Palladium-catalyzed intramolecular carbene insertion into C(sp3)-H bonds

Article abstract of DOI:10.1002/anie.201602020

A palladium-catalyzed carbene insertion into C(sp³)-H bonds leading to pyrrolidines was developed. The coupling reaction can be catalyzed by both Pd⁰ and Pd^II, is regioselective, and shows a broad functional group tolerance. This reaction is the first example of palladium-catalyzed C(sp³)-C(sp³) bond assembly starting from diazocarbonyl compounds. DFT calculations revealed that this direct C(sp³)-H bond functionalization reaction involves an unprecedented concerted metalation-deprotonation step. Pd in action: Palladium has been used to catalyze the C(sp³)-H insertion of metal carbenoids derived from α-diazoesters to form pyrrolidines through intramolecular assembly of C(sp³)-C(sp³) bonds. A reaction mechanism involving a metalation-deprotonation step instead of the usual concerted but asynchronous process is proposed.

Full text of DOI:10.1002/anie.201602020

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201602020
German Edition: DOI: 10.1002/ange.201602020
Palladium Catalysis
3
À
Palladium-Catalyzed Intramolecular Carbene Insertion into C(sp ) H
Bonds
Daniel SolØ,* Francesco Mariani, M.-Lluïsa Bennasar, and Israel Fernµndez*
Abstract:
A
palladium-catalyzed carbene insertion into
potentially useful catalysts for this type of transformation,[5–10]
the use of palladium has been restricted to a couple of
3
À
C(sp ) H bonds leading to pyrrolidines was developed. The
coupling reaction can be catalyzed by both Pd0 and PdII, is
regioselective, and shows a broad functional group tolerance.
This reaction is the first example of palladium-catalyzed
2
À
examples of a-diazo b-ketoester insertion into C(sp ) H
bonds.[11,12] This fact is highly surprising if we take into
account the great success of Pd catalysis in cross-coupling
reactions of diazo compounds with either organic halides or
arylboronic acids.[13]
3
3
À
C(sp ) C(sp ) bond assembly starting from diazocarbonyl
3
À
compounds. DFT calculations revealed that this direct C(sp )
H bond functionalization reaction involves an unprecedented
concerted metalation–deprotonation step.
As part of our research program on the synthesis of
azaheterocycles, we have been exploring different ways to
À
increase the versatility of Pd catalysis in C C bond-forming
reactions.[14,15] In this regard, we decided to investigate the
À
T
he development of practical and green methods for C C
bond formation by the selective functionalization of unac-
À
feasibility of Pd as a catalyst for the carbenoid C H insertion
À
tivated C H bonds is an area of great interest that has been
from diazocarbonyl substrates.
extensively studied over the last years.[1] Among the vast array
Herein we report an operationally simple procedure for
3
3
À
À
of such transformations, the C H insertion of metal carbe-
the Pd-catalyzed intramolecular assembly of C(sp ) C(sp )
noids derived from diazocarbonyl substrates constitutes
bonds starting from a-diazoesters, in which both Pd0 and PdII
3
a
particularly attractive method (Scheme 1).[2] In this
catalysts are effective. Mechanistically, this direct C(sp ) H
À
bond functionalization process is different from those
reported in the literature based on RhII or Cu catalysts.[3,4]
Our investigation began by testing the Pd-catalyzed
cyclization of a-diazoester 1 in order to assess the regiose-
À
lectivity of the C H bond activation process (Scheme 2).
A variety of Pd sources, ligands, additives, and solvents
were investigated (see the Supporting Information for
details). Based on these studies, we established three exper-
À
Scheme 1. Metal-catalyzed C H insertion of diazoesters.
À
approach, the electron-rich C H bonds generally exhibit
higher reactivity toward the carbene center and show an
3
À
activation order of tertiary > secondary @ primary C(sp ) H.
Thus, although the insertion into tertiary and secondary
C(sp ) H bonds has been thoroughly studied, the analogous
process involving primary C(sp ) H and CAr(sp ) H bonds
3
À
3
2
À
À
Scheme 2. Pd-catalyzed cyclization of diazoester 1.
remains comparatively underdeveloped.
À
The C H insertion reactions of diazocarbonyl substrates
have been traditionally carried out in the presence of RhII[3] or
Cu catalysts.[4] Although other metals have emerged as
imental procedures for the cyclization reaction. Thus, treat-
ment of 1 with Pd(OAc)2 (10 mol%) and Cs2CO3 (2 equiv) in
CHCl3 at reflux afforded a 2:1 mixture of pyrrolidine 2 and
tetrahydroquinoline 3, which resulted from the activation of
[*] Prof. Dr. D. SolØ, F. Mariani, Prof. Dr. M.-L. Bennasar
Laboratori de Química Orgànica
3
2
À
À
the C(sp ) H and the aryl CAr(sp ) H bonds, respectively.
The use of Pd2(dba)3 (2.5 mol%) as the catalyst in the
presence of Cs2CO3 (2 equiv) in 1,2-dichloroethane (DCE) at
808C led to a 2.2:1 mixture of 2 and 3. Finally, among the
Facultat de Farmàcia, Universitat de Barcelona
Av. Joan XXIII s/n, 08028 Barcelona (Spain)
E-mail: dsole@ub.edu
Dr. I. Fernµndez
Departamento de Química Orgµnica I
Facultad de Ciencias Químicas
Centro de Innovación en Química Avanzada (ORFEO-CINQA)
Universidad Complutense de Madrid
Ciudad Universitaria, 28040 Madrid (Spain)
E-mail: israel@quim.ucm.es
À
different ligands explored to modify the selectivity of the C
H insertion from 1, when using Pd2(dba)3 (2.5 mol%) as the
precatalyst, we found that the bidentate phosphines 1,3-
bis(diphenylphosphino)propane (dppp), 1,1’-bis(diphenyl-
phosphino)ferrocene (dppf), and 4,5-bis(diphenylphos-
phino)-9,9-dimethyl-xanthene (xantphos; 5 mol%) gave
3
2
À
À
slightly better C(sp ) H to CAr(sp ) H activation ratios
(2.6–2.8:1).
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201602020.
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À
These results showed that a) the C H carbenoid cycliza-
H insertions was observed in the reactions involving meta-
and para-substituted anilines. While 3-chloroaniline 4 f gave
0
II
3
À
tion can be catalyzed by both Pd and Pd , and b) the C(sp )
2
À
À
H insertion is in all cases favored over the CAr(sp ) H
insertion. With this information in hand, we decided to
explore the influence of the introduction of substituents at the
a C H activation selectivity (entries 10 and 11) similar to the
unsubstituted aniline 1, the cyclization reactions of 3-
methoxyaniline 4g proceeded with lower regioselectivity
(entries 12 and 13). Interestingly, higher selectivity was
obtained with both electron-poor 4-chloroaniline 4h
(entry 14) and electron-rich 4-methoxyaniline 4i (entries 15
and 16).
À
aromatic ring on the selectivity of the C H activation process
(see the Supporting Information and Table 1).
À
Table 1: Selected Pd-catalyzed C H insertion reactions of a-diazoesters
The C-H insertion reaction was not limited to N-
4a–i.[a]
methylanilines but also proved suitable for substituted
3
À
C(sp ) H bonds (Table 2).
[a]
À
Table 2: Pd-catalyzed C H insertion of a-diazoesters 7a–c
No. 4 (X)
[Pd](mol%)/
L(mol%)
Solv.
Temp.
Product
(yield [%])[b]
1
2
3
4
5
6
4a (2-I)
4a (2-I)
4b (2-Br)
4b (2-Br)
4c (2-Cl)
4c (2-Cl)
Pd(OAc)2 (10)/– CHCl3 reflux[c]
Pd2(dba)3 (2.5)/– CHCl3 reflux
Pd(OAc)2 (10)/– CHCl3 reflux
Pd2(dba)3 (2.5)/– CHCl3 reflux
Pd(OAc)2 (10)/– CHCl3 reflux
5a (55)
5a (89)
5b (51)
5b (66)
5c (56)
5c (69)
No. 7 (R)
[Pd](mol%)/
L(mol%)
Solv. Temp.
Product
(yield [%])[b]
Pd2(dba)3 (2.5)/
dppf (5)
DCE
808C
1
2
3
4
5
6
7
7a (C6H5)
Pd2(dba)3 (2.5)/– CHCl3 reflux
Pd(OAc)2 (10)/– CHCl3 reflux
8a (66)
cis/trans 5.5:1
8a (46)
cis/trans 4:1
8a (54)
cis/trans 4:1
8b (58)
cis/trans 1.7:1
8b (43)
cis/trans 1.7:1
8c (70)
cis/trans 1.5:1
8c (45)
7
8
9
4d (2-F)
4e (2-Me)
4e (2-Me)
Pd2(dba)3 (2.5)/– DCE
808C
5d (62)
5e (49)
5e (57)
7a (C6H5)
7a (C6H5)
Pd(OAc)2 (10)/– CHCl3 reflux
Pd2(dba)3 (2.5)/
dppf (5)
Pd(OAc)2 (10)/– DCE
DCE
808C
Pd2(dba)3 (2.5)/ DCE
dppf (5)
808C
10
11
12
13
14
4 f (3-Cl)
4 f (3-Cl)
808C[d]
808C
5 f/6 f
(2:1; 39)
5 f/6 f
(2.7:1; 45)
5g/6g
=
7b (CH CH2) Pd2(dba)3 (2.5)/– CHCl3 reflux
Pd2(dba)3 (2.5)/– DCE
=
7b (CH CH2) Pd(OAc)2 (10)/– CHCl3 reflux
808C[d]
808C
7c (C2H5)
7c (C2H5)
Pd2(dba)3 (2.5)/– DCE
Pd(OAc)2 (10)/– DCE
808C[c]
808C[c]
4g (3-MeO) Pd(OAc)2 (10)/– DCE
4g (3-MeO) Pd2(dba)3 (2.5)/– DCE
(1.3:1; 42)
5g/6g
(1.5:1; 47)
cis/trans 1.1:1
4h (4-Cl)
Pd2(dba)3 (2.5)/
dppf (5)
DCE
808C
5h (35)[e]
[a] Reaction conditions: See Table 1. [b] Yields are for those of products
isolated by chromatography. [c] 39 h.
15
16
4i (4-MeO) Pd(OAc)2 (10)/– DCE
4i (4-MeO) Pd2(dba)3 (2.5)/– CHCl3 reflux
808C[d]
5i (25)[f]
5i (38)[f]
[a] Reaction conditions: [Pd]/L (L=ligand; see table) and Cs2CO3
(2 equiv) in CHCl3 or DCE at the indicated temperature for 24 h.
[b] Yields refer to products isolated by chromatography and for entries in
which a product mixture was obtained, the yield refers to the combined
yield. [c] 16 h. [d] 48 h. [e] 1H NMR analysis of the reaction mixture
N-Benzylaniline 7a regioselectively afforded pyrrolidine
8a (5.5:1 cis/trans mixture) in 66% yield when the reaction
was performed in the presence of Pd2(dba)3 (entry 1).[17] The
use of either Pd(OAc)2 (entry 2) or Pd2(dba)3/dppf (entry 3)
as the catalyst afforded slightly lower yields. More impor-
tantly, no competition between allylic insertion and cyclo-
propanation[18–19] was observed in the Pd-catalyzed reactions
of N-allylaniline 7b. Thus, treatment of 7b with either
Pd2(dba)3 (entry 4) or Pd(OAc)2 (entry 5) in CHCl3 at reflux
afforded pyrrolidine 8b (1.7:1 cis/trans mixture). Finally, N-
3
2
1
À
À
showed a ꢀ4:1 C(sp ) H:C(sp ) H activation ratio. [f] H NMR analysis
3
2
À
À
of the reaction mixture showed a ꢀ5:1 C(sp ) H:C(sp ) H activation
ratio.
3
À
To our delight, the C(sp ) H insertion was the only
reaction observed when 2-idoaniline 4a was subjected to the
optimized reaction conditions (entries 1 and 2). It should be
noted that no product resulting from the competitive Pd-
catalyzed reaction of the aryl iodide with the a-diazoester
moiety[13,16] was detected in these reaction mixtures. The best
result was obtained with Pd2(dba)3 in the absence of
phosphine ligands, which afforded pyrrolidine 5a in 89%
yield (entry 2).
propylaniline 7c also underwent a similar regioselective
3
À
insertion at the C(sp ) H bond to give pyrrolidine 8c
(entries 6 and 7). Similar to the reactions involving 2-
iodoaniline 4a, no product resulting from the Pd-catalyzed
reaction of the aryl iodide with the a-diazoester moiety was
detected in any of the Pd-catalyzed reactions of 2-iodoanilines
7a–c.
2-Haloanilines 4b, 4c, and 4d, and 2-methylaniline 4e
According to previous mechanistic studies, it can be
suggested that the Pd-catalyzed transformations described
above likely involve the insertion of the in situ generated Pd–
3
À
also selectively underwent C(sp ) H insertion (entries 3–9).
3
2
À
À
In contrast, competition between the C(sp ) H and CAr(sp )
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metalation into a cesium eno-
late, followed by intermolecular
À
protonation with the HCO3
released in the reaction
medium.[24]
3
À
The initial C(sp ) H activa-
tion in the Pd0 pathway, which
proceeds through the transition
state TS1-A, resembles the
above-discussed CMD process
in the sense that this transfor-
mation also involves a concerted
metalation–deprotonation step.
However, in this case the pro-
À
cess also forms the new C H
bond. Therefore, this single step
can be viewed as a Pd-mediated
À
1,5-H migration from the N
CH3 moiety to the carbenoid
which results in the formal oxi-
dation of the transition metal.[25]
This reaction is more exergonic
(DGR = À24.3 kcalmolÀ1) and
occurs with a lower activation
¼
barrier (DG = 22.4 kcalmolÀ1)
than the CMD process involv-
ing TS1-B. The readily formed
PdII complex INT1-A is finally
converted into pyrrolidine 2 in
Figure 1. Computed reaction profiles for the formation of pyrrolidine 2. Relative free energies (DG298, at
298 K) and bond distances are given in kcalmolÀ1 and angstroms (Š), respectively. All data were
computed at the PCM(CHCl3)-M06L/def2-TZVP//RI-BP86-D3/def2-SVP level. Atom colors: N=blue;
O=red; C=gray; P=orange; Pd=turquoise.
a
highly exergonic (DGR =
À12.9 kcalmolÀ1)
reductive
II
¼
À
carbenoid intermediate into the C H bond. For related Rh -
and Cu-catalyzed transformations, this process has been
proposed to occur in a concerted but asynchronous manner
that directly releases the reaction product and the metal
catalyst in one single step.[20]
elimination through transition state TS2-A (DG = 26.8 kcal
molÀ1) which releases the catalytic species Pd(dppp).
Additional experiments were performed to support the
transition-metal-mediated 1,5-H migration. Thus, when tri-
deuterated aniline [D3]-1 was subjected to the conditions
optimized for the Pd-catalyzed reaction of aniline 1, using
Pd2(dba)3/dppf as the catalyst, a 1.2:1 mixture of pyrrolidine
[D3]-2 and tetrahydroquinoline [D3]-3 was obtained
(Scheme 3). This result confirms that the deuterium atom
was fully transferred to the carbenoid atom, which nicely
Density functional theory (DFT) calculations[21] were
carried out to gain more insight into the mechanism of the
3
À
above Pd-catalyzed C(sp ) H insertions. Thus, Figure 1 shows
the corresponding computed reaction profiles of the process-
es involving INT0-A and INT0-B, the initial Pd0 and PdII–
carbene complexes formed upon reaction of the active
catalytic species Pd(dppp) or Pd(CO3) with 1, respectively.
In the PdII pathway, the initial complex INT0-B evolves to
carbene complex INT1-B via transition state TS1-B with an
activation barrier of 29.1 kcalmolÀ1 in a highly exergonic
transformation (DGR = À15.9 kcalmolÀ1). This saddle point is
associated with the concerted hydrogen migration from the
agrees with the DFT-proposed mechanism. The use of
3
À
Pd(OAc)2 as the catalyst afforded a similar C(sp ) H/CAr-
2
À
(sp ) H ratio with complete preservation of the deuterium
label at the specific position as well. The reaction also
proceeded at room temperature, but required longer times
and higher catalyst loading. Interestingly, the regioselectivity
À
À
N CH3 moiety to the carbonate ligand and Pd C bond
formation (Figure 1). Therefore, this transformation is anal-
ogous to related concerted metalation–deprotonation (CMD)
[22]
À
C H activations which are assisted by acetate or carbo-
nate.[14b,15d] Subsequent exergonic (DGR = À18.3 kcalmolÀ1)
À
insertion of the carbene carbon atom into the Pd C bond via
TS2-B (DG = 11.6 kcalmolÀ1) leads to the formation of the
¼
II
À
Pd complex INT2-B. Final protonolysis of the Pd C bond
would afford pyrrolidine 2 and release the PdII catalyst.[23]
This protonolysis may occur either by intramolecular proton
À
transfer from the HCO3 ligand or, more likely, by trans-
Scheme 3. Pd-catalyzed reactions of [D3]-1.
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3
À
catalyzed C(sp ) H insertion reaction of a-diazoesters to
provide pyrrolidines. Both Pd0 and PdII are effective in this
reaction, which represents the first example of Pd-catalyzed
3
À
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Acknowledgements
We gratefully acknowledge financial support for this work
from MINECO-FEDER (CTQ2012-31391, CTQ2013-44303-
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Keywords: carbene insertion · density functional calculations ·
diazo compounds · palladium catalysis · pyrrolidines
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6467–6470
Angew. Chem. 2016, 128, 6577–6580
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Received: March 1, 2016
Published online: April 15, 2016
6470 www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 6467 –6470