Formation of iridium(III) allene complexes via isomerization of internal alkynes

Article abstract of DOI:10.1021/om4010934

Syntheses of the alkyne complexes (POCOP)Ir(η²-RCi - CR) (1-DPA, R = Ph; 1-Oct, R = C₃H₇; 1-Hex, R = C ₂H₅; POCOP = 2,6-bis(di-tert-butylphosphonito)benzene) are reported. 1-DPA is stable in both the solid state and as a benzene solution; however, both 1-Oct and 1-Hex slowly isomerize to afford the corresponding allene complexes 2-Oct and 2-Hex, respectively. A single-crystal X-ray structure determination of 2-Oct confirmed the assignment of an iridium-bound allene isomerization product. The rates of isomerization were measured using NMR techniques over a range of temperatures to allow determination of thermodynamic parameters. Finally, in contrast to prior reports, initial mechanistic studies have indicated the isomerization process is not catalyzed by the presence of an acid.

Full text of DOI:10.1021/om4010934

Communication
pubs.acs.org/Organometallics
Formation of Iridium(III) Allene Complexes via Isomerization of
Internal Alkynes
Neha Phadke and Michael Findlater*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States.
S
* Supporting Information
ABSTRACT: Syntheses of the alkyne complexes (POCOP)Ir(η2-RCCR) (1-DPA, R =
Ph; 1-Oct, R = C3H7; 1-Hex, R = C2H5; POCOP = 2,6-bis(di-tert-butylphosphonito)-
benzene) are reported. 1-DPA is stable in both the solid state and as a benzene solution;
however, both 1-Oct and 1-Hex slowly isomerize to afford the corresponding allene
complexes 2-Oct and 2-Hex, respectively. A single-crystal X-ray structure determination of
2-Oct confirmed the assignment of an iridium-bound allene isomerization product. The rates of isomerization were measured
using NMR techniques over a range of temperatures to allow determination of thermodynamic parameters. Finally, in contrast to
prior reports, initial mechanistic studies have indicated the isomerization process is not catalyzed by the presence of an acid.
somerization of organic compounds catalyzed by transition-
metal complexes provides a mild and efficient route to
equiv of alkyne; however, only in the case of diphenylacetylene
were we able to cleanly generate the desired η2 species (Scheme
1). The 31P{1H} NMR spectrum of 1-DPA displays a singlet at
I
allenes that cannot be easily accessed under thermal
conditions.1 The isomerization of alkynes to allenes is
particularly valuable due to their use as precursors for organic
synthesis,2 and because the transformation itself may play a role
in organometallic synthesis.3 The repulsive interaction of an
alkyne π orbital with a filled metal d orbital in LnM(η2-RC
CH) renders such complexes less stable relative to the
corresponding vinylidene complexes LnMCCHR,4 and
numerous reports have appeared concerning the isomerization
of terminal alkynes to vinylidenes.5 Analogous orbital
interactions between metal fragments and internal alkynes
would be anticipated to give rise to a similar destabilization.
Indeed, although rare,6 internal alkynes can undergo isomer-
ization to afford disubstituted vinylidenes. However, only a few
reports on metal-mediated isomerization of internal alkynes to
allenes have appeared.
Scheme 1. Alkyne Binding at a 14-Electron Iridium Center
166 ppm that is shifted significantly upfield relative to that for
1
1-H2. The corresponding H NMR spectrum exhibits features
consistent with assignment of 1-DPA as a η2-bound species.15
Corresponding treatment of “(POCOP)Ir” with either oct-4-
yne or hex-3-yne under ambient conditions in benzene solution
gives one major product, which we assign as the η2-alkyne
complex, as determined by 31P{1H} NMR spectroscopy. Over
the course of several days the singlet assigned to the η2-alkyne
complex (ca. 166 ppm for both 1-Oct and 1-Hex) decays and is
replaced by resonances corresponding to two new species. In
the 31P{1H} NMR spectrum of 2-Oct both of the new species
display two doublets and are present in an ∼1:1 ratio. Both
species display a JA‑B coupling constant of 356 Hz arising from
strong 31P−31P coupling of inequivalent phosphorus nuclei. A
similar NMR spectrum is obtained in the case of 2-Hex. On the
basis of the side-to-side inequivalence present in the ligand
framework, we tentatively assigned these new peaks as arising
from two iridium−allene complexes. Gratifyingly, we were able
to grow crystals suitable for study by X-ray diffraction
experiments from cold (−36 °C) solutions of the complex in
7
8
Both ReCl(N2)(dppe)2 and RhCl(C2H4)(As(i-Pr)3)2 have
been shown to form allenes upon reaction with internal alkynes.
The isomerization of preformed metal−alkyne complexes to
allenes can be promoted using acid,9 alumina,10 or silica.11 In
elegant mechanistic studies, Casey and co-workers described
acid-promoted isomerization of alkyne complexes to allene
complexes. Both 1-metallacyclopropene8 and η1-vinyl species3c
were suggested as key intermediates in the isomerization
mechanism. These insights prompted further work, which
explored the interaction of metal hydrides with internal
alkynes.12
Given the general paucity of reports detailing the isomer-
ization of internal alkynes to allenes and the ongoing interest in
the interaction of unsaturated species at late-transition-metal
centers,13 we chose to study the chemistry of an iridium pincer
complex with hex-3-yne, oct-4-yne, and diphenylacetylene.
Treatment of (POCOP)IrH2 in benzene solution with ca. 1
equiv of TBE (tert-butylethylene) affords the coordinatively
unsaturated (not isolated) 14-electron species “(POCOP)Ir”.14
We attempted to trap the η2-alkyne adduct by addition of 1
Received: November 11, 2013
Published: December 24, 2013
© 2013 American Chemical Society
16
dx.doi.org/10.1021/om4010934 | Organometallics 2014, 33, 16−18

Organometallics
Communication
methylene chloride. The solid-state structure of one stereo-
isomer of 2-Oct is shown in Figure 1.
Figure 2. Eyring plot for conversion of 1-Oct to 2-Oct.
Figure 1. ORTEP diagram of 2-Oct with 40% probability thermal
ellipsoids. Key bond lengths (Å) and bond angles (deg): C3−C4 =
1.387(8), C4−C5 = 1.307(8), Ir−C3 = 2.180(6), Ir−C4 = 2.054(5);
C3−C4−C5 = 148.6(6), P1−Ir−P2 = 155.98(5).
Table 1. Temperature Dependence of Rate Constants for
a
Conversion of 1-Oct to 2-Oct
k (s−1
)
T (K)
5.0 × 10−4
4.0 × 10−4
7.0 × 10−5
3.0 × 10−5
3.0 × 10−6
333.0
328.0
318.0
308.0
295.0
In the solid state, the octa-3,4-diene ligand of 2-Oct is bound
unsymmetrically to iridium through the ethyl-substituted C3
C4 π bond with a shorter Ir−C4 and a longer Ir−C3
interaction (Δd = 0.126 Å). The complex 2-Oct adopts a
distorted-trigonal-bipyramidal conformation with a P−Ir−P
pincer bite angle of 156° presumably due to steric interactions
emanating from the bulky tert-butyl phosphine substituents.
Within the allene ligand, the coordinated C3C4 bond is
elongated by 0.08 Å relative to the uncomplexed C4C5
bond, and the allene unit is bent with a C3−C4−C5 angle of
149°. This unsymmetrical binding motif and allene distortion is
in keeping with previously published metal−allene com-
plexes.7,8,12,16 Variable-temperature NMR experiments con-
firmed that the two species present in solution do not
interconvert even at elevated temperatures (80 °C).15
a
Rate constants measured for ∼0.020 M benzene-d6 solutions of 1-
Oct.
solution.17 Mindful of this study and the earlier work of
Casey,3c,9 which determined the crucial role acids could play in
catalyzing the isomerization of alkynes to allenes, we probed
the effects of added base on the observed rate of isomerization.
To a benzene-d6 solution of “(POCOP)Ir” was added
sequentially ca. 1.5 equiv of oct-4-yne and 2 equiv of either 2,6-
di-tert-butylpyridine or triethylamine. The rate of isomerization
was monitored using 31P{1H} NMR spectroscopy. In both
cases, in the presence of an amine, which would be expected to
scavenge protons from solution and thus inhibit catalysis, no
change in the observed rate constant could be detected.
Werner and co-workers invoked a transient zwitterionic
intermediate to account for their observed internal alkyne →
allene isomerization.8 This transient species resembles that of a
transition state postulated by Hoffmann in the concerted
rearrangement of 1-alkynes to vinylidenes.18 However, very
recent studies concerning the iridium-catalyzed isomerization of
alkenes has led us to propose an alternate mechanistic pathway
(Scheme 2).19 Thus, the initially formed π-bound alkyne (A)
undergoes oxidative addition of a Cα−H bond to the iridium
center, resulting in an Ir(III) alkyl hydride possibly stabilized by
π donation from the alkyne (B). Evidence supporting the
formation of an intermediate of structural type B is found in the
crystallographically characterized species [Os(η3-PhC3CHPh)-
(PMe3)4]+.20 Subsequently, the alkyl hydride slips to an allyl
hydride (C), which can easily undergo rotation followed by a
hydride migration to give the η2-allene product D. The absence
of a Cα−H bond in diphenylacetylene may explain the lack of
observed isomerization behavior in 1-DPA.
Chart 1. Proposed Structures of Diastereomers of 2-Oct
The rate of isomerization of 1-Oct or 1-Hex to the allene
complex 2-Oct or 2-Hex is slow at room temperature (ca. 50%
conversion after 72 h). The activation parameters for the
formation of 2-Oct were examined by measuring the observed
rate constant for the reaction over a 40 °C temperature range.
The effect of temperature on the rate of isomerization of 1-Oct
is depicted in Figure 2, and the rate constants are reported in
Table 1. From the Eyring plot an activation entropy (ΔS⧧) of
4.1 eu and an activation enthalpy (ΔH⧧) of 26.2 kcal/mol were
computed for 1-Oct. The rates of isomerization and activation
parameters for 1-Hex are qualitatively similar to those of 1-
Oct.15 The near zero ΔS⧧ implicates a unimolecular transition
structure for the rate-limiting event, which is supported by
experiments in which the amount of added octyne appears to
have no impact on the observed rate constant.
In summary, we have prepared a series of internal η2-alkyne
complexes (POCOP)Ir(η2-RCCR). When R = Ph, the η2-
alkyne was found to be stable in solution and the solid state.
Both alkyl-substituted alkynes (R = C3H7, C2H5) were found to
undergo an isomerization to afford the corresponding η2-allene
complexes. The rates of isomerization were measured over a
Recently, Brookhart and co-workers disclosed that the
hydrogenation of a closely related PONOPIr(CH3) system
could be catalyzed by trace quantities of protons present in
17
dx.doi.org/10.1021/om4010934 | Organometallics 2014, 33, 16−18

Organometallics
Communication
(5) For some recent reviews, see: (a) Puerta, M. C.; Valerga, P.
Coord. Chem. Rev. 1999, 193−195, 977. (b) Metal Vinylidenes and
Allenylidenes in Catalysis: From Reactivity to Applications in Synthesis;
Bruneau, C., Dixneuf, P., Eds.; Wiley-VCH: Weinheim, Germany,
2008. (c) Selegue, J. P. Coord. Chem. Rev. 2004, 248, 1543.
(6) (a) Ikeda, Y.; Yamaguchi, T.; Kanao, K.; Kimura, K.; Kamimura,
S.; Mutoh, Y.; Tanabe, Y.; Ishii, Y. J. Am. Chem. Soc. 2008, 130, 16856.
(b) Otsuka, M.; Tsuchida, N.; Ikeda, Y.; Kimura, Y.; Mutoh, Y.; Ishii,
Y.; Takano, K. J. Am. Chem. Soc. 2012, 134, 17746. (c) Ikeda, Y.;
Mutoh, Y.; Imai, K.; Tsuchida, N.; Takano, K.; Ishii, Y. Organometallics
2013, 32, 4353.
Scheme 2. Proposed Mechanism for Formation of Allene
Complexes
(7) Hughes, D. L.; Pombeiro, A. J. L.; Pickett, C. J.; Richards, R. L. J.
Chem. Soc. Chem. Commun. 1984, 992.
(8) Werner, H.; Schwab, P.; Mahr, N.; Wolf, J. Chem. Ber. 1992, 125,
2641.
(9) Casey, C. P.; Brady, J. T. Organometallics 1998, 17, 4620.
(10) Frank-Neumann, M.; Brion, F. Angew. Chem., Int. Ed. Engl.
1979, 18, 688.
(11) Coughlan, S. C.; Yang, G. K. J. Organomet. Chem. 1993, 450,
151.
broad temperature range, which allowed the thermodynamic
parameters to be determined. Addition of external base did not
have any impact on the observed rate of isomerization, seeming
to preclude a catalytic role being played by trace quantities of
acid.
(12) Wen, T. B.; Zhou, Z. Y.; Lau, C.-P.; Jia, G. Organometallics 2000,
19, 3466.
(13) Findlater, M.; Cartwright-Sykes, A.; White, P. S.; Schauer, C. K.;
Brookhart, M. J. Am. Chem. Soc. 2011, 133, 12274.
(14) For related in situ generation of 14-electron systems see:
(a) Gottker-Schnetmann, I.; White, P. S.; Brookhart, M. J. Am. Chem.
Soc. 2004, 126, 1804. (b) Gottker-Schnetmann, I.; White, P. S.;
Brookhart, M. Organometallics 2004, 23, 1766. (c) Gottker-
Schnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 9330.
(15) See the Supporting Information for details of the synthesis and
characterization of metal complexes and calculations pertaining to rate
and thermodynamic parameters.
(16) For reports of related transition-metal π-allene complexes, see
the following. (a) Au: Brown, T. J.; Sugie, A.; Dickens, M. G.;
Widenhoefer, R. A. Organometallics 2010, 29, 4207. (b) Re: Pu, J.;
Peng, T.-S.; Arif, A. M.; Gladysz, J. A. Organometallics 1992, 11, 3232.
(c) Fe: Omrcen, T.; Conti, N. J.; Jones, W. M. Organometallics 1991,
10, 913. (d) W: Chacon, S. T.; Chisholm, M. C.; Folting, K.;
Huffman, J. C.; Hampden-Smith, M. J. Organometallics 1991, 10, 3722.
(e) Ir: Lundquist, E. G.; Folting, K.; Streib, W. E.; Huffman, J. C.;
Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 855. (f) Pt:
Winchester, W. R.; Jones, W. M. Organometallics 1985, 4, 2228. (g)
Pd: Okamoto, K.; Kai, Y.; Yasuoka, N.; Kasai, N. J. Organomet. Chem.
1974, 65, 427.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and a CIF file giving X-ray crystallographic data
and complete synthetic and characterization details of all metal
complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: michael.findlater@ttu.edu.
■
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
(17) Findlater, M.; Bernskoetter, W. H.; Brookhart, M. J. Am. Chem.
Soc. 2010, 132, 4534.
(18) Silvestre, A.; Hoffmann, R. Helv. Chim. Acta 1985, 68, 1461.
(19) Biswas, S.; Huang, Z.; Choliy, Y.; Wang, D. Y.; Brookhart, M.;
Krogh-Jesperson, K.; Goldman, A. S. J. Am. Chem. Soc. 2012, 134,
13276.
ACKNOWLEDGMENTS
■
Dedicated to Professor Maurice Brookhart on the occasion of
his 70th birthday. M.F. gratefully acknowledges Texas Tech
University for startup funds. The authors thank the National
Science Foundation for funding the purchase of NMR
instrumentation (Grant No. CHE-1048553) used in this
project.
(20) Gotzig, J.; Otto, H.; Werner, H. J. Organomet. Chem. 1985, 287,
247.
REFERENCES
■
(1) For reviews, see (a) Trost, B. M.; Krische, M. J. Synlett 1998, 1.
(b) Fairlamb, I. J. S. Angew. Chem., Int. Ed. 2004, 43, 1048. (c) Uma,
R.; Crevisy, C.; Gree, R. Chem. Rev. 2003, 103, 27. (d) Hashmi, A.;
Stephen, K. In Modern Allene Chemistry; Krause, N., Hashmi, A. S. K.,
Eds.; Wiley-VCH: Weinheim, Germany, 2004; p 877.
(2) (a) Schuster, H. F.; Coppola, G. M. Allenes in Organic Synthesis;
Wiley: New York, 1984. (b) Brandsma, L.; Verkruijsse, H. D. Synthesis
of Acetylenes, Allenes and Cumulenes, A Laboratory Manual; Elsevier:
Amsterdam, 1981.
(3) (a) Leeaphon, M.; Ondracek, A. L.; Thomas, R. J.; Fanwick, P. E.;
Walton, R. A. J. Am. Chem. Soc. 1995, 117, 9715. (b) Wolf, J.; Werner,
H. Organometallics 1987, 6, 1164. (c) Casey, C. P.; Brady, J. T.; Boller,
T. M.; Weinhold, F.; Hayashi, R. K. J. Am. Chem. Soc. 1998, 120,
12500.
(4) Templeton, J. L. Adv. Organomet. Chem. 1989, 29, 1.
18
dx.doi.org/10.1021/om4010934 | Organometallics 2014, 33, 16−18