Mechanistic investigation of the nickel-catalyzed Suzuki reaction of N, O-acetals: Evidence for boronic acid assisted oxidative addition and an iminium activation pathway

Article abstract of DOI:10.1021/ja3079362

The mechanism of a recently reported Suzuki coupling reaction of quinoline-derived allylic N,O-acetals has been studied using a combination of structural, stereochemical, and kinetic isotope effect experiments. The data indicate that C-O activation is fac

Full text of DOI:10.1021/ja3079362

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Communication
Mechanistic Investigation of the Nickel-Catalyzed Suzuki
Reaction of N,O-Acetals: Evidence for Boronic Acid-Assisted
Oxidative Addition and an Iminium Activation Pathway
Kevin T Sylvester, Kevin Wu, and Abigail G. Doyle
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Oct 2012
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Mechanistic Investigation of the Nickel-Catalyzed Suzuki Reaction of
N,O-Acetals: Evidence for Boronic Acid-Assisted Oxidative Addition
and an Iminium Activation Pathway
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Kevin T. Sylvester,‡ Kevin Wu,‡ and Abigail G. Doyle
Department of Chemistry, Princeton University, Princeton, New Jersey 08544
Supporting Information Placeholder
Scheme 1. Possible Mechanisms for Csp3–O Activation
ABSTRACT: The mechanism of a recently reported Suzuki
NiII
coupling reaction of quinoline-derived allylic N,O-acetals has
been studied using a combination of structural, stereochemical,
and kinetic isotope effect experiments. The data indicate that
C–O activation is facilitated by Lewis acid assistance from the
boronic acid coupling partner and an ionic SN1-like mecha-
nism accounts for oxidative addition. In this context, we
demonstrate the first direct observation of oxidative addition
to a quinolinium salt. Notably, this mechanism is distinct from
the more commonly described SN2(‘)-type oxidative addition
of low-valent transition metals to most allylic electrophiles.
LnNi0
(i) SN1
N
Ph
N
N
(PhBO)3
EtO
O
EtO
O
EtO
O
2a
racemic
PhB(OR)3
PhB(OR)3
NiII
N
OEt
(PhBO)3
OEt
(ii) SN2(!)
N
EtO
O
LnNi0
(–)-1
EtO
O
(iii) LA-assisted
SN2(!)
NiII
N
Ph
N
PhB(OR)3
Oxidative addition of a transition metal to an organic elec-
trophile such as an aryl halide or vinyl triflate is a fundamental
elementary step in cross-coupling catalysis. For allylic elec-
trophiles, oxidative addition with a late transition metal gener-
ally occurs by a stereospecific mechanism with inversion or
retention of configuration.1 We recently reported the first ex-
amples of Suzuki cross coupling with allylic N,O- and O,O-
acetals.2 In these reactions, Csp3–O bond activation proceeds
with a nickel catalyst under base-free conditions, enabling a
mild and modular synthesis of various privileged heterocycles
(Eq 1). Coupling reactions with related electron-rich electro-
philes have all required harsh nucleophilic reaction partners
such as Grignard reagents to achieve Csp3–O bond activation
by stereospecific oxidative addition mechanisms.3 As such, we
proposed that a distinct SN1-type mechanism was operative in
the Suzuki couplings. Specifically, we envisioned that the Ni
catalyst could undergo oxidative addition to a transient
(iso)quinolinium or benzopyrylium cation accessed by boronic
acid-assisted ionization of the allylic ether leaving group
(mechanism (i) in Scheme 1). While oxidative additions are
traditionally understood as the reaction of a metal with an X–
Y sigma bond,4 limited precedent for an ionic mechanism is
also available,5 including for the addition of nickel to iminium
LnNi0
(PhBO)3
EtO
O
EtO
O
2a
inversion
and oxocarbenium ions.6 However, this activation mode has
not seen broad application in the design of synthetic methods.7
Arndtsen and Watson have reported Pd and Ni-catalyzed Lew-
is acid-assisted C–C bond-forming reactions with N-
acyliminium precursors;8 however, direct evidence for the
oxidative addition of a metal to an iminium or oxocarbenium
ion in the context of a catalytic bond-forming reaction has yet
to be disclosed.
Since a better understanding of these processes could af-
ford key insights for the development of new C–C bond-
forming reactions with these classic organic electrophiles, we
undertook mechanistic investigations of the reaction between
aryl
boroxines
and
2-ethoxy-1-ethoxycarbonyl-1,2-
dihydroquinoline (EEDQ) 1. We describe herein structural,
isotopic, and stereochemical studies that offer direct evidence
for the involvement of an ionic SN1-type mechanism for allylic
Csp3–O activation. In this context, we demonstrate the first
example of oxidative addition of a low valent metal to a
quinolinium ion.
In our previously reported studies on the catalytic system,2a
we observed the propensity of boroxines to racemize EEDQ.
As such, it was difficult to distinguish the proposed stereocon-
vergent SN1-type oxidative addition (i) from the more classical
mechanisms (ii) and (iii) in Scheme 1, wherein C–C bond
formation takes place with overall inversion of configuration
but is accompanied by an off-cycle racemization pathway. We
Ni(cod)2, PPh3
• Csp
3–O activation
(1)
• Base-free Suzuki
(ArBO)3, 40 °C
X
OEt
X
Ar
1: X = NCO2Et
or X = O
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Scheme 2. Stoichiometric Studies of Oxidative Addition
complex 3 represents a rare example of a structurally-
characterized late transition metal olefin complex to an allylic
3
4
dioxane/t-AmOH
no reaction
60 °C, 12 h
electrophile (Figure 1).11
5
6
7
8
When complex 3 was heated at 60 °C in dioxane/t-AmOH
over 12 hours no oxidative addition was observed to take place
as determined by 1H and 31P NMR spectroscopy. This result is
in agreement with the observation that the C39–O2 bond in 3
exhibits negligible elongation compared to that in free EEDQ
(1.434(4) vs. 1.417(4) Å).12 By contrast, 2-phenyl dihydro-
quinoline 2a was produced in 81% yield within 5 h upon
treatment of 3 with phenyl boroxine at 23 °C. Monitoring this
reaction by 31P NMR spectroscopy revealed clean formation of
olefin complex 4 with no detectable build-up of intermedi-
ates.13 Moreover, complex 3 was found to be a competent cata-
lyst for the phenylation of EEDQ, delivering 2a in 54% isolat-
ed yield at 10 mol% loading.
P
P
Ni(cod)2
DPEPhos
Ni
Ni
P
P
(PhBO)3
9
N
OEt
toluene
23 °C, 1 h
benzene
23 °C, 5 h
10
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N
OEt
N
Ph
EtO
O
EtO
O
EtO
O
1
4
3, X-Ray
97% yield
P
Ni
P
P
P
B(C6F5)3
O
N
Ph2P
PPh2
EtOB(C6F5)3
hexanes
23 °C, 14 h
EtO
O
DPEPhos
The direct oxidative addition pathway (ii) in Scheme 1 is
inconsistent with these initial studies, which clearly demon-
strate a role for the boroxine in the oxidative addition step.
Boronic acids have been used as Lewis acid catalysts for a
wide variety of transformations,14 but a role for them in oxida-
tive addition is less precedented.15 Circumstantial evidence in
favor of a role as Lewis acid includes: (a) when 3 was treated
with the less Lewis acidic phenyl pinacol boronate ester, no
reaction was observed, (b) in competition experiments be-
tween aryl boroxines of differing electronic properties (p-CF3,
p-H, and p-OMe), more electron-deficient boroxines reacted
faster with olefin complex 3 in a 9.5:2.2:1.0 ratio.16
5-borate, X-Ray
82% yield
Unfortunately, in none of these reactions was the presumed
Ni allyl intermediate observed directly. In an attempt to detect
such a species, we sought to identify a system that would be
resistant to reductive elimination. Addition of hindered and
electronically deactivated boroxines to 3 resulted in a color
change from yellow to dark orange without concomitant C–C
bond formation. However, our attempts to characterize any
1
intermediates by H, 13C, and 31P NMR spectroscopy were
Figure 1. Solid-state structure of olefin-complex 3 at 30%
probability ellipsoids. Toluene solvent molecule omitted for
clarity. Hydrogen atoms omitted for clarity, except those at-
tached to C37 and C38.
hampered by the presence of multiple species with broadened
spectra. This problem was addressed by using B(C6F5)3, which
induced precipitation of a dark orange solid, 5-borate, when
combined with equimolar 3 (Scheme 2). Characterization of 5-
borate by NMR spectroscopy and single crystal X-ray diffrac-
tion were both consistent with its formulation as a nickel π-
allyl complex.17 Notably, association of the ethoxide leaving
group to the Lewis acidic borane in 5-borate offers compel-
ling structural evidence for a Lewis acid-assisted oxidative
addition and may have relevance to the course of other allylic
substitution reactions with unactivated ether substrates.18
conjectured that evaluation of a stoichiometric system would
offer a solution to this problem as the rate of oxidative addi-
tion at high concentrations of nickel should be competitive
with off-cycle boroxine-mediated racemization of enantio-
enriched EEDQ.
To evaluate these possibilities, we studied a system com-
prised of Ni, DPEPhos, and EEDQ. DPEPhos was selected as
a ligand because it shares electronic and structural similarities
to PPh3 but is rigid and chelating, which we reasoned should
stabilize the putative allyl complex and facilitate characteriza-
tion of relevant catalytic intermediates.9 For pathway (ii), oxi-
dative addition should occur in the absence of the boronate
nucleophile. Combining Ni(cod)2, DPEPhos, and EEDQ (1:1:1)
in the absence of phenylboroxine led to formation of a yellow
crystalline compound 3 in nearly quantitative yield (Scheme 2).
With access to 3 and 5, we were poised to evaluate the ste-
reochemical outcome of the Lewis acid-assisted oxidative
addition in an effort to distinguish mechanisms (i) and (iii).
Exposing enantioenriched complex 3 (99% ee)19 to 0.17
equivalents of phenyl boroxine resulted in formation of race-
mic 2a, consistent with the stereochemical experiments con-
ducted in the catalytic system (eq 2). However, unlike in the
catalytic system, recovered EEDQ showed high levels of en-
antioenrichment (72% ee). This result is a significant because
it establishes that C–C bond formation is stereoconvergent. It
also suggests that Ni must dissociate from 3 prior to ionization.
Consistent with this proposal, 31P and 19F NMR experiments
revealed fast ligand exchange between the two distinct
However, the H, 13C, and 31P NMR spectra of 3 were con-
1
sistent with complexation of (DPEPhos)Ni to the styrenyl ole-
fin of EEDQ rather than an oxidative adduct. X-ray crystallo-
graphic analysis provided further confirmation of this structur-
al assignment. Although olefin complexes have been invoked
as intermediates in numerous allylic substitution reactions,10
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(DPEPhos)Ni(EEDQ) complexes 6 and 7 (eq 3).17
Scheme 3. Studies with Quinolinium Triflate.
3
P
P
4
5
6
7
Ni
(PhBO)3
(0.17 equiv)
P
Ni
P
Ni(cod)2, DPEPhos
TBSOTf
1
(2)
N
N
N
OEt
N
Ph
dioxane/t-AmOH
23 °C, 16 h
OTf
N
OEt
toluene, 23 °C, 1 h
OTf
EtO
O
EtO
O
EtO
O
EtO
O
EtO
O
8
5-triflate, X-Ray
93% yield
3
99% ee
2a
racemic
3
72% ee
8
80% yield
9
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P
P'
P/P'
Ni
Ni
Ni
P
P'
P/P'
OEt
F
H/F
23 °C
5 min
(3)
N
OEt
OEt
N
OEt
N
O
O
OEt
O
OEt
7: P' = P(4-FC6H4)2
6
crossover products
To test the likelihood that racemization instead occurs
post-oxidative addition by a Ni(0)-mediated displacement of
the Ni-allyl complex,20 we carried out the same reaction at low
[3]. Notably, the concentration of Ni(0) had no impact on the
stereochemical outcome. Moreover, C–C bond formation oc-
curred with conservation of ee (95%) when the stereochemical
experiment was performed with MeMgBr as nucleophile,17
suggesting that a Ni-allyl complex of EEDQ can be configura-
tionally stable. Taken together, these data are most consistent
with the fact that the oxidative addition step in the Suzuki
coupling is SN1-like in character (i).
Figure 2. Solid-state structure of nickel allyl complex 5-
triflate at 30% probability ellipsoids. Dichloromethane mole-
cules, disorder on ethyl group of carbamate, and all hydrogen
atoms omitted for clarity.
A kinetic isotope effect experiment provided further evi-
dence against mechanism (iii). In particular, we observed a
secondary kinetic isotope effect of 1.13 ± 0.02 when 3 and its
isotopologue prepared from 2-d1-EEDQ were treated with 0.8
equivalents of B(C6F5)3. This value is intriguing because it is
most in line with reported KIE values for solvolysis reactions
of activated electrophiles like benzhydryl halides.21 By com-
parison, previously reported secondary kinetic isotope values
for SN2-type oxidative additions typically range from 0.94–
1.05.22 In isolated cases, including for oxidative addition to
electrophiles with highly stabilized leaving groups, isotope
effects as high as 1.53 have been observed.23 Either an SN1 or
radical chain mechanism has been proposed to account for
these data. Although the stereochemical outcome of the aryla-
tion in Eq 2 would be consistent with a radical intermediate,
the fact that radical traps such as TEMPO, and dihydroanthra-
cene do not impede efficient formation of 2a under otherwise
standard reaction conditions argues against such a mecha-
nism.17
carbon of the allyl functionality. Furthermore, exposing allyl
complexes 5-borate or 5-triflate to excess phenyl boroxine in
the presence of cesium fluoride furnished 2a in 65 and 70%
isolated yield, respectively (eq 4). These data demonstrate that
both complexes are competent in the C–C bond-forming step
of the coupling reaction.
P
Ni
(PhBO)3, CsF
P
N
Ph
(4)
N
dioxane/t-AmOH
23 °C, 16 h
X
EtO
O
EtO
O
2a
5-borate: X = B(OEt)Ar3
5-triflate: X = OTf
65% (from 5-borate)
70% (from 5-triflate)
In summary, we have presented a series of experiments
that probe the mechanism of allylic N,O-acetal Csp3–O oxida-
tive addition with Ni. Our data are consistent with a Lewis
acid-assisted oxidative addition mechanism wherein the
boroxine nucleophile serves as the Lewis acid. The C–O acti-
vation reaction appears to proceed by an SN1-like mechanism.
As evidence for this proposal, we demonstrate oxidative addi-
tion of Ni into a quinolinium ion. Future work will be directed
toward evaluating the generality of these results with other
iminium and oxocarbenium ion partners. We anticipate that
this mode of activation will ultimately enable the development
of sp3-cross-coupling reactions complementary to the much
more common transformations of alkyl halides.
These studies support the step-wise mechanism (i) wherein
a Lewis acidic boroxine abstracts the alkoxide leaving group
of EEDQ to generate a quinolinium intermediate that can be
intercepted by Ni from either of its two prochiral faces. To our
knowledge, no structural evidence for oxidative addition of a
low-valent transition metal to a quinolinium or related het-
eroaromatic ion has been reported. To interrogate the feasibil-
ity of this step, we prepared quinolinium triflate 8 from EEDQ
and TBSOTf. Gratifyingly, addition of Ni(cod)2 and DPEPhos
to 8 resulted in rapid precipitation of 5-triflate in 93% yield as
a bright red-orange crystalline solid (Scheme 3). As deter-
mined by single crystal X-ray diffraction, the metrical parame-
ters of 5-triflate are very similar to the Ni allyl complex 5-
borate, with the nickel center lying directly above the central
ASSOCIATED CONTENT
Supporting Information. Experimental procedures, details of
optimization studies, and spectroscopic data for all new com-
pounds. This material is available free of charge via the Internet at
http:// pubs.acs.org.
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Int. Ed. 2008, 47, 5430–5433. (d) Shacklady-McAtee, D. M.;
AUTHOR INFORMATION
Dasgupta, S.; Watson, M. P. Org. Lett. 2011, 13, 3490–3493.
(9) DPEPhos is a competent ligand for the catalytic arylation of
EEDQ; however, coupling reactions with DPEPhos proceed with
depressed rates compared with those using PPh3. See SI for fur-
ther details.
(10) (a) Trost, B. M. Chem. Rev. 1996, 96, 395–422. (b) Hage-
lin, H.; Scensson, M.; Åkermark, B.; Norrby, P.-O. Organometal-
lics 1999, 18, 4574–4583.
Corresponding Author
agdoyle@princeton.edu
Author Contributions
‡These authors contributed equally.
Notes
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(11) For an example of a Pd-olefin complex to an allylic substi-
tution product see: (a) Steinhagen, H.; Reggelin, M.; Helmchen, G.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2108–2110. For an exam-
ple of a complex between Ni and diallylether that does not under-
go oxidative addition see: (b) Yamamoto, T.; Ishizu, J.; Yamamo-
to, A. J. Am. Chem. Soc. 1981, 103, 6863–6869. (c) A Ni-allyl
nitrile olefin complex has been characterized by NMR spectros-
copy: Acosta-Ramírez, A.; Flores-Gaspar, A.; Muñoz-Hernández,
M.; Arévalo, A.; Jones, W. D.; García, J. J. Organometallics 2007,
26, 1712–1720.
(12) Cremin, D. J.; Hergarty, A. F.; Begley, M. J. J. Chem. Soc.
Perkin Trans. 2 1980, 412–420.
(13) The equilibrium constant for exchange of 3 and 2a with 1
and 4 is 0.27 at 23 °C in deuterated benzene. See SI for further
details.
(14) For a recent example of the use of boronic acids as Lewis
acids see: Zheng, H.; Ghanbari, S.; Nakamura, S.; Hall, D. G.
Angew. Chem., Int. Ed. 2012, 51, 6187–6190 and references
therein.
(15) For examples wherein a boronic acid has been proposed as
both activator and coupling partner in a Suzuki reaction see: (a)
Trost, B. M.; Spagnol, M. D. J. Chem. Soc., Perkin Trans. 1 1995,
2083–2097. (b) Tsukamoto, H.; Sato, M.; Kondo, Y. Chem.
Comm. 2004, 1200–1201. (c) Tsukamoto, H.; Uchiyama, T.; Su-
zuki, T.; Kondo, Y. Org. Biomol. Chem. 2008, 6, 3005–3013. (d)
Yu, D.-A.; Shi, Z.-J. Angew. Chem., Int. Ed. 2011, 50, 7097–7100.
(e) Li, M.-B.; Wang, Y.; Tian, S.-K. Angew. Chem., Int. Ed. 2012,
51, 2968–2971.
(16) In competition experiments among the same three borox-
ines with 5-triflate, p-CF3 was favored over p-H and p-OMe by
1.8:1.1:1.0 respectively. Since this ratio is considerably lower
than that observed for the competition experiment starting from 3,
we believe that the data with 3 reflect the electronic bias in oxida-
tive addition rather than transmetalation or reductive elimination.
(17) See SI for full experimental procedures and data.
(18) For recent examples, see: (a) Matsubara, R.; Jamison, T. F.
J. Am. Chem. Soc. 2010, 132, 6880–6881. (b) Nishikata, T.; Lip-
shutz, B. H. J. Am. Chem. Soc. 2009, 131, 12103–12105.
(19) When complex 3 was decomposed in air, recovered EEDQ
showed complete conservation of enantioenrichment by chiral
HPLC.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank Scott Semproni for X-ray crystallographic structure
determination of 3, 5-borate, and 5-triflate, Derek Ahneman and
Jason Shields for preparing the 6-CF3-EEDQ, Thomas Graham
and Daniel Nielsen for helpful discussions, and Lotus Separations
for chiral separations of 1. Financial support provided by Prince-
ton University is gratefully acknowledged.
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π-allyl Ni(II) intermediate
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