Rhodium-Catalyzed oxidative amidation of sterically hindered aldehydes and alcohols

Article abstract of DOI:10.1021/acscatal.6b02541

A rhodium-catalyzed oxidative amidation reaction has been developed with sterically hindered aldehydes and alcohols for the synthesis of amides containing a quaternary carbon at the α position. A variety of amine nucleophiles, both aliphatic and aromatic, are employed and afford the corresponding amides in good to excellent yields. Finally, mechanistic studies are performed to gain insight into both catalytic cycles.

Full text of DOI:10.1021/acscatal.6b02541

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Letter
Rhodium-Catalyzed Oxidative Amidation of
Sterically Hindered Aldehydes and Alcohols
Trang Thi Nguyen, and Kami L. Hull
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02541 • Publication Date (Web): 21 Oct 2016
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Rhodium-Catalyzed Oxidative Amidation of Sterically Hindered
Aldehydes and Alcohols
Trang T. Nguyen and Kami L. Hull*
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Department of Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Avenue, Urbana, IL 61820.
ABSTRACT: A rhodium-catalyzed oxidative amidation reaction has been developed with sterically hindered aldehydes and alcohols for the
synthesis of amides containing a quaternary carbon at the alpha position. A variety of amine nucleophiles, both aliphatic and aromatic, are
employed and afford the corresponding amides in good to excellent yields. Finally, mechanistic studies are performed to gain insight into
both catalytic cycles. Keywords: rhodium, oxidation, amidation, hindered, amine, aniline
Amides are one of the most important functional groups in or-
ganic chemistry, commonly found in natural products, pharmaceu-
ticals and agrochemicals.1 Amide synthesis has been traditionally
accomplished via the coupling of carboxylic acids and amines, using
highly reactive acid chlorides, anhydrides, or coupling reagents.2
This method, while effective, generates large amounts of high mo-
lecular waste, which led the American Chemical Society Green
Chemistry Institute to select ‘amide formation avoiding poor atom
economy reagents’ among the most important tasks facing organic
chemists.3 In recent years, metal-catalyzed oxidative amidation of
aldehydes and alcohols has emerged as a powerful alternative to
traditional methods.4 However, sterically hindered substrates have
proven to be particularly challenging while less nucleophilic
amines, such as anilines, give significantly diminished yields. Reac-
tivity toward more complex compounds like heterocycles has also
been identified as an area for improvement.4e
(dppb) as the most effective (Table S1, entry 4) and further opti-
mization showed that lower equivalents of amine and oxidant were
effective in this transformation (entries 11-16).
These optimized conditions were applied to a reaction of pival-
dehyde with morpholine which generated a very small amount of
product. A recent report has indicated that different conditions are
often required for aliphatic and aromatic amines.6k In our system,
optimization showed that a stronger base, Cs2CO3, slightly im-
proved yield (Table S2, entries 1-7). Unfortunately, H2 was not
observed to form under the reaction conditions and 2.0 equivalents
of methyl methacrylate (MMA) were required to act as a hydrogen
acceptor (entries 8-14). Although the addition MMA reduces the
atom economy of the transformation, it is an inexpensive commod-
ity chemical used in the synthesis of various polymers and gener-
ates the easily removed, low molecular weight methyl isobutyrate as
the sole byproduct. Using this new combination, several ligands
could be employed (entries 15-23); however, tricyclohex-
ylphosphine, an effective and relatively inexpensive ligand, was
chosen for scaling up. These conditions were effective in the ami-
dation reaction of pivaldehyde with aliphatic amines, both second-
ary and primary.
Scheme 1. Unsuccessful amidation of pivaldehyde5
We next explored the amine scope for the amidation reaction of
pivaldehyde (Table 1). Anilines bearing electron donating (3ab) or
electron withdrawing groups (3ac) were effective nucleophiles.
Aryl chlorides (3ad) and ortho-substitution (3ae) were also toler-
ated. Primary aliphatic amines generated products in moderate to
good yields, including α-branched substrate (3aj). When enantio-
merically pure 2j was used, the product 3aj was obtained with 98%
enantiospecificity. A variety of secondary aliphatic amine, both
cyclic and acyclic (3ak) were successfully employed, generating the
tertiary amide products in good to very good yields, except for the
seven-membered ring 3ar. This low yield could be explained by the
increased steric bulk of 2r compared to the analogous six-
membered ring 2q. Heterocyclic amines were converted to amides
in good yields.
Catalytic amidation of aldehydes with amines, a method pio-
neered by Beller,6a is an attractive, atom-economical method for
amide synthesis. Significant progress has been made allowing for
broader substrate scope.6 Nevertheless, sterically hindered aliphatic
aldehydes6c-e and anilines6a-b usually give significantly lower yields.
Aldehydes with α-disubstituted carbons have not been reported in
this reaction; amides containing α-quaternary carbon have been
prepared using less atom-economical methods from N-
chloroamines7 or malonitriles.8 We recently reported a rhodium-
catalyzed oxidative amidation of allylic alcohols and aldehydes
under biphasic conditions to convert amines and anilines to am-
ides. Substrates with mono-substitution at the alpha position were
well tolerated.9 To assess the effectiveness of this method with a
more hindered aldehyde, pivaldehyde and aniline were subjected to
the reported reaction conditions; however, only the corresponding
imine was observed (Scheme 1). By removing water from the set-
up, the desired product was generated in 56% yield (Table S1, en-
try 1). Ligand screening revealed diphenylphosphinobutane
Encouraged by this broad amine scope, we were eager to explore
the aldehyde scope (Table 2). Unfortunately, any increase in the
size of the aldehyde led to significantly diminished yield (3ba) or
no reaction (3ca). In the case of 3ba, the remaining mass balance
was accounted for by the corresponding imine. Optimization
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Table 1. Scope of amines in the amidation of pivaldehydea
imine.5 The effect of water on the reaction is not well understood,
1
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4
but it is consistent with a recent report by Stahl where the use of
molecular sieves, a drying reagent, in an amidation reaction of alco-
hols leads to significantly higher amide/imine ratio.10a However,
molecular sieves and other drying agents were had not effect on the
5
oxidative amidation of 1d
.
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9
To investigate the competition between amide and imine for-
mation futher, a time study was carried out with pivaldehyde and 4-
methoxyaniline. As shown in Figure S2, amide yield increases for
10 hours, while imine yield steadily goes up throughout the course
of the reaction. To determine if the imine can be an intermediate in
the amidation reaction it was subjected to the reaction conditions,
with and without aniline and water. In both cases, imine remained
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unreacted - only a trace amount of the amide product (3aa or 3ab
)
was observed.5 These results suggest that under these reaction con-
ditions the equilibrium between the hemiaminal and imine lays far
to the right and that once the imine is formed it is not converted to
the desired amide at an appreciable rate. This is consistent with the
Rh-catalyst serving a dual purpose: first it acts as a Lewis acid, bind-
ing to the aldehyde and promoting nucleophilic attack by the amine
nucleophile and formation of the Rh-bound hemiaminal interme-
diate that undergoes subsequent b-hydride elimination.
a
Isolated yield. Condition : pivaldehyde (0.64 mmol, 1.0 equiv),
A
aniline (2.5 equiv), [Rh(COD)2]BF4 (3.0 mol %), dppb (3 mol %),
CsOAc (1.5 equiv), styrene (2.0 equiv), THF (1.4 mL); Condition
pivaldehyde (0.64 mmol, 1.0 equiv), amine (2.5 equiv),
[Rh(COD)2]BF4 (3.0 mol %), PCy3 (6 mol %), Cs2CO3 (1.5 equiv),
MMA (2.0 equiv), THF (1.4 mL). b 1.5 equiv amine, 10 h. c 100 ˚C d 48
h.
B
:
efforts as well as attempt to alleviate the steric hindrance in 1c by
placing the phenyl ring one carbon further did not improve the
yield of the desired amide 3da; in all of these reactions the imine
was the primary product.
Scheme 2. Proposed catalytic cycle for aldehyde amidation
The general mechanism for direct amidation of aldehydes in-
volves the formation of a hemiaminal and subsequent oxidation to
the amide.4d In our previous work, the addition of water was ob-
served to significantly improve the yield, as it promoted the equilib-
rium between the imine/enamine and the key hemiaminal. How-
ever, in the present work the addition of water did not increase
amide yields, rather it was found to correlate to hgher yields of the
The proposed catalytic cycle for the oxidative amidation of steri-
cally hindered aldehydes is shown in Scheme 2. The aldehyde first
reacts with the cationic [Rh(I)] catalyst to form a rhodium-bound
aldehyde complex. Nucleophilic attack by the amine and subse-
quent proton transfer results in a Rh(III) species, which then un-
dergoes β-hydride elimination to generate the desired amide and a
rhodium-hydride complex. Insertion of the hydrogen acceptor
followed by reductive elimination regenerates the active Rh(I)
catalyst. Alternatively, the aldehyde could react with amine in an
off-cycle reaction to form the unreactive imine. Control reactions
show that catalyst was not required for imine formation.5 The rela-
tive rate of imine formation increases, when compared to the oxida-
tive amidation reaction, as the steric hindrance of the aldehyde
slows its reaction with the cationic Rh catalyst.
Table 2. Scope of aldehydes in the oxidative amidation with aniline
In order to address the competitive imine formation, we hypoth-
esized that products 3ba-
3da could be prepared from the corre-
sponding alcohols. Previous reports on oxidative amidation of al-
cohols indicated that free aldehyde might not be an intermediate in
the reaction or is only present in very low concentration.10b-c Since
the seminal report by Milstein on direct synthesis of amides from
alcohols,10d this reaction has attracted considerable attention and
significant progress has been made.10 However, the
Reaction conditions for 3ca and 3da: aldehyde (0.082 mmol, 1.0
equiv), aniline (2.5 equiv), [Rh(COD)2]BF4 (3.0 mol %), ligand (3.0
a
mol %), CsOAc (1.5 equiv), styrene (2.0 equiv), THF (0.2 mL).
b
c
Isolated yield; see Table 1, condition
Imine was the main product.
A
. Starting material remained.
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Table 3. Scope of amines in alcohol amidationa
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Scheme 3. Synthesis of herbicide Monalide
trifluoroacetophenone, the desired product 3aa was generated in
high yield. With these new conditions in hand, we proceeded to
explore the amine scope (Table 3). A variety of substituents on the
phenyl ring were tolerated. Remarkably, with more reactive amines
such as 4-methoxyaniline, good yield of product 3ab was achieved
after 1 h reaction time. Heterocyclic substrates also formed amides
in excellent yields. The apparent lower yield of 3at was due to diffi-
culty in separating the product from the amine starting material
rather than low conversion. When a nucleophile containing both
primary and secondary aromatic amines was used, only the primary
reacted, affording secondary amide 3ay in good yield. To demon-
strate the scalability of this reaction, 3aa was prepared on a gram
scale (1.3 g, 7.1 mmol) from 3a and 2a in 89% isolated yield.
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General reaction conditions: alcohol (0.64 mmol, 1.0 equiv), amine
(2.5 equiv), [Rh(COD)2]BF4 (3.0 mol %), Xantphos (3.0 mol %),
CsOAc (1.5 equiv), F3CC(O)Ph (2.2 equiv), THF (1.4 mL). a Isolated
yield b Yield in parenthesis was achieved with 1.5 eq amine in 1 h c Yield
was low due to difficulty in separating product from starting amine. d 48
h.
We could now turn our attention to the original challenge of
imine formation encountered when α-quaternary aldehydes, other
than pivaldehyde, were used. To our delight, products 3ba-
3da
were all prepared from the corresponding alcohols using the opti-
mized conditions in good to excellent yields (Table 4). Interesting-
ly, alcohol 4e required more forcing conditions despite the minimal
structural change from neopentyl alcohol. We attributed this differ-
ence in reactivity to an unfavorable all anti-conformation of 4e. In
comparison, cyclic substrate 4b, which is electronically similar to
4e, afforded the amide in higher yield as the additional substituents
are conformationally locked gauche. Alcohols containing a ketal
group, both cyclic (3fa) and acyclic (3ga), were also excellent sub-
strates for this reaction. To demonstrate the synthetic utility, we
applied the methodology to the synthesis of the herbacide
Monalide (4ed, Scheme 3). As far as we are aware, the results in
Tables 3 and 4 are the highest yields achieved with sterically hin-
dered alcohols and aniline nucleophiles in metal-catalyzed oxida-
tive amidation reactions.
drawback highlighted in Milstein’s original study, namely low reac-
tivity of sterically hindered substrates and less nucleophilic amines
such as aniline,still remains.10b,e-j The few studies that effectively use
aniline nucleophiles all employ benzyl alcohol,10o-v with the excep-
tion of a highly specialized Au-Pd resin which catalyzed reaction of
aliphatic alcohols with anilines in high yields.10w Highly bulky neo-
pentyl alcohol substrate gives significantly diminished yields (10-
20%)10,11 and a single example with good yield using this substrate
was recently reported by Stahl.10a
To probe the reactivity of sterically hindered alcohols and aro-
matic amines in our amidation reaction, screenings were carried
out using neopentyl alcohol and aniline (Table S3). The conditions
optimized for pivaldehyde proved to be ineffective. By changing the
ligand to Xantphos and the hydrogen acceptor to 2,2,2-
To determine whether the changes in ligand and hydrogen ac-
ceptor explain the differences in reactivity of alcohols 4b-d and the
corresponding aldehydes 1b-d, these aldehydes were allowed to
react with aniline under the alcohol reaction conditions. Unfortu-
nately, this did not lead to any improvement in yields, with 3b and
3d affording primarily imine, while 3c remained unreacted.5 The
amine scope also differed when alcohols were employed as sub-
strates. Alkylamines were shown to be effective nucleophiles with
pivaldehyde (Table 1). However, under the optimized reaction
conditions, 4a and benzylamine afford the corresponding amide
3ag in only 20% GC yield (Table S4). Initial screening revealed
that changing the ligand to 1,3-bis(diphenylphosphino)propane
(dppp) and temperature to 100 ˚C improved amide formation, as
3ag was isolated in 58% yield.5 These differences in the reaction
scope suggest that the optimal catalyst for the oxidation of hindered
alcohols to aldehydes is a poor catalyst for the oxidative amidation
reaction between aldehydes and secondary amines. To further
demonstrate the differential reactivity of the two catalysts, a time
study of the reaction between neopentyl alcohol and 4-
methoxyaniline was performed (Figure S3). Unlike with pivalde-
hyde (Figure S2), yield of amide 3ab steadily increases with time,
and only a very small amount of imine byproduct was observed. 11
Table 4. Scope of alcohols in the oxidative amidation with anilinea
Reaction conditions: alcohol (0.64 mmol, 1.0 equiv), aniline (2.5
equiv), [Rh(COD)2]BF4 (3.0 mol %), Xantphos (3.0 mol %), CsOAc
a
b
(1.5 equiv), F3CC(O)Ph (2.2 equiv), THF (1.4 mL). Isolated yield
aniline 4.0 equiv, [Rh(COD)2]BF4 5 mol %, Xantphos 5 mol %,
c
CsOAc 2.0 equiv. [Rh(COD)2]BF45 mol %, Xantphos 5 mol %,
CsOAc 2.0 equiv, 100 ˚C.
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alcohol, followed by β-hydride elimination and transfer hydrogena-
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Scheme 4. Proposed catalytic cycle for alcohol amidation
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amidation reaction for the synthesis of amides from aldehydes and
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aromatic amines. A broader substrate scope was observed with
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ASSOCIATED CONTENT
Supporting Information
.
Experimental procedures and characterization data (PDF)
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* kamihull@illinois.edu
Funding Sources
No competing financial interests have been declared.
ACKNOWLEDGMENT
The authors would like to thank the University of Illinois, Urbana-
Champaign, the ACS PRF (54007-DNI1) and the NSF (CAREER
1555337) for their generous support.
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