[37342-97-5]  · C10H11ClZr  · Chlorobis(cyclopentadienyl)hydridozirconium  · (MW 257.87)

(hydrozirconation reagent; reducing reagent of unsaturated compounds; starting material for organozirconium complexes)

Alternate Name: Schwartz's reagent.

Solubility: insol; hydrolyzed in water; very slowly converted by CH2Cl2 to Cp2ZrCl2.

Form Supplied in: commercially available as white powder (95%).

Analysis of Reagent Purity: IR (nujol) 1390 cm-1 (metal-hydrogen bond). The purity of Cp2Zr(H)Cl is measured by reaction with acetone. A small sample is assayed in a 5 mm NMR tube in C6D6 by treatment with a known amount of excess acetone, and the relative areas of the signal for the mono- and diisopropoxides are determined by 1H NMR.2,3

Preparative Methods: Cp2Zr(H)Cl is prepared by the reaction of Dichlorobis(cyclopentadienyl)zirconium with Lithium Tri-t-butoxyaluminum Hydride,4,5 Sodium Bis(2-methoxyethoxy)aluminum Hydride (Vitride or Red-Al),3 or Lithium Aluminum Hydride.2 Zirconocene dichloride (100 g, 0.342 mol) is dissolved in dry THF (650 mL, heating required) in a 1 L Schlenk flask under argon. To this solution (at or slightly above rt) is added dropwise, over a 45 min period, a filtered solution of LiAlH4 in diethyl ether (prepared from 3.6 g, 94.9 mmol of 95% LiAlH4 and 100 mL dry ether, followed by filtration using a cannula fitted with a piece of glass fiber filter; a Schlenk filtered or commercial clear solution would work as well). The resulting suspension is allowed to stir at room temperature for 90 min. It is then Schlenk filtered under argon using a D frit. The white solid is washed on the frit with THF (4 × 75 mL), CH2Cl2 (2 × 100 mL) with stirring or agitation of a stirbar immersed in the slurry and ether (4 × 50 mL). The resulting white solid is dried in vacuo to give a white powder: 65.5-81 g, 77-92% yield.2

Purification: over-reduction product Cp2ZrH2 reacts rapidly with methylene chloride to form Cp2Zr(H)Cl.2

Handling, Storage, and Precautions: slowly develops a pink color when exposed to light and appears to be photosensitive. This compound should be handled in a fume hood.

Hydrozirconation of Unsaturated Compounds.

Alkenes6 react with Cp2Zr(H)Cl to give alkylzirconium complexes. Hydrozirconation of alkenes proceeds to place the zirconium moiety at the sterically least hindered position of the alkene as a whole. Formation of this product involves either the regiospecific addition of Zr-H to a terminal alkene or addition to an internal alkene followed by rapid rearrangement via Zr-H elimination and readdition to place the metal in the less hindered position of the alkyl chain (eq 1).6,7 This is in sharp contrast to hydroboration or hydroalumination reactions. However, the tendency of zirconium to migrate towards the terminal position in the case of 1-arylalkenes is lower than simple alkenes.8 Alkenes containing a heteroatom such as oxygen undergo C-heteroatom bond cleavage during the hydrozirconation reactions under some conditions (eq 2).9 Alkylzirconium complexes obtained here are easily converted into alkanes,10 alkyl halides (eq 3),6 alcohols (eqs 4 and 5),7 nitriles (eq 6),11 aldehydes, carboxylic acids or esters (eq 7)12 by hydrogenation (or hydrolysis), halogenation, oxidation, successive treatment with isocyanide and iodine, or carbonylation.

Hydrozirconation of alkynes gives alkenylzirconium complexes (eq 8).13 Zr-H addition to the alkyne proceeds with cis stereochemistry. Hydrozirconation of various unsymmetrically disubstituted alkynes occurs readily to give a mixture of alkenylzirconium compounds. Over a period of several hours, this initial mixture of alkenyl species can be converted to one with higher regioselectivity, at room temperature, through catalysis with Cp2Zr(H)Cl.13 It is interesting to note that, whereas alkylzirconium complexes positionally rearrange rapidly, no such process occurs for purified alkenyl analogs. Alkenylzirconium complexes are also easily converted into alkenyl halides13 and aldehydes (eq 9)14 when they are treated with N-Bromosuccinimide and isocyanide/H+, respectively.

In situ generation of Cp2Zr(H)Cl or in situ hydrozirconation is practically useful. There are several in situ procedures which consist of Dichlorobis(cyclopentadienyl)zirconium/Sodium Bis(2-methoxyethoxy)aluminum Hydride (Red-Al), Cp2ZrCl2/t-Butylmagnesium Chloride15 or Cp2ZrCl2/Lithium Triethylborohydride.16 Comparison of hydrozirconation using Cp2Zr(H)Cl and these in situ procedures is shown in Table 1.15,16

(MeC5H4)2Zr(H)Cl reacts about seven times faster with 1-hexene or acetophenone than Cp2Zr(H)Cl.17 Hydrozirconation of other various unsaturated bonds such as Schiff bases,18 ketones,3,4,19 diazoalkanes,20 nitriles,21 carbon dioxide (eq 10),22 1-ene-3-ynes,23 dienes,24 phosphaimines,25 thioketones,26 carbon oxide,27 and isocyanide28 has been investigated.

Hydrozirconation of unsaturated bonds attached to metals also proceeds. Alkenylzinc or alkynylzinc compounds react with Cp2Zr(H)Cl to give unstable 1,1-bimetallic reagents which can be used for highly stereoselective alkenation of aldehydes (eq 11).29 Similarly, addition of Cp2Zr(H)Cl to a solution of (neohexenyl)diisobutylaluminum gives a 1,1-bimetallic complex.30 Its structural analysis by X-ray clearly shows the 1,1-bimetallic system.31 Reaction of stannylacetylenes with Cp2Zr(H)Cl seems to give 1,1-bimetallic compounds, since the iodination product is 1-iodo-1-stannylalkenes.32 The regioselectivity of hydrozirconation of unsaturated bonds attached to metals is noteworthy. Hydrozirconation of an alkynyl ligand attached to Ru33 or Zr,34 and an alkenyl ligand attached to Zr,35 shows the opposite regioselectivity to those described above, which give 1,1-bimetallic compounds (eq 12). However, the reaction of Cp2Zr(H)Cl with Cp(CO)2Re=CHR36 gives a 1,1-bimetallic intermediate with Zr and Re. An acyl ligand on zirconium also reacts with Cp2Zr(H)Cl.37 When Cp2Zr(H)Cl is treated with potassium allyl alkoxide or homoallyl alkoxide, intramolecular hydrozirconation seems to give oxazirconacycles.38

Carbon-Carbon Bond Forming Reactions of Hydrozirconation Products.

Hydrozirconation products of unsaturated compounds can be used for further C-C bond forming reactions. Since alkyl or alkenyl carbon attached to zirconium is not nucleophilic like Grignard, lithium, or aluminum reagents, the direct C-C bond formation is very limited. Only carbonylation (eq 7) and acylation with Acetyl Chloride can be used. However, transmetalation of the alkyl or alkenyl moiety from zirconium to other metals changes the situation. It provides a variety of C-C bond formation reactions of hydrozirconation products.

The alkyl or alkenyl moiety of hydrozirconation products are transferred from zirconium to aluminum when they are treated with Aluminum Chloride. These organoaluminum dichlorides obtained here are converted into ketones or a,b-unsaturated ketones in high yields (eq 13).39 Interestingly, since hydrozirconation of alkenes gives the terminal alkylzirconium exclusively, this method can provide a single ketonic product from a mixture of isomeric alkenes. For alkenylzirconium, through this sequential use of two reactive organometallic species, the ketone is formed by overall cis addition of acyl-H to an alkyne.39 Similar treatment of 1-(trimethylsilyl)-4-bromo-1-butyne with Cp2Zr(H)Cl followed by AlCl3 affords 1-(trimethylsilyl)cyclobutene (eq 14).40

Transmetalation from Zr to Cu produces the corresponding organocopper compounds which can be used for addition to a,b-unsaturated ketones (eq 15).41 It is noteworthy that functionalized lithiocuprates, which contain -CN, -CO2Si(i-Pr)3, -OCOPh, or Cl groups, are easily prepared by this method.42

Migration of an alkenyl group from zirconium to boron (eq 16),43 tin,44 or selenium (eq 17)44 also proceeds. Alkyltin compounds can be prepared from alkylzirconium and Tin(IV) Chloride.45 Imine transfer from zirconium to phosphorus or boron is also possible.46

Catalytic reactions using transition metals have been investigated. Cross-coupling reaction of alkenylzirconium with aryl halides or alkenyl halides is catalyzed by Ni or Pd complexes (eq 18).47 The effect of addition of metal salts such as Zinc Chloride and Cadmium Chloride is remarkable in this reaction.48 Addition of alkenylzirconium to a,b-unsaturated ketones is also catalyzed by Ni (eqs 19 and 20).49

Alkylzirconium compounds react with a,b-unsaturated ketones (eq 21)50 or Allyl Chloride51 in the presence of a catalytic amount of copper compound.

Formation of cationic species from alkyl- or alkenylzirconium compounds with a catalytic amount of Silver(I) Perchlorate is one method to make direct C-C bonds with aldehydes (eq 22).52 Both alkyl and alkenyl groups attack aldehydes to give desired products in high yields. When 3-trimethylsilyl-1-propyne is used as a starting alkyne, (E)-selective terminal 1,3-dienes are formed.53 Alkynes, 1-ethoxyethyne, and (Z)-1-methoxy-1-buten-3-yne give two- and four-carbon homologation of aldehydes, respectively, after treatment with an acid (eq 23).54 A direct C-C bond formation of alkenylzirconium compounds with epoxides also proceeds in the presence a catalytic amount of AgClO4.55

Organozirconium complexes, prepared by either hydrozirconation or the reaction of organolithium reagents with Cp2Zr(H)Cl, can be converted into diene (eq 24),56 alkyne (eq 25),57 silanimine (eq 26),58 and diazobenzene (or hydrazido(1-)) (eq 27)59 ZrII complexes which react with unsaturated compounds to afford various zirconacycles.

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Tamotsu Takahashi & Noriyuki Suzuki

Institute for Molecular Science, Okazaki, Japan

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