Asymmetric Induction via the Structural Indenyl Effect - ACS Publications

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Asymmetric Induction via the Structural Indenyl Effect Robert W. Baker* School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia S Supporting Information *

ABSTRACT: Constrained geometry, planar chiral, dichloro[η5:κS-indenyl-sulfanyl]rhodium complex (pS)-1 reacts with (E)-2-pentene in the presence of silver hexafluoroantimonate to afford syn,syn-1,3-dimethyl-π-allyl complex (pS)-2 in 88% yield. Reaction of (pS)-2 with the sodium or tetrabutylammonium salts of dimethyl malonate anion provided dimethyl 2[(1R,2E)-1-methyl-2-buten-1-yl]-1,3-propanedioate ((R)-3) in 68−70% yield and 66−68% enantiomeric excess. NMR analysis and density functional theory calculations establish that asymmetric induction is substantially determined by the electronic asymmetry of the indenyl ligand polarizing the preferred site of nucleophilic attack on the π-allyl ligand toward the terminal carbon that is syn to the indenyl benzo-ring.

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INTRODUCTION The term indenyl effect was first coined by Basolo and coworkers1 to describe the remarkable increases (ca. 108) in the rates of substitution reactions for the indenyl complexes [η5C9H7Rh(CO)2] and [η5-C9H7Mn(CO)3] when compared to the cyclopentadienyl analogues. Hart-Davis and Mawby2 were the first to observe the indenyl effect in migratory insertion reactions involving the complex [η5-C9H7Mo(CO)3Me] and suggested that the ability of the indenyl ligand to readily undergo ring slippage from η5- to η3-coordination, opening up a new metal coordination site, facilitates associative reaction mechanisms. This distinctive reactivity of indenylmetal complexes has been successfully exploited in a range of catalytic organic transformations.3 In addition to the customarily defined indenyl effect, related ground-state effects are also evident in indenylmetal complexes, such as slip-fold distortions toward η3-coordination, which are apparent in solid-state structures and through 13C NMR spectroscopy,4 as well as conformational effects,5 which have been termed structural indenyl effects by Zanello and co-workers.5e We have recently shown that the structural indenyl effect can control metalcentered chirality in constrained geometry η5:κS-indenylsulfanyl and -sulfinyl rhodacycles of 2-phenylpyridine,6 where there is a substantial preference for the stronger structural trans effect phenyl ligand to be anti to the indenyl benzo-ring. Here, another aspect of the structural indenyl effect is described, where the electronic asymmetry of the indenyl ligand effects asymmetric induction in the reaction of an associated prochiral ligand.

the more electron-rich pentamethylcyclopentadienyl complex fails to react. When the reaction was conducted in the absence of CsOAc a mixture of regioisomeric complexes was obtained, with a preponderance (83:17) of the syn,syn-1-ethyl-3-methylπ-allyl complex, and it was suggested that equilibration of the isomers is possible through an intramolecular proton transfer between coordinated π-allyl and alkene ligands. Applying this reaction to indenyl complex (pS)-18 (Scheme 1) using (E)-2pentene and AgSbF6, in the absence of CsOAc, was examined. While the presence of the chelated sulfur would preclude an intramolecular proton transfer mechanism, it was anticipated that equilibration of isomeric π-allyl complexes could occur through reversible protonation−deprotonation by the HSbF6 generated following C−H activation. syn,syn-1,3-Dimethyl-πallyl complex (pS)-2 was subsequently obtained in good yield (88%) and high purity, as evident from the 1H and 13C NMR spectra (Figures 1SI and 2SI). The distorted η5-coordination of the indenyl ligand in (pS)-2 is confirmed by the presence of scalar couplings in the 13C{1H} NMR spectrum between the 103 Rh nucleus and all five carbon atoms of the indenyl C5 ring, with 1JC,Rh ranging from 2.7 to 5.0 Hz. Coordination of the pendant sulfur is confirmed by a scalar coupling between rhodium and the naphthalene C3′ (see Figure 1 for numbering scheme), with 3JC,Rh = 1.0 Hz, as well as an unresolved 2JC,Rh scalar coupling to C2′ leading to broadening of this signal; in addition there is a scalar coupling from rhodium in the 1H NMR spectrum to the methyl group on sulfur, with 3JH,Rh = 1.6 Hz. Coordination of a syn,syn-1,3-dimethyl-π-allyl ligand is confirmed in the 1H NMR spectrum by the appearance of the H2″ signal at δ 5.44 ppm with 3J = 10.0 Hz scalar couplings to both H1″ and H3″. In the 13C{1H} NMR spectrum, C1″, C2″, and C3″ display 1JC,Rh scalar couplings of 8.3, 5.4, and 12.2 Hz, respectively. The differences in the coupling constants of the π-

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RESULTS AND DISCUSSION Shibata, Tanaka, and co-workers7 have recently reported that the reaction of [(1,3-di(ethoxycarbonyl)-2,4,5trimethylcylopentadienyl)RhCl2]2 with 1-hexene in the presence of silver salts and cesium acetate provides a syn-1-propylπ-allyl complex with high yield and regioselectivity. In contrast, © XXXX American Chemical Society

Received: November 22, 2017

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DOI: 10.1021/acs.organomet.7b00841 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Synthesis of π-Allyl Complex (pS)-2 and Reaction with Dimethyl Malonate Anion

allyl terminal carbons are consistent with observations previously made involving epimeric η5:κS-indenyl-sulfanyl and -sulfinyl rhodacycles of 2-phenylpyridine,6 where both the 1JC,Rh and 1JN,Rh scalar couplings for the 2-phenylpyridine ligand are larger when the C or N atom is anti to the indenyl benzo-ring. To rationalize the formation of (pS)-2, density functional theory (DFT) calculations comparing the Gibbs free energies of the isomeric syn,syn-1,3-dimethyl-π-allyl, syn-1-ethyl-π-allyl, and syn,anti-1,3-dimethyl-π-allyl complex cations were carried out (all geometry optimizations were at the PBE-D2/631G(d)/LANL2DZ(Rh)/C-PCM(THF) level, with potential energies further refined at the PBE-D2/6-311+G(2df,2p)/def2TZVPPD(Rh)/C-PCM(THF) level). The lowest energy syn,syn-1,3-dimethyl-π-allyl complex cation was found to be 19.1 kJ/mol lower in energy than the lowest energy syn-1-ethyl-

Figure 1. Numbering scheme for NMR assignments and key NOE interactions observed in the 1H−1H NOESY spectrum of (pS)-2.

Figure 2. Calculated structures and relative Gibbs free energies of the syn- and anti-SMe isomers of the exo-rotamer of the (pS)-2 complex cation, of the lowest energy 1-ethyl-π-allyl complex cation and the lowest energy syn,anti-1,3-dimethyl-π-allyl complex cation, and the syn- and anti-SMe isomers of the endo-rotamers of the (pS)-2 complex cation. B


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Organometallics

Figure 3. Calculated structures and relative Gibbs free energies of the transition states and products for the reaction of the exo-rotamer of the (pS)-2 complex cation with NH3. Bond distances are in Å.

π-allyl complex cation and 15.1 kJ/mol lower in energy than the lowest energy syn,anti-1,3-dimethyl-π-allyl complex cation (Figure 2). The formation of (pS)-2 is therefore consistent with a thermodynamic outcome resulting from acid-catalyzed equilibration of the π-allyl ligand. Complex (pS)-2 was next treated with the sodium salt of dimethyl malonate anion in THF solution for 20 h at room temperature. The presumed intermediate Rh(I) complex formed in the addition reaction proved to be unstable; however, liberated ligand (R)-3 could be isolated in 68% yield. The absolute configuration of 3 was assigned on the basis of the sign of the specific rotation, and an enantiomeric excess (ee) of 66% determined by 1H NMR analysis in the presence of the chiral shift reagent Eu(hfc)3. The outcome of the reaction was essentially identical when the sodium counterion was replaced with tetrabutylammonium cation. To rationalize the factors that determine the asymmetric induction, a combination of NMR analysis and DFT calculations were used. In the absence of a solid-state structure, the 1H−1H NOESY spectrum of (pS)-2 was examined to determine critical stereochemical issues. Key NOE interactions observed in the spectrum (Figure 3SI) are illustrated in Figure 1 and indicate that the π-allyl ligand has an exo-orientation (i.e., the π-allyl H2″ is syn to the indenyl ligand), and the methyl substituent on sulfur is oriented syn to the indenyl benzo-ring. Since the barrier to pyramidal inversion at sulfur in sulfane−metal complexes is typically low (40−80 kJ/mol), 9 this configuration is likely to be thermodynamically preferred. DFT calculations modeling the

exo-rotamer of the complex cation with the both the syn-SMe and anti-SMe orientations indicate that the syn-orientation is 12.1 kJ/mol lower in energy (Figure 2). This preference appears to be largely electronic in nature. DFT modeling of the corresponding indenyl 2-desmethyl complex cations reveals a similar 11.9 kJ/mol preference for the syn-SMe isomer (Figure 4SI), and this preference is substantially reduced in the tetrahydroindenyl complex cations (vide inf ra). The syn-SMe orientation has also previously been observed in the solid state structures the rac-1 complex8 and a 2-phenylpyridene rhodacyclic complex derived from rac-1.6 DFT modeling of the endo-rotamers of the syn- and anti-SMe-2 complex cations indicate that they are 16.0 and 27.8 kJ/mol higher in energy, respectively, than the exo-rotamer of syn-SMe-2 (Figure 2). Again, the energy difference between the syn- and anti-SMe isomers for the endo-rotamers is around 12 kJ/mol. Given that the yield and enantioselectivity of the reaction of (pS)-2 with dimethyl malonate anion was insensitive to the counterion, DFT calculations of the reaction of the complex cation of (pS)-2 with ammonia were undertaken. Norrby and co-workers10 have recommended ammonia as a good model nucleophile for malonate in DFT calculations of palladiumcatalyzed allylic substitution reactions, which avoids the complications associated with charge separation in combination with the continuum solvent model. The structures and relative Gibbs free energies for syn- and anti-attack by ammonia (with respect to the indenyl benzo-ring) onto syn-SMe-2 and antiSMe-2 are given in Figure 3, together with the resulting C


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Organometallics

for one of the ring-flipped conformers are given in Figure 5, together with the relative energies for the corresponding

products (required for discussions below). Calculations for the reaction of the endo-rotamers were also carried out, with the transition state structures illustrated in Figure 4. The relative

Figure 5. Calculated structures and relative Gibbs free energies of the syn- and anti-SMe isomers of the (pS)-tetrahydro-2 complex cation. Values in parentheses are for the corresponding ring-flipped conformers.

alternative ring-flipped conformations (structures for the alternative ring-flipped conformers are illustrated in Figure 5SI). The energy differences between the syn- and anti-SMe isomers are now seen to be small, consistent with purely steric interactions, which lends support for electronics (i.e., a structural indenyl effect) being largely responsible for the preferred syn-SMe orientation in the indenyl complexes (vide supra). The calculated transition state structures and relative Gibbs free energies for the attack of ammonia onto (pS)-tetrahydro-2 complex cations illustrated in Figure 5 are given in Figure 6, together with the resulting products. Bond lengths and relative energies are also given for the corresponding ring-flipped conformers (structures for the alternative ring-flipped conformers are illustrated in Figure 6SI). Comparing bond distances in the transition states for the indenyl complexes against the corresponding tetrahydroindenyl complexes, the C−N bonds are shorter, and the Rh−C(bond breaking) bonds are longer, for the tetrahydroindenyl analogues, indicating that the tetrahydroindenyl transition states are later than the indenyl. This is also reflected in the close correlation in the relative energies of the transition states and the corresponding products in the case of the tetrahydroindenyl complexes (the average difference between the relative energies being 1.0 kJ/ mol), indicating significant product control of the selectivity. In the case of the indenyl complexes, however, discrepancies between the relative energies of the transition states and the corresponding products are evident, consistent with an earlier transition state. The relative energy of the syn-SMe-4 transition state for anti-NH3 attack is 4.5 kJ/mol higher than the relative energy of the corresponding product; conversely, the relative energy of the anti-SMe-4 transition state for syn-NH3 attack is 3.9 kJ/mol lower than the relative energy of the corresponding product (a less substantial increase in the relative energy of the anti-SMe-4 transition state for anti-NH3 attack is also apparent). The modeling correctly predicts the favored formation of (R)-3, and the magnitude of the free energy differences observed for the ammonia model nucleophile are consistent with the experimentally observed enantioselectivity

Figure 4. Calculated structures and relative Gibbs free energies, with respect to syn-NH3-syn-SMe-4⧧, of the transition states for the reaction of the endo-rotamers of the syn- and anti-SMe-2 complex cations with NH3. Bond distances are in Å.

Gibbs free energies, with respect to the syn-NH3-syn-SMe-2 exorotamer transition state (syn-NH3-syn-SMe-4⧧, Figure 3), for the endo transition states were 11.2−26.7 kJ/mol higher in energy for three of the possibilities, but for the syn-NH3-synSMe-endo-4⧧ transition state, the energy was 1.6 kJ/mol lower. However, should a Curtin−Hammett scenario be operating, preferential reaction via this transition state would, contrary to the experimental result, lead to a low ee of the (S)-3 product. This indicates that the rate for exo−endo rotamer interconversion must be noncompetitive with the rate of nucleophilic addition, and the reaction occurs substantially through the exorotamer. In order to examine the role of electronic versus steric factors, DFT calculations were also carried out for the tetrahydroindenyl derivative of the exo-rotamer of (pS)-2. In addition to the two possible orientations of the methyl substituent on sulfur, two ring-flipped conformations of the tetrahydroindenyl ring were also modeled. The structures and relative Gibbs free energies for the syn- and anti-SMe isomers D


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Organometallics

Figure 6. Calculated structures and relative Gibbs free energies of the transition states and products for the reaction of the (pS)-tetrahydro-2 complex cation with NH3. Bond distances are in Å. Values in parentheses are for the corresponding ring-flipped conformers.

the lengthening of the Rh−C1″ bonds in the indenyl case can be electronic as well as steric in nature, so it does not provide a direct insight into the origin of the energetic discrepancies detailed above. Another metric used to account for the relative reactivity of the terminal carbons of π-allyl palladium complexes are 13C NMR chemical shifts,12 where the more electrophilic terminal carbon usually resonates further downfield. As noted previously, C1″ has a smaller 1JC,Rh scalar coupling than C3″ in the 13C NMR spectrum of (pS)-2; however, C1″ resonates 1.4 ppm upfield from C3″, making it difficult to draw any firm conclusions. Consistent with an early transition state, an examination of the frontier orbitals appears to offer the clearest insight into the origins of the selectivity for the reaction of the indenyl complex. LUMO maps, where the absolute value of the LUMO is mapped onto an electron density surface, for syn- and anti-SMe-2 and syn- and anti-SMe-H4-2 complex cations are given in Figure 7. While the electrophilic site (blue) on the πallyl ligand is symmetrically distributed for the tetrahydroindenyl complexes, in the indenyl cases the electrophilic site is strongly polarized toward the π-allyl terminal carbon that is syn to the indenyl benzo-ring. LUMO maps were also generated using the functionals B3LYP, M06L, and M06, and these display congruent results (Figures 7SI−9SI).

(an energy difference of 4.7 kJ/mol equating to a 74% ee at 298 K). Previous studies focused on steric influences in the nucleophilic attack on π-allyl palladium complexes have related the lengths of the Pd−C bonds to the terminal carbons with their relative reactivity, where the longer Pd−C bond usually corresponds to the more reactive allyl terminus (each 0.01 Å elongation giving approximately a doubling in reactivity),11a although other factors may override this preference.11b In the case of the calculated structures for the exo-rotamer of the (pS)2 complex cation, the Rh−C1″ bond is 0.031 Å longer than the Rh−C3″ bond for the syn-SMe isomer, while it is 0.036 Å longer for the anti-SMe isomer, which is consistent with the calculated preferred nucleophilic attack syn to the indenyl benzo-ring. However, for the tetrahydroindenyl analogues, the Rh−C1″ bonds are also longer than the Rh−C3″ bonds, with differences (averages of the two ring-flipped conformers) of 0.017 Å for the syn-SMe isomers, and 0.046 Å for the anti-SMe isomers, which correlates well with the larger energy differences calculated for syn- versus anti-NH3 attack on the anti-SMe isomers compared with the syn-SMe isomers. For the tetrahydroindenyl complexes these results can be interpreted through purely steric arguments, with steric compression leading to an elongation of one Rh−C(terminal) bond in the π-allyl complex, which is then followed by greater steric relaxation in the transition state and product resulting from the breaking of this bond. While this metric also coincides with the calculated preferences in the indenyl reactions, the origins of

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CONCLUSIONS Planar chiral indenylrhodium dichloride complex (pS)-1, following chloride abstraction with AgSbF6, reacts with the E


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Organometallics

standing over activated 4 Å molecular sieves for at least 72 h prior to use. NMR spectra were recorded at 300 K on a Bruker Avance 300 or Bruker Avance III 500 spectrometer. 1H and 13C NMR chemical shifts were referenced to internal Me4Si. 1H−1H COSY, 1H−1H NOESY, 1H−13C HSQC, and 1H−13C HMBC experiments were used in assigning the NMR spectra. Optical rotations were measured with a PerkinElmer Model 341 polarimeter using a 25 × 5 mm cell. Lowresolution ESI mass spectrometry was carried out with a Bruker amaZon SL ion trap mass spectrometer with enhanced resolution mode in both positive and negative ion modes. High-resolution ESI mass spectrometry was carried out with a Bruker Apex Qe 7T Fourier Transform Ion Cyclotron Resonance mass spectrometer equipped with an Apollo II ESI/MALDI dual source in positive ion ESI mode. The most abundant ion in an isotopic envelope is quoted, with the relative intensities of all ions in each envelope agreeing with that calculated for the proposed formulas. Synthesis of (pS)-2. Complex (pS)-18 (50 mg, 0.102 mmol) was dissolved in DCE (5 mL) and the solution transferred to a tube flask equipped with a J. Young valve. (E)-2-Pentene (35 μL, 3.24 mmol) was added, followed by a solution of AgSbF6 in DCE (2.3 mL of a 0.088 M solution, 0.202 mmol). The flask was sealed and stirred at rt, with protection from light, for 24 h. The mixture was diluted with CH2Cl2 (10 mL), K2CO3 (20 mg, 0.145 mmol) added, and the mixture stirred vigorously for 15 min. The mixture was filtered through Celite, washing the solid residue with CH2Cl2, and the solvent then removed from the filtrate under reduced pressure. The residue was passed through a short column of silica (silica gel 60, 230−400 mesh) eluting with 97:3 CH2Cl2/EtOAc, collecting an orange band, which was then concentrated under reduced pressure. The resulting orange glass was powdered and dried under high vacuum. Yield: 64 mg 1 (88%). [α]20 D = − 437 (c 1.0, CHCl3). H NMR (CDCl3, 500 MHz): δ 0.86 (d, 3H, 3JMe,H1″ = 6.3 Hz, 1″-CH3), 1.72 (d, 3H, 3JMe,H3″ = 6.3 Hz, 3″-CH3), 2.05 (d, 3H, 3JMe,Rh = 1.4 Hz, 2-CH3), 2.53 (d, 3H, 3JMe,Rh = 1.6 Hz, SCH3), 2.76 (dq, 1H, 3JH2″,H1″ = 10.0 Hz, 3JMe,H1″ = 6.3 Hz, H1″), 3.25 (dq, 1H, 3JH2″,3″ = 10.0 Hz, 3JMe,H3″ = 6.3 Hz, H3″), 5.44 (dd, 1H, 3JH2″,H1″ = 10.0 Hz, 3JH2″,H3″ = 10.0 Hz, H2″), 6.06 (s, 1H, H3), 6.99 (br d, 1H, 3JH6,H7 = 8.3 Hz, H7), 7.26 (br d, 1H, 3JH4,H5 = 8.3 Hz, H4), 7.31 (ddd, 1H, 3JH6,H7 = 8.3 Hz, 3JH6,H5 = 7.0 Hz, 4JH6,H4 = 1.0 Hz, H6), 7.58 (ddd, 1H, 3JH4,H5 = 8.3 Hz, 3JH6,H5 = 7.0 Hz, 4JH5,H7 = 0.9 Hz, H5), 7.63 (ddd, 1H, 3JH7′,H8′ = 8.1 Hz, 3JH7′,H6′ = 6.7 Hz, 4JH5′,H7′ = 1.2 Hz, H7′), 7.66 (d, 1H, 3JH3′,H4′ = 8.8 Hz, H3′), 7.69 (br d, 1H, 3 JH7′,H8′ = 8.1 Hz, H8′), 7.72 (ddd, 1H, 3JH5′,H6′ = 8.1 Hz, 3JH7′,H6′ = 6.7 Hz, 4JH6′,H8′ = 1.2 Hz, H6′), 8.09 (br d, 1H, 3JH5′,H6′ = 8.1 Hz, H5′), 8.18 (br d, 1H, 3JH3′,H4′ = 8.8 Hz, H4′). 13C{1H} NMR (CDCl3, 125 MHz): δ 11.39 (2-CH3), 16.45 (1″-CH3), 19.94 (3″-CH3), 25.69 (SCH3), 74.97 (d, 1JC,Rh = 8.3 Hz, C1″), 76.34 (d, 1JC,Rh = 12.2 Hz, C3″), 76.84 (d, 1JC,Rh = 4.6 Hz, C3), 88.02 (d, 1JC,Rh = 5.4 Hz, C2″), 105.90 (d, 1JC,Rh = 4.0 Hz, C7a), 112.23 (d, 1JC,Rh = 4.0 Hz, C2), 112.37 (d, 1JC,Rh = 5.0 Hz, C1), 114.65 (d, 1JC,Rh = 2.7 Hz, C3a), 118.68 (C4), 120.88 (C7), 124.50 (C8′), 127.75 (C6), 128.59 (d, 3JC,Rh = 1.0 Hz, C3′), 128.81 (C5′), 128.96 (C7′), 129.02 (C6′), 131.76 (C5), 131.93 (C8a′), 132.80 (C4′), 133.23 (C1′), 133.74 (C4a′), 144.30 (br s, C2′). + ES-MS: m/z 473.05 ([M − SbF6]+, 100%). − ESMS: m/z 234.86 ([SbF6]−, 100%). + ES-HRMS: m/z 473.08079 ([M − SbF6]+, 100%); calcd for C26H26RhS, 473.08048. Preparation of Dimethyl Malonate Anion Salts. Sodium hydride (220 mg of a 60% mineral oil dispersion, 5.5 mmol) was washed three times with hexane, dried under vacuum, then suspended in THF (10 mL). Dimethyl malonate (0.57 mL, 5.0 mmol) was added dropwise with ice-cooling, and the mixture then allowed to stand overnight. A portion of the supernatant solution (5.4 mL, 2.7 mmol) was added to Bu4NCl (0.744 g, 2.7 mmol), the mixture stirred at rt for 1 h, and then allowed to stand overnight to allow the precipitated NaCl to settle. The supernatant solutions of each salt (ca. 0.5 M) were used in subsequent reactions. Synthesis of (R)-3. To a solution of complex (pS)-2 (30 mg, 0.042 mmol) in THF (3 mL) was added a solution of the sodium salt of dimethyl malonate anion in THF (170 μL of 0.5 M, 0.085 mmol) and the mixture stirred at rt for 20 h. The reaction was diluted with diethyl ether (10 mL), stirred for 10 min, then diluted with pentane (10 mL),

Figure 7. LUMO maps for syn- and anti-SMe-2 and syn- and anti-SMeH4-2 complex cations. Isosurface values range from 0 (red) to 0.015 (blue).

unactivated alkene (E)-2-pentene in the absence of a base to provide thermodynamic C−H activation product (pS)-2, presumably through acid-catalyzed equilibration of the π-allyl ligand. The reaction is also notable with respect to evaluating the electron-donating ability of the indenyl ligand, since an analogous reaction occurs readily with the 1,3-di(ethoxycarbonyl)-2,4,5-trimethylcylopentadienyl ligand but not with the more electron-rich pentamethylcyclopentadienyl ligand.7 Reaction of (pS)-2 with dimethyl malonate anion provided nucleophilic addition product (R)-3 with a 66−68% enantiomeric excess. DFT calculations suggest that asymmetric induction results from an early transition state, with an examination of the frontier orbitals offering the clearest insight into the preference for attack by the nucleophile at the allylic termini that is syn to the indenyl benzo-ring. The manifestation of the structural indenyl effect proposed here has some similarity to other reactions of transition metal-bound π-allyl ligands with nucleophiles where asymmetric electronic effects are proposed to act, such as the reactions of metal-centeredchiral half-sandwich molybdenum complexes13 and asymmetric allylic alkylation reactions catalyzed by palladium complexes with chiral heterobidentate ligands (such as P−N or P−S).14 In both cases, trans effects are evoked to rationalize the preferred direction of nucleophilic attack. While the reaction studied here is stoichiometric, this effect may potentially be exploited in catalytic asymmetric transformations involving chiral indenylmetal complexes.

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EXPERIMENTAL SECTION

General Considerations. Reactions involving air- or moisturesensitive compounds were performed under an atmosphere of nitrogen using standard Schlenk techniques. Tetrahydrofuran (THF) was dried by passing through a column of activated alumina under a nitrogen atmosphere. 1,2-Dichloroethane (DCE) was dried by F


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and stirred a further 10 min. The mixture was filtered through Celite, washing the solid residue with pentane, and the solvent then removed from the filtrate under reduced pressure. Flash chromatography, eluting with 9:1 pentane/ether, provided diester 3 as colorless oil. Yield: 5.8 mg (68%). [α]20 D + 27 (c 0.7, CHCl3). Repeating the experiment with the tetrabutylammonium salt of dimethyl malonate anion afforded diester 3 as a colorless oil (5.9 mg, 70% yield), [α]20 D + 24 (c 0.5, CHCl3). The 1H NMR (CDCl3, 300 MHz) spectrum of diester 3 was identical, within experimental error, to that previously reported.10c The enantiomeric excess of the samples were 66 and 68%, respectively, determined by 1H NMR (CDCl3, 300 MHz) in the presence of Eu(hfc)3 (1.5 equiv., ca. 40 mM final concentration), which separated the diastereotopic methoxy signals of the enantiomers: δ 4.89 and 4.69 (for (S)-3), δ 4.83 and 4.76 (for (R)-3). Computational Details. Calculations were carried out using Spartan ’16 (ver. 2.0, Wave function Inc., Irvine, CA), which interfaces with the Q-Chem (ver. 4.4) program suite15 for DFT calculations. The PBE method16 was chosen as it has been previously used to study a number of metal-mediated reactions;17 in particular, a recent benchmarking study of iridium-mediated reactions17i found that PBE together with Grimme’s DFT-D2 empirical dispersion corrections18 outperformed a number of other methods. Geometry optimizations and vibrational frequency calculations were carried out using the effective core potentials (ECPs) of Hay and Wadt with the double-ζ valence basis set LANL2DZ19 for Rh and the 6-31G(d) basis set for all other elements. Solvation correction was applied in the course of the optimizations with THF as the solvent utilizing the C-PCM model.20 All transition structures contained only one imaginary frequency, with atom displacements consistent with the proposed reaction pathway, and all local minimum structures had no imaginary frequency. To further refine potential energies obtained from the double-ζ valence basis set calculations, single-point energy calculations (PBE-D2 method with C-PCM(THF) solvent correction) were carried out employing the triple-ζ valence basis set def2-TZVPPD21 on Rh, along with the corresponding ECP, and the 6-311+G(2df,2p) basis set for all other elements. To estimate the corresponding Gibbs free energies, thermal corrections calculated at the double-ζ valence basis set level were added to the single-point potential energies. All thermodynamic data were calculated for the standard state (298.15 K and 1 atm). LUMO maps were generated using Spartan ’16. The LUMO maps generated using the B3LYP,22 M06L,23 and M0624 functionals were based on single-point calculations using the PBE-D2 geometries and the 6-311+G(2df,2p)/def2-TZVPPD(Rh) basis sets. Figures 4SI−9SI illustrate calculated structures not shown in the main manuscript, and Table 1SI summarizes the numerical results for all calculated structures.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00841. 1 H, 13C{1H} and 1H−1H NOESY NMR spectra of (pS)2, additional illustrations of calculated structures and LUMO maps, and tabulated numerical results for all calculated structures (PDF) Cartesian coordinates of all calculated structures (XYZ)

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Robert W. Baker: 0000-0001-8702-5072 Notes

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