Ddtz reaction

Heteroatom-stabilized carbene complexes of type 1, first discovered by E.O. Fischer in 1964 [1], nowadays belong to the best investigated classes of transition metal compounds. Such complexes are coordinatively saturated, intensely colored solids = 350-400 nm), which exhibit a sufficient stability for normal preparative use. Especially chromium carbene complexes (2) enjoy increasing importance in organic synthesis, and it must be added that thermal reactions such as benzannulations (i.e. the Ddtz reaction), cyclopropanations and additions to a,j8-unsatu-rated complexes clearly predominate [2J. 

After their discovery by E. O. Fischer and A. Maasbdl in 1964 [1], a large number of carbene complexes with various transition metals such as Cr, Mo, W, Mn, and Fe were prepared [2]. Their synthetic applications in organometallic and organic chemistry increased rapidly, especially with respect to annelation reactions (Ddtz reaction). Among all known Fischer carbene complexes there is no example featuring a diazo functionality. In this contribution we describe the synthesis of a new class of Fischer-type carbene complexes with a diazomethylsilyl substituent in a-position to the carbene carbon-atom. 

Historically, the proposals for the mechanism of the Ddtz reaction have come about largely on the basis of reactivity studies [7]. The most widely cited mechanisms for the Ddtz reaction are presented in Scheme 5-1. The loss of carbon monoxide has been determined to be the in-... 

Photolysis of metal carbene complexes in the presence of carbon monoxide gives metal complexed ketenes by carbonylation by the Ddtz reaction, as in the formation of the isolable vinylketenes 89a and 89b (Eqn (4.48)). " ... 

The Wulff-Ddtz reaction is another approach to the synthesis of the cyclophane system by formal [2 + 2 + 2] cycloaddition. The intramolecular benzannulation of the Fischer carbene complex, which has an alkyne tethered to the a-position of the alkenyl substituent, gave a para-cyclophane in moderate yield under highly diluted conditions (Scheme 8.6) [6]. 

If the Ddtz benzannulation reaction is conducted with ori/zo-disubstituted aryl-carbene complexes, the final aromatization step is blocked and cyclohexadienones can be isolated (Figure 2.34) [356,378,379]. 

In almost all situations the reactions of Fischer carbene complexes of chromium with alkynes lead to the formation of six-membered ring products, but on several occasions five-membered ring annulated products have been observed as minor products or as major products if the formation of six-membered rings is blocked. In an early report by Ddtz, the reaction of the 2,6-dimethylphenyl complex (209) was observed to react with diphenylacetylene to give the complexed and uncomplexed indenes (210) and... 

The mechanism proposed by Ddtz involves the insertion of a carbon monoxide into the vinyl carbene complex intermediate with the formation of the vinyl ketene complex (255). Electrocyclic ring closure of (255) leads to the cyclohexadienone complex (252), which is related to the final tenzannulation product by a tautomerizadon when R is hydrogen. The mechanism proposed by Casey differs from that of Ddtz in that the order of the steps involving carbon monoxide insertion and cyclization to the aryl or alkenyl substiment is reversed. < Specifically, the vinyl carbene complex intermediate (248) first undergoes cyclization to the metallacyclohexadiene (249), followed by cartion monoxide insertion to give the intermediate (251), and finally reductive elimination to give cyclohexadienone intermediate (252). At this time the circumstantial evidence favors the intermediacy of vinyl ketene intermediates since they can be trapped from these reactions and isolated where the metal is dispaced from the vinyl ketene functionality however, there is not any evidence which can rule out the alternative mechanism. 

Ddtz benzannulation Reaction of Fischer chromium carbenes with aikynes to give substituted hydroquinone derivatives. 148... 

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