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Synthesis of Rh h -1,2-azaborete complexes
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The isolation of h -1,2-azaborete complexes from stoichiomet-
ric reactions of iminoboranes, alkynes, and [{(iPr P) RhCl} ], or
3
2
2
alternatively iminoboranes and the p-alkyne complex trans-
[
9]
[
(iPr P) RhCl(PhCCPh)], was a clear sign that such complexes
3 2
acted as intermediates in the catalytic construction of both
Scheme 2. Rhodium-catalyzed syntheses of 1-tert-butyl-2-mesityl-1,2-azabori-
nine (4) and 1-tert-butyl-2-mesityl-4,6-diphenyl-1,2-azaborinine (5).
4
1
1
,2- and 1,4-azaborinine complexes. The ability to isolate h -
,2-azaborete complexes also provides a method for the incor-
poration of two different alkynes regioselectively into the aza-
borinine ring system. The second alkyne, if bulky, was shown
by deuterium labeling to insert into the BÀC bond of the aza-
with 2.8 molar equivalents of phenylacetylene, respectively.
The new 1,2-azaborinines 4 and 5 (Scheme 2) were obtained
and purified by flash chromatography, giving moderate yields.
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borete ring. To better understand h -1,2-azaborete complexes
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The B NMR spectra of 4 (d 37 ppm) and 5 (d 41 ppm) showed
resonances upfield of those previously observed in our work,
which did not allow the distinction between 1,2- (d 47,
(of which only two examples are known), and to provide new
platforms for the late-stage incorporation of functional alkynes
into the azaborinine scaffold, we treated [{(iPr P) RhCl} ] with
3
2
2
5
0 ppm) and 1,4-azaborinine (d 48 ppm) isomers. Signals for
roughly 3 equiv of 2 and either acetylene (1 atm) or phenylace-
tylene (1.0 equiv) at room temperature. h -1,2-Azaborete com-
1
4
the aromatic azaborinine protons were found in the H NMR
spectra (4: d 7.48, 7.27, 6.78, 6.24 ppm; 5: d 7.02, 6.58 ppm). In
general, few reliably diagnostic methods exist for distinguish-
ing between 1,2- and 1,4-azaborinine isomers by one-dimen-
sional NMR techniques, and thus the connectivities of 4 and 5
plexes 6 and 7 were obtained in good yields as orange pow-
ders (Scheme 3). Both compounds showed resonances in the
1
1
31
B (6: d 22.5 ppm; 7: d 21.9 ppm) and P NMR spectra (6: d
1
1
62.9 ppm, J =199 Hz; 7: d 59.8 ppm, J =196 Hz) that are
RhP
RhP
(
and all 1,2-azaborinine compounds reported herein) were con-
nearly indistinguishable from those of previously reported rho-
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firmed by 2D NMR techniques. The exclusive formation of 1,2-
isomers in these reactions was surprising, as earlier work had
suggested that the use of acetylene as the alkyne unit led to
dium h -1,2-azaborete complexes.
Single-crystal X-ray crystallographic analyses confirmed the
proposed structures of 6 and 7 (Figure 4), the latter of which
contains a phenyl group at the 4-position of the azaborete
1
,4-isomers, while functionalized alkynes provided 1,2-iso-
[
9]
4
mers. While 4 features no functionalized carbon atoms, 5
was formed regioselectively, with phenyl groups at the 4- and
ring, in line with previously reported h -1,2-azaborete com-
[
9]
4
plexes. Similar to the reported complexes [h -1,2-{N(tBu)B-
6
-positions, in line with the reported construction of 1,2-di-
[9c]
tert-butyl-4,6-diferrocenyl-1,2-azaborinine.
Crystallographically determined structures of 4 and 5 are
shown in Figure 3. The most striking difference between the
exo
two structures is the relatively small B-N-C
angle in 5
(
121.2(1)8) compared to that of 4 (128.10(9)8). Due to the con-
gested steric environment at the B-N-C1 portion of 5, the ni-
trogen atom is pushed out of the plane of the ring by 0.143
exo
4
(
only 0.009 in 4), which is also shown in the increased C -N-
Scheme 3. Synthesis of rhodium h -1,2-azaborete complexes 6 and 7.
B-C6 torsion angle of 23.0(2)8 (only 0.4(2)8 in 4).
Figure 4. Molecular structures of 6 and 7. Ellipsoids are set at 50% probabili-
ty; hydrogen atoms and some ellipsoids are omitted for clarity. Selected
bond lengths [] and angles [8] for 6: B–N 1.529(3), N–C1 1.439(4), C1–C2
1.424(3), B–C2 1.548(5), N–C3 1.482(3), B–C4 1.554(4), Rh–P 2.2816(8), Rh-
centroid(B-N-C1-C2) 1.885; N-C1-C2 95.8(2), C1-C2-B 88.2(2), C2-B-N 87.4(2),
N-B-C1 88.4(2), C1-N-C3 126.0(2), C3-N-B 140.5(2), N-B-C4 132.6(3), C4-B-C2
137.8(3). For 7: B–N 1.524(3), N–C1 1.450(3), C1–C2 1.433(3), C2–B 1.539(3),
C1–C5 1.472(3), N–C3 1.492(3), B–C4 1.569(3), Rh–P 2.2949(5), Rh-centroid(B-
N-C1-C2) 1.861; N-C1-C2 95.5(2), C1-C2-B 88.5(2), C2-B-N 88.0(2), N-B-C1
88.0(2), C1-N-C3 132.1(2), C3-N-B 134.5(2), N-B-C4 132.0(2), C4-B-C2 138.5(2).
Figure 3. Molecular structures of 4 and 5. Ellipsoids are set at 50% probabili-
ty; hydrogen atoms and some ellipsoids are omitted for clarity. Selected
bond lengths [] and angles [8] for 4: B–N 1.447(2), N–C1 1.384(1), C1–C2
1.359(2), C2–C3 1.415(2), C3–C4 1.360(2), C4–B 1.525(2); N-C1-C2 124.2(1),
C1-C2-C3 120.7(1), C2-C3-C4 119.3(1), C4-B-N 115.5(1), B-N-C1 118.8(1), C4-B-
C6 116.2(1), C6-B-N 128.3(1), B-N-C5 128.10(9). For 5: B–N 1.463(2), N–C1
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.516(2), C1–C2 1.371(2), C2–C3 1.423(2), C3–C4 1.369(2), C4–B 1.395(2); N-
C1-C2 122.1(1); C1-C2-C3 123.8(1), C2-C3-C4 117.0(1), C4-B-N 117.0(1), B-N-C1
17.4(1), C6-B-N 127.1(1), B-N-C5 121.2(1).
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Chem. Eur. J. 2016, 22, 8603 – 8609
8605
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim