Cyclobutadienes reactions with alkynes

In support of this mechanism is the fact that stable cobaltacyclopentadienes 11.23, with an additional PPhs ligand can be prepared (Scheme 11.12). These cobaltacycles undergo further reactions with alkynes, alkenes, azides and diazo compounds to give a variety of cyclic products. Cyclobutadiene complexes 11.26, cyclopentadienone complexes 11.27 and arenes 11.28 can also be formed (Scheme 11.13). ... 

NMR, 3, 542 oxidation, 3, 546 phosphorescence, 3, 543 photoelectron spectra, 3, 542 photolysis, 3, 549 reactions, 3, 543-555 with alkenes, 3, 50 with alkynes, 3, 50 with IH-azepines, 3, 552 with azirines, 3, 554 with cyclobutadiene, 3, 551 with cyclopropenes, 3, 550 with dimethylbicyclopropenyl, 3, 551 with heterocyclic transition metal complexes, 7, 28 29... 

The reaction of alkynes with AIX3 at —78 °C has been shown, by NMR spectroscopy, to generate a zwitterionic cr-cyclobutadiene aluminum species 225 (Scheme 58)211a. Transfer of the cyclobutadiene ligand from 225 to a variety of transition metals has been reported211. 

While one of the first preparations of a cyclobutadiene-metal complex involved the cyclodimerization of diphenylacetylene in the presence of Fe(CO)5 at high temperature212, the thermal reaction of alkynes with Fe(CO)s gives predominantly cyclopentadienone complexes (Section IV.E.l.b). The cyclization of alkynes by a wide variety of metal complexes has been reported (Scheme 59)l 5-21 A—222... 

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

The reaction of alkynes substituted with bulky groups on both carbons with Pd complexes leads to complexes of substituted cyclobutadienes. Treatment of PdCl2(PhCN)2 with t-BuC2Ph gives a cyclobutadiene complex (equation 45). A very interesting case of dimerization to a cyclobutadiene complex is provided by the strained seven-membered ring cyclic aUcyne (equation 46). 

Supporting evidence for the above mechanistic patterns in the majority of metal systems was first established in elegant isotopic labeling studies, which showed clearly that no intermediate with the symmetry of a cyclobutadiene was involved.In one system, the reaction of a cobaltacyclopentadiene with MeC>2CC CC02Me, benzene formation does not involve direct complexation of the third alkyne to the metal. It has therefore been suggested that the conventional insertion process has been here replaced by a direct Diels-Alder reaction with the metallacycle, perhaps as a result of electronic factors (Scheme 25). o ... 

G. M. Whitesides and W. J. Ehmann, 7. Am. Chem. Soc., 1969, 91, 3800 see also G. A. Ville, K. P. C. Vollhardt and M. J. Winter, Organometallics, 1984, 3, 1177 for further studies along these lines, and J. R. Strickler, P. A. Wexler and D. E. Wigley, Organometallics, 1988, 7, 2067 for studies of relevant model systems associated with alkyne trimerizations catalyzed by early transition metals. Note that although cyclobutadiene-like structures are not involved in most alkyne trimerization mechanisms, metal-complexed cyclobutadienes are fairly common side products in alkyne trimerization reactions. As their yields are usually low, their formation does not present practical problems in arene synthesis. 

In its reactions, coordinated cyclobutadiene exhibits aromatic character, undergoing electrophilic substitution, e.g. Friedel-Crafts acylation. A S5mthetic apphcation of (r " -C4H4)Fe(CO)3 in organic chemistry is as a stable source of cyclobutadiene oxidation releases the ligand making it available for reaction with, for example, alkynes as in scheme 23.117. 

Scheme 4-25 summarizes the most common metal-promoted /nfe/molecular cyclocoupling reactions of various species with alkynes. The most prominent organic products include arenes, cyclooctatetraenes, cyclohexadienes (with olefins), pyridines (with nitriles), cyclopen-tenones (with olefin ( CO the Pauson-Khand reaction [98]), pyrones (with CO2), and five-membered heterocycles (with X = S, Se) common organometallic products include cyclobutadiene complexes, cyclopentadienone complexes, and metallacyclopentadienes. 

Compounds 33 and 34 are readily formed from 31 by direct reaction with CpCo(CO)2. A possible reaction sequence is formation of the triene, 31, from alkyne dimerization followed by reaction with the cobalt species to give the three complexes. Reaction of 34 with alkynes yielded only cyclobutadiene complexes alkyne metathesis was not observed, probably since the carbon-to-metal bonds are too strong. 

The reactions of alkynes with transition metal complexes often lead to bewildering mixtures of products. Substituted alkynes often produce rj -cyclobutadiene derivatives (p. 269) metal carbonyls also afford products containing cyclopentadienone and quinone ligands, which incorporate two alkyne units together with one or two carbonyl groups. Some of these systems catalyse the cyclotri- or tetra-merization of alkynes to benzenes or cyclooctatetraenes respectively (Fig. 7.12). 

Alkenes and alkynes undergo cycloaddition reactions with cyclobutadiene produced via decomposition of [Fe(CO)s(cyclobutadiene)]. Further studies on the oxidative decomposition of the optically active complexes (17) and (18) in the presence of dienophiles show that racemic adducts are formed. It would therefore appear that during the addition of the dienophile the iron is not sufficiently close to the cyclobutadiene to maintain the original chiral surroundings of the complex. ... 

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