Dienols hydrozirconation

Dienols with 2 equiv. of 1 gave the corresponding bimetallic alkoxide organozir-conocene complexes however, protolysis allowed recovery of the alcohol functionality (Scheme 8-29) [107]. Alcohols can also be easily converted to ethers. Alkyl, aryl, silyl [85,112,183, 210] and THP [17, 153, 211, 219] ethers are stable under hydrozirconation conditions side products were observed only with the trimethylsUyl group [220, 221]. 

Acetylenic ethers 7 can be hydrozirconated, and subsequent iododezirconation leads to (fc)-iodo enol ethers 8 (Scheme 4.4) [18], These species undergo efficient Sonogashira couplings to give (E)-enynes, which are ultimately converted to stereodefined dienol ethers. These dienes have proven useful in studies of diastereoselective cycloaddition reactions with singlet oxygen, where R in 8 is a nonracemic auxiliary (e. g., menthyl) (Procedure 3, p. 140). 

Tertiary alkylzirconium compounds appear to be generally inaccessible even as intermediates. Tetra-substituted alkenes are not hydrozirconated at all also, zirconium is unable to migrate past a tertiary position in a carbon chain. Attempted hydrozirconation of an internal alkene which is blocked at both ends (20 equation 26) gives only isomerized alkene (21). The only exception, which appears to involve a tertiary intermediate, is found in the hydrozirconation of dienol (22) in equation (27). ... 

Definitive evidence for the role of heteroatom coordination may be seen in the hydrozirconation of al-kenoxide (29 equation 33) where the five-membered ring favors (30) (confirmed by the X-ray crystal structure). A related structure was obtained in equation (34) the 0-coordinated product (31) does not isomerize to the terminal alkylzirconium product (16) below 140 "C. (At 140 C it slowly decomposes to 17, presumably via 16 cf. equation 22 above.) In contrast, hydrozirconation of dienols shows preferred reactivity of the double bond remote from the —OH group an example is shown in Table 3. Probably the first equivalent of (6) reacts with the —OH group both to increase steric bulk and prevent... 

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