Rhodium-catalyzed intermodular hydroiminoacylation of alkenes: Comparison of neutral and cationic catalytic systems

Article abstract of DOI:10.1021/om800827r

Both cationic and neutral rhodium catalytic systems for the hydroiminoacylation of alkenes were studied using NMR spectroscopy and DFT-based methods. With neutral systems, the oxidative addition step was shown to be thermodynamically favored. With the cat

Full text of DOI:10.1021/om800827r

2976
Organometallics 2009, 28, 2976–2985
Rhodium-Catalyzed Intermolecular Hydroiminoacylation of
Alkenes: Comparison of Neutral and Cationic Catalytic Systems
Patricia Marce´,† Cyril Godard,*,‡ Marta Feliz,§ Xiomara Ya´n˜ez,†, Carles Bo,*,‡,§ and
Sergio Castillo´n*,†
Departament de Qu´ımica Anal´ıtica i Qu´ımica Orga`nica and Departament de Qu´ımica F´ısica i Inorga`nica,
UniVersitat RoVira i Virgili, C/Marcel´ı Domingo s/n, 43007 Tarragona, Spain, Institute of Chemical
Research of Catalonia (ICIQ), C/Pa¨ısos Catalans 16, 43007 Tarragona, Spain, and Instituto de
InVestigacio´n en Produccio´n Verde (IPV), UniVersidad de Pamplona-Sede Villa del Rosario, Cu´cuta, Colombia
ReceiVed August 25, 2008
Both cationic and neutral rhodium catalytic systems for the hydroiminoacylation of alkenes were studied
using NMR spectroscopy and DFT-based methods. With neutral systems, the oxidative addition step
was shown to be thermodynamically favored. With the cationic system, the oxidative addition reaction
was shown to be endothermic by DFT calculations, and the corresponding intermediate was not detected
by NMR. The energy barriers in the cationic and in the neutral pathway are relatively similar. This
indicates that the role of chloride when it is coordinated to the rhodium complex is to increase the stability
of the oxidative addition product, enabling the reaction to continue. This is not the case in the cationic
complexes since the low stability of the reaction products promotes the back reaction, and therefore, low
conversions are obtained. The alkene insertion step was also investigated for both systems with styrene
as the substrate. The novel neutral complex 21 was detected and fully characterized by multinuclear
NMR spectroscopy. In the cationic system, the rhodium hydride styrene intermediate 25 was detected
and characterized by NMR spectroscopy. However, no evidence for alkene insertion was found. This
study shows that each step of the catalytic cycle is favored in the case of the neutral system when compared
with the cationic system.
The development of the intermolecular process is, however,
much more challenging, due to the complexity of avoiding the
decarbonylation process. Initially, this was achieved by complet-
ing the reaction under high pressures (or concentrations) of
alkene and/or carbon monoxide.6 Most of the work on inter-
molecular hydroacylation, however, has focused on modifying
chelation procedures to stabilize the metal-acyl intermediate.7,8
Chelation assisted by oxygen,9,10 sulfur,11 or nitrogen atoms7,12
in the aldehyde substrate has been reported. Double chelation
Introduction
Hydroacylation is an atom-economic, transition-metal-
catalyzed process that yields a ketone from an aldehyde and an
alkene via C-H bond activation of the aldehyde followed by
insertion of the alkene (Scheme 1). The synthetic scope of this
reaction is limited by the trend of the metal-acyl intermediates
to undergo decarbonylation, affording reduced substrates. This
problem can be easily circumvented in the intramolecular
version since the coordination of the alkene to give a chelate
facilitates the reaction. After the pioneering work by Sakai1 on
intramolecular hydroacylation, mechanistic studies2 were carried
out and this reaction was used for the synthesis of cylopen-
tanones3 and carbocycles of different sizes.4 An enantioselective
intramolecular hydroacylation process was also successfully used
for the synthesis of enantioenriched cyclopentanones.5
(3) (a) Sakai, Y.; Ishiguro, K.; Funakoshi, K.; Ueno, K.; Suemune, H.
Tetrahedron Lett. 1984, 25, 961. (b) Fairlie, D. P.; Bosnich, B. Organo-
metallics 1988, 7, 936. (c) Funakoshi, K.; Togo, N.; Taura, Y.; Sakai, K.
Chem. Pharm. Bull. 1989, 37, 1776.
(4) (a) Gable, K. P.; Benz, G. A. Tetrahedron Lett. 1991, 32, 3473. (b)
Sato, Y.; Oonishi, Y.; Mori, M. Angew. Chem., Int. Ed. 2002, 41, 1218. (c)
Aloise, A. D.; Layton, M. E.; Shair, M. D. J. Am. Chem. Soc. 2000, 122,
12610.
(5) James, B. R.; Young, C. G. J. Chem. Soc., Chem. Commun. 1983,
1215. (b) Taura, Y.; Tanaka, M.; Funakoshi, K.; Sakai, K. Tetrahedron
Lett. 1989, 30, 6349. (c) Wu, X. M.; Funakoshi, K.; Sakai, K. Tetrahedron
Lett. 1991, 33, 6331. (d) Barnhart, R. W.; Wang, X.; Noheda, P.; Bergens,
S. H.; Whelan, J.; Bosnich, B. J. Am. Chem. Soc. 1994, 116, 1821. (e)
Barnhart, R. W.; McMorran, D. A.; Bosnich, B. Chem. Commun. 1997,
589. (f) Tanaka, K.; Fu, G. J. Am. Chem. Soc. 2001, 123, 11492. (g) Tanaka,
K.; Fu, G. J. Am. Chem. Soc. 2002, 123, 10296. (h) Kundu, K.; McCullagh,
J. V.; Morehead, A. T., Jr. J. Am. Chem. Soc. 2005, 127, 16042.
(6) (a) Marder, T. B.; Roe, D. C.; Milstein, D. Organometallics 1988,
7, 1451.
(7) Suggs, J. W. J. Am. Chem. Soc. 1978, 100, 640.
(8) For a recent review, see: Jun, C.-H.; Jo, E.-A.; Park, J.-W. Eur. J.
Org. Chem. 2007, 1869.
(9) (a) Kokubo, K.; Matsumasa, K.; Nishinaka, Y.; Miura, M.; Nomura,
M. Bull. Chem. Soc. Jpn. 1999, 72, 303. (b) Miura, M.; Nomura, M. J.
Synth. Org. Chem. Jpn. 2000, 58, 578.
(10) The first asymmetric version of the intermolecular hydroacylation
ofnorborneneswitho-hydroxybenzaldehydehasbeenrecentlyreported:Stemmler,
R. T.; Bolm, C. AdV. Synth. Catal. 2007, 349, 1185.
* To whom correspondence should be addressed. Fax: (+34) 977 558446.
E-mail: sergio.castillon@urv.cat (S.C.). Fax: (+34) 977 559563. E-mail:
cyril.godard@urv.cat (C.G.). Fax: (+34) 977 92 0231. E-mail: cbo@iciq.es
(C.B.).
† Departament de Qu´ımica Anal´ıtica i Qu´ımica Orga`nica, Universitat
Rovira i Virgili.
‡ Departament de Qu´ımica F´ısica i Inorga`nica, Universitat Rovira i
Virgili.
§ Institute of Chemical Research of Catalonia (ICIQ).
Universidad de Pamplona-Sede Villa del Rosario.
(1) Sakai, K.; Ide, J.; Oda, O.; Nakamura, N. Tetrahedron Lett. 1972,
13, 1287.
(2) (a) Campbell, R. E.; Miller, R. G. J. Organomet. Chem. 1980, 186,
C-27. (b) Campbell, R. E.; Lochow, C. F.; Vora, K. P.; Miller, R. G. J. Am.
Chem. Soc. 1980, 102, 5824. (c) Milstein, D. J. J. Chem. Soc., Chem.
Commun. 1982, 1357. (d) Milsten, D. Acc. Chem. Res. 1984, 17, 221. (e)
Fairlie, D. P.; Bosnich, B. Organometallics 1988, 7, 946.
10.1021/om800827r CCC: $40.75
2009 American Chemical Society
Publication on Web 04/22/2009

Rh-Catalyzed Hydroiminoacylation of Alkenes
Organometallics, Vol. 28, No. 10, 2009 2977
Scheme 1. Intermolecular Hydroacylation Reaction
following classical steps: the oxidative addition of the aldimine,
followed by the coordination and insertion of the alkene into
the Rh-H bond of the catalyst, and the reductive elimination
of the product and the regeneration of the starting Rh species.22
In the context of the work carried out in our laboratory on
the Rh-catalyzed hydroiminoacylation of alkenes, the catalytic
results revealed a strong difference between the activity of
neutral and cationic catalysts in the one-pot 2-picoline-assisted
intermolecular hydroacylation. Here, we report these catalytic
results and a comparative mechanistic study of the intermo-
lecular hydroiminoacylation of alkenes with the aldimine 3
between neutral and cationic rhodium systems using in situ
NMR techniques and computational DFT-based methods.
methods, in which both the aldehyde and the alkene coordinate
to the rhodium center in a bidentate manner, and alkene
chelation involving an amide group have also been successfully
developed.13 The use of anhydrides as acylating reagents14 or
[RhCp*]15 catalysts does not require chelation assistance.
Suggs showed that 2-amino-picoline-derived aldimines react
with alkenes in the presence of [RhCl(PPh3)3] to give
ketimines.16 Jung developed a one-pot intermolecular hydroa-
cylation procedure that forms in situ the 2-amino-picoline
aldimine derivative, which reacts with an alkene in the presence
of Wilkinson’s catalyst to yield a ketimine.17 Later, the ketimine
is hydrolyzed within the reaction media to provide the ke-
tone (Scheme 2). This procedure avoids the use of a coordinating
atom that is permanently bound to the substrate. This method
was also successfully applied in the hydroacylation of alkynes.18
The mechanism of the intramolecular hydroacylation reaction
has already been studied using both neutral2,19 and cationic
precursors.2e The rate-limiting step of this reaction is the
reductive elimination of the product. Recently, a detailed
computational study provided additional insights into this
mechanism.20 In contrast, the mechanism of intermolecular
hydroacylation has been much less studied but is thought to be
similar to that of the intramolecular version.6-13 When the
reaction was studied using deuterated benzaldehyde and eth-
ene pressure, deuterium in the organic product was found in
the methyl and methylene groups in a ratio of 3:2, indicating
that the alkene insertion was a reversible process.6 In another
study, reductive elimination was proposed to be the rate-
determining step, with other steps of the catalytic cycle described
as reversible processes.2e,21
Results and Discussion
Catalysis. In the course of a study of the one-pot 2-picoline-
assisted intramolecular hydroacylation of alkenes23 and
alkynes,24 the use of Montmorillonite K-10 (MK-10) as a
reusable acid catalyst able to catalyze the imine formation and
ketimine hydrolysis (see first and last steps in Scheme 2), was
previously explored. Furthermore, MK-10 can also be used as
a solid support for immobilizing and recovering the catalyst,
and for this purpose, cationic complexes are required.25 We
therefore studied a series of cationic precursors of the formula
[Rh(cod)(L)2]BF4, where L corresponds to phosphine and
phosphite ligands, in the presence of 2-amino-3-picoline and
MK-10 (Table 1). The hydroacylation reaction described should
be considered as a sequence of three successive reactions: the
formation of aldimine 3, the Rh-catalyzed hydroiminoacylation
leading to ketimine 9, and the hydrolysis of 9 yielding 10. The
rhodium catalyst is thus only involved in the hydroiminoacy-
lation reaction, while MK-10 catalyzes the two other processes.
Comparing the results shown in entries 1-6, it is apparent
that the neutral system [RhCl(PPh3)3] (entry 1) provides a much
higher conversion than any of the cationic catalytic systems
(entries 2-6). Low conversions were obtained when mono-
phosphines with different electronic properties (entries 2-5)
were used. When a monophosphite ligand (entry 6) was used,
no conversion was observed. However, when a source of
chloride was added to the reaction mixture (entry 7), a
comparable conversion to that obtained with the neutral system
was achieved (75%). This result suggested that, under these
conditions, the neutral rhodium catalytic system was restored.
Identical result was obtained when the reaction was performed
starting from aldimine 3, indicating that the nature of the
rhodium catalyst was the key to explain the behavior of these
catalytic systems.
The mechanism of the one-pot 2-picoline-assisted intermo-
lecular hydroacylation procedure has not yet been investigated.
The catalytic cycle proposed for this reaction involves the
(11) (a) Willis, M. C.; Randell-Sly, H. E.; Woodward, R. L.; McNally,
S. J.; Currie, G. S. J. Org. Chem. 2006, 71, 5291. (b) Moxham, G. L.;
Randell-Sly, H. E.; Brayshaw, S. K.; Woodward, R. L.; Weller, A. S.; Willis,
M. C. Angew. Chem., Int. Ed. 2006, 45, 7618. (c) Willis, M. C.; Randell-
Sly, H. E.; Woodward, R. L.; Currie, G. S. Org. Lett. 2005, 7, 2249. (d)
Willis, M. C.; McNally, S. J.; Beswick, C. J. Angew. Chem., Int. Ed. 2004,
43, 340.
(12) Wojcicki, A. AdV. Organomet. Chem. 1973, 11, 87.
(13) Imai, M.; Tanaka, M.; Tanaka, K.; Yamamoto, Y.; Ogata-Imai, N.;
Shimowatari, M.; Nagumo, S.; Kawahara, N.; Suemune, H. J. Org. Chem.
2004, 69, 1144. (b) Tanaka, M.; Imai, M.; Yamamoto, Y.; Tanaka, K.;
Shimowatari, M.; Nagumo, S.; Kawahara, N.; Suemune, H. Org. Lett. 2003,
5, 1365. (c) Tanaka, K.; Tanaka, M.; Suemune, H. Tetrahedron Lett. 2005,
46, 6053.
In light of these results, a mechanistic study using in situ
NMR and DFT calculations was undertaken in order to gain
insight into the difference of behavior between the neutral and
the cationic rhodium systems catalyzing the 2-picoline-assisted
intramolecular hydroacylation of alkenes.
Study of the Oxidative Addition by in situ NMR.
Neutral System. Initially, this process was investigated by NMR
spectroscopy. A THF-d8 solution of RhCl(PPh3)3 was charged
(14) Hong, T.-K.; Barchuck, A.; Krische, M. J. Angew. Chem., Int. Ed.
2006, 45, 6885.
(15) Roy, A. H.; Lenges, C. P.; Brookhart, M. J. Am. Chem. Soc. 2007,
129, 2082.
(16) Suggs, J. W. J. Am. Chem. Soc. 1979, 101, 489.
(17) (a) Park, Y.-J.; Park, J.-W.; Jun, C.-H. Acc. Chem. Res. 2008, 41,
222. (b) Jun, C.-H.; Moon, C. W.; Lee, D. Y. Chem.sEur. J. 2002, 8,
2423. (c) Jun, C.-H.; Lee, D. Y.; Lee, H.; Hong, J.-B. Angew. Chem., Int.
Ed. 2000, 39, 3070. (d) Jun, C.-H.; Hong, J.-B. Org. Lett. 1999, 1, 2161.
(18) (a) Jun, C.-H.; Lee, H.; Hong, J.-B.; Kwon, B. I. Angew. Chem.,
Int. Ed. 2002, 41, 2146. (b) Lee, D. Y.; Hong, B. S.; Cho, E. G.; Lee, H.;
Jung, C.-H. J. Am. Chem. Soc. 2003, 125, 6372. (c) Jun, C.-H.; Lee, H.
Pure Appl. Chem. 2004, 76, 577.
(22) (a) Jun, C.-H.; Lee, H. J. Am. Chem. Soc. 1999, 121, 880. (b) Jun,
C.-H.; Lee, H.; Hong, J.-B. J. Org. Chem. 1997, 62, 1200. (c) Jun, C.-H.;
Hoh, C.-W.; Na, S.-J. Angew. Chem., Int. Ed. 1998, 37, 145. (d) Jun, C.-H.
Chem. Soc. ReV. 2004, 33, 610.
(23) Ya´n˜ez, X.; Claver, C.; Castillo´n, S.; Ferna´ndez, E. Tetrahedron
Lett. 2003, 44, 1631.
(24) Ya´n˜ez, X.; Castillo´n, S. J. Organomet. Chem. 2007, 692, 1628.
(25) Claver, C.; Ferna´ndez, E.; Margalef-Catala`, R.; Medina, P.; Salagre,
P.; Sueiras, J. P. J. Catal. 2001, 201, 70.
(19) Lau, K. Y.; Becker, Y.; Huang, F.; Baenziger, N.; Stille, J. K. J. Am.
Chem. Soc. 1977, 99, 566.
(20) Hyatt, I. E. D.; Anderson, H. K.; Morehead, A. T.; Sargent, A. L.
Organometallics 2008, 17, 135.
(21) (a) Bosnich, B. Acc. Chem. Res. 1998, 31, 667. (b) Barnhart, R. W.;
Bosnich, B. Organometallics 1995, 14, 4343.

2978 Organometallics, Vol. 28, No. 10, 2009
Scheme 2. Proposed Mechanism for the 2-Picoline-Assisted Intermolecular Hydroacylation of Alkenes
Marce´ et al.
Table 1. Hydroiminoacylation Reaction of Aldimine 3, Formed in
situ from Benzaldehyde (1), with 1-Hexene in the Presence of
point, a new doublet signal at δ 33.1 (JRhP ) 116.6 Hz) was
detected as the main species (90%) in solution, and the signal
a
RhClL3 and [Rh(cod)L2]BF4
1
arising from PPh3 increased in intensity. In the H spectrum a
hydride signal was observed at δ -11.14 (q, JRhH ) JPH ) 13
Hz) and readily assigned to the oxidative addition product
5-trans.16,26 It was therefore concluded that, at this temperature,
the oxidative addition reaction occurs very rapidly.
When the reaction was repeated at low temperature, no new
entry
[Rh]
ligand (L)
PPh3
(p-Me-Ph)3P
(p-F-Ph)3P
Ph2EtP
conv (%)b
1
signals were detected between 193 and 243 K by H and 31P
1
2
3
4
[RhCl(PPh3)3]
80c
7
11
7
NMR spectroscopy. However, when the temperature of the
sample was increased to 253 K, the signal corresponding to
[Rh(µ-Cl)(PPh3)2]2 initially detected in the 31P{1H} spectrum
was found to be lower in intensity, and two new doublet of
doublets at δ 54 and 50 (JRhP ) 196 and 170 Hz, JPP ) 47 Hz)
appeared, which were assigned to two inequivalent phosphine
[Rh(cod)L2]BF4
[Rh(cod)L2]BF4
[Rh(cod)L2]BF4
[Rh(cod)L2]BF4
[Rh(cod)L2]BF4
[Rh(cod)L2]BF4
5
9
6
(MeO)3P
(p-Me-Ph)3P
0
7d
75e
a Conditions: benzaldehyde (2.5 mmol), hexene (12.5 mmol),
2-amino-3-picoline (0.8 mmol), [Rh(cod)L2]BF4 (0.05 mmol), free
ligand added (0.05 mmol), MK-10 (80 mg), toluene (2 mL), 110 °C,
2 h. b Conversion of the rhodium-catalyzed reaction expressed as the
1
ligands of a new species. In the corresponding H spectrum,
new low field signals were detected at δ 9.1 (s) and 9.0 (dd,
JHH ) 1.8 and 5 Hz). A COSY experiment showed that the
signal at δ 9.0 was correlated to two proton resonances in the
aromatic region at δ 7.2 and 6.85, which were assigned to the
pyridyl unit of a new aldimine ligand. The chemical shift of
the resonance at δ 9.0 indicated that the pyridyl unit was
coordinated to a Rh center through the N atom. The signal at δ
9.1 was assigned to the NdCHPh group based on its similarity
with the signal corresponding to free aldimine. The new
compound formed was therefore identified as cis-[RhCl(P-
Ph3)2(3)] (12-cis, Scheme 3). Very similar NMR features were
reported for the analogue complex [RhCl(PPh3)2(pyridine)].26,27a,b
At this stage, the temperature of the sample was increased to
258 K. In the 31P{1H} spectrum, the signal corresponding to
[Rh(µ-Cl)(PPh3)2]2 was not detected; the resonances for 12-cis
were found to increase in intensity, and the signal corresponding
to the oxidative addition product 5-trans was detected at δ 32.6
(JRhP ) 116 Hz) and slowly increased in intensity.
ratio of
9 + 10 to 1 + 3 measured by GC (chromatograph
Hewlett-Packard 5890A, HP-5 column, T ) 80 °C for 0.5 min up to
280 °C; rate ) 10 °C/min). c Ratio in %: 1/3/9/10 ) 18:2:10:70.
d BnMe3NCl (0.05 mmol) was added. e Ratio in %: 1/3/9/10
)
22:3:15:60.
Scheme 3. Reactivity of RhCl(PPh3)3 in the Presence of
Aldimine 3 in THF
The detection and NMR characterization of species cis-
[RhCl(PPh3)2(3)] (12-cis) indicated that the oxidative addition
(26) Albinati, A.; Arz, C.; Pregosin, P. S. J. Organomet. Chem. 1987,
335, 379.
into a 5 mm NMR tube fitted with a Young’s tap. In the
corresponding 31P{1H} spectrum, signals arising from this
complex, the rhodium dimer [Rh(µ-Cl)(PPh3)2]2 (11), and free
PPh3 were readily detected (Scheme 3). One equivalent of
aldimine 3 was then added at room temperature under inert
atmosphere, and a new 31P{1H} spectrum was acquired. At this
(27) (a) Heaton, B. T.; Iggo, J. A.; Jacob, C.; Nadarajah, J.; Fontaine,
M. A.; Messere, R.; Noels, A. F. J. Chem. Soc., Dalton Trans. 1994, 2875.
(b) Retondo, E.; Battaglia, G.; Arena, C. G.; Faraone, F. J. Organomet.
Chem. 1991, 419, 399. (c) Elsevier, C. J.; Kowall, B.; Kragten, H. Inorg.
Chem. 1995, 34, 4836. (d) Carlton, L.; De Sousa, G. Polyhedron 1993, 12,
1377.

Rh-Catalyzed Hydroiminoacylation of Alkenes
Organometallics, Vol. 28, No. 10, 2009 2979
Scheme 4. Reactivity of Cationic Complex 13 in the
Presence of Aldimine 3 and BnMe3NCl
9). In order to investigate the role of chlorine, the sample used
in the previous study containing [Rh(NBD)(PPh3)2]BF4 (13) and
aldimine 3 was cooled to room temperature, and 3 equiv of
BnMe3NCl was added under inert atmosphere (Scheme 4, path
b2). When a 1H spectrum was recorded, the signals correspond-
ing to the NBD ligand of 13 were not observed, and three new
signals at δ 4.10, 3.69, and 3.11 were detected and assigned to
an NBD ligand of a new species formed under these conditions.
In the aromatic region, due to the overlapping of signals arising
from PPh3 ligands, the aldimine 3, and BnMe3NCl, no clear
changes could be observed. In the corresponding 31P{1H}
spectrum, a new broad resonance at δ 16.7 was observed as
the only signal. In order to reduce the high fluxionality of this
species, low temperature spectra were recorded. However, at
193 K, the signal was still very broad. It was therefore concluded
that a new Rh complex containing at least one chelating NBD
and one PPh3 ligand was formed. Structure 15 was proposed
for this complex. Given the high fluxionality of the complex
formed and the large shifts observed in 31P NMR, we concluded
that the signal detected at δ 16.7 arose from an average of the
signals corresponding to the coordinated PPh3 ligand from 15
and the free ligand. Indeed, the reported chemical shifts for
complexes containing PPh3, alkenes, chloride, and a nitrogen
donor usually lie above 30 ppm, and although such signals were
not detected, shifts toward this region were always observed
when the temperature was lowered and the ligand exchange rate
was thereby reduced.27c,d
step was initiated by the coordination of the aldimine 3 through
the nitrogen atom of the pyridyl unit (Scheme 3), and that this
process takes place at temperatures higher than 253 K (Scheme
3).
Compound 15 was also formed when the order of addition
of reagents was reversed by adding first BnMe3NCl and second
the aldimine 3 (Scheme 4, path a1). In this case, the H NMR
1
spectrum of a THF-d8 solution of 13 and 3 equiv of BnMe3NCl
showed three new NBD signals at δ 4.32, 3.87, and 2.52,
indicating the formation of a new intermediate a. Surprisingly,
in the corresponding 31P{1H} spectrum, no signals could be
detected. This observation indicated that a highly fluxional
process was occurring under these conditions.
Indeed, when the temperature of the sample was decreased
to 233 K, a new broad signal was observed at δ 20.8. However,
due to the high fluxionality of this species, even at lower
temperatures, their identity could not be determined unambigu-
ously. Such a behavior was previously reported for Rh cationic
systems containing a weakly coordinated halide ligand.28 After
the addition of 2 equiv of aldimine 3 to the solution, only the
signal at δ 16.7 corresponding to the species 15 was detected
in the 31P{1H} spectrum (Scheme 4, path a2).
Cationic System. Reaction of [Rh(NBD)(PPh3)2]BF4
(13) in the Presence of Aldimine 3. In order to study the
oxidative addition process using the cationic system, the
reactivity of the cationic precursor [Rh(NBD)(PPh3)2]BF4 (13)
in the presence of aldimine 3 was first investigated (Scheme
4). A THF-d8 solution containing the complex 13 was charged
1
into an NMR tube, and H and 31P{1H} NMR spectra were
recorded at room temperature. The expected signals for 13 were
readily detected (Table 2). Two equivalents of aldimine 3 was
1
then added under inert atmosphere, and a H spectrum was
immediately recorded at room temperature. The signals corre-
sponding to 3 were readily detected as sharp signals. The
resonances corresponding to the NBD ligand of 13, however,
were this time observed as broad signals. In the 31P{1H}
spectrum, the signal at δ 31.6 was observed as a very broad
resonance, and no Rh-P coupling constant could be measured.
At 333 K, two very broad signals were detected at δ -1.5 and
30.2 in the 31P{1H} spectrum. These resonances were assigned
to free PPh3 and 13, respectively (Scheme 4). Although the
resonances of PPh3 and 13 are usually observed at ca. δ -5
and 31.6, respectively, a fast equilibrium between these species
could explain such a shift. The detection of free PPh3 suggests
that substitution of a PPh3 by 3 had occurred, affording
[Rh(NBD)(PPh3)(3)]BF4 (14) (Scheme 4, path b1). It can be
concluded that a fast equilibrium between 13 and 14 was taking
place. However, it is noteworthy that during this study no
hydride signal was detected, even when the solution was heated,
which indicates that no stable oxidative addition product ([RhH]
species) was formed under these conditions.
When the samples containing 15 were warmed to 343 K, on
the 1H NMR and 31P{1H} spectrum, a hydride signal at δ -11.14
(q, JRhH ) JPH) 13 Hz) and a doublet signal δ 33.1 (JRhP
)
116 Hz), respectively, corresponding to 5-trans were detected.
After several hours at 343 K, total conversion to 5-trans was
achieved. The detection of 5-trans indicated that the oxidative
addition process had occurred in the presence of chloride and
that it was now coordinated to the rhodium center (Scheme 4).
The results described in this section demonstrate that the
cationic rhodium complex [Rh(NBD)(PPh3)2]BF4 (13) does not
react with aldimine 3 to give any stable oxidative addition
products. However, in the presence of BnMe3NCl, the hydride
species 5-trans formed by oxidative addition of 3 is readily
(28) (a) Delis, J. G. P.; Aubel, P. G.; Vrieze, K.; van Leeuwen,
P. W. N. M. Organometallics 1997, 16, 2948. (b) Groen, J. H.; van Leeuwen,
P. W. N. M.; Vrieze, K. J. Chem. Soc., Dalton Trans. 1998, 113. (c) Romeo,
R.; Fenech, L.; Scolaro, L. M.; Albinati, A.; Macchioni, A.; Zuccaccia, C.
Inorg. Chem. 2001, 40, 3293. (d) Albinati, A.; Kunz, R. W.; Amman, C. J.;
Pregosin, P. S. Organometallics 1991, 19, 1800. (e) Gogoll, A.; Ornebro,
J.; Grennberg, H.; Backwall, J. E. J. Am. Chem. Soc. 1994, 116, 3631.
Reaction of [Rh(NBD)(PPh3)2]BF4 (13) in the Presence
of the Aldimine 3 and BnMe3NCl. As shown in Table 1,
cationic rhodium complexes were scarcely active or inactive in
the intermolecular hydroacylation (entries 2-6). However, the
activity was restored when a chloride source was added (entry

2980 Organometallics, Vol. 28, No. 10, 2009
Marce´ et al.
Table 2. Selected NMR Data for the Complexes 5-trans, 13, and 14
1H NMR
31P NMR
complex
T (K)
ligand
δ in ppma
ligand
δ in ppm
JRhP
5-trans
13
15
rt
rt
rt
hydride
NBD
NBD
-11.14 (dt)
2.51 (s, 2H), 4.12 (s, 2H), 4.64 (s, 4H)
3.11 (s, 2H), 3.69 (s, 2H), 4.10 (s, 4H)
PPh3
PPh3
PPh3
33.1 (br s)
31.6 (br s)
16.7 (brs)b
116.6
156
a
The H NMR spectra were recorded at room temperature. b This signal has been proposed to arise from the average of 15 and free PPh3, which are
1
in equilibrium.
Figure 1. Molecular structures and relative energies for isomers
cis and trans of the species 12a and 5a.
Figure 3. Molecular structures and relative energies for 17a, cis
and trans isomers for 20a, intermediates 18a and 19a, and the
corresponding transition state.
Figure 2. Molecular structures and relative energies, referred to as
12a-trans, for the species involved in the reaction coordinate of
the neutral complex.
detected as the only organometallic product. This reaction was
shown to occur via the formation of the Rh phosphine complex
15 (Scheme 4).
Study of the Oxidative Addition by DFT Calculations. The
oxidative addition step of the intermolecular hydroiminoacyla-
tion was examined in both neutral and cationic systems.
Intermediates and the corresponding transition states were
determined for the neutral precursor [Rh(aldimine)(PH3)2Cl]
(12a). Although 14 (Scheme 4) is the only intermediate detected
by NMR from the cationic complex 13 in the presence of
aldimine 3 (Scheme 4, path b), it is reasonable to consider
that this species must lose NBD and that aldimine or phosphine
must coordinate to rhodium before the oxidative addition takes
place. Accordingly, we considered two possible structures:
complex [Rh(aldimine)(PH3)2]+ (17a), where aldimine is coor-
dinated in a chelate manner (Figure 3), and complex
[Rh(aldimine)2(PH3)2]+, where aldimine is coordinated only
through the pyridinic nitrogen. For simplifying the computational
cost, one aldimine has been substituted by pyridine, complex
[Rh(aldimine)(py)(PH3)2]+ (21a) (Figure 4).
For the neutral system, complex 12a and the corresponding
oxidative addition product 5a were considered, in both their
cis and the trans isomers (Figure 1). These results indicated
that 12a-cis, in which the two phosphine ligands are located in
a cis fashion, corresponds to the most stable isomer, which is
precisely the species 12-cis characterized by NMR experiments.
However, the most stable product of the oxidative addition
process is 5a-trans, and therefore, the reaction from 12a-cis to
5a-trans is exothermic by 1.6 kcal · mol-1 (Figure 1). This result
is also in full agreement with the NMR experiments described
earlier (see Supporting Information for a comparison between
X-ray data of the 5-trans iodide complex described by Albinati
et al.26 and our computed DFT geometry). The oxidative
Figure 4. Molecular structures and relative energies for species
involved in the reaction of the cationic complex with an additional
pyridine ligand.
addition process for species 12a-cis to 5a-trans was therefore
studied. During the transition state search, a new intermediate
was characterized (16a, Figure 2) that shows an interaction
between the rhodium center and the C-H bond involved in the
oxidative addition process. Note that in this structure the angle
between the two phosphine ligands increased (101.5°) and that
the C-H bond is elongated (1.102 Å in free aldimine 3, 1.412
Å in 16a). This structure was computed to be above the reactant
by 14.3 kcal · mol-1. The corresponding transition state
(TS16a-5a-trans) was fully characterized and is shown in Figure 2.
Here, we noted a close resemblance to the agostic intermediate,
especially in regard to the P-Rh-P angle of 117.6° and the
C-H bond length (1.759 Å). The energy difference between
the transition state and the agostic intermediate is small, only
6.0 kcal · mol-1, indicating the reactant-like nature of this TS.
For the cationic system, the geometries of the rhodium
squared-planar complex 17a, the possible isomers of the
oxidative addition products (20a-cis and 20a-trans), as well as

Rh-Catalyzed Hydroiminoacylation of Alkenes
Organometallics, Vol. 28, No. 10, 2009 2981
The angle between the two phosphorus atoms and the metallic
center (P-Rh-P) and the carbon-hydrogen (C-H) distance
involved in the oxidative addition process were the two
parameters that changed the most. Regarding the P-Rh-P
angle, in the transition state TS16a-5a-trans, it was found to have
the widest angle (117.6°); this angle was 90.8 and 95.9°
for TS19a-20a-trans and TS21a-22a-cis, respectively. Moreover,
TS16a-5a-trans showed the longest C-H bond (1.76 Å), while
the TS19a-20a-trans and the TS21a-22a-cis showed a C-H distance
equal to 1.58 and 1.51 Å, respectively. This analysis shows
that the three transition states are very similar and that their
structure is close to the corresponding reactant.
These calculations show that, during the oxidative addition,
noticeable changes in the angle P-Rh-P are required. For
instance, in the neutral system 12a-cis, the P-Rh-P angle was
95.1°; in the transition to the intermediate 16a, this angle was
101.5°. When the TS was achieved, the angle increased to
117.6°. Finally, in the oxidative addition product 5a-trans, this
value increased again to 163.2°. Such variations indicate that
the use of monodentate phosphines is suitable for this process.
Furthermore, while the reaction is clearly exothermic only in
the case of the neutral system, the energy barriers in the cationic
and in the neutral pathway are relatively similar. This indicates
that the role of chloride when it is coordinated to the rhodium
complex is to increase the stability of the oxidative addition
product, enabling the reaction to continue. This is not the case
in the cationic complexes since the low stability of the reaction
products promotes the back reaction and, therefore, low conver-
sions are obtained.
Study of the Alkene Insertion and Reductive
Elimination by in situ NMR. Reactivity of 5-trans in the
Presence of Styrene. In order to investigate the insertion of
the alkene into the hydride 5-trans,26 a 5 mm NMR tube was
charged with a solution of 5-trans in toluene-d8, and 5 equiv of
styrene was added under N2 atmosphere. When 1H and 31P{1H}
spectra were recorded at 295 K, only signals corresponding to
the reagents were apparent. The temperature was gradually
increased up to 373 K, and spectra were recorded at intervals
of 10 K. No changes were observed in the 31P{1H} NMR spectra
between 295 and 363 K. At 363 K, two new 31P signals were
detected at δ 45.5 (d, JRh,P ) 163 Hz) and 52.9 (d, JRh,P ) 198
Hz). The latter resonance was readily assigned to the previously
reported dimeric species [Rh(µ-Cl)(PPh3)2]2 (11)29 (Figure 6a).
It was concluded that the signal resonating at δ 45.5 and
exhibiting a Rh-P coupling corresponded to a newly formed
Rh species that contained at least one PPh3 ligand. After a few
minutes at this temperature, the signal corresponding to [Rh(µ-
Cl)(PPh3)2]2 was not observed, and a new resonance was
detected at δ -4.7 (broad singlet), which was readily assigned
to free PPh3. The signal at δ 45.5 and that of free PPh3 increased
in intensity, while the resonance corresponding to the starting
complex 5-trans decreased (Figure 6b).
Figure 5. Calculated pathway for the oxidative addition process.
Relative energies are given in kcal · mol-1 (in parentheses).
the corresponding transition state were determined. First, the
results showed that the most favored pathway, although endo-
thermic, corresponds to the formation of 20a-trans (Figure 3).
The relative energy of the transition state TS19a-20a-trans was
computed to be 21.4 kcal · mol-1 above the corresponding
reactant 17a, while the energy of the cis product was even
higher. Note that the coordination mode of aldimine in 17a does
not allow the reaction to proceed further. So, from the chelating
coordination mode 17a, decoordination and rotation around the
C-N bond are needed to enable the C-H bond in close
proximity to the metal atom. Indeed, two new intermediates,
18a and 19a, that are both less stable than the reactant were
characterized along the reaction coordinate.
Furthermore, as the aldimine 3 is in excess under catalytic
conditions with respect to the Rh catalyst, the coordination of
a second aldimine molecule to the rhodium center could not be
discarded during the catalytic process. For this purpose, a
pyridine molecule was used to model the aldimine in order to
reduce the computational cost. As in the previous cases, reactants
21a, transition state TS21a-22a, and the corresponding product 22a
were characterized. In this case, the isomer 21a-trans is slightly
more stable than 21a-cis, but the energy barrier required to reach
the TS by following the trans complex (37.1 kcal · mol-1) is
much higher than the barrier for the cis 18.5 kcal · mol-1 (Figure
4). In both cases, the products are less stable than the
corresponding reactants.
Figure 5 shows the reaction energy profile for the neutral
complex (black line), the cationic (red line), and the cationic
with pyridine (green line) complexes. In the neutral pathway,
the reaction is initiated by an interaction (16a) between the
rhodium, carbon, and hydrogen involved in the oxidative
addition process. The transition state was found to have an
energy of 20.3 kcal · mol-1 relative to the starting material. The
evolution of this TS affords the complex 5a-trans, which is the
product of the oxidative addition process. The reaction is
exothermic, and thus the formation of this product is favored.
For the cationic complex 17a (red line), the energy barrier is
higher than for the neutral system (black line), and the reaction
is therefore expected to be slower. Furthermore, for this cationic
system, the oxidative addition was found to be endothermic,
and the formation of the product is therefore disfavored.
Moreover, in the cationic pyridine system 21a, the energy barrier
is lower than in the neutral system. In both cationic complexes,
the reaction is endothermic. It is interesting to note that the
transition state geometries are very similar in all three cases;
the distances and angles did not present significant differences.
The identity of the new species formed was investigated using
1H, 13C, COSY, 1H-13C HSQC, and 1H-13C HMBC techniques.
The most relevant data are the following: two proton signals at
δ 2.75 (dd, JHH ) 7 and 14 Hz, 1H) and 3.20 (dd, JHH ) 8 and
14 Hz, 1H) were found to correlate with a 13C signal at δ 38.1
exhibiting a singlet multiplicity, indicating the presence of a
1
CH2 unit; one H resonance at δ 3.13 (q, JHH) 8 Hz, 1H) was
correlated with a doublet 13C signal at δ 52.4 (JRhC ) 15.5 Hz),
and phase discrimination indicated the presence of a CH unit;
HMBC analysis showed that the latter unit was neighboring
(29) Naaktgeboren, A. J.; Nolte, R. J.; Drenth, W. J. Am. Chem. Soc.
1980, 102, 3350.

2982 Organometallics, Vol. 28, No. 10, 2009
Marce´ et al.
Figure 8. Relevant NMR data for complex 23.
Scheme 5. Tautomerism of 24
Scheme 6. Reaction of 5-trans Styrene in the Presence of
Figure 6. Sequence of 31P{1H} spectra of a toluene-d8 solution
containing 5-trans and styrene: (a) at 363 K; (b) at 363 K after a
few minutes; (c) at rt after reaction.
Aldimine 3
When the temperature of the sample was decreased to 295 K
and the sample was taken out of the NMR spectrometer, red
crystals were observed at the bottom of the NMR tube. X-ray
analysis of these crystals confirmed the identity of the complex
[Rh(µ-Cl)(PPh3)2]2.
Figure 7. Selected region of the 13C{1H} spectrum of 23 at room
temperature.
Reaction of 5-trans in the Presence of Styrene and
Aldimine 3. In order to investigate the effect of the presence
of substrate excess, the reaction was repeated in the presence
of 5 equiv of aldimine 3. The temperature was gradually
the previously mentioned CH2 unit and another group containing
a quaternary carbon atom that resonated at δ 86.1 as a doublet
(JRhC ) 15.5 Hz).
1
increased up to 373 K, and the reaction was monitored by H
The higher chemical shift for the latter resonance (86.1 ppm)
indicated the presence of a neighboring heteroatom. Further-
more, the detection of the signals at δ 52.4 and 86.1 as Rh-
coupled resonances indicated that the corresponding carbon
atoms were bound to the Rh center (Figure 7). It was therefore
concluded that a -CH2-CHdC(R′)N- unit was contained in
the new product and was bound to the Rh center through the
CdC double bond in an η2-manner. The absence of P-C
coupling suggested that the vinylic moiety is situated in cis
position to the phosphine ligand(s). After analysis of the NMR
spectra, the product was identified as [RhCl(PPh3)(24a)] (23,
Figure 8). To the best of our knowledge, this species has not
been reported. The detection of this species reveals a tautomeric
process between the organic products 24 and 24a (Scheme 5).
When the temperature of the sample was increased to 373
K, the intensity of 31P signals corresponding to the starting
complex 5-trans and to the dimeric species [Rh(µ-Cl)(PPh3)2]2
(11) rapidly decreased, while those of 23 and free PPh3
increased. After 2 h at this temperature, the only Rh species
detected in solution was 23 (Figure 6c).
and 31P NMR spectroscopy. As previously observed, the
presence of free PPh3 ligand in solution was observed at 353
K. Even at higher temperatures, however, no new 31P signals
were detected. At 363 and 373 K, the signal corresponding to
the starting complex 5-trans decreased in intensity, while that
corresponding to free PPh3 proportionally increased. Even after
a few hours at 373 K, however, the signal corresponding to
5-trans was still visible. In the 1H spectra, the only new signals
detected were assigned to the organic product 24 formed by
the catalytic reaction (Scheme 6).
The presence of 11 in Figure 6a in its further disappearance
(Figure 6b) indicates that 23 is probably formed through
isomerization of 14 induced by 11 and coordination of the
tautomeric form 14a. On the basis of these results, the catalytic
cycle described in Scheme 7 is proposed.
Formation of Cationic Rhodium Hydride Complex
from 5-trans and Study of the Reactivity with Styrene.
Formation of Cationic Rhodium Hydride Complexes. In the
study of the oxidative addition of 3 to cationic rhodium
complexes, no hydride species could be detected (Scheme 4);

Rh-Catalyzed Hydroiminoacylation of Alkenes
Organometallics, Vol. 28, No. 10, 2009 2983
Scheme 7. Proposed Catalytic Cycle for the Hydroiminoacylation of Styrene Catalyzed by Neutral Rh Systems
Scheme 8. Formation of Cationic Hydride Rhodium
however, the very low conversions obtained with these systems
indicate that such species could be formed although to a small
Complexes 20-trans, 22-trans, and 25
extent.
It was therefore decided to form cationic rhodium hydride
complexes in situ and probe their reactivity toward styrene. With
this aim, 5-trans was dissolved in CD2Cl2, cooled to 193 K,
and 1 equiv of AgBF4 was added. The tube was quickly
transferred into the NMR spectrometer that had been precooled
1
to 193 K, and H and 31P{1H} spectra were recorded; only
signals corresponding to the starting complex 5-trans were
observed (Figure 9a).
The tube was then briefly taken out of the spectrometer,
shaken at room temperature for a short time, and reinserted into
the NMR probe. In the 1H spectrum recorded at 193 K, a hydride
signal was then detected at δ -10.9 (dt, JRhH ) 16 Hz, JPH) 9
Hz). In the corresponding 31P{1H} spectrum, a new signal was
The multiplicity of the hydride signal indicated that this species
detected at δ 36.8 (d, JRhP) 114 Hz). The tube was warmed in
contained two equivalent PPh3 ligands. The new complex was
identified as 20-trans (Scheme 8) with one solvent molecule
coordinated to the rhodium center.
the same manner a few times, and it was observed that the new
signals increased in intensity, while the signals for 5-trans
decreased in the same proportion (Figure 9b). The new hydride
product was found to exhibit spectral features similar to those
of 5-trans, indicating small structural differences with 5-trans.
When the temperature of the sample was slowly increased
to room temperature, the broadening of the NMR signals for
the new complex was observed, and at 293 K, the presence of
bulk rhodium metal in the sample was evident, indicating the
decomposition of the complex. The instability of this species
upon warming is in contrast with the report of Suggs, which
described an analogue Rh hydride complex containing a
chelating acyl-quinonoline moiety as an indefinitely stable
species at room temperature.7
In order to probe the presence of a solvent molecule in this
complex, the reaction was repeated, and 5 equiv of pyridine
was added to the solution at low temperature, and new 1H and
31P{1H} spectra were recorded. In the 31P{1H} spectrum, a new
doublet signal was detected at δ 38.5 (JRhP ) 114 Hz). In the
1H spectrum, a new hydride signal was evident as a doublet of
triplets at δ -11.8 (JRhH ) 16 Hz, JPH ) 9 Hz) (Figure 9c). A
set of new aromatic peaks was also detected. When the
temperature was slowly increased, some of the aromatic
resonances were observed to coalesce; this effect was attributed
to the rotation of a pyridine ligand. At room temperature, the
signals corresponding to the new species were found to slightly
broaden, but the complex was found to be stable for a few hours
under these conditions. This new species was fully characterized
by NMR spectroscopy and identified as 22-trans (Scheme 8).
Figure 9. (a) 1H and 31P{1H} spectra of 5-trans in CD2Cl2. (b) 1H
and 31P{1H} of 20-trans formation in CD2Cl2. (c) 1H and 31P{1H}
spectra of cationic complex with pyridine, 22-trans, in CD2Cl2.

2984 Organometallics, Vol. 28, No. 10, 2009
Marce´ et al.
It is noteworthy that species 22-trans corresponds to the
calculated species 22a-trans, which was shown to be more stable
than the cis isomer by DFT calculations. The high energy of
TS21a-22a-trans indicated that the cis pathway is preferred and that
further isomerization proceeds after the CH activation process.
Reaction of [RhH{benzylidene-(3-methylpyridine-2-
yl)amine}(PPh3)2(solv)][BF4] (20-trans) in the Presence of
Styrene. In order to investigate the reactivity of 20-trans in
the presence of styrene, an NMR tube was charged with 5-trans
in CD2Cl2 and reacted with AgBF4 in situ. The formation of
20-trans was monitored by 31P NMR spectroscopy. Three
equivalents of styrene was then added at 193 K, and the tube
was transferred into the precooled spectrometer at 193 K. The
temperature of the sample was then gradually increased, and
the neutral system as a complex containing the coordinated
organic product in its enamine tautomeric form (23), while for
the cationic system, no resulting complex neither hydroiminoa-
cylation product could be detected.
In summary, this study indicates that the role of the chloride
ligand is to increase the stability of the oxidative addition
product. In the case of the cationic complexes, the lower stability
of the oxidative addition product was evidenced. The failure of
the cationic catalysts is likely due to a higher energetic barrier
for one of the following steps of the catalytic cycle.
Experimental Section
General Procedures. All of the starting materials, reagents, MK-
10, and phosphines used were purchased and used without further
purification, except the aldehydes, which were distilled prior to use.
The catalytic reactions were monitored by GC on a Hewlett-Packard
5890A. Conversion was measured using a HP-5 column (25 m ×
0.2 mm L). The oven temperature was set to 80 °C for 0.5 min
and then increased 10 °C/min to 280 °C. All complexes were
synthesized using standard Schlenk techniques under nitrogen
atmosphere. Toluene and THF were distilled over sodium/ben-
zophenone, and dichloromethane was dried over P2O5. All solvents
were deoxygenated before use. RhCl(PPh3)3 was purchased from
Strem Chemicals and AgBF4 from Sigma-Aldrich; both were used
without further purification. The reagents used for the synthesis of
3 were purchased from Sigma-Aldrich. Complex 5-trans was
prepared according to literature methods.26 All other reagents were
used as received from commercial suppliers. All the deuterated
solvents of grade 99.8%D packed in sealed ampules were purchased
1
the reaction was monitored by H and 31P NMR. No new
products were detected up to 233 K. At 243 K, a broad 31P
signal was detected at δ 36.75 (d, JRhP ) 110 Hz). In the
1
corresponding H spectrum, a new broad hydride signal was
detected at δ -10.24. These signals were assigned to the alkene
complex 25. Upon warming, a new hydride was detected at δ
-10.66. Two new doublet signals were also detected in the
31P{1H} spectrum at δ 18.6 (d, JRhP ) 75 Hz) and 14.1 (d, JRhP
) 75 Hz). At 295 K, the hydride signal resonating at δ -10.24
rapidly decreased, while the intensity of the signal at δ -10.66
increased. After a few minutes, the signals corresponding to 25
(Scheme 8) could not be detected. Upon increasing the tem-
perature, the 31P signals resonating at δ 18.6 and 14.1 were found
to broaden and finally to coalesce at δ 16.5 at 295 K. At this
temperature, however, all the signals were found to broaden
rapidly, and rhodium metal was observed in the NMR tube,
indicating that the species present in solution were decomposing.
The identity of the hydride species could not be determined;
however, the presence of a hydride ligand indicated that the
insertion of styrene into the Rh-H bond of 25 had not occurred.
After a few hours at 295 K, the hydride signal was not observed,
although the 31P signal resonating at δ 16.5 was still detected,
indicating that this signal was not correlated with the previously
mentioned hydride species. No evidence for the formation of a
new Rh species or the organic catalytic product was obtained
when a 13C{1H} spectrum was acquired at this temperature. It
was therefore concluded that the insertion of styrene into the
Rh-H bond of the cationic system is a slow process, in contrast
to the neutral system, for which this step is rapid and yields the
intermediate 20-trans.
1
from Euriso-top. H, 13C{1H}, and 31P{1H} spectra were recorded
on a Varian Mercury 400 spectrometer (400.14, 100.63, and 161.98
MHz, respectively). Chemical shifts were referenced to either TMS
as an internal standard (1H and 13C) or 85% H3PO4 as an external
standard (31P). Gas chromatography was performed in a Hewlett-
Packard 5890A apparatus (T ) 80 °C for 0.5 min up to 280 °C;
rate ) 10 °C/min).
Computational Methods: All DFT calculations were performed
using the Amsterdam density functional (ADF2004.01) program
developed by Baerends et al.30-32 Local VWN33 exchange-
correlation potential with nonlocal Becke’s exchange correction34
and Perdew’s correlation correction35 (BP86) were used. Relativistic
corrections were introduced by scalar-relativistic zero order regular
approximation (ZORA).36-38A triple-ꢀ plus polarization basis set
was used on all atoms. For non-hydrogen atoms, a relativistic
frozen-core potential was used, including 4d for rhodium, 2p for
phosphorus and chlorine, 3p for iodine, and 1s for carbon and
nitrogen. The Slater basis sets were extracted from the ADF library.
Transition states were fully characterized by means of vibrational
frequencies analysis. Also, a pseudo-IRC procedure was followed
in order to connect the TS to the corresponding reactant and product,
which consisted of adding or subtracting the imaginary normal mode
vector to the coordinates of the transition state geometry, and
subsequent energy minimization.
Conclusion
Both cationic and neutral rhodium catalyst precursors were
studied in the hydroiminoacylation of alkenes. Catalytic runs
revealed that cationic catalysts were poorly active, while the
neutral catalyst is active under similar conditions. A study by
in situ NMR and DFT calculations allowed us to explain these
differences based on the following facts: (a) The neutral rhodium
complex [RhCl(PPh3)3] reacts at low temperature with aldimine
3 to give the oxidative addition product 5-trans, while the
cationic rhodium complex [Rh(NBD)(PPh3)2]BF4 (13) does not
react with aldimine 3 to give any stable oxidative addition
products even at higher temperature. (b) The oxidative addition
step is an exothermic process for neutral systems, whereas it is
endothermic for cationic systems. (c) The calculated energy
barriers in the cationic and in the neutral pathways are relatively
similar. (d) The coordination and insertion of the alkene is very
rapid for the neutral system, while in the cationic system the
coordination is slow and the insertion was not detected. (e) The
reductive elimination product was fully characterized for
(30) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41.
(31) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S.; Guerra, C. F.;
Baerends, E.; Snijders, J.; Ziegler, T. J. Comput. Chem. 2001, 22, 931.
(32) Guerra, C. F.; Snijders, J.; te Velde, G.; Baerends, E. Theor. Chem.
Acc. 1998, 99, 391.
(33) Vosko, S. H.; Wilk, L.; Nusair, M Can. J. Phys. 1980, 58, 1200.
(34) Becke, A. Phys. ReV. A 1988, 38, 3098.
(35) (a) Perdew, J. P. Phys. ReV. B 1986, 34, 7406. (b) Perdew, J. P.
Phys. ReV. B 1986, 33, 8822.
(36) van Lenthe, E.; Baerends, E.; Snijders, J. J. Chem. Phys. 1993, 99,
4597.
(37) van Lenthe, E.; Baerends, E.; Snijders, J. J. Chem. Phys. 1994,
101, 9783.
(38) van Lenthe, E.; Ehlers, A.; Baerends, E.; Snijders, J. J. Chem. Phys.
1999, 110, 8943.

Rh-Catalyzed Hydroiminoacylation of Alkenes
Organometallics, Vol. 28, No. 10, 2009 2985
General Procedure for the Intramolecular Hydroacylation
of Alkenes Using Cationic Systems and Chlorine Source: A
mixture of benzaldehyde (2.5 mmol), 2-amino-3-picoline (0.8
mmol), 1-hexene (12.5 mmol), MK-10 (80 mg), [Rh(cod)L2] and
PR3 (0.05 mmol), and BnMe3NCl (0.05 mmol) or alternatively
RhCl(PPh3)3 (0.05 mmol) in 2 mL of toluene was heated to 110
°C for 2 h. The resulting mixture was filtered and analyzed by GC.
Typical Procedure for the Preparation of NMR Samples:
Typically, a 5 mm NMR tube fitted with a Young’s valve was
charged with ca. 15 mg of rhodium complex and dissolved in the
appropriate deuterated solvent. Addition of the reagent(s) was
completed under inert atmosphere and low temperature when
required. The sample was then transferred into the NMR spec-
trometer, which was previously set to the appropriate temperature.
(d, 2H, J ) 7.2 Hz, Ph), 7.43 (m, 12H, PPh3), 7.28 (d, 1H, J ) 7.6
Hz, Py), 7.13 (m, 18H, Ph), 6.87 (t, 1H, J ) 7.2 Hz, PPh3), 6.68
(dd, 2H, J ) 7.2 and 8.6 Hz, PPh3), 6.52 (dd, J ) 5.6 and 7.6 Hz,
Py), 2.5 (s, 3H, CH3), -11.2 (q, JRhH ) JPH ) 13 Hz); 13C NMR
(toluene-d8, 100.6 MHz) δ ) 230.5 (dt, JRhC ) 32.6 Hz, JPC ) 7.5
Hz), 184.9-118.9 (Ar), 19.2 (CH3); 31P NMR (CDCl3, 161.9 MHz)
δ ) 33.1 (d, JRhP ) 116.6 Hz).
[RhCl{(2-benzylidene)-3-methylpyridine}(PPh3)2] (12-cis): 1H
NMR (CD2Cl2, 400 MHz, 253 K) δ ) 9.1 (s, 1H, NdCHPh), 9.0
(dd, 1H, J ) 5 and 1.8 Hz, Py), 8.90-6.8 (m, 37H, arom), 2.45 (s,
3H, CH3); 31P NMR (CDCl3, 161.9 MHz, 253 K) δ ) 54 (dd, JRhP
) 196 Hz, JPP ) 47 Hz), 50 (dd, JRhP ) 170 Hz, JPP ) 47 Hz).
[RhCl{(1,3-diphenylpropenyl)-(3-methylpyridin-2-yl)amine}
(PPh3)] (23): 1H NMR (toluene-d8, 400 MHz) δ ) 9.19 (d, 1H, J
) 5.6 Hz, Py), 7.54-6.20 (m, 27H, arom), 3.20 (dd, 1H, J ) 14
and 8 Hz), 3.13 (q, 1H, J ) 8 Hz), 2.75 (dd, 1H, J ) 14 and 7
Hz), 1.30 (s, 3H, CH3); 13C NMR (toluene-d8, 100.6 MHz) δ )
158.9-113.3 (Ar), 86.1 (d, JRhC ) 15.5 Hz), 52.4 (dd, JRhC ) 15.5
Hz, JPC ) 2.3 Hz), 38.1, 16.5 (CH3); 31P NMR (toluene-d8, 161.9
MHz) δ ) 45.5 (d, JRhP ) 163 Hz).
1
Heptanophenone (10): H NMR (CDCl3, 400 MHz) δ ) 7.95
(dd, J ) 5.2 Hz, 1H, H-6 Py), 7.55-7.95 (m, 2H, Ph), 7.35 (dd, J
) 5.2 Hz, 1H, H-4 Py), 6.90-7.10 (m, 3H, Ph), 6.66-6.76 (m,
1H, H-5 Py), 2.60 (t, J ) 7.9 Hz, 2H, hexyl), 2.17 (s, 3H, CH3
Py), 1.10-1.60 (m, 8H, hexyl), 0.80 (t, J ) 6.8 Hz, 3H, CH3 hexyl);
13C NMR (CDCl3, 100.6 MHz) δ ) 171.3 (-CdN), 161.8 (C-2
Py), 148.3 (C-4 Py), 146.3 (C-6 Py), 138.7-120.3 (Ph), 119.4 (C-5
Py), 117.9 (C-3 Py), 31.4, 30.1, 29.8, 21.1 (C-hexyl), 18.8 (CH3
Py), 13.8 (CH3 hexyl), 2.3 (C-hexyl).
[RhH{(2-benzylidene)-3-methylpyridine}(PPh3)2(solv)]BF4
(20-trans): 1H NMR (CD2Cl2, 400 MHz, 233 K) δ ) 8.23 (d, 1H,
J ) 4.4 Hz, Py), 7.61 (d, 2H, J ) 7.6 Hz, Ph), 7.33-7.20 (m,
31H, arom), 7.1-6.78 (m, 4H, Py and Ph), 2.23 (s, 3H, CH3),
-10.86 (overlapping d of t, 1H, JRhH ) 16 Hz, JPH ) 9 Hz); 31P
NMR (CDCl3, 161.9 MHz, 233 K) δ ) 36.8 (d, JRhP ) 118 Hz).
[RhH{(2-benzylidene)-3-methylpyridine}(PPh3)2(pyridine)]BF4
Preparation of 2-Benzylidene-3-methylpyridine (3): The syn-
thesis of this compound was completed according to reported
literature methods.39 A solution of 4.83 g (44.6 mmol) of 2-amino-
3-picoline (2) in 20 mL of THF was prepared, and 5 mL (49.2
mmol) of benzaldehyde (1) was added in the presence of molecular
sieve (4 Å). The reaction mixture was refluxed for 12 h and then
cooled to rt. The molecular sieve was removed by filtration. The
solvent was evaporated in vacuo, and the resulting oily residue was
distillated under vacuum using a Buchi GKR-51 Kugelrohr oven;
a yellow liquid was isolated (yield ) 7.88 g, 90%). Although this
procedure was previously reported, no precise NMR data were
1
(22-trans): H NMR (CD2Cl2, 400 MHz, 298 K) δ ) 8.57 (br s,
free py), 8.22 (d, 1H, J ) 5.2 Hz), 8.15 (br s, 1H, py), 7.81 (dd,
2H, J ) 8 and 1.2 Hz), 7.7-7.1 (m, 42H), 6.94 (t, 2H, J ) 8 Hz),
2.14 (s, CH3), -11.8 (overlapping d of t, 1H, JRhH ) 16 Hz, JP-H
) 9 Hz); 31P NMR (CD2Cl2, 161.9 MHz, 298 K) δ ) 38.5 (d, JRhP
) 114 Hz).
1
1
found to be available. Here, we list the H and 13C data for 3: H
NMR (CDCl3, 400 MHz) δ ) 9.1 (s, 1H, HCdN), 8.3 (dd, J )
1.6 and 4.8 Hz, 1H, Py), 8.00 (dd, J ) 2.4 and 7.2 Hz, 2H, Ph),
7.54 (d, 1H, J ) 7.6 Hz, Py), 7.48 (m, 3H, Ph), 7.07 (dd, 1H, J )
4.8 and 7.2 Hz), 2.46 (s, 3H, CH3); 13C NMR (CDCl3, 100.6 MHz)
δ ) 161.3 (-CdN), 159.2, 145.9, 136.6, 131.5, 129.6, 129.2, 128.9,
128.5, 127.4, 121.7, 17.5.
Acknowledgment. The authors acknowledge the financial
support of MICINN (CTQ2008-001569) (CTQ2008-06549-
C02-02), and Consolider Ingenio 2010, CSD2006-0003
(Ministerio de Educacio´n, Spain), and the ICIQ Foundation.
We thank Servei de Recursos Cient´ıfics (URV) for technical
assistance. Fellowships to P.M. and Juan de la Cierva
fellowship to C.G. from MEC are also acknowledged. We
also thank Prof. Piet W.N.M. van Leeuwen for the critical
reading of the manuscript.
Preparation of [RhHCl{(2-benzylidene)-3-methylpyridine}-
(PPh3)2] (5-trans): A solution of Wilkinson’s catalyst RhCl(PPh3)3
(0.5 g, 0.54 mmol) and 2-benzylidene-3-methylpyridine 2 (0.1 g,
0.51 mmol) in ca. 35 mL of THF was refluxed for 3 h. Half of the
solvent was removed in vacuo, and precipitation of the product
was achieved by addition of cold n-hexane. After filtration, the
product was isolated as a pale yellow solid (0.35 g, 80% yield): 1H
NMR (CDCl3, 400 MHz) δ ) 8.39 (d, 1H, J ) 5.6 Hz, Py), 7.65
Supporting Information Available: Tables giving pictures,
energies, and Cartesian coordinates for all calculated structures,
and comparison of X-ray determined and calculated selected
parameters for 5-trans-I. This material is available free of charge
via the Internet at http://pubs.acs.org.
(39) Chul-Ho, J. Bull. Korean Chem. Soc 1990, 11, 187.
OM800827R