Oxidation hydrozirconation

The addition of any one of several dialkyl chlorophosphates to an arylalkyne-derived vinyl zirconocene in the presence of catalytic amounts of CuBr in THF leads to the corresponding vinyl phosphonate in high yields (78—92% see, for example, Scheme 4.38) [25]. Here, alkyl-substituted acetylenic starting materials do not react beyond the initial hydrozirconation stage. Vinyl phosphonates may be readily converted to acyloins by oxidation to the diol followed by base-induced cleavage. 

Treatment of the optically active gem-borazirconocene alkanes with deuterium oxide followed by alkaline oxidation affords the corresponding optically active 1-deuterio primary alcohols. The enantiomeric excess of the resulting primary alcohols represents the diaster-eoselectivity of the asymmetric hydrozirconation (Scheme 7.13). Based on the cost and availability of optically active ligands, three types were explored monoterpenes, 1,2-diols, and 1,2-amino alcohols. Hydrozirconation of optically pure 1-alkenyl boranes 39 provided optically active 1,1-bimetallics 40. 

Enantioselective, multiple centers 153, 166,189 Hydroamination Intermolecular Alkene 30 Alkyne 1 Intramolecular Alkene 30 Alkyne 13,170 Hydrogen peroxide Oxidation of alcohols 26, 86 Hydrogenolysis of epoxide 1 Hydrozirconation 32... 

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). ... 

With cyclic alkenes, fairly high diastereoselectivity can be achieved. Hydrozirconation of norbomene, followed by TBHP oxidation, gives the exo-alcohol in 95% yield, while the more complex bicyclics camphene (37) and P-pinene (38) give the mixtures shown in equations (39) and (40), respectively. TTie hafnium analog shows the opposite preference with P-pinene and does not react with camphene at all, attributed to significantly greater steric demand. Diastereoselectivity in cyclohexanols obtained by hydrozirconation of ketones has also been examined. ... 

Protonolysis can also be effected with weaker acids in some cases this may be essential. For example, hydrozirconation of alkynic alcohols Me(CH2)nC=C(CH2)mOH with 2 equiv. of (6), followed by hydrolysis with 2% aqueous NH4CI, affords the corresponding cis-alkenol in high yield and selectivity, whereas use of aqueous HCl causes both cis-trans isomerization and allylic rearrangement. Protonolysis by weak acid may be slow enough for other reactions to compete TBHP can be used as an oxidant (see below). 

Alkenyl and dienyl groups can be present in Ti and Zr complexes as well. Vinyltitanium compounds cannot be exploited for nucleophilic additions, however, due to their propensity to undergo oxidative coupling reactions,likewise transition metal derivatives. Vinylzirconium compounds (20) can be prepared from alkynes (hydrozirconation equation 12) by action of zirconium derivatives like (19). ... 

Here, olfeyl-substituted acetylenic starting materials do not react beyond the initial hydrozirconation stage. Vinyl phosphonates may be readily converted to acyloins by oxidation to the diol followed by base-induced cleavage. 

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