Reactivity of η2-(C,O)-bound Ir-ketene complexes: A 1,3-hydride shift is a hydride walk by way of an enol complex

Full text of DOI:10.1021/ja9532392

J. Am. Chem. Soc. 1996, 118, 2097-2098
2097
Scheme 1a
Reactivity of η2-(C,O)-Bound Ir-Ketene
Complexes: A 1,3-Hydride Shift Is a Hydride Walk
by Way of an Enol Complex
D. B. Grotjahn* and H. C. Lo
Department of Chemistry and Biochemistry, Box 871604
Arizona State UniVersity, Tempe, Arizona 85287-1604
ReceiVed September 22, 1995
Metal-ketene complexes1 have been suggested as intermedi-
ates in reductions of CO and CO2 in industrially important
processes.2 Ketene complexes have been implicated as crucial
intermediates in thermal reactions (e.g., the Do¨tz reaction)3 and
in photoinduced transformations of Fischer carbene complexes,
although in the latter case such species have yet to be detected
spectroscopically or isolated.4 Side products in both thermal
and photochemical reactions of Fischer carbene complexes
suggest that intramolecular C-H bond activation of either
carbene or ketene intermediates is possible,5 but this has not
been demonstrated on an isolable ketene complex.1 Here we
report that structurally characterized Ir-ketene complexes 16
undergo C-H bond activation both thermally and photochemi-
cally. Quantitative photochemical conversion of 1 at -25 °C
to a cis-aryl(hydride)Ir(III) complex 2 is cleanly reversed at
ambient temperature. A net 1,3-hydride shift (1 f five-
coordinate acyl 5) is shown to be the result of a four-step hydride
walk, by way of 2, 3, and 4 (Scheme 1). The last intermediate
(4) is a five-coordinate enol complex which, like its acyl
tautomer 5, adds CO stereoselectively.
a
(a) Pyrex-filtered UV light, toluene-d8, -25 °C, e10 min, 100%;
(b) room temperature, e10 min, 100%; (c) same as (a) but in C6D6 at
28-30 °C, 10 h, 78% of 4a from 1a; (d) C6D6, room temperature, 1
day, 100%; (e) for 4a, Ph2CdCdO, C6D6, room temperature, 1 h, 97%
(75% from 1a); (f) for 5a, CO, 1 atm, room temperature, 2 min.
Complex 1a6 could be produced by direct complexation to
Ph2CdCdO7 (88% after chromatography over SiO2). Heating
a solution of 1a in C6D6 (80-90 °C, 8-9 h) led to 5a in 88-
92% yield. The 1H NMR spectrum of 5a8 showed resonances
for nine aromatic protons, a one-proton singlet at δ 4.52, and
absorptions for two P(i-Pr)3 ligands. In the 31P{1H} NMR
spectrum, an AB pattern with large coupling (313.3 Hz)
indicated inequivalent, mutually trans phosphines. In the 13C
spectrum (C6D6), only two triplets were seen, at δ 195.92 (J )
3.2 Hz) and 139.03 (J ) 7.0 Hz), supporting the assignment of
an acyl carbon and an aryl carbon bound to the IrCl[P(i-Pr)3]2
unit. Save for the lack of observable coupling to the proton
resonating at δ 4.52 ppm, these data might be consistent with
formulation of the new product as 6 {M ) IrCl[P(i-Pr)3]2},9a
but NOE information8,9b and the facile addition of CO discussed
below confirm structure 5a. In contrast to 6, 5a is derived
(formally) from 1a by ortho-metalation and a 1,3-hydride shift.
How has this hydride shift occurred? In the thermolysis
reaction just described, intermediates were not detected in NMR
spectra of the mixture. However, irradiating a solution of 1a
in toluene-d8 with Pyrex-filtered UV light at -25 °C for only
10 min produced 2a in quantitative yield based on integration
against an internal standard. In the 1H NMR spectrum of 2a,8
an upfield triplet (δ -11.21, JPH ) 10.5 Hz) and resonances in
the region 6.5-7.9 ppm for nine protons pointed to oxidative
addition of an ortho-CH bond to the metal. One singlet in the
31P{1H} NMR spectrum10 implicated the symmetry of an η2-
(C,O)-bound ketene complex, and 13C NMR and IR data were
consistent with reduced back-bonding in 2a compared with that
in 1a.11 Remarkably, on standing at ambient temperature for
10 min, 2a reverted to 1a quantitatively. Furthermore, geometric
isomers 1b,c stereospecifically furnished the corresponding
thermally unstable products 2b,c.8 Thus, the behavior of 1 and
2 is a rare example of a photochromic organometallic reaction.12
Although 2a reverted to 1a in a dark reaction, prolonged
irradiation of 1a at higher temperatures (28-30 °C, 10 h) gave
(1) (a) Geoffroy, G. L.; Bassler, S. L. AdV. Organomet. Chem. 1988,
28, 1-83. (b) For an especially good leading reference, see: Hofmann, P.;
Perez-Moya, L. A.; Steigelmann, O.; Riede, J. Organometallics 1992, 11,
1167-1176 and references therein. (c) Review of recent ketene chemistry:
Tidwell, T. T. Acc. Chem. Res. 1990, 23, 273-279.
(2) (a) Keim, W. Catalysis in C1 Chemistry; D. Reidel: Dordrecht, 1983.
(b) Rofer-dePoorter, C. K. Chem. ReV. 1981, 81, 447-474. (c) Hermann,
W. A. Angew. Chem., Int. Ed. Engl. 1982, 21, 117-130. (d) Wolczanski,
P. T.; Bercaw, J. E. Acc. Chem. Res. 1980, 13, 121-127. (e) Blyholder,
G.; Emmett, P. H. J. Phys. Chem. 1960, 64, 470-472.
(3) (a) Do¨tz, K.-H. In Organometallics in Organic Synthesis; de Meijere,
A., tom Dieck, H, Eds.; Springer: Berlin, 1988. (b) Wulff, W. D. In
ComprehensiVe Organic Synthesis; Trost, B., Fleming, I., Eds.; Pergamon:
Oxford, 1991; Vol. 5, pp 1065-1113. (c) Kim, O. K.; Wulff, W. D.; Jiang,
W.; Ball, R. G. J. Org. Chem. 1993, 58, 5571-5573. (d) Harvey, D. F.;
Grenzer, E. M.; Gantzel, P. K. J. Am. Chem. Soc. 1994, 116, 6719-6732.
(4) (a) Review: Schmalz, H.-G. Nachr. Chem., Tech. Lab. 1994, 42,
608-612. (b) Hegedus, L. S.; deWeck, G.; D’Andrea, S. J. Am. Chem.
Soc. 1988, 110, 2122-2126. (c) Merlic, C. A.; Xu, D.; Gladstone, B. G. J.
Org. Chem. 1993, 58, 538-545.
(5) (a) Macomber, D. Organometallics 1984, 3, 1589-1591. (b) Blan-
dinieres, S.; Parlier, A.; Rudler, H. J. Chem. Soc., Chem. Commun. 1990,
23-24. (c) Challener, C. A.; Wulff, W. D.; Anderson, B. A.; Chamberlin,
S.; Faron, K. L.; Kim, O. K.; Murray, C. K.; Xu, Y.-C.; Yang, D. C.;
Darling, S. D. J. Am. Chem. Soc. 1993, 115, 1359-1376. (d) Wang, S. L.
B.; Su, J.; Wulff, W. D.; Hoogsteen, K. J. Am. Chem. Soc. 1992, 114,
10665-10666.
(9) (a) The compound 6 [M ) Fe(CO)3] was formed (38%) by reaction
of Ph2CdCdO with Fe(CO)5, and its 1H NMR spectrum exhibited a narrow
doublet (J ) 5.6 Hz) at 4.42 ppm for aH: Bkouche-Waksman, I.; Ricci, J.
S., Jr.; Koetzle, T. F.; Weichmann, J.; Herrmann, W. A. Inorg. Chem. 1985,
24, 1492-1499 and references to earlier work. (b) Key NOE data8 for 5a
which discounted 6: 90% saturation of the singlet at δ 4.52 led to strong
enhancement (7.0%) of the two-proton doublet for the ortho protons of the
C6H5 ring, as well as 1.8% enhancement of the one-proton doublet for the
nearest proton on the metalated ring.
(10) A complex of ClIr[P(i-Pr)3]2 with a (C,C)-bound ketene bearing
different substituents on the terminal carbon would be expected to show
two 31P resonances. See also ref 6.
(11) For 2a: 1H NMR (toluene-d8, -27 °C) δ 192.93 (t, JPH ) 1.9 Hz,
C1), 85.63 (s, C2); IR (toluene-d8, NaCl) 2336 (Ir-H), 1742, 1464. For
1a: 1H NMR (C6D6, ambient) δ 143.39 (t, JPH ) 3.2 Hz, C1), 74.26 (s,
(6) Grotjahn, D. B.; Lo, H. C. Organometallics 1995, 14, 5463-5465.
(7) Ph2CdCdO and P(i-Pr)3 were added to [(µ-Cl)Ir(cyclooctene)2]2 in
C6D6 at 25 °C (molar ratio 1.0:2.0:0.5), and 1a was isolated after 0.5 h.
(8) 1H, 13C, and 31P NMR and IR data for all new compounds are
available as supporting information. Thermally stable compounds were
further characterized by elemental analysis.
C2); IR (KBr) 1636, 1589 cm-1
.
(12) Schulz, M.; Werner, H. Organometallics 1992, 11, 2790-2795.
Vollhardt, K. P. C.; Weidman, T. W. J. Am. Chem. Soc. 1983, 105, 1676-
1677.
0002-7863/96/1518-2097$12.00/0 © 1996 American Chemical Society

2098 J. Am. Chem. Soc., Vol. 118, No. 8, 1996
Communications to the Editor
rise to a mixture containing products 5a (6%), the known trans-
(CO)IrCl[P(i-Pr)3]2 (11%),13,14 and 4a (78%, versus internal
standard). The new complex 4a bears equivalent 31P nuclei,
and its 1H NMR spectrum showed resonances for nine aromatic
protons as in 2a or 5a, but in addition, a sharp one-proton singlet
at 7.07 ppm, which disappeared on shaking with D2O.15a
Moreover, 4a exhibited a weak, broad IR absorption near 3350
cm-1, as seen in spectra of organic enols,15b and this shifted to
2470 cm-1 on exchange with D2O. 13C NMR and HMBC data
for 4a indicated that the resonances for the HOCdC unit
appeared at δ 150.07 (t, J ) 6.5 Hz, C1) and 127.29 (t, J ≈ 1
Hz, C2). The spectral evidence in favor of an enol formulation
was corroborated by tautomerization of 4a to 5a on standing in
solution in the dark (half-life ca. 15 h) or on attempted
chromatography over silica gel. Similarly, 4a-O-d isomerized
to 5a-C2-d over 1 day. Control experiments were undertaken
to characterize the prototropy in 4a and 5a. In the dark, 5a
showed no evidence16 of deuteration at C2 after 2 days with
D2O at 30 °C, or after 8 h at 80 °C. Addition of DCl (0.25
equiv) to the mixture resulted in 60% deuteration of 5a at C2
after 2 h at 80 °C (with some decomposition). Irradiation of
5a did not lead to 4a, and irradiation in the presence of D2O (5
h) did not give rise to detectable deuteration at C2.15c,16
Furthermore, the alcohol function in 4a could be trapped
quantitatively with Ph2CdCdO1c to give chelate 7, with a red-
10. Second, whereas 11 tautomerized to 9 on silica gel, 10
could be isolated unchanged in 58% overall yield from 1a. The
far greater stability of 1020 and the downfield shift of its
hydroxylic proton are both attributed to an intramolecular
hydrogen bond21ab which based on these two criteria must be
stronger in 10 than that in the five-coordinate 4a.
We have been unable to detect 3 (Scheme 1), but it is a
reasonable link between 2 and 4: isomerization of 2a could
lead to 3a.22 From 3a, enol 4a could be produced by insertion
of the ketene CdO function into the Ir-H bond. In contrast,
insertion of the CdC bond into M-H bonds was reported in
two intermolecular reactions,23 where the geometric constraints
of the intramolecular reaction 3 f 4 were absent. From the
evidence gathered to date, the ketene functionalization repre-
sented by isomerization of 1a to 5a occurs by way of a four-
step hydride walk, instead of by direct 1,3-hydride shift.24
Despite the importance of acyl ligands, and studies in the
last decade on their deprotonation (ususally with strong bases),25
the chemistry of tautomeric enol complexes involving σ-bound
metal substituents21 is virtually unexplored. The results here
demonstrate for the first time in high-yield1,9 reactions on
isolable species that ketene complexes can be a source of enol
and acyl26 ligands through thermal or photochemical C-H bond
activation and subsequent hydride walk.
Acknowledgment. We thank the donors to Petroleum Research
Fund, administered by the ACS, for partial support of this work and
Johnson Matthey Aesar Alfa for a loan of iridium salts and Cambridge
Isotope Labs for a grant of 13CO. Dr. Ron Nieman and Camil Joubran
assisted with vital 2D NMR experiments.
shifted IR absorption for the ester carbonyl of 1630 cm-1 17
.
Further studies involving ligand additions to 4a and 5a gave
a clearer picture of the bonding and reactivity of these ketene-
derived species. Bubbling CO into a solution of 5a for 2 min
produced a mixture of diasteromeric, chromatographically
separable adducts 8 and 9 in a ratio of 4:1 (Scheme 1).18 Similar
treatment of enol 4a with CO gave the adducts 10 and 11 (eq
1, ratio 3.5:1), each exhibiting a sharp singlet for the enolic
proton at δ 8.33 and 5.95, respectively.15a,19 Significantly, the
Supporting Information Available: Spectral data and preparations
of 15 compounds (22 pages). This material is contained in many
libraries on microfiche, immediately follows this article in the microfilm
edition of this journal, can be ordered from the ACS, and can be
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ordering information and Internet access instructions.
JA9532392
(20) In solution, pure 10 was consumed very slowly (∼2% per day),
forming 9, whereas 11 (as a mixture with 10) isomerized to 9 with a half-
life of about a day.
(21) (a) Casey, C. P.; O’Connor, J. M.; Haller, K. J. J. Am. Chem. Soc.
1985, 107, 3172-3177. (b) O’Connor, J. M.; Uhrhammer, R.; Rheingold,
A. L.; Roddick, D. M. J. Am. Chem. Soc. 1991, 113, 4530-4544. (c) Many
π-complexes of enols are known: see over 20 references in footnote 5 of
ref 21b.
(22) Addition of P(i-Pr)3 (3 equiv) did not slow the conversion of 1a to
4a. Possible routes from 2 to 3 and 4 could involve dissociation of the
ketene CdO bond, or P(i-Pr)3, or chloride, but present data do not allow
us to distinguish rigorously between these possibilities. An intramolecular
rearrangement is suggested by the observation that changing the initial
concentration of 1a by a factor of 2.8 did not change the time required to
consume 90% of 1a, and disappearance of 1a followed first-order kinetics.
We thank a reviewer for suggesting bimolecular pathways from 2 to 3, or
chloride dissociation, but the latter seems unlikely in benzene or toluene
solvent.
isomeric enols showed very different chemical behavior. First,
on saturating the solution of 10 and 11 with D2O at room
temperature, the O-H signal of the minor adduct 11 disappeared
at least 100 times more rapidly than that of the major isomer
10: times for completion were e5 min for 11 and 0.5 days for
(13) Werner, H.; Ho¨hn, A. Z. Naturforsch., Teil B 1984, 39B, 1505-
1509.
(14) The fate of the lost elements of Ph2C is still undetermined, but traces
of two fluorescent components are seen by TLC.
(23) Lindner, E.; Berke, H. Z. Naturforsch, Teil B 1974, 29B, 275-276.
Ungva´ry, F. J. Chem. Soc., Chem. Commun. 1984, 824.
(24) (a) It is not known if alternative isomerization of 1 to an undetected
η2-(C,C)-bound isomer, followed by metalation at an ortho position, leads
to 2, 4, and 5. If metalation occurred on a C,C-bound isomer of 1, then
both 1b and 1c would provide the same mixture of isomeric metalation
products. However, at -25 °C metalation of 1b,c to 2b,c is completely
regiospecific. At the higher temperatures used to form 4a, mixtures of
products result from irradiation of 1b,c, from equilibration between 1b,c
(dark reaction, 25 °C, half-life ca. 14 days; 70 °C, complete in 1 h), which
is greatly accelerated on irradiation (28-30 °C, complete in 3 h). Further
experiments to determine whether the metal moves by way of η2-(C,C) or
η1-O isomers will be necessary,24b but based on steric considerations alone,
the latter possibility is preferred. (b) Fluxionality in allene complexes:
Foxman, B.; Marten, D.; Rosan, A.; Raghu, S.; Rosenblum, M. J. Am. Chem.
Soc. 1977, 99, 2160-2165. In aldehyde complexes: Quiro´s Mende´z, M.;
Seyler, J.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. Soc. 1993, 115, 2323-
2334.
(15) (a) The chemical shift of the enolic OH in 4a is constant over a
concentration range of 5.8-100 mM, as is the corresponding shift of the
OH in 10 over 20-100 mM, excluding significant involvement of
intermolecular hydrogen bonding. (b) For studies of sterically hindered enols,
see: Hart, H.; Rappoport, Z.; Biali, S. E. In The Chemistry of Enols;
Rappoport, Z., Ed.; Wiley: Chichester, 1990; pp 481-589. (c) Enol photo-
chemistry: Weedon, A. C. In ref 15b, pp 591-638.
(16) Estimated lower limit of detection is 10%.
(17) For a similar observation in a crystallographically characterized
chelate, see: Esteruelas, M. A.; Lahoz, F. J.; Lo´pez, A. M.; On˜ate, E.;
Oro, L. A. Organometallics 1994, 13, 1669-1678.
(18) Key 13C NMR data: for 8 (C6D6), δ 234.95 (t, J ) 8.1 Hz, acyl C),
180.67 (t, J ) 9.3 Hz, CO); for 9, δ 214.03 (t, J ) 4.5 Hz, acyl C), 177.77
(t, J ) 9.1 Hz, CO). IR for 8: 2004 (CO), 1665 (acyl) cm-1. IR for 9:
2023 (CO), 1640 (acyl) cm-1. Assignments were fully corroborated by
labeling with 13CO.8,19 At room temperature in solution, 8 isomerized to 9
(half-life ca. 30 h).
(25) Davies, S. G. Pure Appl. Chem. 1988, 60, 13-20. Liebeskind, L.
S.; Welker, M. E.; Fengl, R. W. J. Am. Chem. Soc. 1986, 108, 6328-
6343.
(19) 13CO was added to 5a or 4a, giving the 13CO isotopomers of 8-11.
Chelate 7 opened to a single adduct, in which the 13CO and phenyl ligands
(26) Irradiation of the ketene complex η2-(C,O)-[Ph(CH3)CdCdO]IrCl-
2
6
were trans. Typical values for JCC across Ir in these complexes for trans-
[P(i-Pr)3]2 gave thermally stable Ir(H)(Cl)[COC(Ph)dCH2][P(i-Pr)3]2,
and cis-disposed carbons were 30.8-36.8 and 1.5-2.0 Hz, respectively.8
The enolic proton resonance in the spectrum of 10-13CO appeared as a
doublet (J ) 1.8 Hz).
whose structure is currently under investigation. Key 1H NMR data: δ
2
-11.00 (t, JPH ) 10 Hz, Ir-H), 6.46 (dt, JPH ) 2.7 Hz, JHH ) 3.6 Hz)
2
and 4.82 (dt, JPH ) 2.6 Hz, JHH ) 3.6 Hz) (1 H each, CdCH2).