Hydrozirconation

Hydrozirconation usually takes place readily at or slightly above room temperature, in benzene or toluene. The reactivity pattern of alkenes in hydrozirconation is the same as in hydroboration, except that tetrasubstituted alkenes are unreac-tive.457,460 A unique feature of hydrozirconation of alkenes is that, with some exceptions, terminally zirconated addition products are formed exclusively. Since isomeric alkenes cannot be isolated, zirconium moves rapidly along the chain without dissociation. Tertiary zirconium intermediates seem to be so unstable that the isopropylidene group never undergoes hydrozirconation [Eq. (6.75)],460 and hence, 1-methylcyclohexene fails to react.460 The reason is apparently steric, since the alkene must fit into the somewhat bent sandwich structure of the reagent (41)  

Exceptions are cycloalkenes, which do form secondary zirconium derivatives,460 as well as arylalkenes461 [Eq. (6.76)] and conjugated dienes 462,463 

In the latter cases, steric and electronic factors facilitate the formation of small amounts of secondary substituted products. 

Hydrozirconation (H—Zr) has recently been developed as a procedure that appears to have broad potential for reducing and functionalizing alkenes, alkynes and 1,3-dienes [45]. 

The reagents involved are inexpensive, readily prepared and the yields are high. Hydrozirconation converts unsaturated C—C bonds (even non activated ones) via 7r-bonded intermediates to cr-bonded Zr derivatives. 

The reaction is cleanly achieved by heating the unsaturated molecule with H(Cl)ZrCp2. Formally this complex is a high valent complex, two factors favorable for the stability of M—alkyl bonds K2 large). Moreover, it is readily prepared from Cp2ZrCl2 and L1A1H4 or NaAlH2(OR)2 in THF. 

Cp2ZrH(Cl) = ([Zr]-H) reacts with unsaturated C—C bonds to give isolable addition products (in opposition to boranes and alanes, organo-Zr are rather stable in dry air) that can be hydrolyzed with diluted acidic solutions. The scope of hydrozirconation with respect to chemoselectivity and substrate structure lies between those of hydroalumination and hydroboration. The facile isomerization of secondary alkylzirconium derivatives into primary alkyl derivatives occurs very readily (even below room temperature, note the difference with A1 and B ). 

Tetrasubstituted olefins fail to react, as do some trisubstituted cyclic olefins. Whereas acetylenes react 70—100 times faster than the corresponding olefins. 

Monosubstituted alkenes and certain alkynes can undergo hydroalumination with iBujAl in the presence of a catalytic amount of Cp2ZrCl2, providing a convenient alternative to hydrozirconation [75]. 

In deciding on the material to be covered in this chapter, limitations had to be set The first section will present the synthesis of various zirconocene hydrides. The focus of the subsequent sections is to present a general synopsis of the different aspects of the hydrozirconation reactions using 1 not only on carbon-carbon multiple bonds but also on heteropolar multiple bonds. Those aspects of hydrozirconation that were covered in previous reviews [1-5, 27] and in the excellent chapter by Labinger in Comprehensive Organic Synthesis [28] are summarized or briefly mentioned here. For other aspects of organozirconium chemistry not covered by the above-mentioned reviews, the reader is referred to a number of monographs and reviews [29, 30], 

The insolubility of 1 has the major inconvenience that it prevents studying in detail the mechanistic factors which would operate during this hydrometalation process, as in general only the organic substrate and final zirconocene product are observed by NM R spectroscopy. 

The vacant orbital in 16e -zirconocene(IV) complexes allows a ir-interaction with an incoming alkene or aUcyne. However no metal alkene/alkyne backbonding is possible with the d°-Zr-metal center. As a consequence, the metal-olefin interaction is not stabilized, and formation of the thermodynamically favored o-bound organozirconocene complex ( 10 kcal/mol) is then observed [36]. The product is the result of an overall cis addition of the zirconocene metal fragment and the hydrogen across the carbon-carbon multiple bonds. 

To overcome the problem of the insolubility of 1, different strategies have been developed. Synthesis of alternative soluble hydride reagents, substitution of cyclopen-tadienyl rings and/or changing the chloro-auxiHaiy ligand were widely investigated. 

Hydrido zirconocene complexes of the type [Cp 2ZrHX] (X = Cl [42], OR [43], NR2 [44], and PRj [45]) are rare and considerably more inconvenient and/or expensive to prepare and isolate in pure form than [Cp2Zr(H)Cl]n. 

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The alkenylzirconium 685, prepared by hydrozirconation of a terminal alkyne with hydrozirconocene chloride, reacts with alkenyl halide to afford the conjugated diene 686(545]. The Zr reagent can be used even in the presence of the carbonyl group in 687, which is sensitive to Al and Mg reagents. 

Hydrides. Zirconium hydrides react easily with unsaturated molecules. This process, termed hydrozirconation, replaces the hydrogen with the unsaturated group ... 

Mixed-Metal Systems. Mixed-metal systems, where a zirconium alkyl is formed and the alkyl group transferred to another metal, are a new apphcation of the hydrozirconation reaction. These systems offer the advantages of the easy formation of the Zr—alkyl as well as the versatiUty of alkyl—metal reagents. For example, Cp2ZrRCl (R = alkyl or alkenyl) reacts with AICI3 to give an Al—alkyl species which may then be acylated with... 

SCHWARTZ Hydrozirconalion Hydrozirconation with Cp2Zr(CI)H and Michael addition of the Zr reagent... 

Several trialkoxy(2-butenyl)zirconium(IV)6,7i 18 and 2-butenylbis(cyclopentadienyl)zirco-nium(IV)18,19 124 complexes have been investigated with respect to the diastereoselectivity on addition to aldehydes. Chlorobis(cyclopentadienyl)-(3-tributylstannyl-2-propenyl)zirconium(IV), prepared by hydrozirconation of tributyl-(l,2-propadienyl)tin, accomplishes the (E)-selective, Wittig-like 1,2-propenylidenation of aldehydes and methyl ketones125. 

Wipf P, Kendall C (2005) Hydrozirconation and Its Applications. 8 1-25 Wu GG, Huang M (2004) Organolithium in Asymmetric Process. 6 1-36 Wu YT, de Meijere A (2004) Versatile Chemistry Arising from Unsaturated Metal Carbenes. 13 21-58... 

A variety of other addition reactions occurring regioselectively are also known. These include hydrocyanation, hydroalumination, hydrosilylation, and hydrozirconation. 

Scheme 8-2 Intermediate in the hydrozirconation of ethylene proposed by Morokuma and coworkers based on ah initio MO calculations...

The substituted-cyclopentadiene derivatives analogs of [Cp2Zr(H)Cl]n (1), (MeCp)2Zr(H)Cl (2) [38], Cp CpZr(H)Cl (3) [39], and [(Me2Si)(C5H5)2Zr(H)Clj [40] are more soluble in non-polar organic solvents, and consequently they are more reactive. Thus, the direct competition between 1 and 2 for the hydrozirconation reaction of 1-hexene affords predominantly (86%) the product derived from the methylated derivative (Scheme 8-3) [41]. 

Hydrozirconation across Carbon-Carbon Multiple Bonds 257... 

Selective monoalkylation of Cp2ZrCl2 to form Cp2Zr(R)Cl in which R is alkyl or alkenyl is best accomplished by hydrozirconation with [Cp2Zr(H)Cl]n (1). Most syn-thehc applications involve the use of terminal organozirconocene complexes. The hydrozirconahon reachon on carbon-carbon double or triple bonds with 1 is generally clean and gives high yields of essentially 100% cis addition products. 

Hydrozirconation across Carbon-Carbon Double Bonds... 

Note that multiple hydrozirconation reactions can be conducted on the same substrate in a one-pot reaction (Scheme 8-10) [90-93]. 

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