Hydrogen-bond acceptance of bifunctional ligands in an alkyne-metal π complex

Article abstract of DOI:10.1021/ja0774616

Experiment and theory have been used to study reactive alkyne π complexes, intermediates in anti-Markovnikov alkyne hydration by CpRu bis(phosphine) catalysts with heterocyclic substituents. Each heterocycle accepts a hydrogen bond from an acetylene C-H, as revealed by NMR coupling constants between alkyne ¹³C and ¹H nuclei as well as between alkyne ¹³C and pyridine ¹⁵N (^2hJ_CN). Moreover, further alkyne transformations occur at temperatures from 50 to 90 °C below what is needed to convert a control compound without the heterocycles. Copyright

Full text of DOI:10.1021/ja0774616

Published on Web 12/08/2007
Hydrogen-Bond Acceptance of Bifunctional Ligands in an Alkyne-Metal π
Complex
Douglas B. Grotjahn,* Valent´ın Miranda-Soto, Elijah J. Kragulj, Daniel A. Lev, Gu¨lin Erdogan,
Xi Zeng, and Andrew L. Cooksy
Department of Chemistry and Biochemistry, 5500 Campanile DriVe, San Diego State UniVersity,
San Diego, California 92182-1030
Received September 27, 2007; E-mail: grotjahn@chemistry.sdsu.edu
Scheme 1. Precursor Synthesis
Typically, organometallic catalyst selectivity and reactivity have
been controlled by variations in the metal and ancilliary ligands
used, focusing on steric and electronic properties of the latter.1,2 In
contrast, nature’s metalloenzymes use secondary interactions such
as hydrogen bonding or proton transfer in the active site.3 An
increasing number of synthetic catalysts and related systems show
the benefits of secondary interactions,4 which are generally difficult
to control. Using hydrogen bonding or proton transfer from or to
imidazolyl5 and pyridylphosphines,4d,6,7 our group has reported rate
enhancements in anti-Markovnikov alkyne hydration and alkene
isomerization by factors of up to 10000. In studying the origin of
remarkable rate enhancements in alkyne hydration, we have
discovered hydrogen bonding between the terminal alkyne proton
and a heterocyclic nitrogen. To probe this unusual structural feature,
we report the first use of scalar coupling across a hydrocarbon C-H
bond (2hJCN), a technique used to characterize hydrogen bonds in
biological samples.8
temperatures consistent with an equilibrium between 7b and 6b
with Ea ) 15.9 kcal mol-1
.
Initial DFT calculations on the alkyne hydration pathway first
suggested the importance of structures like 5 (Scheme 1).9 However,
in 5 the orientation of the alkyne alone does not allow us to conclude
that there is C-H-N hydrogen bonding: in 5c, a complex lacking
the nitrogens, the same alkyne orientation is seen in both an X-ray
In summary, when imidazole derivative 4b reacts with acetylene,
alkyne complex 5b is not seen, because of rapid conversion to 7b,
whereas, when starting with pyridyl analogue 4a, alkyne complex
5a can be observed at -40 °C, but at temperatures closer to 0 °C
it evolves to 6a. In contrast, control alkyne complex 5c requires
temperatures 50-90 °C higher for conversion to vinylidene 6c, all
showing the profound effects and assistance of the pendant bases
on these transformations.
Bonding in vinylidenes 6a and 6c was compared using isoto-
pomers derived from H13C13CH. Significantly, nearly identical
values for 1JCC (Scheme 2) are consistent with the same C-C bond
length and degree of backbonding in each case.13
To clarify the structure of 5a, various unsuccessful attempts were
made to grow crystals at -40 °C. Instead, spectroscopic charac-
terization14 of alkyne bonding in 5a and 5c was made using H13C13-
CH, resulting in 5a- and 5c-(13C)2. At -40 °C the ten-line AA′XX′
pattern could be analyzed13 to show that there were significant
differences in couplings involving the hydrogens (Scheme 2). Data
above for 1JCC in 6a and 6c show that the electronic effects of the
two ligands are identical, consistent with almost identical 1JCC values
in the π complexes. We could find no experimental data on effects
of hydrogen bonding on alkyne NMR coupling constants and only
a single theoretical paper (on HCCH-OH2).15 Comparing alkyne
and hydrogen-bonded alkyne, the changes in JCH and JCH seen
by us experimentally (+1.5 and +1.9 Hz) resemble those predicted15
for HCCH-OH2 (+2.55 and +1.65 Hz).16 In short, the differences
in NMR couplings are for those couplings to hydrogen, and we
attribute this to effects of hydrogen bonding in 5a. Attempts were
made to engage the CH bonds of π complex 5c in hydrogen
bonding. Instead, addition of imidazole 8 led to addition to the
alkyne (9).
-
crystal structure (with BF4 counterion)10 and our calculations. It
appeared from the literature10 that to make an alkyne complex like
5 and study its structure before it isomerized to a vinylidene
complex, we would need the smallest alkyne, phosphine, and
ancilliary Cp ligand possible, making 5a,b with ligands 1a and 1b
our synthetic targets, with 1c as a control.
Thus, ligands 1a,11 1b,9 and 1c were each converted to 2 using
CpRu(COD)Cl. In the case of 2c, ionization of the chloride (Scheme
1) was done in the presence of HCCH to give 5c, which could be
isolated and stored cold, but it converted smoothly to 6c after 3 h
at 50 °C (Scheme 2).
In contrast, early experiments made it very clear that extraor-
dinary measures would be needed to isolate or even observe 5a or
5b. First (Scheme 1), the chlorides in 2a and 2b were ionized in
the presence of water12 to give aquo complexes 3a or 3b. Storage
of 3a or 3b under vacuum led to loss of water and mixtures of 3
and 4, with a greater amount of 4 in the case of 4b. When cooled
below -40 °C in CD2Cl2, chelate complex 4a did not react at an
appreciable rate with acetylene (Scheme 2), but near -40 °C was
converted to symmetrical π complex 5a, which isomerized smoothly
to vinylidene 6a at higher temperatures (0 °C, 3 h). In the imidazole
case, 4b was inert to acetylene until ca. 10 °C, where unsymmetrical
addition product 7b was the only product detected. Complex 7b
reasonably arises from unseen vinylidene 6b. Interestingly, the
NMR spectrum of 7b (e.g., two sets of sharp resonances for the
imidazoles at -40 °C) shows coalescence behavior at higher
1
2
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J. AM. CHEM. SOC. 2008, 130, 20-21
10.1021/ja0774616 CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Alkyne Hydrogen Bonding and Enhanced Reactivity of
Bifunctional Complexes
experiments show a pronounced kinetic acceleration by the
heterocycles on isomerization of alkyne to vinylidene (5 to 6), and
importantly, 3a is a very competent catalyst (>99% yield of hexanal
from 1-hexyne after 5 h at 70 °C using 2 mol % 3a).
In summary, the presence of C-H-N hydrogen bonding in an
alkyne π complex was revealed using NMR coupling information,
from both data within the alkyne ligand as well as between alkyne
and pyridine (2hJCN). Ongoing experimental and theoretical studies
are designed to elucidate further the favorable effects of bifunctional
ligands in alkyne hydration and related reactions, and these will
be reported in due course.
Acknowledgment. We thank the NSF for continuing support
under Grants CHE 0415783 and 0719575, Cambridge Isotope
Laboratories, Inc. for labeled building blocks, Professor John Love
for first pointing us to biomolecular NMR literature, and Dr. LeRoy
Lafferty for assisting with NMR experiments.
Supporting Information Available: Details of compound prepara-
tion and characterization and calculations. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
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Because NMR coupling constants are a novel tool for studying
alkyne hydrogen bonding, we wanted to provide additional evidence
for structure 5a. The remarkable discovery of scalar couplings across
hydrogen bonds has stimulated a great deal of experimental work
on proteins, DNA, and supramolecular interactions8 and theoretical
investigations of simple systems, but there appears to have been
no use in coordination or organometallic chemistry. Ligand 1a-
15N was made9 and converted to 3a-(15N)2 and 4a-(15N)2. Addition
of acetylene at low temperature led to 5a-(15N)2, whose 13C NMR
(14) IR spectroscopy has been a major tool for studying hydrogen bonding of
organic alkyne derivatives (e.g., refs 14a-e), but the high reactivity of
5a thus far has precluded observing its IR spectrum. (a) Steiner, T. AdV.
Mol. Struct. Res. 1998, 4, 43-77. (b) Kreevoy, M. M.; Charman, H. B.;
Vinard, D. R. J. Am. Chem. Soc. 1961, 83, 1978-1983. (c) Jeng, M. L.
H.; DeLaat, A. M.; Ault, B. S. J. Phys. Chem. 1989, 93, 3997-4000. (d)
Jeng, M. L. H.; Ault, B. S. J. Phys. Chem. 1989, 93, 5426-5431. (e)
Sundararajan, K.; Sankaran, K.; Viswanathan, K. S. J. Mol. Struct. 2004,
733, 187-192.
1
spectrum acquired at -50 °C with decoupling of both H and 31P
allowed observation of a somewhat broadened17 doublet (2hJCN
)
3 ( 0.5 Hz) as expected for coupling of one natural abundance
acetylene 13C to the nearest 15N nucleus, whereas a similar spectrum
of 5a showed a singlet.
(15) Pecul, M.; Leszczynski, J.; Sadlej, J. J. Chem. Phys. 2000, 112, 7930-
7938.
1
2
(16) Our calculations predict that JCH and JCH increase from 5c to 5a, and
ascribe the shift to the Fermi contact contribution. This contribution
increases with the s-character of the CH bonding MO, as the CCH bond
angle becomes more linear (5c, 154°; 5a, 162°) with hydrogen bonding.
These effects are a subject of future study.
Previous theoretical work by Del Bene et al. on a simple system18
showed that when the C-H-N angle is near 135°, 2hJCN would be
approximately one-third the magnitude when the hydrogen bond
is linear. Indeed, DFT calculations on 5a itself show a slightly
unsymmetrical structure with two C-H-N angles (124.5 and
135.7°).19 Determining the strength of the hydrogen bonds in 5a
remains a subject for future study, but as an estimate, we note that
calculations on conversion of 5c to 6c indicate that ∆G ) -17.9
kcal mol-1 whereas for similar conversion of 5a to 6a, ∆G ) -12.7
kcal mol-1. The difference may be attributed to thermodynamic
stabilization of 5a by two hydrogen bonds. Despite this effect,
(17) At -20 °C, coupling was obscured by broadening, presumably due to
alkyne rotation. See for example: Carbo´, J. J.; Crochet, P.; Esteruelas,
M. A.; Jean, Y.; Lledos, A.; Lopez, A. M.; Onate, E. Organometallics
1h
2002, 21, 305-314.
J
was not observed, but this could be expected
NH
to be less than 1 Hz.
(18) Del Bene, J. E.; Perera, S. A.; Bartlett, R. J.; Yanez, M.; Mo, O.; Elguero,
J.; Alkorta, I. J. Phys. Chem. A 2003, 107, 3222-3227.
(19) In addition, preliminary calculation of 2hJCN gave values of -3.4 and -6.7
Hz, averaging to -5 Hz. Comparison of the other experimentally
determined NMR coupling constants shown in Scheme 2 with calculated
values shows that the latter are consistently about 20% too high.
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