Lewis Acid Activation of Carbodicarbene Catalysts for Rh-Catalyzed Hydroarylation of Dienes

Article abstract of DOI:10.1021/jacs.5b03510

The activation of carbodicarbene (CDC)-Rh(I) pincer complexes by secondary binding of metal salts is reported for the catalytic site-selective hydro-heteroarylation of dienes (up to 98% yield and >98:2 γ:α). Reactions are promoted by 5 mol % of a readily

Full text of DOI:10.1021/jacs.5b03510

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Communication
Lewis Acid Activation of Carbodicarbene Catalysts
for Rh-Catalyzed Hydroarylation of Dienes.
Courtney C. Roberts, Desiree M. Matias, Matthew J. Goldfogel, and Simon J. Meek
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 11 May 2015
Downloaded from http://pubs.acs.org on May 11, 2015
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Lewis Acid Activation of Carbodicarbene Catalysts for Rh-
Catalyzed Hydroarylation of Dienes.
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Courtney C. Roberts, Desirée M. Matías, Matthew J. Goldfogel, and Simon J. Meek*
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290.
Supporting Information Placeholder
electronically and sterically modify catalyst reactivity profiles
ABSTRACT: The activation of carbodicarbene-Rh(I) pincer
through binding of a second Lewis acid to the carbon(0) either
complexes by secondary binding of metal salts is reported for
temporarily or permanently. The use of carbon(0) bimetallic
the catalytic site-selective hydro-heteroarylation of dienes (up
complexes as catalysts, or the application of Lewis acids to
to 98% yield and >98:2 γ:α). Reactions are promoted by 5
mol % of a readily available tridentate carbodicarbene-Rh
complex in the presence of an inexpensive lithium salt. The
alter catalyst reactivity by secondary binding to carbon(0)
,
ligands in catalysts, has not been reported.10 11
reaction is compatible with a variety of terminal and internal
dienes and tolerant of esters, alkyl halides, and vinyl boron
functional groups. X-ray data and mechanistic experiments
provide support for the role of the metal salts on catalyst acti-
vation and shed light on the reaction mechanism. The in-
creased efficiency (120 to 22 °C) made available by catalytic
amounts of metal salts to catalysts containing C(0) donors is a
significant aspect of the disclosed studies.
Herein, we report the remarkable increase in catalyst reac-
tivity (120 °C to 22 °C) of (CDC)-Rh(I) complexes in inter-
molecular hydroarylations caused by catalytic amounts of
metal salt additives. Reactions proceed in the presence of 5
mol % of a readily available Rh(I) complex with terminal and
1,4-disubstituted dienes and a variety of N-heterocyclic
arenes.12 Mechanistic evidence is provided through protona-
tion studies that show secondary binding to the carbon(0) do-
nor results in a more electron deficient Rh center and
Development of catalytic methods that directly employ
readily available unsaturated hydrocarbons represents an im-
portant objective in chemical synthesis. A subgroup of these
reaction types is catalytic intermolecular hydroarylation, a
highly atom-economical process involving the net C–H addi-
, ,
tion across an unsaturated C–C bond.1 2 3 Metal π-acid cata-
lysts are effective promoters for such transformations, wherein
the C=C bond is rendered electrophilic and susceptible to ad-
dition by arene nucleophiles.4 Reactions typically proceed at
elevated temperatures (70–135 °C) in the presence of a cation-
ic Pt,4d-f or Au4e-i catalyst with electron-rich alkenes, and are
generally inhibited by Lewis-basic functionality,4i a problem
also common to catalytic hydroamination.5 To address some
of these limitations we initiated a program to develop new
catalysts and catalytic methods that enable the addition of
nucleophiles to C–C double bonds. We previously developed
Rh(I) complexes supported by pincer carbodicarbene (CDC)
ligands that efficiently catalyze the hydroamination of 1,3-
dienes with aryl and alkyl amines (35–120 °C).6
Divalent carbon(0) compounds, such as CDCs and carbodi-
phosphoranes (CDPs), represent an emerging area in ligand
development for transition metal catalysis and provide strate-
gies to access new modes of reactivity.7 Principal to their
reactivity is a divalent carbon(0) supported by two L-type
donor groups.8 Unlike their carbon(II) analogs, N-heterocyclic
carbenes (NHCs), the reactivity profile of carbon(0) ligands is
centered around two lone-pairs of electrons that are available
for binding to Lewis acids. Homo- and heterobimetallic tran-
sition metal complexes of carbon(0) have been reported, and
primarily employ CDP ligand frameworks and coinage metals
(I–IV Chart 1).9 Strong electron donor properties paired with
the ability to form dinuclear species provides a framework to
increased positive charge at a bound alkene. An outline of
the activation strategy for our studies is shown in Scheme 1a.
Reversible binding of a Lewis acid to a (CDC)-Rh(I) complex
will result in decreased electron density at Rh(I) when a Lewis
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conditions were chosen that employ cost effective LiBF4 at 50
acid is bound, rendering the Rh(I) more activating towards π
1
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3
4
5
6
°C. Of note, catalytic hydroarylation in the presence of 10
mol % 2,6-di-t-Bu-pyridine as a Brønsted acid scavenger does
not inhibit the reaction (entry 12, Table 1).15
acids.
We began our investigations with Rh–Cl complex 4
(Scheme 1b). Treatment of indole and diene 1 with 5 mol % 4
and 5 mol % NaBArF in dioxane at 80 °C resulted <2% con-
4
version to product; chloride is efficiently substituted by diene
1 in the presence of NaBArF . Use of 5 mol % AgBF4 in place
4
of NaBArF4 as a halide scavenger (in situ generation of AgCl)
delivers 3 in >98% conversion (>98:2 γ:α). In order to deter-
mine if AgCl was responsible for the increased catalyst reac-
tivity, cationic Rh(I)-styrene complex 5 was synthesized.13
Hydroarylation in the presence of 5 mol % styrene complex 5
in dioxane at 80 °C for 12 h affords <2% conversion to 3; sty-
rene in 5 is displaced by diene 1 at 22 °C in 30 min (Scheme
1c). Use of 5 mol % 5 and 5 mol % AgCl under identical con-
ditions affords 3 in >98% conversion and >98:2 γ:α. These
initial observations demonstrate the ability of AgCl as an addi-
tive to increase the activity of the (CDC)-Rh complexes for
hydroarylation.
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aSee SI for experimental details. bYields of purified products are an av-
erage of two runs. c91:9 C2:C3 site-selectivity on pyrrole. e1H NMR
yield; determined by analysis of 400 or 600 MHz 1H NMR spectra of
unpurified mixtures with DMF as an internal standard. e85:15 C2:C3 site-
f
selectivity on pyrrole. 48 h reaction.
The scope of LiBF4 as an additive was investigated for a
range of N-heterocycles to phenyl and cyclohexyl 1,3-dienes 1
and 6. As shown in Table 2, phenyl substituted 1,3-diene un-
dergoes site-selective hydroarylation with N-Me and 7-Cl
indole at 50 °C (entries 1–2) to afford 7 (63%, >98:2 γ:α) and
8 (71%, 96:4 γ:α). More nucleophilic 2,4-dimethylpyrrole
reacts at 22 °C in the presence of 5 mol % Rh(I) 5 and LiBF4
to deliver 9 in 88% yield, >98:2 γ:α, and 91:9 C2:C3 site-
selectivity on the pyrrole ring. Indoles bearing electron-
withdrawing groups require a slightly higher temperature (60
°C) to achieve good conversion; treatment of 6-NO2 indole
with 1 in the presence of 5 and LiBF4 reacts to generate 10 in
57% yield and 91:9 γ:α (entry 4). 3-Methyl indole directs the
diene addition to the 2-position of indole (entry 5) to afford 11
in 33 % yield and 92:8 selectivity. Similarly, increasing the
sterics and decreasing the nucleophilicity of the N-
hetereoarene results in a less efficient reaction; TIPS-pyrrole
undergoes catalytic hydroarylation at 70 °C to yield 12 in 38%
yield and >98:2 γ:α (entry 6, Table 1). The reaction does not
improve if >5.0 mol % LiBF4 is used. Alkyl dienes are equal-
ly effective reaction partners for Rh-catalyzed hydroarylation
as illustrated by the reactions of cyclohexyl-1,3-diene 6 with a
variety of substituted indoles (entries 7–10, Table 2); reactions
proceed efficiently with catalytic LiBF4 (5 mol %) at 50 °C
and deliver the alkylated indoles (13–16) in good yields (66–
91%) and excellent site-selectivity (>98:2 γ:α). 2,4-Dimethyl
pyrrole affords 2-substituted pyrrole 17 in 53% yield and
>98:2 diene γ:α selectivity (entry 11) at 22 °C in 24 h. Again,
alkyl 1,3-dienes react more sluggishly with indoles bearing
larger groups on nitrogen and require longer reaction times
and higher temperatures; 60 °C for 48 h is required to generate
substituted N-Bn indole 18 in 85% yield and 87:13 γ:α site-
selectivity.
aSee SI for experimental details; all reactions performed under N2 atm.
bConversion to product; values determined by analysis of 400 or 600 MHz
1H NMR spectra of unpurified mixtures with DMF as an internal standard.
d
cYields of purified products are an average of two runs. Dioxane as sol-
vent. e10 mol % 2,6-di-t-Bu-pyridine.
We next examined the effect of other Lewis acid additives
on catalyst activity, the results of these studies are shown in
Table 1. Control reaction with Rh(I)-styrene complex 5 at 120
°C in dioxane, leads to only 6% conversion to 3 (entry 1). As
illustrated in entries 2–4, reaction of 5 mol % 5 with an
equimolar amount of Cu-, Ag-, or Au chloride in Et2O at 50
°C for 24 h affords indole 3 in high yield (up to 96%) and se-
lectivity (up to 95:5 γ:α). Lithium salts were also found to
promote catalytic hydroarylation with similar efficiency (en-
tries 5–6); 5 mol % LiBArF -OEt2 and LiBF4 deliver 3 in 76%
4
and 94% yield and >98:2 and 97:3 γ:α, respectively.7c,14 Nota-
bly, sodium salts are not effective at increasing catalyst reac-
tivity, likely due to the decreased Lewis acidity of sodium
compared to lithium (entry 7, Table 1). Decreasing the reac-
tion temperature to 22 °C results in <2% conversion to 3 ex-
cept in the case of AuCl (60%, 81:19 γ:α, entry 10), and LiBF4
(>98%, 85:15 γ:α, entry 11). To achieve both high conver-
sion and site-selectivity in further studies, optimal reaction
We next extended the utility of the Rh(I)-catalyzed protocol
to more challenging site-selective additions to internal
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dienes.16 Products delivered through these catalytic reactions
afford functionalized differentially allyl substituted heterocy-
tion at C(0) leads to shortening of the CO bond length, indi-
cated by an increase in the IR stretching frequency (vCO
1
2
3
4
5
6
7
8
=
2016 cm-1), representing a significant decrease in π-back dona-
tion and decrease in electron density at Rh. Tetrafluoroboric
acid is a less effective activator, compared to LiBF4, for diene
hydroarylation.18 This is likely due to ligand protonation being
less reversible than binding LiBF4, which would indicate the
importance of a reversible interaction provided by Lewis acid
additives such as AgCl and LiBF4. Coinage metal salts also
present the possibility of a bimetallic interaction,9 although
this is unlikely with lithium. Together, the enhanced ligand
substitution and reactivity suggests that the metal additives
bind to the C(0) of the olefin complex promoting ligand sub-
stitution (styrene by THF) and also addition of a nucleophile
to the bound C=C bond.19 Further analysis of the catalytic
reaction mechanism with LiBF4 through deuterium labeling
studies with C-3 deuterium labeled N-Me-indole results in
formation of d1-7 with >95% deuterium transfer (Scheme 3d).
cles (Scheme 2). Reactions proceed with <2% conversion to
product at 120 °C for 24 h without LiBF4. 1,4-Aryl-alkyl sub-
stituted dienes undergo catalytic hydro-heteroarylation with
indole and 2,4-dimethyl pyrrole to deliver functionalized het-
erocycles (19–21) in good yield but with a slight diminution in
diene site-selectivity (>98:2–85:15 γ:α). The Rh-catalyzed
synthesis of p-chorostyrene derivatives 22 and 23 is notable as
such electron deficient dienes are not compatible with Au-
catalyzed methods;4c 5 mol % 5 and 5 mol % LiBF4 at 60 °C
delivers 22 and 23 in 73% and 95% yield. The reaction
demonstrates good functional group tolerance to alkyl halides
(24), boronate esters (25), and esters (26). Addition of indole
to symmetrical hexa-2,4-diene at 22 °C affords 27 in 64%
yield in 67:33 β:α selectivity.
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c
a–bSee Table 2. 85:15 C2:C3 site-selectivity on pyrrole. d98:2 C2:C3
site-selectivity on pyrrole. e1H NMR yield; determined by analysis of 400
or 600 MHz 1H NMR spectra of unpurified mixtures with DMF as an
internal standard.
To gain insight into the catalyst structure and the effect of
the metal salt additive, we obtained an X-ray structure of cati-
onic (CDC)-Rh-styrene complex 5 (Scheme 3a). Complex 5
has a square-planar structure, with an sp2 hybridzed C(0) do-
nor and a C(0)–Rh bond length of 2.07 Å. The styrene ligand
exhibits significant metal–alkene π-back-donation demonstrat-
ed by an elongated styrene C=C bond (1.395 Å).17 To investi-
gate the electronic changes that occur to the Rh–alkene bond
upon binding a second species to the carbon(0) donor, we at-
tempted to analyze the 13C NMR spectrum of the coordinated
styrene in complex 5 in the presence of a Lewis acid additive
(Scheme 3b). However, when complex 5 is treated with a
metal salt, loss of styrene is observed; treatment of (CDC)-
Rh(I)–styrene complex 5 with LiBArF , or AuCl in d8-thf at 22
4
°C for 24 h leads to tetrahydrofuran bound 28 and free styrene
(AuCl: 81% styrene, and LiBArF : 6% styrene (17% in 72 h)).
4
No loss of styrene (<2%) is observed without any additive.
These results indicate olefin substitution is facilitated by bind-
ing of a metal salt to the CDC, which destabilizes the olefin
complex by decreasing π-back donation, leading to styrene
substitution by a weakly donating tetrahydrofuran molecule.
In addition, rapid substitution of styrene occurs when complex
5 is reacted with 1 equivalent of indole and LiBF4 at 40 °C.
In summary, these studies describe a key attribute of car-
bon(0) donor ligands that expands their limited use in cataly-
sis.6, We show the potential for tuning ligand donation in
20
CDCs through secondary binding of Lewis acids, which ena-
bles the use of cationic (CDC)-Rh-based complexes as cata-
lysts for diene hydroarylation. Notably, simple lithium salts
emerged as effective catalytic Lewis acids that promote reac-
tions under mild conditions for a range of heteroarenes with
terminal and internal dienes. Development of related reactions
that utilize CDC catalyst activation, as well as enantioselective
variants, are in progress.
1
No Rh–H signals are observed in the H NMR spectrum, sug-
gesting a C–H activation mechanism is unlikely, indicating an
alkene activation pathway. To quantify the electronic changes
that occur at Rh(I) upon binding to the carbon(0) donor, pincer
(CDC)-Rh(I)–CO complex 296 was treated with HBF4-OEt2 to
cleanly yield 30 in >98% conversion (Scheme 3c). Protona-
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ASSOCIATED CONTENT
1
2
3
4
Supporting Information. Experimental procedures and spectral
and analytical data for all products. This material is available free
of charge via the Internet at http://pubs.acs.org.
(5) For a recent review on metal-catalyzed hydroamination, see:
Huang, L.; Arndt, M.; Gooßen, K.; Heydt, H.; Gooßen, L. J. Chem.
Rev. 2015, 115, 2596–2697.
(
6
) Goldfogel, M. J.; Roberts, C. C.; Meek, S. J. J. Am. Chem. Soc.
2014, 136, 6227–6230.
) For review of carbon-based ligands that includes carbon(0) do-
AUTHOR INFORMATION
5
6
7
8
9
Corresponding Author
(7
nors, see: (a) Melaimi, M.; Soleilhavoup, M.; Bertrand, G. Angew.
Chem. Int. Ed. 2010, 49, 8810–8849. For selected examples of car-
bon(0) ligands, see: (b) Dyker, C. A.; Lavallo, V.; Donnadieu, B.;
Bertrand, G. Angew. Chem. Int. Ed. 2008, 47, 3206–3209. (c) Lavallo,
V.; Dyker, C. A.; Donnadieu, B.; Bertrand, G. Angew. Chem. Int. Ed.
2008, 47, 5411–5414. (d) Melaimi, M.; Parameswaran, P.; Don-
nadieu, B.; Frenking, G.; Bertrand, G. Angew. Chem. Int. Ed. 2009,
48, 4792–4795.
(8) Frenking, G.; Tonner, R. Carbodicarbenes in Contemporary
Carbene Chemistry; Moss, R. A., Doyle, M. P. Eds.; John Wiley &
Sons: Hoboken, New Jersey, 2014; pp 216–236.
(9) (a) Vicente, J.; Singhal, A. R.; Jones, P. G. Organometallics
2002, 21, 5887–5900. (b) Alcarazo, M.; Lehmann, C. W.; Anoop, A.;
Thiel, W.; Fürstner, A. Nature Chem. 2009, 1, 295–301. (c) Reitsa-
mer, C.; Schuh, W.; Kopacka, H.; Wurst, K.; Peringer, P. Organome-
tallics 2009, 28, 6617–6620. (d) Alcarazo, M.; Radkowski, K.;
Mehler, G.; Goddard, R.; Fürstner, A. Chem. Commun. 2013, 49,
3140.
sjmeek@unc.edu
Notes
Authors declare no competing financial interests.
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ACKNOWLEDGMENT
Financial support was provided by the University of North Caro-
lina at Chapel Hill, the Petroleum Research Fund of the American
Chemical Society (52447-DNI1), and an Eli Lilly New Faculty
Award. C.C.R. is an NSF graduate research fellow. We are
grateful to M. Joannou (UNC) for helpful discussions and assis-
tance solving the X-ray structure of Rh complex 5.
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4
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Ph
5.0 mol %
Ph
Cl
BArF
4
1
2
3
4
5
6
7
8
Me
P
Rh
P
N
N
H
N
Ph
Me
γ
α
Me
+
N
H
N
Cl
Me
N
Ph
Me
Ph
5.0 mol % LiBF4
95% yield, >98:2 γ:α
Me
Et2O, 50 °C, 18 h
Ph
Ph
BArF
4
P
Rh
P
N
N
N
LA
N
L
9
LA = Lewis acid
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ph
ACS Paragon Plus Environment