (q, J=
6 Hz, 2H), 1.99 (s, 3H), 1.6–1.5 (m, 12H), 1.17 ppm (t, J=6 Hz, 3H);
13C NMR (C6D6, 75 MHz): d=151.3 (CH), 60.6 (CH2), 53.1 (C), 44.2
(CH2), 36.5 (CH2), 29.6 (CH), 14.2 ppm (CH3); MS (FAB): m/z calcd for
C13H22NO: 208.1701 [M+H]+; found: 208.1709.
Synthesis of N-1-adamantyl-substituted amidinium salt 1d: The general
procedure was followed with ethyl N-1-adamantylformamidinate (4.63 g,
22 mmol). The crude N-1-adamantyl substituted amidine was dissolved in
hexanes. The solution was cooled to À158C, giving a microcrystalline
1
white solid (yield: 5 g; 82%). M.p.: 93–958C; H NMR (C6D6, 300 MHz):
d=7.24 (s, 1H), 3.95 (br s, 1H), 3.49–3.46 (m, 3H), 2.92 (m, 2H), 2.74
(m, 1H), 2.4–2.2 (m, 1H), 2.02 (s, 3H), 1.78 (s, 6H), 1.61 (s, 6H), 1.36
(m, 1H), 1.13 (m, 2H), 1.02 ppm (m, 1H); 13C NMR (C6D6, 75 MHz):
d=149.9 (CH), 61.7 (CH2), 53.2 (C), 50.7 (br, CH2), 47.4 (br, CH2), 45.1
(CH2), 37.0 (CH2), 30.3 (CH), 28.4 (CH), 26.5 (CH2), 23.3 ppm (CH2);
MS (FAB): m/z calcd for C17H29N2O: 277.2280 [M+H]+; found: 277.2276.
Crude amidinium1d was obtained as a white powder. It was washed with
water (three times) and with technical-grade diethylether (three times),
dissolved in CH2Cl2, and the solution was dried with sodium sulfate.
After filtration, a concentrated CH2Cl2 solution was layered with hex-
anes, and the triflate salt 1d was crystalized upon cooling at À208C over-
General procedure for the synthesis of N-bridgehead amidinium salts
1b–d: A stoichiometric mixture of 3-piperidinemethanol and ethyl N-
substituted formamidinate was stirred at 1608C for 3 h in a flask equip-
ped with a Dean–Stark condenser to remove ethanol. After completion
of the reaction, the residue was dried under vacuum. The resulting crude
amidine alcohol was dissolved in dried CH2Cl2 (30 mL) and the solution
was cooled to À788C. Diisopropylethylamine (3.1 mL, 18 mmol) was
added to the solution, followed by the dropwise addition of trifluorome-
thanesulfonic anhydride (2.5 mL, 15 mmol). The solution was stirred for
30 min at À788C and then warmed to room temperature. The volatiles
were removed under reduced pressure giving an orange solid, which was
washed with diethylether until no colored impurities remained.
1
night (yield: 6.5 g; 87% ). M.p. 192–1948C; H NMR (CDCl3, 300 MHz):
d=8.62 (s, 1H), 4.13 (m, 1H), 3.63 (dd, J=16 and 6 Hz, 1H), 3.46–3.31
(m, 3H), 2.93 (d, J=12 Hz, 1H), 2.85 (m, 1H), 2.23 (m, 3H), 2.06 (m,
6H), 1.96 (m, 1H), 1.76 (m, 1H), 1.70 (m, 6H), 1.58 (m, 1H), 1.33 ppm
(m, 1H); 13C NMR(CDCl3, 75 MHz): d=163.3 (CH), 62.9 (C), 51.6
(CH2), 49.8 (CH2), 47.8 (CH2), 40.5 (CH2), 35.4 (CH2), 29.6 (CH), 28.7
(CH), 24.3 (CH2), 22.8 ppm (CH2); MS (FAB): m/z calcd for C17H27N2:
259.2174 [M]+; found: 259.2178.
Synthesis of N-2,6-diethylphenyl-substituted amidinium salt 1b: The gen-
eral procedure was followed with ethyl N-2,6-diethylphenylformamidi-
nate (6.5 g, 31.7 mmol). The intermediate N-2,6-diethylphenyl substituted
amidine was obtained as a red oil. 1H NMR (C6D6, 300 MHz): d=7.28–
7.05 (m, 3H), 6.9 (s, 1H), 4.10 (broad, 1H), 3.40 (m, 2H), 2.73 (q, J=
5 Hz, 2H), 2.8–2.5 (m, 4H), 1.48 (m, 1H), 1.31 (t, J=5 Hz, 6H), 1.28–
0.90 ppm (m, 4H); 13C NMR (C6D6, 75 MHz): d=152.8 (CH), 136.2 (2
C), 126.6 (CH), 123.1(CH), 64.2 (CH2), 49.1 (br CH2), 45.0 (br, CH2),
38.5 (br, CH2), 27.4 (CH2), 25.5 (CH2), 24.7 (CH), 15.3 ppm (CH3); MS
(FAB): m/z calcd for C17H27N2O: 275.2124 [M+H]+; found: 275.2130.
Amidinium 1b was purified as follows: the crude solid was dissolved in
dichloromethane and washed with water (three times). The organic
phase was dried over MgSO4, and the volatiles were removed under
vacuum. The resulting sticky orange oil was dissolved in dichlorome-
thane, and diethylether was layered. Overnight, at À208C 1b was ob-
Synthesis of aminal 3: A solution of ammonia borane (35 mg, 1.14 mmol)
in dichloromethane (5 mL) was added over 5 min to a suspension of ami-
dinium1a (498 mg, 1.14 mmol) in THF (5 mL) at À788C. After warming
to room temperature, the volatiles were removed in vacuo. Extraction of
the residue with toluene (2ꢂ5 mL) gave a waxy oil (yield: 258 mg, 82%).
1H NMR (C6D6, 500 MHz): d=7.01 (t, J=7.5 Hz, 1H), 6.95 (dd, J=7.5
and 1.5 Hz, 1H), 6.88 (dd, J=7.5 and 1.5 Hz, 1H), 4.80 (d, J=11 Hz,
1H), 3.80 (d, J=11 Hz, 1H), 3.38 (m, 2H), 3.16 (m, 2H), 3.03 (d, J=
13 Hz, 2H), 2.79 (d, J=13 Hz, 1H), 2.65 (d, J=11 Hz, 1H), 2.52 (m,
1H), 1.22 (m, 4H), 1.15 (d, J=6.5 Hz, 3H), 1.05 (d, J=6.5 Hz, 3H), 1.04
(d, J=6.5 Hz, 3H), 0.96 ppm (d, J=6.5 Hz, 3H); 13C NMR (CDCl3,
75 MHz): d 20.3 (CH2), 24.2 (2 CH3), 24.3 (CH3), 25.4 (CH3), 26.9 (CH2),
28.0 (CH), 28.6 (CH), 28.9 (CH), 50.1 (CH2), 51.8 (CH2), 53.9 (CH2), 70.0
(CH2), 123.9z (CH), 125.8 (CH), 127.5 (CH), 141.8 (C), 148.5 (C),
149.5 ppm (C); MS (FAB):
1
tained as colorless crystals (yield: 9.8 g; 76%). M.p. 144–1468C; H NMR
m/z calcd for C19H30N2Na: 309.2307 [M+Na]+; found: 309.2320.
(CDCl3, 300 MHz): d=8.41 (s, 1H), 7.36 (t, J=7.5 Hz, 1H), 7.21 (d, J=
7.5 Hz, 1H), 7.14 (d, J=7.5 Hz, 1H), 4.23 (m, 1H), 3.80 (dd, J=15 and
6 Hz, 1H), 3.60–3.30 (m, 3H), 3.20 (d, J=12 Hz, 1H), 3.04 (m, 1H), 2.62
Synthesis of nBuLi adduct 5: Solution of n-butyllithium in hexanes
(2.5m; 0.537 mL, 1.34 mmol) was added to a stirred suspension of 1a
Chem. Eur. J. 2013, 19, 14895 – 14901
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14899

G. Bertrand et al.
(584 mg, 1.34 mmol) in diethyl ether 5 mL). After stirring the resulting
yellow solution 10 min at room temperature, the volatiles were removed
in vacuo, and the product was extracted with pentane (3ꢂ5 mL). The sol-
vent was removed, and the resulting oil was dried in vacuo (yield:
334 mg, 73%). 1H NMR (C6D6, 300 MHz): d=7.2–7.0 (m, 3H), 4.36 (d,
J=9.5 Hz, 1H), 3.96 (m, 1H), 3.65 (d, J=9.5 Hz, 2H), 3.04–2.60 (m,
6H), 2.52 (d, J=13 Hz, 2H), 2.22 (m, 2H), 1.80 (m, 2H), 1.31 (d, J=
5 Hz, 6H), 1.25 (d, J=6 Hz, 6H), 1.59–0.80 ppm (m, 7H); 13C NMR
(C6D6, 75 MHz): d=15.4 (CH3), 18.5 (CH2), 23.2 (CH2), 24.2 (CH3), 24.6
(CH3), 25.0 (CH3), 25.4 (CH3), 27.9 (CH), 28.2 (CH), 29.0 (CH), 31.5
(CH2), 36.2 (CH2), 47.4 (CH2), 54.0 (CH2), 55.5 (CH2), 78.6 (CH), 123.9
(CH), 124.6 (CH), 124.8 (CH), 144.1 (C), 149.7 (C), 150.0 ppm (C); MS
Acknowledgements
Thanks are due to the NSF (CHE-1316956), and DOE (DE-SC0009376)
for financial support, the Extreme Science and Engineering Discovery
Environment, which is supported by NSF (OCI-1053575; D.M.), and the
Department of Chemistry and Pharmacy of the Ludwig Maximilian Uni-
versity of Munich for a scholarship (F.S.).
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63.
ACHTUNGTRENNUNG
(FAB): m/z calcd for C23H38N2Na: 365.2933 [M+Na]+; found: 365.2921.
Synthesis of iridium complex 7: A solution of potassium hexamethyldisi-
lazide (72 mg, 0.36 mg) in THF (3 mL) was added to a suspension of ami-
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(CH), 29.8ACHTUNGTRENNUNG(3 CH), 34.1 (CH2), 36.6 (3 CH2), 39.6 (3 CH2), 42.4 (CH2),
42.6 (CH2), 50.2 (CH2), 55.7 (Cq), 83.4 ppm (N3CH). The freshly prepared
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À788C. The resulting solution was warmed to room temperature and stir-
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gave 7 as an orange powder, which was washed with pentane (2ꢂ3 mL)
and dried in vacuo (yield: 177 mg, 82%). M.p. 155–1608C (decomp);
1H NMR (C6D6, 500 MHz): d=5.53 (dd, J=12 and 6.5 Hz, 1H), 5.13 (td,
J=8 and 2.5 Hz, 1H), 4.95 (dd, J=14 and 8 Hz, 1H), 3.21 (td, J=8 and
1 Hz, 1H), 3.17 (dd, J=12 and 9 Hz, 1H), 2.93 (td, J=11 and 3 Hz, 1H),
2.77 (td, J=7.5 and 1 Hz, 1H), 2.70 (dq, J=13.5 and 1.5 Hz, 2H), 2.59
(d, J=12 Hz, 1H), 2.51–2.43 (m, 4H), 2.34–2.26 (m, 3H), 2.11 (m, 6H),
1.94–1.84 (m, 2H), 1.70 (d, J=11.5 Hz, 3H), 1.62 (d, J=11.5 Hz, 3H),
1.57–1.50 (m, 2H), 1.44–1.18 ppm (m, 5H); 13C NMR (C6D6, 125 MHz):
d=25.1 (CH2), 28.1 (CH2), 28.6 (CH2), 30.9 (3 CH), 31.6 (CH2), 32.3
(CH2), 35.3 (CH), 36.1 (CH2), 36.8 (3 CH2), 42.9 (3 CH2), 48.1 (CH), 49.7
(CH2), 51.5 (CH2), 54.9 (CH2), 60.8 (CH), 61.2 (C), 82.1 (CH), 83.4
(CH), 223.5 ppm (C); MS (FAB): m/z calcd for C25H39N2IrCl: 595.2431
[M+H]+; found: 595.2446.
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Synthesis of amidine 10: Diethylether was added to a solid mixture of
amidinium1a (831 mg, 1.91 mmol) and lithium hexamethyldisilazide
(334 mg, 2.0 mmol) at À788C. The suspension was warmed to room tem-
perature and stirred overnight. The volatiles were removed in vacuo. The
product was extracted with toluene (three times). The resulting red solu-
tion was concentrated under vacuum and layered with pentane, giving
red crystals overnight at À208C (yield: 550 mg; 78%). M.p. 131–1338C;
1H NMR (CDCl3, 300 MHz): d=7.37 (d, J=4 Hz, 1H), 7.11 (d, J=3 Hz,
1H), 7.1–7.0 (m, 4H), 7.07 (s, 1H), 4.51 (d, J=7 Hz, 1H), 4.44 (d, J=
7 Hz, 1H), 4.28 (dd, J=3 and 11 Hz, 1H), 3.63 (m, 1H), 3.43 (m, 1H),
3.32 (d, J=11 Hz, 2H), 3.25 (d, J=5 Hz, 1H), 3.10 (m, 2H), 2.80 (m,
4H), 2.7–2.4 (m, 5H), 1.9–1.6 (m, 5H), 1.3–1.0 (m, 3H), 1.38 (d, J=6 Hz,
3H), 1.33 (d, J=6 Hz, 6H), 1.20 (d, J=6 Hz, 3H), 1.15 (d, J=6 Hz, 6H),
1.13 (d, J=6 Hz, 6H), 0.29 (s, 9H), 0.17 ppm (s, 9H); 13C NMR (CDCl3,
75 MHz): d=168.7 (CH), 149.4 (C), 146.8 (C), 146.3 (C), 144.8 (C), 137.5
(2C), 124.8 (CH), 124.5 (CH), 123.8 (2CH), 122.9 (2CH), 82.4 (CH), 78.5
(CH), 55.4 (CH2), 54.6 (CH2), 53.2 (CH2), 47.6 (CH2), 46.2 (CH2), 43.2
(CH), 30.9 (CH2), 30.2 (CH), 28.3 (CH), 27.8 (CH2), 27.6 (CH), 27.5
(CH), 27.1 (CH), 24.8 (CH2), 24.5 (CH3), 23.5 (CH3), 23.2 (6CH3), 20.4
(CH2), 6.5 (3CH3), 4.6 ppm (3CH3); MS (EI): m/z calcd for C38H57N4:
569.4583 [MÀN
ACHTUNGTRENNUNG
((SiCH3)3)2]+; found: 569.4567.
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14900
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Chem. Eur. J. 2013, 19, 14895 – 14901

Studies of Anti-Bredt Amidinium Salts
FULL PAPER
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Received: June 26, 2013
Published online: September 17, 2013
Chem. Eur. J. 2013, 19, 14895 – 14901
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14901