Double Hydroboration of Quinolines via Borane Catalysis: Diastereoselective One Pot Synthesis of 3-Hydroxytetrahydroquinolines

Article abstract of DOI:10.1002/adsc.201901050

Described herein is an organoborane-catalysed consecutive borylative reduction of quinolines and isoquinolines to furnish tetrahydro(iso)quinolines bearing a C(sp³)?B bond β to the nitrogen atom. The installed C?B bond is oxidatively transformed to the hydroxy group in one pot. The present double hydroboration is proposed to proceed via a stepwise ionic mechanism involving a boronium ion. The stereo-outcome was found to be dependent on the position (C2 vs C4) of the substituents in quinolines. (Figure presented.).

Full text of DOI:10.1002/adsc.201901050

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DOI: 10.1002/adsc.201901050
Double Hydroboration of Quinolines via Borane Catalysis:
Diastereoselective One Pot Synthesis of 3-
Hydroxytetrahydroquinolines
Eunae Kim,a, c Hyun Ji Jeon,a, b Sehoon Park,a, b,* and Sukbok Changa, b,*
a
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea
E-mail: sbchang@kaist.ac.kr; sehoon.park@gtiit.edu.cn
Center for Catalytic Hydrocarbon Functionalization, Institute for Basic Science (IBS), Daejeon 34141, Korea
Korea Research Institute of Chemical Technology (KRICT), Daejeon 34141, Korea
b
c
Manuscript received: August 21, 2019; Revised manuscript received: September 12, 2019;
Version of record online: ■■■, ■■■■
Dedicated to Professor Eric N. Jacobsen on the occasion of his 60th birthday
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201901050
can be precisely added. Moreover, an additional
synthetic benefit of this hydrosilane reduction ap-
proach can be achieved in that a silyl group can be
lines and isoquinolines to furnish tetrahydro(iso)
Abstract: Described herein is an organoborane-
catalysed consecutive borylative reduction of quino-
quinolines bearing a C(sp3)À B bond β to the nitro-
gen atom. The installed CÀ B bond is oxidatively
transformed to the hydroxy group in one pot. The
present double hydroboration is proposed to proceed
via a stepwise ionic mechanism involving a
boronium ion. The stereo-outcome was found to be
dependent on the position (C2 vs C4) of the
substituents in quinolines.
incorporated into products by forming a new CÀ Si
bond during the course of the reductive process.
Indeed, our group recently reported the B(C6F5)3-
catalyzed silylative reduction of pyridines, quinolines,
and α,β-unsaturated nitriles and imines to afford C
(sp3)À Si moiety-containing products (Scheme 1b).[5]
On the other hand, a number of elegant catalytic
hydroboration methodologies for generating six-mem-
bered N-heteroarenes have been developed by using
organohydroboranes such as pinacolborane to selec-
tively afford N-borylated 1,2- or 1,4-dihydro-products
(Scheme 1c).[6] For example, in situ generated metal
hydride species based on Mg, Rh, Zn, La, and Th were
found to promote 1,2-hydroboration of pyridines[6a–f]
while 1,4-selectivity was observed via an ionic outer-
Keywords: Lewis acidic borane; hydroboration; con-
secutive reaction; ionic mechanism; diastereoselectiv-
ity
Tetrahydroquinolines are a prevalent synthetic motif in sphere pathway catalysed by Lewis or Brønsted
numerous biologically active natural products and acids.[6g–m] Despite these recent advances in the
pharmaceutical compounds (Scheme 1a).[1] As a result, dearomative hydroboration of N-heteroarenes, to our
there have been considerable interests in the develop- best knowledge, no catalytic procedures have been
ment of efficient and selective catalytic routes to revealed thus far for the sp3 CÀ B bond-forming
tetrahydroquinolines and their derivatives.[2] Hydro- hydroboration of N-heteroaromatics.[7]
genation of quinolines is undoubtedly the most
Herein we report a B(C6F5)3-catalyzed double
straightforward and atom-economic approach to access hydroboration of quinolines to give rise to β-borylated
tetrahydroquinolines.[3] However, this procedure often tetrahydroquinolines (Scheme 1d). It represents the
leads to exhaustively reduced products with inferior first catalyst system toward consecutive borylative
functional group tolerance, thus limiting the resultant reduction of quinolines. This transition metal-free
molecular diversity in the reduced N-heterocyclic borane catalysis was found to proceed via a stepwise
products to serve as a synthetic intermediate. In this ionic mechanism, which eventually leads to 2,3-cis or
regard, reduction of N-heteroaromatics with hydro- 3,4-trans products depending on the substituent’s
silanes is an attractive alternative[4] mainly because the position on the N-aromatic skeleton.
stoichiometric amounts of reductants, hydrosilanes,
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Table 1. Optimization of Reaction Conditions.[a]
entry changes from the “standard conditions” Yield of 1
(%)[b]
1
2
3
4
5
6
7
8
None
82
80
70
60
77
79
38
80
Addition of PPh3 (5 mol%)
Addition of DABCO (5 mol%)
C6D6 instead of CDCl3
°
°
25 C instead of 85 C
°
°
110 C instead of 85 C
HBpin instead of HBcat
HBcat (2.2 equiv.) instead of HBcat
(4.0 equiv.)
9
HBcat (1.1 equiv.) instead of HBcat
(4.0 equiv.)
0.5 h instead of 24 h
<1 (87)[c]
10
44 (46)[d]
[a] Reactions were carried out in 0.5 mmol scale in a J. Young
NMR tube. Conversion of quinoline in all entries was
quantitative.
Scheme 1. 3-Hydorxytetrahydroquinolines as a structural motif
of pharmaceutical drugs and B(C6F5)3-catalyzed dearomative
hydroboration reactions.
[b] Determined by 1H NMR of the crude reaction mixture on the
basis of 1,1,2,2-tetrachloroethane (TCE) as an internal
standard.
[c] Combined yield of dihydroquinolines: [1,2-(20%)/1,4-
(67%)].
[d] [1,2-(9%)/1,4-(37%)].
At the outset, we performed optimization studies
for the presupposed B(C6F5)3-catalyzed consecutive
borylative reduction of quinoline (Table 1). The
desired double hydroboration took place with 4 equiv.
of catecholborane (HBcat) in the presence of B(C6F5)3 otherwise same conditions (entry 10). The formation of
°
catalyst (5 mol%) at 85 C in CDCl3, thus providing two regioisomers suggests that the initial step likely
1,3-bis-borylated tetrahydroquinoline 1 in 82% NMR involves 1,2-hydroboration as a competing hydride
yield after 24 h (entry 1).[8] Based on the fact that transfer path.
frustrated Lewis pairs are capable of activating a BÀ H
Under the optimized conditions, the borylative
bond of organohydroboranes,[9] catalytic reactions in reduction of quinolines substituted on the benzofused
the presence of Lewis bases were examined (entries 2– ring was first investigated (Table 2).[10] The initially
3). A catalytic amount of PPh3 did not bring any formed 1,3-bis-borylated tetrahydroquinolines were
change in yielding 1 (entry 2) while another Lewis converted to the 3-hydroxytetrahydroquinolines upon
base DABCO led to slightly decreased reaction the one-pot oxidation with sodium perborate tetrahy-
efficiency (entry 3). A reaction in benzene instead of drate. Quinolines with alkyl, aryl, or halide groups at
chloroform caused a significant decrease in yield of 1 the C5 to C8 positions were found to display minimal
°
(entry 4). When the reaction was conducted at 25 C, impact on the formation of the desired β-borylated
the desired product was obtained in 77% (entry 5) products (2–10). An ethereal sp2 CÀ O bond (aryloxy
while it remained rather similar even at higher temper- group) was tolerated under the present reductive
°
ature (110 C, entry 6 vs. 1).
conditions to give the product 11 in 79% yield. The
Notably, the identity of hydroboranes was shown to current catalysis proved to have high fidelity towards
be more critical as evidenced by the inferior yield with sp3 CÀ B bond formation at the β-position to give the
HBpin instead of HBcat (entries 1 vs. 7). The decrease C3-borylated tetrahydroquinoline possessing a newly
of HBcat quantity to 2.2 equiv. still gave 1 in good formed tetra-substituted carbon (12). Two isomeric
yield (entry 8), while treating a stoichiometric amount benzoquinolines underwent the borylative reduction
of HBcat generated a mixture of 1,2- and 1,4- with similar efficiency (13–14). It needs to be
dihydroquinolines, and product 1 was not formed emphasized that the present metal-free catalytic system
(entry 9). Noteworthy is that a reaction for a short time operates even in the gram-scale reaction with good
(0.5 h) furnished not only 1 (44%) but also a mixture efficiency (for 3, 4, 12, and 14: 60–85%).
of 1,4-(37%) and 1,2-(9%) dihydroquinolines under
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Table 2. Substrate Scope of Quinolines.[a]
24 h), thus giving moderate yields of β-silylated
azacyclic products.[5a]
Next, we were curious to see whether a diastereose-
lective transformation can be achieved by subjecting
quinolines having substituent(s) at the C2 and/or C4
positions (Table 3). To obtain better diastereoselectiv-
Table 3. Diastereoselective Borylative Reduction.[a]
[a] Reaction condition: substrate (0.5 mmol), HBcat (2.0 mmol)
°
and B(C6F5)3 (5 mol%) in CHCl3 (0.5 mL) at 50 C for 15 h:
isolated yields. Diastereomeric ratio (dr) was determined by
1H NMR analysis of the catalytic crude reaction mixture.
[b] NaBO3À 4H2O (5 equiv.) was used.
ity in case when it is, reactions were conducted at
[a] Reaction condition: substrate (0.5 mmol), HBcat (2.0 mmol)
°
slightly lower temperature (50 C for 15 h) in the
°
and B(C6F5)3 (5 mol%) in CHCl3 (0.5 mL) at 85 C for 15 h:
presence of 5 mol% of B(C6F5)3. To our delight, 2-
methylquinoline was converted to the corresponding β-
hydroxyl product (20) in 53% with excellent diastereo-
meric ratio (dr=99:1). The initially formed β-bory-
lated intermediate was oxidized in a stereo-retentive
manner.[11] As observed in the B(C6F5)3-catalyzed
silylative reduction,[5a] the stereo-relationship of 20
between the preexisting methyl and a newly formed C
(sp3)À B bond was determined to be cis. In addition,
isolated yields.
[b] Gram-scale synthesis.
[c] NaBO3À 4H2O (3 equiv.) was used.
[d]
°
At 50 C.
[e] Determined by 1H NMR of the crude reaction mixture on the
basis of 1,1,2,2-tetrachloroethane (TCE) as an internal
standard.
Gratifyingly, a range of isoquinolines bearing reactions of 2-methylquinolines substituted with meth-
substituents at the benzene ring also underwent yl or halide groups at the benzofused ring also afforded
°
smoothly the consecutive borylation at 85 C to provide the corresponding cis-isomeric products mainly
the corresponding 2,4-bis-borylated tetrahydroisoqui- although stereoselectivity was observed to be slightly
nolines in high yields (15–19). It is interesting to note decreased (for 21–23: dr=84:16–92:8).[12] On the
that when compared to the present borylative con- other hand, 4-substituted quinolines were initially
version, the silylative reduction of isoquinolines was reduced in a trans-manner and then the β-borylated
°
operative only under rather harsher conditions (100 C, intermediates were oxidized to trans-3-hydroxyteter-
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Scheme 2. 1H NMR-reaction progress monitoring.
ahydroquinolines (24–27: dr=85:15–89:11). Notably, an isomerization of 1,2-intermediate (1a’) occurs
when 2,4-dimethylquinoline was subjected to the leading to 1,4-isomer (1b’) during the reaction prog-
standard conditions, a consecutive borylation occurred ress (Scheme 2b,d).
to give the desired β-hydroxy product 28 in excellent
Based on the previous reports and the present
diastereoselectivity (dr=99:1). In this reaction, the mechanistic insights, we propose an outer-sphere ionic
boryl transfer step is assumed to determine the stereo- pathway for the present consecutive hydroboration of
relationship between the C4-methyl and the incorpo- quinolines involving a boronium ion as a key species
rated CÀ B moiety while the relative stereochemistry (Scheme 3).[6g,14–15] Initially, B(C6F5)3 forms a quinoline
between the C2-methyl and the CÀ B bond is proposed
to be determined at the hydride transfer step by [HB
(C6F5)3]À . These mechanistic implications underline a
stereoselective sequential transfer of the boryl and
hydride groups under the current borane catalysis.
To gain more insights, the consecutive hydrobora-
tion of quinoline with excessive HBpin or HBcat were
1
°
monitored at 25 C by H NMR (Scheme 2). In 15 min,
the reaction with HBpin as a reducing agent showed
74% conversion to provide a mixture of dihydroquino-
lines and teterahydroquinoline, wherein 1,2-intermedi-
ate (1a) was 49% and 1,4-intermediate (1b) was 13%.
Upon prolonged reaction time (2 h), a full conversion
was observed to give only 5% of 1a along with 76%
of 1b. Such a notable change in the ratio of 1a to 1b
during the course of reaction progress suggests that an
isomerization of 1,2-intermediate takes place to lead to
the thermodynamically more favoured 1,4-intermediate
(Scheme 2a,c).[13] Interestingly, the use of HBcat
resulted in a faster reaction, thus reaching a quantita-
tive conversion in 10 min giving 1,4-intermediate as a
major component (1b’, 64%) along with β-borylated
Scheme 3. Proposed mechanism.
tetrahydroquinoline 1 in 14%. This mixture was
converted eventually to 1 in 93% yield in 24 h. This
result again may corroborate our working proposal that
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adduct A as an off-cycle resting species[5,16] and the
constructive path begins upon the activation of a BÀ H
bond of HBcat by B(C6F5)3 to generate a boronium ion
pair bearing a borohydride anion B. It is postulated
that the subsequent hydride transfer from the [HB
(C6F5)3]À species occurs mainly at the C4-position of
the bound quinoline to afford 1,4-intermediate C. In
sequence, this 1,4-adduct C undergoes a turnover-
limiting C3-selective hydroboration to liberate a
borylated tertrahydroquinoline G presumably via an
iminium intermediate F. As mentioned above, an
initially formed allylamine adduct D will be isomer-
ized to its 1,4-isomer C although detailed pathways are
not known at the present stage.[17]
In summary, we have developed the B(C6F5)3-
catalyzed borylative reduction of quinolines with a
new sp3 CÀ B bond formation at the C3-position. The
resultant 1,3-bis-borylated tetrahydroquinolines are
converted to 3-hydroxytetrahydroquinolines by an one-
pot oxidation. An ionic stepwise pathway involving a
boronium species is proposed that enables the diaster-
eoselective dearomative borylation cascade. This met-
al-free catalytic procedure is anticipated to offer an
access to pharmaceutically interesting β-hydroxy-tetra-
hydro- quinolines from quinolines in a stereoselective
manner.
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Experimental Section
General Procedure
step 1) In a 2.5 mL reaction vial, catecholborane (2.0 mmol,
4.0 equiv.) was added to a solution of B(C6F5)3 (5.0 mol%) in
CHCl3 (0.5 mL) under Ar atmosphere. Quinoline (0.50 mmol)
°
was then added, and the reaction mixture was stirred at 85 C
for 15 h. After completion of the reaction, the mixture was
allowed to cool down to room temperature and concentrated
under reduced pressure.
step 2) To the above crude reaction mixture dissolved in THF
(2.0 mL) and H2O (2.0 mL) was added sodium perborate
tetrahydrate (2.5 mmol, 5.0 equiv.). Then, the mixture was
stirred at room temperature for 1 h. The mixture was concen-
trated under reduced pressure and diluted with H2O (10 mL).
The resulting solution was extracted with EtOAc (10 mL×3)
and the combined organic layers were washed with brine
(30 mL). The organic phase was dried over anhydrous MgSO4,
filtered, and concentrated under reduced pressure. The residue
was purified by flash column chromatography on silica gel
(ethyl acetate/n-hexane, 1/1) to give the corresponding 3-
hydroxytetrahydroquinoline products.
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Acknowledgements
This research was supported by the Institute for Basic Science
(IBSR010-D1). We thank Dr. Dongwook Kim for single-crystal
X-ray diffraction experiments with synchrotron radiation
performed at the BL2D-SMC in Pohang Accelerator Labora-
tory.
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hydroboration of styrenes, wherein B(C6F5)3 catalyst did
not enable β-selective hydroboration of styrene sub-
strates. This result may support our assumption that the
1,2-adduct in our case is not directly reduced to the final
product of β-borylated tetrahydroquinoline such as G
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[12] Stoichiometric borylative reductions using BH3 are [18] CCDC numbers for compounds
3 (1952314), 23
known to produce 3,4-trans-products since BH3 is added
in a cis-manner, while the catalytic borylative reduction
via an outer-sphere ionic pathway involves the trans-
addition of HBcat.
(1941403), and 25 (1941402) contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
[13] a) M. Lounasmaa, A. Tolvanen, Comprehensive Hetero-
cyclic Chemistry II (Eds.: A. R. Katritzky, C. W. Rees, E.
F. V. Scriven), Pergamon, Oxford, 1996, Vol. 5, pp 135–
Adv. Synth. Catal. 2019, 361, 1–7
6
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Double Hydroboration of Quinolines via Borane Catalysis:
Diastereoselective One Pot Synthesis of 3-
Hydroxytetrahydroquinolines
Adv. Synth. Catal. 2019, 361, 1–7
E. Kim, H. J. Jeon, S. Park*, S. Chang*