Pauson-Khand reactions diene-alkene

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

The idea that alkenes possessing electron-withdrawing groups are not adequate substrates for Pauson-Khand reactions has in recent years turned out not to be precise. Carretero has reported several examples involving electron-deficient alkenes 3), such as os/Tunsaturalcd ketones, esters, nitriles, sulfoxides and sulfones. In these reactions they reach good yields of PK products (4) and isolate small amounts of dienes 5, that come from a /J-climinalion competitive reaction (Scheme 5) [26,27]. 

Besides [m + n] cycloadditions and [m 4- m + m +. ..] cyclo- and cocyclooligomerizations of alkenes, dienes or alkynes (see Sections 1.5.8.3.5, and 1.5.8.3.6.), transition metal complexes can also catalyze the cycloaddition of more than two different components. Most important is carbonylative ring synthesis with carbon monoxide as the C, unit. Several methods of this type use transition metals stoichiometrically, others catalytically. For some of the stoichiometric methods, developments towards catalytic versions are under way (e g., the Pauson-Khand reaction, see below). 

An interesting cyclization reaction was reported that involved the reaction of dienes, diynes, or ene-ynes with transition metals to form cyclopentenone derivatives in the presence of carbon monoxide.363 in a simple example, ene-yne 444 was heated with dicobalt octacarbonyl and CO to give a 68% yield of 445.364 jjjj transformation has become an important synthetic tool known as the Pauson-Khand reaction.365 jhe mechanism probably involves insertion of the alkene (or alkyne) into the transition metal bond, which is why it is presented in this section. Formally, it is a [2+2+l]-cycloaddition, but the accepted mechanism is the one proposed by Magnus,364 and shown in Figure 13.8.366 n has been stated that further study is required to... 

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