Isomerizations tetrakis palladium

In another reduction, the propargylic phosphate 64 is reduced with samarium(II) iodide in the presence of tetrakis(triphenylphosphine)palladium and tert-butanol as a proton source the allene 65 is produced almost exclusively, <1% of the isomeric alkyne 66 being present in the product mixture [19]. 

Indeed, under photolytic conditions, bis(silyl)nitrilimine 9 isomerizes into bis(silyl)carbodiimide 14 in quantitative yield [3b]. More surprisingly, when a THF solution of diazo derivative 3 is stirred overnight at room temperature in the presence of a catalytic amount of tetrakis(triphenylphosphine) palladium, bis(stannyl)carbodiimide 15 is obtained in 90 % yield [16a] (Scheme 5). 

The scheme illustrates that E-olefins lead to virtually pure trans-adducts. However, Z-olefins produce mixtures of as- and tra .9-methylenecyclopentanes, the loss of stereospecificity being partially due to Z/E isomerization of the olefin under the reaction conditions. The stepwise nature of the addition also contributes to the loss in stereochemical integrity. Although in these examples the palladium catalyst used was the prereduced tetrakis(triphenylphosphane)palladi-um(0), studies have shown that there is no significant difference in yield between this approach and the use of palladium acetate with triisopropyl phosphite. Indeed, this latter system is often much easier to apply since the nature of the phosphane can be altered and the handling of air-sensitive complexes is avoided. The procedure with teti akis(triphenylphosphane)palladium(0) as catalyst is exemplified by the addition to coumarin. 

The first examples of a Pd-catalyzed addition of terminal alkynes to allenes have been reported by Trost and Kottirsch (Scheme They observed that different palladium complexes efficiently catalyze the addition of terminal alkynes I to 3-substituted allenecarboxylates 23 to produce a mixture of the a,)8-unsaturated ester 24 and the two isomeric /S.-y-unsaturated esters (Z)- and (E)-25 (Scheme 9). Remarkably, the type of Pd catalyst used played a crucial role in determining the regiochemistry of addition to the 1,3-disubstituted allenes 23. Thus, with the Pd(OAc)2 TDMPP system (cat. A) a 76 24 ratio of 24 and E/Z)-25 was produced, whereas with a combination of tetrakis(carbo-methoxy)palladacyclopentadiene (TCPC) and TDMPP (cat. B) these products were obtained in a ratio of 9 91 (Scheme 9). ° ... 

A variety of a-nitroepoxides undergo conversion to the corresponding 1,2-diketones or, in some cases, a-nitroketones with tetrakis(triphenylphosphine)palladium(0) (Scheme [io],[ii] Proposed reaction mechanism is shown in Scheme 16. 2,3-Epoxy alcohols also undergo the Pd(0)-catalyzed reaction to be isomerized to a- or /3-hydroxy ketones or both, depending on the nature of the substituents on the phenyl ring (Scheme... 

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