NHC-Copper(I) Halide-Catalyzed Direct Alkynylation of Trifluoromethyl Ketones on Water

Article abstract of DOI:10.1002/chem.201601581

An efficient and easily scalable NHC-copper(I) halide-catalyzed addition of terminal alkynes to 1,1,1-trifluoromethyl ketones, carried out on water for the first time, is reported. A series of addition reactions were performed with as little as 0.1-2.0?mo

Full text of DOI:10.1002/chem.201601581

DOI: 10.1002/chem.201601581
Communication
&
Synthetic Methods
NHC–Copper(I) Halide-Catalyzed Direct Alkynylation of
Trifluoromethyl Ketones on Water
[a]
[b]
[c]
[d]
[a]
´
Paweł Czerwinski, Edyta Molga, Luigi Cavallo,* Albert Poater,* and Michał Michalak*
Abstract: An efficient and easily scalable NHC–copper(I)
halide-catalyzed addition of terminal alkynes to 1,1,1-tri-
fluoromethyl ketones, carried out on water for the first
time, is reported. A series of addition reactions were per-
formed with as little as 0.1–2.0 mol% of [(NHC)CuX] (X=
Cl, Br, I, OAc, OTf) complexes, providing tertiary propargyl-
Figure 1. Examples of trifluoromethyl carbinol-containing compounds.
ic trifluoromethyl alcohols in high yields and with excel-
lent chemoselectivity from a broad range of aryl- and
more challenging alkyl-substituted trifluoromethyl ketones
arthritis (Figure 1). Besides that this structural motif is also
(TFMKs). DFT calculations were performed to rationalize
a matter of interest for synthetic organic chemists due to, inter
the correlation between the yield of catalytic alkynylation
alia, the unquestionable role of Mosher’s acid as a chiral shift
and the sterics of N-heterocyclic carbenes (NHCs), ex-
reagent.
pressed as buried volume (%VBur), indicating that steric
To date, there are few reports devoted to direct addition of
effects dominate the yield of the reaction. Additional DFT
acetylides to trifluoromethyl ketones (TFMKs). For this purpose
calculations shed some light on the differential reactivity
a number of catalytic systems have been developed, including
of [(NHC)CuX] complexes in the alkynylation of TFMKs.
copper–bisphosphine
complexes
(10 mol%
CuOtBu/
The first enantioselective version of a direct alkynylation
in the presence of C1-symmetric NHC–copper(I) complexes
is also presented.
Xantphos),[5]AgF/PCy3 (10 mol%),[6] Ag–Ti nanoparticles,[7] and
ligand-free copper(I) salt in DMSO.[8] Recently, [(NHC)CuCl]
(under continuous flow conditions in a toxic mixture of sol-
vents DMF/benzene/THF)[9] and cationic heteroleptic
[(NHC1)(NHC2)AgBF4] complexes[10] were also used. An enantio-
selective direct alkynylation was also reported using Ti(iPrO)4/
ZnEt2/cinchona alkaloid co-catalytic system[11] and lithium bi-
naphtholate.[12] Carreira et al. have also presented an autocata-
lytic process for the addition of a Li/Zn acetylide in an asym-
metric fashion for the synthesis of anti-HIV drug Efavirenz.[13]
It should be clearly pointed out that all of the above-men-
tioned methods have limited applicability with respect to
TFMKs, mainly to aromatic ones. Therefore, the broad applica-
bility of trifluoromethyl carbinols has stimulated the search of
new, efficient protocols for their synthesis. In this regard, the
development of reactions in/on water is of high importance.
From the standpoint of organic synthesis, the use of water as
the reaction medium has certain advantages. Water is safe,
benign, environmentally friendly, and inexpensive in compari-
son to common organic solvents. Moreover, the use of water
as the reaction medium has beneficial effects on reactivity and
selectivity, which was demonstrated many times, including in
Diels–Alder reactions[14] and also alkynylation of carbonyl and
imine groups.[15] Herein, we wish to present for the first time
an efficient, easily scalable direct alkynylation of trifluoromethyl
ketones, catalyzed by well-defined [(NHC)CuX] complexes on
water under mild conditions.
The synthesis of fluorine-containing compounds has attracted
much attention in the last decades due to their unique physi-
cal and biological properties.[1] Nearly one-third of all com-
pounds present on the pharmaceutical and agrochemical
market contain fluorine atom(s).[1,2] In particular, a-trifluoro-
methyl carbinol moiety is present in many pharmaceuticals, in-
cluding the critical anti-HIV drug Efavirenz[1,3] and glucocorti-
coid agonist BI 653048[4] used for the treatment of rheumatoid
´
[a] P. Czerwinski, Dr. M. Michalak
Institute of Organic Chemistry, Polish Academy of Sciences
01-224 Warsaw, Kasprzaka 44/52 (Poland)
E-mail: michal.michalak@icho.edu.pl
[b] E. Molga
Faculty of Chemistry, Warsaw University of Technology
Noakowskiego 3, 00-664 Warsaw (Poland)
[c] Prof. L. Cavallo
KAUST Catalysis Center, Physical Sciences and Engineering Division
King Abdullah University of Science and Technology
23955-6900 Thuwal (Saudi Arabia)
E-mail: luigi.cavallo@kaust.edu.sa
[d] Dr. A. Poater
Institut de Química Computacional i Catàlisi and Departament de Química
Universitat de Girona, Campus Montilivi
17071 Girona, Catalonia (Spain)
Recently, we have developed an alkyne addition to an enan-
tiomerically pure nitrone catalyzed by well-defined NHC–cop-
per(I) complexes on water.[16] We anticipated that the applica-
E-mail: albert.poater@udg.edu
Supporting information and ORCID(s) for the author(s) for this article can
be obtained under http://dx.doi.org/10.1002/chem.201601581.
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tion of the same conditions can lead to a practical and eco-
nomical method for the synthesis of trifluoromethyl propargyl
alcohols. Considering the method developed by us and the
electronic and chemical properties of TFMKs, application of the
same conditions could be questionable. It is well-established
that TFMKs could form hydrates.[17] Depending on the nature
of the substituent, the equilibrium can be shifted strongly to-
wards the hydrated form.[18] Furthermore, the strong acidic
character of TFMKs bearing an alkyl substituent (pKaꢀ15)
might also be problematic. One can suppose that undesired
side reactions (e.g., self-condensation) resulting from the pres-
ence of a base, which is needed for the activation of alkynes
to form acetylides, can be privileged, decreasing the yield of
the major product.
Table 1. Influence of the NHC carbene ligand and the counterion on the
addition of phenylacetylene (2a) to trifluoroacetophenone (1a)[a]
.
Entry
Catalyst [2 mol%]
Conversion [%][b]
Yield [%][c]
1
2
3
4
5
6
7
8
[IMesCuCl]
[IMesCuBr]
[SIMesCuCl]
[SIMesCuBr]
[PyIMesCuCl]
[IPrCuI]
[IPrCuBr]
[IPrCuCl]
[SIPrCuCl]
SIPr·HCl+Cu2O
[IPr*CuCl]
54
56
49
49
76
88
88
93
94
44
92
93
93
93
92
92
53
49
48
45
45
68
82
85
87 (97,[d] 98,[e] 93[f])
The solution to the above-mentioned problems can be
achieved by appropriate selection of the ligand. It is well es-
tablished that NHC–copper(I) complexes (Figure 2) can form
9
88
38
89
86
85
84
88
87
47
10
11
12
13
14
15
16
17
[SIPrCuI]
[SIPrCuBr]
[SIPrCu][OAc]
[SIPrCu][OTf]
[SIPrAgCl]
[IPrAuCl]
[a] All reactions were conducted using trifluoroacetophenone (1.0 mmol),
phenylacetylene (1.2 mmol, 1.2 equiv), Et3N (0.2 mmol), and 2 mol% of
the respective [NHCMX] complex. [b] Conversion was determined by GC
with tetradecane as internal standard. [c] Isolated yield after chromatog-
raphy. [d] The reaction was conducted on a 10.0 mmol scale, 16 h, 508C,
and 2 mol% of catalyst was used. [e] The reaction was conducted on
a 10.0 mmol scale, 48 h, 508C, and 0.1 mol% of catalyst was used. [f] The
reaction was conducted on a 20.0 mmol scale, 48 h, 808C, and 0.5 mol%
of catalyst was used, purified by distillation.
Figure 2. NHC–metal complexes used in the alkynylation reaction.
stable, monomeric (as confirmed by X-ray analysis)[19], highly
reactive (e.g., in 1,2,3-triazole formation)[20], and moisture-in-
sensitive acetylides under mild conditions, in contrast to the
polymeric ones, generated from copper(I) salts themselves.
Moreover, NHC ligands could also increase the nucleophilicity
of the C(sp) carbon of the acetylide, permitting the addition to
the electrophile in contrast to polymeric, inactive copper(I)
acetylides.
A series of copper(I) complexes bearing different counterions,
including halides, acetate, and triflates, afforded alcohol 1a
with yields around 90%. Similar reactivity was observed in the
reaction catalyzed by [SIPrAgCl], whereas gold(I) complex
[IPrAuCl] was less effective. Additional investigations were con-
ducted to confirm the unique effect of NHC ligands. Recently,
Cazin has reported the possible use of copper(I) oxide as
a source of metal for the synthesis of respective copper(I) com-
plexes on water.[21] Encouraged by these results, we proved
that the addition can be performed with the copper(I) complex
formed in situ. However, the yield of the reaction is lower in
comparison with the reaction conducted with [SIPrCuCl] com-
plex (entry 9 versus 10), whereas copper(I) halides did not cata-
lyze the alkynylation. The CÀH activation/addition sequence
catalyzed by the complex generated in situ is of high impor-
tance from the perspective of the enantioselective reaction
since chiral NHC copper complexes cannot be isolated due to
their instability.
In the initial studies, trifluoroacetophenone (1a) and phenyl-
acetylene (2a) were chosen as the model substrates to probe
the steric and electronic properties of the NHC ligands forming
the copper(I) complex. All of the reactions were conducted on
water using 2 mol% of the NHC–copper(I) complex. Detailed
studies are summarized in Table 1. First we investigated two
families of copper(I) complexes bearing SIMes and IMes li-
gands. The conversion and yield were moderate, around 50%,
irrespective of the ligand and counterion. A slightly better
result was obtained with [PyIMesCuCl] possessing a chelating
pyridine arm, furnishing the alcohol 1a with 68% yield
(entry 5). Further investigation proved that the yield could be
increased to 88% in the reactions with [IPrCuX] complexes.
Similarly satisfactory results were obtained in the reaction per-
formed with the sterically congested complex [IPr*CuCl]
(entry 11). The best results, both in terms of yield and conver-
sion, were obtained when [SIPrCuX] complexes were applied.
Bearing in mind that alcohol 1a would serve as a convenient
precursor of Mosher’s acid, we carried out the alkynylation on
a large scale. Pleasingly, we found that the loading of [IPrCuCl]
could be lowered to 0.1 mol%, allowing the isolation of alco-
hol 1a with virtually quantitative yield. However, the reaction
time had to be extended to 48 h. When the addition was per-
formed on a 10 mmol scale using 0.5 mol% of [IPrCuCl], the
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product was isolated with 97% yield after chromatography.
Pleasingly, the same reaction conducted on a 20 mmol scale
afforded the product with 93% yield after simple Kugelrohr
distillation (Table 1). It should be pointed out that only
48.5 mg of [IPrCuCl] was needed to produce 5.1 g of alcohol
3a.
the former alkyne ligand and 1a, and finally the release of the
product. The highest in energy transition state corresponds to
the CÀC bond formation, with an overall energy barrier of
26.1 kcalmolÀ1 for the [IPrCuCl] catalyst, and 29.4 kcalmolÀ1 for
the [IMesCuCl] and [SIMesCuCl] systems. Even though the ge-
ometry of the latter transition state for the three studied sys-
tems is rather similar (see the SI for further details), the two
preceding intermediates are roughly 3–4 kcalmolÀ1 less stable
for [IPrCuCl] relative to [IMesCuCl] and [SIMesCuCl] because of
the higher sterical hindrance around the metal centre due to
the sterically demanding IPr ligand (steric maps included in
Figure S2 in the SI). The higher stability of the IMes- and
SIMes-based intermediates results in higher energy barriers,
which explains the poor yield for both systems, whereas the
reduced stability of the IPr-based intermediates results in
a lower barrier, explaining the experimental yields above 90%.
Next, we examined the role of fluorine atom(s) as the acti-
vating group. It was proved that the presence of at least two
fluorine atoms is needed to achieve a high yield (Scheme 1).
The yield of the addition of phenylacetylene to fluoroaceto-
phenone dramatically decreased to 16%, whereas acetophe-
none appeared to be completely unreactive under the reaction
conditions.
To examine whether the increase of the steric hindrance of
the NHC ligand leading to higher yields of the alkynylation
was just a matter of arbitrary fact, or following a regular quan-
titative trend, we performed density functional theory (DFT)
calculations. Preliminary calculations showed that there was
virtually no correlation (R2 =0.025) between the yield and the
dissociation energy of the halogen, acetate or triflate. Thus, we
switched our efforts towards correlating the yield to the steric
bulkiness of the NHC using the percent of buried volume
(%VBur), a general descriptor initially developed for NHCs.[22]
Although the first correlation with the whole dataset in Table 1
already gave a reasonably good correlation, with an R2 =0.803,
removing the two data for Ag- and Au-based systems and the
highly sterically demanding [IPr*CuCl], the agreement is much
better (R2 =0.980) with 13 systems still described (see Fig-
ure S1, in the Supporting Information). Moreover, removing
the complexes bearing OAc and OTf counterions, which are
quite mobile, does not modify the agreement at all (R2 =
0.986), confirming this correlation between the sterics and the
yield of alkynylation is not an artefact (see Figure S1 in the SI
to verify that the distribution data are equally spread). Overall,
due to the paucity of available experimental data, we correlat-
ed the yields with the %VBur with meaningful statistical signifi-
cance.
To rationalize why [IMesCuCl] and [SIMesCuCl] show such
poor catalytic behaviour in comparison to [IPrCuCl], we investi-
gated the reaction pathway corresponding to the addition of
2a to 1a (see Figure 3). The mechanism is summarized in
three main parts: first the deprotonation of the entering
alkyne assisted by NEt3, next the CÀC bond formation between
Scheme 1. Influence of fluorine atom(s) on the addition reaction. [a] Conver-
sion based on GC with tetradecane as internal standard. [b] The reaction
was performed at 1008C for 16 h.
Figure 3. Reaction pathway (energies in kcalmolÀ1) of the addition of phenylacetylene to trifluoroacetophenone by the action of catalysts [IPrCuCl] (in light
grey), [IMesCuCl] (in black), and [SIMesCuCl] (in dark grey).
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Taking into consideration the commercial availability of the
[IPrCuCl] complex and the need to keep the reaction time rea-
sonable, we chose [IPrCuCl] (2 mol%) and Et3N (20 mol%) on
water as the optimal conditions to examine the scope of sub-
strates. First, we focused on the use of aromatic and heteroaro-
matic TFMKs. In the series of aromatic TFMKs, the yield was
high when phenylacetylene was used as the other partner
(Scheme 2). Electron-donating and -withdrawing groups were
tolerated under the reaction conditions. However, the yield of
alcohol 3i was moderate, due to diminished electrophilicity.
Further experimentation revealed that alcohol 3i could be iso-
lated with virtually quantitative yield upon extension of the re-
action time to 48 h or at higher temperature (1008C). No com-
petitive formation of the hydrate was observed in this case.
Next, we explored the possibility of the synthesis of a key
building block of efavirenz in racemic form. After some experi-
mentation, it was found that a protecting group on the nitro-
gen atom has a beneficial effect on the yield. Trifluoromethyl
propargyl alcohol 3r could be isolated in 76% yield using
a trityl-protected aniline derivative with only 2 mol% of IPr*-
CuCl and 2 mol% of N,N,N,N-tetramethylguanidine, whereas
PMB-protected ketone gave a poor yield (26%; see the SI).
Then, we unambiguously proved that the alkynylation can
be highly chemoselective. When respective TFMKs were treat-
ed with phenylacetylene, carbinols 3k–3n were exclusively
formed and no product arising from the addition to aldehyde
or ketone was detected. Further investigations revealed that
the addition could be performed efficiently in the presence of
aldehyde, ketone, halogen, ferrocenyl, cyclopropyl, hydroxyl,
and acetal substituent.
Finally, to our delight, we found that sulfur- and oxygen-con-
taining heterocyclic TFMKs were tolerated under the reaction
conditions to afford the products with moderate to excellent
yields. Other electrophiles, such as a cyclic difluoro ketone,
isatin, and an aldehyde, also participate in the alkynylation, al-
though higher temperatures and longer reaction times were
needed to achieve a good yield (Scheme 2, 3ab–3ad).
The last part of our investigations was devoted to more
challenging alkyl-substituted TFMKs. This group of substrates is
seldom examined. The alkyl-substituted TFMKs are thought to
Scheme 2. Scope of electrophiles in the alkynylation. [a] [IPrCuCl] (2 mol%) and N,N,N,N-tetramethylguanidine (TMG; 20 mol%) were used. [b] The reaction
was conducted without external base.
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be difficult substrates due to possible undesired side reactions,
such as self-condensation or hydration. Gratifyingly, all tested
alkyl-substituted TFMKs appeared to participate well in alkyny-
lation with alkyl- or aryl-substituted terminal alkynes, affording
products with excellent yields in contrast to examples reported
previously (Scheme 3).[9] A broad range of functional groups,
Scheme 4. Enantioselective addition catalyzed by C1-symmetric NHC–cop-
per(I) complexes generated in situ.
and phosphonyl groups, were tolerated under the reaction
conditions. DFT calculations were performed to rationalize the
correlation between the yield of catalytic alkynylation and the
sterics of N-heterocyclic carbenes (NHCs), expressed with
buried volume (%VBur), showing an excellent agreement of
the theoretical calculations and the synthetic studies. Addition-
al DFT calculations shed light on the differential reactivity of
[(NHC)CuX] complexes in the alkynylation of TFMKs. The pres-
ent work represents the first example of a direct enantioselec-
tive alkynylation catalyzed by an NHC–copper(I) complex.
Scheme 3. The scope of addition to alkyl-substituted TFMKs.
including alkene, cyclopropyl, nitrile, and phosphonyl, was tol-
erated under the reactions conditions. We also observed a rare
example of nucleophilic 1,2-addition to enone, catalyzed by
the copper. When trifluoromethyl enone was reacted with phe-
nylacetylene, only product 4a arising from the 1,2-addition
pathway was isolated in 68%. Gratifyingly, we proved that the
addition could also be performed with an alkyne bearing an
amine functional group. In this case, the respective product 3z
was isolated with 62% yield without the need to use an exter-
nal base (Scheme 2).
Acknowledgements
The authors are grateful to Polish Ministry of Science and
Higher Education for financial support of the research (grant
Iuventus Plus IP2012 064172). A.P. thanks the Spanish MINECO
for project CTQ2014-59832-JIN, and L.C. funding from King Ab-
dullah University of Science and Technology (KAUST).We would
like to thank Prof. Samir Zard (Ecole Polytechnique) for helpful
discussion concerning the synthesis of trifluoromethyl ketones
and Dr. P. Kwiatkowski and Dr. W. Chaładaj for a generous gift
of samples of trifluoromethyl ketones and alkynes.
Taking into account the significant role of anti-HIV Efavirenz
in the drug market and Mosher’s acid in the determination of
diastereomeric excess, we initiated the studies to explore the
addition in enantioselective fashion. The preliminary results are
summarized in Scheme 3. To our delight, when the copper
source (Cu2O) was mixed with a chiral ligand effecting
a double activation, for example, containing an NHC core for
the activation of the terminal alkyne by forming a nucleophilic
copper acetylide and a thiourea or urea moiety for the activa-
tion of the trifluoromethyl ketone by hydrogen bonding, the
adduct was formed in moderate yield with 22% ee (Scheme 4).
The enantioselectivity could be further improved to 31% ee by
changing the tert-leucinol-derived ligand. This part of research
is now ongoing with promising results.
Keywords: alkynylation · copper · ketones · N-heterocyclic
carbenes · on water
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim