Subtle reactivities of boron and aluminum complexes with amino-linked n-heterocyclic

Article abstract of DOI:10.1021/om200878e

This paper describes the synthesis and characterization of trimethylaluminum (2a), dimethylaluminum (3a), and triphenylboron complexes (7) supported by functional aminelinked NHC ligands. The chemical reactivity studies with carbodiimide and isocyanate were preformed with 2a and 3a, illustrating the noninnocent nature in Alcarbene bonding. However, the boroncarbene interaction is quite robust against addition of these substrates even at higher temperature conditions. The subtle difference in chemical reactivity between aluminum and boron is attributed from the metalcarbene bond covalency. Development of the catalytic method for SuzukiMiyaura coupling utilizing triphenylboron reagent supported by amino-NHC is also presented.

Full text of DOI:10.1021/om200878e

Article
pubs.acs.org/Organometallics
Subtle Reactivities of Boron and Aluminum Complexes with Amino-
Linked N-Heterocyclic Carbene Ligation
Chia-Cheng Tai,† Ya-Ting Chang,†,‡ Jie-Hong Tsai,† Titel Jurca,† Glenn P. A. Yap,§ and Tiow-Gan Ong*,†
†Institute of Chemistry, Academia Sinica, Nangang, Taipei, Taiwan, Republic of China
‡National Taiwan Science and Technology University, Taipei, Taiwan, Republic of China
§Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: This paper describes the synthesis and character-
ization of trimethylaluminum (2a), dimethylaluminum (3a), and
triphenylboron complexes (7) supported by functional amine-
linked NHC ligands. The chemical reactivity studies with
carbodiimide and isocyanate were preformed with 2a and 3a,
illustrating the noninnocent nature in Al−carbene bonding.
However, the boron−carbene interaction is quite robust against
addition of these substrates even at higher temperature conditions.
The subtle difference in chemical reactivity between aluminum and
boron is attributed from the metal−carbene bond covalency.
Development of the catalytic method for Suzuki−Miyaura coupling
utilizing triphenylboron reagent supported by amino-NHC is also
presented.
triphenylboron reagent supported by amino-NHC is also
reported.
INTRODUCTION
■
The judicious employment of N-heterocyclic carbenes (NHCs)
as supporting scaffolds has resulted in significant advances in
transition metal mediated catalysis.1 However, the studies of
NHCs in group 13 elements such as boron and aluminum
remain limited to a few selected cases.2−5 Structural and
reactivity studies of these complexes can pave the way for future
catalysis, materials, and medicinal applications. For example, the
elegant concept of frustrated Lewis pairs has spurred great
interest in the use of bulky NHC-B(C6F5)3 for hydrogen
RESULT AND DISCUSSION
■
Synthesis and Characterization of Amino−NHC
Organoaluminum Complexes. Preliminary work was
initiated with the synthesis of amino-NHC trialkyl aluminum
complexes. Amino-NHC 1 was readily introduced to AlR3 in
THF to furnish 2a (R = Me) and 2b (R = Et) in high yield. The
spectroscopic and structural features for both compounds have
been reported by our group previously.4c At this juncture, we
became interested in uncovering the basic chemical nature of
these main group complexes supported by the amino-NHC
scaffold. Heating AlMe3 complex 2a in toluene was attempted
at 110 °C, generating 3a with a peak disappearance associated
with a CH3 ligand and N-H proton. Single-crystal X-ray
diffraction analysis of 3a displayed a tetrahedrally distorted
dimethyl aluminum center featuring a bidentate coordination
mode with the amide and carbene site (Figure 1). The
formation of a half-chair, six-membered-ring metallacycle upon
chelation twists the ideal coordination geometry of Al with a
bite angle of C(7)−Al−N(1) = 93.83(7)° and also slightly
shortens the bond length of Al−carbene to 2.0585(18) Å in
comparison to 2a (2.074(2) Å). The Al−N(1) bond length
(1.8554(15) Å) is considered to be normal for a typical Al−
amide bonding interaction.
activation.6 Recent seminal works by Fensterbank, Laco
̂
te,
Malacria, and Curran have demonstrated nontoxic NHC
boranes that are stable radical hydrogen atom donors,5e−h
leading to the subsequent application as promising co-initiators
in radical photopolymerization.7 Furthermore, juxtaposing
recent development in our and other laboratories, Al-NHCs
are also finding their role as cooperative bimetallic catalysts.8
Previously, we have reported amino-linked NHC−aluminum
complexes and the noninnocent nature of Al−NHC bonding.4c
Although the use of these functionalized NHC ligands with
transition metals is widespread, examples of the main group
elements appear to be rare.9 Delving into group 13 chemistry,
we were curious as to whether the amino-functionalized NHC
would lead to a distinct reactivity difference between the
aluminum and boron complexes. Herein, we attempted to
synthesize Al and B complexes supported by this ligand
framework and examine their intrinsic carbene bonding nature
against unsaturated substrates. Development of the catalytic
method for Pd-mediated Suzuki−Miyaura coupling utilizing
Received: October 6, 2011
Published: January 11, 2012
© 2012 American Chemical Society
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Reaction of Organoaluminum with Carbodiimide and
Isocyanate. Along these lines, studying the reactivities of 2
Scheme 2
Figure 1. Molecular diagram of 3a with selected atoms depicted as
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and angles (deg):
Al−N(1) 1.8554(15), Al−C(20) 1.980(2), Al−C(19) 1.9832(19),
Al−C(7) 2.0585(18), C(7)−N(2) 1.348(2), C(7)−N(3) 1.356(2),
N(1)−Al-C(20) 114.30(8), N(1)−Al−C(19) 118.35(8), C(20)−Al−
C(19) 111.00(9), N(1)−Al−C(7) 93.83(7), C(20)−Al−C(7)
108.73(8), C(19)−Al−C(7) 108.83(8), N(2)−C(7)−N(3)
104.12(14).
In contrast, the ethyl derivative 2b is relatively thermally
stable. We are unable to generate Al−amide bonding via alkane
Scheme 1
and 3 with electrophilic substrates would perhaps delineate the
intrinsic bonding nature of these group 13 elements with
carbene. Exposure of a solution of 2a in toluene to 4-
methylphenyl isocyanate resulted in the formation of a new
species, 4. An X-ray diffraction study of the crystal of complex 4
revealed the insertion of a carbonyl moiety into the Al−carbene
bond, generating zwitterions containing aluminate and
imidazolium (Figure 3). Similar insertion chemistry has been
previously observed with organo-f-block complexes.10 The
bond distances (1.342(2) and 1.334(2) Å) and angles
(107.85(14)°) in N(1)−C(10)−N(2) are indicative of
electronic delocalization of the heterocyclic ring and are
consistent with typical imidazolium salts. Likewise, the isolation
of the single crystal of 5 provides evidence that 2a also reacted
with dicyclohexylcarbodiimide in a similar fashion to the
isocyanate (Figure 4). The structural parameters in 5 are no
different from those of compound 4; thus they warrant no
further discussion.
Further studies of 3, the corresponding dimethyl aluminum
counterpart with isocyanate, were performed to see if the
chelating effect would actually increase the stability of the Al−
carbene bond toward electrophiles. Reaction of 3 with
tolyisocyanate (∼3 equiv) at room temperature in THF
resulted in the immediate precipitation of a white powder, 6.
We were unsuccessful in obtaining meaningful NMR spectra
for product 6, even after numerous attempts of purifying the
white precipitate from the other intractable impurities via
repeat recrystallization in different solvent settings. Close
monitoring by NMR at low temperature did not provide us any
meaningful supporting evidence with regard to the nature of
the reaction. Fortunately, X-ray structural analysis of crystalline
6 revealed a rather unexpected product. The two isocyanate
molecules are inserted separately into the Al−carbene and Al−
elimination of 2b after prolonged heating at elevated
temperature (100−150 °C). Perhaps the reason the diethyl
aluminum complex 3b does not form is due to unfavorable
steric crowding arising from the interaction of the ethyl ligand
with the tert-butyl amino pendant arm. Examination of a space-
filling model of 2b illustrates that there is likely insufficient
space around the Al metal center for tert-butyl amine to
penetrate the ethyl ligand environment and approach the Al
atom (see Figure 2).
Figure 2. Molecular structures with space-filling models for 2b (left)
and 2a (right). Atomic dimensions are the atomic van der Waals radii.
The Al atom is shown in red, N in blue, and C in gray.
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Figure 5. Molecular diagram of 6 with selected atoms depicted as
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity. Al−O(1) 1.782(4), Al−O(2) 1.828(4), Al−
C(2) 1.963(6), Al−C(1) 1.968(6), N(1)−C(12) 1.355(6), N(2)−
C(12) 1.295(6), C(12)−C(29) 1.496(8), N(2)−C(12)−C(29)
126.6(5).
Figure 3. Molecular diagram of 4 with selected atoms depicted as
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and angles (deg):
Al−O(1) 1.8626(13), Al−C(28) 1.9763(19), Al−C(27) 1.979(2),
Al−C(29) 1.989(2), C(10)−N(1) 1.342(2), C(10)−N(2) 1.334(2),
C(10)−C(19) 1.509(2), C(19)−N(4) 1.283(2), C(19)−O(1)
1.295(2), C(25)−N(4) 1.417(2), N(2)−C(10)−N(1) 107.85(14).
parallel to its aluminum equivalent. Amino-linked NHC 1 was
introduced to Ph3B in THF at room temperature to afford 7 in
good yield (70%). The 1H NMR spectroscopic features of 7 are
notably different from its amino-NHC counterpart, displaying a
distinct signal for the tert-butyl amine side arm at 0.92 ppm,
which is shifted upfield from 0.99 ppm for 1. Two additional
broad peaks at 7.04 and 6.93 ppm attributed to aryl substituents
at the boron center with an integration of 15 protons were also
observed. Moreover, the peak seen at −8.8 ppm in the 11B
NMR spectra evidenced the complexation of the boron to the
amino-NHC.11 Single crystals of 7 suitable for X-ray analysis
were grown by cooling an ether solution of the boron
compound, the structural model of which is presented in
Figure 6. Interestingly, we also found no analogous report of
the corresponding crystal structure of NHC-BPh3 similar to 7
for the purpose of comparison. The sp3-hybridized boron
center coordinating to the carbene site appears as a distorted
tetrahedron with dihedral angles C36−B1−C7 of 112.09o and
C24−B1−C30 of 110.79o. The boron−carbene bond length is
1.666(3) Å, in line with previously reported values of 1.663 Å
for 1,3-bis(2,6-diisopropylphenyl)imidazolylidene-B(C6F3)3,6a
but longer than the NHC-B(C6F3)3 adduct, which contains
1,3,4,5-tetramethylimidazolylidene with a B−C bond distance
of 1.6407 Å.12 The longer B−C bond is attributed to the
greater steric demands invoked by the dangling amino pendant
arm of 7. Notably, the B−N(1) distance of 4.90 Å illustrated no
close intramolecular contact.
First, the boron-NHC Lewis pairs exhibited no reactivity
with carbodiimide addition. Nonetheless, complex 8 was
conveniently formed when the reaction of 7 with 1 equiv of
toly isocyanate was performed. 11B NMR spectroscopic studies
of the reaction mixture revealed a peak at −9.06 ppm,
illustrating that the boron−carbene bond remains intact (see
compound 8; vide supra). On the basis of its structural analysis
(Figure 7), complex 8 was formulated as a tetrahedral boron
center coordinating to carbene bearing a pendant urea group,
which is consistent with the NMR analysis. Again, the results
demonstrated the stability of the boron−NHC bonding
Figure 4. Molecular diagram of 5 with selected atoms depicted as
thermal ellipsoids at the 50% probability level. Hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles (deg): Al−
N(1) 1.9525(14), Al−C(1) 1.9940(18), Al−C(3) 1.9970(19), Al−
C(2) 2.0034(18), C(13)−N(1) 1.358(2), C(13)−N(2) 1.340(2),
C(13)−C(28) 1.506(2), C(28)−N(4) 1.359(2), C(28)−N(5)
1.293(2), N(2)−C(13)−N(1) 106.81(14).
amide bonds of 3, generating an eight-membered metallacycle
(6) puckered in half-chair conformation as represented in
Figure 5.
Synthesis and Reactivity of Amino-NHC Triphenylbor-
on Complex. Because of the noninnocent nature of Al−NHC
in the amino-linked NHC system toward electrophiles, we are
curious as to whether boron element would exhibit reactivity
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Scheme 3
DFT computations on truncated systems MMe3·(C{(NCH3)-
N(CHCH)N(CH2CH2NHCH3)}), where M = Al and B,
respectively, were performed using the B3LYP functional and 6-
311+G** basis set for geometry optimization and cc-pVTZ
basis set for bond order calculation.13 The bond order
calculation reveals an Al−C bond order of 0.4649 and B−C
bond order of 0.8418. The significant difference in bond orders
further evidences the disparity in M−C reactivity when
contrasting Al and B NHC analogues.14
Catalytic Suzuki−Miyaura Coupling. Palladium-cata-
lyzed cross-coupling reactions are among the most common
and effective strategies employed by chemists for constructing
C−C bonds in molecular synthesis,15 which has been
recognized with the Nobel Prize for Chemistry in 2010. The
Suzuki−Miyaura coupling reaction represents one example of a
palladium-catalyzed reaction that permits a bond attachment of
aromatic substituents onto an aromatic molecule in a gentle
manner using boron reagents. A recent successful role played
by NHC-boron complexes in organometallic transformation in
Suzuki−Miyaura coupling,11 as well as the high stability of
boron−carbene bonds, has prompted us to further explore the
catalysis feasibility. The Suzuki−Miyaura coupling reaction of
aryl halides with complex 7 was evaluated. Initial screening
attempts were undertaken to determine the viability of various
reaction conditions for C−C coupling of 4-bromoacetophe-
none in toluene/H2O (10:1 volume) at 80 °C. As seen in Table
1, Pd(OAc)2 is found to be the most effective catalyst (entry 2),
with a 92% yield, while adding triphenylphosphine ligand
(entry 3) only diminished the efficiency of the reaction. More
importantly, no addition of base was required in this cross-
coupling reaction. A precarious yield (16%) in the control
experiment using plain Ph3B (entry 4) under similar conditions
illustrated the importance of amino-NHC’s dual role of
auxiliary scaffold in enhancing both the activity of palladium
and the basicity of the boron reagent. Next, we further
expanded the scope of the cross-coupling of substrate 7 with
different substituted aryl bromide (see Table 2). In general, a
high yield of coupled products can be obtained with electron-
withdrawing substituted aryl bromide (entries 1 and 3), but a
moderate yield was observed for 1-bromo-4-methylbenzene
(entry 2). Electron-rich substrates with an amino group and
ortho-bromopyridine (entries 4−6) afforded a lower yield, but
surprisingly, meta-bromopyridine delivered a 90% yield (entry
7). Finally, product 21 can be obtained under similar conditions
via 1-chloro-4-methylbenzene in a slightly decreased yield
(55%). Notably, there are few issues regarding the role of
amino-NHC that are worthy of comment. First, we are curious
Figure 6. Molecular diagram of 7 with selected atoms depicted as
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity. B(1)−C(24) 1.631(3), B(1)−C(36) 1.644(2),
B(1)−C(30) 1.651(3), B(1)−C(7) 1.666(3), N(2)−C(7) 1.3618(19),
N(3)−C(7) 1.360(2), N(3)−C(7)−N(2) 103.46(13).
Figure 7. Molecular diagram of 8 with selected atoms depicted as
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms
are omitted for clarity. B(1)−C(10) 1.653, B(1)−C(38) 1.633(2),
C(10)−N(1) 1.357(2), C(10)−N(2) 1.359 (2), N(1)−C(10)−N(2)
104.18(14).
interaction compared with unsaturated substrates such as
carbodiimide and isocyanate even at higher temperatures.
The subtle difference in chemical reactivity between
aluminum and boron is perhaps attributed to the metal−
carbene bond covalency. The Al−carbene interaction in this
case is mainly ionic, making the Al derivatives prone to react
with electrophiles, as the electron density mostly resides on the
carbene fragment. Yet, the manifestation of covalent character
in the boron−carbene bond increases the stability of boron
complex 7. The inherent differences between Al−C and B−C
bonds and their respective reactivities as observed by our
experimental work are further echoed with theoretical studies.
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Table 1. C−C Cross-Coupling in Various Optimization Conditions
b
entry
catalyst
ligand
boronic substrate
yield (%)
a
1
PdCI2(PPh3)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
N/A
N/A
PPh3
N/A
7
50
92
85
16
a
2
7
a
3
7
a
4
BPh3
a
The reaction was carried out using 1 equiv of boronic substrate and 1 equiv of 10 with 5 mol % catalyst, toluene/H2O, 10:1 (volume ratio), 80 °C,
b
18 h. Isolated yield.
Table 2. Scope for Cross Coupling: Aryl Transfer
NHC ligands. On the basis of the chemical reactivity studies,
we have found that the bonding of Al−carbene is more prone
to reaction with an unsaturated substrate, while the boron−
carbene interaction is quite robust. In addition, the catalysis
results suggest that the development of a catalytic method for
Suzuki−Miyaura coupling is also feasible for boron reagents
supported by amino-NHC ligands.
EXPERIMENTAL SECTION
■
General Procedures. All air-sensitive manipulations were
performed under an atmosphere of nitrogen using Schlenk techniques
and/or a glovebox. Toluene, hexanes, THF, and ether were purified by
passage through a column of activated alumina using a solvent
purification system purchased from Innovative Technology, Inc.
Deuterated benzene and toluene were dried by vacuum transfer
from activated molecular sieves. Ph3B, AlEt3 (1.8 M in heptane
solution), and AlMe3 (1.0 M in heptane solution) were purchased
1
from Aldrich Chemical Co. and used without further purification. H,
11B, and 13C NMR spectra were run on a Bruker 300 MHz, Bruker 400
MHz, and Bruker 500 MHz spectrometer using the residual proton of
the deuterated solvent as internal reference. Compound 1 was
synthesized according to literature preparation.4c
Synthesis of AlMe3·(C{(NMesityl)N(CHCH)N(CH2CH2NHt-Bu)})
(2a). AlMe3 (1.0 M) in heptane solution (1.739 mL, 1.739 mmol) was
added to amino-NHC 1 (500 mg, 1.739 mmol) in THF solution at 20
°C. After stirring for 2 h, the solvent was removed in vacuo to afford a
bright yellow solid. The crude product was further purified by
recrystallization from ether/hexanes (50:50) at −20 °C. Yield: 90%
1
(560 mg). H NMR (C6D6, 400 MHz, 25 °C): δ 6.72 (s, 2H, C6H2),
6.69 (s, 1H, CH), 5.94 (s, 1H, CH), 4.03 (t, J = 5.6 Hz, 2H, CH2),
2.64 (t, J = 6.0 Hz, 2H, CH2), 2.07 (s, 3H, ArCH3), 1.92 (s, 6H,
ArCH3), 0.86 (s, 9H, Bu), 0.29 (t, J = 8 Hz, NH), −0.47 (s, 9H,
a
The reaction was carried out using 1 equiv of boronic substrate and 1
t
equiv of 1x with 5 mol % catalyst, toluene/H2O, 10:1 (volume ratio),
80 °C, 18 h. Isolated yield.
AlCH3). 13C NMR (C6D6, 75 MHz, 25 °C): δ 175.93 (NCN), 139.04
(Ar), 135.89 (Ar), 135.30 (Ar), 129.18 (Ar), 122.20 (CH), 121.36
(CH), 50.91 (CH2), 50.16 (NCMe3), 43.76 (CH2), 29.05 (tBu), 21.08
(ArCH3), 17.49 (ArCH3), −7.02 (AlMe3). Anal. Calcd for C21H36AlN3:
C, 70.55; H, 10.15; N, 11.75. Found: C, 70.12; H, 10.30; N, 11.59.
Synthesis of AlEt3·(C{(NMesityl)N(CHCH)N(CH2CH2NHt-Bu)})
(2b). AlEt3 (1.0 M) in hexanes solution (10.0 mmol, 10 mL) was
added to amino-NHC 1 (10.0 mmol, 4.430 g) in THF solution at
room temperature. After stirring for 12 h, the solvent was removed in
vacuo to afford a white powder. The crude solid was extracted with
toluene (30 mL), and the white insoluble precipitate was removed by
filtration. The filtrate was dried in vacuo to afford a white solid, which
was further purified by recrystallization from ether/hexanes (50:50
b
if a carbene transfer from boron to palladium occurred during
the reaction. On the basis of H NMR characterization, the
1
reaction of independently heating a mixture of Pd(OAc)2 or
PdCl2 with 7 in the absence of aryl halide at 80−110 °C gave
no experimental evidence to substantiate the existence of
NHC-ligated palladium species in the catalysis. Finally, it
should be noted that the performance of 7 in mediating the
coupling reaction is comparable to the reported NHC-boron
complexes.11 Therefore, we ruled out the importance of the
amino side arm of 7 in augmenting the catalysis.
1
volume) at −30 °C. Yield: 70% (2.80 g). H NMR (C6D6, 500 MHz,
25 °C): δ 6.79 (d, J = 1.5 Hz, 1H, CH), 6.69 (s, 2H, C6H2), 5.90 (d, J
= 1.5 Hz, 1H, CH), 4.04 (t, J = 5.5 Hz, 2H, CH2), 2.64 (m, 2H, CH2),
2.06 (s, 3H, CH3), 1.90 (s, 6H, CH3), 1.45 (t, J = 8 Hz, 9H,
AlCH2CH3), 0.85 (s, 9H, tBu), 0.27 (t, J = 8.0 Hz, 1H, NH), 0.09 (q, J
= 8 Hz, 6H, AlCH2). 13C NMR (C6D6, 125 MHz, 25 °C): 176.1 (br,
CONCLUSION
■
In conclusion, this paper describes the synthesis and character-
ization of aluminum and boron complexes supported by amino-
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Ccarbene), 139.2 (Ar), 135.8 (Ar), 135.1 (Ar), 129.3 (Ar), 122.2 (NCH),
121.4 (NCH), 50.8 (NCH2), 50.1 (NCMe3), 43.6 (NCH2), 28.9 (tBu),
21.0 (Ar-Me), 17.5 (Ar-Me), 11.5 (CH2CH3), 0.8 (CH2). HR-MS (EI):
m/z [(M − Et)+] calcd for C22H37AlN3 370.2803; found 370.2798.
Me2Al-(κ2-C,N)-(C{(NMesityl)N(CHCH)N(CH2CH2Nt-Bu)}) (3a).
Compound 2a in a toluene solution (200 mg 0.559 mmol) was
heated at 110 °C for a day. The solvent was removed in vacuo to afford
a yellow residue, which was further purified by recrystallization at −20
°C from ether/hexanes (50:50) to afford clear crystals with a 90%
C36H42BN3: C, 81.80; H, 8.20; N, 7.95. Found: C, 81.89; H, 8.14; N,
7.93.
Reaction of 7 with Tolyl Isocyanate: Formation of 8. A
solution of tolyl isocyanate (56 mg, 0.42 mmol) in 5 mL of ether was
added to a solution of 7 (148 mg, 0.28 mmol) in ether and allowed to
stir at room temperature for 8 h. The solvent was removed in vacuo to
afford a white oil. The crude product was washed with hexanes and
further purified by recrystallization from ether/hexanes (1:1) at −20
°C to affored clear colorless crystals with a yield of 98% (182 mg). 1H
NMR (CDCl3, 400 MHz, 25 °C): δ 7.83 (s, 1H, ArH), 7.28−6.82 (br,
18H, ArH), 6.75 (s, 1H, ArH), 6.42 (s, 2H, C6H2), 6.16 (s, 1H, CH),
4.01 (t, J = 5.5 Hz, 2H, NCH2), 2.95 (t, J = 5.5 Hz, 2H, NCH2), 2.32
(s, 3H, CH3), 2.12 (s, 3H, CH3), 1.84 (s, 6H, CH3), 1.24 (s, 9H, tBu).
13C NMR (CDCl3, 100 MHz, 25 °C): δ 156.59 (Ar), 138.50 (Ar),
135.77 (Ar), 135.26 (Ar), 134.83 (Ar), 133.99 (Ar), 132.93 (Ar),
129.35 (Ar), 128.60 (Ar), 126.16 (Ar), 123.77 (Ar), 122.75 (NCH),
121.99 (NCH), 120.61 (Ar), 55.60 (NCMe3), 51.26 (NCH2), 43.78
(NCH2), 30.16 (tBu), 21.68 (Ar-Me), 21.58 (Ar-Me), 18.18 (Ar-Me).
11B NMR (CDCl3, 53 MHz, 25 °C): δ −9.06 (Ccarbene-BPh3). HR-MS
(MALDI) [M + Na]+ calcd for C44H49BN4O 683.3892; found
660.3914.
Suzuki−Miyaura Coupling Procedure. In a typical reaction, a
mixture of aryl bromide (1.0 mmol), 7 (1.0 mmol), and Pd(OAc)2
catalyst precursor (5 mol %) in 10 mL of a mixture of toluene/water in
10:1 volume ratio was stirred at 80 °C for 10−18 h under nitrogen
atmosphere. The solution was allowed to cool to room temperature
for GC and NMR analysis utilizing 1,3,5-trimethoxybenzene as an
internal standard. For isolation of the products, the solvent was
removed completely under vacuum. A mixture of ether/water was
added for extraction to afford the crude coupling product. The crude
mixture was purified by flash chromatography utilizing a 4:1 mixture of
hexanes/ether as eluent to afford the cross-coupling product.
X-ray Crystallography. Details of the X-ray data collection,
solution, and refinement for 3a, 4, 5, 6, 7, and 8 are presented in the
SI, Tables A, B, and C. Suitable crystals were mounted using viscous
oil flash cooled to the data collection temperature. Data were collected
on a Nonius Kappa CCD diffractometer (Mo Kα = 0.71073 Å).
Multiscan absorption corrections were applied. Unit-cell parameters,
equivalent reflections, and systematic absences in the diffraction data
are consistent, uniquely, with P21/n (=P21/c) for 3a, 4, 7, and 8 and
with the enantiomeric space groups P41 and P43 for 6. The anomalous
dispersion parameter in 6 refined to nil within estimated error,
suggesting P43 as the correct space group choice. One severely
disordered diethyl ether molecule of solvation per compound molecule
in 6 was treated as diffused contributions.16 Although the unit cell
parameters are suggestive of a monoclinic cell, no symmetry higher
than triclinic was observed for 5 and solution in the centrosymmetric
space group option yielded chemically reasonable and computationally
stable results of refinement. No overlooked symmetry for 5 was
suggested by the ADDSYM program of PLATON.16 Antibumping
restraints were applied for compound 8. The structures were solved by
direct method (SHELXS-97)17 and refined using the least-squares
methods on F2. CIFs were deposited with the CSD under numbers
CCDC 839342−839347.
1
(172 mg) yield. H NMR (C6D6, 400 MHz, 25 °C): δ 6.69 (s, 2H,
C6H2), 6.01 (d, J = 1.2 Hz, 1H, CH), 5.85 (d, J = 1.2 Hz, 1H, CH),
3.48 (m, 2H, CH2), 3.26 (m, 2H, CH2), 2.00 (s, 3H, ArCH3), 1.89 (s,
6H, ArCH3), −0.55 (s, 6H, AlMe2). 13C NMR (C6D6, 125 MHz, 25
°C): δ 173.98 (NCN), 139.39 (Ar), 135.47 (Ar), 135.06 (Ar), 129.38
(Ar), 121.43 (CH), 120.88 (CH), 55.40 (CH2), 53.52 (NCMe3), 45.41
(CH2), 31.23 (tBu), 20.97 (ArCH3), 17.61 (ArCH3), −6.30 (AlMe3).
Anal. Calcd for C20H32AlN3: C, 70.35; H, 9.45; N, 12.31. Found: C,
70.30; H, 9.83; N, 12.01.
Reaction of 2a with Isocyanate: Formation of 4. p-
Tolylisocyanate (68 mg, 0.508 mmol) was added to a solution of 2a
(200 mg, 0.559 mmol) in THF at −20 °C. The solution was stirred for
2 h and dried in vacuo to afford a yellow solid. The yellow residue was
washed by hexanes to afford a white powder, which was further
purified by recrystallization from ether at −20 °C, affording a yield of
1
70% (192 mg). H NMR (C6D6, 400 MHz, 25 °C): δ 7.41 (d, J = 8
Hz, 2H, C6H2), 7.00 (d, J = 8 Hz, 2H, C6H2), 6.63 (s, 2H, C6H2), 6.43
(d, J = 1.6 Hz, 1H, CH), 5.72 (d, J = 1.6 Hz, 1H, CH), 4.18 (t, J = 5.2
Hz, 2H, CH2), 2.49 (br s, 2H, CH2), 2.05 (s, 6H, ArCH3), 2.02 (s, 3H,
t
ArCH3), 1.99 (s, 3H, ArCH3), 0.78 (s, 9H, Bu), 0.37 (t, 1H, NH),
−0.40 (s, 9H, AlMe3). 13C NMR (C6D6, 100 MHz, 25 °C): δ 145.02
(NCN), 144.56 (Ar), 144.36 (Ar), 140.15 (Ar), 135.06 (Ar), 133.72
(Ar), 132.95 (Ar), 129.32 (Ar), 129.27 (Ar), 124.92 (Ar), 121.73
(CH), 120.36 (CH), 51.20 (NCMe3), 50.02 (CH2), 42.20 (CH2),
28.94 (tBu), 21.07 (ArCH3), 20.98 (ArCH3), 18.11 (ArCH3), −5.90
(AlMe3).
Reaction of 3 with Carbodiimide: Formation of 5.
Dicyclohexylcarbodiimide (105 mg, 0.508 mmol) was added to a
solution of 3 (200 mg, 0.559 mmol) in THF at −20 °C. The solution
was allowed to stir overnight and dried in vacuo to afford a brown oil.
The brown oil was washed by hexanes to afford a bright yellow
powder, which was further purified by recrystallization from ether/
1
hexanes (50:50) at −20 °C, affording a yield of 70% (221 mg). H
NMR (C6D6, 400 MHz, 25 °C): δ 6.72 (s, 2H, C6H2), 6.68 (d, J = 1.6
Hz, 1H, CH), 5.93 (d, J = 1.6 Hz, 1H, CH), 4.02 (t, 2H, CH2), 3.12
(m, 2H, NCH), 2.63 (m, 2H, CH2), 2.07 (s, 3H, ArCH3), 1.92 (s, 6H,
ArCH3), 1.87 (m, 4H, CH2), 1.56 (m, 4H, CH2), 1.34 (m, 6H, CH2),
t
1.06 (m, 6H, CH2), 0.85 (s, 9H, Bu), 0.29 (m, 1H, NH), −0.47 (s,
9H, AlMe3). 13C NMR (C6D6, 75 MHz, 25 °C): δ 176.72 (NCN),
139.65 (Ar), 139.17 (Ar), 135.95 (Ar), 135.33 (Ar), 129.25 (Ar),
122.34 (CH), 121.18 (CH), 55.74 (NCH), 50.97 (CH2), 50.15
(NCMe3), 43.78 (CH2), 35.40 (CH2), 29.00 (tBu), 25.82 (CH2), 24.87
(CH2), 21.05 (ArCH3), 17.48 (ArCH3), −7.16 (AlMe3).
Synthesis of (BPh3)·(C{(NMesityl)N(CHCH)N(CH2CH2NHt-Bu)})
(7). A solution of triphenylborane (167 mg, 0.69 mmol) in THF (5
mL) was added to a solution of 1 (200 mg, 0.69 mmol) in THF at
room temperature, and the mixture was allowed to stir for 2 h. The
solvent was removed in vacuo to afford a white solid, which was further
purified by recrystallization from ether/hexanes (1:1) at −20 °C to
ASSOCIATED CONTENT
■
1
S
afford clear colorless crystals with a yield of 77% (281 mg). H NMR
* Supporting Information
(CDCl3, 400 MHz, 25 °C): δ 7.57 (d, J = 1.6 Hz, 1H, CH), 7.04−6.93
(br, 15H, BAr3), 6.72 (d, J = 1.6 Hz, 1H, CH), 6.40 (s, 2H, C6H2),
3.73 (t, J = 6.4 Hz, 2H, NCH2), 2.35 (q, J = 8 Hz, 2H, NCH2), 2.09 (s,
Crystal data as CIF files for compounds 3a, 4, 5, 6, 7, and 8;
table of atomic coordinates and information for all the
optimized species for computational studies. This material is
available free of charge via the Internet at http://pubs.acs.org.
t
3H, CH3), 1.89 (s, 6H, CH3), 0.92 (s, 9H, Bu). 13C NMR (CDCl3,
100 MHz, 25 °C): δ 138.79 (Ar), 135.58 (Ar), 135.24 (Ar), 134.37
(Ar), 128.89 (BAr), 126.33 (BAr), 123.86 (BAr), 122.46 (NCH),
121.42 (NCH), 51.30 (NCH2), 50.40 (NCMe3), 41.65 (NCH2),
29.08 (tBu), 21.00 (Ar-Me), 18.63 (Ar-Me). 11B NMR(CDCl3, 53
MHz, 25 °C): δ −8.75 (Ccarbene-BPh3). HR-MS (MALDI) [M + H]+
calcd for C36H43BN3 528.3550; found 528.3570. Anal. Calcd for
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tgong@chem.sinica.edu.tw.
642
dx.doi.org/10.1021/om200878e | Organometallics 2012, 31, 637−643

Organometallics
Article
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ACKNOWLEDGMENTS
■
We are grateful to the Taiwan National Science Council (NSC:
97-2113-M-001-024-MY2) and Academia Sinica for their
generous financial support.
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