Hydrozirconation alkene

Why is the prohibition against secondary alkylzirconium relaxed for phenyl substituents Buchwald et al. suggest that the flat phenyl group is less sterically demanding than an alkyl, while others have proposed an electronic effect favoring nzylic zirconium compounds. Evidence supports the latter for internal aromatic alkenes hydrozirconation initially gives mostly the benzylic isomer (25) (based on the alcohol products of oxidation), which slowly (48-96 h at 40 C) converts to the terminal isomer (26 equation 30). ... 

Conjugate addition of alkenes. Hydrozirconation of alkcncs followed by addition of a catalytic amount of CuBr S(CH3)2 results in in situ preparation of alkyl cuprates that undergo 1,4-addition to an enone. 

This type of reaction is also observed in Main Group chemistry e.g. hydroboration (p. 66), hydroalumination (p. 80), hydrostannation (p. 108). It and its reversal are central to catalytic cycles such as those involved in the hydrogenation (p. 182) and hydroformylation (p. 387) of alkenes. Hydrozirconation (p. 290) provides another example of its use in stoichiometric organic synthesis. Its reverse... 

Formation of the least-hindered alkylzirconium complex during the course of the hydrozirconation-isomerization process of alkenes can be altered by the presence of aromatic rings, additional conjugative double bonds, chelating heteroatoms [81, 94] or groups that can be lost by elimination [95, 96]. 

Note that hydrozirconation of 2-vinylfuran gives only the internal product [86] (Scheme 8-16) which probably is the result of the combination of the effects described in this section (i) O-coordination, (ii) aromatic stabihzation, (iii) reduced steric effect of the flat furan ring, which favors the reverse expected regiochemistry in the hydrozirconation reaction of alkenes with [Cp2Zr(H)Cl] (1). 

Hydrozirconation of terminal alkynes R-C=CH (R= aryl, alkyl) with 1 affords terminally ( )-Zr-substituted alkenes with high efficiency and excellent stereochemical and regiochemical control (>98%). These alkenylzirconocene complexes are of particular interest for synthetic use [136, 143, 144]. Moreover, beside the electropositive halogen sources [145] and heteroatom electrophiles [3] used in the pioneering studies to directly cleave the Zr-C bond, ( )-vinyl-Zr complexes were recently transformed into a number of other trans-functionalized alkenes such as ( )-vinyl-sul-fides[146], vinylic selenol esters [147], vinyl-sulfones [148], vinyl-iodonium [149], vinyl-(R0)2P(0) [150], and vinilic tellurides [143]. 

As for alkenes, the rate of hydrozirconation of alkynes decreases with increasing substitution on the alkyne. An unsymmetrical diyne reacts with 1 preferentially at the less-substituted triple bond [85]. 

The chemoselectivity of Schwartz s reagent (1) toward alkynes, alkenes, nitriles, and carbonyl groups, and thus its general functional group compatibility, can be modulated. However, it is important to keep in mind that the presence of functional groups may have regiochemical consequences on the hydrozirconation reaction. 

As predicted from the comparative rates for C=C over C=C hydrozirconation cited earlier, a (poly)enyne is selectively hydrozirconated at the alkyne moiety, whatever the position of the alkene function [138, 210] in the molecule. It can be exempUfied by the chemoselective hydrozirconation of 1,3-butenyne. One exception to this chemoselectivity has been reported, which showed the terminal alkene to react with 1 but leaving the TMS-substituted alkyne function intact (Scheme 8-25). 

Scheme 8-26 Hydrozirconation of alkenes with nitrile function...

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

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). 

Negishi previously reported that a wide variety of Lewis-acidic compounds catalyzed hydrozirconation of alkenes such as 1-decene 108 with /-BuZrCp2Cl.126 It was found that the reaction of 1-decene with 1.1 molar equiv. of TIB A, in the presence of 2-5 mol% of chlorine-containing late transition metals, led to the formation of 1-iododecane 110 after treatment of the product with iodine (Table 9). 

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]. 

Another major protocol for the generation of three-membered zirconacycles was initially devised by Erker [47—49] and was extensively developed by Buchwald [36—44] (Erker—Buchwald protocol) (Scheme 1.14). No alkenes or alkynes are used as temporary ligands in this protocol. Unless hydrozirconation is used to generate the initial organyl-zirconocene derivatives, even final alkene or alkyne ligands are not usually derived from the corresponding ji-compounds. Thus, the synthetic values of the two representative protocols are quite different and often complementary to each other. 

The only other alkenyl carbenoid with a proton trans to the halide that can readily be generated by deprotonation is the parent 1-lithio-l-chloroethene 57 [43] (Scheme 3.13). Insertion into organozirconocenes arising from hydrozirconation of alkenes and alkynes, followed by protonation, affords terminal alkenes and ( )-dienes 59, respectively [38]. The latter provides a useful complement to the synthesis of 54 in Scheme 3.12 since the stereocontrol is >99%. 

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 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... 

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