Sulfinylmethyl Phosphines as Chiral Ligands in the Intermolecular

Jul 7, 2009 - Synopsis. A new family of enantiomerically pure p-tolyl and tert-butyl sulfinylmethyl phosphine ligands with an extra chiral center in t...
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Organometallics 2009, 28, 4571–4576 DOI: 10.1021/om900438v

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Sulfinylmethyl Phosphines as Chiral Ligands in the Intermolecular Pauson-Khand Reaction Catalina Ferrer, Antoni Riera,* and Xavier Verdaguer* Unitat de Recerca en Sı´ntesi Asim etrica (URSA-PCB), Institute for Research in Biomedicine (IRB Barcelona), and Departament de Quı´mica Org anica, Universitat de Barcelona, c/Baldiri Reixac 10, E-08028 Barcelona, Spain Received May 25, 2009

A new family of enantiomerically pure p-tolyl and tert-butyl sulfinylmethyl phosphine ligands is described. Ligands with an extra chiral center in the central carbon atom were prepared by phosphinylation of benzyl and homobenzyl sulfoxides. Ligand exchange reaction of these compounds with Co2-alkyne complexes afforded up to 6:1 dr. The resulting bridged complexes were tested in the intermolecular Pauson-Khand reaction to provide up to 97% ee.

Introduction Since its discovery in 1973, the ability to synthesize cyclopentenones in a straightforward way aroused interest in the Pauson-Khand reaction (PKR)1 and has led to the development of new complexes and catalysts capable of performing the reaction in an enantioselective fashion.2 Apart from cobalt, other metals, such as Ru,3 Ir,4 Ni,5 or Rh,6 have also been successfully used in the intramolecular version of this reaction. Currently, one of the major challenges still to be tackled is the development of an efficient catalyst for the asymmetric intermolecular version of the PKR. In our efforts to find efficient ligands for the asymmetric PKR,7 we have recently reported a new family of N-phosphine sulfinamide (PNSO) ligands that give high yields and high ee’s in the asymmetric intermolecular PKR (Figure 1).8 *Corresponding author. E-mail: [email protected]; [email protected]. (1) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E.; Foreman, M. I. J. Chem. Soc., Perkin Trans. 1 1973, 977–981. (2) For general reviews see: (a) Laschat, S.; Becheanu, A.; Bell, T.; Baro, A. Synlett 2005, 2547–2570. (b) Gibson, S. E.; Mainolfi, N. Angew. Chem., Int. Ed. 2005, 44, 3022–3037. (c) Gibson, S. E.; Stevenazzi, A. Angew .Chem., Int. Ed. 2003, 42, 1800–1810. (d) Bonaga, L. V. R.; Krafft, M. E. Tetrahedron 2004, 60, 9795–9833. (e) Shibata, T. Adv. Synth. Catal. 2006, 348, 2328–2336. (f) Rivero, M. R.; Adrio, J.; Carretero, J. C. Synlett 2005, 26–41. (g) Struebing, D.; Beller, M. Top. Organomet. Chem. 2006, 18, 165–178. (3) Morimoto, T.; Chatani, N.; Fukumoto, Y.; Murai, S. J. Org. Chem. 1997, 62, 3762–3765. (4) Shibata, T.; Takagi, K. J. Am. Chem. Soc. 2000, 122, 9852–9853. (5) Pages, L.; Llebaria, A.; Camps, F.; Molins, E.; Miravitlles, C.; Moreto, J. M. J. Am. Chem. Soc. 1992, 114, 10449–10461. (6) Jeong, N.; Sung, B. K.; Choi, Y. K. J. Am. Chem. Soc. 2000, 122, 6771–6772. (7) (a) Verdaguer, X.; Moyano, A.; Pericas, M. A.; Riera, A.; Maestro, M. A.; Mahia, J. J. Am. Chem. Soc. 2000, 122, 10242–10243. (b) Verdaguer, X.; Pericas, M. A.; Riera, A.; Maestro, M. A.; Mahia, J. Organometallics 2003, 22, 1868–1877. (c) Verdaguer, X.; Lledo, A.; LopezMosquera, C.; Maestro, M. A.; Pericas, M. A.; Riera, A. J. Org. Chem. 2004, 69, 8053–8061. (d) Sola, J.; Riera, A.; Verdaguer, X.; Maestro, M. A. J. Am. Chem. Soc. 2005, 127, 13629–13633. (8) (a) Sol a, J.; Reves, M.; Riera, A.; Verdaguer, X. Angew. Chem., Int. Ed. 2007, 46, 5020–5023. (b) Reves, M.; Achard, T.; Sola, J.; Riera, A.; Verdaguer, X. J. Org. Chem. 2008, 73, 7080–7087. r 2009 American Chemical Society

The main feature of these bidentate ligands is that the phosphorus atom provides metal affinity, while the sulfur moiety is the source of chirality. However, few examples of phosphorus-sulfoxide bidentate ligands have been described in the literature.9,10 One of the few examples of phosphorussulfoxide bidentate ligands described is the p-tolylsulfinylmethyl phosphine 1 (Figure 1). When pure, compound 1 is stable toward oxygen migration, although it can be prepared in only 23% yield.11 Considering the success achieved with the use of PNSO ligands in the asymmetric intermolecular PKR, we thought it important to study the effect of changing the central nitrogen for a carbon atom and the introduction of an extra chiral center in the ligand. Thus, here we describe the synthesis of a family of ligands of general formula III (PCSO ligands), their coordination behavior toward alkyne-dicobalt complexes, and the intermolecular asymmetric PKR of the resulting complexes.

Results and Discussion The general strategy for the preparation of PCSO ligands was the reaction of the corresponding R-sulfinyl carbanions with Ph2PCl (Scheme 1). Thus, ligand 1 was prepared by alkylation of commercially available (R)-p-tolyl methyl sulfoxide (2) with chlorodiphenyl phosphine at -78 °C, followed by protection in situ with borane to prevent oxygen migration. Using this strategy, the yield of compound 1-BH3 was improved from 23% to 43%. For the preparation of PCSO ligands bearing an additional chiral center on the central carbon atom, benzyl and homobenzyl sulfoxides 3 and 5 were used as starting (9) (a) Hiroi, K.; Suzuki, Y.; Kawagishi, R. Tetrahedron Lett. 1999, 40, 715–718. (b) Wenschuh, E.; Fritzsche, B. J. Prakt. Chem. 1970, 312, 129–134. (10) For general reviews on chiral sulfoxides see: (a) Fernandez, I.; Khiar, N. Chem. Rev. 2003, 103, 3651–3705. (b) Mellah, M.; Voituriez, A.; Schulz, E. Chem. Rev. 2007, 107, 5133–5209. (11) Alcock, N. W.; Brown, J. M.; Evans, P. L. J. Organomet. Chem. 1988, 356, 233–247. Published on Web 07/07/2009

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Ferrer et al.

Figure 1. Phosphino-sulfinamide (PNSO) and sulfinylmethyl phosphines (PCSO) ligands. Scheme 1. Synthesis of p-Tolyl PCSO Ligands

Figure 2. X-ray crystal structure of 6-BH3. Scheme 2. Synthesis of tert-Butyl PCSO Ligands

materials (Scheme 1). Enantiomerically pure sulfoxides 3 and 5 were obtained by reaction of (1R,2S,5R)-(-)-menthyl (S)-p-toluenesulfinate with a Grignard reagent.12 Reaction of the sulfoxide anions with chlorodiphenyl phosphine at -78 °C and in situ protection with borane led to the desired ligands 4-BH3 and 6-BH3. As expected, phosphinylation of lithium R-sulfinyl carbanions with chlorodiphenyl phosphine proved to be highly diastereoselective, providing the corresponding SS,CR isomer as a major product. Ligand 4-BH3 was obtained as a 13:1 mixture of diastereomers, from which the major SS,CR compound was isolated by crystallization. Moreover, the reaction with p-tolyl homobenzyl sulfoxide was completely stereoselective. The relative configuration for these compounds was firmly established by single-crystal X-ray analysis of 6-BH3 (Figure 2). Addition of Ph2PCl occurs with stereoselectivity similar to the addition of D2O, MeI, ketones, and imines to lithium R-sulfinyl carbanions.13,14 Electrophilic assistance of the Li ion bound to the sulfoxide oxygen with concomitant inversion of the R-sulfinyl carbanion accounts for the stereochemistry observed.15 Vedejs and co-workers used the reaction of lithium R-sulfinyl carbanions with Ph2PCl in the synthesis of thiol esters; however, they used racemic sulfoxides, and the intermediate phosphine-sulfoxides were not isolated.16 Thus, with the help of borane-phosphorus protection, optically pure phosphine-sulfoxides 4 and 6 could be isolated for the first time. For the synthesis of the analogous optically pure tert-butyl PCSO ligands, a similar methodology was pursued (Scheme 2). In this case, addition of the Grignard reagents to Ellman’s (R)tert-butanethiosulfinate gave the corresponding enantiomerically pure sulfoxides 7 and 10.17 Metalation of tert-butyl benzyl (12) Maitro, G.; Vogel, S.; Sadaoui, M.; Prestat, G.; Madec, D.; Poli, G. Org. Lett. 2007, 9, 5493–5496. (13) Solladie, G.; Moine, G. J. Am. Chem. Soc. 1984, 106, 6097–6098. (14) Crucianelli, M.; Bravo, P.; Arnone, A.; Corradi, E.; Meille, S. V.; Zanda, M. J. Org. Chem. 2000, 65, 2965–2971. (15) Marsh, M.; Massa, W.; Harms, K.; Baum, G.; Boche, G. Angew. Chem., Int. Ed. Engl. 1986, 25, 1011–1012. (16) Vedejs, E.; Meier, G. P.; Powell, D. W.; Mastalerz, H. J. Org. Chem. 1981, 46, 5253–5254. (17) Cogan, D. A.; Liu, G.; Kim, K.; Backes, B. J.; Ellman, J. A. J. Am. Chem. Soc. 1998, 120, 8011–8019.

sulfoxide 7 with MeLi at -78 °C and reaction with Ph2PCl gave the desired PCSO ligands in 65% yield as a 6:1 diastereomeric mixture. The selectivity was increased to 20:1 using n-BuLi at 0 °C. However, the yield in this case was only 25%. The major diastereomer SR,CS was purified by flash chromatography. Following the same experimental procedure, the analogous tert-butyl homobenzyl sulfoxide 10 provided only trace amounts of the desired PCSO ligand along with diphenyl styryl phosphine as the major reaction product (Scheme 2). The desired product may have been unstable due to a competing sulfoxide β-elimination pathway. Deprotection of the borane group with DABCO and complexation of the PCSO ligands to dicobaltalkyne complexes were carried out in toluene at 75 °C in a one-pot procedure (Table 1). Reaction of ligand 1-BH3 with trimethylsilylacetylene dicobaltcarbonyl gave only the corresponding pentacarbonyl complex, in which only the phosphorus atom was coordinated to cobalt, in a 1:1 mixture of diastereomers (Table 1, entry 1). Extended heating at higher temperature produced small amounts of bridged complex and led to decomposition of the starting material. Conversely, ligand exchange reaction with 4-BH3 with either trimethylsilylacetylene dicobaltcarbonyl (Table 1, entry 2) or phenylacetylene dicobaltcarbonyl (Table 1, entry 3) gave the corresponding bridged complexes in good yield as a 1:1 mixture of diastereomers. Ligand 6-BH3 proved to be somewhat more selective, and complexes 13a/13b and 14a/14b were obtained in a 2:1 diastereomeric ratio (Table 1, entries 4 and 5). Complexes 13a/13b and 14a/14b derived from ligand 6-BH3 were more prone to decomposition than those derived from ligand 4-BH3, and they were isolated in lower yield. All these diastereomeric mixtures were separated by silica gel chromatography except for complex 14b. Diastereomeric complexes can also be distinguished from one another by 1H NMR spectroscopy by means of the terminal

Article

Organometallics, Vol. 28, No. 15, 2009 Table 1. Complexation of p-Tolyl PCSO Ligands

entry 1 2 3 4 5

ligand 1-BH3 4-BH3 4-BH3 6-BH3 6-BH3

R H Ph Ph Bn Bn

X TMS TMS Ph TMS Ph

t (h) 16 1 6 1 1

yield (%)

d.r.a

complex

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Table 2. Pauson-Khand Reaction of Optically Pure Complexes with Norbornadiene

entry

complex

L

X

t (h)

yield (%)

eea

product

1 2 3 4 5 6 7 8b

11a 11b 12a 12b 13a 13b 14a 16b

4 4 4 4 6 6 6 9

TMS TMS Ph Ph TMS TMS Ph Ph

16 16 6 1 0.5 0.5 0.5 5

84 87 79 90 77 94 98 75

75 66 88 97 83 96 87 88

(þ)-17 (-)-17 (þ)-18 (-)-18 (þ)-17 (-)-17 (þ)-18 (-)-18

b

1:1 1.2:1 1:1 2:1 2:1

62 60 36 33

a As determined by 1H NMR spectroscopy. bonylic complex was obtained.

b

11a/11b 12a/12b 13a/13b 14a/14b

Only the pentacar-

Scheme 3. Complexation of Ligand 8-BH3

b

a Enantiomeric excess determined by either chiral GC or HPLC. Reaction carried out at room temperature.

Scheme 4. Isomerization of Ligand 8-BH3 and Complexation of Ligand 9-BH3 with Phenylacetylene Dicobaltcarbonyl

alkyne resonance. In the Xa series of diastereomers this proton appears at lower field than the one corresponding to the Xb series.18 Similar results to those for the complexation of ligand 1-BH3 were obtained in the reaction of ligand 8-BH3 with phenylacetylene dicobaltcarbonyl. Only coordination of the phosphorus atom was observed, which yielded a 1:1 mixture of pentacarbonylic complexes 15a/15b (Scheme 3). In this case, complexes 15a and 15b were quite stable and were isolated in 92% yield; however, they could not be separated by either crystallization or chromatography. Extended heating for longer periods or higher temperature led only to the decomposition of materials. Treatment of 15a/15b with N-methylmorpholine N-oxide (NMO) in CH2Cl2 at room temperature to create a coordinative vacancy at the cobalt atom that would be occupied by a sulfur atom led only to oxidation of the latter. The difficulty in the coordination of sulfur to the metal center in complexes 15a/15b may be attributed to the steric hindrance found in the closed bridged complexes (Scheme 3). When PCSO ligands 4-8 act as a bridged P,S ligands on the dicobalt complexes, the substituents on sulfur and carbon adopt a cis disposition. While this is not an obstacle for p-tolyl PCSO ligands, the extra bulk of the tert-butyl analogue 8-BH3 prevents the formation of the bridged species. In order to check this hypothesis, we tested ligand 9-BH3 in (18) The stereochemistry of the bridged complexes was assigned on the basis of the optical rotation of the resulting Pauson-Khand adducts (vide infra).

the complexation reaction. A 6:1 diastereomeric mixture of ligands 8-BH3 and 9-BH3 was treated with MeLi at 0 °C for 1 h and then quenched with water (Scheme 4). This treatment resulted in the isomerization of ligand 8-BH3 to give a 1:1 mixture of diastereomers 8-BH3 and 9-BH3, which were separated by chromatography. The reaction of ligand 9-BH3 with phenylacetylene dicobaltcarbonyl proved to be quite selective, and a 1:6 mixture of complexes 16a/16b was obtained in 30% yield (Scheme 4). Ligand 9 provided a trans disposition between the tert-butyl and the phenyl groups in 16a/16b, which allowed the coordination of the sulfinyl fragment to the cobalt center. Nevertheless, the pentacarbonylic complex, in which the sulfur moiety is not coordinated, was isolated in 27% yield. Finally, diastereomerically and optically pure complexes obtained in the complexation reactions were tested in the intermolecular PKR with norbornadiene under thermal activation in toluene at 80 °C (Table 2). Complexes containing PCSO ligands are very active in this process, since all the corresponding product enones were obtained in good yield and good to excellent enantiomeric excess. As a general trend, complexes of the Xa series provided dextrorotatory product enones, while Xb complexes gave levorotatory ones. This observation allowed us to establish the absolute configuration of the bridged series of complexes Xa and Xb. In a similar fashion to that for N-phosphino sulfinamide ligands, dextrorotatory products are indicative of sulfoxide

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(or sulfide) bonding to the Pro-S metal center (Xa series), while levorotatory products entail sulfoxide coordination to the Pro-R cobalt atom (Xb series).7b,19 Complexes of the Xb series tended to provide a more stereoselective reaction (>95% ee) than the Xa series (Table 2, entries 4 and 6) except for complexes 11a and 11b (75% and 66% ee, respectively). In general, we may conclude that p-tolyl PCSO ligands proved to be more selective than their PNSO analogues (II). In contrast, the tert-butyl PCSO ligands proved to be more difficult to synthesize and were less selective than their PNSO analogues (I) in the asymmetric intermolecular PKR.8

Conclusions Here we have synthesized a new family of enantiomerically pure p-tolyl and tert-butyl sulfinylmethyl phosphine (PCSO) ligands. Selective phosphinylation of benzyl and homobenzyl sulfoxides allowed the preparation of ligands with an extra chiral center in the central carbon atom. The ligand exchange reaction of these ligands to Co2-alkyne complexes afforded moderate diastereoselectivities (up to 6:1). The relevance of the relative stereochemistry on the sulfur and carbon centers for the formation of bridged complexes has been disclosed. Thus, for the tert-butyl PCSO analogue, only the SR,CR diastereomer provides the desired bridged complex 16a/16b. Finally, the resulting PCSO cobalt complexes have been tested in the intermolecular PKR, in which the p-tolylsulfinyl ligands 4 and 6 provided up to 97% and 96% ee.

Experimental Section General Methods. All reactions were carried out under a nitrogen atmosphere in dried solvents. THF was dried over sodium/benzophenone, toluene over sodium, and dichloromethane over CaH2. Thin-layer chromatography was carried out using TLC-aluminum sheets with silica gel (Merk 60 F254). Chromatography purifications were carried out using flash grade silica gel (SDS Chromatogel 60 ACC, 35-70 μm). NMR spectra were recorded at 23 °C on a Varian Mercury 400 and on a Varian Unity 300 apparatus. 1H NMR and 13C NMR spectra were referenced either to relative internal TMS or to residual solvent peaks. 31P NMR spectra were referenced to phosphoric acid. Signal multiplicities in the 13C spectra have been assigned by DEPT and HSQC experiments. Optical rotations were recorded on a Perkin-Elmer polarimeter at the sodium D line at room temperature (concentration in g/mL). Melting points were determined using a B€ uchi melting point apparatus and were not corrected. IR spectra were recorded in a FT-IR apparatus. HRMS were recorded using an electrospray ionization spectrometer. HPLC chromatography was performed on an Agilent Technologies Series 1100 chromatograph with UV detector. Enantiomeric purity of sulfoxides 3 and 5 was checked by HPLC chromatography (Chiralcel OD-H) and was shown to be higher than 98% ee, as compared to a racemic standard. Sulfoxide 1020 has been previously described in the literature. (R)-Diphenyl(p-tolylsulfinylmethyl)phosphine, Borane Complex 1-BH3. To a solution of (R)-methyl p-tolylsulfoxide (100 mg, 0.65 mmol) in diethyl ether at -10 °C was added a 1.8 M solution of PhLi (0.39 mL, 0.71 mmol). After stirring for 30 min at this temperature, the solution was cooled to -78 °C and Ph2PCl  (19) Castro, J.; Moyano, A.; Pericas, M. A.; Riera, A.; AlvarezLarena, A.; Piniella, J. F. J. Am. Chem. Soc. 2000, 122, 7944–7952. (20) Casey, M.; Manage, A. C.; Nezhat, L. Tetrahedron Lett. 1988, 29, 5821–5824.

Ferrer et al. (0.13 mL, 0.71 mmol) was added. After 5 min the solution was warmed to room temperature and left for 2 h. Then BH3 3 SMe2 (0.07 mL, 0.71 mmol) was added, and the reaction was left for 30 min more. The solution was quenched with water, and after extractive workup (water/EtOAc) and silica gel chromatography (10:1 hexane/EtOAc) 97 mg (43%) of 1-BH3 was obtained as a white solid. Mp: 141-142 °C. IR (film): νmax 1046, 1436, 2387 cm-1. 1H NMR (400 MHz, CDCl3): δ 0.77-1.62 (br, 3H, BH3), 2.36 (s, 3H), 3.55 (dd, J=14 and 6 Hz, 1H), 3.84 (dd, J= 14 and 8 Hz, 1H), 7.21 (d, J=8 Hz, 2H), 7.36-7.53 (m, 8H), 7.647.83 (m, 4H) ppm. 13C NMR (75 MHz, CDCl3): δ 21.3 (CH3), 56.3 (d, JP=27 Hz, CH2), 124.1 (2CH), 126.9 (d, JP=56 Hz, C), 127.1 (d, JP=56 Hz, C), 128.9 (d, JP=10 Hz, 2CH), 129.0 (d, JP= 10 Hz, 2CH), 130.0 (2CH), 131.6 (d, JP=2 Hz, CH), 131.9 (d, JP= 2 Hz, CH), 132.6 (d, JP=12 Hz, 2CH), 132.7 (d, JP=12 Hz, 2CH), 141.6 (d, JP = 6 Hz, C), 142.2 (C) ppm. 31P NMR (121 MHz, CDCl3): δ 13.8 (s) ppm. HRMS-ESI: m/z calcd for C20H23BOPS 353.1300, found 353.1291 [Mþ þ H]. General Procedure for the Alkylation of Optically Pure Sulfoxides with Chlorodiphenyl Phosphine. A solution of 1 equiv of the sulfoxide in THF (volume necessary to make the concentration of the sulfoxide 0.5 M) was cooled at -78 °C, and 1.3 equiv of MeLi (1.6 M solution in diethyl ether) was added dropwise. After 30 min the solution was added via cannula very slowly to a solution of 1.1 equiv of Ph2PCl in THF (volume necessary to make the concentration of the phosphine 0.5 M) at -78 °C. After 30 min 1.3 equiv of BH3 3 SMe2 was added, and the reaction was slowly warmed to room temperature for 3 h. The solution was quenched with water, and after extractive workup (water/EtOAc) and silica gel chromatography (10:1 hexane/ EtOAc) the PCSO ligands were obtained. Diphenyl((1R)-phenyl-(S)-(p-tolylsulfinyl)methyl)phosphine, Borane Complex 4-BH3. Following the general procedure, (R)benzyl p-tolylsulfoxide (3)12 (100 mg, 0.43 mmol), MeLi (0.35 mL of a 1.6 M solution, 0.57 mmol), Ph2PCl (0.09 mL, 0.48 mmol), and BH3 3 SMe2 (0.05 mL, 0.57 mmol) were reacted to give 55 mg (30%) of 4-BH3 as a 13:1 diastereomeric mixture as a white solid. The diastereomers were separated by precipitation with hexane and EtOAc. Mp: 149-150 °C. [R]D =þ1.36 (c 0.05, CHCl3). IR (film): νmax 1057, 1435, 2362 cm-1. 1H NMR (400 MHz, CDCl3): δ 1.04-1.64 (br, 3H, BH3), 2.29 (s, 3H), 4.22 (d, J=11 Hz, 1H), 6.84 (d, J=7 Hz, 2H), 6.93 (d, J=8 Hz, 2H), 7.03 (d, J=8 Hz, 2H), 7.07-7.11 (m, 2H), 7.19-7.23 (m, 1H), 7.34-7.38 (m, 2H), 7.427.49 (m, 3H), 7.54-7.58 (m, 1H), 7.62-7.67 (m, 2H), 7.87-7.91 (m, 2H) ppm. 13C NMR (75 MHz, CDCl3): δ 21.6 (CH3), 70.0 (d, JP=26 Hz, CH), 124.5 (2CH), 126.4 (d, JP=55 Hz, C), 127.0 (d, JP=5 Hz, C), 127.3 (d, JP=55 Hz, C), 127.9 (CH), 128.7 (d, JP= 1 Hz, 2CH), 128.9 (d, JP =10 Hz, 2CH), 129.0 (d, JP =10 Hz, 2CH), 129.5 (2CH), 131.7 (d, JP=2 Hz, CH), 131.8 (d, JP=4 Hz, 2CH), 132.2 (d, JP=2 Hz, CH), 133.3 (d, JP=9 Hz, 2CH), 134.3 (d, JP = 9 Hz, 2CH),139.8 (d, JP = 7 Hz, C), 141.8 (C) ppm. 31 P NMR (121 MHz, CDCl3): δ 23.0 (s) ppm. HRMS-ESI: m/z calcd for C26H27BOPS 429.1613, found 429.1615 [Mþ þH]. Diphenyl((1R)-2-phenyl-1-(S)-(p-tolylsulfinyl)ethyl)phosphine, Borane Complex 6-BH3. Following the general procedure, (R)2-phenylethyl p-tolylsulfoxide (5)21 (600 mg, 2.46 mmol), MeLi (2.30 mL of a 1.6 M solution, 3.69 mmol), Ph2PCl (0.50 mL, 2.70 mmol), and BH3 3 SMe2 (0.30 mL, 3.19 mmol) were reacted to give 430 mg (40%) of 6-BH3 as a single diastereomer as a white solid. Mp: 108-109 °C. [R]D=þ0.56 (c 0.02, CHCl3). IR (film): νmax 1056, 1436, 2388 cm-1. 1H NMR (400 MHz, CDCl3): δ 0.78-1.63 (br, 3H, BH3), 2.34 (s, 3H), 3.02-3.18 (m, 2H), 4.12 (ddd, J=10, 7, and 5 Hz, 1H), 6.81-6.83 (m, 2H), 7.047.05 (m, 3H), 7.08 (d, J=8 Hz, 2H), 7.15 (d, J=8 Hz, 2H), 7.20 (td, J=8 and 2 Hz, 2H), 7.31-7.35 (m, 1H), 7.51-7.60 (m, 5H), 8.11 (dd, J=8 and 2 Hz, 1H), 8.13 (dd, J=8 and 2 Hz, 1H) ppm. 13 C NMR (75 MHz, CDCl3): δ 21.4 (CH3), 31.78 (d, JP=4 Hz, (21) Blakemore, P. R.; Marsden, S. P.; Vater, H. D. Org. Lett. 2006, 8, 773–776.

Article CH2), 64.2 (d, JP = 22 Hz, CH), 125.5 (2CH), 125.7 (d, JP = 55 Hz, C), 126.48 (CH), 127.0 (d, JP =55 Hz, C), 128.3 (2CH), 128.4 (d, JP =10 Hz, 2CH), 128.6 (2CH), 128.8 (d, JP =10 Hz, 2CH), 129.5 (2CH), 131.1 (d, JP=2 Hz, CH), 131.9 (d, JP=2 Hz, CH), 132.9 (d, JP=9 Hz, 2CH), 134.1 (d, JP=9 Hz, 2CH), 137.8 (d, JP = 2 Hz, C),137.9 (d, JP = 7 Hz, C), 141.8 (C) ppm. 31 P NMR (121 MHz, CDCl3): δ 22.8 (s) ppm. HRMS-ESI: m/z calcd for C27H29BOPS 443.1770, found 443.1770 [Mþ þ H]. HPLC analysis of 6-BH3 performed with a Chiralcel OD-H column, eluting with 2% i-PrOH in heptane at 1.0 mL/min and monitored by UV at 254 nm, gave peaks at 25.1 min (minor enantiomer) and 34.5 min (major enantiomer), which integrated to reveal 99% ee (as compared to a racemic standard). Diphenyl((1S)-(R)-tert-butylsulfinyl(phenyl)methyl)phosphine, Borane Complex (8-BH3). Following the general procedure, (S)benzyl tert-butylsulfoxide (7)22 (1.0 g, 5.04 mmol), MeLi (4.10 mL of a 1.6 M solution, 6.56 mmol), Ph2PCl (1.02 mL, 5.55 mmol), and BH3 3 SMe2 (0.62 mL, 6.56 mmol) were reacted to give 1.3 g (65%) of 8-BH3 as a 6:1 diastereomeric mixture as a white solid. Alternatively, when the reaction was carried out using n-BuLi (2.73 mL of a 2.4 M solution, 6.56 mmol) at 0 °C, 495 mg (25%) of a 20:1 diastereomeric mixture was obtained as a white solid. The diastereomers were separated by silica gel chromatography (15:1 toluene/EtOAc). Mp: 128-129 °C. [R]D = -0.42 (c 0.5, CHCl3). IR (film): νmax = 1056, 1437, 2400, 2959, 3059 cm-1. 1 H NMR (400 MHz, CDCl3): δ 1.01 (s, 9H), 5.02 (d, J=10 Hz, 1H), 7.05-7.07 (m, 2H), 7.12-7.24 (m, 5H), 7.27-7.31 (m, 1H), 7.39-7.44 (m, 2H), 7.50-7.59 (m, 3H), 8.06-8.11 (m, 2H) ppm. 13 C NMR (75 MHz, CDCl3): δ 24.0 (3CH3), 57.9 (d, JP=3 Hz, C), 60.1 (d, JP =24 Hz, CH), 125.7 (d, JP =54 Hz, C), 128.2 (2CH), 128.3 (2CH), 128.8 (d, JP = 56 Hz, C), 128.7-128.8 (m, 4CH), 130.4 (d, JP=1 Hz, C), 130.6 (d, JP=4 Hz, CH), 130.83 (d, JP= 2 Hz, CH), 132.0 (d, JP=6 Hz, CH), 132.7 (d, JP=9 Hz, 2CH), 134.8 (d, JP =10 Hz, 2CH) ppm. 31P NMR (121 MHz, CDCl3): δ 27.8 (s) ppm. HRMS-ESI: m/z calcd for C23H27BOPS 393.1613, found 393.1622 [M--H]. Diphenyl((1R)-(R)-tert-butylsulfinyl(phenyl)methyl)phosphine, Borane Complex 9-BH3. To a solution containing a 6:1 diastereomeric mixture of ligands 8-BH3 and 9-BH3 (790 mg, 2.01 mmol) in THF (15 mL) was added MeLi (1.34 mL of a 3 M solution, 4.03 mmol) at 0 °C. The reaction was stirred at room temperature for 1 h, and then it was quenched with water. After extractive workup (water/EtOAc), a 1:1 diastereomeric mixture of ligands 8-BH3 and 9-BH3 was obtained in quantitative yield as a white solid. The diastereomers were separated by silica gel chromatography (15:1 toluene/EtOAc). Mp: 141-142 °C. [R]D = -0.34 (c 0.5, CHCl3). IR (film): νmax 1045, 1104, 1437, 2395 cm-1. 1H NMR (400 MHz, CDCl3): δ 1.00 (s, 9H), 4.53 (d, J=11 Hz, 1H), 7.14-7.22 (m, 5H), 7.37-7.43 (m, 2H), 7.44-7.50 (m, 3H), 7.56 (td, J=7 and 1 Hz, 1H), 7.66-7.75 (m, 2H), 7.84-7.91 (m, 2H) ppm. 13C NMR (75 MHz, CDCl3): δ 24.0 (3CH3), 58.0 (d, JP = 28 Hz, CH), 58.2 (d, JP=7 Hz, C), 125.2 (d, JP=53 Hz, C), 127.0 (d, JP=53 Hz, C), 128.1 (d, JP=2 Hz, 2CH), 128.5 (d, JP=2 Hz, C), 128.7 (d, JP =2 Hz, CH), 128.8 (d, JP =3 Hz, 2CH), 128.9 (d, JP=3 Hz, 2CH), 131.7 (d, JP=2 Hz, CH), 132.4 (CH), 132.5 (d, JP =5 Hz, 2CH), 133.4 (d, JP =9 Hz, 2CH), 135.1 (d, JP = 9 Hz, 2CH) ppm. 31P NMR (121 MHz, CDCl3): δ 27.3 (s) ppm. HRMS-ESI: m/z calcd for C23H29BOPS 395.1764, found 395.1764 [Mþ þ H]. General Procedure for the Preparation of the Dicobalt-Tetracarbonyl Complexes of Optically Pure PCSO Ligands. To a Schlenk tube containing 1 equiv of the corresponding boraneprotected PNSO ligand in toluene (volume necessary to make the concentration of the ligand 0.1 M) was added 1.5 equiv of DABCO and 1.1 equiv of the corresponding dicobalt-hexacarbonyl acetylene complex. The mixture was kept at 75 °C for the time indicated in Table 1. The crude product was concentrated (22) Kelly, P.; Lawrence, S. E.; Maguire, A. R. Eur. J. Org. Chem. 2006, 4500–4509.

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under vacuum and purified by silica gel chromatography (20:1 hexane/EtOAc or toluene) to give the PCSO complexes as red solids. Co2(μ-TMSC2H)(CO)4(μ-C26H23OPS) (11a/11b). Following the general procedure, diphenyl((1R)-phenyl-(S)-(p-tolylsulfinyl)methyl) phosphine borane complex (4-BH3) (400 mg, 0.94 mmol), DABCO (157 mg, 1.40 mmol), and trimethylsilylacetylene dicobalt complex (395 mg, 1.03 mmol) were reacted. Chromatography on silica gel (10:1 hexane/EtOAc) afforded 430 mg (62%) of a 1:1 diastereomeric mixture of complexes 11a/11b. The diastereomers could be partially separated by silica gel chromatography (toluene). 11a: IR (film): νmax 1970, 1998, 2027 cm-1. 1H NMR (400 MHz, C6D6): δ 0.43 (s, 9H), 1.73 (s, 3H), 4.37 (d, J=7 Hz, 1H), 6.52 (br s, 2H), 6.58-6.62 (m, 4H), 6.70-6.74 (m, 1H), 6.94-6.96 (m, 3H), 7.00 (d, J=8 Hz, 1H), 7.04-7.06 (m, 3H), 7.52-7.58 (m, 4H), 7.81-7.86 (m, 2H) ppm. 13 C NMR (75 MHz, C6D6): δ 0.8 (3CH3), 20.8 (CH3), 90.6 (d, JP =8 Hz, CH), 93.4 (d, JP =9 Hz, CH), 97.8 (d, JP =9 Hz, C), 125.1 (2CH), 127.4 (2CH), 127.9 (CH), 128.1 (d, JP=7 Hz, 2CH), 128.8 (d, JP=9 Hz, 2CH), 129.3 (2CH), 130.2 (d, JP=2 Hz, CH), 130.7 (d, JP=2 Hz, CH), 131.2 (d, JP=2 Hz, 2CH), 132.2 (d, JP= 6 Hz, C), 132.6 (d, JP=9 Hz, 2CH), 133.9 (d, JP=33 Hz, C), 135.4 (d, JP=13 Hz, 2CH), 135.6 (d, JP=38 Hz, C), 141.3 (C), 144.3 (d, JP=11 Hz, C) ppm. 31P NMR (121 MHz, C6D6): δ 64.3 (s) ppm. HRMS-ESI: m/z calcd for C35H33O5Co2NaPSSi 765.0111, found 765.0116 [Mþ þ Na]. 11b: IR (film): νmax 1974, 2000, 2031 cm-1. 1 H NMR (400 MHz, C6D6): δ 0.44 (s, 9H), 1.72 (s, 3H), 4.62 (d, J=8 Hz, 1H), 5.90 (d, J=10 Hz, 1H), 6.45 (br s, 2H), 6.56 (t, J= 7 Hz, 2H), 6.62 (d, J=8 Hz, 2H), 6.65-6.69 (m, 1H), 6.92-6.99 (m, 6H), 7.30 (d, J=8 Hz, 2H), 7.34-7.39 (m, 2H), 7.77-7.82 (m, 2H) ppm. 13C NMR (75 MHz, C6D6): δ 1.0 (3CH3), 20.9 (CH3), 84.0 (d, JP=12 Hz, CH), 85.0 (d, JP=10 Hz, CH), 95.8 (d, JP= 10 Hz, C), 124.9 (2CH), 127.3 (2CH), 127.9 (CH), 128.1 (d, JP= 7 Hz, 2CH), 128.5 (d, JP =9 Hz, 2CH), 129.0 (2CH), 129.9 (d, JP=1 Hz, CH), 130.7 (d, JP=2 Hz, CH), 130.8 (d, JP=6 Hz, C), 131.6 (d, JP=2 Hz, 2CH), 132.3 (d, JP=11 Hz, 2CH), 132.8 (d, JP =31 Hz, C), 134.8 (d, JP =38 Hz, C), 135.2 (d, JP =13 Hz, 2CH), 141.2 (C), 144.9 (d, JP = 10 Hz, C) ppm. 31P NMR (121 MHz, C6D6): δ 57.0 (s) ppm. HRMS-ESI: m/z calcd for C35H33O5Co2NaPSSi 765.0111, found 765.0108 [MþþNa]. Co2(μ-PhC2H)(CO)4(μ-C26H23OPS) (12a/12b). Following the general procedure, diphenyl((1R)-phenyl-(S)-(p-tolylsulfinyl)methyl) phosphine borane complex (4-BH3) (200 mg, 0.47 mmol), DABCO (78 mg, 0.70 mmol), and phenylacetylene dicobalt complex (200 mg, 0.51 mmol) were reacted. Chromatography on silica gel (10:1 hexane/EtOAc) afforded 210 mg (60%) of a 1:1 diastereomeric mixture of complexes 12a/12b. The diastereomers could be partially separated by silica gel chromatography (toluene). 12a: IR (film): νmax 1973, 2003, 2030 cm-1. 1H NMR (400 MHz, C6D6): δ 1.74 (s, 3H), 4.29 (d, J=7 Hz, 1H), 6.53 (br s, 2H), 6.58-6.65 (m, 4H), 6.72-6.76 (m, 1H), 6.81 (d, J=7 Hz, 1H), 6.95-6.97 (m, 3H), 7.02-7.13 (m, 6H), 7.47 (d, J=8 Hz, 2H), 7.56-7.61 (m, 2H), 7.85 (d, J= 7 Hz, 2H), 7.87-7.92 (m, 2H) ppm. 13C NMR (75 MHz, C6D6): δ 20.7 (CH3), 78.0 (d, JP=13 Hz, CH), 90.4 (d, JP=5 Hz, CH), 103.8 (C), 124.8 (2CH), 127.2 (CH), 127.3 (2CH), 128.1 (d, JP= 9 Hz, 2CH), 128.6 (d, JP = 9 Hz, 2CH), 128.8 (2CH), 129.2 (2CH), 130.0 (d, JP=3 Hz, 2CH), 130.2 (CH), 130.6 (CH), 131.3 (d, JP=2 Hz, 2CH), 131.8 (d, JP=6 Hz, C), 132.7 (d, JP=12 Hz, 2CH), 133.8 (d, JP=32 Hz, C), 134.8 (d, JP=22 Hz, C), 135.0 (d, JP=13 Hz, 2CH), 141.2 (C), 141.5 (C), 143.9 (d, JP=11 Hz, C) ppm (one carbon signal is missing). 31P NMR (121 MHz, C6D6): δ 64.9 (s) ppm. HRMS-ESI: m/z calcd for C38H29O5Co2NaPS 769.0029, found 769.0022 [Mþ þ Na]. 12b: IR (film): νmax 1976, 2004, 2033 cm-1. 1H NMR (400 MHz, C6D6): δ 1.78 (s, 3H), 4.50 (d, J=8 Hz, 1H), 5.71 (d, J=9 Hz, 1H), 6.51 (br s, 2H), 6.62-6.73 (m, 5H), 6.99-7.09 (m, 8H), 7.35-7.43 (m, 5H), 7.86-7.92 (m, 4H) ppm. 13C NMR (75 MHz, C6D6): δ 21.3 (CH3), 71.8 (d, JP = 10 Hz, CH), 82.4 (d, JP = 13 Hz, CH), 104.4 (d, JP=16 Hz, C), 125.3 (2CH), 127.7 (2CH), 127.8 (CH),

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Organometallics, Vol. 28, No. 15, 2009

128.5 (CH), 128.7 (d, JP=9 Hz, 2CH), 128.9 (d, JP=9 Hz, 2CH), 129.3 (2CH), 129.4 (2CH), 130.4 (CH), 130.5 (d, JP = 4 Hz, 2CH), 131.1 (CH), 131.2 (d, JP=5 Hz, C), 132.1 (2CH), 132.7 (d, JP=11 Hz, 2CH), 133.1 (d, JP=30 Hz, C), 134.9 (d, JP=37 Hz, C), 135.5 (d, JP=14 Hz, 2CH),141.7 (C), 142.4 (C), 145.1 (d, JP= 11 Hz, C) ppm. 31P NMR (121 MHz, C6D6): δ 58.3 (s) ppm. HRMS-ESI: m/z calcd for C38H29O5Co2NaPS: 769.0029, found 769.0026 [Mþ þ Na]. Co2(μ-TMSC2H)(CO)4(μ-C27H25OPS) (13a/13b). Following the general procedure, diphenyl((1R)-2-phenyl-1-(S)-(p-tolylsulfinyl)ethyl) phosphine borane complex (6-BH3) (30 mg, 0.07 mmol), DABCO (12 mg, 0.1 mmol), and trimethylsilylacetylene dicobalt complex (29 mg, 0.07 mmol) were reacted. Chromatography on silica gel (10:1 hexane/EtOAc) afforded 19 mg (36%) of a 2:1 diastereomeric mixture of complexes 13a/13b. The diastereomers could be partially separated by silica gel chromatography (50:1 hexane/ EtOAc). 13a: IR (film): νmax 1967, 1995, 2025 cm-1. 1H NMR (400 MHz, C6D6): δ 0.48 (s, 9H), 1.87 (s, 3H), 2.65-2.82 (m, 2H), 5.08-5.12 (m, 1H), 6.06 (d, J=7 Hz, 2H), 6.42 (d, J=11 Hz, 1H), 6.66-6.72 (m, 4H), 6.76-6.86 (m, 3H), 7.02-7.11 (m, 4H), 7.587.63 (m, 2H), 7.83-7.88 (m, 2H), 7.99 (d, J = 8 Hz, 2H) ppm. 13 C NMR (75 MHz, C6D6): δ 1.3 (3CH3), 21.3 (CH3), 33.1 (d, JP= 4 Hz, CH2), 83.3 (d, JP=13 Hz, CH), 90.2 (d, JP=9 Hz, CH), 97.8 (d, JP =9 Hz, C), 126.5 (CH), 127.9 (2CH), 128.5-128.9 (m, 8CH), 129.9 (2CH), 130.1 (CH), 130.9 (CH), 133.1 (d, JP=12 Hz, 2CH), 133.5 (d, JP=31 Hz, C), 135.0 (d, JP=13 Hz, 2CH), 137.1 (d, JP=36 Hz, C), 138.8 (d, JP=3 Hz, C), 142.7 (d, JP=6 Hz, C), 143.4 (C) ppm. 31 P NMR (121 MHz, C6D6): δ 56.1 (s) ppm. HRMS-ESI: m/z calcd for C36H35O5Co2NaPSSi 779.0268, found 779.0274 [Mþ þ Na]. 13b: IR (film): νmax 1972, 1998, 2034 cm-1. 1H NMR (400 MHz, C6D6): δ 0.52 (s, 9H), 1.88 (s, 3H), 2.63-2.84 (m, 2H), 4.53-4.57 (m, 1H), 5.91 (d, J=7 Hz, 2H), 6.20 (d, J=10 Hz, 1H), 6.61-6.79 (m, 5H), 6.85-7.01 (m, 6H), 7.50-7.54 (m, 2H), 7.77-7.81 (m, 2H), 7.93 (d, J=8 Hz, 2H) ppm. 13C NMR (75 MHz, C6D6): δ 1.1 (3CH3), 20.9 (CH3), 33.3 (CH2), 81.3 (d, JP =12 Hz, CH), 86.5 (d, JP =10 Hz, CH), 96.8 (d, JP=10 Hz, C), 126.3 (CH), 127.1 (2CH), 127.9 (2CH), 128.3 (4CH), 128.4 (2CH), 129.4 (CH), 129.5 (2CH), 130.7 (CH), 132.1 (d, JP=12 Hz, 2CH), 133.9 (d, JP=32 Hz, C), 135.9 (d, JP= 13 Hz, 2CH), 137.4 (d, JP=36 Hz, C), 138.5 (C), 142.7 (C),143.2 (d, JP =7 Hz, C) ppm. 31P NMR (121 MHz, C6D6): δ 56.3 (s) ppm. HRMS-ESI: m/z calcd for C36H35O5Co2NaPSSi 779.0268, found 779.0277 [Mþ þ Na]. Co2(μ-PhC2H)(CO)4(μ-C27H25OPS) (14a/14b). Following the general procedure, diphenyl((1R)-2-phenyl-1-(S)-(p-tolylsulfinyl)ethyl) phosphine borane complex (6-BH3) (300 mg, 0.68 mmol), DABCO (114 mg, 1.02 mmol), and phenylacetylene dicobalt complex (289 mg, 0.75 mmol) were reacted. Chromatography on silica gel (10:1 hexane/EtOAc) afforded 172 mg (33%) of a 2:1 diastereomeric mixture of complexes 14a/14b. One diastereomer could be partially separated by silica gel chromatography (50:1 hexane/ EtOAc). 14a: IR (film): νmax 1972, 2000, 2060 cm-1. 1H NMR (400 MHz, C6D6): δ 1.85 (s, 3H), 2.71-2.90 (m, 2H), 4.88-4.93 (m, 1H), 6.16 (d, J=7 Hz, 2H), 6.23 (d, J=10 Hz, 1H), 6.63-6.87 (m, 10H), 7.02-7.05 (m, 3H), 7.52-7.57 (m, 2H), 7.78-7.85 (m, 5H), 7.97 (d, J=8 Hz, 2H) ppm. 13C NMR (75 MHz, C6D6): δ 20.8 (CH3), 32.4 (d, JP=4 Hz, CH2), 75.9 (d, JP=10 Hz, CH), 80.07 (d, JP=13 Hz, CH), 105.78 (d, JP=14 Hz, C), 126.1 (CH), 127.4 (2CH), 127.5 (2CH), 128.1-128.4 (m, 7CH), 128.8 (2CH), 129.4 (2CH), 129.7 (CH), 130.1 (CH), 130.2 (CH), 130.4 (CH), 132.5 (d, JP = 12 Hz, 2CH), 132.6 (d, JP=33 Hz, C), 134.4 (d, JP=13 Hz, 2CH),

Ferrer et al. 136.5 (d, JP=36 Hz, C), 138.4 (d, JP=4 Hz, C), 141.3 (C), 141.8 (d, JP=7 Hz, C), 142.9 (C) ppm. 31P NMR (121 MHz, C6D6): δ 55.6 (s) ppm. HRMS-ESI: m/z calcd for C39H31O5Co2NaPS 783.0191, found 783.0209 [Mþ þ Na]. Co2(μ-PhC2H)(CO)5(C23H25OPS) (15a/15b). Following the general procedure, diphenyl((1S)-(R)-tert-butylsulfinyl(phenyl)methyl) phosphine borane complex (8-BH3) (100 mg, 0.25 mmol), DABCO (43 mg, 0.38 mmol), and phenylacetylene dicobalt complex (108 mg, 0.28 mmol) were reacted. Chromatography on silica gel (10:1 hexane/EtOAc) afforded 164 mg (92%) of a 1:1 diastereomeric mixture of complexes 15a/15b: IR (film): νmax = 2007, 2059 cm-1. 1H NMR (400 MHz, C6D6): δ 0.70 (s, 9H), 0.75 (s, 9H), 4.85 (d, J=4 Hz, 1H), 4.92 (d, J=3 Hz, 1H), 5.03 (d, J= 3 Hz, 1H), 5.24 (d, J=4 Hz, 1H), 6.79-7.08 (m, 26H), 7.24-7.37 (m, 8H), 7.43-7.48 (m, 2H), 8.06-8.10 (m, 2H), 8.39-8.43 (m, 2H) ppm. 13C NMR (75 MHz, C6D6): δ 23.7 (3CH3), 23.7 (3CH3), 57.7 (C), 57.7 (C), 61.2 (d, JP = 5 Hz, CH), 61.7 (d, JP=8 Hz, CH), 71.5 (CH), 73.2 (CH), 85.4 (C), 87.4 (C), 126.6138.9 (m, 48C) ppm. 31P NMR (121 MHz, C6D6): δ 67.1, 67.8 (s) ppm. HRMS-ESI: m/z calcd for C34H31O4Co2PS 684.0345, found 684.0353 [M - 2CO]. Co2(μ-PhC2H)(CO)4(μ-C23H25OPS) (16b). Following the general procedure, diphenyl((1R)-(R)-tert-butylsulfinyl(phenyl)methyl) phosphine borane complex (9-BH3) (200 mg, 0.51 mmol), DABCO (85 mg, 0.76 mmol), and phenylacetylene dicobalt complex (216 mg, 0.56 mmol) were reacted. Chromatography on silica gel (10:1 hexane/EtOAc) afforded 106 mg (30%) of a 1:6 diastereomeric mixture of complexes 16a/16b. The major diastereomer 16b was isolated by crystallization using hexane/toluene: IR (film): νmax 2027, 1999, 1971 cm-1. 1H NMR (400 MHz, C6D6): δ 0.95 (s, 9H), 3.92 (d, J=9 Hz, 1H), 6.03 (d, J=8 Hz, 1H), 6.34 (d, J=9 Hz, 1H), 6.67 (td, J=7 and 1 Hz, 1H), 6.75-6.86 (m, 3H), 6.91-6.96 (m, 2H), 6.99-7.13 (m, 6H), 7.42-7.47 (m, 2H), 7.71 (d, J=7 Hz, 1H), 7.79 (d, J=7 Hz, 2H), 7.95-8.00 (m, 2H) ppm. 13C NMR (75 MHz, C6D6): δ 24.7 (3CH3), 66.4 (d, JP = 8 Hz, C), 77.7 (d, JP=10 Hz, CH), 81.1 (d, JP=10 Hz, CH), 106.2 (d, JP=18 Hz, C), 127.3 (d, JP=20 Hz, 2CH), 128.1 (d, JP=9 Hz, 2CH), 128.2 (2CH), 128.3 (d, JP=9 Hz, 2CH), 128.8 (2CH), 129.8 (CH), 130.0 (d, JP =4 Hz, 2CH), 130.6 (CH), 131.0 (CH), 131.8 (CH), 133.7 (d, JP =12 Hz, 2CH), 133.9 (d, JP =13 Hz, 2CH), 134.14 (d, JP =25 Hz, C), 141.4 (C) ppm (two carbon signal are missing due to overlapping). 31P NMR (121 MHz, C6D6): δ 79.2 (s) ppm. HRMS-ESI: m/z calcd for C35H31O5Co2NaPS 735.0186, found 735.0183 [Mþ þ Na]. General Procedure for the Pauson-Khand Reaction of Optically Pure Complexes with Norbornadiene. Optically pure tetracarbonyl complexes (1 equiv), norbornadiene (10 equiv), and toluene (volume necessary to make to concentration of the complex 0.1 M) were charged in a Schlenk flask under nitrogen, and the reaction was stirred at 80 °C for the time indicated in Table 2. The Pauson-Khand adducts 178b and 188b were obtained after silica gel chromatography (95:5 hexane/ EtOAc).

Acknowledgment. We thank MICINN (CTQ200800763/BQU) and IRB Barcelona for financial support. Supporting Information Available: NMR spectra of all compounds. X-ray structure of 6-BH3. This material is available free of charge via the Internet at http://pubs.acs.org.