Zirconocene complexes carbonylation

Enones and enoates undergo 1,2-reduction [115, 191]. Lipshutz et al. reported the effective protection of carbonyl functions by the triisopropylsilyl acyl silane group (TIPS), which allowed the selective conversion of alkenes or alkynes to the corresponding zirconocene complexes [24]. The aldehyde could subsequently be regenerated by desilylation with TBAF [186]. 

Clear formation of ketene—zirconocene complexes upon treatment of acylzirconocene chlorides with a hindered amide base indicates that the carbonyl group of the acylzirconocene chloride possesses usual carbonyl polarization (Scheme 5.10). However, these zirconocene—ketene complexes are exceptionally inert due to the formation of strongly bound dimers [13a], Conversion of the dimer to zirconocene—ketene—alkylaluminum complexes by treating with alkylaluminum and reaction with excess acetylene in toluene at 25 °C has been reported to give a cyclic enolate in quantitative yield. Although the ketene—zirconocene—alkylaluminum complex reacts cleanly with acetylene, it does not react with ethylene or substituted acetylenes [13b]. Thus, the complex has met with limited success as a reagent in organic synthesis. 

Butenolides,1 Addition of the Schwartz reagent to a protected propargylic alcohol (2) followed by carbonylation provides an acyl zirconocene complex (3). This is not isolated but treated in situ with iodine to provide an intermediate (a) that cyclizes to a 3,5-disubstituted butenolide (4). Optically active substrates undergo this sequence with no loss of optical purity. 

By the nature of its molecular mechanism, the carbonyl-insertion reaction represents a typical reaction mode of o alkyltransition metal complexes. Formation of the new C—C cr-bond takes place during a 1,2-alkyl-migration step, transforming an alkylmetal carbonyl moiety [cts-M(CO)R] into an acylmetal unit (M—COR) (89). In general, (s-cir-diene)-zirconocene complexes 5 appear to exhibit a substantial alkylmetal character (90). Therefore, it is not too surprising that some members of this class of compounds [in contrast to most other dienetransition metal complexes (97)] react with carbon monoxide with C—C bond formation (45). However, as demonstrated by X-ray structural data for 5 (Tables V... 

Reaction of (5-ci,s-Conjugated diene)zirconocene Complexes with Carbon Monoxide—Relative Carbonylation Rates and Product Yields... 

For these complexes, the rate of carbonylation increases rapidly with increasing tr-alkyl character. Thus, (5-cis-butadiene)ZrCp2 (5a) is carbony-lated 2.5 times faster than the zirconocene complex of 1,2,5,6-tetramethyl-3,4-bis(methylene)tricyclo[3.1.0.0 ] hexane (51) (ambient temperature and 1 bar CO pressure) taken as a standard. Introduction of methyl groups at the internal carbon centers C2/C3 of the diene chain increases the carbonylation rate by a factor of 5 with phenyl groups at these positions increasing the rate by a factor of 250 (Table VIII). The organometallic reaction products have yet to be isolated or completely identified. Ultimately, zirconium enolate complexes (20) are probably formed in these... 

Zirconocene complexes (with s-cis geometry) of isoprene, 2,3-dimethylbutadiene, and 3-methyI-I,3-pentadiene are reported to give exclusively 1 1 addition with carbonyl substrates even if these are used in excess and the reaction temperature is fairly high (ca. 100 C). On the contrary, zirconocene complexes of 5-ci5-butadiene, 1,3-pentadiene, and 2,4-hexadiene ca, 1 1 mixture of the s-cis and s-trans isomers) easily accept (equation 53) 2 equiv. of either butanal or 3-pentanone, at low temperature ca. 30 C) in high yields (95%). This can be exploited for the stepwise insertion of two different electrophiles... 

Similar reactions were conducted with 16a and various aldehydes [28]. It can be emphasized that the [3+2] cycloaddition reactions take place selectively on the carbonyl group allowing therefore the preparation of diversely functionalized zwitterionic zirconocene complexes such as compounds 21 a-e possessing either a free phosphino group (21 b) or free alkenyl group (21 c- e) (Eq. 4) ... 

The carbonylation reactions of zirconocene complexes of type ZrR2Cp2 (R = alkyl, aryl, and H) are discussed in a review and some relevant material is incorporated into reviews on Ti for the literature years 1981 and 1982. ... 

In a somewhat different area of organozirconium chemistry, Erker s group has recently reported the formation of Cp2Zr(CO)2 (2) in very good yield via the carbonylation of the zirconocene diene complexes 26a and 26b (57). In cases where the diene was structurally less complex (e.g.,... 

In 1982, a most remarkable reaction was discovered by Bercaw et al. which involved the reductive coupling of the terminal carbonyl ligands of [CpFe(CO)2]2- When 1 equivalent of the permethylated zirconocene dinitrogen complex [(rj-CsMe Zr k (29) was treated with 2 equivalents of [CpFe(CO)2]2 in toluene above -20°C, 3 equivalents of N2 evolved... 

A monocarbonyl complex of zirconocene has been proposed by Floriani and co-workers as a transient intermediate in the carbonylation of the bridging oxymethylene complex (Cp2ZrCl)2(Ai-CH20) (57) (104,105). The assignment of 58 as the proposed intermediate to the final carbonylation product, [Cp2Zr(/i-CHO=CHO)]2 (59), was based on the appearance of a metal carbonyl band at 1970 cm-1 together with the subsequent CO-induced loss of Cp2ZrCl2. 

A wide variety of five-membered zirconacydes 8 may be formed by the formal co-cycliza-tion of two 7i-components (3 and 6 alkene, alkyne, allene, imine, carbonyl, nitrile) on zir-conocene ( Cp2Zr ) (Scheme 3.2) [2,3,8]. The co-cydization takes place via the r 2-complex 5 of one of the components, which is usually formed by complexation of 3 with a zircono-cene equivalent (path a) ( Cp2Zr itself is probably too unstable to be a true intermediate) or by oxidation on the metal (cyclometallation/p-hydrogen elimination) (path b). Two additional routes to zirconocene r 2-complexes are by the reverse of the co-cyclization reaction (i. e. 8 reverting to 5 or 9 via 7), and by rearrangement of iminoacyl complexes (see Section... 

Treatment of a stannylenyne/enene compound with a low-valent zirconocene derivative results in a bicydization reaction leading to the 1,1-stanniozirconocene complex 99 (Scheme 7.29) in moderate yield [168], The product of carbonylation of 99 can be readily... 

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