Article pubs.acs.org/Organometallics
Decarbonylation of Aliphatic Aldehydes by a TpMe2Ir(III) Metallacyclopentadiene Arián E. Roa,†,‡ Verónica Salazar,*,‡ Joaquín López-Serrano,† Enrique Oñate,⊥ Margarita Paneque,*,† and Manuel L. Poveda† †
Instituto de Investigaciones Químicas (IIQ) and Departamento de Química Inorgánica, CSIC and Universidad de Sevilla, Avenida Américo Vespucio 49, 41092 Sevilla, Spain ‡ Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Hidalgo, Carretera Pachuca a Tulancingo Km 4.5, 42184 Mineral de la Reforma, Hidalgo, México ⊥ Departamento de Química Inorgánica and Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza and CSIC, 50009 Zaragoza, Spain S Supporting Information *
ABSTRACT: The Ir(III) compound TpMe2Ir[C(CO2Me) C(CO2Me)C(CO2Me)C(CO2Me)](OH2) (1) reacts thermally with aliphatic aldehydes RC(O)H (R = Me, tBu) to lead to the decarbonylation products TpMe2Ir[C(CO2Me)C(CO2Me)C(CO2Me)C(CO2Me)](CO) (2) and RH. In turn, formaldehyde reacts with 1, yielding a product resulting from the hydrogenation of one of the double bonds of the iridacycle. Theoretical calculations reveal the role of the metallacycle as a shuttle for the transfer of the aldehyde H atom. Under photochemical (UV) irradiation, the decarbonylation reaction becomes catalytic for a variety of aliphatic aldehydes.
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ligand as a result.10 To be able to perform this reaction, the iridium (or other metal) center needs to have an open, or readily accessible, coordination site and an R′ group that will abstract the aldehyde hydrogen. The overall process is represented in eq 1, and this reactivity has been successfully
INTRODUCTION Aldehyde decarbonylation, RC(O)H → RH + CO, is a very useful, thermodynamically downhill, organic reaction that is normally carried out with the help of transition metal complexes, mainly Ru(II), Rh(I), and Ir(I) species. First discovered as a stoichiometric reaction,1 it soon became a catalytic process,2 whose mechanism is particularly well understood.3 Catalyst improvements are continuously appearing in the literature, and more importantly the decarbonylation step is being linked to other processes, such as coupling with different partners, for example, 2-phenylpyridines (to give pyridyl-substituted biaryls),4 terminal alkynes (producing 1,2substituted alkenes),5 norbornenes (to afford substituted norbornanes),6 and others.7 It is generally accepted that the metal-mediated decarbonylation reaction starts with the oxidative addition of the aldehydic C−H bond to form an hydrido-acyl compound,8 which would subsequently experience the migratory deinsertion of the carbonyl and the reductive elimination of R−H. To the best of our knowledge, only on certain occasions has a different pathway been proposed, as is the case of the theoretical investigation on the decarbonylation of aldehydes mediated by first-row transition metals, for which it was proposed a mechanism starting with a C−C bond activation.9 With Ir(III) complexes (or other high oxidation state metal systems),8 aldehydes may decarbonylate to give a carbonyl © 2012 American Chemical Society
employed in the synthesis of Ir(III) complexes with very bulky alkyl ligands.10 Our group has also reported on the decarbonylation of aldehydes by the Ir(I)-2,3-dimethylbutadiene complex TpMe2Ir[CH 2C(Me)C(Me)CH 2 )] (Tp Me2 = hydrotris(3,5dimethylpyrazolyl)borate) to give, through a series of Ir(III) intermediates, a Ir(III) carbonyl as the final product (eq 2).11 If we consider that this reaction takes place through the unsaturated Ir(III) species A, the relationship between eqs 2 and 1 is easily revealed. The iridacyclopentadiene Tp Me2 Ir[C(CO 2 Me)C(CO2Me)C(CO2Me)C(CO2Me)](OH2) (1)12 contains a Received: November 8, 2011 Published: January 9, 2012 716
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clearly indicate an η1-O-coordination of the aldehyde.15 Besides these resonances, those corresponding to both the metallacycle and the TpMe2 ligand are within the expected range and need no further comment (see Experimental Section). If compound 3 is heated at higher temperatures (120 °C), it transforms into the mixture of eq 3, thus showing the role of this species as a plausible intermediate in the decarbonylation reaction. As described below, we propose that the decarbonylation reaction starts by the formation of a σ-C−H complex, whose formation can be facilitated by the approach of the aldehyde through the more favorable O-coordination. In any case, the formation of 3 is reversible, and if it is stirred in solution, in the presence of excess water, 1 is regenerated. Also, during its purification by column chromatography, it partially reverts, thus accounting for the low yield of isolated product, even when the reaction of eq 4 is quantitative, as deduced by the NMR spectra of the crude mixture. Unexpectedly, formaldehyde reacts with complex 1 in a different way. Instead of species 2 and molecular hydrogen, the iridacyclopent-2-ene 4, where all the atoms of H2CO have been incorporated, is the only product of the reaction (eq 5).
very labile water ligand, which can be easily displaced, and that is the origin of most of its reactivity observed to date. In its reactions (C6H12, 60−90 °C) with Lewis bases that have high affinity for Ir(III) centers, such as carbon monoxide or acetonitrile, simple adducts of formula TpMe2Ir[C(CO2Me) C(CO2Me)C(CO2Me)C(CO2Me)](L) (L = CO, 2; L = NCMe) are formed.12 More interesting reactions occur with alkenes and alkynes, where these unsaturated substrates become incorporated into the iridacycle.13,14 In this contribution we report on the decarbonylation of some aliphatic aldehydes RC(O)H (R = H, Me, tBu) by the Ir(III) complex 1.
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RESULTS AND DISCUSSION Compound 1 reacts, under forcing conditions (C6H12, 120 °C, 18 h), with the aliphatic aldehydes RC(O)H (R = Me, tBu), with clean formation of the carbonyl species 2 and extrusion of the corresponding hydrocarbon R−H (eq 3). Methane and 2As can be observed, the two hydrogen atoms of the formaldehyde have stereoselectively hydrogenated one of the two olefin moieties of the iridacyclopentadiene, with both atoms finishing in a cisoid disposition and pointing toward the carbonyl ligand (NOESY evidence that is corroborated by a single-crystal X-ray diffraction study described below). Compound 4 is easily characterized by its NMR data. The two H atoms incorporated into the metallacycle give rise to two doublets in the 1H NMR spectrum at 4.84 and 4.12 ppm (3JHH = 9.8 Hz), while their supporting 13C nuclei resonate at 57.7 ppm (1JCH = 126 Hz, Ir-CHR-CHR-) and 13.5 ppm (1JCH = 135 Hz, Ir-CHR-). In the same 13C NMR spectrum, the carbonyl ligand is responsible for a resonance at 163.4 ppm (the corresponding signal for compound 2 is located at 159.4 ppm). Figure 1 shows an ORTEP view of a molecule of 4 along with some selected bond lengths and angles. In one of the two crystallographically inequivalent molecules in the asymmetric unit, the iridacyclopentene unit is characterized by the C−Ir−C bite angle of 81.16(14)o and the Ir−C bond lengths of 2.051(3) (sp2 carbon) and 2.129(4) Å (sp3 one).12 The C(7)−C(10) bond distance, 1.334(5) Å, is typical for a CC double bond. Figure 1 clearly shows the syn disposition with respect to the carbonyl ligand, deduced spectroscopically, of the hydrogen atoms of C(1) and C(4). With respect to the mechanism for the formation of 4, it can be said that the hydrogenation of the carbon−carbon double bond must take place before the final formation of the carbonyl ligand (complex 2 does not react with hydrogen even under more forcing conditions than those needed for eq 5 to take place). In fact, all the carbonyl species studied in the TpMe2Ir(III) system have never experienced a thermally activated substitution reaction (extrusion of this ligand is likely necessary for a reaction with H2 to occur) and only
methylpropane have been identified in the volatile fraction by GC and GC-MS spectroscopies, respectively. In accord with the examples of the reactions of aldehydes with Ir(III) species given in the Introduction, we had expected the formation of the carbonyl complexes “TpMe2Ir[C(CO2Me)C(CO2Me)C(CO2Me)CH(CO2Me)](R)(CO)”. A DFT study, to be discussed below, permits understanding this apparent discrepancy. In order to get more information about these decarbonylations, the reaction with tBuC(O)H was carried out under milder conditions. At 60 °C, a new compound is observed, which contains the same metallacycle as 1, and a coordinated aldehyde (eq 4). This new compound, 3, is easily characterized
by its NMR spectra. The most relevant data correspond to the aldehyde moiety: the resonance of the proton at 9.71 ppm, shifted only 0.27 ppm downfield with respect to the free aldehyde, together with the resonance of the aldehyde carbonyl carbon, at 223.9 ppm (vs 206.5 ppm in the free aldehyde), 717
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the conditions specified, they all experienced almost complete decarbonylation. In that respect, it is interesting to point out that the opposite reaction, i.e., the thermodynamically uphill, photochemical catalytic carbonylation of hydrocarbons, has been reported.18 Theoretical support for the above experimental observations was obtained by means of a DFT study on a model system, in which the TpMe2 ligand was replaced by Tp (hydrotrispyrazolylborate) and the CO2Me fragments of the iridacycle unit were replaced by hydrogen atoms. Two aldehydes were considered: acetaldehyde and formaldehyde (the latter was chosen to account for the double hydrogenation of the iridacyclopentadiene ring). While the model aquo complex 1C plus the appropriate aldehyde was chosen as the origin of free energies, the calculations indicate that water substitution to yield the corresponding O-coordinated aldehyde complexes (3C) is only slightly endoergic (ΔG = 1.9 kcal·mol−1 and 2.0 kcal·mol−1 for the acetaldehyde, 3-MeC, and formaldehyde, 3H C, adducts, respectively). Species 3-MeC maps onto compound 3, which could be isolated in agreement with the relative stability calculated for the model complexes. The first elemental step in the decarbonylation mechanism was considered to be hydrogen migration from the corresponding aldehyde to one of the α-carbons in the iridacycle. For the acetaldehyde system, a transition state, TS(A→B)-MeC, was located that connects the hydrogenated intermediate (B-MeC) product with its precursor (A-MeC), which has a σ-C−Hcoordinated aldehyde and a relative energy of 21.1 kcal·mol−1
Figure 1. X-ray structure of complex 4 (50% displacement ellipsoids, hydrogen atoms omitted for clarity). Selected bond lengths (Ǻ ) and angles (deg): Ir(1)−N(1) 2.134(3), 2.137(3); Ir(1)−N(3) 2.142(3), 2.148(3); Ir(1)−N(5) 2.134(3), 2.143(3); Ir(1) −C(1) 2.124(4), 2.129(4); Ir(1)−C(10) 2.051(3), 2.057(3); Ir(1)−C(28) 1.838(4), 1.836(4); C(7)−C(10) 1.334(5), 1.3404(5); Ir(1)−C(1)−C(4) 109.0(2), 109.8(2); C(1)−C(4)−C(7) 110.5(3), 110.7(3); C(4)− C(7)−C(10) 118.1(3), 118.4(4); Ir(1)−C(10)−C(7) 117.0(3), 117.0(3); C(1)−Ir(1)−C(10) 81.16(14), 81.21(14).
photochemically can it be displaced from the iridium center. It should be emphasized that eq 5 is a very unusual reaction, as formaldehyde always decarbonylates in effective transition metal decarbonylating systems, with formation of H2 as the coproduct.8,16 As mentioned before, the carbonyl adduct 2, once formed, is very stable, due to the inertness of the Ir(III)−CO bond in this type of compound;17 thus this decarbonylation cannot be made catalytic: by heating compound 2 with an excess of aldehyde, no consumption of any amount of substrate is observed. Nevertheless, the capability of thermally stable M−CO bonds to dissociate under photochemical (UV) irradiation is well known. Thus, we decided to study the photochemical reaction of 2 with ca. 5 equiv of acetaldehyde and tert-butylaldehyde, to find that, under these conditions, they catalytically decarbonylate to generate the corresponding hydrocarbon (CH4 and 2methylpropane, respectively) in good yields. To test the generality of this process, we have carried out the same reaction with a variety of aldehydes as shown in Table 1. Under
Figure 2. Calculated geometries for the σ-C−H complex with acetaldehyde A-MeC (left) and the transition state TS(A→B)-MeC (right) for hydrogen migration to the metallacycle unit to yield B-MeC. C and H atoms of the Tp ligands have been omitted for clarity.
(Figure 2). The overall free energy barrier amounts to 22.3 kcal·mol−1. Examination of intermediate B-MeC suggests that back migration of the hydrogen on the α-carbon of the iridacycle unit onto the acyl-Me could give the expected decarbonylation products (after facile rotation of the acyl ligand, B′-MeC). Indeed a transition state was located for this step, TS(B′→2C), which has a relative energy ΔG = 39.0 kcal·mol−1. The overall reaction is very exoergic with an energy return of 36.6 kcal·mol−1 (Figure 3). Competitive back migration of the hydrogen on the α-carbon of the iridacycle unit onto the oxygen atom of the acyl ligand is easier and generates the hydroxycarbene C-MeC reversibly with an energy barrier of 10 kcal·mol−1. A related hydroxycarbene was isolated as an intermediate in the decarbonylation of aldehydes by the Ir(I)2,3-dimethylbutadiene complex Tp Me2 Ir[CH 2C(Me)C(Me)CH2)].11 However, calculations indicate that in this case formation of C-MeC is unproductive, since further evolution to the decarbonylation products from C-MeC would require H migration from the OH to the methyl
Table 1. Photochemical Catalytic Decarbonylation of Aldehydes Mediated by TpMe2Ir[C(CO2Me) C(CO2Me)C(CO2Me)C(CO2Me)](CO) (2)a entry
aldehyde
decarbonylated product
1 2 3 4 5 6 7 8
CH3C(O)H CH3CH2C(O)H CH3(CH2)3C(O)H (CH3)2CHCH2C(O)H (CH3)3CC(O)H C6H5CH(CH3)C(O)H C6H5CH2CH2C(O)H C6H5CH(CH3)CH2C(O)H
CH4 CH3CH3 CH3(CH2)2CH3 (CH3)2CHCH3 (CH3)2CHCH3 C6H5CH2CH3 C6H5CH2CH3 C6H5CH(CH3)2
a Reaction conditions: Compound 2 (0.020 g, 0.024 mmol); C6H12 (1 mL); aldehyde (0.120 mmol); UV irradiation (Photochemical Reactors lamp model 3010, 125 W, 36 h).
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Figure 3. Calculated free energy profile for the decarbonylation of acetaldehyde by 1C including the calculated geometry (C and H atoms of the Tp are omitted) of the transition state for hydrogen back migration from the metallacycle unit of B′-MeC to yield 2C and methane. B-MeC and B′-MeC denote two rotamers of the same species, which are differentiated by the orientation of their acyl ligands.
moieties of the hydroxycarbene ligand, and this step has a prohibitive barrier in excess of 60 kcal·mol−1. When the formaldehyde system was calculated, no transition state for C−H activation of the aldehyde was located. All attempts to find such a transition state or a minimum for a σC−H precursor analogous to A-MeC led to a minimum, in which the hydrogen of the aldehyde had already been transferred to the iridacycle unit of the metal complex. This species, B-HC, is analogous to B-MeC. In this case, the second hydrogen of formaldehyde, now present in the formyl ligand of B-HC, can also be transferred to the iridacycle unit. The calculated energy barrier from the appropriate rotamer of B-HC (B′-HC) is only 8.7 kcal·mol−1, and the overall energy return for the formation of the doubly hydrogenated product, 4C, is 47.9 kcal·mol−1. According to the calculations, hydrogen back migration from another rotamer of B-HC (B″-HC) to its formyl ligand to yield the decarbonylation product 2C and molecular hydrogen is also possible, but the energy barrier (30.5 kcal·mol−1 from B″-HC) is greater than that for the second hydrogenation of the iridacycle unit. These findings are summarized in Figure 4.
Figure 4. Calculated free energy profiles for the formation of 2C and dihydrogen (dotted line) and the double hydrogenation of the iridacyclopentadiene unit of 1C (solid line) in the reaction of 1C and formaldehyde. Calculated geometries for the key transition states of each process are also included (C and H atoms of the Tp ligands have been removed for clarity).
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CONCLUDING REMARKS The Ir(III) metallacyclopentadiene 1 reacts thermally, in an unexpected way, with the aliphatic aldehydes acetaldehyde and tert-butylaldehyde, by promoting the decarbonylation of the substrate with formation of the carbonyl Ir(III) metallacyclopentadiene, 2, with concomitant generation of the corresponding hydrocarbon. These reactions start by Ocoordination of the aldehyde, and computational studies suggest that they may proceed via a mechanism in which the metallacycle unit acts as a shuttle for the hydrogen atom in the CHO moiety of the aldehyde, transferring it to the alkyl moiety. In the case of formaldehyde, double hydrogenation of the metallacycle is kinetically and thermodynamically preferred to molecular hydrogen elimination. Under UV irradiation, these reactions become catalytic, and a variety of aliphatic aldehydes have been shown to participate in this process.
EXPERIMENTAL SECTION
General Considerations. All the reactions were carried out under nitrogen, following standard Schlenk techniques. The lamp used for the photochemical irradiation was a Photochemical Reactors, model 3010 (125 W). Microanalyses were carried out by the Microanalytical ́ Service of the Instituto de Investigaciones Quimicas (Sevilla, Spain). The NMR Instruments were Bruker DRX-500, DRX-400, and DPX300 spectrometers. Spectra were referenced to external SiMe4 (δ 0 ppm) using the residual protio solvent peaks as internal standards (1H NMR experiments) or the characteristic resonances of the solvent nuclei (13C NMR experiments). Spectral assignments were made by means of routine one- and two-dimensional NMR experiments where appropriate. The GC instrument was an Agilent 6890, Series Plus+, fitted with a Restek RT-Qplot 30 m × 0.32 mm column. The GC-MS apparatus was a ThermoQuest (trace GC 2000 series + Automassmulti) fitted with a Teknokroma TRB-1 30 m × 0.25 mm × 0.25 μm 719
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hydrocarbon (ethylbenzene) formed revealed a yield of isolated product of 80%. Structural Analysis. X-ray data for compound 4 were collected on a Bruker Smart APEX diffractometer equipped with a normal focus, 2.4 kW sealed tube source (Mo radiation, λ = 0.71073 Å) operating at 50 kV and 30 mA. Data were collected over the complete sphere. Each frame exposure time was 10 s, covering 0.3° in ω. Data were corrected for absorption by using a multiscan method applied with the SADABS program.19 The structure was solved by direct methods. Refinement was performed by full-matrix least-squares on F2 with SHELXL97,20 including isotropic and subsequently anisotropic displacement parameters. In the last cycles of refinement a molecule of disordered diethyl ether and 0.8 molecule of disordered dichloromethane were observed in the asymmetric unit. Crystal data: C28H36B1Ir1N6O9, 0.5 C4H10O1, 0.4 C1H2Cl2, Mw 874.67, irregular prism, light yellow (0.20 × 0.14 × 0.12), triclinic, space group P1̅, a = 10.9591(5) Å, b = 15.4387(7) Å, c = 23.2626(11) Å, α = 78.0530(10)o, β = 84.7740(10)o, γ = 69.8550(10)o, V = 3614.2(3) Å3, Z = 4, Dcalc = 1.607 g cm−3, F(000) = 1751, T = 100(2) K, μ = 3.813 mm−1; 45 253 measured reflections (2θ: 3−58°, ω scans 0.3°), 17 154 unique (Rint = 0.0314); min./max. transmn factors 0.7289/0.8621. Final agreement factors were R1 = 0.0295 (14 108 observed reflections, I > 2σ(I)) and wR2 = 0.0743; data/restraints/parameters 17 154/52/932; GoF = 1.020. Largest peak and hole: 1.734 and −1.473 e/Å3. Computational Details. All calculations were performed using the GAUSSIAN 09 series of programs21 using the B3Lyp functional.22,23 An effective core potential24 and its associated double-ζ LANL2DZ25 basis set were used for iridium. C, H, B, N, and O atoms were represented by means of the 6-31G(d,p) basis set.26−28 The structures of the reactants, intermediates, transition states, and products were fully optimized in the gas phase without any symmetry restriction. Frequency calculations were performed on all optimized structures at the same level of theory to characterize the stationary points and the transitions states, as well as for the calculation of gas-phase enthalpies (H), entropies (S), and Gibbs energies (G) at 298.15 K. The nature of the intermediates connected was determined by perturbing the transition states along the TS coordinate and optimizing to a minimum.
column. The IR apparatus was a Bruker Tensor 27. Complex 1 was obtained by the published procedure.12 Thermal Reaction of 1 with RC(O)H (R = CH3, tBu). To a stirred suspension of compound 1 (0.10 g, 0.126 mmol) in C6H12 (5 mL) was added the corresponding aldehyde (0.126 mmol). The mixture was heated at 120 °C for 18 h, after which time a sample of the gases of the reaction was analyzed by GC, to show formation of the corresponding hydrocarbon (CH4 and CH3CH(CH3)CH3, respectively), by comparison with a real sample of the gas. The solution was evaporated under reduced pressure, and the solid residue analyzed by NMR to show quantitative formation of the known compound 2. Formation of the Adduct 3. Compound 1 (0.100 g, 0.126 mmol) was dissolved in CH2Cl2 (3 mL), tert-butylaldehyde was added (0.068 mL; 0.63 mmol), and the mixture was stirred for 18 h at 60 °C. After this period of time, the solvent was evaporated, and the solid residue purified by column chromatography (silica gel, diethyl ether/hexane, 50:50). The title compound was isolated in 46% yield as a brown solid. 1 H NMR (CDCl3, 25 °C): δ (ppm) 9.71 (s, 1H, C(O)H), 5.74, 5.52 (s, 3H, 2:1, CHpz), 3.74, 3.34 (s, 3H each, CO2Me), 2.39, 2.38, 2.04, 1.81 (s, 3:6:3:6 H, respectively, Mepz), 1.07 (s, 9H, tBu). 13C{1H} NMR (CDCl3, 25 °C): δ (ppm) 223.9 (C(O)H), 173.3, 167.1 (CO2Me), 157.2, 150.8 (C-CO2Me), 165.2, 150.9, 143.9, 143.7 (1:2:2:1, Cqpz), 108.4, 107.0 (1:2, CHpz), 52.0, 51.0 (CO2Me), 44.8 (CMe3) 23.3 (CMe3), 13.8, 13.5, 13.1, 12.8 (2:1:1:2, Mepz). Anal. Calcd for C32H44BIrN6O9: C, 44.70; H, 5.16; N, 9.77. Found: C, 44.75; H, 5.13; N, 9.67. Reaction of 1 with Formaldehyde. Compound 1 (0.100 g, 0.126 mmol) and an excess of para-formaldehyde (0.012 g, 0.36 mmol) were suspended in cyclohexane (3 mL), and the mixture was stirred for 18 h at 80 °C. The solvent was removed under vacuum, and the residue was extracted with a mixture of pentane and methylene chloride and cooled to −20 °C. Compound 4 was isolated as brown crystals, in 85% yield. IR (KBr, cm−1): 2051 (ν CO). 1H NMR (CDCl3, 25 °C): δ (ppm) 5.83, 5.74, 5.68, (s, 1H each, CHpz), 4.84 (d, 1H, 3JH−H = 9.8 Hz, Ir-CHR), 4.12 (d, 1H, 3JH−H = 9.8 Hz, Ir-CHR-CHR), 3.71, 3.64, 3.45, 2.61, (s, 3H each, CO2Me), 2.49, 2.41, 2.36, 2.32, 2.28, 2.11 (s, 3H each, Mepz). 13C{1H} NMR, (CDCl3, 25 °C): δ (ppm) 178.4, 173.1, 172.1, 166.6 (CO2Me), 163.4 (Ir-CO), 156.33, 151.0, 150.0, 144.2, 143.6, 142.8 (Cqpz), 149.9 (Ir-CR), 127.4 (Ir-CHRCHR), 109.8, 107.1, 106.6 (CHpz), 57.7 (1JC−H = 126.1 Hz, Ir-CHR-CHR), 51.4, 51.3, 50.9, 50.0 (CO2Me), 16.1, 14.9, 13.6, 12.9, 12.6, 12.2 (Mepz), 13.5 (1JC−H = 134.7 Hz, Ir-CHR). Anal. Calcd for C28H36BIrN6O9: C, 41.85; H, 4.52; N, 10.46. Found: C, 41.80; H, 4.52; N, 10.76. Photochemical Reaction of 2 with Aldehydes. A mixture of the carbonyl adduct 2 (0.020 g, 0.024 mmol) and the corresponding aldehyde (0.120 mmol) (see Table 1) was suspended in C6H12 (1 mL) in a small ampule (ca. 3 mL volume), provided with a Young-type Teflon tap. The mixture was placed close to an hν (125 W) lamp and irradiated for 36 h. As this reaction does not occur under thermal conditions, no precautions were taken to avoid the usual heating caused by irradiation. After this time, the samples were analyzed to show full conversion to the hydrocarbon. For the case of entries 1 and 2, a sample of the gas over the reaction mixture was analyzed by GC, by comparison with real samples. Entries 3−5 were also studied by analysis of the gas phase by GC-MS, the corresponding peaks of the chromatographs being analyzed by MS. For entries 6−8, once the reaction finished, the volatile fraction was distilled trap-to-trap, and the resulting solution that had condensed in the trap was analyzed by GCMS. In all cases, the solid residues obtained after evaporation of the volatile fractions were analyzed by NMR to show the presence of adduct 2 as the only organometallic species and, in some instances, only small amounts of unreacted aldehyde (
