Regioselective Amine–Borane Cyclization: Towards the Synthesis of 1,2-BN-3-Cyclohexene by Copper-Assisted Triazole/Gold Catalysis

Article abstract of DOI:10.1002/anie.201604986

The combination of triazole/gold (TA-Au) and Cu(OTf)₂is identified as the optimal catalytic system for promoting intramolecular hydroboration for the synthesis of a six-membered cyclic amine–borane. Excellent yields (up to 95 %) and regioselectivities (5-exo vs. 6-endo) were achieved through catalyst control and sequential dilution. Good functional-group tolerance was attained, thus allowing the preparation of highly functionalized cyclic amine–borane substrates, which could not be achieved using other methods. Deuterium-labeling studies support the involvement of a hydride addition to a gold-activated alkyne with subsequent C?B bond formation.

Full text of DOI:10.1002/anie.201604986

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International Edition: DOI: 10.1002/anie.201604986
German Edition: DOI: 10.1002/ange.201604986
B,N-Heterocycles
Regioselective Amine–Borane Cyclization: Towards the Synthesis of
1,2-BN-3-Cyclohexene by Copper-Assisted Triazole/Gold Catalysis
Stephen E. Motika, Qiaoyi Wang, Novruz G. Akhmedov, Lukasz Wojtas, and Xiaodong Shi*
Abstract: The combination of triazole/gold (TA-Au) and
Cu(OTf)2 is identified as the optimal catalytic system for
promoting intramolecular hydroboration for the synthesis of
a six-membered cyclic amine–borane. Excellent yields (up to
95%) and regioselectivities (5-exo vs. 6-endo) were achieved
through catalyst control and sequential dilution. Good func-
tional-group tolerance was attained, thus allowing the prepa-
ration of highly functionalized cyclic amine–borane substrates,
which could not be achieved using other methods. Deuterium-
labeling studies support the involvement of a hydride addition
Discrete subgroups of the six-membered 1,2-aminoborane
can be identified based on oxidation states. This includes the
fully oxidized aromatic aminoborane, cyclohexadiene B
(Scheme 1), and cyclohexene A. Dewar and Marr[5] and
White[6] first reported the synthesis of a monocyclic, aromatic
azaborine, though these early works offered poor functional-
group tolerance and operated under harsh reaction condi-
tions. Later, work by Ashe and Liu led to the effective revival
of this field through seminal reports on the synthesis of the
fully aromatic 1,2-dihydro-1,2-azaborine (direct benzene
isostere) through the synthesis and successive oxidation of
B.[7] A key synthetic step involved a ring-closing metathesis
À
to a gold-activated alkyne with subsequent C B bond for-
mation.
À
(RCM) of di-allylic N B precursors to afford the key cyclic
B
oron-containing heterocycles have attracted attention
scaffold.[8] Although this method has maintained a state-of-
the-art status for simpler six-membered cyclic aminoborane,
relatively expensive and highly reactive reagents are required
with overall modest yields.
because of their broad utility in chemical, biological, and
materials research.[1] A prominent subgroup is the cyclic 1,2-
À
amino-borane, which contains a N B subunit that is isoelec-
tronic to a C C double bond. The application of these
[2]
À
In the last several years, our group has developed new
transition-metal catalysts containing the 1,2,3-triazole
ligand.[9] Those extended efforts have led to the discovery of
1,2,3-triazole/gold (TA-Au) complexes as a new class of
catalysts with improved stability.[10,11] By using these catalysts,
we recently disclosed a unique strategy to access five-
compounds as hydrogen storage materials[3] or as precursors
for the preparation of the aromatic 1,2-azaborine has
garnered significant attention over the past few decades
(Scheme 1).[4] The preparation of these compounds still
remains challenging though, because of limited effective
synthetic approaches. For this reason, new strategies which
offer good functional-group tolerance, high efficiency, and
scalability are desired. In this work, we report the synthesis of
1,2-BN-3-cyclohexene through a copper-assisted triazole/
gold-catalyzed intramolecular hydroboration.
À
membered N B-containing heterocycles through TA-Au-
catalyzed alkyne hydroboration.[12] Key to this discovery
was the need for heightened catalyst stability in the presence
of a CN-modified amine–borane. Thus, when coupling the
enhanced stabilizing effects of the TA ligand and a sterically
influential XPhos primary ligand, excellent conversions and
yields were attained, thus providing the cyclic amine–borane 2
(Figure 1a). All other tested catalyst permutations absent of
triazole led to rapid catalyst decomposition, therefore high-
lighting the crucial role of the triazole ligand in stabilizing the
gold cation in a reducing reaction environment.
Scheme 1. 1,2-Aminoborane analogues.
[*] S. E. Motika, Dr. L. Wojtas, Prof. Dr. X. Shi
Department of Chemistry, University of South Florida
Tampa, FL 33620 (USA)
E-mail: xmshi@usf.edu
Dr. Q. Wang, Dr. N. G. Akhmedov
Department of Chemistry, West Virginia University
Morgantown, WV 26505 (USA)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201604986.
Figure 1. Challenges in the synthesis of 1,2-BN-cyclohexene. DCE=1,2-
dichloroethane, Tf=trifluoromethanesulfonyl.
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We then sought to extend this protocol to access a modular
studies of a TA-Au-catalyzed propargyl ester rearrange-
ment.[15] The kinetic data clearly supports the formation of an
alkyne–gold p complex as the turnover-limiting step, and
explained the observed chemoselectivity when using the TA-
Au catalyst (activation alkyne over allene).
À
synthetic platform for six-membered N B cycles (Scheme 1;
C). According to the literature, there are only two reported
examples regarding the syntheses and applications of 1,2-BN-
3-cyclohexadiene (C and D).[7c,3d] For this reason, there is also
little known about this 1,2-aminoborane subgroup, which
alone warrants further investigation.
With this mechanistic insight, we postulate that the
addition of a Lewis-acidic metal cation will aid in the
dissociation of the triazole ligand to yield an active gold
catalytic system. Moreover, it is possible that the presence of
triazole will beneficially assist in the deauration step through
the formation of TA-Au.[16] Thus, the metal-assisted TA-Au
catalysts may achieve improved reactivity with the retention
of stability toward the borohydride. To test this idea, we
charged 3a with a combination of a gold catalyst and other
Lewis acids. As expected, the addition of different metal
cations significantly improved the TA-Au catalyst reactivity.
Finally, with the combination of 10 mol% [(ArO)3PAu(TA-
H)]OTf and 10 mol% Cu(OTf)2, 100% conversion of 3a was
achieved with 4a and 5a obtained in greater than 95% yield.
Furthermore, no conversion of the starting material was
observed when only a Lewis acid was used.[17] Several results
obtained under alternative reaction conditions are summar-
ized in Table 1. Typical cationic gold ([LAu]+; entry 4) gave
poor conversions and low yields of the cyclic aminoborane
because of the rapid catalyst decomposition. The use of
TA-Au alone (entry 2) also gave subdued conversions and
yields because of the reduced reactivity of the gold catalyst.
Interestingly, using PPh3 as the primary ligand, the corre-
sponding TA-Au catalyst gave slightly better yield than
When treating the homopropargyl amine derivatives 3a
and 3b under the previously optimized reaction conditions,
less than 15% conversion was obtained along with complete
gold decomposition (based on 31P NMR data) after 6 hours
(Figure 1b). Moreover, the reaction of 3a gave a mixture of
the 6-endo product 4a and 5-exo product 5a in a 1:1 ratio.
Absolute structural confirmation for the cyclization products
4a and 4b were obtained using single-crystal X-ray analysis
(Figure 2). The structure of product 5 was characterized using
comprehensive NMR analysis (see the Supporting Informa-
tion). Notably, for both 4a and 4b, only cis-isomers were
obtained based on X-ray crystallography and NMR spectros-
copy.
Figure 2. X-ray structures of BN-cyclohexene. Thermal ellipsoids shown
at 50% probability.[21]
This result revealed two major challenges which were
absent in our seminal NB-heterocycle synthesis: a) more
subdued reaction kinetics requiring a more reactive catalyst
which is still capable of withstanding reduction from the
present borohydride; b) poor regioselectivity (desired 6-endo
over kinetically favored 5-exo).
Table 1: Optimization of catalysts.[a,b]
To circumvent these issues, we first re-examined the
general design of the triazole/gold catalyst. As shown in
Scheme 2, the triazole effectively stabilizes the gold cation
Entry Variations from the standard
reaction conditions
Conv. Yield [%] 4a/5a
[%]
(4a+5a)
1
2
3
none
100
70
<70 <55
96
34
2:1
1:1
1:1
without Cu(OTf)2
Other tested M(OTf)n salt
instead of Cu(OTf)2
[c]
4
5
6
7
[Au]=10% LAuNTf2
<40 <18
1:1
1:1
1:1
1:1
L=PPh3, IPr, XPhos, (ArO)3P
10 mol% [PPh3Au(TA-H)]OTf
without Cu
10 mol% [XPhosAu(TA-H)]OTf
without Cu
63
75
65
48
41
57
10 mol% [(ArO)3PAu(TA-H)]OTf
without Cu
Scheme 2. Proposed new strategy: M+-assisted TA-Au activation.
7
8
9
10
11
[Au]=10 mol% [PPh3Au(TA-H)]OTf
[Au]=10 mol% [XPhosAu(TA-H)]OTf
[Au]=10 mol% [(ArO)3P(TA-H)]OTf
[Au]=10 mol% [(ArO)3P(TA-Me)]OTf
[Au]=5 mol% [(ArO)3P(TA-Ph)]OTf
70
84
90
88
58
34
55
85
84
53
2:1
2:1
2:1
2:1
2:1
through formation of a coordinatively saturated [L-Au-TA]+
complex, thereby reducing the net concentration of
[L-Au]+.[13] This feature explains the observed reduced
reactivity of TA-Au compared to the corresponding [L-Au]+
catalyst.[14] In fact, monitoring TA-Au-catalyzed reactions by
31P NMR spectroscopy showed TA-Au as the dominant signal
throughout the reaction (resting state). Using React IR and
NMR spectroscopy, we recently reported the mechanistic
[a] General reaction conditions: 3a (0.2 mmol), [Au cat.] (5–10 mol%),
Cu(OTf)2 (5–10 mol%), dichloromethane (0.2m), RT. [b] 1H NMR yields
determined using 1,3,5-trimethoxybenzene as an internal standard.
[c] M(OTf)n =Ga(OTf)3, Zn(OTf)2, In(OTf)3, etc.; Ar=2,4-di-tert-butyl-
phenyl, DCM=dichloromethane, TA-Ph=N1-phenylbenzotriazole.
2
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Table 3: Reaction scope.[a,b]
XPhosAu(TA) (entries 5 and 6). This outcome suggests that
the electronic and steric influences inherent to XPhos, which
were pivotal in our previous study, play less of a role in this
system. We then turned to an even weaker s-donating
phosphite ligand, (ArO)3P, coupled with the TA ligands. As
expected, significantly improved conversion and yield were
observed (entry 7). Finally, the application of N-phenyl-
substituted benzotriazole (TA-Ph) gave the optimal result,
with 100% conversion of 3a and formation of products (4a
and 5a) in greater than 95% overall yield. Notably, excellent
stereoselectivity was observed for both 4a and 5a (d.r. > 20:1,
cis only confirmed by X-ray). This result further validated the
Lewis acid assistance in TA-Au activation.
Our next focus was to improve the poor regioselectivity.
As seen in the screening conditions, the addition of Cu(OTf)2
enhanced the final regioselectivity (from 1:1 to 2:1). This
result suggested that the underlying mechanistic pathways
might be different between the 5-exo and 6-endo routes. We
speculated that the product variability could be a result of
a competitive intermolecular versus intramolecular hydro-
boration.
To test this hypothesis, we conducted the reaction at dilute
concentrations. As expected, different ratios were observed
(Table 2). Fortunately, under dilute conditions, the desired 4a
was observed as the major product. With a 20% copper
Table 2: Tuning the additive and concentration.[a,b]
[a] General reaction conditions: 3a (0.2 mmol), [Au cat.] (10 mol%),
Cu(OTf)2 (10 mol%), dichloromethane (0.01m:0.2m=terminal/internal
alkyne), RT. [b] Yield of isolated product. TBS=tert-butyldimethylsilyl.
Cu(OTf)2 [M] Yield [%] 4a/5a Cu(OTf)2 [M] Yield [%] 4a/5a
(mol%)
4a 5a
(mol%)
4a 5a
could not be assessed though, as the starting material was
unstable at room temperature.[18]
none
10
20
0.2
0.2
0.2
18 19
60 31
73 24
1:1
2:1
3:1 none
3:1 20
10
5
0.01 80 16 4.5:1
0.05 34 16 2.5:1
Generally, both neutral and electron-withdrawing groups
on aryl-substituted internal alkynes worked well, thus giving
the products in excellent yields (4b, 4 f, 4g, 4h; Table 3). In
the case of the more-electron-rich aryl alkyne 4i, incomplete
conversion was observed, but a high yield was attained (based
on recovered starting material). The heterocycle-substituted
alkyne 4k and TMS-substituted alkyne 4j both worked well,
and further highlights the mild nature of this method.
It was evident that primary amine derivatives underwent
the reaction with much lower reaction rate, thus leading to
incomplete conversions (Table 3). The terminal alkyne 4p
gave low overall conversion, while a less reactive internal
alkyne provided no reaction at all under the optimized
reaction conditions. In general, alkyl-substituted internal
alkynes gave reduced regioselectivity (4n/5n = 3:1; 4o/5o =
5:1), even under dilute conditions. As discussed above, it is
reasonable to suggest that the five-membered-ring formation
may occur by an intermolecular process given the strong
dilution effect. However, for the six-membered-ring cycliza-
tion, both steric and/or electronic factors could strongly
influence the final regioselectivity (i.e. Markovnikovꢀs rule),
thus leading to the observed reduced selectivity. Overall, the
exact reaction path of this new transformation is interesting
0.05 34 45
0.01 90 7
1:1.5
13:1
10
0.05 72 21
[a] General reaction conditions: 3a (0.2 mmol), [Au cat.] (10 mol%),
Cu(OTf)2, dichloromethane, RT. [b] Yield determined by 1H NMR
spectroscopy using 1,3,5-trimethoxybenzene as internal standard.
loading at 0.01m, excellent regioselectivity was obtained
(13:1) with 4a being isolated in an excellent yield (90%).
With the optimal reaction conditions in hand, we explored
the reaction scope. The results are shown in Table 3. Various
terminal and internal alkynes could be tolerated under the
optimal reaction conditions, thus providing the BN-cyclo-
hexenes in excellent yields and regioselectivities. Initially,
terminal alkyne substrates containing different nitrogen
substitution were assessed. Excellent yields were observed
in all cases, even in the presence of increasingly more reactive
benzylic hydrogen atoms (4a, 4c, and 4d). A sterically
hindering cyclohexyl group at the nitrogen position showed
no adverse effect (4m; 90% yield). The Lewis-basic OTBS
group was well tolerated, thus giving the desired cyclization
product in excellent yields (4e and 4l). The aniline derivative
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and may be complicated. Detailed mechanistic investigations
are ongoing.
Following the successful optimization of cyclization con-
ditions, we submitted the cyano-hydride product 4 to LiAlH4
with the benzene derivative observed in very low yields
(< 5%). With the goal to develop a practical synthesis of BN-
containing organic compounds (gram-scale synthesis and
good functional group tolerability), our group is currently
focused on investigations towards determining the optimal
reaction conditions for both oxidation (to azaborine) and
reduction (to BN-cyclohexane). Those results will be reported
in due course.
À
for B CN cleavage. As shown in Figure 3, the desired
In summary, we report a novel cyclization protocol
towards the synthesis of 1,2-BN-3-cyclohexenes. The combi-
nation of a Lewis acid and a triazole/gold complex provides
improved catalyst reactivity with good stability. Critical
dilution led to excellent regioselectivity, thus suggesting
a possible intermolecular reaction pathway for the 5-exo-dig
cyclization. Although final reaction conditions for oxidation
and reduction of this BN-cyclohexene have not been revealed
at this moment, the reported work initiated the critical step in
developing the gold-catalyzed hydroboration as a novel and
Figure 3. Post-cyclization functionalization approaches. X-ray struc-
tures[21] with thermal ellipsoids shown at 50% probability. TFA=tri-
fluoroacetic acid.
À
practical protocol for the synthesis of N B-containing com-
pounds with diverse substitute patterns and excellent overall
efficiency.
reduced products (6a–c) were observed in excellent yields.
Treating the borane 6c with TFA (1.0 equiv) gave the
dehydrogenation product 7. Structures of 6 and 7 were
unambiguously confirmed by X-ray crystallography.[19]
Some mechanistic probes were performed using deute-
rium-labeled samples. To confirm the initial hydride addition
mechanism, we synthesized two deuterium-labeled precursors
to monitor potential isotope scrambling (Figure 4). For the
Acknowledgments
We acknowledge the NSF (CHE-1619590) and NSFC
(21228204) for financial support.
Keywords: boranes · copper · heterocycles · gold ·
reaction mechanisms
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[18] For aniline derivatives, only trace amounts of the desired acyclic
precursor could be observed by crude NMR spectroscopy.
Isolation proved futile, as complete decomposition was evident
under various purification protocols.
À
[19] Investigations towards N B dehydrogenation were performed
À
following B CN cleavage. At this point, modest yields of the
desired dehydrogenated product have been attained. We include
the trifluormethylation, as this represents a better leaving group
compared to ÀCN. The TFA adducts have been the most
À
effective in further N B dehydrogenation and will published in
due course.
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(7) contain the supplementary crystallographic data for this
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Cambridge Crystallographic Data Centre.
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Received: May 21, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
B,N-Heterocycles
S. E. Motika, Q. Wang, N. G. Akhmedov,
L. Wojtas, X. Shi*
&&&& — &&&&
Regioselective Amine–Borane
Cyclization: Towards the Synthesis of 1,2-
BN-3-Cyclohexene by Copper-Assisted
Triazole/Gold Catalysis
Golden combo: The combination of tria-
zole/gold (TA-Au) and copper salts is the
optimum catalytic system for intramo-
lecular hydroboration to synthesize six-
membered cyclic amine–boranes. Excel-
lent yields and regioselectivities were
achieved, and good functional-group tol-
erance was attained. Deuterium-labeling
studies support the involvement of an
initial hydride addition to a gold-activated
À
alkyne and subsequent C B bond for-
mation.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!