Cyclopropanation and isomerization reactions of β-diketiminato boron complexes

Article abstract of DOI:10.1021/om300337r

The reaction of HC[(CBu^t)(NAr)]₂Li with BCl ₃ yielded the azaallyl boron dichloride [ArN=C(Bu^t)C(H) C(Bu^t)N(Ar)]BCl₂ (1), which can be converted to the β-diketiminato boron dichloride HC[(C

Full text of DOI:10.1021/om300337r

Communication
pubs.acs.org/Organometallics
Cyclopropanation and Isomerization Reactions of β-Diketiminato
Boron Complexes
Liang Xie, Hongfan Hu, and Chunming Cui*
State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: The reaction of HC[(CBut)(NAr)]2Li with BCl3
yielded the azaallyl boron dichloride [ArNC(But)C(H)C(But)-
N(Ar)]BCl2 (1), which can be converted to the β-diketiminato
boron dichloride HC[(CBut)(NAr)]2BCl2 (2) upon heating at 40
°C. Reaction of 1 with the bulky lithium salts LiN(SiMe3)2 and
MesLi (Mes = 2,4,6-Me3C6H2) resulted in the cyclopropanation of
the CBut group via the deprotonation of the methyl group, while reactions with PhLi and LiNEt2 gave substitution products.
β-Diketiminato ligands are some of the most versatile
supporting ligands in organometallic chemistry and homoge-
neous catalysis.1 The employment of this type of ligand in
boron chemistry has allowed the isolation of unique stable
cationic, dicationic, and multiply bonded boron species.2 In
addition, the ligands have proved to be excellent for the
stabilization of low-valent heavier group 13 carbene analogues3
and other interesting group 13 species.1a,4 The unusual
stabilization effects can be attributed to the electron
delocalization of the monoanionic ligand backbone, steric
protection of the N substituents, and high stability of the ligand
framework. However, recent studies have shown that the
ligands may undergo C−N bond rupture and C−H bond
activation of the β-methyl groups under reducing conditions.5
Our previous studies on the boron compounds supported by
the monoanionic ligand HC[C(Me)(NAr)]2 (Ar = 2,6-
Me2C6H3) indicated that the C−H bond of the β-methyl
group may take part in chemical transformations.2d Current
great interest in the preparation of stable borylenes6 prompted
us to investigate boron compounds incorporating this type of
ligand with the expectation of generating the elusive N-
heterocyclic borylenes. Herein, we report on the synthesis and
reactions of boron compounds containing the {HC[C(But)-
(NAr)]2}− framework. Interestingly, the ligand not only
exhibits two types of bonding modes but also undergoes a
cyclopropanation reaction upon treatment with bulky lithium
salts, demonstrating the unique behavior of the ligand in boron
chemistry.
Scheme 1. Synthesis and Isomerization of 1
Single crystals suitable for X-ray diffraction studies were
obtained from THF/n-hexane at −40 °C. The structure of 1 is
shown in Figure 1, along with selected bond parameters. The
Figure 1. Molecular structure of 1. Selected bond lengths (Å) and
angles (deg): B1−C1 = 1.665(3), B1−N1 = 1.593(3), B1−Cl1 =
1.859(3), B1−Cl2 = 1.527(3), C1−C2 = 1.527(3), N1−C2 =
1.303(3), C1−C15 = 1.519(3), N2−C15 = 1.280(3); C1−B1−N1 =
83.16(16), C1−C2−N1 = 99.42(19), B1−N1−C2 = 93.97(18), B1−
C1−C2 = 83.44(17).
The boron dichloride 1 was prepared in high yield by the
reaction of HC[C(But)(NAr)]2Li7 with BCl3 in diethyl ether
1
(Scheme 1). Compound 1 has been fully characterized by H,
11B, and 13C NMR and EI mass spectroscopy, elemental
analysis, and an X-ray single-crystal analysis. The 13C NMR
spectrum of 1 exhibits two signals at δ 176.0 and 209.9 ppm for
the imino carbon atoms, indicating their different chemical
environments. The 11B resonance observed at δ 8.17 ppm
corresponds to a four-coordinate boron atom. The structure
has been confirmed by an X-ray single-crystal analysis.
most noticeable structural feature of 1 is the coordination mode
of the ligand, in which, unlike the normally observed N,N
coordination in group 13 compounds,4 the boron atom is
coordinated to one of the nitrogen atoms and to the γ-carbon
Received: April 24, 2012
Published: June 11, 2012
© 2012 American Chemical Society
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Communication
atom. The B−C bond length (1.665(3) Å) is significantly
longer than those found in the reported amidinate boranes
(1.613(3)−1.628(2) Å), while the B−N bond (1.593(3) Å) is
comparable to those observed in the reported aminoboranes
(1.583(3)−1.606(2) Å).8 The coordination of the boron to the
C1−C2−N1 aza-allylic fragment leads to the slightly
lengthened C2−N1 bond length of 1.303(3) Å, in comparison
to the uncoordinated C15−N2 double-bond length of 1.280(3)
Å. However, the C1−C2 bond length of 1.527(3) Å is in the
typical range for a C−C single bond and is even slightly longer
than the C1−C15 single-bond length of 1.519(3) Å. These data
indicate a very small degree of electron delocalization over the
C1−C2−N1 framework. Several examples of β-diketiminato
complexes with this type of bonding mode have also been
reported.9
Scheme 2. Cyclopropanation of 1
X-ray single-crystal analysis. The 11B NMR spectra of 3 and 4
display similar resonances at δ 28.7 and 28.5 ppm, respectively,
which are very close to those reported for cyclic diaminobor-
anes.11 The 1H NMR spectra of the two compounds show that
one hydrogen atom in the CBut group is missing with the
formation of a CH2 fragment, identified by the unique
resonances at δ 0.19 and 0.41 ppm, respectively. The structures
of 3 and 4 have been finally determined by X-ray single-crystal
analysis.
Interestingly, compound 1 is subject to isomerization to 2
under thermal conditions. Heating a solution of 1 in toluene at
40 °C for 24 h resulted in the clean formation of 2, indicating
that 2 is the thermodynamically stable isomer. The 11B NMR
spectrum of 2 exhibits a signal at δ 6.74 ppm, which is shifted
upfield relative to that observed for 1 (δ 8.17 ppm).
The structures of 3 and 4 are shown in Figures 3 and 4,
respectively. The two structures both contain a C1−C2−C3
1
Furthermore, the H and 13C NMR spectra of 2 only show
one set of signals for the N−C−C−C−N backbone and their
substituents, indicating the normal N,N coordination of the β-
diketiminato ligand. The molecular structure of 2 has been
determined by X-ray single-crystal analysis, which is shown in
Figure 2. Unlike the β-diketiminato boron dichloride HC[C-
Figure 3. Molecular structure of 3. Selected bond lengths (Å) and
angles (deg): B1−N1 = 1.412(3), B1−Cl1 = 1.799(5), N1−C1 =
1.426(13), C1−C2 = 1.541(9), C2−C3 = 1.506(8), C1−C3 =
1.520(9), C1−C6 = 1.496(9), C6−C7 = 1.348(11), C7−N1′ =
1.517(11); N1−B1−N1′ = 122.7(4), N1−B1−Cl1 = 118.7(2), N1′−
B1−Cl1 = 118.7(2), C1−C2−C3 = 59.8(5), C2−C1−C3 = 58.9(4),
C1−C3−C2 = 61.3(5).
Figure 2. Molecular structure of 2. Selected bond lengths (Å) and
angles (deg): B1−N1 = 1.5297(19), B1−N2 = 1.569(2), B1−Cl1 =
1.9099(18), B1−Cl2 = 1.8416(18), C1−N1 = 1.3579(18), C1−C2 =
1.390(2), C2−C3 = 1.4124(19), N2−C3 = 1.3353(17); N1−B1−N2
= 109.79(12), B1−N1−C1 = 114.72(11), N1−C1−C2 = 117.93(13),
C1−C2−C3 = 124.17(13), C2−C3−N2 = 117.63(13), B1−N2−C3 =
116.90(12).
10
(Me)(NC6F5)]2BCl2 reported by Cowley and co-workers, in
which the C3N2B six-membered ring is planar, the boron atom
in 2 is out of the C3N2 plane, probably due to the steric
hindrance of the Ar and But groups. In addition, the Cl1 atom is
nearly perpendicular to the C3N2 plane with a long B1−Cl1
bond distance of 1.9099(18) Å.
Treatment of 1 and 2 with 2 equiv of MesLi (Mes = 2,4,6-
Me3C6H2) in toluene at 40 °C resulted in the cyclopropanation
of one of the CBut groups to yield compound 3 instead of
substitution of the chloride ligand (Scheme 2). Reaction of 1
with LiN(SiMe3)2 also led to a similar cyclopropanation
reaction with an additional substitution reaction by the lithium
salt to yield 4. Reaction of 3 with MesLi in hot toluene led to
the formation of a complicated mixture, which could not be
characterized. Compound 3 and 4 have been fully characterized
by 1H, 11B, and 13C NMR spectroscopy, elemental analysis, and
Figure 4. Molecular structure of 4. Selected bond lengths (Å) and
angles (deg): B1−N1 = 1.483(3), B1−N2 = 1.454(3), B1−N3 =
1.457(3), N2−C1 = 1.410(7), C1−C2 = 1.538(7), C2−C3 =
1.496(6), C1−C3 = 1.536(6), C1−C6 = 1.476(6), C6−C7 =
1.343(6), C7−N3 = 1.505(6); N2−B1−N3 = 116.2(2), N1−B1−N2
= 122.5(2), N1−B1−N3 = 121.3(2), C1−C2−C3 = 60.8(3), C2−
C1−C3 = 58.3(3), C1−C3−C2 = 60.9(4).
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Communication
cyclopropane framework with C−C bond lengths in the range
of 1.496(6)−1.541(9) Å and C−C−C bond angles in the range
of 58.3(3)−61.3(5)°. The three-coordinate boron atoms in 3
and 4 are planar (the sums of the angles around the boron
atom are 360.1 and 360.0°, respectively, in 3 and 4). The B−N
bond distances in 3 (1.412(3) Å) are noticeably shorter than
the corresponding distances in 4 (1.454(3) and 1.457(3) Å)
because the existence of the exocyclic B1−N1 bond in 4 leads
to reduced B−N π interactions in the six-membered ring. The
C1−N1 and C1−N2 bond distances in 3 (1.426(13) Å) and 4
(1.410(7) Å) are much shorter than C7−N1′ (1.517(11) Å) in
3 and C7−N3 (1.505(6) Å) in 4, probably due to the nitrogen
lone pair interactions with the C1−C2−C3 three-membered
rings.
In contrast, reactions of 1 and 2 with the less hindered PhLi
(Ph = C6H5) and LiNEt2, under the same conditions as for the
formation of 3 and 4, resulted in nucleophilic substitution on
the boron atom instead of cyclopropanation. Reaction of 1 with
2 equiv of LiNEt2 afforded the monoamino-substituted product
5, while the reaction with PhLi gave the diphenyl-substituted
borane 6 (Scheme 3). Compounds 5 and 6 have been
type of cyclopropanation reaction has not been reported for
bulky β-diketiminato ligands with β-But groups.
In summary, the initial formed azaallyl boron dichloride 1
undergoes an isomerization reaction to give the corresponding
β-diketiminato compound 2. The bulky Lewis bases MesLi and
LiN(SiMe3)2 deprotonate one of the β-But groups of 2 to yield
the cyclopropanation products 3 and 4, while the less sterically
hindered bases LiNEt2 and PhLi lead to the formation of the
normal substitution products 5 and 6. These types of
isomerization and cyclopropanation reactions have not been
reported for other elemental and metal complexes supported by
β-diketiminato ligands, indicating the unique bonding and
chemical properties of this class of boron compounds. The
investigation of the reduction chemistry of 1 and 2 with various
reducing reagents is currently in progress.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, a figure, and a table giving synthetic procedures and
characterization data for the new compounds reported in this
paper and CIF files giving crystallographic data for compounds
1−5. This material is available free of charge via the Internet at
http://pubs.acs.org.
Scheme 3. Reaction of 1 with PhLi and LiNEt2
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: cmcui@nankai.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
characterized by 1H, 11B, and 13C NMR and EI mass
spectroscopy and elemental analysis. The NMR spectra of 6
show only one set of signals for the substituents on the C3N2B
backbone, indicating that its structure is similar to that of 2.
We are grateful to the National Natural Science Foundation of
China and the 973 program (Grant No. 2012CB821600) for
the support of this work.
1
However, the H NMR spectrum of 5 displays one set of
REFERENCES
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