Cyclobutadiene complexes from acetylenes

Cyclobutadiene-metal complexes have also been suggested as intermediates in a number of other reactions, notably in the formation of benzenes by trimerization of acetylenes 2, 18, 57, 90) and also in other reactions. Conclusive evidence is still lacking, but the inertness of the known cyclobutadiene complexes towards acetylenes in Diels-Alder type reactions makes this rather unlikely. Intermediates with open-chain structures such as (XCVI) appear more attractive. Arnett and Bollinger (2) have isolated by-products from the dicobalt-octacarbonyl-catalyzed trimerization of diisopropylacetylene which are very similar to some, e.g., (LI), obtained in the thermal decomposition of tetramethylcyclobutadienenickel chloride complexes (29). Again, here, however, the evidence is by no means conclusive and a variety of intermediates other than a cyclobutadiene-metal complex can be postulated to explain the observed products however, see also the Appendix. 

Attention should be drawn to the remarkable structures of many of the compounds formed from acetylenes during complex formation. Acetylenes by self condensation can give complexes of the elusive cyclobutadiene... 

Instances in which cyclobutadiene complexes are the major products from the reactions of acetylene complexes with additional alkyne are uncommon. These generally have been found to be significant products with sterically hindered alkynes and with palladium and platinum metals. For example, phenyl tert-butyl acetylene was converted to the corresponding cyclobutadiene complex (one isomer) upon treatment with (PhCN)2 PdCl2 (Hos-okawa and Moritani, 1969) [Eq. (71)]. With sterically less demanding tolane,... 

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

As was mentioned in Section III,A a very useful method of synthesis of olefin complexes involves the displacement of carbon monoxide from metal carbonyls by olefins. Under similar conditions acetylenes usually react to give new ligands, e.g., cyclobutadienes, cyclopentadienones, and quinones (see Sections V,E and VI,C), and it is not surprising, therefore, that the range of known acetylene complexes is smaller than the range of olefin complexes. 

Cyclobutadiene-metal complexes were obtained from reactions of the corresponding acetylenes with metal complexes i9U,2i) Orbital symmetry principles would suggest that these complexes are either formed via stepwise processes or involved the intervention of bimetallic species (32). The stepwise routes are particularly attractive. Acetylene ligands can reasonably be expected to undergo a [( 2s 2 -f-ji2s] cycloaddition with the metal center generating the metalocyclodiene intermediate 33. Cyclobutadiene can then be extruded from the metal center with the aid of another metal. 

Other preparative methods make use of cyclobutadiene intermediates which react with acetylene compounds. The first method was reported by Criegee and his coworker 6) who prepared tetramethylcyclobutadiene from the 1,2-diiodo derivative (4). In the second method, the cyclobutadiene irontricarbonyl complex is oxidized by Ce,v to produce free cyclobutadiene (5) 7). This reaction is widely used for the synthesis of cyclobutadiene. 

It is well known that palladium chloride is an active catalyst for the cyclization of acetylene to form cyclobutadiene as well as benzene derivatives. In this reaction an intermediate complex was isolated which has a palladium carbon a-bond, the formation of which was explained by an insertion mechanism, not by concerted cyclotrimerization. When this complex obtained from butyne and palladium chloride was decomposed by various means, 5-vinyl-l,2,3,4,5-penta-methylcyclopentadiene and 5-(l-chlorovinyl)-l,2,3,4,5-pentamethylcyclopenta-diene were obtained in addition to hexamethylbenzene... 

Liberation of excess cyclobutadiene from its iron complex in the presence of acetylenes leads to a double addition, affording the previously unknown tetracyclic system (831 R = Ph or OMe). Trapping with phenyl vinyl ketone affords (832X nhich has been converted by reduction, alkylation, and photolysis into the new tri-cyclo[3,1,1,0 ]heptanols (833). ... 

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