Olefins, Pauson-Khand reaction

We tried to take advantage of the mentioned activation in other type of reactions[5]. Since the Co mediated carbonylative cycloaddition of alkynes and olefins (Pauson-Khand reaction) is under extensive study, we advanced that the mentioned triple bonallow reaction completion under milder temperatures than those usually required. While no reaction was found... 

An important advance was reported by Schore and Croudace [13], who showed for the first time that carbon-tethered enyne precursors undergo an intramolecular Pauson-Khand reaction (PKR) in good yields with complete control of regioselectivity. In this connection, it was not essential to use strained olefins as starting materials. 

The asymmetric reactions discussed in this chapter may be divided into three different types of reaction, as (1) hydrometallation of olefins followed by the C—C bond formation, (2) two C C bond formations on a formally divalent carbon atom, and (3) nucleophilic addition of cyanide or isocyanide anion to a carbonyl or its analogs (Scheme 4.1). For reaction type 1, here described are hydrocarbonyla-tion represented by hydroformylation and hydrocyanation. As for type 2, Pauson-Khand reaction and olefin/CO copolymerization are mentioned. Several nucleophilic additions to aldehydes and imines (or iminiums) are described as type 3. 

Zhang has proposed a mechanism for the rhodium-catalyzed Alder-ene reaction based on rhodium-catalyzed [4-1-2], [5-i-2], and Pauson-Khand reactions, which invoke the initial formation of a metallacyclopentene as the key intermediate (Scheme 8.1) [21]. Initially, the rhodium(I) species coordinates to the alkyne and olefin moieties forming intermediate I. This intermediate then undergoes an oxidative cycHzation forming the metallacyclopentene II, followed by a y9-hydride elimination to give the appending olefin shown in intermediate III. Finally, intermediate III undergoes reductive elimination to afford the 1,4-diene IV. 

Enantiomerically pure trans-2-phenylcyclohexanol, first used by Whitesell as a chiral auxiliary has become a popular reagent in a number of asymmetric transformations. Some recent applications include asymmetric azo-ene reactions, [4 + 2]-cycloaddition reactions, ketene-olefin [2 + 2]-reactions, enolate-imine cyclocondensations, Pauson-Khand reactions," palladium annulations and Reformatsky reactions. Despite its potential, use of this chiral auxiliary on a preparative scale is currently limited by its prohibitive cost. 

An application of the Pauson-Khand reaction for the synthesis of a carbaprostacyclin analogue (Scheme 11) [44] illustrates the power of organometallic methods for the activation of olefins and acetylenes. 

Oxidative alkoxycarbonylation asymmetric carbonylation, 11, 467 catalyst development, 11, 467 mechanism, 11, 466 Oxidative amination, olefins, 10, 155 Oxidative cleavage, mechanisms, 1, 103 Oxidative promoters, in Pauson-Khand reaction with dicobalt octacarbonyl, 11, 337... 

Murai s group developed a series of netv Ru3(CO)i2-catalyzed qrcloaddition reactions involving CO. In the intramolecular Pauson-Khand reaction, the olefinic moiety is replaced by a carbonyl [84] or imine group [85] to give either y-lactones or y-lactams (Eqs. 11.40 and 11.41). 

The Pauson-Khand reaction is a powerful tool for the synthesis of cyclopentanones, 246, from a>-alkenylacetylenes, 245, and carbon monoxide.176 Enyne cyclization has been catalyzed with nitriles using catalytic (77S-CsH5)2Ti(PMe3)2 95177-179 and other variants have since been discovered where the desired cyclopentenones can be directly prepared from the enyne and CO using (77S-CsHs)2Ti(CO)2 68 (Scheme 33) 176,180-184 Addition of PMe3 to the latter reaction mixture has proved to be beneficial. Stoichiometric reactions established that the initial step in the catalytic cycle is reductive coupling of the alkyne and the olefin to form the titanacycle. Carbon monoxide insertion followed by reductive elimination generates the observed product. 

The chapter ends with a rather miscellaneous collection of reactions such as one example of an electrocyclic ring closure 19. There are more important examples of ring closing olefin metathesis (RCM) and other organometallic reactions such as the Pauson-Khand reaction and co-trimerisation. 

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. 

The Pauson-Khand Reaction Cycloadditions of Olefins, Acetylenes, and CO... 

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