Intermolecular hydroamination of 1,3-dienes catalyzed by bis(phosphine)carbodicarbene-rhodium complexes

Article abstract of DOI:10.1021/ja502275w

A carbodicarbene (CDC)-based pincer ligand scaffold is reported, along with its application to site-selective Rh(I)-catalyzed intermolecular hydroamination of 1,3-dienes with aryl and alkyl amines. To the best of our knowledge, this is the first example of the use of a well-defined CDC complex as an efficient catalyst. Transformations proceed in the presence of 1.0-5.0 mol % Rh complex at 35-120 °C; allylic amines are obtained in up to 97% yield and with >98:2 site selectivity.

Full text of DOI:10.1021/ja502275w

Communication
pubs.acs.org/JACS
Intermolecular Hydroamination of 1,3-Dienes Catalyzed by
Bis(phosphine)carbodicarbene−Rhodium Complexes
Matthew J. Goldfogel,‡ Courtney C. Roberts,‡ and Simon J. Meek*
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
S
* Supporting Information
Scheme 1. Synthesis and X-ray Structure of (CDC)-Rh(I)
a
ABSTRACT: A carbodicarbene (CDC)-based pincer
Complexes 6−8
ligand scaffold is reported, along with its application to
site-selective Rh(I)-catalyzed intermolecular hydroamina-
tion of 1,3-dienes with aryl and alkyl amines. To the best
of our knowledge, this is the first example of the use of a
well-defined CDC complex as an efficient catalyst.
Transformations proceed in the presence of 1.0−5.0 mol
% Rh complex at 35−120 °C; allylic amines are obtained
in up to 97% yield and with >98:2 site selectivity.
arbon-based donors represent an important class of
Cligands for transition metals that promote multiple
reaction types.1 A significant objective in developing catalytic
reactions is the design and synthesis of new classes of ligands.
a
See SI for experimental details; all reactions were performed under
Accordingly, the development of new classes of carbon-based
N2 atm. Selected bond lengths for 8 (Å): Rh1−C1, 2.043; Rh1−N5,
ligands for use in transition-metal catalysis is an important goal
2.027; Rh1−P1, 2.240; Rh1−P2, 2.230; C1−C2, 1.398; C1−C3, 1.387;
C2−N1, 1.352; C2−N2, 1.374; C3−N3, 1.359; C3−N4, 1.379.
in chemical synthesis. Carbodicarbenes (CDCs),2,3 also
referred to as bent allenes, are a family of compounds that
contain a divalent carbon(0) center, captodatively stabilized by
two carbene donors. These ligands can effectively bind to
transition metals, and their σ- and π-electron-donating
properties have been established both experimentally4 and by
theoretical calculations5 to be stronger than those of N-
heterocyclic carbenes (NHCs). Surprisingly, only monodentate
CDC complexes have been characterized to date (Au (1),4d,6
Ru (2),7 Fe,8 Rh,4a−c,9 Pd9); in addition, there is an absence of
reports demonstrating the ability of CDCs to act as effective
ligands for transition-metal catalysis. Furthermore, metal
complexes supported by tridentate CDC-based ligands have
not been prepared, despite the utility of pincer scaffolds in
promoting a number of important reactions.10 In light of these
limitations, we initiated a program for the study of a new class
of tridentate bis(phosphine)carbodicarbenes and examined
their ability to yield catalysis and effect a number of useful
transformations.
Herein we report the synthesis, structure, and catalytic
activity of easily prepared tridentate bis(phosphine)(CDC)-
Rh(I) complexes that effect formation of allylic amines via
selective intermolecular hydroamination of 1,3-dienes with aryl
and alkyl amines. General catalytic procedures for the synthesis
of functionalized unsaturated N-containing molecules by the
direct addition of amines to C−C π-bonds offer desirable,
atom-economical transformations for chemical synthesis.11
Transition-metal-catalyzed intermolecular addition of amines
to dienes to selectively afford allylic amines has been
studied;12,13 however, poor control of site selectivity and the
lack of a general catalytic system capable of both aryl and alkyl
amine additions limit the field.14 Catalytic protocols have
focused on the use of aryl and alkyl amines in order to obtain
high site selectivity.12 The (CDC)-Rh(I)-promoted hydro-
aminations described herein proceed with low catalyst loadings
(1−5 mol %) and are tolerant of both alkyl and aryl amines;
levels of site selectivity and efficiency are similar to those
obtained with previous intermolecular metal-catalyzed meth-
ods. Notably, the identity of the phosphine substituents (aryl vs
alkyl) plays an important role in determining the catalyst
activity.
We initiated our studies by the synthesis of the required 1,4-
diazepenium salts. As shown in Scheme 1, phosphination of
Received: March 6, 2014
Published: April 17, 2014
© 2014 American Chemical Society
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Communication
heterocyclic base 3 with Ph2PCl (or i-Pr2PCl) in the presence
of Et3N affords 1,4-diazepenium salts 4 and 5 in 85% and 71%
yield, respectively. Both salts are bench stable and purified by
silica gel column chromatography. The tetrafluoroborate salts 4
and 5 may then undergo cyclometalation by treatment with a
suspension of [Rh(cod)Cl]2 in THF at 22 °C, followed by
deprotonation of the corresponding cationic Rh(III)-H with
NaOMe in THF at 22 °C, to afford square-planar (CDC)-
Rh(I) complexes 6 and 7 as orange/yellow solids, each in 98%
yield.15 The 13C NMR signal of the CDC carbon(0) appears as
a doublet of triplets in the 13C{1H} NMR spectrum: 72.98 ppm
for 6 (1JRh = 36.0 Hz, 1JP = 11.7 Hz) and 73.74 ppm for 7 (1JRh
Table 1. Evaluation of (CDC)-Rh(I) Complexes in
Hydroamination
a
b
c
entry
complex; mol %
additive; mol %
conv (%)
yield (%)
1
6; 5
7; 5
6; 5
7; 5
6; 5
6; 5
6; 5
6; 1
8; 5
−
−
−
<2
<2
75
73
70
40
60
63
72
<2
<2
nd
nd
66
65
59
31
51
59
67
nd
nd
2
3
AgBF; 5
AgBF; 5
AgPF6; 5
AgSbFe; 5
AgOTf; 5
AgBF; 1
−
4
1
5
= 36.3 Hz, JP = 10.4 Hz). These values are consistent with
those previously reported by Bertrand and Furstner,4,6 with the
6
̈
7
upfield shift indicating the electron-rich nature of the divalent
carbon(0). To elucidate some of the structural features of
(CDC)-Rh complexes (Scheme 1), we obtained the X-ray
crystal structure of acetonitrile complex 8.16 As indicated by the
ORTEP diagram, the Rh1−C1 bond length is 2.043 Å. Bond
lengths of the CDC ligand indicate a CDC structure with
average C3−C1 bond lengths of 1.395 Å, in comparison to the
shorter N2−C2 carbene in NHCs (average 1.365 Å). The Rh1−
N5 bond length of 2.029 Å indicates the strong trans influence
of the CDC carbon.17
8
9
10
11
HBF4·OEt2; 5
AgBF; 5
−
a
See SI for experimental details; all reactions performed under N2 atm;
b
>98% site selectivity in all cases. Values determined by analysis of 400
or 600 MHz 1H NMR spectra of unpurified mixtures with DMF as an
c
internal standard. Yields of purified products are an average of two
runs; nd = not determined.
To gain insight into the electronic nature of the ligand, we
treated 4 with 1 equiv of HBF4·OEt2 in CD2Cl2 at 22 °C (eq
hydroamination with 5 mol % Rh(I)-NCMe complex 8 (entry
9) affords 11 with 72% conversion, similar to that achieved with
catalysts generated in situ with silver(I) salts, suggesting that a
cationic Rh(I) complex is the active catalyst. Control reactions
with HBF4·OEt2 and AgBF4 (entries 10 and 11) exclude an
acid- or silver(I)-catalyzed process.
1
1), which generated dication 9. The symmetrical H NMR
Next, we examined the influence of changing the identity of
the amine on the activity of (CDC)-Rh(I)-catalyzed hydro-
amination. As the representative examples in Table 2
Table 2. (CDC)-Rh-Catalyzed Hydroaminations of Phenyl-
a
1,3-butadiene with Aryl and Secondary Alkyl Amines
complex; temp
time
(h)
conv
b
yield
c
confirms protonation at the central carbon, in accord with
previously described systems.4d,5e This demonstrates the
presence of significant electron density at the central carbon
of cation 4 and supports its reactivity as a CDC. The electron-
donating properties of the CDCs derived from 4 and 5 were
further evaluated through the carbonyl stretching frequencies of
10a,b (eq 2). The cationic Rh(I) complexes exhibit infrared
νCO values (10a, 1986 cm−1; 10b, 1970 cm−1) lower than those
observed for analogous cationic Rh(I) pincer complexes.18
With Rh(I) complexes in hand, we began to investigate
whether 6 and 7 are effective catalysts for hydroamination. As
the data in Table 1 illustrate, the ability of Rh(I) complexes 6
and 7 to catalyze the hydroamination of phenyl-1,3-butadiene
with aniline requires an additive; <2% conversion is observed
(entries 1 and 2). In contrast, as shown in entries 3 and 4, when
5 mol % (CDC)-Rh and 5 mol % AgBF4 are used (80 °C,
C6H5Cl), the reactions proceed to deliver allylic amine 11
(>98% Markovnikov site selectivity) in 66% and 65% isolated
yield, respectively. Less-coordinating ions (PF6, SbF6, and OTf)
are less efficient (31−59% yield; entries 5−7). Gratifyingly, the
hydroamination can be effected with 1 mol % 6 to deliver 11
(63% conversion) in slightly diminished yield (59%). Catalytic
entry
amine; product
mol %
(°C)
(%)
(%)
1
2
3
4
5
6
7
8
9
C6H5NH2; 11
6; 1
7; 2
7; 3
6; 3
7; 5
7; 3
6; 5
7; 2
7; 5
7; 5
60
60
60
50
60
80
80
80
80
80
24
24
48
48
48
48
48
48
48
48
88
96
68
86
89
92
80
58
74
71
91
64
85
80
89
p-CF3C6H4NHa; 12
p-MeOC6H4NH2; 13
o-BrC6H4NH2; 14
o-MeC6H4-NH2; 15
morpholine; 16
pyrrolidine; 17
d
75
Bn2NH; 18
56
72
6
Bn(Me)NH; 19
e
10
a−c
n-Pr2NH; 20
14
d
See Table 1. With 20 mol % NH4BF4 additive; 11% without
NH4BF4. 12% conv at 100 °C.
e
demonstrate, Rh complexes 6 and 7 catalyzed hydroamination
of phenyl 1,3-butadiene with various aryl and alkyl amines to
generate allylic amines with >98% γ-selectivity. Entries 2 and 3
of Table 2 illustrate that allylic aryl amines with electron-
withdrawing (12) and electron-donating (13) groups can be
accessed with high site selectivity; the reaction of p-CF3-
substituted aniline proves to be slightly more efficient. Sterically
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hindered o-bromoaniline and o-toluidine (entries 4 and 5)
require 3−5 mol % of 6 and 7 to generate allylic amines 14 and
15 with complete site selectivity in 85% and 80% yield,
respectively. As shown in entries 6 and 7, cyclic alkyl amines
morpholine and pyrrolidine are tolerated and react to furnish
allylic amines 16 (89% yield) and 17 (75% yield); however,
pyrrolidine requires the use of 20 mol % NH4BF4 additive.
Moreover, secondary alkyl amines bearing benzyl (entries 8 and
9) and n-propyl (entry 10) groups can participate in Rh-
catalyzed site-selective hydroamination, albeit with varying
efficiency. Two points regarding Table 2 merit mention. First,
the optimal complex (6 vs 7) and reaction conditions in each
case vary depending on the amine structure.19 Second, in
general, (CDC)-Rh-catalyzed hydroaminations with alkyl
amines require higher temperatures (80 °C) to proceed
compared to aryl amines (50−60 °C).
Table 3. (CDC)-Rh(I)-Catalyzed Hydroaminations of
Dienes with Aniline
a
To further evaluate the catalytic properties of (CDC)-Rh(I),
we investigated the reaction with respect to the electronics of
the aryl diene component. A notable aspect of these studies is
the observed increased reactivity of complex 6 with electroni-
cally disparate dienes. As illustrated in Scheme 2, Rh-catalyzed
Scheme 2. Site-Selective Rh(I)-Catalyzed Hydroaminations
of Electronically Different Aryl Dienes
a
See SI for experimental details; all reactions performed under N2 atm
b
with 2 equiv of diene; up to >98% site selectivity. Yields of purified
products are an average of two runs. 3:2 mixture of γ:α addition. 4
c
d
equiv of diene was used.
Scheme 3. (CDC)-Rh-Catalyzed Hydroaminations of 1,3-
a
Dienes with Secondary Alkyl Amines
addition of aniline to p-MeO-substituted diene occurs at
significantly lower temperature compared to that of p-F-
substituted diene (35 vs 60 °C) to afford 21 and 22,
respectively, each in >85% yield.
Rhodium-catalyzed diene hydroaminations promoted by
pincer CDC complexes display significant synthetic scope. As
the representative examples in Table 3 indicate, Rh complexes
6 and 7 promote the hydroamination of alkyl diene substrates
to deliver allylic amine products bearing di- or trisubstituted
olefins (up to >98% γ-selectivity). Under optimal reaction
conditions (5 mol % 6 at 60 °C), cyclohexylbutadiene is
efficiently converted to 23 in 89% yield (entry 1). It is worthy
of note that n-alkyl-derived substrates undergo efficient catalytic
hydroamination to generate allylic amines but as mixtures of
constitutional isomers; 24 (5 mol % 6, 70 °C; entry 2) is
generated in 70% yield as an inseparable 3:2 mixture of γ:α
addition products. This underscores a current limitation of Rh
complexes 6 and 7 toward site-selective hydroamination of
sterically unbiased 1,3-dienes. As illustrated in entries 3, 7, and
8, trisubstituted 1,3-dienes undergo site-selective (>98%) Rh-
catalyzed hydroamination (5 mol % 6 or 7, 65−80 °C, 48 h) to
deliver the corresponding allylic amines in good yield: 25
(97%), 29 (77%), and 30 (69%). The Rh-catalyzed protocol is
also effective for the generation of cyclic allylic amines, as
demonstrated by the formation of 28 (entry 6) in 96% yield. It
should be noted that a number of functional groups are
compatible under the relatively mild reaction conditions,
including alkenes (entry 3), esters (entry 4), alcohols (entry
5), and N-tosyl amines (entry 8).
a
See Table 3.
Rh(I) hydroamination protocol. As noted (vide supra), catalytic
hydroamination with aliphatic amines generally requires higher
temperatures (70−120 °C) than that with aryl amines. Site-
selective formation of aliphatic allylic amines 31 (62%) and 32
(91%) from dibenzyl amine and morpholine proceeds
efficiently in the presence of 5 mol % 6 (70 and 100 °C).
Incorporation of ester functionality is also tolerated, as catalytic
hydroamination (5 mol % (CDC)-Rh 6 and 5 mol % AgBF4)
delivers 33 (120 °C, 48 h) and 34 (100 °C, 48 h) in modest to
excellent yields (30% and 91%, respectively).
In conclusion, we have developed a tridentate carbodicarbene
ligand scaffold that enables efficient Rh-catalyzed site-selective
intermolecular hydroamination of 1,3-dienes compatible with
both alkyl and aryl amines. The reactions described represent
the first examples of a CDC−transition metal complex
functioning as an effective catalyst. Further investigations into
the mechanistic details of (CDC)-Rh(I)-catalyzed hydro-
The four representative examples in Scheme 3 further
underline the generality and synthetic utility of this (CDC)-
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AUTHOR INFORMATION
■
Corresponding Author
sjmeek@unc.edu
Author Contributions
‡M.J.G. and C.C.R. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(13) For related catalytic intermolecular hydroamidations of 1,3-
dienes, see: (a) Brouwer, C.; He, C. Angew. Chem., Int. Ed. 2006, 45,
■
Financial support was provided by the University of North
Carolina 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 assistance solving the X-ray structure of Rh complex 8, and
S. Moore for assistance with HRMS.
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