Hydrozirconation of an alkyne

Cross-coupling ofalkynes. A route to isomerically pure 1,3-dienes involves hydrozirconation of an alkyne followed by conversion to the alkyne complex 3, which couples with a second alkyne to give a metallacyclopentadiene 4. Treatment of 4 with dilute acid provides the diene 5, usually as a single isomer (>97%). Overall yields are in the range 53-80%. 

Of course, these findings do not automatically mean that such functional groups cannot be tolerated that depends upon relative rates. For instance, hydrozirconation of nitriles can be relatively slow complete reaction of RCN under standard conditions required 72 h, 14 h and 1.5 h, respectively for R = Me, Ph, Bn, Consistent with previous indications that alkynes are more reactive than alkenes towards (6), hydrozirconation of an alkyne can be carried out in the presence of a nitrile group (see Table 5), whereas methacrylonitrile gives only C N hydrozirconation (equation 20). ... 

Vinyl cuprates.3 An attractive route to vinyl cuprates involves transmetalla-tion of vinylzirconates, available by hydrozirconation of 1-alkynes with the Schwartz reagent in THF at 25°. Transmetallation can be effected with CH3Li (3 equiv.)... 

Similarly, alkynic esters have been successfully hydrozirconated (Table 5). HCsC(CH2)3C02Bu gave the alkenylzirconium product in much higher yield than the corresponding methyl ester, which exhibited carboxy reduction. Clearly the reactivities of the C C and ester groups are in close balance here, with steric factors sufficient to tip it towards the former. Hydrozirconation of an alkenic ester would not be expected to work well no example is reported. In the reaction of an enone with (6 equation 21) little if any hydrozirconation of the C =C bond occurs. ... 

The total synthesis of apoptolidin was accomplished in the laboratory of K.C. Nicolaou. The key C12-C28 vinyl iodide fragment was prepared using the Schwartz hydrozirconation of an internal alkyne followed by trapping of the alkenylzirconium intermediate with iodine (I2). The vinyl iodide was formed as a 6 1 mixture of regioisomers. Under the reaction conditions, the methyl orthoester was converted to the methyl glycoside moiety at C21, which was presumably facilitated by the complexation of Zr with the pyranoside oxygen atom. 

An access to chiral allylic alcohols starts from hydrozirconation of 1-alkynes which is... 

The most common routes to alkenyltin compounds involve reaction between an alkenyl-metallic compound and a tin halide, or the hydrostannation of an alkyne (see Section 4.4). In recent years, the stannylmetallation of alkynes has become increasingly important. Direct hydrogenation of an alkynylstannane to a vinylstannane does not appear to be practicable, presumably because the catalyst is poisoned by the tin compound, but cis addition of dihydrogen can be achieved by hydrozirconation followed by hydrolysis,1 or in some cases, by cyclotitanation followed by hydrolysis2 (Section 8.2.2 below). 

Hydrozirconation of functionalized alkyne 8 in CH2C32 with the Schwartz reagent followed by in situ transmetallation to Me2Zn and addition of N-diphenyl-phosphinoylimine 15 provides the functionalized trans-aminocydopropane 16 in 60% yield [15]. The cydopropane formation can be rationalized by the formation of a transient zinc-carbenoid species from CH2CI2, which subjected the intermediate allylic amine derivative to an efficient Simmons-Smith-type cydopropanation. 

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]. 

Similarly, alkenylzirconium species prepared by the hydrozirconation of alkynes add in a conjugated fashion to enones. Formation of an intermediate zincate prior to transmetalation to the copper species facilitates the Michael addition (Scheme 2.62) [135]. This methodology has been applied to the preparation of protected misoprostol 129 (Scheme 2.63) [136, 137]. 

The kinetic product distribution can be further shifted in excess reagent to get organozirconium derivatives with high regioselectivity. Even subtle steric differences such as in 2-pentyne ensure high selectivity [Eq. (6.77), a]. As Eq. (6.77) demonstrates, hydrozirconation of alkynes is an exclusive syn process. 

Hydrozirconation of terminal triple bonds is an essential method to obtain alkenes with defined stereochemistry. In the case of internal alkynes the zirconocene moiety adds to the sterically less hindered position of the triple bond. C/s-selectivity is high, but the regioselectivity is sometimes moderate depending on the nature of the substrate.8... 

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