Hydrozirconation steric effect

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]. 

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). 

The relative reactivity is measured qualitatively. At RT the rate of hydrozirconation of various alkenes is 1-octene > cis-4-octene > trans-4-octene > methylenecyclo-hexane > cyclopentene > cyclohexene > 3-methyl-2-butene. In competitive studies, a terminal alkene reacts 70 times faster that a disubstituted alkene. These relative rates are attributed to steric effects. Double bonds y to zirconium are less reactive than dienes since ( -Cp)2Zr(CH2CH2CH=CH2)Cl can be isolated from the reaction of ((j -Cp)2ZrHCl and butadiene . 

In the case of unsymmetrically disubstituted alkynes, mixtures of two cis-adducts are obtained in a ratio that depends on steric effects. For example, hydrozirconation of 5-methyl-2-hexyne with (1) for 2 hr. gives a mixture of (2) and (3) in the ratio 55 45. When a few milligrams of (1) is added to the original... 

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). ... 

In hydrozirconation of alkenes by Cp2ZrHCl, terminal alkenes insert in the anti-Markovnikov direction to give a stable 1° alkyl. Internal alkenes, such as 2-butene, insert to give an unstable 2° alkyl, that -eliminates to give 1- and 2-butene. The 1-butene can now give a stable 1° alkyl that is the final product. This is particularly noteworthy because the free terminal alkene is less stable than the internal alkene. Tlie outcome arises because the 1° alkyl is thermodynamically more stable than a 2° alkyl for steric reasons. The 1° alkyl, R, can subsequently be functionalized in a number of ways to give a variety of RX derivatives. Hydrozirconation is also effective with less reactive substrates, such as nitriles, where addition of Zr-H across the C=N bond is possible. ... 

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