Schwartz's reagent

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Schwartz's reagent
IUPAC name

chloridobis(η5-cyclopentadienyl)hydridozirconium

Other names

Cp2ZrClH, zirconocene hydride

Identifiers
CAS number 37342-97-5
Properties
Molecular formula C10H11ClZr
Molar mass 257.87 g/mol
Appearance White solid
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Schwartz's reagent is the common name for the chemical compound with the formula (C5H5)2ZrHCl, sometimes called zirconocene hydrochloride or zirconocene chloride hydride, and is named after Jeffrey Schwartz, a chemistry professor at Princeton University. This metallocene is used in organic synthesis for various transformations of alkenes and alkynes.[1][2][3]

Preparation

The complex was first prepared by Wailes and Weigold.[4] It can be purchased or readily prepared by reduction of zirconocene dichloride with lithium aluminium hydride:

(C5H5)2ZrCl2 + 1/4 LiAlH4 → (C5H5)2ZrHCl + 1/4 "LiAlCl4"

In practice this reaction also affords (C5H5)2ZrH2, which is treated with methylene chloride to give the mixed hydride chloride.[5] An alternative procedure that generated Schwartz's Reagent from dihydride has also been reported.[6]

Uses in organic synthesis

Schwartz's reagent can be used for a number of reactions. It has been shown that it can be used to reduce amides to aldehydes. Reducing tertiary amides with Schwartz's reagent can reach efficient yields, but primary and secondary amides will show decreased yields. The use of Schwartz's reagent in this manner will not require any added heat and can be done quickly, and reduction of the alcohol form is not a problematic side reaction as it can be with other reducing agents. Schwartz's reagent will selectively reduce the amide over any readily reducible esters that may be present in the reaction mixture.[7]

Vinylation of ketones in high yields is a possible use of Schwartz's reagent.[8]

Schwartz's reagent is used in the synthesis of some macrolide antibiotics,[9][10] (-)-motuporin,[11] and antitumor agents.[12]

Hydrozirconation

Schwartz's reagent reacts with alkenes and alkynes via the process called hydrozirconation, which results in the addition of the Zr-H bond across the C=C or C≡C bond.

The selectivity of the hydrozirconation of alkynes has been studied in detail.[13][14] Generally, the addition of the Zr-H proceeds the syn-addition. The rate of addition to unsaturated carbon-carbon bonds is terminal alkyne > terminal alkene ~ internal alkyne > disubstituted alkene [15] Acyl complexes can be generated by insertion of CO into the C-Zr bond resulting from hydrozirconation.[16] Upon alkene insertion into the zirconium hydride bond, the resulting zirconium alkyl undergoes facile rearrangement to the terminal alkyl and therefore only terminal acyl compounds can be synthesized in this way. The rearrangement most likely proceeds via β-hydride elimination followed by reinsertion.

Two locations of attack are possible

Computational studies indicate that hydrozirconation occurs from the interior portion.[17][18]

References

  1. ↑ D. W. Hart and J. Schwartz (1974). "Hydrozirconation. Organic Synthesis via Organozirconium Intermediates. Synthesis and Rearrangement of Alkylzirconium(IV) Complexes and Their Reaction with Electrophiles". J. Am. Chem. Soc. 96 (26): 8115–8116. doi:10.1021/ja00833a048. 
  2. ↑ J. Schwartz, and J. A. Labinger (2003). "Hydrozirconation: A New Transition Metal Reagent for Organic Synthesis". Angew. Chem. Int. Ed. 15 (6): 330–340. doi:10.1002/anie.197603331. 
  3. ↑ Donald W. Hart, Thomas F. Blackburn, Jeffrey Schwartz (1975). "Hydrozirconation. III. Stereospecific and regioselective functionalization of alkylacetylenes via vinylzirconium(IV) intermediates". J. Am. Chem. Soc. 97 (3): 679–680. doi:10.1021/ja00836a056. 
  4. ↑ P. C. Wailes and H. Weigold (1970). "Hydrido complexes of zirconium I. Preparation". Journal of Organometallic Chemistry 24 (2): 405–411. doi:10.1016/S0022-328X(00)80281-8. 
  5. ↑ S. L. Buchwald, S. J. LaMaire, R. B. Nielsen, B. T. Watson, and S. M. King, "Schwartz's Reagent", Org. Synth. ; Coll. Vol. 9: 162 
  6. ↑ Peter Wipf, Hidenori Takahashi, and Nian Zhuang (1998). "Kinetic vs. thermodynamic control in hydrozirconation reactions". Pure Appl. Chem. 70 (5): 1077–1082. doi:10.1351/pac199870051077. 
  7. ↑ M. W. Leighty, J. T. Spletstoser, and Gunda I. Georg (2011). "Mild Conversion of Tertiary Amides to Aldehydes Using Cp2ZrHCl (Schwartz’s Reagent)". Org. Synth. 88: 427–437. 
  8. ↑ H. Li, and P.J. Walsh (2005). "Catalytic Asymmetric Vinylation and Dienylation of Ketones". JACS 127: 8355–8361. doi:10.1021/ja0425740. 
  9. ↑ Matthew O. Duffey, Arnaud LeTiran, and James P. Morken (2003). "Enantioselective Total Synthesis of Borrelidin". J. Am. Chem. Soc. 125: 1458–1459. doi:10.1021/ja028941u. 
  10. ↑ J. Wu, and J.S. Panek (2011). "Total Synthesis of (-)-Virginiamycin M2: Application of Crotylsilanes Accessed by Enantioselective Rh(II) or Cu(I) Promoted Carbenoid Si-H Insertion". J. Org. Chem. 76 (24): 9900–9918. doi:10.1021/jo202119p. 
  11. ↑ T. Hu, and J.S. Panek (1999). "Total Synthesis of (-)-Motuporin". J. Org. Chem. 64: 3000–3001. doi:10.1021/jo9904617. 
  12. ↑ K. C. Nicolaou, et. al. (2003). "Total Synthesis of Apoptolidin: Completion of the Synthesis and Analogue Synthesis and Evaluation". J. Am. Chem. Soc. 125: 15443–15454. doi:10.1021/ja030496v. 
  13. ↑ R. C. Sun, M. Okabe, D. L. Coffen, and J. Schwartz (1998), "Conjugate Addition of a Vinylzirconium Reagent: 3-(1-Octene-1-yl)cyclopentanone", Org. Synth. ; Coll. Vol. 9: 640 
  14. ↑ Panek, J. S.; Hu, T. (1997). "Stereo- and Regiocontrolled Synthesis of Branched Trisubstituted Conjugated Dienes by Palladium(0)-Catalyzed Cross-Coupling Reaction". J. Org. Chem. 62 (15): 4912–4913. doi:10.1021/jo970647a. 
  15. ↑ Peter Wipf and Heike Jahn (1996). "Synthetic applications of organochlorozirconocene complexes". Tetrahedron 52 (40): 12853–12910. doi:10.1016/0040-4020(96)00754-5. 
  16. ↑ Christopher A. Bertelo, Jeffrey Schwartz (1975). "Hydrozirconation. II. Oxidative homologation of olefins via carbon monoxide insertion into the carbon-zirconium bond". J. Am. Chem. Soc. 97 (1): 228–230. doi:10.1021/ja00834a061. 
  17. ↑ E. Y. Pankratyev, T. V. Tyumkina, L. V. Parfenova,S. L. Khursan, L. M. Khalilov, and U. M. Dzhemilev (2011). "DFT and Ab Initio Study on Mechanism of Olefin Hydroalumination by XAlBui2 in the Presence of Cp2ZrCl2 Catalyst. II.(1) Olefin Interaction with Catalytically Active Centers". Organometallics 30 (22): 6078–6089. doi:10.1021/om200518h. 
  18. ↑ Juping Wang, Huiying Xu, Hui Gao, Cheng-Yong Su, Cunyuan Zhao, and David Lee Phillips (2010). "DFT Study on the Mechanism of Amides to Aldehydes Using Cp2Zr(H)Cl". Organometallics 29 (1): 42–51. doi:10.1021/om900371u. 

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