On the origin of diastereofacial selectivity in the interaction of β-pinene with rhodium carbonyl: A density functional study

Journal of Molecular Structure: THEOCHEM 816 (2007) 109–117 www.elsevier.com/locate/theochem

On the origin of diastereofacial selectivity in the interaction of b-pinene with rhodium carbonyl: A density functional study Valber D. Silva a, E.N. Dos Santos b, Elena V. Gusevskaya b, Willian R. Rocha a

b,*

Departamento de Quı´mica Fundamental – CCEN, Universidade Federal de Pernambuco, 50740-901 Recife – PE, Brazil b Departamento de Quı´mica – ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte – MG, Brazil Received 28 February 2007; received in revised form 3 April 2007; accepted 4 April 2007 Available online 10 April 2007

Abstract In this work we have performed electronic structure calculations at the density functional theory (DFT) level in order to understand the structural, electronic and energetic factors involved in the diastereofacial selectivity in the interaction of b-pinene with [HRh(CO)3], used as a model for the unmodified rhodium catalyst of hydroformylation. Analysis of the nature of the metal–ligand interaction of the p-complexes revealed that the catalyst coordination through the more sterically hindered face of b-pinene generates a stronger p-complex, however, being thermodynamically less stable due to the smaller steric hindrance for the catalyst coordination on the other face of the olefin. Our energetic results show that despite the coordination of the olefin to the more sterically hindered face being thermodynamically less favorable, the lower activation energy (8.5 kcal mol1), the higher stability of the metal–alkyl product (10.1 kcal mol1) and the absence of an isomerization pathway for this coordination mode, are responsible for the dramatic preference for this pathway, observed experimentally.  2007 Elsevier B.V. All rights reserved. Keywords: Diastereofacial selectivity; Density functional theory; Hydroformylation

1. Introduction Monoterpenes available from renewable resources are an abundant and sustainable supply of building blocks for the fine chemicals industry [1,2]. A number of oxygenated derivatives of considerable commercial importance can be produced by their catalytic transformations. For example, the hydroformylation of monoterpenes is an important pathway for the synthesis of aldehydes of interest to perfumery, flavor and pharmaceutical industry [3,4]. In the particular case of b-pinene (see Fig. 1), this monoterpene has been hydroformylated using rhodium and cobalt carbonyls [5–7] such as Rh6(CO)16 and Co2(CO)8, phosphine-modified rhodium catalysts [5,8–10] and heterobimetallic Pt–Sn catalyst [11]. Under hydroformylation conditions, as shown in Fig. 1, several reactions can take *

Corresponding author. Fax: +55 31 34995700. E-mail address: [email protected] (W.R. Rocha).

0166-1280/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.04.005

place: hydroformylation of b-pinene yielding the diastereomeric mixture of aldehydes 1 and 2; double bond migration or isomerization of b- to a-pinene, which can also be hydroformylated generating the aldehydes 3 and 4, as well as hydrogenation of both pinenes. The appearance of these different aldehydes is directly related to the fact that in both isomers, b- and a-pinene, the double bond exhibits two diastereotopic faces as shown in Scheme 1. The approach of the catalytic species from the top face, the most hindered one, or from the bottom face will govern the diastereofacial selectivity. For instance, products 2 and 4 in Fig. 1, comes from the attack of the catalyst from the top face, while products 1 and 3 results from the attack from the bottom face. The discrimination between the two diastereotopic faces of b-pinene is expected to depend on the different steric hindrance of the faces to the approach of the catalyst. However, several experimental observations leaded to the conclusion that the diastereoselectivity is not a straightforward consequence of the steric peculiarities of

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Fig. 1. Possible products formed from b-pinene under hydroformylation conditions.

Scheme 1.

the substrate. Azzaroni et al. [7] performed the hydroformylation of (1S,5S)-()- and (1R,5R)-(+)-b-pinene in the presence of [Rh4(CO)12], in which the [HRh(CO)3] catalytic species is generated in situ by the cluster fragmentation. They found 10-formylpinane, with the formyl group trans to the gem-dimethyl carbon bridge, 2, as a main product. This aldehyde results from the top coordination of b-pinene on rhodium and the authors have suggested that the diastereoselectivity is influenced not only by the steric difference between the two faces of the double bond, but also by the relative energies of the transition states along the insertion of the olefin into the Rh–H bond. Kalck and Urrutigoı¨ty [12] suggested that an agostic interaction between the methyl group and rhodium atom, occurring in a rhodiumalkyl intermediate formed by the hydrogen transfer to the top coordinated olefin, compensates sufficiently in energy for the steric hindrance of the substrate. We have recently studied, experimentally, the effects of phosphorus ligands and reaction conditions on the diastereoselectivity of the rhodium catalyzed hydroformylation

of b-pinene [8]. It was found that with unmodified rhodium catalyst the hydroformylation of b-pinene at 60 C leaded to comparable amounts of cis (1) and trans (2) isomers of 10-formylpinane. However, at higher temperatures (100–120 C) the reaction was much more stereoselective: 90–97% of 10-formylpinane was formed with the trans configuration. As it was mentioned before, the formation of trans aldehyde 2 requires the catalyst coordination to the more sterically hindered face of the olefin, syn to the gem-dimethyl carbon bridge (‘top’ coordination). It is therefore surprising the extremely high preference for this pathway. Why with unmodified catalyst the coordination through the less hindered face of the olefin is so disfavored? In this work we report a theoretical investigation of the interaction of b-pinene with the unmodified rhodium catalyst [HRh(CO)3]. Electronic structure calculations at the density functional theory (DFT) level were performed in order to understand the structural and electronic factors involved in the diastereofacial selectivity of the interaction of b-pinene with [HRh(CO)3] as well as the energetics involved in such interactions and along the olefin insertion reaction. 2. Theoretical details Full geometry optimizations and frequency calculations were performed at the gradient-corrected density functional theory (DFT) level [13] using the exchange functional according to Becke [14] and the correlation functional suggested by Perdew [15]. Stegman and Frenking [16] have shown that the BP86 functional gives energetic results compared with calculations at the MP2 and CCSD(T) level of theory. We have also shown that the

V.D. Silva et al. / Journal of Molecular Structure: THEOCHEM 816 (2007) 109–117

BP86 functional furnish energetic results compared to the MP4(SDQ) level [17] for rhodium-phosphine complexes. In a recent comparative theoretical study [18], the performance of several exchange-correlation functionals to describe the insertion of ethylene into the Rh–H bond, in the complex [Rh(CO)3(H)(C2H2)], was tested. It was shown that the BP86, B3P86 and PBE functionals provides reliable energetic results, as good as the ones computed at the more expensive CCSD(T)//CISD level of theory. So, we have reasons to believe in the accuracy of the energy differences reported in this work. The inner shell electrons (1s, 2s, 2p, 3s, 3p, 3d) of rhodium were treated by the effective core potential of Hay and Wadt [19], and the valence electrons (4s, 4d and 5s) were included explicitly in the calculation, using the associated double-n basis set in which the original [55/5/5] contraction scheme was changed to a more flexible [441/2111/31] contraction. The use of this contraction scheme was based on the work of Frenking and coworkers [20] who have shown that this contraction scheme for the valence electrons of second-row transition metal gives good structural results. The 6-31G(d) all electron basis set [21,22] was employed for the atoms of the ligands. The transition state structures were located using the quadratic synchronous transit approach of Schlegel and Peng [23] and characterized through harmonic frequency calculations, showing one imaginary frequency. In order to understand the nature of the metal–ligand interactions in the p-complexes we analyzed the BP86 wave function, using the charge decomposition analysis (CDA) of Dapprich and Frenking [24]. The CDA method consists of using linear combination of fragment orbitals (LCFO) of properly chosen fragments A and B for the interpretation of the interactions in a molecule AB. The interaction is divided into three main contributions: (i) the mixing between the occupied orbitals of A and empty orbitals of B, which indicates the magnitude of electron donation from A to B (A fi B), (ii) the mixing between the occupied orbitals of B with the empty orbitals of A, which gives the extent of back-donation, i.e., the electron donation from B to A (A ‹ B), and (iii) the mixing between the occupied orbitals of A and the occupied orbitals of B, which indicates the extent of charge polarization in the region of the bonding. Eqs. (1)–(3) show the linear combination of fragment orbitals, used in the CDA procedure to obtain the charge donation, qdi, charge backdonation, qbi and charge polarization, qri, between two fragments of the molecule. qd i ¼

occ:;A X X vac:;B k

qbi ¼

qri ¼

mi cli cmi hUl jUm i

ð2Þ

mi cki cmi hUk jUm i

ð3Þ

3. Results and discussion The optimized geometries for the catalytic species, R1, and b-pinene, R2, are shown in Fig. 2. The catalyst exhibits a square-planar geometry, with the angles around the rhodium atom deviating from the optimal value of 90 expected for a d8 compound. The \(CO–Rh–CO) angle between the carbonyls in a cis position is 99.7 and the angle involving trans carbonyls is 160.7. The Rh–CO dis˚ , trans to the hydride ligand, is slightly tance of 1.958 A ˚ obtained for the cis greater than the value of 1.928 A Rh–CO bond. b-Pinene has a strained geometry, with the

m

occ:;A X X occ:;B k

ð1Þ

This method has been shown to be very useful to understand the nature of the metal–ligand interaction in transition metal compounds [25]. All calculations reported here have been carried out with the Gaussian-98 program [26].

n

occ:;B X X vac:;A l

mi cki cni hUk jUn i

111

m

Fig. 2. Optimized structures of the rhodium(I) hydride catalyst, R1, and b-pinene, R2. Angles are given in degrees and bond distances in angstrom.

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\(C1–C7–C5), \(C1–C6–C5) and \(C1–C2–C3) angles of 85.1, 86.5 and 113.7, respectively. The presence of the methyl group above the double bond diminishes the accessible surface for the catalyst approach, it can be seen by the small C10–C8 distance of ˚ . The coordination of b-pinene to the catalyst 3.670 A through the bottom face generates the metal–alkene complex BOTTOM, shown in Fig. 3. This intermediate has a trigonal-bipyramidal structure, with the \(CO–Rh–CO) angle in the equatorial plane being 119.1. The Rh–CO bond distances in BOTTOM are slightly elongated as compared with the original catalyst R1. Upon coordination, the ˚ (from 1.347 C8–C2 distance in alkene increases by 0.059 A ˚ to 1.406 A) due to a change in hybridization of C2 and C8

atoms, acquiring a partial sp3 character on going from free olefin R2 to coordinated compound BOTTOM. This change in hybridization forces the backbone of the alkene to move away from the metallic center, which can be seen ˚ in R2 by the decrease in the C10–C8 distance from 3.670 A ˚ to 3.329 A in BOTTOM. It is important to note that the alkene coordinates unevenly through the two carbon atoms, with the distance between the rhodium center and the branched carbon C2 ˚ ) in BOTTOM being longer than the distance (2.421 A ˚ ). In order to involving the normal carbon C8 (2.289 A understand the reasons of this unsymmetrical coordination, we have performed a natural bond orbital (NBO) analysis [27] of substituent effects on the C@C bond polar-

Fig. 3. Optimized structures for the metal–alkene p-complex (BOTTOM), transition states (TS1-B and TS2-B) and metal–alkyl complexes (PROD1-B and PROD2-B) obtained along the bottom face coordination of b-pinene to the catalyst. Angles are given in degrees and bond distances in angstrom.

V.D. Silva et al. / Journal of Molecular Structure: THEOCHEM 816 (2007) 109–117

izarion in some alkenes. The results are shown in Table 1. As it can be seen, the double bond is progressively polarized towards the less substituted carbon, when substitutions are made on the other carbon. For instance, in ethylene, the C@C bond has equal contribution of the Pp orbitals of each carbon. In b-pinene, the p-bond has contributions of 47% from C2 and 53% from C8. Therefore, the unsymmetrical coordination of b-pinene to the catalyst occurs due to the steric and electronic effects in the alkene itself. This is a result of the progressive double bond polarization upon substitution at C2. Once the metal–alkene complex BOTTOM is generated, the insertion of the alkene into the Rh–H bond may proceed through two pathways, with the hydrogen being transferred either to substituted carbon C2 or to unsubstituted carbon C8. The hydrogen transfer to C2 passes through the four-center transition state TS1-B, while the transfer to C8 occurs through the four-center transition state TS2-B, as shown in Fig. 3. In both structures, the Rh–H bond is stretched and bends towards the carbon atom of the alkene. The C2@C8 bond is elongated in both structures and the carbon–hydrogen distance decreases. The transition state structures TS1-B and TS2-B, have imaginary frequencies of 632.0 and 704.1 cm1, respectively. Analysis of the nuclear displacements associated with these imaginary modes clearly shows a concerted movement in which the Rh–H bond is breaking and the C–H bond is forming. After the hydride transfer is completed, the metal–alkyl compounds PROD1-B and PROD2-B are formed. PROD1-B is generated in the pathway in which the hydride is transferred to the substituted carbon C2 and PROD2-B is originated through the hydride transfer to C8. As can be seen in Fig. 3, both structures deviated significantly from the expected square-planar geometry and intramolecular interactions appear between the transferred hydrogen atom and the metallic center. As we can see, the closer the hydrogen is to the metallic center, the smaller is the angle between the carbonyls, moving the structure away from the planar ˚ arrangement. In PROD1-B, the Rh–H distance is 2.341 A and the CO–Rh–CO angle is more opened (138.8) than in PROD2-B (128.6), in which the hydrogen is in a closer ˚ ). We believe that contact with the rhodium atom (2.115 A these intramolecular interactions may even contribute for blocking coordination sites on rhodium making more difficult subsequent steps of the catalytic cycle.

113

The coordination of b-pinene through the most hindered face generates the metal–alkene intermediate TOP, which is shown in Fig. 4. The coordination is also unsymmetrical, ˚ ) than the with the Rh–C2 distance being longer (2.398 A ˚ Rh–C8 one (2.254 A). Unlike the intermediate BOTTOM, the TOPstructure only allows the rotation of the olefin in the counter-clockwise direction! Rotation of the olefin in the clockwise direction would give rise to a strong steric repulsion between the gem-dimetil bridge and the carbonyl ligand trans to the hydride. Every Attempt to optimized this structure leaded to the dissociation of b-pinene from the coordination sphere of rhodium. Therefore, when the olefin is coordinated to the metal through the top face, the hydride can only be transferred to the substituted carbon C2. The transition state for the alkene insertion (or hydride transfer) step, TS-T, is also shown in Fig. 4. This is a four-center transition state having an imaginary frequency of 604.8 cm1, exhibiting a concerted motion in which the C2–H bond is forming and the Rh–H bond is breaking, as in the case of the transition state structures originated from the bottom coordination. In the metal–alkyl compound generated in this pathway, PROD-T, the angle between the trans carbonyls is more opened (148.0) than in PROD1-B and PROD2-B (138.8 and 128.6, respectively). It is important to see that in PROD-T the intramolecular interaction takes place not between the transferred hydrogen atom and the metallic ˚ , but between center, which are at the distance of 3.164 A the hydrogen bonded to the C1 atom and the metallic cen˚ . It is also important ter, which exhibit a distance of 2.632 A to note the correlation between the intramolecular interaction distance, between the hydrogen and rhodium, and the angle between the carbonyls. If we compare the carbonyl angles in compounds PROD2-B, PROD1-B and PRODT, it can be see that they increase by ca. 10 and the distance of the agostic interaction also increases from ˚ in PROD2-B, to 2.341 A ˚ in PROD1-B and than 2.115 A ˚ in PROD-T. The characterization of this intrato 2.632 A molecular interaction as an agostic interaction [28], should be view with care. The structural criteria for an agostic interaction may be satisfied however, as pointed out by Popelier and Logothetis [29], other criteria, based on the topology of the electron density, should also be satisfied and so, further studies in this regard is necessary. The data on the relative energies of the intermediates and transition states formed along the interaction of

Table 1 Natural bond orbital analysis of the substituent effects on the C@C bond polarization of some alkenesa Compound

Bond orbital p(C1@C2)

Ethylene Propene 2-Methylpropene b-Pinene a

[0.7071(2Pz)]C1 [0.6960(2Pz)]C1 [0.6871(2Pz)]C1 [0.6707(2Pz)]C1

pa (C1@C2) + + + +

[0.7071(2Pz)]C2(50% C1 + 50% C2) [0.7180(2Pz)]C2(48.4% C1 + 51.6% C2) [0.7266(2Pz)]C2(47.2% C1 + 52.8% C2) [0.7418(2Pz)]C2(46.9% C1 + 53.1% C2)

C1 is the carbon in which the substitution is taking place.

[0.7071(2Pz)]C1 [0.7180(2Pz)]C1 [0.7266(2Pz)]C1 [0.7418(2Pz)]C1

   

[0.7071(2Pz)]C2(50% C1  50% C2) [0.6960(2Pz)]C2(51.6% C1  48.4% C2) [0.6871(2Pz)]C2(52.8% C1  47.2% C2) [0.6707(2Pz)]C2(53.1% C1  46.9% C2)

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Fig. 4. Optimized structures for the metal–alkene p-complex (TOP), transition state (TS-T) and metal–alkyl complex (PROD-T) obtained along the top face coordination of b-pinene to the catalyst. Angles are given in degrees and bond distances in angstrom.

b-pinene with the catalyst are shown in Table 2 and Fig. 5. As it can be seen, the coordination of the alkene to the bottom face, the less hindered one, is more favorable then the top coordination. The BOTTOM structure is 3.5 kcal mol1 more stable than the TOP structure. In order to obtain a better insight into the nature of the metal–ligand interactions, we have analyzed the BP86 wave function using the charge decomposition analysis (CDA) method [24] and, the results for the p-complexes BOTTOM and TOP are quoted in Table 3. As it can be seen the top coordination results in a more effective interaction with the metallic fragment. The extent of donated charges from the b-pinene to the metallic fragment and the magnitude of the back-donated charges from the metallic fragment to the bpinene are greater in the case of top coordination. As a result, the top coordination leads to stronger interaction energy (33.4 kcal mol1) than the bottom coordination (29.1 kcal mol1). The stronger p-coordination for the

Table 2 Calculated total energies, Etot. (a.u.), zero point energies, ZPE (a.u.) and relative energies, Erel. (kcal mol1) Species

Etot.

ZPE

Erel.a

R1 R2 BOTTOM TS1-B TS2-B PROD1-B PROD2-B TOP TS-T PROD-T

450.255338 390.629384 840.900455 840.882835 840.881202 840.901615 840.898740 840.894994 840.879390 840.907132

0.029862 0.230619 0.263177 0.261596 0.261102 0.266150 0.265189 0.263214 0.261229 0.266879

0.0 0.0 9.9 1.2 2.2 10.6 8.8 6.4 3.3 14.1

a

(8.2) (1.9) (2.6) (7.0) (5.8) (4.7) (3.8) (10.1)

Values in parenthesis are ZPE corrected relative energies.

TOP complex is reflected in the Rh-C and C@C bond lengths, of this compound, compared with the values computed for the BOTTOM compound for these same bond

V.D. Silva et al. / Journal of Molecular Structure: THEOCHEM 816 (2007) 109–117

115

Fig. 5. Overall energy profile for the interaction of b-pinene with [HRh(CO)3]. The ZPE corrected BP86 relative energies are given in parenthesis in kcal mol1.

Table 3 Summary of the charge decomposition analysis (CDA) for the pcomplexes TOP and BOTTOMa P P Donation BackDonation: P rep: P res: Ebonding (kcal mol1)

TOP

BOTTOM

0.481 0.213 0.356 0.026 33.4

0.465 0.190 0.337 0.026 29.1

a

The CDA analysis were performed assuming the [HRh(CO)3] as the acceptor fragment.

lengths (see Figs. 3 and 4). The CDA results show that once the p-complex TOP is formed, the interaction energy is stronger than for the BOTTOM compound. However, in order to generate the TOP complex from the isolated fragments (Catalyst and b-pinene), a strong steric hindrance should be overcome. The substituent fragment at C2 moves away from the double bond plane, resulting in a more opened Rh–C2–C1 angle of 114.9, compared with the value of 110.8 for the BOTTOM compound (see Figs. 3 and 4). In summary, despite the TOP coordination generates a stronger p-complex, the BOTTOM compound is thermodynamically more stable due to smaller steric effects for the catalyst coordination. The olefin insertion pathway through the transition state structure TS1-B, generated by the bottom coordination, has an activation energy, relative to the metal–alkene structure, of 10.1 kcal mol1. The activation energy for the hydride transfer passing through the TS2-B transition state is only 0.7 kcal mol1 above. Taking the metal–alkene complex BOTTOM as reference, the products generated along the bottom coordination (PROD1-B and PROD2B) are thermodynamically unfavorable. The reaction endo-

thermicity is 1.2 kcal mol1 for the formation of PROD1-B and 2.4 kcal mol1 for PROD2-B. Despite the top coordination being less favorable thermodynamically, the olefin insertion in TOP intermediate occurs with an activation energy of 8.5 kcal mol1, which is 1.6 kcal mol1 lower than the activation energy for the insertion in the BOTTOM intermediate through TS1-B and 2.3 kcal mol1 lower than in the route through TS2-B. Unlike the ‘‘bottom’’ route, the product generated through the top coordination is thermodynamically favorable, taking the TOP structure as reference. The reaction is exothermic, with the PROD-T structure being 5.4 kcal mol1more stable than the metal–alkene intermediate TOP. The product generated through the top coordination is 3.1 kcal mol1 more stable than the more stable product found in the ‘‘bottom’’ route, PROD1-B. These energetic trends can be better visualized in the schematic reaction profile shown in Fig. 5. Further transformations of PROD-T and PROD1-B, after the migratory insertion of the alkyl group to the coordinated CO and hydrogenolysis, lead to aldehydes 2 (trans) and 1 (cis), respectively. If the same steps occurred with PROD2-B, a branched aldehyde containing the CHO group bound to C2 would be formed. However, such product has not been identified experimentally in the product mixture, which means that it is formed in trace amounts if formed at all. On the other hand, the intermediate PROD2-B must be the only one which yields the isomeric alkene, i.e. a-pinene, as a branched alkyl-complex from the TOP intermediate is not formed. It is noteworthy that the differences in activation energies for the migratory insertion of the alkene into the Rh–H bond are relatively small for all three pathways (10.1 kcal mol1 for TS1-B, 10.8 kcal mol1 for TS2-B, and 8.5 kcal mol1 for TS-T). On the other hand, for the

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reverse pathways (b-elimination of hydrogen) the differences in energies are greater: 8.9 kcal mol1 for TS1-B, 8.4 kcal mol1 for TS2-B, and 13.9 kcal mol1 for TS-T. In the later case the reverse process is in fact a c-elimination of hydrogen. Thus, the activation energy for the hydrogen elimination in the intermediate PROD-T is about 5 kcal mol1 higher than for the b-elimination in the intermediates PROD2-B and PROD1-B. We think it reasonable to believe that the activation barrier for the b-elimination in PROD2-B to form the co-ordinated a-pinene is on roughly the same order of magnitude as the values calculated in the present work (8–15 kcal mol1). Indeed, the isomerization is the main reaction pathway with unmodified rhodium catalysts: more than 50% of b-pinene is converted into a-pinene under a wide range of experimental conditions [6]. This example shows that a diasteroselectivity control in hydroformylation is a much more complex issue than merely a determination from which face the olefin coordination is more favored. It is obvious that the relative concentrations of the metal–alkene complexes as well as the energy barrier for the migratory insertion step must be considered. A less obvious issue is that b-elimination may play an important role in the observed ratio between the diasteroisomers. The relative concentration of the metal–alkene complex BOTTOM must be higher as its relative energy lies 3.5 kcal mol1 down compared to the TOP complex, but the energy barrier for the migratory insertion for the TOP intermediate is 1.6 kcal mol1 lower. Thus, it is surprising that aldehyde 2 to aldehyde 1 ratios as high as 30:1 have been observed under certain experimental conditions8. Such a high preference for aldehyde 2 may be related to the fact that only the intermediate resulted from the bottom coordination can form a-pinene. In other words, the top coordination can lead only to aldehyde 2 as a product, while the bottom coordination can lead to both aldehyde 1 and a-pinene. a-Pinene is formed in selectivity as high as 65% under these conditions. As the isomerization is a drain pathway for the intermediate which leads to aldehyde 1, if the values of the amounts of apinene and aldehyde 1 are considered together, the sum surpasses the amounts of aldehyde 2 formed. In summary, our calculations shows that despite the coordination of b-pinene to the catalyst through the top face being thermodynamically less favorable than the bottom coordination, the activation energy for the insertion reaction is lower and the metal–alkyl product generated in this pathway is more stable than the products formed through the bottom coordination. However, inspection of Fig. 5 suggests that the reaction is thermodynamically controlled, since the most stable metal–alkyl product, PROD-T, comes from a higher in energy coordination intermediate, TOP, and higher in energy transition state, TS-T. That is, the different stability of the metal–alkyl products generated in both pathways plays a major role in determining the diastereofacial selectivity in the interaction of b-pinene with the catalyst.

4. Conclusion In this work we have performed electronic structure calculations at the density functional theory (DFT) level in order to understand the structural, electronic and energetic factors involved in the diastereofacial selectivity in the interaction of b-pinene with [HRh(CO)3], used as a model for the unmodified rhodium catalyst of hydroformylation. The migratory olefin insertion reaction was also investigated and all stationary points along the reaction coordinate were fully optimized. Analysis of the nature of the metal–ligand interaction of the p-complexes revealed that the catalyst coordination through the more sterically hindered face of b-pinene generates a stronger p-complex, however, being thermodynamically less stable due to the smaller steric hindrance for the catalyst coordination on the other face of the olefin. The coordination of the olefin through the bottom face, which leads to the cis aldehyde is thermodynamically preferred by 3.5 kcal mol1. However, the coordination through the top face, which leads to the trans aldehyde, gives rise to a lower activation energy (8.5 kcal mol1) for the olefin insertion reaction. In addition, the metal–alkyl product generated in this route is more stable. The reaction is thermodynamically controlled, with the different stability of the metal–alkyl products generated in both pathways playing a major role in determining the diastereofacial selectivity. Acknowledgements The authors thank the Brazilian agencies CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico) and FAPEMIG (Fundac¸a˜o de Amparo a` Pesquisa do Estado de Minas Garais) for the financial support. References [1] K.A.D. Swift, Top. Catal. 27 (2004) 143. [2] J.L.F. Monteiro, C.O. Veloso, Top. Catal. 27 (2004) 169. [3] W.E. Erman, Chemistry of the Monoterpenes: An Encyclopedic Handbook, Marcel Dekker, New York, 1985. [4] A.J. Chalk, in: P.N. Rylander, H. Greenfield, R.L. Augustine (Eds.), Catalysis of Organic Reactions, vol. 22, Marcel Dekker, New York, 1988, p. 43. [5] I. Cipre´s, Ph. Kalck, D.-C. Park, F. Serein-Spirau, J. Mol. Catal. 66 (1991) 399. [6] E.N. Dos Santos, C.U. Pittman Jr., H. Toghiani, J. Mol. Catal. 83 (1993) 51. [7] F. Azzaroni, P. Biscarini, S. Bordoni, G. Longoni, E. Venturini, J. Organomet. Chem. 508 (1996) 59. [8] H.J.V. Barros, M.L. Ospina, E. Arguello, W.R. Rocha, E.V. Gusevskaya, E.N. Dos Santos, J. Organomet. Chem. 671 (2003) 150. [9] K. Soulantica, S. Sirol, S. Koinis, G. Pneumatikakis, P. Kalck, J. Organomet. Chem. 498 (1995) C10. [10] S. Sirol, Ph. Kalck, New J. Chem. 21 (1997) 1129. [11] E.V. Gusevskaya, E.N. Dos Santos, R. Augusti, A.O. Dias, C.M. Foca, J. Mol. Catal. A 152 (2000) 15. [12] Ph. Kalck, M. Urrutigoı¨ty, Coord. Chem. Rev. 248 (2004) 2193. [13] See for example: R.G. Parr, W. Yang, Density-Functional Theory of Atoms and Molecules, Oxford Univ. Press, Oxford, 1989. [14] A.D. Becke, Phys. Rev. A 38 (1988) 3098.

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