Arylation of Aldehydes to Directly Form Ketones via Tandem Nickel Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b01782

A nickel-catalyzed arylation of both aliphatic and aromatic aldehydes proceeds with air-stable (hetero)arylboronic acids, with an exceptionally wide substrate scope. The neutral condition tolerates acidic hydrogen and sensitive polar groups and also preserves α-stereocenters of some chiral aldehydes. Interestingly, this nickel(0) catalysis does not follow common 1,2-insertion of arylmetal species to aldehydes and β-hydrogen elimination.

Full text of DOI:10.1021/acs.orglett.9b01782

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Arylation of Aldehydes To Directly Form Ketones via Tandem Nickel
Catalysis
Chuanhu Lei,‡ Daoyong Zhu,◊ Vicente III Tiu Tangcueco,◊ and Jianrong Steve Zhou*,†
†State Key Laboratory of Chemical Oncogenomics, Key Laboratory of Chemical Genomics, School of Chemical Biology and
Biotechnology, Peking University Shenzhen Graduate School, Nanshan District, Shenzhen 518055, China
‡Center for Supramolecular Chemistry and Catalysis and Department of Chemistry, College of Sciences, Shanghai University,
Shanghai 200444, China
◊Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, 21 Nanyang Link, Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: A nickel-catalyzed arylation of both aliphatic and aromatic
aldehydes proceeds with air-stable (hetero)arylboronic acids, with an
exceptionally wide substrate scope. The neutral condition tolerates acidic
hydrogen and sensitive polar groups and also preserves α-stereocenters of
some chiral aldehydes. Interestingly, this nickel(0) catalysis does not
follow common 1,2-insertion of arylmetal species to aldehydes and β-
hydrogen elimination.
Scheme 2. Ligand Effect on Arylation of a Model Aldehyde
(Yield of 2a Is Indicated)
rylation of aldehydes was traditionally performed by
Aaddition of Grignard reagents under basic conditions,
followed by oxidation of the resulting carbinols (Scheme 1a).
But carbinol oxidation often involves harsh acidic or basic
Scheme 1. Catalytic Arylation of Aldehydes for Preparation
of Aryl Ketones
conditions, which have poor compatibility with sensitive
structures and acidic hydrogen. Alternatively, Grignard
addition to acid chlorides can be performed to access ketones
directly. Arylation of aldehydes with bench-stable organo-
borons to produce diaryl ketones is well documented with the
assistance of homogeneous catalysts of Ru,1 Rh,2 Pd,3 and Pt
(Scheme 1b).4 For example, Genet et al. reported Rh-catalyzed
conversion of aryl aldehydes to biaryl ketones, using ArBF3K or
ArB(OH)2,2 but the conditions cannot be applied to aliphatic
aldehydes containing enolizable α-hydrogens. In particular, Gu
et al. also reported arylation of aromatic aldehydes with RBpin
esters catalyzed by nickel(0) and N-heterocyclic carbenes,
which proceeded via a key step of nickel(0) oxidative addition
Received: May 22, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01782
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Scheme 3. Arylation of Aliphatic Aldehydes
Scheme 4. Arylation of Chiral Aldehydes and Comparison
with Other Synthetic Methods
Scheme 5. Arylation of Aromatic Aldehydes and Enals
of formyl CH bonds (Scheme 1c).5 Unfortunately, the
examples were restricted to aryl aldehydes, while aliphatic
aldehydes suffered from fast self-aldol condensation, a side
reaction. Moreover, excess amounts of aldehydes, ketones, aryl
iodides, and air or peroxides were needed as sacrificial oxidants
to remove metal hydrides under these conditions.
Arylation of aldehydes with aryl electrophiles is another
common approach to access ketones. For example, Hartwig et
al. reported Pd-catalyzed arylation of tert-butyl hydrazones
using aryl bromides, and a strong base, sodium tert-butoxide,
was needed.6 After acidic hydrolysis, aryl ketones were
released. In another example, Xiao et al. reported Pd-catalyzed
arylation of aliphatic aldehydes using aryl halides via a key step
of arylation of in situ formed enamines (Scheme 1d),7 but α-
branched aldehydes cannot be used as substrates. Recently,
MacMillan et al. reported nickel-catalyzed arylation of various
aldehydes, including enolizable ones, using aryl bromides
under blue LED irradiation (Scheme 1e).8 Unfortunately, in all
of the conditions above, bases-sensitive functional groups were
incompatible. Recently, Newman et al. also disclosed arylation
of both aromatic and aliphatic aldehydes with aryl triflates, but
an example carrying an unprotected NH-indole resulted in
poor yield.9
Finally, Cheng et al. disclosed nickel-catalyzed arylation of
aryl aldehydes using aryl iodides with zinc powder as terminal
B
DOI: 10.1021/acs.orglett.9b01782
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Letter
Scheme 6. Mechanistic Studies
was the best solvent for this transformation, while in DMA,
THF, and toluene the yield of 2a dropped to 74%, 54%, and
29%, respectively. Under similar conditions, palladium(0)
complexes and nickel(II) salts gave no ketone 2a at all (for
details, see the Supporting Information).
When the model reaction of aldehyde 1a and phenylboronic
acid was carried out on a 2 mmol scale using 2 mol % nickel,
79% yield of 2a was isolated. Phenylboroxine was also tested
and afforded ketone 2a in 57% yield. From other arylboron
reagents, the yields of 2a are 0% from PhBpin, 17% from
PhBcat, 3% from PhBnep, and 21% from Ph3B, respectively.
Notably, aldol condensation of aldehyde 1a was the main side
reaction in these reactions.
Using the optimal conditions, a diverse set of aliphatic
aldehydes smoothly reacted to give aryl ketones (Scheme 3).
The main side reaction was reduction of aldehydes. α-
Branched aldehydes were also well tolerated (2f−i). However,
the reaction of hindered pivalaldehyde only afforded 10% of
the desired ketone. In reactions of model aldehyde 1a,
electronically diverse arylboronic acids coupled smoothly.
Notably, electron-deficient arylboronic acids usually gave
moderate yields of aryl ketones (2o−r). Moreover, heteroaryl
rings such as furan, thiophene, and benzothiophene were well
tolerated (2s−u). Unfortunately, primary or secondary
alkylboronic acids did not afford the desired ketones, while
alkenyl ones furnished low yields; for example 1-cyclopentenyl
boronic acid gave only <20% yield of the ketone. In those
reactions, aldol condensation was the side reaction observed.
As a demonstration of synthetic utility, arylation of aldehyde 1f
readily provided ketone 2v, which was used as a synthetic
intermediate toward a glucagon receptor modulator.13
Figure 1. Phenylation kinetics of 1a in the presence of cyclohexanone
(top) and its absence (bottom).
reductant, but aliphatic aldehydes gave poor yields.10 Thus,
efficient, general methods for arylation of aldehyde with good
compatibility of sensitive functional groups are still desirable.11
Herein, we report a general arylation method that provides
aryl ketones from aliphatic aldehydes, including enolizable
ones, and (hetero)aryl aldehydes under nearly neutral
conditions. We initiated our study by examining arylation of
enolizable aldehyde 1a and phenylboronic acid using a catalytic
cocktail of (PPh3)4Ni(0) and a bulky, strongly donating
bisphosphine dcype as shown in Scheme 2. To our surprise,
the reaction did not form aryl alkenes 2al as we anticipated.12
Instead, aryl ketone 2a was produced under many conditions,
along with side products derived from reduction of aldehyde
(1ar) and phenyl addition (2aa).
The side reaction, reduction of aldehyde 1a, clearly indicates
that during the nickel catalysis a nickel hydride species is likely
produced. Thus, we examined the effect of added hydride
acceptors and found that addition of acetone or cyclohexanone
remarkably improved the yield of ketone 2a. In comparison,
trifluoroacetone and acetophenone were less effective accept-
ors, giving 49% and 60% of 2a, respectively.
The efficiency of the reaction was highly dependent on the
nature of ancillary ligands on the nickel catalystonly strongly
donating, bulky bisphosphines, such as dcype and dcypp,
generated active catalysts. Other diphosphines (binap, dppe,
dppp, dppb, and dppf) and monophosphines (PPh3, PCy3, Pt-
Bu3, Davephos, and XPhos) did not afford 2a at all. DMSO
(S)-Citronellal 3a containing a β-tertiary stereocenter was
also readily arylated to ketone 4a in 84% yield (Scheme 4a). In
reactions of a chiral cyclopropyl carboxaldehyde 3b (Scheme
4b), three stereocenters were virtually unchanged during
C
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6o), aryl fluorides (6a, 6n and 6p), and aryl chlorides (6d),
while acidic protons of free phenols (6b), alcohols (6k) and
unprotected indoles (6l) were compatible. The reactions of
α,β-unsaturated aldehydes, however, gave moderate yields of
the ketones 6w−x owing to competitive reduction of the olefin
and reduction of aldehydes. Finally, the reaction of cyclopropyl
carboxaldehyde proceeded smoothly without ring opening
(6y).
Scheme 7. Possible Reaction Pathways
To understand the catalytic pathway, we monitored the
phenylation kinetics of 1a in the presence of cyclohexanone at
90 °C by GC. The conversion of aldehyde 1a and yields of
product 2a and other byproducts are summarized in Figure 1.
Several key observations were made. (a) Ketone 2a was
continuously formed and its yield reached 88% after 6 h,
whereas the yield of carbinol 2aa quickly reached plateau after
1 h (24% yield), which then slowly decreased to 10% after 6 h.
(b) At the same time, cyclohexanol cyl progressively
accumulated and reached 51% yield after 6 h, indicating that
cyclohexanone was the main hydrogen acceptor. (c) A small
amount of byproduct 1ar was also detected (6% yield after 6
h). (d) To account for the formation of 2a, some DMSO also
served as a hydride scavenger because a distinct smell of Me2S
was noted after the reaction.14(e) In the absence of
cyclohexanone, aldehyde 1a was the main hydrogen acceptor
giving 1ar in 50% yield after 6 h. (f) In arylation with
phenylboroxine, we found that 1 equiv of water significantly
accelerated the formation of carbinol 2aa, especially in the first
hour (see the Supporting Information).
To gain additional support for the intermediacy of carbinols,
carbinol 2aa was then added to a catalytic phenylation of
aldehyde 1f. It almost fully converted to aryl ketone 2a in 97%
yield after 12 h (Scheme 6a). Moreover, when 2aa was
subjected to transfer hydrogenation with cyclohexanone, to our
surprise, both (dcype)Ni(0) catalyst and 1 equiv PhB(OH)2
were necessary. When PhB(OH)2 was omitted, no dehydro-
genation of 2aa was detected, suggesting that the carbinol
oxidation is mechanistically distinct from simple alcohol
dehydrogenation (Scheme 6b).15 Most likely, 2aa reacted
with PhB(OH)2 to in situ form boronic ester 2ab, which then
underwent nickel(0)-catalyzed retro-hydroboration to produce
ketone 2a (Scheme 7a). The resulting borylnickel hydride was
then trapped by cyclohexanone to complete the catalytic loop
at the end (Scheme 7a).16 The need for the nickel(0) catalyst
also discounted an uncatalyzed Oppenauer-type oxidation
(Scheme 7b).17
arylation. In the third example, (S)-N-Boc-prolinal 3c (98%
ee) was smoothly arylated by both electron-rich and poor
arylboronic acids as well as a thienylboronic acid (Scheme 4c),
while the ee values of products were almost unchanged from
the aldehyde. One exception was noticed in arylation using an
electron-deficiency p-fluorophenyl ring (4c3), in which the ee
dropped from 98% to 92%.
Furthermore, we have considered several possible pathways
for the formation of carbinols. (a) The arylation can only be
catalyzed by nickel(0) complexes, but not nickel(II) complexes
at all. This rules out a simple pathway involving 1,2-insertion of
arylnickel(II) to aldehydes (Scheme 7c).10 (b) The inter-
mediacy of aryl carbinols also precludes another pathway
involving oxidative addition of formyl C−H bonds by
nickel(0), followed by arylation5,18 (Scheme 7d). Aryl esters,
which are likely byproducts in this putative pathway, were
never detected. Also consistent with this, a H/D competition
experiment using a 1:1 mixture of 1a and deuterated 1ad
resulted in an apparent kH/kD value of 1.0 (Scheme 6c).
Furthermore, no decarbonylation of aldehydes18,19 was
detected after aldehydes were heated with 20 mol %
(dcype)nickel(0) catalyst for 12 h at 120 °C. For example,
1a only afforded only self-aldol condensation (1as) at 50%
conversion, while heating 1-naphthlenealdehyde led to several
side products at 10% conversion. (c) The participation of aryl
A comparison was made with Genet’s Rh-catalyzed arylation
using phenylboronic acid,2b which resulted in 65% yield of 4c1,
but the ee decreased to 90% even after we optimized the base,
K2CO3 (Scheme 4d). Another procedure of Genet using
KArBF3 did not use any base,2a but it only afforded 4c1 in 17%
yield and 87% ee, unfortunately. Furthermore, we noticed that
Genet’s methods only gave <10% yield of ketones when linear
aldehyde 1a was used. In a third comparison, Pd-catalyzed
arylation of in situ formed enamines prefers aldehydes without
α-branching as substrates,7 so a reaction of 4c resulted in a
very complex mixture containing <5% of 4c1.
Besides aliphatic aldehydes, the current arylation procedure
was successfully applied to aromatic ones after adjusting the
ratio of aldehydes to arylboronic acids to 1:1.2 (Scheme 5).
Arylboronic acids of diverse electronic properties efficiently
added to these aldehydes. Furthermore, heteroaryl aldehydes
of thiophene, furan, indole, and quinoline also reacted well
(6i−m). The conditions were compatible with esters (6f and
D
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carbinols also ruled out a pathway involving ligand-to-ligand
hydrogen transfer20 on the nickel center, directly between a
bound aldehydes and a ketone acceptor as Gu et al. reported
previously (Scheme 7e).
(Phosphine)nickel(0) complexes were known to form
isolable η2 complexes A with aldehydes (Scheme 7f),21
which were implicated as key intermediates in several nickel-
catalyzed processes,22 including addition of alkylboranes and
arylboron reagents to aldehydes that formed carbinols.23 When
aldehyde 1a was added to an equimolar mixture of Ni(PPh3)4
and dcype, the solution quickly turned from red to light yellow,
and two doublets were detected by 31P{1H} NMR spectros-
copy at 66.8 and 56.3 ppm (2JP−P = 67.4 Hz), characteristic of
an η2-complex.
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ASSOCIATED CONTENT
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* Supporting Information
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Corresponding Author
*E-mail: jrzhou@pku.edu.cn.
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Jianrong Steve Zhou: 0000-0002-1806-7436
Notes
The authors declare no competing financial interest.
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We thank Peking University Shenzhen Graduate School,
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