Schwartz's reagent is the common name for the
organozirconium compound
Organozirconium chemistry is the science of exploring the properties, structure, and reactivity of organozirconium compounds, which are organometallic compounds containing chemical bonds between carbon and zirconium. Organozirconium compounds have ...
with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
(C
5H
5)
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
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
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
Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li Al H4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic ...
:
: (C
5H
5)
2ZrCl
2 + LiAlH
4 → (C
5H
5)
2ZrHCl +
LiAlCl4
This reaction also affords (C
5H
5)
2ZrH
2, which is treated with
methylene chloride to give Schwartz's reagent
LiAl(O-t-Bu)
3H can be used in place of LiAlH
4.
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 (C
5H
5)
2ZrHCl from zirconocene dichloride by using LiH. This method can also be used to synthetize isotope-labeled molecules, like olefines by employing Li
2H or Li
3H as reducing agents.
Schwartz's reagent has a low solubility in common organic solvents. The trifluoromethanesulfonate (C
5H
5)
2ZrH(OTf) is soluble in THF.
Structure
The complex adopts the usual "clam-shell" structure seen for other Cp
2MX
n 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
Solid-state NMR (ssNMR) spectroscopy is a technique for characterizing atomic level structure in solid materials e.g. powders, single crystals and amorphous samples and tissues using nuclear magnetic resonance (NMR) spectroscopy. The anisotropic pa ...
spectroscopy also indicates a dimeric structure. The X-ray crystallographic structure for the methyl compound (C
5H
5)
4Zr
2H
2(CH
3)
2 compound is analogous.
Uses in organic synthesis
Schwartz's reagent reduces
amides to
aldehydes.
Vinylation of
ketone
In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...
s 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 Hydrometalation (hydrometallation) is a type of chemical reaction in organometallic chemistry in which a chemical compound with a hydrogen to metal bond (M-H, metal hydride) adds to compounds with an unsaturated bond like an alkene (RC=CR) forming ...
. Substrates for hydrozirconation are
alkenes and
alkyne
\ce
\ce
Acetylene
\ce
\ce
\ce
Propyne
\ce
\ce
\ce
\ce
1-Butyne
In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
s. With terminal alkynes the terminal vinyl zirconium product is predominantly formed. Secondary reactions are
nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic additions di ...
s,
transmetalation Transmetalation (alt. spelling: transmetallation) is a type of organometallic reaction that involves the transfer of ligands from one metal to another. It has the general form:
:M1–R + M2–R′ → M1–R′ + M2–R
where R and R′ can be, but ...
s,
conjugate additions,
coupling reactions,
carbonylation and
halogenation
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, ...
.
Computational studies indicate that hydrozirconation occurs from the interior portion.
When treated with one equivalent of Cp
2ZrClH,
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
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
,
bromoalkane
Organobromine compounds, also called organobromides, are organic compounds that contain carbon bonded to bromine. The most pervasive is the naturally produced bromomethane.
One prominent application of synthetic organobromine compounds is the u ...
s, and
ketone
In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...
s:
:
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
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
group is replaced by nucleophilic carbon forming a
cyclopropane
Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane itself ...
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
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
*
*
*
References
External links
{{commons category
Examples in organic synthesisat the University of Connecticut website
Metallocenes
Organozirconium compounds
Cyclopentadienyl complexes
Reagents for organic chemistry
Chloro complexes
Metal hydrides
Zirconium(IV) compounds