Crystal clear structural evidence for electron derealization in 1,2-dihydro-1,2-azaborines

Article abstract of DOI:10.1021/ja8024966

The first examples of "pre-aromatic" 1,2-dihydro-1,2-azaborine heterocycles have been structurally characterized, enabling the direct comparison of delocalized bonds of 1,2-dihydro-1,2-azaborines to their corresponding formal double and single bonds in no

Full text of DOI:10.1021/ja8024966

Published on Web 05/14/2008
Crystal Clear Structural Evidence for Electron Delocalization in
1,2-Dihydro-1,2-azaborines
Eric R. Abbey, Lev N. Zakharov,* and Shih-Yuan Liu*
Department of Chemistry, UniVersity of Oregon, Eugene, Oregon 97403
Received April 10, 2008; E-mail: lsy@uoregon.edu
The isolation and structural description of benzene marked the
birth of the concept of aromaticity.2,3 Since this important discovery
more than a century ago, derivatives of benzene, that is, arenes,
have been playing a pivotal role not only in the field of chemistry
but also in other scientific disciplines.4,5 1,2-Dihydro-1,2-azaborine
(from now on abbreviated as 1,2-azaborine) is related to benzene
by substitution of a single CC bond unit of benzene with an
isoelectronic BN bond.6
Figure 1. Strategy for determining electron delocalization in 1,2-azaborines:
direct structural comparison.
Scheme 1. Synthesis of BN-Heterocycle Precursor 1
Despite its seemingly simple structure, relatively little is known
about this family of heterocycles compared to their isoelectronic
analogues. Dewar7,8 and White9 pioneered the chemistry of 1,2-
azaborine and its ring-fused polycyclic derivatives in the 1960s.10
Recent contributions by Ashe,11–18 Piers,19–24 and Paetzold25 have
facilitated the preparation of novel BN-heterocycles and sparked a
renewed interest in these compounds. As part of a program directed
toward the development and application of 1,2-azaborines as
versatile arene surrogates, we recently addressed a limitation
associated with existing synthetic methods for monocyclic 1,2-
azaborines, that is, the narrow scope with respect to the boron
substituent, by presenting the first general solution for the synthesis
of B-substituted 1,2-azaborines.26
Because of the similarity between 1,2-azaborine and the quintes-
sential aromatic compound, benzene, the characterization of its
aromaticity has been of substantial interest. In the past, assertions
of aromaticity of 1,2-azaborines have heavily relied on computa-
tional studies.27–30 Recent synthetic achievements have provided
experimental data to supplement the theoretical calculations. For
instance, Ashe has demonstrated that 1,2-azaborines can undergo
electrophilic aromatic substitutions.17 The 1H NMR chemical shifts
of 1,2-azaborines are consistent with the presence of aromatic ring
current effects.11b
The geometrical structure of conjugated compounds (i.e., their
bond length equalization due to delocalization) provides another
crucial criteria of aromaticity.31 For instance, geometry-based
indices of aromaticity have been developed to quantify the extent
of aromaticity in carbo- and heterocycles.32 To date, only five X-ray
crystal structures of non-π-bound 1,2-azaborines have been
reported.14,17,18,26 All of these structures exhibit a planar geometry,
with intra-ring C-C, C-B, C-N, and B-N bond lengths ranging
from 1.35-1.41, 1.50-1.53, 1.37-1.39, and 1.43-1.45 Å, respec-
tively. While these bond distances are consistent with a delocalized
picture of this six-membered heterocycle, the lack of structural data
of directly comparable reference compounds renders this description
somewhat arbitrary and ambiguous. We envisioned that the
synthesis of reference heterocycles A-D (Figure 1) and their direct
structural comparison with the presumed delocalized structure E
would provide an unambiguous picture of electron delocalization
in 1,2-azaborines.
To the best of our knowledge, no structural data are currently
available for monocyclic aminoboranes A-D illustrated in Figure
1.33 Furthermore, there are no examples of heterocycles C and D
in the literature. In this Communication, we present the synthesis
and structural characterization of a complete set of compounds
described in Figure 1 (A-E; R1 ) t-Bu, R2 ) NPh2). Direct
comparison of these structures unambiguously reveals localized
bonding in heterocycles A-D and delocalized bond distances in
the aromatic molecule E.
In our early work on B-substituted 1,2-azaborines, we noticed
that heterocycle 1 containing N-t-Bu and B-NPh2 substituents
furnishes highly crystalline solids suitable for single crystal X-ray
diffraction. Thus we chose this substitution pattern for our compara-
tive structural investigation.
The synthesis of 1 is described in Scheme 1. Condensation of
tert-butylallylamine with allylboron dichloride produced the diene
precursor F. Ring-closing metathesis34 of this intermediate using
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2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
the first generation Grubbs catalyst gave product G in 84% isolated
yield. Nucleophilic attack at the B-Cl bond in G with potassium
diphenylamide furnished the desired heterocycle 1.
We envisioned that heterocycle 1 could serve as a universal
precursor toward the four remaining target structures (2-5 in
Scheme 2). Indeed, dehydrogenation of 1 using Pd black26 generated
structure 2 (eq 1). Conversely, compound 1 could be hydrogenated
using the same catalyst to furnish 3 (eq 2). Prior to our studies, no
methods were available to synthesize heterocycle 4 and 5. Recog-
nizing that precursor 1 contains an allylamine fragment, we thought
that we could take advantage of existing isomerization methods to
produce the enamine isomer 4. Ruthenium complexes have been
shown to transform N-allylamines and N-allylamides into their
corresponding isomers in a catalytic fashion.35–37 Thus, we began
to investigate the use of commercially available ruthenium catalysts
for the synthesis of 4. We were surprised to discover a striking
difference in reactivity between two of the surveyed ruthenium
complexes. While Ru(H)(Cl)(CO)(PPh3)3 generated the desired
N-vinyl isomer 4 selectively (eq 3), we were delighted to observe
that attempts at the same transformation with Ru(Cl)2(PPh3)3
provided the B-vinyl isomer 5, also with high selectivity (eq 4).
The synthetic access to compounds 1-5 in Scheme 2 paves the
road for their structural analysis. Gratifyingly, we were able to
obtain structural data for all five heterocycles via single crystal
X-ray diffraction. Selected structural parameters are summarized
in Figure 2 and Table 1.38 Analysis of the data reveals the following
trends without exception: (1) all nonaromatic structures (i.e., 1, 3,
4, and 5) have BsN bond distances consistent with significant
double bond character (∼1.41 Å), which lengthen to 1.45 Å after
oxidation to 2 (red entries, Table 1); (2) formal CdC double bonds
in 1, 4, and 5 lengthen significantly upon aromatization (blue entries,
Table 1); (3) all formal single bonds shorten upon delocalization
(Table 1); (4) the puckered conformations in nonaromatic hetero-
cycles 1, 3, 4, and 5 become planar upon formation of 1,2-azaborine
2 (Figure 2, and Table 1, entry planarity).
Figure 2. ORTEP illustrations, with thermal ellipsoids drawn at the 35%
probability level, of BN-heterocycles 1-5.
Table 1. Selected Bond Distances and Deviations from Planarity
(Å) for BN-Heterocycles 1-5
a Root mean square deviation of intra-ring atoms from the
least-squares plane (in Å).
from planarity of 0.164 Å compared to 0.226 Å in 3 (Table 1).
However, it is less planar than 1,2-disubstituted 1,4-cyclohexadiene
structures, which are almost completely planar.41 The twisting
observed in 1 might be due to steric interactions between the
relatively bulky N-t-Bu and the B-NPh2 substituents. The bond
lengths in 1 (in Å) N(1)-B ) 1.405(2), and C(4)-C(5) ) 1.319(2)
are consistent with localized double bonds. The slight shortening
of the formal single bonds C(3)-C(4) and C(5)-C(6) in 1 compared
to 3 (1.493 Å vs 1.511 and 1.508, respectively) is consistent with
a contraction due to a change in hybridization from sp3 to sp2.
Partially conjugated 4 and 5 represent the two possible BN-
heterocyclic isomers equivalent to 1,3-cyclohexadiene. The bond
distances in 5 (in Å) N(1)-B ) 1.407(2), B-C(3) ) 1.559(2),
C(3)-C(4) ) 1.338(2) indicate a localized short-long-short
“diene” bond sequence. The torsion angle between the double bonds
of this “diene” N(1)-B-C(3)-C(4) is -30.9(2)°, indicating a
nonplanar geometry. Similarly, the structural parameters in 4 (in
Å) C(5)-C(6) ) 1.319(3), C(6)-N(1) ) 1.432(3), N(1)-B )
1.417(3), are also consistent with a localized “diene” moiety. The
valueofthetorsionangleof-25.2(3)°showsthattheC(5)-C(6)-N(1)-B
fragment in 4 is nonplanar as well.
Heterocycle 3 serves as a good reference point for structural
comparisons. The C-C, C-B, and C-N bond distances in 3 are
consistent with formal single bonds (Figure 2, and Table 1).39 The
B-N bond distance of 1.403(2) Å is consistent with a strong double
bond character (sum of covalent radii ) 1.56 Å). Thus, 3 is
isoelectronic and isostructural with a 1,2-disubstituted cyclohexene.
Structural comparisons in the Cambridge Crystallographic Database
(CCD) indicate that the half-chair conformation of 3 is also adopted
by many 1,2-disubstituted cyclohexenes.40
Compound 1 is analogous to a 1,2-disubstituted 1,4-cyclohexa-
diene. It is more planar than 3, with a root-mean-square deviation
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C O M M U N I C A T I O N S
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Direct comparison of partially conjugated “dienes” 4 and 5 with
fully delocalized 1,2-azaborine 2 highlights the clear contrast
between localized bonding in “dienes” 4 and 5 and delocalized
bonding in 2 (Table 1, bold entries). Upon forming the six
π-electron species 2, the N(1)-B bond lengthens, and both the
B-C(3) and C(6)-N(1) bonds shorten. This observation is
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contributing to the overall structure of 1,2-azaborine E, which is
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Structures 1-5 reveal with unprecedented detail the geometrical
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The boron in boron-containing heterocycles has been shown to
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bulky N-t-Bu and B-NPh2 substituents.
In summary, we have successfully structurally characterized the
first examples of “pre-aromatic” 1,2-azaborine heterocycles, en-
abling the direct comparison of delocalized bonds of 1,2-azaborines
to their corresponding formal double and single bonds in nonaro-
matic systems. The comprehensive data presented in this study
provide an unprecedented look into the structural changes that occur
in six-membered BN-heterocycles on their road to aromaticity, and
they establish with little ambiguity that 1,2-azaborines such as 2
possess delocalized structures consistent with aromaticity.
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Acknowledgment. Support has been provided by University
of Oregon and the Medical Research Foundation of Oregon.
Funding for the University of Oregon Chemistry Research and
Instrumentation Services has been furnished in part by the National
Science Foundation (Grant CHE-0234965). HR-MS data were
obtained at the Mass Spectroscopy Facilities and Services Core of
the Environmental Health Sciences Center at Oregon State Uni-
versity. Financial support for this facility has been furnished in part
by the National Institute of Environmental Health Sciences, NIH
(P30 ES00210).
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Supporting Information Available: Experimental procedures for
the synthesis of compounds 1-5, compound characterization data, CIF
files for structures 1-5. This material is available free of charge via
the Internet at http://pubs.acs.org.
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References
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