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Psoats/sandbox
Names
IUPAC name
chloridobis(η5-cyclopentadienyl)hydridozirconium
Other names
Cp2ZrClH, zirconocene hydride
Identifiers
UNII
Properties
C10H11ClZr
Molar mass 257.87 g/mol
Appearance White solid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Schwartz's reagent is the common name for the chemical compound with the formula (C5H5)2ZrHCl, sometimes described zirconocene hydrochloride or zirconocene chloride hydride. The synthesis of Schwartz' reagent was completed in the 1970s and is named after its discoverer Jeffrey Schwartz, who is currently a professor in Chemistry at Princeton University. This metallocene is used in organic synthesis for various transformations of alkenes and alkynes.[1][2][3]

Crystalline Structure via Cl-NMR

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Solid state NMR data using chlorine-35 has been studied by Jochen Autschbach, Robert W. Schurko and others at University of Windsor, Canada, and The State University of New York in order to find the crystalline structure of Schwartz's Reagent. The SSNMR showed that in the solid state the reagent is an oligomer due to the bridging of the hydride ligands. To confirm experimental findings, quantum mechanical calculations were employed using density functional theory and Hartree-Fock methods on the molecular structures. The ligands were shown to have chlorine bridging. [4]

Dimer crystal structure of Schwartz's reagent

Uses of Schwartz's reagent

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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. [5]

Vinylation of ketones in high yields is a possible use of Schwartz's reagent. [6]

Schwartz's reagent is used in the synthesis of some macrolide antibiotics[7][8], (-)-motuporin [9], and also antitumor agents. [10]

Hydrozirconation

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Schwartz's reagent reacts with alkenes and alkynes via the process called hydrozirconation which formally 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.[11][12] 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 [13] Acyl complexes can be generated by insertion of CO into the C-Zr bond resulting from hydrozirconation.[14] 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.

Mechanism and Computational Support

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Two locations of attack are possible
File:Mechanism of hydrozirconation with Schwartz's reagent.png
Mechanism of hydrozirconation

Schwartz’s reagent has been used in a number of organometallic reactions. One main reaction this reagent is used in is hydrozirconation. The importance of this reaction leads the chemist to theorize a plausible mechanism. Computation studies have been completed to develop a possible mechanism of a reaction when using Schwartz’s reagent. In order to employ density functional theory to study organometallic compounds, the bridging needs to be strong so one can get good valid results. The quantum mechanical analysis employed various levels of theory largely using density functional theory and Møller–Plesset perturbation theory. The first route included the attachment to the zirconium in the reagent while the second route was the attachment on the alkene. It was found that the attachment of the alkene to Schwartz’s reagent takes place from the interior portion. The study also discussed their catalytic cycle which included processes such as alkene hydrozinconation and transmetalation. It is interesting to note that the hydrozirconation process within the catalytic cycle had the highest activation energy. [15]

Another study computational analysis once again using density functional theory provides further support that the hydrozirconation process takes place from the inside portion of the Schwartz's reagent.[16]

Preparation

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The complex was first prepared by Wailes and Weigold.[17] It can be purchased or readily prepared by reduction of zirconocene dichloride with lithium aluminium hydride in THF solvent. The reaction is run under an argon atmosphere. There is a hydride shift from the lithium aluminum hydride to the substrate by the elimination of chloride and form Schwartz’s reagent.

Synthesis of Schwartz's reagent

In practice this reaction also makes (C5H5)2ZrH2, due to additional hydride shifts. This can be corrected when treated with methylene chloride to give the mixed hydride chloride.[18] An alternative procedure that generated Schwartz's Reagent from dihydride has also been reported.[19]

References

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  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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Aaron J. Rossini, Ryan W. Mills, Graham A. Riscoe, Erin L. Norton, Stephen J. Geier, Ivan Hung, Shaohui Zheng, Jochen Autschbach, and Robert W. Schurko (2009). "Solid-State Chlorine NMR of Group IV Transition Metal Organometallic Complexes". J. Am. Chem. Soc. 131 (9): 3317–3330. doi:10.1021/ja808390a.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ^ H. Li, and P.J. Walsh (2005). "Catalytic Asymmetric Vinylation and Dienylation of Ketones". JACS. 127: 8355–8361. doi:10.1021/ja0425740.
  7. ^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ 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.
  9. ^ T. Hu, and J.S. Panek (1999). "Total Synthesis of (-)-Motuporin". J. Org. Chem. 64: 3000–3001. doi:10.1021/jo9904617.
  10. ^ 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. {{cite journal}}: Explicit use of et al. in: |author= (help)
  11. ^ R. C. Sun, M. Okabe, D. L. Coffen, and J. Schwartz (1998). "Conjugate Addition of a Vinylzirconium Reagent: 3-(1-Octene-1-yl)cyclopentanone". Organic Syntheses{{cite journal}}: CS1 maint: multiple names: authors list (link); Collected Volumes, vol. 9, p. 640.
  12. ^ 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/jo970647.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ Peter Wipf and Heike Jahn (1996). "Synthetic applications of organochlorozirconocene complexes". Tetrahedron. 52 (40): 12853–12910. doi:10.1016/0040-4020(96)00754-5.
  14. ^ 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.
  15. ^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  16. ^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ^ 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.
  18. ^ S. L. Buchwald, S. J. LaMaire, R. B. Nielsen, B. T. Watson, and S. M. King. "Schwartz's Reagent". Organic Syntheses{{cite journal}}: CS1 maint: multiple names: authors list (link); Collected Volumes, vol. 9, p. 162.
  19. ^ Peter Wipf, Hidenori Takahashi, and Nian Zhuang (1998). "Kinetic vs. thermodynamic control in hydrozirconation reactions" (PDF). Pure Appl. Chem. 70 (5): 1077–1082. doi:10.1351/pac199870051077.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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Category:Metallocenes Category:Organozirconium compounds Category:Cyclopentadienyl complexes Category:Reagents for organic chemistry

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