Esters, alkynic hydrozirconation

After hydrozirconation with Cp2ZrEt2 and reaction with an chloroformic ester, alkynes directly (requiring no catalyst) afford conjugated esters. ... 

True to its nature as an early transition metal, zirconium displays considerable Lewis acidity and binds to hard Lewis bases such as carbonyl groups, thus facilitating hydride transfer and reduction. In general, amides, ketones, aldehydes, and nitriles are not compatible with alkene and alkyne hydrozirconation conditions. Alkynes are selectively hydrozirconated in the presence of esters, but only stericaUy hindered triisopropylsilylesters survive the slower hydrozirconation of alkenes. Acylsilanes are also readUy tolerated in hydrozirconation, and carbamates, acetals, epoxides, sUylethers, alkyl- and phenylethers. 

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]. 

The synthetic potential of alkenylzirconium complexes is partially due to the fact that the hydrozirconation of alkynes can be carried out in the presence of some synthetically useful functional groups such as halide [80,153, 211, 212], acetals, amides, imides, carbamates, sulfides [186], ester, cyano [95, 213] and chiral propargyl amino functionalities [214]. 

Carboxylic acids can be protected as oxazolines [96, 105-107, 186, 191] or as ester functions. Alkynic esters such as silyl esters [153, 211], tert-butyl esters [216], and even benzyl esters [153, 211] have been successfully hydrozirconated when the reactive site was a terminal alkyne or vinyl group (Scheme 8-27). 

Alkynes are universally hydrozirconated as well the only failure in the literature is that of a perfluori-nated compound, C7Fi5(2sCH. Alkynes appear to be more reactive than alkenes, both by qualitative comparison and from the results on enynes (Table 6 also see Section 3.9.3.3.1), Furthermore, hydrozirconation of alkynes can compete with reduction of unsaturated functional groups such as nitriles and esters (Table 5), which is generally not true for alkenes. Dienes can be cleanly monohydrozirconated if one of the double bonds is terminal other cases are considered in Section 3.9.3.3.I. 

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. ... 

Hydrozirconation of alkenes as a route to rr-alkylzirconiums is limited by other reactive functional groups e.g., alkynes react more readily than alkenes with (/j -Cp)2ZrHCl, and other reactive functional groups with ( / -Cp)2ZrHCl include alcohols, aldehydes, ketones, carboxylic acids, carboxylic acid esters, nitriles and thiocarbonyls . Alcohols may be protected either as alkyl ethers or as trialkylsilyl ethers. 

Instead of the commercially available CpZrHCl, this reagent can be prepared in situ from Cp2ZrCl2 and lithium triethylborohydride. The reaction mixture hydrozirconates terminal acetylenes without affecting e.g. an ester or trimethylsilyl functionality in the alkyne. An application of the Schwartz reagent in natural product synthesis is the hydrozirconation of the terminal triple bond of Me3SiC=C(CH2)4C=CH. A review on hydrozirconation has been published . ... 

Organotitanium and -zirconium chemistry already has an established place in organic synthesis and many reactions are covered elsewhere in the Practical Approach series. Examples include reductive coupling of carbonyl compounds with low valent titanium to form 1,2-diols or alkenes methylenation of ester carbonyl groups with titanocene methylidene (Cp2Ti=CH2) zirconium-catalysed methylalumination of alkynes and hydrozirconation of alkynes and alkenes with the Schwartz reagent, Cp2ZrHCl. ... 

From this preliminary discussion, the hydrozirconation reaction of alkynes is a valuable process but the scope of functional groups present in the carbon skeleton is rather limited to specific esters. In order to circumvent the limitations, acylsi-lanes were designed as a synthetic equivalent to carbonyl groups. Treatment of 6 with Cp2Zr(H)Cl afforded a 80% yield of the desired olefin 7 with no detectable amount of the carbonyl 1,2-adduct (Scheme 12.9) [13]. 

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