Copper(i)-catalyzed diastereoselective hydroxytrifluoromethylation of dienes accelerated by phosphine ligands

Article abstract of DOI:10.1039/c3sc50582k

A diastereoselective 1,4-hydroxytrifluoromethylation of dienes is catalyzed by copper cyanide-phosphine complexes. This reaction is significantly accelerated by bulky monodentate phosphines, and the discovery enables expedient access to a variety of CF₃-containing allylic alcohol derivatives.

Full text of DOI:10.1039/c3sc50582k

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Copper(I)-catalyzed diastereoselective
hydroxytrifluoromethylation of dienes accelerated
by phosphine ligands†
Cite this: DOI: 10.1039/c3sc50582k
*
Deng-Fu Lu, Cheng-Liang Zhu and Hao Xu
Received 28th February 2013
Accepted 19th March 2013
A diastereoselective 1,4-hydroxytrifluoromethylation of dienes is catalyzed by copper cyanide–phosphine
complexes. This reaction is significantly accelerated by bulky monodentate phosphines, and the discovery
enables expedient access to a variety of CF3-containing allylic alcohol derivatives.
DOI: 10.1039/c3sc50582k
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been reported.13 Herein, we describe a copper(I)-catalyzed
diastereoselective 1,4-hydroxytriuoromethylation of dienes, a
Introduction
Peruorinated alkyl groups (especially the CF3 group) are oen
associated with unique electronic, lipophilic and metabolic
properties that are desirable in drug discovery.1 Therefore,
extensive research has been devoted to the selective incorpo-
ration of the CF3 group into organic molecules. A tremendous
amount of progress has been made in this area,2 most notably
including: (a) transition metal catalyzed or mediated CF3 group
transfer to alkyl, aromatic, vinyl, and alkynyl compounds;3 (b)
metal-free innate triuoromethylation of heterocycles;4 (c) light-
induced CF3 group incorporation in aromatics and a-tri-
uoromethylation of carbonyl compounds;5 (d) CF3-substituted
carbene insertion in olens and imines;6 and (e) CF3 group
transfer to aldehydes, ketones, and activated imines.7 In addi-
tion to these achievements, selective CF3 group transfer to
unfunctionalized olens has recently emerged as an important
research topic with increased interest.
reaction that is greatly accelerated by bulky monodentate phos-
phine ligands (Scheme 1B). This discovery enables expedient
transformations from readily available unfunctionalized dienes
to allylic alcohol derivatives with CF3-containing stereogenic
centers, which are otherwise difficult to access. The ligand
accelerated effect also allows for the control of reactivity and
selectivity by catalyst engineering.
Results and discussion
We selected 1,4-cyclohexadiene 1 as a model for catalyst
discovery (Table 1). Initial screening revealed that copper(I)
catalysts effectively transfer both the CF3 and acyloxy moieties
in Togni's reagent14 3 to diene 1, affording the 1,4-anti addition
product 2 as the major diastereomer.15 We observed a strong
counter anion effect: Cu(MeCN)4PF6 and CuCl promote fast and
non-selective processes to furnish 2 in low yield and dr,
although both of them are effective catalysts for allylic tri-
uoromethylation of monosubstituted olens (entries 1 and 2).8
Since a unique cyanide effect was noted in our recently
discovered iron-catalyzed olen aminohydroxylation,16 we
explored the CuCN-based catalyst in this transformation in
In 2011, the groups of Buchwald, Liu, and Wang indepen-
dently reported the allylic triuoromethylation of mono-
substituted olens (Scheme 1A).8 Sodeoka et al. subsequently
reported the triuoromethylation of terminal allyl silanes.9
Recently, Buchwald and Zhu10 disclosed an intramolecular oxy-
triuoromethylation of terminal olens with tethered nucleo-
´
philes and the groups of Szabo and Sodeoka independently
discovered the hydroxytriuoromethylaiton of styrenes.11 The
groups of Studer and Akita also reported a transition-metal free
TEMPONa-mediated and a photoredox oxytriuoromethylation
of olens respectively.12 In spite of these useful synthetic
methods, the diastereoselective 1,4-functionalization of dienes
with generation of CF3-containing stereogenic centers has not
Department of Chemistry, Georgia State University, 100 Piedmont Ave SE, Atlanta,
Georgia 30303, USA. E-mail: hxu@gsu.edu; Tel: +1-404-413-5553
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data for all new compounds, and selected NMR spectra. CCDC
926058. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3sc50582k
Scheme 1 CF3 group transfer to olefins and dienes.
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Table 1 Catalyst investigation for the hydroxytrifluoromethylation of dienes
Entrya
Cu(X)n
Ligand (15 mol%)
T (h)
Conversionb (%)
Yieldb (%)
drb
1
2
Cu(MeCN)4PF6
CuCl
CuCN
CuCN
CuCN
CuCN
CuCN
CuCN
CuCN
NA
NA
NA
4
5
6
7
8
9
9
4
4
12
12
6
4
4
12
6
12
12
90
80
32
56
Trace
Trace
53
47
40
23
77
1.2
2.0
NA
NA
3.0
5.0
4.2
7.0
6.7
NA
5.5
3
<10
<10
>95
>95
>95
50
>95
Trace
<20
4c
5c
6c
7c
8c
9cd
10cd
11cd
Cu(MeCN)4PF6
CuCl
NA
13
9
a
All reactions were carried out under argon. b Conversion, yield and dr were determined using 19F NMR with PhCF3 as an internal standard. c The
catalyst was prepared by premixing CuCN with ligand for 20 min. The PtBu3 was generated in situ by deprotonation of PtBu3$HBF4 salt with
d
(iPr)2EtN. BINAP ¼ 2,20-bis(diphenylphosphino)-1,10-binaphthyl.
order to enhance the chemo- and diastereoselectivity (entries group is transferred to the terminal position (entries 3 and 4,
3–9). Even though CuCN and the CuCN–phenanthroline 4 E/Z > 20 : 1).20 Concordantly, cyclic and acyclic disubstituted
complex barely promote the reaction (entries 3–4), we discov- dienes also act as good substrates (entries 5 and 6): the
ered that phosphine ligands can signicantly accelerate this 1,4-addition product contains an E-olen that has the CF3 and
conjugate addition: the CuCN–PPh3 complex promoted the acyloxy groups in two allylic positions (E/Z > 20 : 1).21 Dienes with
desired reaction with an improved yield and dr (entry 5, 53% other functional groups also participate in the reaction. The CF3
yield, dr ¼ 3.0), and bulkier PCy3 6 allowed for a further increase group transfer to isoprene proved successful (entry 7, E/Z >
in dr (entry 6, 47% yield, dr ¼ 5.0). However, PnBu3 7 resulted in 20 : 1).21 We note that the CF3 group is preferentially transferred
decreased selectivity (entry 7, 40% yield, dr ¼ 4.2). Racemic to the disubstituted terminal olen of isoprene. Furthermore, an
BINAP 8 was also found to mediate a much slower addition, isoprene derivative is also a viable substrate (entry 8, E/Z >
affording 2 with a further enhanced dr but a low yield (entry 8, 20 : 1).21 We further discovered that an allylic alcohol functional
23% yield, dr ¼ 7.0).17 This reaction failed to reach completion group can be tolerated in this reaction (entry 9).
even aer 24 h.
Further evaluation of acyclic internal dienes revealed the
Inspired by the effectiveness of bulky monodentate phos- stereoconvergent nature of this 1,4-addition: a mixture of
phines in cross-coupling reactions,18 we tried to further opti- trisubstituted dienes (entry 10, s-cis/s-trans ¼ 1 : 1) underwent
mize this reaction with PtBu3 9 and were pleased to nd that the convergent conjugate addition with 3 to afford a pair of
CuCN–9 complex effectively promoted a fast and selective CF3 regioisomers (E/Z > 20 : 1).20 The corresponding allylic alcohol
group transfer to generate 2 with a decent yield and dr (entry 9, with a CF3-containing quaternary center can be separated from
77% yield, dr ¼ 6.7). To our surprise, the rate acceleration effect its isomer upon hydrolysis. We subsequently observed that the
of phosphines uniquely applies to CuCN catalysts: Cu(MeCN)4- CF3 group transfer to another pair of internal dienes with Ph-
PF6–PtBu3 and CuCl–PtBu3 complexes are unable to promote the and Me-substituents occurs smoothly, affording an anti-addi-
reaction to full conversion (entries 10–11).
With the optimal catalyst in hand, we explored the substrate eoselectivity (entry 11, E/Z > 20 : 1, dr > 20 : 1).22
scope of this hydroxytriuoromethylation with a variety of dienes
The unique effectiveness of CuCN–PtBu3 in this reaction is
tion product with an acceptable yield and an excellent diaster-
(Table 2). Analogous to cyclohexadiene (entry 1), the diaster- mechanistically intriguing; therefore, we carriedout a few control
eoselective CF3/acyloxy group transfer to cyclopentadiene occurs reactionstoprobeforthepossiblemechanism(Scheme2).First,a
smoothly (entry 2, 67% isolated yield aer the hydrolysis, and screen of a variety of copper(II) catalysts revealed that they poorly
dr > 20 : 1).19 Monosubstituted dienes with either an alkyl or aryl promote the desired transformation, resulting in the recovery of
substituent readily participate in the reaction, where the CF3 dienes and Togni's reagent 3. This suggests that copper(I)
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Table 2 Substrate scope for the copper-catalyzed diastereoselective hydroxy-
trifluoromethylation of dienes
Entrya
1
Substrate
Product
Yieldb (%)
73
2c
67
3
4
72
68
Scheme 2 Control experiments for mechanistic insight. TEMPO ¼ 2,2,6,6-tet-
ramethyl-1-piperidinyloxy radical.
discovered that in the absence of the CuCN–PtBu3 catalyst the
diene 1, Togni's reagent 3, and TEMPO were fully recovered in the
aforementioned reaction. These results indicate that the CF3
radical is involved in the 1,4-addition and copper(I) is critical for
its generation. We then applied Baran's innate heteroaryl tri-
uoromethylation protocol to diene 1 to ensure that the CF3
radical is generated.4 We were surprised to nd that no new
CF3-incorporation product was detected and diene 1 was recov-
ered. This result suggests the ligand transfer from a copper
complex to a CF3 radical–diene adduct is equally important for
the 1,4-hydroxytriuoromethylation of dienes. We subsequently
used a DCM–MeOH (10 : 1) mixture as the solvent for the CuCN–
PtBu3 catalyzed reaction and did not identify any methoxy group
incorporated product. The absence of methoxy group incorpo-
rated product in the presence of MeOH indicates that the
involvement of a free allylic cation intermediate is unlikely.
The collective mechanistic evidence, including the control
experiments, the observed ligand acceleration effect and the
stereoconvergent nature of this reaction, suggest the critical
roles of the CF3 radical and copper(I) catalysts in this reaction.
Based on these results, we put forward a mechanistic working
hypothesis that best corroborates the experimental data
(Scheme 3). The CuCN–PtBu3 complex can presumably cleave
the I–CF3 bond, and reversibly generate high-valent copper
species.23 There are two possible complexes that can be in
equilibrium: (a) a copper(III) complex A1 that is covalently
bonded with a carboxylate and a CF3 group;24 and (b) a copper(II)
complex A2 that is tightly associated with a CF3 radical.25 The
coordination of the electron rich and bulky PtBu3 ligand to
CuCN evidently enhanced copper's reducing ability, and
presumably accelerates the I–CF3 bond cleavage. It is important
5
73
6
7
74
67
8
9
58
68
10cd
67
58
11d
a
b
All reactions were carried out under argon. Isolated yield, and dr
were determined using 19F NMR. The desired product was isolated
c
aer hydrolysis. d CuCN–PtBu3 (20 mol%) was applied.
to note that
a similar reactivity of copper(I)–phosphine
catalysts have an important redox role in the reaction. When we complexes was reported by Shi et al. in olen di-amination with
applied a stoichiometric amount of TEMPO to the CuCN–PtBu3 di-tert-butyldiaziridinone.26 The CF3 radical associated with A2
catalyzed hydroxytriuoromethylation of diene 1, we detected a can then undergo atom transfer radical addition to diene 1 to
CF3 radical trapping product 10 (74% yield) by NMR, with a small generate an allylic radical complex B1, where the olen is bound
amount of 1,4-addition product 2 (12% yield). We further with copper(II) through p-coordination. B1 is possibly in
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Scheme
3
Mechanistic working hypothesis of CuCN–PtBu3-catalyzed
1,4-hydroxytrifluoromethylation of 1,4-cyclohexadiene.
equilibrium with B2, another copper(III) complex with a s allyl
fragment. Presumably, the bulky PtBu3 ligand shis the equi-
librium from B1 to B2 and induces a carboxylate ligand transfer
from the copper to the allyl fragment, affording the 1,4-addition
product with the regeneration of copper(I) catalyst.27
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Conclusions
In summary, we have discovered a copper(I) catalyzed diaster-
eoselective 1,4-hydroxytriuoromethylation of dienes. This
CF3/acyloxy group transfer process is evidently accelerated by
bulky monodentate phosphine ligands. We also demonstrated
that a variety of dienes are suitable substrates for this new
reaction. Preliminary mechanistic studies revealed the origin of
ligand accelerated catalysis, as well as the critical role of the
CF3 radical in this process. Our current research is focused on
the better understanding of the mechanistic details and the
application of this method in medicinal agent synthesis.
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Acknowledgements
This work was supported by Georgia State University and the
American Chemical Society Petroleum Research Fund (ACS PRF
51571-DNI1).
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