Pauson intermolecular

Scheme 10.67 Intermolecular Pauson-Khand reactions with PuPHOS and CyPHOS.

The intermolecular Pauson-Khand reaction of the resulting S/P-cobalt complexes with norbornadiene was studied under thermal and A -oxide activation conditions. Thus, heating the diastereomerically pure complex (R = Ph, R = Cy) with ten equivalents of norbornadiene at 50 °C in toluene afforded the corresponding exo-cyclopentenone in a quantitative yield and with an enantio-selectivity of 99% ee. Under similar conditions, the analogous trimethylsilyl complex (R = TMS, R = Cy) afforded the expected product in a high yield but with a lower enantioselectivity of 57% ee. In order to increase this enantio-selectivity, these authors performed this reaction at room temperature in dichloromethane as the solvent and in the presence of NMO, which allowed an enantioselectivity of 97% ee to be reached. These authors assumed that the thermal activation promoted the isomerisation of the S/P ligand leading to a nonstereoselective process. 

Scheme 10.69 Intermolecular Pauson-Khand reactions of amido-alk5mes with PuPHOS and CamPHOS-derived ligands.

Scheme 16.48 Intermolecular Pauson-Khand reaction using NMO as a promoter.

Khand and Pauson reported a Co-mediated intermolecular [2-I-2-I-1] cycloaddition of an alkyne, an alkene and carbon monoxide (the Pauson-Khand reaction) [4, 26] wherein an alkyne-Co2(CO)6 complex, which had been prepared from Co2(CO)g... 

In the 1990s, successful catalytic asymmetric intermolecular Pauson-Khand reac-... 

Concerning the regiocontrol of intermolecular Pauson cyclization in which a homo-allylic donor group is present in the alkene subunit [195], there seems to be some electronic contributions. 

The intramolecular Pauson-Khand reaction. It was not until 10 years after the first reports of the intermolecular PK reaction that Schore and co-workers published their findings on an intramolecular variant of the reaction.8... 

Abstract The Pauson-Khand [2 + 2 + 1 ] cycloaddition is one of the best ways to construct a cyclopentenone. It implies the formation of three new bonds and one or two cycles in the intermolecular or intramolecular versions, respectively. Furthermore some groups have enhanced the synthetic power of this transformation by combining the PKR with other processes. In addition, some unexpected results imply that successive events have occurred, usually after the cycloaddition process. This review aims to point out the most recent advances in cascade reactions in which the Pauson-Khand and PK-type... 

For a recent study on regiochemistry of the intermolecular PKR, see Kerr WJ, McLaughlin M, Pauson PL, Robertson SM (2001) J Organomet Chem 630 104... 

Highly efficient asymmetric intermolecular Pauson-Khand reactions have been developed by using the chiral phosphine ligand (/ )-(+)-g yphos and N-methylmorpholine A-oxide as a promoter (see Section II,C).178... 

Considerable efforts have been made to develop asymmetrical variants of the classical Pauson-Khand reaction. Initial investigations have shown that compounds derived from cobalt complexes of type 1, in which a carbonyl ligand is replaced by a chiral phosphane (glyphos), react with high enantioselectivity [22], However, the procedure is too complex to be of preparative value. The concept of Kerr et al., who achieved significant enantioselectivities (max. 44 % ee) in intermolecular Pauson-Khand reactions by... 

The high value of the Pauson-Khand reaction in the synthesis of natural products and other complex compounds has been frequently demonstrated [3]. One of the most impressive examples is the synthesis of the marine natural product (+)-epoxydictymene by Schreiber and co-work-ers [33], The synthetic strategy (Scheme 11) uses an intermolecular Nicholas reaction [34]... 

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). ... 

Asymmetric intermolecular Pauson Khand reactions have been reahzed using a number of chiral auxiliaries chelating to the metal and/or attached to the alkyne. One example using a camphor-derived hgand is seen in Scheme 253. Moderate asymmetric induction has been observed using chiral amine A-oxides as the promoter. For example, (-F)-indohzino[3,4-b]quinoline A-oxide gave up to 53% ee. 

Intramolecular Pauson Khand reactions are synthetically more useful compared to their intermolecular counterparts. Enynes separated by either three or four atoms (1,6- or 1,7-enynes) cyclize in an intramolecular fashion upon complexa-tion with Co2(CO)s, and subsequent heating to form bicycUc enones. Angular tricyclic ring systems have been prepared in... 

An intermolecular Pauson-Khand reaction of a dihydrofuran intermediate was used to prepare a benz[ indenone, allowing for the first synthesis of a zirconocene benz[/]indene complex <03OL3153>. A stereoselective intermolecular Pauson-Khand reaction of 3-(R)-methyldihydrofuran was used to construct the desired diastereoisomer of oxabicyclo[3.3.0]octanone ring shown below in the total synthesis of the sesterterpene terpestacin <03JA11514>. 

Numerous synthetic applications of the intermolecular Pauson-Khand reaction have been reported. Pauson has reported a number of very direct applications of cycloadditions of ethylene in the synthesis of prostanoids and jasmone analogs (e.g. equations 15 and This is a reliable entry to 2-sub-... 

Considerable effort has been devoted to achieving the intermolecular catalytic Pauson-Khand reaction. The mthenium complex-catalyzed reaction of an alkyne with an alkene such as ethylene or 2-norbornene under CO gave hydroquinone derivatives [79], with CO (2 mol) being introduced into the products (Eq. 11.36). This reaction is the first example of the preparation of hydroquinone derivatives by the reaction of alkynes and alkenes with CO, while hydroquinone is synthesized by the ruthenium-catalyzed reaction of 2 mol acetylene with 2 mol CO (Eq. 11.37) [80]. 

A reaction which corresponds to the intermolecular Pauson-Khand reaction was accomplished by the Ru3(CO)i2-catalyzed reaction of cyclobutenediones with alkenes under CO (Eq. 11.38) [81]. 

Scheme 11.3 Intermolecular Pauson-Khand reaction with alkyne and 2-pyridylsilylalkene.

Gibson SE, Mainolfi N. The intermolecular Pauson-Khand reaction. Angew. Chem. Int. Ed. 2005 44 3022-3037. 

Enantioselectivities up to 44 % were reached in intermolecular PKRs when chiral aminoxides R 3N—>0 were used [19]. Although the mechanism is not known, it seems likely that the chiral A-oxide discriminates between the prochiral carbonyl cobalt units, either oxidizing one carbon monoxide selectively to produce a vacant site for the alkene insertion, or stabilizing a vacant site on one of the cobalts preferentially. This approach was modified by application of chiral precursor substrates [20]. Albeit the synthesis of the latter is cumbersome, the concept was successfully applied in several total syntheses, for example of hirsutene [21], brefeldine A [22], /9-cuparenone [23], and (+)-15-norpentalenene [24] (eq. (10)). Stoichiometric amounts of the mediator compound Co2(CO)8 are still necessary in this useful version of the Pauson-Khand reaction. 

Preston, A. J., Parquette, J. R. A pyridylsilyl group expands the scope of the intermolecular Pauson-Khand reactions. Chemtracts200Z, 16, 435-438. 

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