Synthesis of Tertiary and Other Sterically Demanding Alkyl and Aryl Complexes of Iridium by Aldehyde C-H Bond Activation

Full text of DOI:10.1021/ja970245k

J. Am. Chem. Soc. 1997, 119, 5269-5270
5269
Scheme 1
Synthesis of Tertiary and Other Sterically
Demanding Alkyl and Aryl Complexes of Iridium
by Aldehyde C-H Bond Activation
Peter J. Alaimo, Bruce A. Arndtsen, and
Robert G. Bergman*
Chemical Sciences DiVision, Lawrence Berkeley
National Laboratories and Department of Chemistry
UniVersity of California, Berkeley, California 94720
compounds.16,19-21 Here we report that reactions between 1a,b
and aldehydes (RCHO) occur rapidly with decarbonylation at
room temperature to produce methane and iridium salts of the
general formula [Cp*(PMe3)Ir(R)(CO)][X] (2a-e, g-m) (X )
OTf, BArf) (Scheme 1). In spite of the fact that these reactions
usually occur within minutes at 25 °C and in most cases at
reasonable rates even at -60 °C, the reactions are exceptionally
clean and selective. High yields (quantitative by NMR; 63-
87% following two recrystallizations) are obtained with a variety
of aldehydes, and no evidence for attack of Ir at any hydrogen
other than that attached to the carbonyl carbon atom is observed.
Most unusual is the fact that even when the alkyl group is
tertiary or highly hindered, the reaction still proceeds cleanly
to the metal alkyl (or aryl) product 2. Such selectivity for the
aldehyde C-H bond has been observed in other decarbonylation
systems.22-24
Following anion metathesis of 2e to form 2f, single-crystal
X-ray diffraction studies were performed on tertiary alkyl
complexes 2f and 2h. The structures of 2f and 2h clearly show
the proposed formulation; however, both were plagued by severe
disorder, resulting in geometric data that should be considered
with care. For this reason, a single-crystal X-ray diffraction
study was performed on mesityl carbonyl complex 2l. Although
this too was successfully modeled with 5% disorder in the
cation, these data are significantly more reliable than the other
two structures. ORTEP diagrams of each of these cations are
illustrated in Figure 1 and full crystallographic data may be
obtained from the Supporting Information.
ReceiVed January 27, 1997
The first syntheses of transition metal complexes containing
tertiary alkyl ligands were reported as late as 1972.1-3 Since
then, very few tertiary alkyl complexes of transition metals have
been described.4-9 Such complexes have been difficult to
synthesize and isolate due to their propensity to undergo further
reactions such as â-elimination, giving stable transition metal
hydrides.10 In group 9, we are aware of a handful that have
been isolated,11-14 but none of iridium.15 Herein, we report a
method for the synthesis of two cationic tertiary alkyl (and other
highly hindered) carbonyl complexes of iridium which are quite
stable; thus far, none has been observed to decompose at room
temperature. This work represents an example16 of the rational
application of C-H bond activation17-19 to the synthesis of a
class of complexes of chemical or structural interest. During
the course of this work we have also examined the reactions of
R,â-unsaturated aldehydes. These lead to isolable π-complexes
as kinetic products which, upon thermolysis, undergo subsequent
C-H bond activation and migratory deinsertion to give ther-
modynamically more stable (carbonyl)(vinyl)iridium complexes.
In earlier studies, we showed that the iridium(III) complexes
Cp*(PMe3)Ir(Me)(OTf) (1a) and [Cp*(PMe3)Ir(Me)(CH2-
Cl2)][BArf] (1b) (Cp* ) η5-C5Me5, OTf ) OSO2CF3, BArf )
B(3,5-C6H3(CF3)2)4-) intermolecularly activate C-H bonds in
a number of alkanes, arenes, and functionalized organic
(1) Bower, B. K.; Tennent, H. G. J. Am. Chem. Soc. 1972, 94, 2512-
2514.
The 31P{1H} NMR chemical shifts of complexes 2a-m in
CD2Cl2 depend to some extent upon the nature of the group
coordinated to the metal. They appear at about -34 ppm except
for bulky R groups (e.g., 2,4,6-trimethylphenyl, 1-adamantyl,
tert-butyl). For complexes containing such sterically demanding
ligands, the 31P{1H} NMR shifts appear further upfield (ca. -45
ppm). The terminal carbonyl stretching frequencies vary little
(2) Kruse, W. J. Organomet. Chem. 1972, 42, C39-C42.
(3) Gill, D. F.; Shaw, B. L. J. Chem. Soc., Chem. Commun. 1972, 65-
66.
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1990, 112, 4600-4601.
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Trans. 1985, 921-929.
in these compounds, appearing between 1997 and 2035 cm-1
,
typical for late transition metal η1-carbonyl complexes.25 Full
spectroscopic data are consistent with the formulations given.
Treatment of methyl triflate complex 1a with R,â-unsaturated
aldehydes was explored to determine whether π-coordination
of the double bond is competitive with activation of the
aldehydic C-H bond. Reaction of 1a with R-methylcinnam-
aldehyde, ((E)-PhC(H)dC(Me)CHO), once again leads to
decarbonylation of the organic reactant, affording the vinyl
carbonyl complex [Cp*Ir(PMe3)(CO)(2-(Z)-1-phenylpropenyl)]-
[OTf] 2m. However, upon treatment of 1a with acrolein
(8) Giering, W. P.; Rosenblum, M. J. Organomet. Chem. 1970, 25, C71-
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(9) Schumann, H.; Jeske, G. Z. Naturforsch. B: Chem. Sci. 1985, 1490-
1494.
(10) Ogoshi, H.; Watanabe, E.; Koketsu, N.; Yoshida, Z. Bull. Chem.
Soc. Jpn. 1976, 49, 2529-2536. (b) A referee has emphasized that some
of these are quite stable; see ref 1.
(11) Bresciani-Pahor, N.; Randaccio, L.; Zangrando, E.; Summers, M.
F.; J. H. Ramsden, J.; Marzilli, P. A.; Marzilli, L. G. Organometallics 1985,
4, 2086-2090.
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Chem. Ber. 1993, 126, 1193-1200.
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1973, 153-154.
(20) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462-
10463.
(15) Our group prepared a tertiary cubyliridium species, but did not isolate
it: Sponsler, M. B.; Weiller, B. H.; Stoutland, P. O.; Bergman, R. G. J.
Am. Chem. Soc. 1989, 111, 6841-6843.
(21) Arndtsen, B. A.; Bergman, R. G. J. Organomet. Chem. 1995, 504,
143-146.
(22) Abu-Hasanayn, F.; Goldman, M. E.; Goldman, A. S. J. Am. Chem.
Soc. 1992, 114, 2520-2524.
(16) Luecke, H. F.; Arndtsen, B. A.; Burger, P.; Bergman, R. G. J. Am.
Chem. Soc. 1996, 118, 2517-2518.
(23) Gomez, M.; Kisenyi, J. M.; Sunley, G. J.; Maitlis, P. M. J.
Organomet. Chem. 1985, 296, 197-207.
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R. Z. Anorg. Allg. Chem. 1991, 606, 141-155.
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352-354.
(25) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1980; pp 768-775.
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Acc. Chem. Res. 1995, 28, 154-162.
S0002-7863(97)00245-X CCC: $14.00 © 1997 American Chemical Society

5270 J. Am. Chem. Soc., Vol. 119, No. 22, 1997
Communications to the Editor
Scheme 3
but in that case may rearrange more rapidly to the C-H
activation product. The reaction with acrolein is the first case
in our Ir(III) C-H activation systems in which we have observed
formation of an adduct that continues on to a C-H activation
product.
We believe that formation of complexes 2a-m proceeds by
initial C-H activation of the aldehydic proton to afford acyl
intermediate Cp*(PMe3)Ir(COR)(OTf) (complex A in Scheme
3). This may occur by oxidative addition to give an iridium-
(V) intermediate, followed by reductive elimination of meth-
ane.26 Iridium(V) compounds are not common, but a few have
28
been isolated including (Cp*)Ir(H)2(SiEt3)227 and (Cp*)Ir(Me)4
synthesized by Maitlis and co-workers, which are surprisingly
stable, lending credence to the intervention of an intermediate
of this type. The reaction could alternatively occur by σ-bond
metathesis between the iridium-methyl and the aldehydic C-H
bonds, giving the same acyl intermediate A, as is common in
many early transition metal d0 systems.29 Demonstrating that
the aldehydic proton is indeed the one being activated, reaction
between 1a and [1-D1]acetaldehyde results in quantitative
production of CH3D and no detectable CH4 by 1H NMR
spectroscopy.30 Rearrangement of A by carbonyl migratory
deinsertion would give observed products 2a-m.
In conclusion, we have found a mild route to a variety of
cationic alkyl and aryl carbonyl complexes of iridium by the
C-H bond activation of aldehydes. This reaction demonstrates
a high degree of selectivity for the aldehydic C-H bond and
allows the formation of complexes having stable bonds between
a metal and several extremely sterically demanding alkyl and
aryl ligands. While most aldehydes react immediately to form
products 2, the reaction with acrolein provides us with an
example of a substrate that first coordinates to the cationic
iridium starting material and subsequently undergoes C-H
activation.
Figure 1. ORTEP diagrams of (a) [Cp*(PMe3)Ir(tBu)(CO)][BPh4] 2f,
(b) [Cp*(PMe3)Ir(1-adamantyl)(CO)][BArf] 2h, (c) [Cp*(PMe3)Ir-
(mesityl)(CO)][OTf] 2l showing the major component in each.
Scheme 2
Acknowledgment. We thank Dr. Fred Hollander, Director of the
U.C. Berkeley X-ray Diffraction Facility (CHEXRAY) for determina-
tion of the crystal structures of 2f, 2h, and 2l. We are grateful for
support of this work by the Director, Office of Energy Research, Office
of Basic Energy Sciences, Chemical Sciences Division, U.S. Department
of Energy, under contract no. DE-AC03-76SF00098.
(CH2dCHCHO), slow reaction over 10 days at room temper-
ature results in formation of π-complex 3, which is easily
isolated in 63% yield (Scheme 2). A new aldehyde proton
resonance for this species is seen at 8.23 ppm (cf. 9.55 ppm in
free acrolein), which appears as a broad doublet, as it is coupled
both to the R-proton of acrolein and to the 31P nucleus on the
iridium center. Also indicative of the structure illustrated for
3 is the 1H NMR resonance for the iridium-bound methyl group
which appears as a doublet (3JPH ) 6.4 Hz) at 0.81 ppm (cf.
0.92 ppm in 1a) indicating that loss of methane has not occurred.
The difference between these two reactions is presumably due
to the reduced steric hindrance about the C-C double bond of
acrolein compared to that in R-methylcinnamaldehyde, resulting
in the observed competition between alkene coordination and
C-H activation. The π-complex 3 is found to be the kinetic
product of the reaction, as subsequent thermolysis in CH2Cl2
(75 °C, 24 h) affords the expected vinyl carbonyl product 4
(Scheme 2) in 57% isolated yield. It is possible, therefore, that
the π-complex is also formed with R-methylcinnamaldehyde,
Supporting Information Available: Spectroscopic and analytical
data for complexes 2a-m, 3, and 4 and X-ray structural data for 2f,
2h, and 2l (44 pages). See any current masthead page for ordering
and Internet access instructions.
JA970245K
(26) Strout, D. L.; Zaric, S.; Niu, S.; Hall, M. B. J. Am. Chem. Soc.
1996, 118, 6068-6069.
(27) Fernandez, M.-J.; Maitlis, P. M. Organometallics 1983, 2, 164-
165.
(28) Isobe, K.; Bailey, P. M.; Maitlis, P. M. J. Chem. Soc., Chem.
Commun. 1981, 808-809.
(29) ActiVation and Functionalization of Alkanes; Hill, C. L., Ed.; John
Wiley and Sons: New York, 1989.
(30) This experiment demonstrates that these reactions do not proceed
by a so-called “dissociative” reaction involving initial cyclometallation in
solution, cf.: Hinderling, C.; Plattner, D. A.; Chen, P. Angew. Chem., Int.
Ed. Engl. 1997, 36, 243-244.