Cyclobutenyl complexes

The reaction of NiCl2, LiCp, and allylmagnesium chloride affords the 7c-allyl complex , Ni(Cp-f X / C3H5). The Pd and Pt analogs are obtained from [M(t) -Cp-> )C1]2 and Na[Cp] several ring-substituted analogs " are also listed in Table 9. Analogous 7t-cyclohexenyl and -cyclobutenyl complexes have also been reported the latter is derived from nucleophilic attack by [Cp] on a coordinated... 

The [2 + 2] cycloaddition of the propynyl tungsten complex (12b) has been found to be stereospecific with the cis and trans isomers of eAyl prc nyl ether." ITie reaction with c/s-propenyl ethyl ether gave rise to the cyclobutenyl complex (18), in addition to a minor amount of the ring-opened dienyl carbene complex (19). The stereochemistry about the double bonds in the dienyl complex (19) revealed that it was derived from a thermal electrocyclic conrotatory ring opening of the c/s-cyclobutenyl carbene complex (18) and thus the stereochemistry is completely retained in this cycloadditirxi. In support of this analysis it was found that cyclobutenyl complex (18) could be thermally opened to the dienyl complex by heating in an inert solvent at 50 C. In the cycloaddition of the propynyl complex (12) with trans-... 

Subsequent reaction with a nucleophile affords a metal-vinylidene complex. This subject has been reviewed by Bruce. Reactions with electrophilic alkenes initially lead to a cyclobutenyl complex in a two-step process via a paramagnetic intermediate. Subsequently, the ring opens in a concerted fashion to a butadiene derivative. 

Tetraphenylcyclobutadienepalladium and -nickel complexes and tetra-methylcyclobutadienenickel chloride react readily with nucleophilic reagents to give 7r-cyclobutenyl complexes 12, 30, 31, 65, 91), a reaction reminiscent of those described by Chatt et al. for diene-palladium and diene-platinum halide complexes (Section VI, F). Non-halogen-containing cyclobutadiene complexes, however, appear inert under similar conditions so that this reaction is very dependent on the other ligands present. Some similarity between cyclobutadiene-metal and diene-metal complexes appears to exist but how far the parallel can be drawn remains to be seen. The reactions are fully discussed in Section VI. 

Analogous reversible reactions occur with tetraphenylcyclobutadiene-palladium halides (12,69, 70,91) to form cyclobutenyl complexes isomeric... 

Nucleophilic attack. The complex [Ni2Cl4(C4Me4)2] reacts with sodium cyclopentadienide to give a cyclobutenyl complex. Other derivatives also react in the same manner. 

Maiths and co-workers (1620,1621) reported the synthesis of a number of novel palladium(II) organometallic complexes formed from ring-opening reactions of pentaaryl cyclobutenyl complexes (Fig. 212). Thus, addition of... 

In the synthesis of alkylidenecyclobutenes from propargyl alcohols, stoichiometric experiments show that the first step involves [2+2] oxidative head-to-head coupling of the alkynes, leading to an isolable cyclobutadiene-ruthenium complex. Addition of acid generates a cyclobutenyl metal intermediate which undergoes carboxylate addition on the less substituted allylic carbon atom (Scheme 7). 

The topic of 7r-allyl-transition metal chemistry is considerably broader than complexes involving the tt-CsHs ligand itself. A variety of complexes is now known in which the 7r-allyl radical is part of a carbocyclic ring system. Some examples of 7r-cyclopropenyl-, 7r-cyclobutenyl-, and tt-cyclopentenyl- transition metal complexes are presented below. 

The cyclobutadienepalladium complexes (XXXVI) react with alcohols, usually in the presence of weak bases, to give cyclobutenylpalladium complexes (LXXXIV) isomeric with (XXXV) (see Section VI, F) these reactions are reversed on treatment of (LXXXIV) with hydrogen halides and constitute another preparation of the cyclobutadienepalladium complexes. A related reaction on (w-cyclopentadienyl)(7r-alkoxytetraphenyl-cyclobutenyl)palladium and -nickel complexes (XLIV) (64, 65) is also discussed in Section VI, F. The mechanism of this reaction is not established, but the initial step is probably protonation of the ether. 

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