Kinetic and thermodynamic preferences for the diastereoselective oxidative addition of H2 to trans-Ir(P*R3)2(CO)Cl: Monodentate chiral phosphines may impart exceptional degrees of diastereoselectivity

Article abstract of DOI:10.1021/ja026059i

Studies on square planar iridium complexes of the type trans-Ir(P*R₃)₂(CO)Cl, where P*R₃ is PhP[(C₅Me₄)]₂, PhP[Me₂C₄H₆], or PhP[Prⁱ₂C₄H₆], demonstrate that monodentate chiral phosphines impart exceptional degrees of diastereoselectivity in the oxidative addition of H₂. Thus, the oxidative addition of H₂ to the two faces of the meso isomer (R,S)-trans-Ir(P*R₃)₂(CO)Cl proceeds with a kinetic diastereoselectivity which exceeds that for related square planar iridium complexes employing bidentate chiral phosphine ligands. Furthermore, the kinetically favored dihydride is not favored thermodynamically, and the magnitude of the inversion of the kinetic and thermodynamic selectivities is greater than has previously been observed using bidentate phosphines. Copyright

Full text of DOI:10.1021/ja026059i

Published on Web 06/07/2002
Kinetic and Thermodynamic Preferences for the Diastereoselective Oxidative
Addition of H2 to trans-Ir(P*R3)2(CO)Cl: Monodentate Chiral Phosphines May
Impart Exceptional Degrees of Diastereoselectivity
Jun Ho Shin and Gerard Parkin*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
Received February 28, 2002; Revised Manuscript Received April 24, 2002
Oxidative addition of dihydrogen is a crucial step in transition-
metal-catalyzed reactions involving H2. Olefin hydrogenation is one
such reaction, and the asymmetric version catalyzed by chiral
rhodium-phosphine complexes has experienced considerable com-
mercial impact, as exemplified by Monsanto’s synthesis of L-
DOPA.1,2 A particularly interesting feature of these asymmetric
rhodium-catalyzed reactions is that the selectivity is counter to the
preference of the catalyst to bind to a particular face of the prochiral
olefin,1,3,4 such that the reactions have been proposed to occur by
an “anti-lock-and-key” mechanism.4b Oxidative addition of H2 is
generally recognized to be the rate-determining and enantiomer-
determining step in these reactions,4 and the selectivity is therefore
dictated by the dramatically enhanced reactivity of H2 towards the
minor component olefin adduct. To account for the overall
selectivity, the susceptibility of the diastereomeric adducts towards
oxidative addition of H2 has been estimated to differ by a substantial
Figure 1. Kinetics plots for oxidative addition of H2 to trans-Ir(P*R3)2-
(CO)Cl and reductive elimination from trans-Ir(P*R3)2(CO)ClH2 (P*R3 )
PhP[(C5Me4)]2).
factor of ca. 600 in rate constant.4a However, well-defined examples
of oxidative addition of H2 that exhibit selectivity of this magnitude
are unprecedented. In this paper, we provide the first report that
chiral monophosphine ligands are capable of imparting a high
Scheme 1
degree of diastereoselectivity in the oxidative addition of H2 to a
metal center and that the selectivity exceeds that for certain bidentate
phosphine ligands.
The reaction of H2 with Vaska’s complex, trans-Ir(PPh3)2(CO)-
Cl, is the classic example of oxidative addition. Vaska-type
complexes, therefore, provide an excellent system to determine the
ability of chiral phosphine ligands to impart diastereoselectivity in
the oxidative addition of H2. For this purpose, we have employed
the monodentate chiral phosphines PhP[(C5Me4)]2,5 PhP[Me2C4H6],6
and PhP[Pri2C4H6]7 (Scheme 1).
Trans-Ir(P*R3)2(CO)Cl derived from racemic P*R3 consists of
consideration, as illustrated in Figure 1.11 Of most interest, there is
a significant difference in the barrier for oxidative addition of H2
to the two faces of the meso isomer, (R,S)-trans-Ir(P*R3)2(CO)Cl.
For example, the rate constant for oxidative addition of H2 to one
face of (R,S)-trans-Ir(P*R3)2(CO)Cl (P*R3 ) PhP[Pri2C4H6]) is
a factor of at least ∼60 greater than that to the other face.12
However, despite the extreme kinetic selectivity, the kinetically
favored product is not the thermodynamic product, and the kinetic
product transforms to a 1:3 equilibrium mixture with the thermo-
dynamic product over a period of days (as illustrated in Figure 1
for PhP[(C5Me4)]2).13 The kinetics of reductive elimination of
H2 from the (R,r,S)- and (R,s,S)-trans-Ir(P*R3)2(CO)ClH2 diaster-
eomers exhibit even greater differences than those for the oxida-
tive addition (Figure 2). Thus, the rate constants for reductive
elimination of H2 from the R,r,S and R,s,S diastereomers of trans-
Ir(P*R3)2(CO)ClH2 (P*R3 ) PhP[Pri2C4H6]) differ by a factor of
∼170.11,14
a pair of R,S and R,R/S,S diastereomers,8 and oxidative addition of
H2 to this mixture yields three diastereomers (one of which exists
as an enantiomeric pair). The formation of three diastereomers from
a mixture composed of two diastereomers is a consequence of
oxidative addition of H2 to the meso isomer, (R,S)-trans-Ir(P*R3)2-
(CO)Cl, resulting in a structure in which the iridium is a
“pseudoasymmetric center”.9 The term “pseudoasymmetric center”
is used to describe a stereogenic center in an achiral molecule, and
is given the notation r or s. Thus, the two diastereomers derived
from addition of H2 to (R,S)-trans-Ir(P*R3)2(CO)Cl may be
classified as R,r,S and R,s,S, differing only in the configuration at
iridium (Scheme 1).10
Significantly, the barriers to both oxidative addition and reductive
elimination are highly dependent upon the diastereomer under
* To whom correspondence should be addressed. E-mail: parkin@
chem.columbia.edu.
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J. AM. CHEM. SOC. 2002, 124, 7652-7653
10.1021/ja026059i CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We thank the U. S. Department of Energy,
Office of Basic Energy Sciences (DE-FG02-93ER14339) for
support of this research, and Nicholas Johnson (Chirotech Technol-
ogy Ltd.) for a gift of (3R,6R)- and (3S,6S)-(2,7-dimethyloctane-
3,6-diol) used in the preparation of PhP[Pri2C4H6].
Supporting Information Available: Experimental details (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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Figure 2. Free energy surface for addition of H2 to the two faces of (R,S)-
trans-Ir(P*R3)2(CO)Cl (P*R3 ) PhP[Pri2C4H6]). Values in kcal mol-1 at
300 K and the r/s configuration is arbitrary.
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The extremely high kinetic facial selectivity for oxidative addition
of H2 observed here contrasts markedly with the selectivity that
has been observed previously for related square planar Vaska-type
complexes that employ bidentate chiral phosphine ligands. For
example, oxidative addition of H2 to cis-Ir(chiraphos)(CO)Br yields
a 2.1:1 mixture of diastereomers,15 while the corresponding reaction
of [(S)-binap]Ir(PPh3)Cl gives a 1:1 mixture of diastereomers.16
Thus, chiral monophosphine ligands are capable of achieving a
kinetic diastereoselectivity which greatly exceeds that of certain
bidentate phosphine ligands.
In light of previous studies on iridium complexes, it is also
significant that the kinetic and thermodynamic products of oxidative
addition of H2 to (R,S)-trans-Ir(P*R3)2(CO)Cl are different. For
example, Eisenberg has noted that the kinetic and thermodynamic
products for oxidative addition of H2 to the two faces of cis-Ir-
(chiraphos)(CO)Br are the same.15,17 Furthermore, Landis has also
observed that the kinetic and thermodynamic diastereomers for
oxidative addition of H2 to a series of [Ir(bisphosphine)(1,5-COD)]+
complexes to give [Ir(bisphosphine)(1,5-COD)H2]+ are generally
the same; only for [Ir(chiraphos)(1,5-COD)]+ are the kinetic (1.8:
1) and thermodynamic (1:6.1) selectivities of H2 addition inverted.18
The kinetic diastereoselectivity for the monodentate phosphine
systems reported here (∼60:1 for P*R3 ) PhP[Pri2C4H6]), thus
greatly rivals the one other system, namely [Ir(chiraphos)(1,5-
COD)]+ (1.8:1), for which the kinetic and thermodynamic products
of oxidative addition are different.19 The favorable kinetic dis-
crimination that (R,S)-trans-Ir(P*R3)2(CO)Cl exhibits towards a
molecule as small as H2 thus bodes well for further applications of
chiral monodentate phosphine ligands in asymmetric catalysis, an
area that has been neglected by comparison to the use of
multidentate phosphine ligands, until recently.20
In conclusion, the oxidative addition of H2 to the two faces of
the meso isomer (R,S)-trans-Ir(P*R3)2(CO)Cl proceeds with a high
degree of kinetic diastereoselectivity, thus demonstrating that
monodentate phosphine ligands are capable of providing an effective
kinetic discrimination for a substrate as small as H2. However, (i)
the kinetically favored dihydride complex is not favored thermo-
dynamically, and (ii) the kinetic discrimination is significantly
greater than the thermodynamic discrimination. The magnitude of
the inversion of the kinetic and thermodynamic selectivities is
greater than has previously been experimentally observed for other
iridium complexes of bidentate phosphine ligands. Considering the
small size of the H2 reactant, these observations underscore the
importance of not assuming that the energies of diastereomeric
intermediates reflect the energies of related diastereomeric transition
states in asymmetric transformations.
(5) Shin, J. H.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Inorg. Chem.
2001, 40, 5626-5635.
(6) Burk, M. J.; Feaster, J. E.; Harlow, R. L. Organometallics 1990, 9, 2653-
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(7) Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow, R. L. J. Am. Chem.
Soc. 1993, 115, 10125-10138.
(8) For brevity, we use R and S nomenclature to describe the enantiomers of
P*R3 although the chirality is due to the configuration of two stereogenic
carbon centers.
(9) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic
Compounds; Wiley-Interscience: New York, 1994; pp 123-124.
(10) The designator (R,r,S)- is arbitrarily assigned to the thermodynamically
more stable of the two isomers.
(11) Rate and equilibrium constant data at 300 K for PhP[Pri2C4H6] deriva-
tives: kRR ) kSS ) 1.0(1) × 10-2 M-1 s-1, kRrS ) 6.0(3) × 10-4 M-1 s-1
,
kRsS ) 3.5(4) × 10-2 M-1 s-1; k-RR ) 8.3(2) × 10-6 s-1, k-RrS ) 7.0(2)
× 10-7 s-1, k-RsS ) 1.2(1) × 10-4 s-1; KRR ) KSS ) 1.2(1) × 103 M-1
,
KRrS ) 8.6(2) × 102 M-1, KRsS ) 2.9(1) × 102 M-1. See Supporting
Information for data for other derivatives.
(12) One outcome of the markedly different rates of oxidative addition is that
the combined amount of (R,R)- and (S,S)-trans-Ir(P*R3)2(CO)ClH2 isomers
formed at the point at which H2 uptake is complete exceeds the amount
of (R,R)- and (S,S)-trans-Ir(P*R3)2(CO)Cl present initially due to phosphine
exchange between the square planar isomers. However, it should be noted
that the rate of oxidative addition is not influenced by addition of P*R3,
consistent with the notion that reaction with H2 occurs with the square
planar complex.
(13) The oxidative addition of H2 to trans-Ir(P*R3)2(CO)Cl (P*R3 ) PhP[(C5-
Me4)]2) is characterized by an inverse equilibrium isotope effect, with
KH/KD ) 0.55 at 300 K. For other examples of such isotope effects, see:
Hascall, T.; Rabinovich, D.; Murphy, V. J.; Beachy, M. D.; Friesner, R.
A.; Parkin, G. J. Am. Chem. Soc. 1999, 121, 11402-11417.
(14) It should be noted that the observed rate constants for the slow elimination
of H2 from the thermodynamically favored (R,r,S)-trans-Ir(P*R3)2(CO)-
ClH2 isomers correspond to an upper limit. Specifically, it is possible
that the reductive elimination from (R,r,S)-trans-Ir(P*R3)2(CO)ClH2 is so
slow that it occurs by an alternative mechanism. For example, loss of H2
from (R,r,S)-trans-Ir(P*R3)2(CO)ClH2 could be catalyzed by isomerization
to (R,s,S)-trans-Ir(P*R3)2(CO)ClH2 (possibly involving P*R3 dissociation).
Alternatively, reductive elimination could be facilitated by bimolecular
dihydride transfer to four coordinate Ir(P*R3)2(CO)Cl. See, for example:
Kunin, A. J.; Johnson, C. E.; Maguire, J. A.; Jones, W. D.; Eisenberg, R.
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(17) It should be noted that high geometric (i.e., nonfacial) diastereoselectivities
for oxidative addition of H2 parallel to the P-Ir-Br and P-Ir-CO bonds
of cis-Ir(chiraphos)(CO)Br have been observed, and that the kinetic and
thermodynamic selectivities for these two modes of addition are inverted
(ref 15).
(18) Kimmich, B. F. M.; Somsook, E.; Landis, C. R. J. Am. Chem. Soc. 1998,
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(19) High kinetic selectivities were obtained for (R,R)-Me-DuPhos (47:1), (R)-
BINAP (>50:1), and (R)-Tol-BINAP (>50:1), but the kinetic and
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