Sequential insertion of carbon monoxide and imines into nickel-methyl bonds: A new route to imine hydroacylation

Article abstract of DOI:10.1021/om000685b

The sequential insertion of carbon monoxide and imines has been achieved at ambient temperatures with cationic nickel complexes, yielding the nickel-chelated amides (bipy)Ni[η²-C(H)TolN(R)COCH₃]⁺X^-(bipy = 2,2′-b

Full text of DOI:10.1021/om000685b

© Copyright 2000
Volume 19, Number 23, November 13, 2000
American Chemical Society
Communications
Sequ en tia l In ser tion of Ca r bon Mon oxid e a n d Im in es
in to Nick el-Meth yl Bon d s: A New Rou te to Im in e
Hyd r oa cyla tion
J ason L. Davis and Bruce A. Arndtsen*
Department of Chemistry, McGill University, 801 Sherbrooke Street West,
Montreal, Quebec, Canada H3A 2K6
Received August 8, 2000
Summary: The sequential insertion of carbon monoxide
and imines has been achieved at ambient temperatures
with cationic nickel complexes, yielding the nickel-
chelated amides (bipy)Ni[η2-C(H)TolN(R)COCH3]+X-
(bipy ) 2,2′-bipyridyl; Tol ) p-tolyl; R ) alkyl, aryl; X
) OTf, PF6, SbF6, B(3,5-C6H3(CF3)2)4). The addition of
KCN/ methanol to these products of CO/ imine insertion
leads to cleavage of the amide ligand, providing a facile
method to achieve the overall hydroacylation of imines
under mild conditions.
ing analogous reactions are much more rare.4-7 Ex-
amples of the latter typically involve electropositive
metal centers, which undergo insertion with a regio-
chemistry to generate a robust M-X bond rather than
a new carbon-nitrogen bond.4,5 The development of late-
transition-metal complexes that can tolerate functional-
ity during insertion reactions suggests they might be
prime candidates for insertion of substrates such as
imines.8-11 Recently, both we12 and others13 have dem-
onstrated that palladium complexes of the form L2Pd-
(CH3)(RNdC(H)R′)+X- (L2 ) bidentate neutral ligand;
The migratory insertion of imines into metal-acyl
bonds provides a potential method to generate amides
from simple and readily available R′(H)CdNR building
blocks. However, while the insertion of carbon-contain-
ing unsaturated molecules (e.g., olefins or alkynes) has
been explored extensively,1-3 reports of heteroatom
containing substrates (e.g., CdX; X ) O, NR) undergo-
(4) For examples of imine insertion: (a) Obora, Y.; Ohta, T.; Stern,
C. L.; Marks, T. J . J . Am. Chem. Soc. 1997, 119, 3745. (b) Coles, S. J .;
Hursthouse, M. B.; Kelly, D. G.; Toner, A. J .; Walker, N. M. Can. J .
Chem. 1999, 77, 2095.
(5) For examples of aldehyde insertions: (a) Hanna, T.; Baranger,
A. M.; Bergman, R. G. J . Org. Chem. 1996, 61, 4532. (b) Han, R.;
Hillhouse, G. L. J . Am. Chem. Soc. 1997, 119, 8135.
(6) Products consistent with imine insertion have been suggested
in several systems: (a) Alper, H.; Amaratunga, S. J . Org. Chem. 1982,
47, 3595. (b) Reduto, A. C.; Hegedus, L. S. Organometallics 1995, 14,
1586. (c) Muller, F.; van Koten, G.; Vrieze, K.; Heijdenrijk, D.
Organometallics 1989, 8, 33.
(1) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(2) For reviews of olefin polymerization, see: (a) J ohnson, L. K.; Ittel,
S. D.; Brookhart, M. Chem. Rev. 2000, 100, 1169. (b) Britovsek, G. J .
P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428.
(c) Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663 and
references therein.
(7) Imine insertion into metal hydrides are known and have been
postulated in catalytic hydrogenations: (a) Fryzuk, M. D.; Piers, W.
E. Organometallics 1990, 9, 986. (b) Debad, J . D.; Legzdins, P.; Lumb,
S. A.; Batchelor, R. J .; Einstein, F. W. B. Organometallics 1995, 14,
2543. (c) Willoughby, C. A.; Buchwald, S. L. J . Am. Chem. Soc. 1994,
116, 11703 and references therein. (d) J ames, B. R. Catal. Today 1997,
37, 209.
(8) Abu-Surrah, A.; Rieger, B. Angew. Chem., Int. Ed. Engl. 1996,
35, 2475.
(9) Younkin, T. R.; Connor, E. F.; Henderson, J . I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460.
(3) Brandsma, L.; Vasilevsky, S. F.; Verkruijsse, H. D. Applications
of Transition Metal Catalysts in Organic Chemistry; Springer: Berlin,
1997.
10.1021/om000685b CCC: $19.00 © 2000 American Chemical Society
Publication on Web 10/12/2000

4658 Organometallics, Vol. 19, No. 23, 2000
Communications
Ta ble 1. Syn th esis of Com p lexes 2a -h a n d 4a -g
Sch em e 1
yield (%)
entry
imine
R
X
2a
4b
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
Ph
CH2Ph
Me
Me
Me
PF6
PF6
PF6
SbF6
BArfc
OTfd
PF6
PF6
Cl
64
84
92
86
85
89
68
78
82e
90
95
98
88
91
36
25
Me
iPr
tBu
Me
Isolated yields. NMR yields. c BArf ) B(3,5-C6H3(CF3)2)4.
OTf ) OSO2CF3. e Yield of (bipy)Ni(CH3)Cl.
a
b
d
R ) alkyl, aryl; R′ ) aryl) can mediate the sequential
insertion of carbon monoxide and imines to generate
palladium-bound amides. These provide the first well-
defined examples of imine insertion into a late metal-
carbon bond.6
In an attempt to explore the scope of this reaction,
as well as develop a metal complex capable of mediating
imine insertion under more mild conditions, the syn-
thesis and reactivity of the nickel complexes (bipy)Ni-
(CH3)(RNdC(H)Tol)+X- (bipy ) 2,2′-bipyridyl, Tol )
p-C6H4CH3) has been investigated. We report herein the
first example of the sequential insertion of CO and
imines into nickel-methyl bonds, which ultimately
leads to a controlled, metal-mediated route to synthesize
amides from imines.14 Methods for the generation,
characterization, and cleavage of the amide fragments
from the nickel center, as well as a comparison of the
relative rates of insertion to the analogous palladium
complexes, are described below.
plexes 2a -h to 1 atm of carbon monoxide results in the
rapid insertion of CO to form the nickel-acyl complexes
3a -h . Unlike 2a -h , nickel-acyl complexes 3a -g do
react upon standing for several hours in CD2Cl2 solu-
tion, resulting in the formation of the product of imine
insertion into the nickel-acyl bond (4a -g) (Scheme 1).
The same reaction at 70 °C results in complete conver-
sion to 4a -g in under 30 min.
1
Complexes 4a -g have been characterized by H and
13C NMR, IR, and, in the case of 4b, X-ray structural
analysis. In all cases, only one regioisomer of complexes
4a -g is obtained, in which imine insertion has occurred
to form an amide bond and a new nickel-carbon bond.
The formation of a robust amide bond likely provides
the driving force for imine insertion into the nickel-
acyl bond, which does not exist in nickel-methyl
complexes 2a -h .
The reaction of [R(H)NdCH(Tol)]+X- (1a -h ) with
(bipy)Ni(CH3)2 in THF at -40 °C results in the immedi-
ate liberation of methane and formation of the cationic
nickel-imine complexes 2a -h (eq 1, Table 1).15 Com-
The yield of the imine insertion products is sensitive
to the imine and counteranion employed. Use of more
strongly coordinating counterions dramatically reduces
the yield of 4 (Table 1, entries 6 and 9). In the case of
1i, the chloride counteranion is found to coordinate
preferentially over imine in both the nickel-methyl (2)
and -acyl complexes (3), and no evidence for imine
insertion is observed. Replacing the primary alkyl
substituent on imine nitrogen (entries 2-6) with a
secondary iPr substituent greatly reduces both the rate
of insertion and the yield (entry 7), while the N(tBu)-
substituted imine (3h ) does not undergo insertion (entry
plexes 2a -h can be isolated by precipitation with
pentane, affording solids which are stable at room
1
temperature under nitrogen. The H and 13C NMR for
2a -h display a downfield shift in the imine resonances
upon coordination,16 consistent with η1-binding of the
imine through the nitrogen.12
No evidence for imine insertion into the Ni-CH3 bond
of complexes 2a -h was observed at ambient tempera-
ture, and heating to 70 °C resulted only in the decom-
position of the complex to liberate free imine. Thus,
similar to the analogous palladium complexes,12 2a -h
have a significant kinetic barrier to imine insertion into
the nickel-alkyl bond. However, the exposure of com-
1
8). Monitoring the reaction of 3g (or 3h ) by H NMR
(CD2Cl2) reveals the competitive formation of the pro-
tonated iminium salt 1g in 21% yield (1h : 40% yield)
in the product mixture. Similarly, the use of a less basic
N-phenyl-substituted imine in 3a also leads to the low-
yield formation of 1a (6% yield) in addition to 4a , as
does the more coordinating OTf- counteranion in 3f
(14) For examples of metal-mediated imine hydroacylation, see: (a)
Vasapollo, G.; Alper, H. Tetrahedron Lett. 1988, 29, 5113. (b) Alper,
H.; Amaratunga, S. Tetrahedron Lett. 1981, 22, 3811. (c) Zhou, Z.;
J ames, B.; Alper, H. Organometallics 1995, 14, 4209. (d) Antebi, S.;
Alper, H. Can. J . Chem. 1986, 64, 2010. (e) Morimoto, T.; Achiwa, K.
Tetrahedron: Asymmetry 1995, 6, 2661.
(10) (a) Mecking, S.; J ohnson, L. K.; Wang, L.; Brookhart, M. J . Am.
Chem. Soc. 1998, 120, 888. (b) J ohnson, L. K.; Mecking, S.; Brookhart,
M. J . Am. Chem. Soc. 1996, 118, 267.
(11) Morimoto, T.; Chatani, N.; Murai, S. J . Am. Chem. Soc. 1999,
121, 1758.
(12) Dghaym, R. D.; Yaccato, K. J .; Arndtsen, B. A. Organometallics
1998, 17, 4.
(13) Kacker, S.; Kim, J .; Sen, A. Angew. Chem., Int. Ed. 1998, 37,
1251.
(15) Wilke, G.; Herrmann, G. Angew. Chem., Int. Ed. Engl. 1966,
6, 581.
(16) Complex 2c: 1H NMR (CD2Cl2) δ 8.56 (s, dC(H)Tol); 13C NMR
δ 169.7 (s, dC(H)Tol). Free MeNdC(H)Tol: 1H NMR (CD2Cl2) δ 8.42
(dC(H)Tol); 13C NMR δ 160.6 (dC(H)Tol).

Communications
Organometallics, Vol. 19, No. 23, 2000 4659
Ta ble 2. Ra tes of Im in e In ser tion w ith 3a -c a n d
5a -c
imine in 2a -c in acetonitrile solvent. In contrast to the
behavior of palladium complexes 5a -c in CD3CN, where
(bipy)Pd(CH3)(RNdC(H)Tol)+X- and (bipy)Pd(CH3)-
(NCCD3)+X- can be clearly distinguished at ambient
1
temperature by H NMR,12 the analogous equilibrium
with nickel complexes 2a -c and the solvated complex
is much more rapid, resulting in broadened 1H NMR
resonances at temperatures down to -40 °C.
The sequential insertion of carbon monoxide and
imine into the nickel-methyl bond of 2 allows the
formation of nickel-bound amides under very mild
conditions (i.e., ambient temperature and 1 atm of CO).
In addition, the amide ligand in 4a -c can be readily
cleaved from the nickel center, providing a metal-
mediated route to generate amides from imines and CO.
The addition of KCN to a methanol solution of 4a results
in the immediate generation of a colorless solution.
Removal of the solvent, followed by an ether extraction
and acid wash with 10% HCl, provides amide 6a in
>95% isolated yield. A similar process can be performed
with complexes 4b,c, in all cases leading to the produc-
tion of amides in near-quantitative yields. Overall, this
reaction represents a relatively straightforward two-step
method to hydroacylate imines via the sequential inser-
tion of carbon monoxide and imine into a nickel-methyl
bond (eq 2). To our knowledge, the only other report of
a
b
kNi
∆Gq(40 °C)
kPd
∆Gq(90 °C)
R
(10-3 s-1
)
(kcal/mol) (×10-4 s-1
)
(kcal/mol) ∆∆Gq
a
b
c
Ph
1.5
22.7
23.8
23.1
0.84
0.93
2.3
28.2
28.1
27.4
5.5
4.3
4.3
PhCH2
CH3
0.27
0.89
a
b
Rate of disappearance of 3a -c at 45 °C in CD2Cl2. Rate of
disappearance of 5a -c (X ) BArf) at 90 °C in d5-PhBr.18
(1f: 42% yield). The generation of protonated iminium
salts with less basic or more slowly inserting imines,
or more strongly coordinating anions, implies that
decoordination of the imine to form (bipy)Ni(COCH3)-
L+ (L ) CO, X) may lead to decomposition. Consistent
with this observation, the addition of excess PhNdC(H)-
Tol to the reaction of 2a with CO completely suppresses
the formation of the protonated imine products and
generates complex 4a in quantitative yield.17
The insertion of imines into the nickel-acyl bond of
3a -g occurs with a lower barrier than that of the
analogous palladium complexes, which require pro-
longed heating at 70 °C. To quantify the influence of
the metal center on insertion rates, the first-order rate
constants for the insertion of imines with a number of
isoelectronic nickel- and palladium-acyl complexes
have been determined. As shown in Table 2, the nickel
complexes all display a ca. 4.3-5.5 kcal/mol lower
barrier for imine insertion than the analogous pal-
ladium complexes.
The lower barrier to imine insertion with nickel
complexes is not surprising and is similar to that
observed in ethylene insertion into isoelectronic cationic
nickel and palladium complexes.19,20 It has been sug-
gested that the more rapid insertion of olefins with first-
row transition metals may arise from either a weakened
metal-olefin bond or perhaps a weaker metal-alkyl
bond.21 While we do not have sufficient evidence to
distinguish between these possibilities for imine inser-
tion, our data are consistent with a more labile imine
σ-coordination in the nickel complexes. This is shown
qualitatively by the rapid equilibrium coordination of
imine hydroacylation with carbon monoxide is that by
Alper and co-workers,14a-d which utilizes cobalt carbonyl
catalysts at elevated temperatures.
In conclusion, we have presented the first example
of imine insertion into a nickel-acyl bond to form the
five-membered metallocycles 4a -g. This sequential
insertion of carbon monoxide and imine occurs with a
significantly lower barrier with nickel complexes 2a -g
than the isoelectronic palladium complexes and provides
a new and controlled route to generate both nickel-
bound and free amides. Considering that these amides
are formed via the well-defined insertion of imines into
a Ni-C bond, the induction of asymmetry into this
process, and the incorporation of other insertion mono-
mers into the amide products, should prove to be viable
extensions of this work. Studies directed toward the
latter will be the subject of future reports.
Ack n ow led gm en t. We thank the NSERC (Canada),
the FCAR (Quebec), and ESTAC for their financial
support of this research.
(17) The added PhNdC(Tol) does not significantly perturb the rate
of formation of 4a , suggesting that imine insertion is an intramolecular
process from 3a . A manuscript describing the complete mechanistic
details of imine insertion with the isoelectronic palladium complexes
is currently in preparation.
(18) Dghaym, R. D.; Arndtsen, B. A. Unpublished results.
(19) Rix, C. F.; Brookhart, M. J . Am. Chem. Soc. 1995, 117, 1137.
(20) Svejda, S. A.; J ohnson, L. K.; Brookhart, M. J . Am. Chem. Soc.
1999, 121, 10634.
(21) (a) Han, Y.; Deng. L.; Ziegler, T. J . Am. Chem. Soc. 1997, 119,
5939. (b) Deng, L.; Margi, P.; Ziegler, T. J . Am. Chem. Soc. 1997, 119,
1094.
Su p p or tin g In for m a tion Ava ila ble: Text and figures
giving details of the synthesis and characterization data for
2a -h and 4a -g. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM000685B