Diastereoselective Synthesis of Planar Chiral Cobalt Metallocene

NOTE pubs.acs.org/Organometallics

Diastereoselective Synthesis of Planar Chiral Cobalt Metallocene Based Oxazoline Platinacycles M. Emin G€unay, David L. Hughes, and Christopher J. Richards* School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, U.K.

bS Supporting Information ABSTRACT: Reaction of (η5-(S)-2-(4-methylethyl)oxazolinylcyclopentadienyl)(η4-tetraphenylcyclobutadiene)cobalt with cisbis(dimethylsulfoxide)dichloroplatinum gave a 4.7:1 ratio of diastereomeric platinacycles, and the S,Rp-configuration of the major isomer was determined by X-ray crystallography. The same reaction on the corresponding t-Bu-substituted oxazoline derivative proceeded with a 5.6:1 selectivity, with the S,Rp-configuration of the major isomer determined by circular dichroism. Dimethylsulfoxide- and triphenylphosphine-ligated complexes are inactive as catalysts for the allylic imidate rearrangement of a trichloroacetimidate.

M

etallocene-based planar chiral palladacycles are of growing importance in asymmetric synthesis.1 Notable examples include the cobalt oxazoline palladacycles (COP) 2a/b, obtained by highly diastereoselective palladation of (S)-valine-derived oxazoline 1 (Scheme 1).2 These palladacycles have been applied as catalysts for the enantioselective rearrangement of trihaloacetimidates,3 in addition to acting as either catalysts or reagents in a number of other enantioselective reactions.4 Also prepared by diastereoselective C
H activation is the ferrocene imidazoline palladacycle (FIP) 3 (Figure 1), which has also been applied as a catalyst for enantioselective trifluoroacetimidate rearrangement.5 Several other related diastereoselective palladation methodologies are known,6 and planar chiral palladacycles may also be obtained by enantioselective palladation.4d,7 In contrast there are far fewer examples of planar chiral platinacycles, and almost all of the examples known have been obtained from essentially non-stereoselective reactions.8 The one exception is the highly diastereoselective synthesis of 4, a platinacycle that has been applied as a catalyst to the Friedel
Crafts alkylation of indoles.9 In view of the selective synthesis of COP catalyst 2a, we chose to investigate the synthesis of related planar chiral platinacycles. In this paper we report on our preliminary investigation into the synthesis and properties of these novel metallocene-based metallacycles.

’ RESULTS AND DISCUSSION A mixture of 1 and cis-bis(dimethylsulfoxide)dichloroplatinum was heated at reflux in xylenes for 3 days (Scheme 2). Three products were separated by column chromatography, and following crystallization from dichloromethane/hexane, the major product was identified as the (S,Rp)-platinacycle 5 (Figure 2). As this and other related nitrogen-based DMSO-coordinated platinacycles contain a trans nitrogen
sulfur geometry,8,10 the minor product was identified as the platinacycle (S,Sp)-6, in part r 2011 American Chemical Society

by there being only three cyclopentadienyl proton signals in the 1 H NMR spectrum. The isolated yields of 5 (45%) and 6 (10%) are in agreement with the 4.7:1 ratio of these platinacycles in the 1 H NMR of the crude reaction mixture prior to chromatography. A third product was identified as the non-palladacycle complex 7, for which the platinum coordination geometry was not determined. Previous examples of mononitrogen adducts obtained from cis-bis(dimethylsulfoxide)dichloroplatinum have been characterized as both cis- and trans-isomers.11 Repetition of the complexation with a shorter reaction time of 15 h, and examination of the crude reaction mixture by 1H NMR, revealed a ratio of coordination complex (7) to platinacycles (5/6) of 2.6:1 and a ratio of 5:6 of 5.4:1. Furthermore, heating a 1:9 mixture of 5 and 6 at reflux in xylenes for 24 h resulted in essentially no change in diastereomeric ratio. These results indicate that platination, under the conditions used with an aprotic solvent, is irreversible, the formation of major and minor diastereoisomers resulting from oxazoline-mediated kinetic control. This is in contrast to the corresponding palladation reaction outlined in Scheme 1, the protic solvent employed (acetic acid) resulting in reversible metalation and oxazoline-mediated thermodymanic control.12 Heating at reflux 1 and cis-PtCl2(DMSO)2 in methanol with NaOAc for 96 h resulted only in the recovery of starting material 1, and heating at reflux 1 and cis-PtCl2(DMSO)2 in toluene for 96 h resulted only in the formation of coordination complex 7. Repetition of the platination methodology with the t-Busubstituted oxazoline 8 again resulted in the formation of three complexes, two of which were platinacycles in a 5.6:1 ratio. Following chromatography, the major platinacycle isomer was isolated in 22% yield (Scheme 3). The configuration of the new element of planar chirality in this platinacycle was determined by circular dichroism. The CD spectra of a small number Received: April 26, 2011 Published: June 30, 2011 3901

dx.doi.org/10.1021/om200353n | Organometallics 2011, 30, 3901–3904

Organometallics

NOTE

Scheme 1. Synthesis of Cobalt Oxazoline Palladacycles

Figure 1. Ferrocene-based planar chiral metallacycles. Figure 2. Representation of the X-ray crystal structure of (S,Rp)-5.

Scheme 2. Diastereoselective Platination of Oxazoline 1

of ferrocene-based planar chiral palladacycles have been determined,13 and all follow a general rule where the Rp metallacycle configuration is associated with a positive value of molecular circular dichroism (Δε) for the band arising from the major absorption in the visible region (λmax ∼500 nm). In the same way the CD spectrum of (S,Rp)-5 contained a positive molecular circular dichroism value at 480 nm (Figure 3), as did the CD spectrum of the major platinacyle 9 derived from the t-Bu oxazoline, enabling this to be assigned the S,Rp-configuration. The same sense of diastereoselectivity observed in the platination of the i-Pr- and t-Bu-substituted oxazolines is in contrast to palladation with Pd(OAc)2/AcOH, which results in different diastereoisomers from substrates 1 and 8 (S,Rp and S,Sp, respectively).12,14 Given the success of palladacycle 2b as an asymmetric catalyst for the allylic imidate rearrangement, we explored the possibility of generating the corresponding chloride-bridged platinacycle by prolonged heating of (S,Rp)-5 under vacuum. This failed to result

Scheme 3. Diastereoselective Platination of t-Bu Oxazoline 8

in removal of the DMSO ligand, as did the reaction of (S,Rp)-5 with sodium acetylacetonate or sodium hexafluoroacetylacetonate; in all cases only starting material was recovered. Complex 10, the monomeric triphenylphosphine adduct of 2b is a known catalyst for the rearrangement of (E)-trichloroacetimidates,3e albeit with a lower activity than 2b, and the corresponding platinum complex 11 was generated from the reaction of (S,Rp)-5 with triphenylphosphine (Scheme 4). The trans nitrogen
phosphorus geometry of 11 was confirmed by the upfield shift to 2.89 ppm of the coordinated phosphine adjacent cyclopentdieneyl proton in the 1 H NMR spectrum, as observed similarly in the corresponding palladium complex 10 (3.13 ppm). Addition of 1 mol % of platinacycles (S,Rp)-5 and 11 to separate solutions of (E)-2,2,2trichloroacetimidic acid hex-2-enyl ester in acetonitrile, followed by heating at 70 °C for 48 h, resulted only in ∼3% conversion to the corresponding trichloroacetamide in addition to recovered starting material. The same outcome was observed in a control reaction containing neither platinacycle. Despite the recent interest in exploiting the relativistic effects of platinum(II) for the activation of carbon
carbon π-bond containing substrates,15 with the exception of complex 4, platinacycles have not been employed as catalysts in this respect. Platinum-based pincer complexes have been used as fixed oxidation state Lewis acid catalysts for the activation of other functional groups with comparable activity to the corresponding palladacycles.16 It is anticipated that coordination of the C
C double 3902

dx.doi.org/10.1021/om200353n |Organometallics 2011, 30, 3901–3904

Organometallics

NOTE

(1H, m, CH), 4.26 (2H, dd, J 8.6, 3.8, CH), 4.63
4.66 (2H, m, CpH), 5.48 (1H, dd, J 2.4, 1.2, CpH), 7.14
7.23 (12H, m, m/p-PhH), 7.46 (8H, d, J 6.4, o-PhH). 13C{1H} NMR (δ, 100 MHz, CDCl3): 14.0 (CH3), 18.8 (CH3), 29.0 (CH(CH3)2), 44.3 (S(O)CH3), 46.4 (S(O)CH3), 65.4 (CHCH2), 72.7 (CHCH2), 75.6 (C4Ph4), 82.1 (CpC), 84.0 (CpC), 88.5 (CpC), 88.7 (CpC), 90.1 (CpC), 126.5 (PhC), 128.3 (PhC), 129.3 (PhC), 135.4 (PhC), 175.5 (CdN). MS (m/z, EI): 899.1 (Mþ).

Chloro[(η5-(S)-(Sp)-2-(20 -(40 -methylethyl)oxazolinyl)cyclopentadienyl,1-C,30 -N)(η4-tetraphenylcyclobutadiene)cobalt]dimethylsulfoxideplatinum(II), 6. Yield: 0.016 g, 10%. Mp: 202 °C.

Figure 3. Circular dichroism spectra of (S,Rp)-5 and (S,Rp)-9.

Scheme 4. Synthesis of Triphenylphosphine Adduct 11

Anal. Found: C, 54.82; H, 4.30; N, 1.41. Calcd for C41H39ClCoNO2PtS: C, 54.76; H, 4.37; N, 1.56. [R]24D
122 (c 0.01, CH2Cl2). IR (KBr): νmax 1588 (CdN) cm
1. 1H NMR (δ, 400 MHz, CDCl3): 0.06 (3H, d, J 6.8, CHCH3), 0.73 (3H, d, J 7.2, CHCH3), 2.47
2.57 (1H, m, CH(CH3)2), 2.67 (3H, s, S(O)CH3), 3.27 (3H, s, S(O)CH3), 3.95
3.99 (1H, m, CH), 4.28 (1H, dd, J 8.8, 5.6, CH), 4.40 (1H, app t, J 9.2, CH), 4.70 (1H, app t, J 2.6, CpH), 4.77 (2H, dd, J 2.4, 1.6, CpH), 5.32
5.33 (1H, m, CpH), 7.11
7.20 (12H, m, m/p-PhH), 7.47 (8H, d, J 8.0, o-PhH). 13C{1H} NMR (δ, 100 MHz, CDCl3): 13.3 (CH3), 20.0 (CH3), 29.9 (CH(CH3)2), 45.2 (S(O)CH3), 45.8 (S(O)CH3), 66.7 (CHCH2), 73.2 (CHCH2), 76.5 (C4Ph4), 85.2 (CpC), 86.4 (CpC), 92.0 (CpC), 126.7 (PhC), 128.2 (PhC), 129.4 (PhC), 135.8 (PhC), 180.0 (CdN). MS (m/z, EI): 899.1 (Mþ).

Dichloro[(η5-(S)-(2-(4-methylethyl)oxazolinyl)cyclopentadienyl,3-N)(η4-tetraphenylcyclobutadiene)cobalt]dimethylsulfoxideplatinum(II), 7. Yield: 0.035 g, 22%. Mp: 142 °C. Anal.

bond of (E)-2,2,2-trichloroacetimidic acid hex-2-enyl ester to platinum will result in activation of this substrate toward intramolecular nucleophilic attack, the rate- and enantioselectivitydetermining step of palladacycle-catalyzed rearrangement.3f Thus the absence of rearrangement in the presence of 5 and 11 is likely due to the significantly slower rate of ligand substitution in these platinum(II) complexes. In conclusion, we have demonstrated that bulky, oxazolineappended, cobalt metallocenes undergo platination with cisPtCl2(DMSO)2 and that despite the harsh reaction conditions required, useful levels of diastereoselectivity result. The resulting metallacycles contain a unique stereochemical environment for platinum, and the potential for exploiting this in asymmetric catalysis is the subject of an ongoing study.

’ EXPERIMENTAL SECTION Xylenes were dried over sodium wire. Petroleum ether refers to that fraction boiling in the range 40
60 °C. Column chromatography was performed on silica gel (Davisil 40
63 μm). All reactions were performed under a nitrogen atmosphere.

Chloro[(η5-(S)-(Rp)-2-(20 -(40 -methylethyl)oxazolinyl)cyclopentadienyl,1-C,30 -N)(η4-tetraphenylcyclobutadiene)cobalt]dimethylsulfoxideplatinum(II), 5. cis-PtCl2(DMSO)2 (0.142 g;

0.34 mmol) was added to a solution of 12 (0.100 g; 0.17 mmol) in xylenes (15 mL), and the mixture heated at reflux for 3 days with stirring. Following evaporation of the solvent in vacuo the residue was column chromatographed (1:1 Et2O/petroleum ether) to give the products as dark orange, crystalline solids [Rf values 5 (0.63), 6 (0.45), 7(0.24)]. Yield: 0.069 g, 45%. Mp: 256 °C. Anal. Found: C, 54.68; H, 4.44; N, 1.47. Calcd for C41H39ClCoNO2PtS: C, 54.76; H, 4.37; N, 1.56. [R]24D þ410 (c 0.0016, CH2Cl2). IR (KBr): νmax 1580 (CdN) cm
1. 1H NMR (δ, 400 MHz, CDCl3): 0.61 (3H, d, J 6.8, CHCH3), 0.73 (3H, d, J 6.8, CHCH3), 2.35
2.45 (1H, m, CH(CH3)2), 2.45 (3H, s, S(O)CH3), 3.18 (1H, app t, J 9.0, CH), 3.27 (3H, s, S(O)CH3), 3.37
3.42

Found: C, 52.80; H, 4.26; N, 1.52. Calcd for C41H40Cl2CoNO2PtS: C, 52.63; H, 4.31; N, 1.50. [R]24D þ364 (c 0.07, CH2Cl2). IR (KBr): νmax 1621 (CdN) cm
1. 1H NMR (δ, 400 MHz, CDCl3): 0.83 (3H, d, J 6.4, CHCH3), 0.84 (3H, d, J 6.8, CHCH3), 2.65
2.75 (1H, m, CH(CH3)2), 3.14 (1H, app t, J 9.6, CH), 3.27 (6H, s, S(O)CH3), 3.82 (1H, app t, J 7.8, CH), 4.04
4.10 (1H, m, CH), 4.82 (2H, brs, CpH), 5.21 (1H, brs, CpH), 6.42 (1H, brs, CpH), 7.16
7.23 (12H, m, m/p-PhH), 7.42 (8H, d, J 7.2, o-PhH). 13C{1H} NMR (δ, 100 MHz, CDCl3): 14.9 (CH3), 19.5 (CH3), 30.5 (CH(CH3)2), 43.8 (S(O)CH3), 43.9 (S(O)CH3), 67.7 (CHCH2), 71.2 (CHCH2), 77.1 (C4Ph4), 82.0 (CpC), 85.8 (CpC), 86.0 (CpC), 86.61 (CpC), 86.64 (CpC), 127.0 (PhC), 128.4 (PhC), 129.2 (PhC), 135.1 (PhC), 166.8 (CdN). MS (m/z, EI): 935.1 (Mþ).

Chloro[(η 5-(S)-(R p)-2-(20 -(4 0 -dimethylethyl)oxazolinyl)cyclopentadienyl,1-C,30 -N)(η4-tetraphenylcyclobutadiene)cobalt]dimethylsulfoxideplatinum(II), 9. cis-PtCl2(DMSO)2

(0.100 g; 0.24 mmol) was added to a solution of 814 (0.100 g; 0.17 mmol) in xylenes (10 mL), and the mixture heated at reflux for 3 days with stirring. Following evaporation of the solvent in vacuo the residue was column chromatographed (1:1 Et2O/petroleum ether) to give the product 9 [Rf = 0.29] as a dark orange, crystalline solid (0.033, 22%). Mp: 138 °C. Anal. Found: C, 55.31; H, 4.37; N, 1.60. Calcd for C42H41ClCoNO2PtS: C, 55.23; H, 4.52; N, 1.53. [R]24D þ306 (c 0.34, CH2Cl2). IR (KBr): νmax 1581 (CdN) cm
1. 1H NMR (δ, 400 MHz, CDCl3): 0.83 (9H, s, (CH3)3), 2.38 (3H, s, S(O)CH3), 2.80 (1H, app t, J 8.5, CH), 3.30 (3H, s, S(O)CH3), 3.41 (1H, d, J 8.0, CH), 4.36 (1H, d, J 8.5, CH), 4.62
4.65 (2H, m, CpH), 5.53
5.54 (1H, m, CpH), 7.15
7.20 (12H, m, m/p-PhH), 7.47
7.50 (8H, m, o-PhH). 13C{1H} NMR (δ, 100 MHz, CDCl3): 26.4 (C(CH3)3), 35.2 (C(CH3)3), 43.7 (S(O)CH3), 47.3 (S(O)CH3), 68.1 (CHCH2), 75.1 (CHCH2), 75.4 (C4Ph4), 82.5 (CpC), 83.9 (CpC), 88.3 (CpC), 88.6 (CpC), 90.2 (CpC), 126.5 (PhC), 128.3 (PhC), 129.3 (PhC), 135.4 (PhC), 177.1 (CdN). MS (m/z, EI): 913.0 (Mþ).

Chloro[(η5-(S)-(Rp)-2-(20 -(40 -methylethyl)oxazolinyl)cyclopentadienyl,1-C,30 -N)(η4-tetraphenylcyclobutadiene)cobalt]triphenylphosphineplatinum(II), 11. Triphenylphosphine (0.008 g,

0.03 mmol) was added to a solution of 5 (0.027 g, 0.03 mmol) in CH2Cl2 (10 mL), and the reaction mixture was stirred for 12 h at room temperature. The solvent was removed in vacuo, and the residue column 3903

dx.doi.org/10.1021/om200353n |Organometallics 2011, 30, 3901–3904

Organometallics chromatographed (100% CH2Cl2) to give the product as a yellow solid (0.015 g, 46%). Mp: 168 °C. Anal. Found: C, 63.07; H, 4.40; N, 1.16. Calcd for C57H48ClCoNOPPt: C, 63.19; H, 4.47; N,1.29. [R]24D þ938 (c 0.15, CH2Cl2). IR (KBr): νmax 1601 (CdN) cm
1. 1H NMR (δ, 400 MHz, CDCl3): 0.74 (6H, d, J 6.8, CHCH3), 2.79
2.88 (1H, m, CH(CH3)2), 2.89 (1H, d, J 2.0, CpH), 3.30 (1H, app t, J 9.0, CH), 3.65 (1H, dt, J 9.2, 3.0, CH), 4.28
4.31 (2H, m, CH and CpH), 4.56 (1H, d, J 2.8, CpH), 7.10
7.45 (35H, m, PhH). 13C{1H} NMR (δ, 100 MHz, CDCl3): 13.9 (CH3), 19.0 (CH3), 28.9 (CH(CH3)2), 65.7 (CHCH2), 72.4 (CHCH2), 75.0 (C4Ph4), 80.7 (CpC), 85.5 (CpC), 86.6 (CpC), 86.9 (CpC), 91.2 (d, J 8.4, CpC), 126.4 (PhC), 127.9 (PhC), 128.1 (PhC), 128.2 (PhC), 129.1 (PhC), 130.5 (PhC), 131.1 (PhC), 131.7 (PhC), 135.3 (PhC), 135.4 (PhC), 135.6 (PhC), 174.8 (CdN). MS (m/z, EI): 1083.3 (Mþ).

’ ASSOCIATED CONTENT

bS

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank RJ Bennie Ram John Jesudason (Queen Mary, University of London) for preliminary experiments, Myles Cheesman for assistance with the CD spectra, the Scientific € I_TAK) for and Technological Research Council of Turkey (TUB financial support (M.E.G.), and the EPSRC National Mass Spectrometry Centre (University of Wales, Swansea). ’ REFERENCES (1) (a) Richards, C. J. In Chiral Ferrocenes in Asymmetric Catalysis: Synthesis and Applications; Dai, L.-X.; Hou, X.-L., Eds.; Wiley-VCH: Weinheim, 2010; pp 337
368. (b) Nomura, H.; Richards, C. J. Chem. Asian J. 2010, 5, 1726. (2) Stevens, A. M.; Richards, C. J. Organometallics 1999, 18, 1346. (3) (a) Overman, L. E.; Owen, C. E.; Pavan, M. M.; Richards, C. J. Org. Lett. 2003, 5, 1809. (b) Anderson, C. E.; Overman, L. E. J. Am. Chem. Soc. 2003, 125, 12412. (c) Kang, J.; Kim, T. H.; Yew, K. H.; Lee, W. K. Tetrahedron: Asymmetry 2003, 14, 415. (d) Kirsch, S. F.; Overman, L. E.; Watson, M. P. J. Org. Chem. 2004, 69, 8101. (e) Nomura, H.; Richards, C. J. Chem.—Eur. J. 2007, 13, 10216. (f) Watson, M. P.; Overman, L. E.; Bergman, R. G. J. Am. Chem. Soc. 2007, 129, 5031. (4) (a) Kirsch, S. F.; Overman, L. E. J. Am. Chem. Soc. 2005, 127, 2866. (b) Kirsch, S. F.; Overman, L. E. J. Org. Chem. 2005, 70, 2859. (c) Lee, E. E.; Batey, R. A. J. Am. Chem. Soc. 2005, 127, 14887. (d) Roca, F. X.; Motevalli, M.; Richards, C. J. J. Am. Chem. Soc. 2005, 127, 2388. (e) Kirsch, S. F.; Overman, L. E.; White, N. S. Org. Lett. 2007, 9, 911. (f) Overman, L. E.; Roberts, S. W.; Sneddon, H. F. Org. Lett. 2008, 10, 1485. (g) Olson, A. C.; Overman, L. E.; Sneddon, H. F.; Ziller, J. W. Adv. Synth. Catal. 2009, 351, 3186. (h) Rodrigues, A.; Lee, E. E.; Batey, R. A. Org. Lett. 2010, 12, 260. (i) Cannon, J. S.; Kirsch, S. F.; Overman, L. E. J. Am. Chem. Soc. 2010, 132, 15185. (j) Cannon, J. S.; Kirsch, S. F.; Overman, L. E.; Sneddon, H. F. J. Am. Chem. Soc. 2010, 132, 15192. (5) (a) Weiss, M. E.; Fischer, D. F.; Xin, Z.; Jautze, S.; Schweizer, W. B.; Peters, R. Angew. Chem., Int. Ed. 2006, 45, 5694. (b) Fischer, D. F.; Barakat, A.; Xin, Z.; Weiss, M. E.; Peters, R. Chem.—Eur. J. 2009, 15, 8722. (6) Selected examples: (a) Sokolov, V. I.; Troitskaya, L. L.; Reutov, O. A. J. Organomet. Chem. 1977, 133, C28. (b) Dunina, V. V.; Gorunova, O. N.; Livantsov, M. V.; Grishin, Y. K.; Kuz’mina, L. G.; Kataeva, N. A.; Churakov, A. V. Inorg. Chem. Commun. 2000, 3, 354. (c) Xia, J.-B.; You,

NOTE

S.-L. Organometallics 2007, 26, 4869. (d) Jautze, S.; Seiler, P.; Peters, R. Angew. Chem., Int. Ed. 2007, 46, 1260. (7) (a) Sokolov, V. I.; Troitskaya, L. L. Chimia 1978, 32, 122. (b) Sokolov, V. I.; Troitskaya, L. L.; Reutov, O. A. J. Organomet. Chem. 1979, 182, 537. (c) Ryabov, A. D.; Firsova, Y. N.; Goral, V. N.; Ryabov, E. S.; Shevelkova, A. N.; Troitskaya, L. L.; Demeschik, T. V.; Sokolov, V. I. Chem.—Eur. J. 1998, 4, 806. (d) Dunina, V. V.; Razmyslova, E. D.; Gorunova, O. N.; Livantsov, M. V.; Grishin, Y. K. Tetrahedron: Asymmetry 2003, 14, 2331. (e) G€unay, M. E.; Richards, C. J. Organometallics 2009, 28, 5833. (8) (a) Lopez, C.; Caubet, A.; Perez, S.; Solans, X.; Font-Bardia, M. Chem. Commun. 2004, 540. (b) Lopez, C.; Caubet, A.; Perez, S.; Solans, X.; Font-Bardia, M.; Molins, E. Eur. J. Inorg. Chem. 2006, 3974. (c) Ryabov, A. D.; Panyashkina, I. M.; Polyakov, V. A.; Howard, J. A. K.; Kuz’mina, L. G.; Datt, M. S.; Sacht, C. Organometallics 1998, 17, 3615. (9) Huang, H.; Peters, R. Angew. Chem., Int. Ed. 2009, 48, 604. (10) (a) Annunziata, R.; Cenini, S.; Demartin, F.; Palmisano, G.; Tollari, S. J. Organomet. Chem. 1995, 496, C1. (b) Cravotto, G.; Demartin, F.; Palmisano, G.; Penoni, A.; Radice, T.; Tollari, S. J. Organomet. Chem. 2005, 690, 2017. (c) Lopez, C.; Perez, S.; Solans, X.; Font-Bardia, M.; Calvet, T. New J. Chem. 2010, 34, 676. (11) (a) Braddock, P. D.; Romeo, R.; Tobe, M. L. Inorg. Chem. 1974, 13, 1170. (b) Bonivento, M.; Canovese, L.; Cattalini, L.; Marangoni, G.; Michelon, G.; Tobe, M. L. Inorg. Chem. 1981, 20, 1493. (c) Otto, S.; Chanda, A.; Samuleev, P. V.; Ryabov, A. D. Eur. J. Inorg. Chem. 2006, 2561. (12) Yeamine, M. R.; Richards, C. J. Tetrahedron: Asymmetry 2007, 18, 2613. (13) (a) Komatsu, T.; Nonoyama, M.; Fujita, J. Bull. Chem. Soc. Jpn. 1981, 54, 186. (b) Sokolov, V. I.; Nechaeva, K. S.; Reutov, O. A. J. Organomet. Chem. 1983, 253, C55. (c) Wu, Y. J.; Cui, X. L.; Du, C. X.; Wang, W. L.; Guo, R. Y.; Chen, R. F. J. Chem. Soc., Dalton Trans. 1998, 3727. (d) Cui, X. L.; Wu, Y. J.; Du, C. X.; Yang, L. R.; Zhu, Y. Tetrahedron: Asymmetry 1999, 10, 1255. (e) Wu, Y.-J.; Ding, L.; Wang, W.-L.; Du, C.-X. Tetrahedron: Asymmetry 1998, 9, 4035. (f) Sokolov, V. I.; Bulygina, L. A.; Babievskii, K. K.; Yamskov, I. A. Russ. Chem. Bull., Int. Ed. 2007, 56, 1464. (14) Prasad, R. S.; Anderson, C. E.; Richards, C. J.; Overman, L. E. Organometallics 2005, 24, 77. (15) F€urstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46, 3410. (16) Fossey, J. S.; Richards, C. J. Organometallics 2004, 23, 367.

3904

dx.doi.org/10.1021/om200353n |Organometallics 2011, 30, 3901–3904