Aromatic vs aliphatic C-H bond activation by rhodium(I) as a function of agostic interactions: Catalytic H/D exchange between olefins and methanol or water

Article abstract of DOI:10.1021/ja029197g

The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)₂(solv)_n]BF₄ (coe = cyclooctene, solv = solvent), producing the C-H activated complexes 4a-c (solv = (a) acetone, (b) THF, (c) methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R = D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1 (analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative addition) and electrophilic (H/D exchange) C-H activation proceed through η²-C,H agostic intermediates. In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically different C-H activation processes, in accordance with the experimental observations.

Full text of DOI:10.1021/ja029197g

Published on Web 08/14/2003
Aromatic vs Aliphatic C-H Bond Activation by Rhodium(I) as
a Function of Agostic Interactions: Catalytic H/D Exchange
between Olefins and Methanol or Water
Boris Rybtchinski, Revital Cohen, Yehoshoa Ben-David, Jan M. L. Martin,* and
David Milstein*
Contribution from the Department of Organic Chemistry, The Weizmann Institute of Science,
76100 RehoVot, Israel
Received November 3, 2002; Revised Manuscript Received May 29, 2003; E-mail: david.milstein@weizmann.ac.il; comartin@wicc.weizmann.ac.il
Abstract: The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)2(solv)n]BF4 (coe
) cyclooctene, solv ) solvent), producing the C-H activated complexes 4a-c (solv ) (a) acetone, (b)
THF, (c) methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin
saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R )
D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated
methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1
(analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H
activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D
exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both
PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative
addition) and electrophilic (H/D exchange) C-H activation proceed through η2-C,H agostic intermediates.
In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange
with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H
coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically
different C-H activation processes, in accordance with the experimental observations.
without the direct involvement of η2-C,C intermediates.6-8 In
Introduction
the case of aliphatic C-H bond activation, a growing body of
Mechanistic studies of C-H activation by late transition
metals revealed a number of important characteristics of this
process.1 Jones and Feher inferred from competition experiments
and kinetic isotope effects that coordination of benzene at
Cp*Rh(PMe3) lowers the barriers to C-H activation.2 The η2-
C,C benzene intermediate in the course of C-H activation was
evidence suggests that C-H bond coordination to a metal center
precedes the bond cleavage step in virtually all cases.1 It is
generally recognized that C-H bond activation in aromatic
hydrocarbons is favored over the aliphatic ones due to kinetic
(η2-C,C arene coordination) and thermodynamic (strong M-aryl
bond) factors.1 An important consequence of agostic bonding
first observed by Perutz et al.3a and was characterized in solution
and η2-C,H bonding in general is the increased acidity of the
and shown to be at equilibrium with aryl hydride complexes.3,4
Such an intermediate was recently isolated and shown to
C-H bond due to the donation from C-H bond σ orbital to a
metal center.1b In a few cases this leads to facile deprotonation8
undergo facile C-H activation; X-ray and NMR evidence for
and H/D exchange with D2O.8a As aliphatic σ complexes9 draw
great attention in view of their relevance to saturated hydro-
carbon activation, aromatic η2-C,H σ-bonding is less studied.
η2-C,C coordination during the reaction was provided.5 The
importance of η2-C,H coordination was also demonstrated, and
C-H activation in arenes may proceed through such complexes
(1) (a) Crabtree, R. H. Chem. ReV. 1985, 85, 245. (b) Crabtree, R. H. J. Chem.
Soc., Dalton Trans. 2001, 2437. (c) Shilov, A. E.; Shul’pin, G. B. Chem.
ReV. 1997, 97, 2879. (d) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.;
Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. (e) Labinger, J. A, Bercaw,
J. E. Nature 2002, 417, 507.
(6) Aromatic C-H activation proceeding directly through η2-C,H adducts: (a)
Peterson, T. H.; Golden, J. T.; Bergman, R. G. J. Am. Chem. Soc. 2001,
123, 455. (b) Asplund, M. C.; Snee, P. T.; Yeston, J. S.; Wilkens, M. J.;
Payne, C. K.; Yang, H.; Kotz, K. T.; Frei, H.; Bergman, R. G.; Harris, C.
B. J. Am. Chem. Soc. 2002, 124, 10605.
(2) For a review, see: Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22,
91.
(7) Aromatic C-H oxidative addition proceeding directly through η2-C,H
agostic complexes: (a) Lavin, M.; Holt, E. M.; Crabtree, R. H. Organo-
metallics 1989, 8, 99. (b) Toner, A. J.; Grundemann, S.; Clot, E.; Limbach,
H.-H.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc.
2000, 122, 6777.
(3) (a) Belt, S. T.; Duckett, S. B.; Helliwell, M.; Perutz, R. N. J. Chem. Soc.,
Chem. Commun. 1989, 928. (b) Belt, S. T.; Dong, L.; Duckett, S. B.; Jones,
W. D.; Partridge, M. G.; Perutz, R. N. J. Chem. Soc., Chem. Commun.
1991, 266. (c) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.;
Jones, W. D.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 7685.
(4) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2000, 122, 10846.
(8) Aromatic C-H activation proceeding by deprotonation of η2-C,H agostic
complexes: (a) Vigalok, A.; Uzan, O.; Shimon, L. J. W.; Ben-David, Y.;
Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 12539. (b)
Gusev, D. G.; Madott, M.; Dolgushin, F. M.; Lyssenko, K. A.; Antipin,
M. Y. Organometallics 2000, 19, 1734.
(5) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem.
Soc. 2001, 123, 12724.
(9) Hall, C.; Perutz, R. N. Chem. ReV. 1996, 96, 3125.
9
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J. AM. CHEM. SOC. 2003, 125, 11041-11050
11041

A R T I C L E S
Rybtchinski et al.
Scheme 1
Scheme 2
of doublets at -22.66 ppm (1JRhH ) 33.2 Hz, 2JPH ) 26.5 Hz)
in H NMR. The aromatic ipso carbon gives rise to a broad
doublet centered at 150.40 ppm (1JRhC ) 36.8 Hz) in the 13C-
{1H} NMR spectrum. All the compounds are stable in polar
solvents and slightly soluble in water. An acetone-water
mixture (5:1 v/v) is an excellent solvent.
We have recently reported a system that exhibits an intriguing
mode of reversible C-H activation (Scheme 1).10 In this system
the reversible C-H bond elimination proceeds in a stepwise
manner with increasing the temperature. Spin saturation transfer
experiments involving selective irradiation of the hydride ligand
revealed that initially only one hydrogen atom of the methylene
bridge, Rh-CH2-aryl, is involved in the exchange, and then
upon temperature increase the two methylene protons are
exchanged and with further temperature increase the ortho
methyl protons start to exchange (Scheme 1). We suggested
that agostic bonding might be responsible for this behavior.
Complex 2 did not show any H/D exchange with deuterated
solvents (THF, acetone, methanol, water).
We now report on a PC-Rh system based on the ligand 3,
which exhibits behavior in cyclometalation and H/D exchange
with ROH (R ) H, Me) very different from that of complex 2.
Moreover, this system promotes catalytic H/D exchange into
olefins with MeOD and D2O. To clarify the observed reactivity,
we performed a computational DFT study of the C-H activation
process in both aliphatic (based on ligand 1) and aromatic (based
on ligand 3) PC-Rh systems. The differences in agostic η2-
C,H bonding were found to account for the observed striking
reactivity difference between the two systems.
1
Spin saturation transfer (SST) experiments involving selective
irradiation of the hydride ligand in 4a-c at 23 °C indicated
chemical exchange between the hydride and the ortho proton
(Scheme 2). Quantitative SST analysis of the exchange processes
in complex 4 is hampered by the fact that solvent dissociation
precedes the C-H elimination step.12 Comparing SST in
complexes 2c and 4c (methanol as a solvent) revealed that under
the same conditions (23 °C) complex 2c (Scheme 1) exhibited
exchange involving only one proton of the methylene group.
Exchange with the second methylene proton and ortho methyl
group started to take place only at higher temperatures. This is
a surprising observation, in view of the expected easier C-H
reductive elimination of 2 due to the weaker Rh-benzyl as
compared with Rh-aryl bonds and the more stable five- vs six-
membered chelate.
A striking difference between complexes 2 and 4 was
observed also regarding H/D exchange phenomena. Thus,
complex 2 did not show any H/D exchange in MeOD or D2O
at room temperature or upon heating. Yet, H/D exchange took
place in complex 4 in CD3OD, completely substituting the
hydride and the aromatic ortho-hydrogen atom for D within 24
h at room temperature (or within 2 h at 65 °C). Since the
methanol CD3 group remained intact and no formaldehyde or
carbonyl complexes were observed, only the deuterium atoms
of the O-D group are responsible for the exchange. In acetone-
d6 and THF-d8 no significant H/D exchange took place.
Significantly, the acetone-water system is similar to methanol
both in SST and H/D exchange (hydride and ortho-H exchange
takes place when D2O is used), confirming that the O-D group
is critical for the exchange. As both complexes 2 and 4 are
hydrides, and in both of them coordinated methanol or water
molecules are present, the H/D exchange in 4 is unlikely to
proceed through a process involving the hydride, by a Hδ-‚‚‚Hδ+
type interaction.13 Moreover, if Hδ-‚‚‚Hδ+ type interaction were
responsible for the H/D exchange, it would be more facile in
the case of 2 which is slightly more hydridic. We explain the
Experimental Results
Cationic rhodium(I) precursors, [Rh(coe)2(solv)n]BF4 (coe )
cyclooctene, solv ) solvent), were obtained by chloride
abstraction from [Rh(coe)2Cl]2, according to a known proce-
dure.11 When [Rh(coe)2(solv)n]BF4 was reacted at room tem-
perature with the ligand 3 in THF, acetone, or methanol,
quantitative formation of the C-H activation products 4a-c,
respectively, was observed within 1 h (Scheme 2). Evaporation
and redissolving in a polar solvent of choice resulted in the
clean compounds 4a-c, which were characterized by NMR
(complex 4c also by elemental analysis and MS) and possess
similar spectral characteristics. For example complex 4c gives
rise to a broad doublet centered at 109.82 ppm (1JRhP ) 175.8
Hz) in 31P{1H} NMR. The Rh-H signal appears as a doublet
(12) We performed quantitative analysis of the exchange rate constants in
complex 2 in various solvents and at various temperatures (see ref 10). It
was found that the constants are representative of a complex process
involving solvent dissociation. Our computational results support this
conclusion. Thus, the C-H reductive elimination/oxidative addition mech-
anism in complexes 2 and 4 appears to be difficult to assess by SST
quantitative analysis (although in complex 2 rates of Various exchange
processes can be compared).
(10) Rybtchinski, B.; Konstantinovsky, L.; Shimon, L. J. W.; Vigalok, A.;
Milstein, D. Chem. Eur. J. 2000, 17, 3287.
(11) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 3089.
(13) For an Ir-H/N-H exchange postulated to proceed via an Hδ-‚‚‚Hδ+
interaction, see: Koelliker, R.; Milstein, D. J. Am. Chem. Soc. 1991, 113,
8524.
9
11042 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

C−H Bond Activation by Rhodium(I)
A R T I C L E S
Table 1. Relative Energies (∆Ee) and Free Energies (∆G298) of the Various Complexes of the Aryl- and Benzyl-PC/Rh Systems at the
mPW1k/SDB-cc-pVDZ//mPW1k/SDD Level of Theory
complex
∆E (kcal/mol)
e
∆G298 (kcal/mol)
Aryl-PC/Rh System
Me-CHactiv3MeOH (5)
0.00
21.59
24.45
21.02
4.65
0.00
7.59
11.10
9.67
Me-CH-activated (6 + MeOH)
Me-CH-agostic (7 + MeOH)
Me-CC-Eta2 (8,8enant + MeOH)
Me-openArm-3MeOH (9)
4.51
Me-TS-CHagostic-to-CHactivated (TS(6-7) + MeOH)
Me-TS-Agostic-to-Eta2 (TS(7-8) + MeOH)
Me-TS-Eta2-to-Eta2(enantiomer) (TS(8,8enant) + MeOH)
Me-TS-openArm3MeOH to Agostic2MeOH (TS(7-9))
26.73
28.22
25.10
20.45
12.07
14.87
12.69
19.11
Benzyl-PC/Rh System
Me-CHactivated-3MeOH-exo (10)
Me-CHactivated-3MeOH-endo (10*)
0.00
5.13
0.00
5.24
Me-CHactivated-2MeOH-exo (11 + MeOH)
Me-CHactivated-2MeOH-endo (11* + MeOH)
Me-CH-agostic-exo (12 + MeOH)
23.05
25.95
33.14
29.96
26.47
8.57
8.73
12.19
19.97
17.70
15.15
9.30
Me-CH-agostic-endo (12* + MeOH)
Me-CC-Eta2 (13 + MeOH)
Me-openArm3MeOH (14)
Me-TS-CHagostic-to-CHactivated-exo (TS(12-11) + MeOH)
Me-TS-CHagostic-to-CHactivated-endo (TS(12*-11*) + MeOH)
Me-TS-Agostic-Exo-to-Agostic-Endo (TS(12-12*) + MeOH)
Me-TS-CH3-in-place-rotation (TS(12-12) + MeOH)
Me-TS-CHagostic-to-CC-Eta2 (TS(12-13) + MeOH)
Me-TS-Eta2-to-Eta2(enantiomer) (TS(13-13enant)+ MeOH)
Me-TS-openArm3MeOH-to-Eta22MeOH (TS(14-13))
35.73
34.83
33.36
36.79
36.89
28.13
25.84
21.43
21.26
21.91
23.71
24.02
16.50
25.22
Scheme 3
compounds were deuterated utilizing D2O in the presence of a
transition metal catalyst15,16 To the best of our knowledge
selective H/D exchange of olefins with D2O or MeOD catalyzed
by metal complex has not been described. Such a process can
be useful for the synthesis of selectively deuterated olefins.
Computational Results
H/D exchange phenomena in terms of stronger aromatic C-H
agostic interactions in the case of 4. This interaction increases
the C-H bond acidity significantly, sufficiently to promote H/D
exchange with weak acids such as D2O and MeOD (see
Computational Results and Discussion). Reaction of 4 with bases
(KOtBu, NEt3) leads to decomposition.
Vinylic H/D Exchange in Olefins. Complex 4 catalyses H/D
exchange of the vinylic protons of styrene in the presence of
D2O or MeOD at 60 °C (Scheme 3). ExclusiVe Vinylic H/D
exchange is obserVed, no other protons being substituted by
deuterium. In methanol a catalytic H/D exchange process with
about 80% exchange and 50 TON takes place after heating for
24 h at 60 °C. Similar reactivity and TON were observed using
acetone/D2O (5:1 v/v, respectively) mixture. Notably, complex
2 does not catalyze H/D exchange with olefins under the same
conditions.
Other terminal olefins such as tert-butylethylene and (trim-
ethylsilyl)ethylene also show H/D exchange, the process being
slower and less efficient than that with styrene. Internal olefins
such as cyclooctene and cyclohexene do not undergo exchange.
Reversible olefin insertion into Rh-H bond coupled with the
H/D exchange in 4 described above can be responsible for the
observed phenomena. The intimate mechanism and scope of
the olefin deuteration is under current investigation. It should
be noted that general procedures utilizing D2O for the incor-
poration of deuterium into organic compounds are often limited
to activated positions.14 Only in a few cases unactivated organic
To clarify our observations regarding C-H activation/
reductive elimination, density functional theory (DFT) calcula-
tions were performed at the mPW1k/SDB-cc-pVDZ//mPW1k/
SDD level of theory. Calculations were carried out first for the
aromatic and aliphatic PC system with H substituents on the
phosphines (H-PC systems) to reduce the computational time.
Afterward, the calculations were performed for the Me-PC-
Rh systems (using H-PC geometries as a starting point) to
identify plausible reaction pathways. Methyl substituents on the
t
phosphine were chosen as the ones mimicking Bu in their
electronic properties and rendering the computational time
reasonable. In the experimental aliphatic PC system the para-
Me group is not involved in the interaction with the metal center;
thus, only the two Me groups ortho to the phosphine arm were
introduced in the computational model system. A summary of
the computational results is given in Table 1. We chose the
C-H activation products, complexes 5 and 10, as reference zero
points as these complexes correspond to the ones observed
experimentally and are the most stable species. It should be
noted that these complexes have 3 bound MeOH molecules
while the reaction intermediates have only 2 MeOH molecules
bound to the metal center. Thus, the relative energies of the
intermediates do include the solvent binding energy and
(15) K2PtCl4 was reported to catalyze exchange in arenes and alkanes: (a)
Garnett, J. L.; Hodges, R. J. J. Am. Chem. Soc. 1967, 89, 4546. (b) Shilov,
A. E.; Shteinman, A. A. Coord. Chem. ReV. 1977, 24, 97.
(16) Cp*(PMe3)IrCl2 catalyzes H/D exchange utilizing D2O in a variety of
organic substrates: Klei, S. R.; Golden, J. T.; Tilley, T. D.; Bergman, R.
G. J. Am. Chem. Soc. 2002, 124, 2092.
(14) For recent review on H/D exchange involving C-H activation see: (a)
Junk, T.; Catallo, W. J. Chem. Soc. ReV. 1997, 26, 401. See also ref 1b.
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11043

A R T I C L E S
Rybtchinski et al.
Figure 1. . Aryl-(Me)PC/Rh system: Reaction profile (∆G298) for C-H activation and exchange at the mPW1k/SDB-cc-pVDZ//mPW1k/SDD level of
theory.
Scheme 4
therefore are high in energy relative to the starting compound.
The free energy (∆G298) values, unlike the ∆Ee values, include
the variation in entropy as obtained from the thermal corrections
calculated using the rigid rotor harmonic oscillator approxima-
tion.
Aromatic Me-PC/Rh System. The reaction pathway found
for the aromatic system is presented in Scheme 4 and Figure 1.
The lowest energy pathway proceeds through species with
two (intermediates) or three (products) coordinated methanol
molecules. A pathway involving only one coordinated methanol
molecule was found to be much higher in energy.
mol higher in free energy. Subsequently, 6 undergoes reductive
elimination, with a barrier of 4.5 kcal/mol, to form an agostic
intermediate 7.17 Then formation of an η2-aryl complex 8 takes
place with a barrier of 3.8 kcal/mol. Complex 8 can rearrange
to its enantiomer 8-enant (∆Gq ) 3 kcal/mol). This intercon-
version opens a channel for an exchange between the hydride
and the ortho hydrogen atom.18 The barriers of all the processes
are relatively low (see Scheme 4 and Figure 1), in agreement
with the experimental observation of the hydride/ortho-H
(17) A high-energy process (barrier of 15.8 kcal/mol), involving concerted C-H
reductive elimination and MeOH loss to form 5 directly from 7, was also
found.
(18) No exchange pathway involving rotation of the Ph ring was found to take
place directly from the agostic complex 7.
The C-H reductive elimination path starts with dissociation
of a methanol molecule from 5 to produce 6, which is 7.6 kcal/
9
11044 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

C−H Bond Activation by Rhodium(I)
A R T I C L E S
Scheme 5
exchange at room temperature. Complete opening of the system
to give 9 appears to be a less favorable pathway for this
exchange (∆Gq ) 8 kcal/mol).19,20
CH2-aryl and ortho CH3-aryl). Pathway I involves an “exo-
endo ring flip” reaction, which allows the exchange of the
hydride with only one hydrogen of the Rh-CH2-aryl group.
The flip between the exo and the endo conformations (12 and
12*, respectively) is followed by C-H oxidative addition in
12* to form the endo isomer 11*. In this pathway only one of
the Rh-CH2-aryl protons is geometrically preferable for the
exchange (see below). Pathway II involves an “in-place rota-
tion” process of the methyl group bound in an exo-agostic mode
to the Rh center in complex 12. This process has a 1.8 kcal/
mol higher energy barrier than the ring flip process. Pathway
III, η2-flip, involves a conversion between the exo-agostic
complex 12 and the η2-C,C aryl one 13. Complex 13 can be
converted to its enantiomer 13-enant, opening a channel to the
C-H activation in the ortho-Me group (Scheme 5, Figure 2).
As the barrier of the exo-endo flip, pathway I, is slightly lower
than that of the in-place rotation of the Me group, pathway II
(∆Gq ) 1.95 vs 3.7 kcal/mol), the selective exchange can take
place. The kinetic barrier of the η2-flip reaction was found to
be comparable to that of the Me in-place rotation (∆Gq ) 4.05
vs 3.7 kcal/mol). However, in 12 two C-H bonds are available
for bonding to the metal center in the course of Me in-place
rotation upon clockwise and counterclockwise motion. On the
other hand, only one arene C-C bond, which is situated between
the agostic Me and the phosphine arm in 12, is available for
η2-C,C bonding. Therefore, the rate of Me rotation should be
increased by approximately a factor of 2. The tendency in the
DFT computed reaction pathways (the rates increase in the order
I < II < III) fits the selectivity observed experimentally. It
should be noted that the experimentally observed SST selectivity
in 2c is in the temperature range of 13 °C,10 implying that the
difference in the kinetic barriers is small, in good agreement
with the computed small differences between the reaction
Aliphatic Me-PC/Rh System. Similarly to the aromatic
system the lowest energy pathways proceed through species
with two or three coordinated methanol molecules (the path
with one coordinated methanol molecule was found to be
much higher in energy). However, in contrast to the aromatic
systems, the aliphatic one allows a variety of possible geometries
in the course of the C-H reductive elimination/activation
(Scheme 5).
The main difference between the aliphatic system and its
aromatic counterpart is higher barriers for the reductive elimina-
tion process from the Rh(III) hydride to the Rh(I) C-H agostic
complex, ∆GqRE ) 12.7 in the aliphatic system vs 4.5 kcal/mol
in the aromatic one. This is in agreement with the experimentally
observed exchange selectivity in the aliphatic system (complex
2c) in which not all the processes responsible for the observed
reversible C-H activation pattern can happen at room temper-
ature. We found two isomers for the C-H activation product:
exo and endo, complexes 10 and 10* respectively. Complex 10
is more stable and most probably corresponds to the experi-
mentally observed complex 2c. All the agostic complexes found
computationally also have endo and exo isomers, and the
pathways accounting for the exchange processes were found to
proceed through these agostic intermediates.
Three different pathways were found (Scheme 5, Figure 2)
for the exchange reactions corresponding to the experimentally
observed SST phenomena (exchange between Rh-H, Rh-
(19) The value of ∆Gq for this barrier is probably underestimated due to hydrogen
bonding in the TS between the H atom of the dissociated methanol and
the O atom of the coordinated methanol.
(20) No analogue of 9 with two coordinated MeOH molecules was found.
Repeated attempts to locate this minimum resulted in the convergence to
8.
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A R T I C L E S
Rybtchinski et al.
Figure 2. . Benzyl-(Me)PC/Rh system: Three reaction profiles (∆G298) for C-H activation and exchange at the mPW1k/SDB-cc-pVDZ//mPW1k/SDD
level of theory.
barriers. In addition, the experimentally found ratio for the
exchange of one and two methylene protons (k2H/k1H ) 1/20),10
which translates to a barrier difference of 1.8 kcal/mol, is in
excellent agreement with the one found computationally for the
exo-endo flip path and in-place rotation (∆∆Gq ) 1.8 kcal/
mol). Thus, pathway I (exo-endo flip) leads to the selective
exchange of only one methylene proton at room temperature,
pathway II (in-place rotation) to the exchange of the two
methylene protons at slightly elevated temperatures and pathway
III (η2-flip) to the exchange at higher temperatures with the
second ortho-methyl group.
Significantly, distinctively different reactivity is found for
endo and exo isomers. We found that most of the productive
exchange phenomena proceed through the agostic exo-isomer
12. It should be noted that, in addition to the agostic C-H in
complex 12*, another C-H bond of the agostically bound Me
group appears to be involved in the interaction with the metal
center. Although this interaction seems to be rather weak (Figure
3, Table 2), it is a noticeable one in 12* and absent in 12. This
interaction may account for the higher stability (by 2.3 kcal/
mol) of 12* in comparison to 12. Neither in-place rotation nor
η2-C,C coordination was found to start from the endo isomer
12*, the only productive pathways being C-H activation and
isomerization into exo-isomer 12. Thus, the specific geometry
of the η2-C,H bound Me group in 12* predisposes one of the
nonagostic C-H bonds to the interaction with the metal center.
It is in line with our previous suggestion10 and can account for
the selective exchange phenomena in 2c (see below).
Wiberg NBO (natural bond orders) of 7, 12, and 12* are given
in Table 2, and optimized structures are presented is Figure 3.
The geometry of the metal-C-H agostic triangles (Figure 3,
Table 2) indicates that the rhodium center is situated closer to
the arene C-H bond in 7 than to that of the CH3 group in 12
or 12*, indicative of stronger agostic bonding in 7.21 Bond order
analysis (Table 2) further confirms a higher degree of bonding
between the metal center and the arene C-H. There is also a
noticeable predisposition to bonding of one of the nonagostic
H of the Me group. Significantly, analysis of the partial charges
reveals that in the aliphatic case (12 and 12*) the agostically
bound H acquires some hydridic character (partial charge of
+0.198 on agostic H atom vs +0.246 to +0.278 for the other
methyl H atoms22), whereas in the aromatic case the electron
density on the agostic H atom slightly decreases (partial charge
of +0.241 on the agostic H atom vs +0.220 to +0.240 for the
other aromatic H atoms).23 In 12 and 12* the metal is bound
tightly to agostic H and is relatively remote from the agostic
carbon; in 7 the metal is situated close to both the C and H
atoms of the agostic bond. It is in accordance with the metal
trajectory in the agostic bonding derived by Crabtree et al.: the
stronger the agostic bond, the shorter an M-C bond is observed
and vice versa.21 Stronger agostic bonding in 7 may be a
consequence of various factors: involvement of the aromatic
ring π-system in the agostic C-H bonding;23 chelating ring size
differences; better polarizability of the sp2 vs sp3 hybridized
orbital. It appears that due to strong agostic bonding in 7, the
C-H bond becomes sufficiently acidic to exchange with water
Structure of the Agostic Complexes. An important feature
of both the aromatic and aliphatic PC-Rh systems is that the
agostic intermediates are consistently found in all the pathways
involving C-H reductive elimination/activation. Since they
appear to be the key intermediates in these processes, we present
here a comparison of their characteristics. Bond lengths and
(21) Crabtree, R. H.; Holt, E. M.; Lavin, M.; Morehouse, S. M. Inorg. Chem.
1985, 24, 1986.
(22) Natural population analysis (NPA) [ref 45].
(23) A slightly arenium character of 7 is demonstrated by the bond lengths and
bond orders alternation in the aromatic ring, although most of the positive
charge is still localized on the metal center, similarly to the system reported
in ref 8a.
9
11046 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

C−H Bond Activation by Rhodium(I)
A R T I C L E S
Table 2. Bond Angles (deg) Bond Lengths (Å) and Bond Orders of the Agostic Complexes in Aryl- and Benzyl-PC/Rh Systems at the
mPW1k/SDD Level of Theory
7
12
12*
bond angles
bond lengths
Rh-H1-C2
Rh-C2-H1
C7-C2-C3
C5-C6-C7
C7-C2-H1
C5-C6-H8
98.24
51.40
120.07
120.49
119.85
119.67
159.12
179.00
2.259
1.784
1.154
1.079
1.398
1.393
1.395
1.397
1.393
1.410
0.253
0.144
0.746
0.908
1.341
1.421
1.423
1.436
1.442
1.398
Rh-H1-C4
Rh-C4-H1
Rh-C4-C5
C6-C5-C10
C8-C7-C11
Rh-C4-H3
113.21
41.29
96.96
119.70
119.03
94.30
178.83
174.73
2.455
1.763
3.334
2.757
1.150
1.095
1.085
Rh-H3-C4
Rh-C4-H3
Rh-C4-C5
C6-C5-C10
C8-C7-C11
Rh-C4-H1
106.99
45.69
126.02
119.86
119.14
70.54
179.04
-178.27
2.398
1.794
2.280
3.071
1.094
C6-C7-C2-H1
C4-C5-C6-H8
Rh-C2
C9-C8-C7-C11
C7-C6-C5-C4
Rh-C4
C9-C8-C7-C11
C7-C6-C5-C4
Rh-C4
Rh-H1
Rh-H1
Rh-H3
C2-H1
Rh-H2
Rh-H1
C6-H8
Rh-H3
Rh-H2
C2-C3
C4-H1
C4-H1
C3-C4
C4-H2
C4-H3
1.151
1.086
C4-C5
C4-H3
C4-H2
C5-C6
C6-C7
C7-C2
bond orders
Rh-C2
Rh-C4
Rh-H1
Rh-H2
Rh-H3
C4-H1
C4-H2
C4-H3
0.179
0.128
0.014
0.003
0.788
0.873
0.92
Rh-C4
Rh-H3
Rh-H1
Rh-H2
C4-H1
C4-H3
C4-H2
0.184
0.127
0.021
0.009
0.887
0.783
0.904
Rh-H1
C2-H1
C6-H8
C2-C3
C3-C4
C4-C5
C5-C6
C6-C7
C7-C2
Thus, both agostic complexes 7 and 12 (12*) can lead to the
C-H bond oxidative addition, but only the arene C-H agostic
bonding in 7 results in high C-H bond acidification (see
Discussion).
Discussion
Control of C-H Oxidative Addition and Reductive
Elimination by η2-C,H Coordination: PC-Rh Systems.
The proposed two-step model24 describes the metal/C-H
oxidative addition as follows. The first step, coordination,
involves forward donation from a C-H bonding σ orbital to a
vacant metal-based orbital of appropriate symmetry, resulting
in the formation of an agostic bond (intramolecular case) or an
adduct (σ complex, intermolecular case). The second step,
insertion, involves back-donation from an occupied metal-based
orbital to a C-H antibonding σ*CH orbital, resulting in metal
alkyl hydride formation. Back-donation to σ*CH should be
significant in order to cause C-H oxidative addition, otherwise
agostic complexes25 or σ complexes9 are more stable than the
corresponding insertion products. Recent theoretical26 and
experimental1b-e studies extensively support this model.
In both aromatic and aliphatic PC-Rh systems described in
this work C-H oxidative addition takes place. The agostic η2-
C,H complexes in both cases are close in energy to the C-H
oxidative addition TS (Schemes 4 and 5, Figures 1 and 2). These
early TSs reflect that the intermediacy of agostic complexes
(24) Hoffmann, R.; Saillard, J. Y. J. Am. Chem. Soc. 1984, 106, 2006. See
also: Low, J. J.; Goddard, W. A. J. Am. Chem. Soc. 1986, 108, 6115.
(25) For reviews on agostic complexes, see: (a) Brookhart, M.; Green, M. L.
H.; Wong, L.-L. Prog. Inorg. Chem. 1988, 36, 1. (b) Crabtree, R. H. Angew.
Chem., Int. Ed. Engl. 1993, 32, 789.
Figure 3. . Optimized geometries of the agostic complexes: 7, 12 (exo),
and 12* (endo) as obtained at the mPW1k/SDD level of theory.
(26) (a) Niu, S.; Hall, M. B. Chem. ReV. 2000, 100, 353. See also: (b) Cundari,
T. J. Am. Chem. Soc. 1994, 116, 340. (c) Musaev, D. G.; Morokuma, K.
J. Am. Chem. Soc. 1995, 117, 799. (d) Margl, P.; Ziegler, T.; Bloechl, P.
E. J. Am. Chem. Soc. 1995, 117, 12625. (d) Siegbahn, P. E. M. J. Am.
Chem. Soc. 1996, 118, 1487.
and methanol in a way resembling a previously reported (PCP)-
Rh(CO)+ system.8a In the case of 12 the agostic C-H appears
to be less acidic and does not exchange with D2O and MeOD.
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11047

A R T I C L E S
Rybtchinski et al.
Table 3. Activation Parameters for C-H Reductive Elimination at
promotes C-H oxidative addition in both aliphatic and aromatic
case. This is despite the differences in η2-C,H bonding,
indicating that back-donation to the σ* orbital of η2-C,H bond
is sufficient for oxidative addition to occur in both aromatic
and aliphatic PC-Rh cases and the relatively low electron
density of the [Rh(PR3)(MeOH)2]+ fragment is adequate for
C-H oxidative addition. In the aromatic system the pattern of
agostic interactions and C-H oxidative addition is relatively
simple. We could not find a direct C-H activation pathway
starting from η2-C,C arene π-bound complex. In all cases the
aromatic C-H oxidative addition was found to proceed through
agostic intermediates. Interestingly, our computational results
indicate that the formation of η2-C,C π-complex is not essential
for the aromatic C-H oxidatiVe addition, similarly to several
reported cases of intermolecular6 or intramolecular7 C-H
activation. It should be noted that chelation in the PC-Rh
system can impose certain geometric restrictions on the possible
modes of metal-substrate interaction.7 However, the optimized
geometries of both η2-C,H and η2-C,C complexes, 7 and 8,
respectively, do not show significant strain and are almost
equivalent energetically. Importantly, when in computational
studies we attempted to detect a direct C-H activation pathway
starting from the η2-C,C π complex, agostic species preceding
C-H oxidative addition were consistently found. It was reported
that activation of vinylic C-H in ethylene bound to a Cp*Ir-
(PMe3) fragment proceeds not through an η2-C,C π-complex
intermediate but through a C-H σ complex.27 Aromatic C-H
oxidative addition may proceed not only through η2-C,C arene
π complex but also directly through a C-H σ complex.6 It has
been reported that an η2-C,C complex appears to be the most
energetically stable intermediate in the course of C-H oxidative
addition of benzene by a cationic Pt(II) complex; however, it
was proposed that this π complex rearranges into a η2-C,H one,
which is essential for C-H activation.4 It is likely that, in most
cases of arene C-H oxidative addition, η2-C,C π-bonding may
bring the arene into the vicinity of the metal, and the resulting
π complex leads to the η2-C,H bonding necessary for C-H
activation. It is also possible that C-H oxidative addition takes
place directly through a C-H σ complex, the π complex being
a resting state, as we found in the Rh-PC system. In any case,
η2-C,H seems to be essential for arene C-H oxidative addition.
the mPW1k/SDB-cc-pVDZ//mPW1k/SDD Level of Theory
q
q
q
∆G (kcal/mol)
∆H (kcal/mol)
∆S (eu)
benzyl-PC/Rh-endo
benzyl-PC/Rh-exo
aryl-PC/Rh
9.1
12.7
4.5
8.3
12.1
4.3
-2.5
-2.0
-0.7
should be considered rather a geometric predisposition29 of a
specific C-H to η2-C,H bonding, imposed by η2-C,H coordina-
tion of the other C-H bond. This weak interaction, however,
can control the C-H oxidative addition pattern. It suggests that
the overall influence of C-H bond coordination on C-H
oxidative addition may be more complex than expected.
As the C-H reductive elimination is concerned, both aryl-H
and benzyl-H reductive elimination processes are characterized
by comparable small negative activation entropies (Table 3),
whereas the enthalpies are different. Benzylic C-H reductive
elimination is expected to be more facile than arene C-H
reductive elimination (M-benzyl bond is weaker than M-aryl).
In addition, five-membered rings (aromatic PC-Rh) are at least
as stable as six-membered ones (aliphatic PC-Rh). It is
surprising, therefore, that in our PC systems there is an opposite
trend for aryl-H vs benzyl-H reductive elimination. Factors other
than M-C bond strength may be involved in controlling the
relative rates of the reductive elimination. One of the factors
may be the higher directionality of the sp3-hybridized benzylic
C-M bond vs sp2 aryl C-M bond. However, as the activation
entropies for the reductive elimination in both systems are
comparable, the directionality of the bonds does not appear to
play a significant role. We suggest the following explanation.
The products of the reductive elimination, i.e., the agostic
complexes, both in the aromatic and aliphatic cases, are very
close in energy to the transition state, which is a late transition
state regarding reductive elimination. Thus, formation of a strong
aryl C-H bond and better metal η2-C-H bonding in the aryl
agostic intermediate correlates with a lower energy TS and can
explain the lower activation enthalpy in the aromatic case.
Formation of stronger Rh-Caryl bond resulting in acidification
of the C-H proton, and its scrambling by the metal center can
also account for the observed lower activation barrier in the
aryl-PC system (vide infra).
η2-C,H Bond Coordination in Electrophilic vs Nucleophilic
Arene C-H Bond Activation: Agostic C-H Bond Acidity
and C-H Bond Oxidative Addition. It is generally recognized
that the coordination of a C-H bond to a metal can result in its
enhanced acidity due to the donation from the C-H σ orbital
to the metal. A number of recent examples indicate that arene
C-H agostic bonding may result in a C-H acidity high enough
to show exchange with water and deprotonation with amines.8
Thus, arene C-H σ complexes with late transition metals
demonstrate that such bonding may alter the properties of the
C-H bond to an extent resembling insertion of a heteroatom
into it. Interestingly, the agostic complex 7 can be viewed as
the one demonstrating both oxidative addition (nucleophilic
metal insertion into a C-H bond) and electrophilic (H/D
exchange due to the C-H bond acidity) reactivity patterns.
Analysis by Cundari of experimental and theoretical reaction
In the aliphatic PC-Rh system, the wide range of bonding
possibilities and the lack of symmetry result in a complex pattern
of agostic interactions. Thus, complex 2c demonstrates Various
types of C-H coordination occurring in one species. Moreover,
the agostic interactions control the reVersible C-H oxidatiVe
addition in the system, leading to energetically distinct C-H
actiVation processes, which are obserVed experimentally and
confirmed in the computational study. η2-C,H bonding was
suggested to play a critical role in alkane C-H bond activation.1
Regarding the simplest alkane molecule, methane, theoretical
calculations predict various modes of methane coordination
including η2-C,H, η2-H,H, and η3-H,H,H interactions9,26 and
methane rotation involving alternating η2-C,H bonds.28 It should
be noted that the involvement of an additional C-H bond in
the interaction with the metal center in the agostic intermediate
12* is weak, far from being a true η2-H,H or η3-H,C,H one. It
(29) For an example of a weak agostic bond as a result of geometric
predisposition, see: Cooper, A. C.; Clot, E.; Huffman, J. C.; Streib, W.
E.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999,
121, 97.
(27) Stoutland, P. O.; Bergman R. G. J. Am. Chem. Soc. 1988, 110, 5732.
(28) (a) Martin. R. L. J. Am. Chem. Soc. 1999, 121, 9459. (b) Iron, M. A.; Lo,
H. C.; Martin, J. M. L.; Keinan, E. J. Am. Chem. Soc. 2002, 124, 7041.
9
11048 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

C−H Bond Activation by Rhodium(I)
A R T I C L E S
and in 13C{1H} NMR the 13C signal of the deuterated solvents was
used as a reference. 31P NMR chemical shifts are reported in ppm
downfield from H3PO4 and referenced to an external 85% solution of
phosphoric acid in D2O. Saturation transfer experiments were performed
by selective saturation of the hydride resonance. The response of the
signal of the methylene protons was detected by comparison (difference
spectrum) with a control experiment, in which the signal free area was
irradiated under the same conditions. Abbreviations used in the
description of NMR data are as follows: Ar, aryl; br, broad; dist,
distorted; s, singlet; d, doublet; m, multiplet.
Electrospray (ES) mass spectrometry was performed at the Weiz-
mann Institute of Science using a MicroMass LCZ Detector 4000 with
CV ) 43 V, temperature ) 150 °C, and EE ) 4.2 V. Elemental analysis
was performed at Mikroanalytisches Laboratorium, 45470 Mu¨lheim,
Germany.
coordinates in oxidative addition reactions involving low-valent
metals complexes and in σ-bond metathesis reactions involving
high-valent ones revealed that there is a good degree of
similarity between C-H activation mechanisms in both cases,
early on in the reaction coordinate.26b Cationic metal centers
were shown to form strong η2 C-H σ bonds, due to the
enhanced forward electron donation from the C-H σ orbital to
the empty metal orbital.1 As a consequence, they are expected
to increase C-H bond acidity. When this electron donation is
high enough, an electrophilic mechanism may operate, i.e., H+
is expelled or abstracted by a base. In case of sufficient back-
donation from a metal to C-H σ* orbital, oxidative addition
takes place. Significantly, complex 4 demonstrates both types
of C-H activation occurring concurrently. Once a σ complex
is formed, both electrophilic and nucleophilic (oxidatiVe addi-
tion) mechanisms may be operating. The question of the inherent
nature of C-H activation remains a challenge to answer in some
systems.30 On the basis of our combined experimental and
computational study, it might be envisaged that both electro-
philic and oxidative addition mechanisms may be operating in
the C-H bond activation by cationic metal centers, depending
on the interplay between forward and back-donation upon C-H
bond coordination.
Preparation of Ligand 3. An acetone solution (75 mL) of tBu2PH
(4.42 g, 30.276 mmol) was added in a glovebox to an acetone solution
(75 mL) of PhCH2Br (4.134 g, 25.23 mmol). The reaction mixture was
refluxed under argon for 8 h, and then 150 mL of pentane was added,
resulting in precipitation. The solid was collected, washed with pentane,
and dissolved in 100 mL of degassed H2O. An aqueous solution (100
mL) of NaOAc (25 g) was added, and the resulting solution was
extracted with ether. The resulting colorless etheral solution was dried
over Na2SO4 and filtered, and the solvent was removed under vacuum.
The compound was further purified by distillation at 0.4 mm/Hg, bp
96-100 °C, resulting in 4.30 g (75% yield) of 1 as a viscous colorless
liquid.
Conclusions
1
31P{1H} NMR (CDCl3): 36.06 (s). H NMR (CDCl3): 7.35-7.30
Our combined experimental and theoretical study presents a
comparison of C-H bond activation/reductive elimination in
aliphatic and aromatic PC-Rh systems. The agostic interactions
in the aliphatic case create a complex reactivity pattern due to
multiple bonding possibilities. Upon formation of an arene
agostic C-H bond it becomes sufficiently acidic to promote
H/D exchange with water and alcohols. This was utilized in an
unprecedented catalytic process in which the vinylic hydrogens
of terminal olefins were selectively exchanged with deuterium
using D2O or MeOD as deuterium donors. The aromatic PC
system demonstrates both nucleophilic (oxidative addition) and
electrophilic (H/D exchange) type of C-H activation, while the
aliphatic PC-Rh system shows only nucleophilic C-H activa-
tion by oxidative addition, which is a function of the different
agostic interactions in the two systems. Thus, strong C-H bond
precoordination was shown to be critical for both electrophilic
and nucleophilic mechanisms of C-H activation in the same
system (aryl PC/Rh). It suggests that fine-tuning of the electron
density of the metal center and the reaction media can be utilized
for selective C-H activation by various mechanisms, as
exemplified in the reported selective deuteration of olefins.
(m, 2H, Ar-H), 7.25-7.19 (m, 2H, Ar-H), 7.13-7.07 (m, 1H, Ar-H),
2
3
2.83 (d, JPH ) 3.2 Hz, 2H, Ar-CH2-P), 1.12 (d, JPH ) 10.9 Hz,
18H, P-C(CH3)3. 13C{1H} NMR (CDCl3): 141.58 (d, JPC ) 10.2 Hz,
C
ipso, Ar), 129.50 (d, JPC ) 8.4 Hz, Ar), 128.18 (s, Ar), 125.24 (br s,
Ar), 31.75 (d, 1JPC ) 21.8 Hz, P-C(CH3)3), 29.71 (d, 2JPC ) 13.0 Hz,
1
P-C(CH3)3), 28.41 (d, JPC ) 23.2 Hz, Ar-CH2-P). The assignment
of the signals in 13C{1H} NMR is confirmed by 13C DEPT-135 NMR.
Reaction of [Rh(coe)2(solv)n]BF4 with Ligand 1. Formation of
the C-H Activation Products 4a-c. A THF (3 mL) solution of AgBF4
(24 mg, 0.124 mmol) was added to a THF solution (4 mL) of [Rh-
(coe)2Cl]2 (44 mg, 0.062 mmol). Immediate precipitation of AgCl took
place, which was removed by filtration after allowing the mixture to
stand at room temperature for 15 min, resulting in a clear orange
solution of [Rh(coe)2(THF)n]BF4. A THF (3 mL) solution of ligand 1
(30 mg, 0.128 mmol) was added to this solution, resulting in color
change to orange-red. The resulting mixture was kept at room
temperature for 1 h followed by solvent evaporation under vacuum.
The residue was washed with pentane and dissolved in 3 mL of MeOH,
resulting in colorless solution. After keeping the solution for 1 h at
room temperature, the solvent was evaporated under vacuum and the
residue was then redissolved in either THF, methanol, or acetone, to
yield quantitatively the corresponding solvento-complexes 4a-c.
Complex 4a. 31P{1H} NMR (acetone-d6): 111.35 (d, 1JRhP ) 171.8
Experimental Methods
1
Hz). H NMR (acetone-d6): 7.36 (br s, 1H, Ar-H), 7.01 (br d, JHH
)
4.3 Hz, 1H, Ar-H), 6.82 (br m, 1H, Ar-H), 6.77 (dist t, JHH ) 4.3 Hz,
General Methods. All experiments with metal complexes and the
phosphine ligand were carried out under an atmosphere of purified
nitrogen in an MBraun MB 150B-G glovebox. The complex [Rh-
(coe)2Cl]2 was prepared according to a literature procedure.31
1H, 13C, and 31P NMR spectra were recorded at 400, 100, and 162
MHz, respectively, at 295 K, using a Bruker Avance-400 NMR
spectrometer. 1H NMR and 13C{1H} NMR chemical shifts are reported
in ppm downfield from tetramethylsilane. 1H NMR chemical shifts are
referenced to the residual hydrogen signal of the deuterated solvents,
2
2
1H, Ar-H), 3.25 (br dd, JHH ) 15.4 Hz, JPH ) 10.2 Hz, 1H, Ar-
2
CH2-P, left part of AB system), 3.06 (br dd, JPH ) 10.0 Hz, 1H,
3
Ar-CH2-P, right part of AB system), 1.36 (d, JPH ) 13.6 Hz, 9H,
3
P-C(CH3)3), 1.27 (d, JPH ) 13.2 Hz, 9H, P-C(CH3)3), -22.41 (dd,
1JRhH ) 34.5 Hz, 2JPH ) 24.1 Hz, 1H, Rh-H). 13C{1H} NMR (acetone-
1
d6): 150.90 (br d, JRhC ) 32.7 Hz, Cipso-Rh), 149.58 (s, Ar), 149.48
(s, Ar), 135.02 (br s, Ar), 124.99 (s, Ar), 123.97 (br s, Ar), 36.21 (d,
1
1JPC ) 17.2 Hz, P-C(CH3)3), 35.72 (d, JPC ) 27.4 Hz, P-C(CH3)3),
1
33.55 (br d, JPC ) 33.7 Hz, Ar-CH2-P), P-C(CH3)3 signals
overlapped with the signals of the solvent.
(30) For two recent examples, see: (a) Zhong, H. A.; Labinger, J. A.; Bercaw,
J. E. J. Am. Chem. Soc. 2002, 124, 1378. (b) Tellers, D. M.; Yung, C. M.;
Arndtsen, B. A.; Adamson, D. R.; Bergman, R. G. J. Am. Chem. Soc. 2002,
124, 1400.
Complex 4b. 31P{1H} NMR (THF-d8): 109.82 (br d, 1JRhP ) 176.6
1
Hz). H NMR (THF-d8): 7.33 (br d, JHH ) 6.9 Hz, 1H, Ar-H), 6.92
(31) Herde, J. L.; Senoff, C. V. Inorg. Nucl. Chem. Lett. 1971, 7, 1029.
(br d, JHH ) 7.7 Hz, 1H, Ar-H), 6.81 (m, 2H, Ar-H), 3.08 (dd, 2JHH
)
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11049

A R T I C L E S
Rybtchinski et al.
16.6 Hz, 2JPH ) 9.8 Hz, 1H, Ar-CH2-P, left part of AB system), 3.06
(dd, JPH ) 9.6 Hz, 1H, Ar-CH2-P, right part of AB system), 1.37
set-relativistic effective core potential (RECP) combination35 on the
transition metals. The SDB-cc-pVDZ basis set combines the Dunning
cc-pVDZ basis set36 on the main group elements with the Stuttgart-
Dresden basis set-RECP combination35 on the transition metals, with
an f-type polarization exponent taken as the geometric average of the
two f-exponents given in the Appendix to ref 37.
Geometry optimizations for minima were carried out using the
standard Schlegel algorithm38 in redundant internal coordinates until
in the neighborhood of the solution and then continued using analytical
second derivatives.39 Optimizations for transition states were carried
out by means of the QST3 approach,40 with an initial guess for the
transition state being generated from manual manipulation of the
geometry using MOLDEN.41 In cases where this approach failed to
converge, we used analytical second derivatives at every step.
Zero-point and RRHO (rigid rotor-harmonic oscillator) thermal
corrections (to obtain ∆S, ∆H, and ∆G values) were obtained from
the unscaled computed frequencies.
Where necessary, the Grid ) UltraFine combination, i.e., a pruned
(99 590) grid in the integration and gradient steps and a pruned (50 194)
grid in the CPKS (coupled perturbed Kohn-Sham) steps, was used as
recommended in ref 42.
Zero-point and RRHO (rigid rotor-harmonic oscillator) thermal
corrections were obtained from the unscaled computed frequencies.
Where necessary to resolve ambiguities about the nature of a
transition state, intrinsic reaction coordinate (IRC43) calculations were
carried out. In some cases where IRC calculation failed for technical
reason, displacements were made along the normal coordinate for the
imaginary frequencies and optimizations started from there. While this
pseudo-IRC procedure is less unambiguous than IRC, it at least offers
some form of corroboration.
2
3
3
(d, JPH ) 13.7 Hz, 9H, P-C(CH3)3), 1.22 (d, JPH ) 13.3 Hz, 9H,
P-C(CH3)3), -24.07 (br s, 1H, Rh-H). 13C{1H} NMR (THF-d8):
150.23 (m, Cipso-Rh, Ar), 135.28 (s, Ar), 125.10 (s, Ar), 123.95 (s,
1
Ar), 123.71 (s, Ar), 123.55 (s, Ar), 36.29 (d, JPC ) 16.9 Hz,
P-C(CH3)3), 35.42 (d, 1JPC ) 26.9 Hz, P-C(CH3)3), 33.98 (br d, 1JPC
) 33.8 Hz, Ar-CH2-P), 30.14 (m, P-C(CH3)3), 29.44 (br s,
P-C(CH3)3).
1
Complex 4c. 31P{1H} NMR (methanol-d4): 109.82 (br d, JRhP
)
1
175.8 Hz). H NMR (methanol-d4): 7.43 (br d, JHH ) 7.3 Hz, 1H,
Ar-H), 6.98 (br d, JHH ) 7.2 Hz, 1H, Ar-H), 6.82 (m, 2H, Ar-H), 3.25
(ddd, 2JHH ) 16.7 Hz, 2JPH ) 10.2 Hz, 3JRhH ) 2.2 Hz, 1H, Ar-CH2-
2
P, left part of AB system), 3.06 (dd, JPH ) 9.9 Hz, 1H, Ar-CH2-P,
3
right part of AB system), 1.33 (d, JPH ) 13.6 Hz, 9H, P-C(CH3)3),
3
1
1.21 (d, JPH ) 13.0 Hz, 9H, P-C(CH3)3), -22.66 (dd, JRhH ) 33.2
Hz, 2JPH ) 26.5 Hz, 1H, Rh-H). 13C{1H} NMR (methanol-d4): 150.40
(br d, 1JRhC ) 36.8 Hz, Cipso-Rh), 150.25 (m, Ar), 135.41 (s, Ar), 135.16
1
(s, Ar), 125.35 (m, Ar), 124.02 (m, Ar), 36.23 (d, JPC ) 17.0 Hz,
P-C(CH3)3), 35.48 (d, 1JPC ) 27.6 Hz, P-C(CH3)3), 34.28 (br. d, 1JPC
) 34.1 Hz, Ar-CH2-P), 30.09 (m, P-C(CH3)3), 29.33 (br s,
P-C(CH3)3).
ES MS positive mode (m/e): calcd for C16H29OPRh (one MeOH
molecule remains coordinated) MH+, 371.28; found MH+, 371.12. ES
MS negative mode (m/e): calcd for BF4, 86.80; found, 86.91.
Anal. Calcd for C16H29OPRh (assuming one MeOH molecule remains
coordinated): C, 41.95; H, 6.38. Found: 41.93; 6.44.
Deuteration Experiments. In a typical experiment, complex 4 (15
mg, 0.040 mmol) was dissolved in a solvent of choice (CD3OD or
acetone/D2O (5:1 v/v) mixture). Styrene (about 70 equiv) was added
to the solution of 4, and the resulting mixture was placed in the screw-
cap NMR tube and was heated for 24 h at 65 °C. Toluene-d8 (20 µL)
was added to the reaction mixture as a standard, and the integrals of
the signals of vinylic deuterons in 2D NMR were compared to those of
tolune-d8. In all the cases exclusive vinylic deuteration was observed.
Formation of styrene-d3 was confirmed by GC-MS.
For interpretative purposes, atomic partial charges and Wiberg bond
indices44 were obtained by means of a natural population analysis
(NPA)45 at the mPW1k/SDD level.
The energetics for our final reaction profile were validated by single-
point energy calculations, using the mPW1k/SDD reference geometries,
at the higher level of theory mPW1k/SDB-cc-pVDZ.
Acknowledgment. This work was supported by the Israel
Science Foundation (Grant No 83/00), the MINERVA founda-
tion, Munich, Germany, the Tashtiyot program of the Ministry
of Science (Israel), the Israel Inter-University Computing Center,
and the Helen and Martin Kimmel Center for Molecular Design.
We thank Dr. Leonid Konstantinovsky for the help with NMR
measurements. D.M. holds the Israel Matz Professorial Chair
in Organic Chemistry. J.M.L.M. is a member of the Lise
Meitner-Minerva Center for Computational Quantum Chemistry.
Computational Methods
All calculations were carried out using the Gaussian 98 program
revision A.1132 running on Compaq ES40 and XP1000 workstations
as well as on a mini-farm of Pentium IV Xeon 1.7/2.0 GHz PC’s
running Red Hat Linux 7.2 in our group, on an experimental Linux
PC Farm at the Faculty of Physics, and on the (Israel) Inter-University
Computing Center (IUCC) SGI Origin 2000.
The mPW1k (modified Perdew-Wang 1-parameter for kinetics)
exchange-correlation functional of Truhlar and co-workers33 was
employed in conjunction with the SDD and SDB-cc-pVDZ basis sets
(see below). The mPW1k functional was very recently shown28b,33,34
to yield more reliable reaction barrier heights than other exchange-
correlation functionals.
Supporting Information Available: XYZ coordinates of all
computed structures and SST spectrum of complex 4b (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
The SDD basis set is the combination of the Huzinage-Dunning
double-ú basis set on lighter elements with the Stuttgart-Dresden basis
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R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
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