Zirconocene alkenyl complexes

Casey was able to prepare related zirconocene alkenyl complexes according to Scheme 8.18. Alkene coordination was established by a number of NMR techniques. While zwitterionic compounds 38 allowed the determination of the alkene dissociation energy, AG = 10.5 kcal mol , very similar to that of 35. Thermally more stable complexes were obtained by protonation of 37 with [HNMePh2][B(C5F5)4[. Dynamic NMR spectroscopy and line shape analysis allowed the measurement of the barriers of alkene dissociation (AG = 10.7 and 11.1 kcal mol ), as well as for the site epimerisation ( chain skipping ) at the zirconium center (AG = 14.4 kcal mol" ) (Scheme 8.19) [77]. 

The bis-zirconocene complex CpjClZrCHjCHjZrCpjCl has been isolated upon double hydrozirconation of acetylene with 1 [102]. Recently, the preparation of a heterogeneous bis-zirconocene catalyst was succesfully achieved from zirconocene dichloride complexes containing alkenyl or alkynyl substituents [224]. 

Terminal alkyne complexes of zirconium, while challenging to isolate, have been implicated in a number of organometallic transformations. Mixing an alkenyl zirconocene 218 with a transient dialkyl zirconocene 111 furnishes a yu-acetylide complex 219 (Scheme 33).112 This reaction is believed to proceed by initial transmetallation to form a zirconocene alkenyl alkyl 220, which undergoes subsequent /3-hydrogen abstraction to generate the terminal alkyne complex 221. This proposed intermediate can be trapped with PMe3 222 from the alkylation of the zirconocene alkenyl bromide with butyllithium. Comparison of the spectroscopic features of this product to the... 

Rosenthal U, Ohff A, Michalik M et al (1993) Transformation of the first zirconocene alkyne complex without an additional phosphane ligand into a dinuclear o-alkenyl complex by hydrogen transfer from ti -C5H5 to the alkyne ligand. Angew Chem Int Ed Engl 32 1193-1195... 

The first X-ray structure of a zirconocene(II)—alkene complex, zir-conocene(II)-stilbene stabilized by phosphine, was reported by Takahashi and coworkers [7]. Basically, the formation of zirconocene alkene or alkyne complexes may be achieved in three different ways (a) by a reductive elimination process starting from alkyl, alkenyl or aryl zirconocene(lV) complexes (Eq. 1, and Eqs. 5-6) [1-5,8,9], or (b) by ligand exchange reactions involving carbonyl or phosphine complexes (Eq. 7), or (c) by -C-C bond cleavage of zircona-cyclopentanes or zirconacyclopentenes (Eqs. 8-9) [10,11]. 

Carbon monoxide rapidly inserts into the carbon—zirconium bond of alkyl- and alkenyl-zirconocene chlorides at low temperature with retention of configuration at carbon to give acylzirconocene chlorides 17 (Scheme 3.5). Acylzirconocene chlorides have found utility in synthesis, as described elsewhere in this volume [17]. Lewis acid catalyzed additions to enones, aldehydes, and imines, yielding a-keto allylic alcohols, a-hydroxy ketones, and a-amino ketones, respectively [18], and palladium-catalyzed addition to alkyl/aryl halides and a,[5-ynones [19] are examples. The acyl complex 18 formed by the insertion of carbon monoxide into dialkyl, alkylaryl, or diaryl zirconocenes may rearrange to a r 2-ketone complex 19 either thermally (particularly when R1 = R2 = Ph) or on addition of a Lewis acid [5,20,21]. The rearrangement proceeds through the less stable... 

Cationic r 2-acylzirconocenes [Cp2Zr(COR)(L)+] have been reported to show attractive reactivity [39]. A cationic r 2-acylzirconocene can be generated in quantitative yield by treating a cationic zirconocene complex [Cp2Zr(R)(L)+] (R = alkyl or alkenyl) with carbon monoxide (<23 °C, 1 atm) (Scheme 5.36). The cationic acylzirconocene complexes undergo regiose-... 

Protonation of (/u.-alkenyl)bis(zirconocene) complexes under nonnucleophilic conditions takes a different regio-chemical course than the B(C6p5)3 addition (equation 27). The X-ray crystal structure of (60) is consistent with distorted square-pyramidal pentacoordinated geometry at the central atom Cl. 

Interestingly, high regioselectivity see Regioselectivity) and stereoselectivity see Stereoselectivity) have been observed in the reaction of zirconocene eqnivalents and nnsaturated molecules. Thus, alkyl substituents prefer to be fi to Zr, whereas aryl and alkenyl ones prefer to be a to Zr. In cases where there are two carbon-bonnd substituents, they nsnally prefer to be trans to one another, presumably to minimize steric interactions like in the l-zirconacyclopent-3-yne (101), the first metallocyclopentyne complex. ... 

Czisch, P., Erker, G., Korth, H. G., Sustmann, R. Transfer and coupling of zirconocene-bound alkenyl ligands. An alternative route to (s-trans-T -conjugated diene)ZrCp2 complexes. Organometallics 1984, 3, 945-947. 

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) ... 

Photochemical [2 + 2]-cycloaddition of unbridged bis(alkenyl-Cp)zirconocene complexes provides a unique approach for the synthesis of 6 -hydrocarbyl-bridged -precursor leads to the corresponding meso-ansa-zhconoccne. 

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