Oxidation Pauson-Khand reaction

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. 

A combination of a metathesis and a Pauson-Khand reaction, which leads to tricyclic compounds starting from diene-ynes, has been described by Perez-Castells and colleagues [262]. Treatment of the Co-complex 6/3-86, obtained from the corresponding alkyne in 75 % yield, with 5 mol% of the Ru-catalyst 6/3-13 for 18 h, followed by addition of an N-oxide as trimethylamine-N-oxide (TMANO) or NMO as copromoters, gave 6/3-87 in 81% yield. 

Under the conditions of the cobalt-mediated carbonylative A-oxide-promoted cocyclization (Pauson-Khand reaction) at room temperature, compound 547 provides exocyclic 1,3-diene 548 as the major product (>98%) together with only traces of the corresponding carbonylative product 549. Owing to the relative instability of the diene, it is more efficient to perform a one-pot cobalt cyclization/Diels-Alder process after A-oxide-promoted cyclization of the cobalt complexes. Compound 550 is obtained as a single diastereomer in 39% overall yield if MTAD is used as a dienophile (Scheme 90) <2003JOC2975>. 

Hetero Pauson-Khand reactions with an aldehyde or ketone component have been shown to afford synthetically versatile y-butyrolactones. Buchwald [50] and Crowe [51] independently showed that aliphatic enones and enals react with CO under Cp2Ti(PMe3)2 mediation (Scheme 11). CO insertion and thermal (or oxidative) decomposition gave diastereomerically pure bicyclic y-butyrolactones and stable Cp2Ti(CO)2. Imines did not react under the reaction conditions. 

Several reports have appeared on the effect of additives on the Pauson-Khand reaction employing an alkyne-Co2(CO)6 complex. For example, addition of phosphine oxide improves the yields of cyclopentenones 119], while addition of dimethyl sulfoxide accelerates the reaction considerably [20]. Furthermore, it has been reported that the Pauson-Khand reaction proceeds even at room temperature when a tertiary amine M-oxide, such as trimethylamine M-oxide or N-methylmorpholine M-oxide, is added to the alkyne-Co2(CO)6 complex in the presence of alkenes [21]. These results suggest that in the Pauson-Khand reaction generation of coordinatively unsaturated cobalt species by the attack of oxides on the carbonyl ligand of the alkyne-Co2(CO)6 complex [22] is the key step. With this knowledge in mind, we examined further the effect of various other additives on the reaction to obtain information on the mechanism of this rearrangement. 

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. 

Amine—bis(amide) ligands, in chromium(III) models, 5, 376 Amine 7V-oxide promoters, in Pauson—Khand reaction with dicobalt octacarbonyl, 11, 337 Amines... 

Dicobalt octacarbonyl, in Pauson—Khand reaction homogeneous catalysis, 11, 340 metal-coupled promoters, 11, 339 non-oxidative promoter-assisted, 11, 338 oxidative promoter-assisted, 11, 337 physical promoters, 11, 339 solid-supported promoters, 11, 339 Dicobalt triple-decker sandwiches, preparation, 3, 14 (+)-Dictamnol, via [5+2]-cycloadditions, 10, 613-614 Dicyclohexylborane, for alkene hydroboration, 9, 150... 

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... 

Formation of the tricyclo[3.3.0.0.]decane 209 by the reaction of [3.2.0]bicyclo-heptadiene 205 with propyne complex (206) is an example [81], The Pauson Khand reaction is explained by the following simplified mechanism. At first the oxidative cyclization of 205 and 206 generates the cobaltacyclopentene 207, to which insertion of CO gives 208. Finally, reductive elimination of208 affords the cyclopentenone 209. 

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... 

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. 

Initially, the Pauson-Khand reaction involved heating the substrate in a hydrocarbon solution at elevated temperatures and, as such, was not applicable to labile polyfunctional substrates. Later it was discovered that this cycloaddition could be greatly accelerated under the action of mild oxidants (morpholine... 

Dihydrofiirans have seen considerable use as substrates in the Pauson-Khand reaction. The parent compound reacts in excellent yield with acetylene, terminal and internal alkynes. Yields in this system respond very well to the use of catalytic reaction conditions (equation 4). Another unusual experimental modification has also been found by Pauson to be useful in this system addition of tri-n-butylphosphine oxide nearly doubles the product yield in certain cases (equation 37). The role of the added substance is unclear. Addition of phosphine oxide does not always improve reaction efficiency at this time there are no guidelines to indicate when its use might be beneficial. Substituted dihydrofurans give somewhat lower but still acceptable yields the poor regioselectivity in unsymmetrical cases is the more significant difficulty with these substrates (equation 38). 

Metallacycles have been claimed to play pivotal roles in many transition metal-mediated multi-component coupling reactions [1]. For example, [2 -i- 2 -i- 2] alkyne cyclo-trimerization leading to benzenes - the Reppe reaction - has been considered to proceed via metallacyclopentadiene and elusive metallacycloheptatriene intermediates ("common mechanism ), while metallacyclopentenes have been proposed as intermediates for the [2 -i- 2 -i- 1] cyclo-coupling reactions of an alkyne, an alkene, and CO leading to a cyclopentenone (the Pauson-Khand reaction). A metallacyclic compound - which is defined here as a carbocyclic system with one atom replaced by a transition metal element - can be generally formed by oxidative cyclization of two unsaturated molecules with a low-valent transition metal fragment [2-4]. Alter-... 

Further, replacement of the acetylenic part in the Pauson-Khand reaction by a carbonyl or imine group has been successfully achieved. a,/3-Unsaturated imines react with CO in the presence of Ru3(CO)i2 catalyst to give carbonylative [4 -i- 1] cycloadducts, y-lactams, in high yields [86], A possible mechanism is shown in Scheme 11.4. Coordination of a,/3-unsaturated imine to "Ru(CO)4 gives 9, which is converted into 10 via oxidative cyclization. Subsequent carbonylation of 10 gives 11, the reductive elimination of which gives 12 (Eq. 11.42). 

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