Carbene-metal hydrides can be much less acidic than phosphine-metal hydrides: Significance in hydrogenations

Article abstract of DOI:10.1021/ja101233g

Acidities of iridium hydride intermediates were shown to be critical in some transformations mediated by the chiral analogues of Crabtree's catalyst, 1-3. To do this, several experiments were undertaken to investigate the acidities of hydrogenation mixtures formed using these iridium-oxazoline complexes. DFT calculations indicated that the acidity difference for Ir-H intermediates in these hydrogenations were astounding; iridium hydride from the N-heterocyclic carbene catalyst 1 was calculated to be around seven pK _a units less acidic than those from the P-based complexes 2 and 3. Consistent with this, the carbene complex 1 was shown to be more effective for hydrogenations of acid-sensitive substrates. In deuteration experiments, less "abnormal" deuteration was observed, corresponding to fewer complications from acid-mediated alkene isomerization preceding the D ₂-addition step. Finally, simple tests with pH indicators provided visual evidence that phosphine-based catalyst precursors give significantly more acidic reaction mixtures than the corresponding N-heterocyclic carbene ones. These observations indicate carbene-for-phosphine (and similar) ligand substitutions may impact the outcome of catalytic reactions by modifying the acidities of the metal hydrides formed.

Full text of DOI:10.1021/ja101233g

Published on Web 04/08/2010
Carbene-Metal Hydrides Can Be Much Less Acidic Than
Phosphine-Metal Hydrides: Significance in Hydrogenations
Ye Zhu, Yubo Fan, and Kevin Burgess*
Department of Chemistry, Texas A & M UniVersity, Box 30012, College Station, Texas 77841
Received February 10, 2010; E-mail: burgess@tamu.edu
Abstract: Acidities of iridium hydride intermediates were shown to be critical in some transformations
mediated by the chiral analogues of Crabtree’s catalyst, 1-3. To do this, several experiments were
undertaken to investigate the acidities of hydrogenation mixtures formed using these iridium-oxazoline
complexes. DFT calculations indicated that the acidity difference for Ir-H intermediates in these
hydrogenations were astounding; iridium hydride from the N-heterocyclic carbene catalyst 1 was calculated
to be around seven pKa units less acidic than those from the P-based complexes 2 and 3. Consistent with
this, the carbene complex 1 was shown to be more effective for hydrogenations of acid-sensitive substrates.
In deuteration experiments, less “abnormal” deuteration was observed, corresponding to fewer complications
from acid-mediated alkene isomerization preceding the D2-addition step. Finally, simple tests with pH
indicators provided visual evidence that phosphine-based catalyst precursors give significantly more acidic
reaction mixtures than the corresponding N-heterocyclic carbene ones. These observations indicate carbene-
for-phosphine (and similar) ligand substitutions may impact the outcome of catalytic reactions by modifying
the acidities of the metal hydrides formed.
Introduction
Despite the considerations outlined above, there is a
tendency to regard homogeneous catalytic alkene hydrogena-
Transition-metal hydrides are ubiquitous in catalysis.1 They
are frequently formed as intermediates even in cases where the
catalyst precursor does not contain a M-H bond, and they may
also be formed en route to catalyst deactivation. The reactivity
of transition-metal hydrides is widely variable. Different
coordination modes are open to hydride (and dihydrogen)
ligands, they may dissociate to donate hydrides or protons, and
they can participate in hydrogen bonding.2,3 In some cases the
importance of metal hydrides in catalysis is obvious. In most
Heck couplings,4 for instance, transtion-metal hydrides reduc-
tively eliminate acidic molecules, hence it is necessary to add
a base to avoid complications that would be caused by
accumulation of acid. In other cases, basic additives have equally
profound effects, but the underlying reasons may be obscure.
For instance, some asymmetric homogeneous hydrogenation
reactions give significantly better enantioselectivities in the
presence of base for nonevident5 or undefined6 reasons. Acidities
of transition-metal hydrides,7 therefore, must also account for
some obscure ligand effects;8 in other words, ligands complexed
to metal centers influence the acidities of transition-metal
hydrides, and this in turn can impact catalysis.
tions as pH neutral because the starting materials, dihydrogen
and alkene, are not acidic. Some hydrogenations may be
described as “ionic”9-12 because heterolytic cleavage of
hydrogen occurs; however, to the best of our knowledge, no
one has commented on how activation of hydrogen with
transition-metal catalysts impacts the pH of the medium in
catalytic hydrogenations.
This paper demonstrates that catalytic hydrogenation reac-
tions need not be intrinsically neutral and that this can be
significant to the products formed. Specifically, we draw
attention to iridium-mediated hydrogenations of alkenes by
analogues of Crabtree’s catalyst [(COD)Ir(PCy3)(py)]+ as a
situation in which significant concentrations of protons may be
generated. Further, the outcome of these hydrogenation reactions
can be affected by the acidities of catalytic intermediates.
Results and Discussion
(1) Dedieu, A. Transition Metal Hydrides: Recent AdVances in Theory
and Experiment; VCH: New York, 1991.
(2) McGrady, G. S.; Guilera, G. Chem. Soc. ReV. 2003, 32, 383–392.
(3) Crabtree, R. H. Science 1998, 282, 2000–2001.
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New York, 1985.
(5) Sandoval, C. A.; Ohkuma, T.; Muniz, K.; Noyori, R. J. Am. Chem.
Soc. 2003, 125, 13490–503.
Direct experimental evidence for the mechanism of hydro-
genations by Crabtree’s catalyst analogues is unavailable and
difficult to obtain via kinetic or spectroscopic methods.13-15
However, calculations first by Andersson16 on a simplified
system with a N,P-ligand set, then from our laboratories15,17
(6) Takahashi, H.; Achiwa, K. Chem. Lett. 1987, 1921–1922.
(7) Qi, X.-J.; Liu, L.; Fu, Y.; Guo, Q.-X. Organometallics 2006, 25, 5879–
5886.
(8) Kollar, K.; Sandor, P.; Szalontai, G.; Heil, B. J. Organomet. Chem.
1990, 393, 153–158.
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A R T I C L E S
Zhu et al.
Figure 1. Postulated turnover-limiting step in hydrogenations with Crab-
tree’s catalyst analogues.
on the carbene-oxazoline complex 1 converge on very similar
preferred reaction pathways. This involves loss of the COD
ligand and complexation with the alkene substrate and two
molecules of dihydrogen. The rate-limiting step in the catalytic
cycle is transformation of this tetrahydride A into the σ-alkyl
species B (Figure 1).
Figure 2. Postulate for acidities of (a) the P-Ir-H complexes relative to
(b) carbene-Ir-H systems.
This mechanism involves alternation between iridium(III) and
-(V) oxidation states. It indicates why Crabtree’s catalyst
analogues are able to hydrogenate “coordinatively unfunction-
alized” tri- and tetrasubstituted alkenes, whereas rhodium
complexes such as Wilkinson’s catalyst do not. Specifically,
the positively charged and higher oxidation state Ir complexes
are more electrophilic than neutral and lower oxidation state
Rh complexes, so they have more affinity to the π-electron
density of alkene substrates. Moreover, the small steric demands
of a tetrahydride ligand set facilitate coordination of alkenes
with three or four substituents that are intrinsically hindered.
Finally, hydrogenations with these Ir complexes are not
particularly sensitive to oxygen, as expected for high oxidation
state Ir intermediates. Involvement of intermediates A also
explains why the catalytic cycle is so hard to follow spectro-
scopically because these tetrahydrides are in a rapid, dynamic
equilibrium with dihydrido-dihydrogen complexes.15,17
Figure 2 contrasts the factors driving dissociation of protons
from the iridium(V) intermediates A (shown in abbreviated
form) where the ligating group X is either a phosphine or an
N-heterocyclic carbene intermediate. In both cases, an irid-
ium(III) species forms; that is, the metal is reduced as its electron
density increases. We postulate that this dissociation is easier
when X is a P-ligand than a carbene because P-ligands are (i)
inferior σ-donors, hence are less able to stabilize Ir(V),18 and
(ii) superior π-acceptors, thus better able to stabilize Ir(III).
Consequently, Crabtree’s catalyst and P-ligated derivatives
should be more acidic than the corresponding carbenes.
Here we used density functional theory (TPSS functional;19
see Supporting Information) to calculate acidity differences for
the key metal hydride complexes involved in hydrogenations
Table 1. pKa Values for Transition Metal Hydrides
pKa
MeCN
calca
CH2Cl2
complex
exptl
∆pKab
∆pKab
1
2
3
4
5
6
7
8
9
[HNi(dmpe)2]+
24.4 ( 0.2
31.1
14.7 ( 0.3
22.2
21.7
32.3
13.8
24.6
11.3
9.8
11.5
17.4
36.1
11.9
22.5
4.0
14.8
1.5
0
1.7
7.6
26.3
13.4
24.0
5.5
16.3
1.4
0
1.8
7.4
30.0
[HPt(dmpe)2]+
[HNi(dppe)2]+
[HPt(dppe)2]+
C
D
E
F
G
a Experimental pKa of Ni and Pt complexes as references, and all
have a standard deviation of (2.2. b Calculated relative pKa with the
catalyst intermediate D as reference.
with Crabtree’s catalyst analogues. These data were then
compared to reference systems to estimate absolute pKa values.20
Almost all reliable experimental pKa’s for metal hydrides in
the literature are measured in acetonitrile, so the calculated pKa’s
were first obtained by simulating reactions in this medium.
In validation work, calculations were performed for several
metal hydrides for which pKa’s have been measured in MeCN
(Table 1, entries 1-4). The calculated pKa differences were then
related to absolute pKa values using the literature experimental
data for each of the other three metal hydride controls, then
averaged to give the data shown in Table 1, entries 1-4. These
numbers are consistent with the experimental data with a
maximum deviation of 2.7 for entry l, which concerns a complex
of pKa 21.7. This close agreement for the control complexes
indicates a high degree of confidence for application of the same
technique to calculate acidities for the key intermediates in
hydrogenations by Crabtree’s catalyst analogues.
Calculated data for the hydrogenation intermediates are shown
in Table 1, entries 5-9. Considering first the data in MeCN,
Crabtree’s catalyst C was calculated to be less acidic than the
P-oxazoline complex D, so the latter was used as a “bottom-
line reference”. The N,P-ligands in D and E are alike; hence
these complexes would be expected to have similar acidities,
and the calculations are consistent with this. Moreover, higher
acidity for the diphenylphosphinite complex D relative to E was
(9) Magee, M. P.; Norton, J. R. J. Am. Chem. Soc. 2001, 123, 1778–
1779.
(10) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.; Zhu, G. J. Am.
Chem. Soc. 2005, 127, 7805–7814.
(11) Noyori, R.; Ohkuma, T. Pure Appl. Chem. 1999, 71, 1493–1501.
(12) Noyori, R. Acta Chem. Scand. 1996, 50, 380–390.
(13) Mazet, C.; Smidt, S. P.; Meuwly, M.; Pfaltz, A. J. Am. Chem. Soc.
2004, 126, 14176–14181.
(14) Dietiker, R.; Chen, P. Angew. Chem., Int. Ed. 2004, 43, 5513–5516.
(15) Cui, X.; Fan, Y.; Hall, M. B.; Burgess, K. Chem.sEur. J. 2005, 11,
6859–6868.
(16) Brandt, P.; Hedberg, C.; Andersson, P. Chem.sEur. J. 2003, 9, 339–
347.
(17) Fan, Y.; Cui, X.; Burgess, K.; Hall, M. B. J. Am. Chem. Soc. 2004,
126, 16688–16689.
(18) Chianese, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H.
Organometallics 2003, 22, 1663–7.
(19) Tao, J. M.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys.
ReV. Lett. 2003, 91, 146401–146404.
(20) Qi, X.-J.; Fu, Y.; Liu, L.; Guo, Q.-X. Organometallics 2007, 26, 4197–
4203.
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Acidities of Carbene-Metal Hydrides in Hydrogenations.
A R T I C L E S
Figure 3. Relative acidities of putative intermediates in hydrogenations.
Figure 4. Hydrogenation of the acid-sensitive enol ether 4 with complexes
1-3 gives progressively less of the anticipated ether product 5.
also predicted because the diphenylphosphine in E is a superior
σ-donor, better able to stabilize Ir(V); again, the calculations
support this (Figure 3).
reaction gave only the expected hydrogenation products when
our N,carbene-iridium catalyst 1 was used.23
In the present work we expanded the scope of our studies to
include catalyst precursors 1-3 in hydrogenation of the acid-
sensitive enol ether 4. We predicted these catalyst precursors
would give progressively more acidic intermediates in hydro-
genations. This assertion was based on the assumption that the
carbene ligand is a better σ-donor than either of the P-ligands
and, of those, the PCy2 system has superior σ-donating
properties.18,24-26 The degree of back-bonding to these ligands
is likely to be minor and to follow the opposite trend.27-29
The most important comparison in Table 1 is between the
data for the N,P-complexes D/E and the corresponding carbene
F. Hydrides D and E were calculated to be 7.6 to 5.9 pKa units
more acidic than the carbene intermediate F, as predicted from
the concepts outlined in Figure 2. A 7.6 pKa difference is similar
t
to the acidity difference between formic acid and BuOH.
Finally, it is interesting to compare intermediates from
Crabtree’s and Wilkinson’s catalysts. The hydrogenation inter-
mediate G from Wilkinson’s catalyst was far less acidic than
that from any of the iridium complexes. It was 26.3 pKa units
less acidic than D and 24.8 pKa units less acidic than the
tetrahydride C from Crabtree’s catalyst.
All the calculations discussed so far are for acetonitrile
solvent, but hydrogenation reactions with Crabtree’s catalyst
analogues are generally run in dichloromethane. No pKa data
for useful control metal hydrides in dichloromethane have been
published; hence calculated pKa values in that medium cannot
be related to absolute values. However, pKa differences between
the complexes are accessible, and these data are shown in the
right-hand column of Table 1, relative to the intermediate that
was calculated to be the most acidic, i.e., the diphenylphos-
phinite oxazoline D. The calculated acidity differences for
complexes C-G in dichloromethane (right-hand column) are
very similar to those for acetonitrile (first column). This indicates
that the differences in pKa values for the complexes are
preserved, no matter how the absolute pKa values vary between
these two solvents.
1
Figure 4 shows H NMR spectra of crude materials isolated
from these reactions. The desired product 5, and almost nothing
else, was formed when the carbene complex 1 was used. The
dicyclohexylphosphinite complex 2 gave much less of the de-
sired ether product 5 and relatively more byproducts. At the
other extreme, the diphenylphosphinite complex 3 gave less than
5% of 5 (i.e., almost none by NMR) and mostly undesired
impurities.
The byproducts shown in Figure 4 could not be conveniently
isolated and characterized. However, hydrogenation of the enol
ether 6 gives mainly two products: 7, from direct addition of
hydrogen, and 8 via acid-mediated rearrangement and hydro-
(21) Cheruku, P.; Gohil, S.; Andersson, P. G. Org. Lett. 2007, 9, 1659–
1661.
(22) Cheruku, P.; Diesen, J.; Andersson, P. G. J. Am. Chem. Soc. 2008,
130, 5595–5599.
(23) Zhu, Y.; Burgess, K. AdV. Synth. Catal. 2008, 350, 979–983.
(24) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics
1999, 18, 2370–5.
It was a study on enol ether substrates that originally led us
to suspect that more protons were produced when N,P-Ir-catalyst
precursors were used relative to the corresponding carbene
catalyst. Specifically, Andersson et al. had noted21,22 that alkyl
enol ethers gave complex mixtures when hydrogenated using
one of their N,P-iridium catalysts, but we observed the same
(25) Tolman, C. A. Chem. ReV. 1977, 77, 313–348.
(26) Viciano, M.; Mas-Marza, E.; Sanau, M.; Peris, E. Organometallics
2006, 25, 3063–3069.
(27) Tonner, R.; Heydenrych, G.; Frenking, G. Chem.-Asian J. 2007, 2,
1555–1567.
(28) Lee, M.-T.; Hu, C.-H. Organometallics 2004, 23, 976–983.
(29) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309.
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A R T I C L E S
Zhu et al.
Scheme 1. Rationale for the Deuterium Scrambling
Table 2. Deuteration of Styrene Derivativesa
genation.23 This reaction gave the direct hydrogenation product
7 only if a base, e.g., K2CO3, was added.23 Complete conversion
and almost quantitative yields were achieved using all three
catalyst precursors 1-3 in these reactions. Product ratios
obtained (1H NMR) when 6 was hydrogenated using catalysts
1-3 are shown below. More of the acid-derived byproduct 8
was formed from the dicyclohexylphosphinite complex 2 than
from the carbene, and most accumulated when the diphe-
nylphosphinite complex 3 was used as the catalyst precursor.
Thus the amount of direct hydrogenation product follows the
order 1 > 2 > 3; that is, the amount of 7 decreases as the
anticipated acidities of the predicted intermediates increase.
A second set of evidence for the acidity effects comes from
deuterium labeling studies. Several years ago we observed
deuteration of alkenes catalyzed by chiral analogues of
Crabtree’s catalyst gave some label incorporation at sites
other than those corresponding to direct addition.30 In the
light of the data presented above, we postulated that abnormal
D-incorporation arises via addition of D+ to the alkene, loss
of H+ to give isomerism, and then addition of D2 (Scheme
1). Table 2 shows experiments designed to test this hypothesis
by using complexes 1 and 3. According to this hypothesis, 3
should generate more protons in the hydrogenation reactions
and give more “abnormal” deuteration. Comparing entries 1
with 2, and 4 with 5, for substrate 9 indicates levels of
abnormal deuterium incorporation (in red) were greater
without K2CO3 (indicating acidic conditions favor abnormal
D-incorporation) and that the N,carbene-catalyst 1 (entries
1 and 2) gave significantly less abnormal incorporation than
the N,P-catalyst 3 (entries 4 and 5). This is consistent with
less generation of acid from the hydrogenation reactions
involving the carbene 1 relative to the N,P-complex 3.
The same trends observed for deuteration of substrate 9 were
seen for 10 except that addition of K2CO3 did not significantly
change the levels of abnormal deuterium incorporation. How-
ever, the levels were decreased when a stronger base, Proton
Sponge 11, was added to the deuteration mediated by catalyst
3 (compare entries 4-6). Conversely, when the acid 12 was
added instead, the levels of abnormal deuteration increased
(compare entries 1-3).
a Relative to the maximum, 1.00, with the results of indirect
addition shown in red, determined by 2H NMR of the crude
deuteration products.
sample, (ii) after the reaction was completed (4 h). Control
experiments demonstrate methyl red gives a red coloration under
acidic conditions in this environment (frame e). Hydrogenations
using the carbene catalyst 1 (frames c and d) and the N,P-system
3 (frames g and h) show the latter is clearly more acidic. Frames
c and d suggest acidic intermediates formed after only 5 min in
the reaction.
We suspect that proton concentrations in hydrogenation
reactions are particularly important for tetrasubstituted alkene
substrates such as 14.31 High levels of abnormal deuteration
were observed for the catalyst precursors that are most likely
to generate acid, i.e., 2 and 3 (Figure 6). Further, carbene 1 did
not hydrogenate these same tetrasubstituted alkenes. This could
Perhaps the most dramatic illustration of pH in the hydro-
genation reactions came from simple experiments using substrate
13 and the acid-base indicator “methyl red” (orange under basic
conditions and red in acid) (Figure 5). Methyl red was added
(i) 5 min after the hydrogenation reaction began and, to another
(30) Perry, M. C.; Cui, X.; Powell, M. T.; Hou, D.-R.; Reibenspies, J. H.;
(31) Schrems, M. G.; Neumann, E.; Pfaltz, A. Angew. Chem., Int. Ed. 2007,
46, 8274–8276.
Burgess, K. J. Am. Chem. Soc. 2003, 125, 113–123.
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Acidities of Carbene-Metal Hydrides in Hydrogenations.
A R T I C L E S
Figure 6. Abnormal deuterium distributions for the tetrasubstituted alkene
14 were less for the catalyst precursor 2 than with 3, whereas minimal
conversion was obtained when the carbene 1 was used.
generate protons than the N,P-systems 2 and 3 (in that order).
We also have shown that the acidities of catalytic intermedi-
ates in hydrogenation reactions may impact the product
distributions for acid-sensitive substrates, like enol ethers.
For some other alkenes, acid-mediated isomerization might
compete with direct addition of hydrogen in ways that is not
apparent until deuterium labeling is used. Asymmetric
hydrogenations of tetrasubstituted alkenes, for instance, are
especially vulnerable to this complication because the direct
addition of hydrogen is relatively slow due to steric effects,
while protonations are fast because they give carbocations
that are both benzylic and trisubstituted. It will be interesting
to see how acidities of M-H bonds in hydrogenations can
impact other acid-sensitive substrates. Acidities of metal
hydride intermediates are important in many transition-metal-
catalyzed reactions; in some cases, dramatically altered
catalytic behavior on phosphine-to-carbene ligand substitution
will be indicative of this.
Figure 5. Hydrogenation with catalyst 1: (a) Methyl red with Et3N in
CH2Cl2 (control for base); (b) without indicator (after 4 h); (c) with methyl
red added after 5 min; and (d) with methyl red added after 4 h.
Hydrogenation with catalyst 3: (e) Methyl red with acid 12 in CH2Cl2
(control for acid); (f) without indicator (after 4 h); (g) with methyl red added
after 5 min; and (h) with methyl red added after 4 h.
Acknowledgment. We thank The National Science Foundation
(CHE-0750193) and The Robert A. Welch Foundation for the finan-
cial support for this work. The supercomputer facilities, BRAZOS
and HYDRA, were provided by Texas A&M University. The SGI
Altix 3-node cluster, MEDUSA, at the Department of Chemistry
was supported by the NSF (NSF-DMS 0216275).
be because of steric factors. However, if acid was generated
and the alkene isomerized to a 1,1-disubstituted form, then
complex 1 should reduce this material. Consequently, it is
possible that the carbene complex 1 simply generates less acid
than 2 and 3; hence the reaction does not proceed in the former
case.
Supporting Information Available: Details of the calculation
information and experimental procedures for the preparation of
compounds 1 to 14, details of the deuterium labeling experi-
Conclusions
The data presented here show that hydrogenations of
alkenes using chiral analogues of Crabtree’s catalyst can be
sensitive to protons generated by the catalytic intermediates.
Further, the carbene catalyst precursor 1 is less inclined to
2
ments and H NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA101233G
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