Pauson with allenes

Scheme 8. Pauson-Khand reactions with allenes.

Due to its reliability, the SN2 substitution is often used in applications which require the highly enantioselective formation of the allene for example, Brummond et al. [19g] prepared the yneallene 19 (a starting material for intramolecular allenic Pauson-Khand cycloadditions) through the anti-selective SN2 substitution of the chiral propargylic mesylate 18 with a suitable magnesium cuprate (Scheme 2.6). 

In a similar manner, Brummond et al. demonstrated the first total synthesis of 15-deoxy-A12,14-prostaglandin J2 (162) that was completed using a silicon-tethered allenic Pauson-Khand reaction to obtain the highly unsaturated cyclopentenone substructure [36]. Treatment of alkynylallene 160 with molybdenum hexacarbonyl and dimethyl sulfoxide affords the desired cycloadduct 161 in 43% yield (Scheme 19.30). Trienone 161 was obtained as a 2 1 Z E mixture of isomers in which the Z-isomer could be isomerized to the desired E-isomer. The silicon tether was cleaved and the resulting product converted to 15-deoxy-A12,14-prostaglandin J2 (162). 

Allenyne represents an interesting substrate for the intramolecular Pauson-Khand(-type) reaction, where an allene moiety acts as an ene component. Here, there are two possible reaction pathways (Scheme 11.21) (i) the reaction of an external tr-bond of allene moiety gives a bicyclic dienone (type A) or (ii) the reaction of an internal 7i-bond gives a bicyclic cyclopentenone with an alkylidene substituent (type B). 

Parasites, antimicrobials for, 12, 458 Pauson-Khand reaction allenic substrates, 11, 352 and allenynes, 10, 356-357 with aminocarbonylation, 11, 531 asymmetric catalysts, in desymmetrizations, 11, 357 catalytic... 

Several authors have demonstrated that a Pauson-Khand type formation of methylenecyclopente-nones from enynes, allenes and carbon monoxide occurs with stoichiometric ammounts of [Fe2(CO)y] or [MoCCO) ], and catalytically with [Cp2Ti(CO>2] [19, 29]. Gazes et al. recently reported that cobalt-mediated inter- and intramo-... 

Cyclopropene can also be used as the aUcene component and affords bicyclo[3.1.0]hexen-2-ones upon reaction with alkyne dicobalt octacarbonyl complexes in the presence of NMO (Scheme 250). Vinyl ethers and vinyl esters serve as ethene equivalents in Pauson-Khand reactions. For example, reaction of vinyl benzoate with complex (169) furnished cyclopentenone (170) (Scheme 251). This reaction was used in a synthesis of (-l-)-taylorine and nortaylorine. Allenes participate in intermolecular Pauson-Khand reactions affording alkylidene-substituted cyclopentenones (Scheme 252). ... 

Cycloaddition. A synthesis of bicyclic dienones by the Pauson-Khand reaction of an allene/yne is based on catalysis by Co2Rh2. Two molecules of an allene combine with CO to form 4-alkylidene-2-cyclopentenones. ... 

This reaction has found its way into organic synthesis fi and is attractive because of its generality and selectivity. In Drummond s synthesis of hydroxymethylacylfulvene,3 0 for example, a Pauson-Khand reaction of the allene-alkyne 446 gave a 69% yield of 447. In this case, molybdenum hexacarbonyl was used. In Takano s synthesis of dendrobine, 448 was treated with dicobalt octacarbonyl and then NMO to give an 89%... 

Abstract Recent density functional theory computations of cobalt-catalyzed hydroformylation of propene, A -vinyl acetamide, 1,3-butadiene, acetylene, propyne, and allene and the urea formation from methyl amine as well as Pauson-Khand reaction have been reviewed. The detailed catalytic mechanism and regioselectivity have been discussed and compared with the available experimental data. It shows that modem computational chemistry provides not only qualitative but also quantitative aspects of catalytic reactions. 

Most Pauson-Khand reactions have been conducted with an alkene, an alk5me, and CO. However, PKRs have been developed with aUenynes (Scheme 17.36). Narasaka reported the first example of an intramolecular allenyne PKR catalyzed by an iron complex, and Brum-mond has extensively studied the allenic Pauson-Khand reaction. These reactions have been catalyzed by the combination of Mo(CO) and DMSO or by [Rh(CO)2Cl]2, and several examples are shown in Equations 17.77 and 17.78. - The reactions in Equations 17.77 and 17.78 illustrate the different regiochemistry of the products from reactions catalyzed by molybde num and rhodium. Computational studies indicate that different geometries of octahedral Mo(0), and square-planar Rh(I) species account for the different regioselectivities. ... 

---