Reductive coupling zirconocene

Fig. 1 Proposed bimolecular zirconocene complex leading to reductive coupling...

Waymouth and coworkers used chiral zirconocene complexes such as 56 with Et3Al as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicyclic compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by (i-alkoxidc ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12]. 

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... 

In the analogous reaction of differently substituted azines RR C=NN=CRR , the products depend strongly on the metal used (Ti and Zr) as well as on the substituents R and R [43], With R = R = Me and M = Ti, substitution of the alkyne by the azine and subsequent CH activation of the complex 78 is observed. With R = Ph and R = H, the acetylene is also substituted and, through a reductive coupling of two azine molecules, the binudear Ti(III) complex 79 is formed. Using the zirconocene 2a, and with azine substituents R = Ph and R = H, no substitution of the alkyne is observed, but one of the C=N double bonds of the azine inserts into the Zr—C bond of the starting complex to yield complex 80. 

The chemistry of 306 differs from that of its seven-membered analogue in that its reaction with rram-stilbene gives reductive coupling product 308 in addition to the stilbene adduct of zirconocene, the sole product of the latter (vide infra). This was attributed to differences in ring strain.116... 

Zirconaaziridines prepared through the addition of imine to zirconocene are generally not of high purity and cannot be isolated in high yield reductive coupling of imines often competes. However, this method is preferred for preparing zirconaaziridines in situ for reaction with electrophiles that can be... 

For the preparation of 5-membered zirconacycles, although it is possible to use di-Grignard or dilithium reagents for the preparation of zirconacyclopentanes [ 10], the reductive coupling of unsaturated organic substrates, either intramolecular or intermolecular, on a zirconocene(II) species is the most common way. 

Phospholes and other related heterocycles are an important class of main group compounds. The chemistry of phospholes and their preparation has been reviewed extensively by Mathey.3 We provide details here for a simple, one-pot procedure for the preparation of 1-phenyl-2,3,4,5-tetramethylphosphole applying zirconocene chemistry.4 The procedure involves reduction of (n-CsHsteZrCIa with butyllithium in the presence of 2-butyne which (as reported initially by Negishi, et al.s) forms a zirconium metallacycle. Addition of dichlorophenylphosphine to this reaction mixture produces the phosphole. One other procedure for the preparation of 1-phenyl-2,3,4,5-tetramethylphosphole has been reported by Nief, et al.2 That procedure involved aluminum chloride - coupling of 2-butyne, followed by reaction with dichlorophenylphosphine to form a chlorophospholium tetrachloroaluminate which was then reduced with tributylphosphine to produce the phosphole in 68% yield. 

Zirconocene-aryne complexes can also be transmetallated, opening the way to a broad spectrum of synthetic operations such as oxidation, halogenation and cross-coupling (Scheme 5). For example, transmetallation of zirconocene-aryne complex 26 with the palladium complex generated from Pd(0) and m-bromo(trifluoromethyl)benzene regioselectively affords palladium(ll) complex 27, which after reductive elimination can be iodinated to 28 [22]. Alternatively, treatment of 26 with B(OEt)3 results in the formation of 29, and its iodination and oxidation leads to 30 [23]. 

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