Zircona-cyclopentadiene

Eq. 2.8. Regioselective formation of a zircona-cyclopentadiene using a trimethylsilyl-substituted acetylene. 

Eq. 2.16. Selective formation of the diiodination product from a zircona-cyclopentadiene using CuCl. 

Eq. 2.19. Heterocycle formation from zircona-cyclopentadienes by heteroatom transfer. 

Examples of a rather classical cyclopropanation reaction involving Sim-mons-Smith-type reagents or dichlorocarbene and a double bond of zircona-cyclopentadienes were reported by Takahashi and coworkers (Scheme 27) [38]. 

Takahashi and Li (Chapter 2) focus on the preparation and reactions of zircona-cyclopentadienes which, for a quarter of a century since their discovery in 1974 by... 

Through a net [2 + 2+1] cycloaddition, iminocyclopentadi-enes can be obtained through the initial formation of a zircona-cyclopentadiene derivative (from two alkynes and a low valent zirconocene component), and subsequent reaction with an aryl isocyanate and BF3-Et20 (eq 65). Quite remarkably, no other Lewis acid investigated provided this cycloaddition only EtAlCL led to a productive reaction, although different adducts were obtained. 

Synthesis of Substituted Benzenes The coupling reaction of zircona-cyclopentadiene with halogenated arenes provides a direct method of aromatic ring extension. As shown in Scheme 11.25, the reaction of zirconacyclopentadienes 4 with diiodobenzene in the presence of CuCl and DMPU afforded 1,2,3,4-tetra-substituted naphthalenes 60 in good yields. When tetraiodobenzene was used under similar conditions, 1,2,3,4,5,6,7,8-octa-substituted anthracene derivatives 61 could also be prepared [28]. 

Lithiated chloromethyltrimethylsilane was reported to insert into zircona-cyclopentadienes to give zirconacyclohexadiene derivatives (Eq. 25) [27]. This has been so far the only example of carbenoid insertion into 5-membered met-allacyclopentadienes. 

Highly substituted pyran derivatives were synthesized with high regiose-lectivity from the reaction of zirconacycloipentadienes with diethyl ketoma-lonate in the presence of two equivalents of BiCl3 (Eq. 71) [80]. When zircona-cyclopentadienes were treated with azodicarboxylates in the presence of CuCl, dihydropyridazine derivatives could be prepared (Eq. 71) [80]. 

The coupling reaction of zirconocene alkyne complexes with a second alkyne provides a general method for the preparation of asymmetrically substituted zircona-cyclopentadienes. The overall transformation is the chemoselective and regioselec-tive intermolecular cross-coupling reaction of two alkynes (Scheme 12.33) [44]. 

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