chemistry

Schwartz's reagent

Schwartz's reagent is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Schwartz's reagent rather than just read about it. In short: Schwartz's reagent is the common name for the organozirconium 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.

Key takeaways

Schwartz's reagent belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Schwartz's reagent to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Schwartz's reagent from memory before moving on to harder problems.

Reference excerpt

Schwartz's reagent is the common name for the organozirconium 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.

Preparation The complex was first prepared by Wailes and Weigold. 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 This reaction also affords (C5H5)2ZrH2, which is treated with methylene chloride to give Schwartz's reagent An alternative procedure that generated Schwartz's reagent from dihydride has also been reported. Moreover, it's possible to perform an in situ preparation of (C5H5)2ZrHCl from zirconocene dichloride by using LiH. This method can also be used to synthesize isotope-labeled molecules, like olefines by employing Li2H or Li3H as reducing agents. Schwartz's reagent has a low solubility in common organic solvents. The trifluoromethanesulfonate (C5H5)2ZrH(OTf) is soluble in THF.

Structure The complex adopts the usual "clam-shell" structure seen for other Cp2MXn complexes. The dimetallic structure has been confirmed by Microcrystal electron diffraction. The results are consistent with FT-IR spectroscopy, which established that the hydrides are bridging. Solid state NMR spectroscopy also indicates a dimeric structure. The X-ray crystallographic structure for the methyl compound (C5H5)4Zr2H2(CH3)2 compound is analogous.

Uses in organic synthesis Schwartz's reagent reduces amides to aldehydes. Vinylation of ketones in high yields is a possible use of Schwartz's reagent. Schwartz's reagent has been used in the synthesis of some macrolide antibiotics, (−)-motuporin, and antitumor agents.

Hydrozirconation Hydrozirconation is a form of hydrometalation. Substrates for hydrozirconation are alkenes and alkynes. With terminal alkynes the terminal vinyl zirconium product is predominantly formed. Secondary reactions are nucleophilic additions, transmetalations, conjugate additions, coupling reactions, carbonylation and halogenation. Computational studies indicate that hydrozirconation occurs from the interior portion. When treated with one equivalent of Cp2ZrClH, diphenylacetylene gives the corresponding alkenylzirconium as a mixture of cis and trans isomers. With two equivalents of hydride, the endproduct was a mixture of erythro and threo zircono alkanes:

In 1974 Hart and Schwartz reported that the organozirconium intermediates react with electrophiles such as hydrochloric acid, bromine and acid chlorides to give the corresponding alkane, bromoalkanes, and ketones:

The corresponding organoboron and organoaluminum compounds were already known, but these are air-sensitive and/or pyrophoric whereas organozirconium compounds are not.

Scope In one study the usual regioselectivity of an alkyne hydrozirconation is reversed with the addition of zinc chloride:

One example of a one-pot hydrozirconation - carbonylation - coupling is depicted below:

With certain allyl alcohols, the alcohol group is replaced by nucleophilic carbon forming a cyclopropane ring: The selectivity of the hydrozirconation of alkynes has been studied in detail. Generally, the addition of the Zr–H proceeds via the syn-addition. The rate of addition to unsaturated carbon-carbon bonds is terminal alkyne > terminal alkene ≈ internal alkyne > disubstituted alkene Acyl complexes can be generated by insertion of CO into the C–Zr bond resulting from hydrozirconation. 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.

Further reading Hart, D. W.; Schwartz, J. (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. Bibcode:1974JAChS..96.8115H. doi:10.1021/ja00833a048. Schwartz, J.; Labinger, J. A. (2003). "Hydrozirconation: A New Transition Metal Reagent for Organic Synthesis". Angew. Chem. Int. Ed. 15 (6): 330–340. doi:10.1002/anie.197603331. Hart, Donald W.; Blackburn, Thomas F.; Schwartz, Jeffrey (1975). "Hydrozirconation. III. Stereospecific and regioselective functionalization of alkylacetylenes via vinylzirconium(IV) intermediates". J. Am. Chem. Soc. 97 (3): 679–680. Bibcode:1975JAChS..97..679H. doi:10.1021/ja00836a056.

References

External links

Examples in organic synthesis at the University of Connecticut website

Illustrations

Worked examples

Example 1 — a first encounter with Schwartz's reagent

Start with the simplest possible case. Write down what Schwartz's reagent claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Schwartz's reagent before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Schwartz's reagent ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Schwartz's reagent

In research: Schwartz's reagent appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Schwartz's reagent in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Schwartz's reagent is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chloro complexes, Cyclopentadienyl complexes, Dimers (chemistry), so understanding it makes those chapters shorter.
In everyday life: Look for Schwartz's reagent outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Schwartz's reagent in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Schwartz's reagent means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Schwartz's reagent out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Schwartz's reagent in simple terms?

Why does Schwartz's reagent matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Schwartz's reagent?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Schwartz's reagent.