Zirconium hydrozirconation

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... 

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. 

Styrene hydrozirconation led to a ratio for terminal and benzylic zirconocene products of around 85 15 [97]. Experimental evidence on alkyl-substituted styrene suggested that both electronic [98,99] and steric effects [41,86] are important for the formation of the benzyhc and terminal zirconium isomers. Migration of the metal fragment during the reaction of the zirconocene moiety might occur, perturbing the terminal/benzyl regioisomer ratio of the isolated products (Scheme 8-11) [67, 83, 98-102]. 

Derivatives of zirconium with a Zr-H bond also can add to alkenes and alkynes. This reaction is known as hydrozirconation.230 The reagent that is used most frequently... 

AICI3 is a moisture-sensitive and strong Lewis acid. It is a first choice for Friedel-Crafts-type reactions, which provide numerous important transformations in laboratory and industry. It can also be applied to the transformation of alkenes to ketones via alkylaluminum halides.303 Hydrozirconation of an olefin and subsequent transmetalation from zirconium to aluminum gives the corresponding alkylaluminum dichloride, and the subsequent acetylation by acetyl chloride affords the corresponding ketone in high yield (Scheme 66). 

As Scheme 16 shows, vinyl(methyl)zinc compounds 17 were obtained from the hydrozirconation of terminal alkynes with Cp2ZrHGl, followed by treatment with dimethylzinc. The initially formed vinyl zirconium complex undergoes rapid transmetallation with ZnMe2 to yield the product.44... 

Acylzirconocene chloride derivatives are easily accessible in a one-pot procedure through the hydrozirconation of alkene or alkyne derivatives with zirconocene chloride hydride (Schwartz reagent) [Cp2Zr(H)Cl, Cp = cyclopentadienyl] and subsequent insertion of carbon monoxide (CO) into the alkyl— or alkenyl—zirconium bond under atmospheric pressure (Scheme 5.1) [2],... 

The first example of a stable 1,1-bidentate Lewis acid based on boron and zirconium has been reported [35]. The synthesis of 22 is outlined in Scheme 7.12. Treatment of hex-l-yne with HBBr2 Me2S followed by conversion of the dibromoboronic ester to the corresponding alkenyl boronic acid and esterification with propane-1,3-diol provided the alkenyl boronic ester. Hydrozirconation of this compound with 3 equivalents of the Schwartz reagent, Cp2Zr(H)Cl [57], afforded the desired product 22 in 86% yield. 

The amination of styrene, however, led to two products (1-phenyl-1-ethylamine and 2-phenyl-l-ethylamine) in a 1 3 ratio [113], indicating that the hydrozirconation was not completely regioselective [114,115]. Since it is well known that hydrozirconation of trisubsti-tuted alkenes places zirconium at the least hindered carbon of the chain by a process involving zirconium migration, this class of alkenes was not investigated [5,116], On the other hand, hydrozirconation/amination of 3-methyl-l,2-butadiene gave an allylic amine. Reaction of the latter could either occur at the terminal carbon or proceed with... 

As an example of the selective reactivity of borazirconocene alkenes, their hydrolysis was examined [1]. The carbon—zirconium bond is more reactive than the carbon—boron bond towards various electrophiles, and so hydrolysis can be expected to occur with preferential cleavage of the former bond. Since hydrolysis of alkenylzirconocenes is known to proceed with retention of configuration [4,127—129], a direct utility of 45 is the preparation of (Z)-1-alkenylboronates 57 (Scheme 7.17) [12]. Though the gem-dimetalloalkenes can be isolated, in the present case it is not necessary. The desired (Z)-l-alkenylboronates can be obtained in a one-pot procedure by hydrozirconation followed by hydrolysis with excess H20. The reaction sequence is operationally simple and is compatible with various functional groups such as halides, acetals, silanes, and silyloxy protecting groups [12]. 

Alkynylzinc bromides 133 are also cleanly hydrozirconated with Cp2Zr(H)Cl leading to 1,1-bimetalloalkenes of zinc and zirconium (134), which react smoothly with aldehydes to afford allenes in satisfactory yields (Scheme 7.40) [232,233]. 

For the asymmetric 1,4-addition of alkenyl groups in aprotic media, alkenyl zirconium reagents can be used, which are generated by hydrozirconation of terminal alkynes (Figure 3.30). Under these conditions, alkenyl groups derived from alkylacetylenes are efficiently installed, but those from arylacetylenes are not as effective (entry 3). 

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) ... 

An efficient synthesis of ethers involves hydrozirconation of thioketones with hydrochlorobis(cyclopentadienyl)zirconium [251]. 

The steric demand of the zirconocene moiety leads to rearrangements in the case of hydrozirconation of internal double bonds. Regardless of position or configuration of the double bond in the substrate, zirconium migrates to the terminal position of the alkyl chain via insertion and /TH-elimination steps. Such isomerization does not occur upon hydrozirconization of internal alkynes. 

The hydrozirconation of alkynes is a well-established reaction, giving vinylic zirconium species of known regio-and stereochemistry.176 These species react with aryltellurium halides leading to vinylic tellurides with the ( )-stereochemistry 98 (Scheme 61),177,178 so complementing the other general routes to these compounds which give preferentially the (Z)-products (Sections 9.13.5.2.3, 9.13.5.2.5). 

The -elimination approaches to cyclopropanes [Scheme 11, Eq. (i)] would be a quite general method, regardless of the way in which the precursor zirconium intermediate was formed. The hydrozirconation reaction [25] can be considered in this respect, since it provides an easy and frequently used route to organozirconocenes. The residual formation of cyclopropane rings through the hydrozirconation-y-elimination sequence had already been observed in 1976... 

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