Transmetallation palladium complexes

Palladium-catalyzed annulation reactions of conjugate acceptors and allenyl boronic ester provide substituted cyclopentenes in high yields and diastereose-lectivities (Scheme 6.24). These reactions are hypothesized to commence by the conjugate addition of a nucleophilic propargyl-palladium complex. Transmetalation of allenylboronic acid pinacol ester with a Pd(II) catalyst proceeds via an SE2 mechanism to provide the propargyl-palladium complex, which on conjugate attack on the electrophile furnishes an allene intermediate. Finally, endo carbopalladation of the pendant allene and protodepalladation generates the cyclopentene [28]. 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

Yasuda S, Yorimitsu H, Oshima K (2008) Synthesis of aryliron complexes by palladium-catalyzed transmetalation between [CpFe(CO)2l] and aryl Grignard reagents and their chemistry directed toward organic synthesis. Organometallics 27 4025 027 Jonas K, Schieferstein L (1979) Simple route to Li- or Zn-metalated r -cyclopentadien-yliron-olefin complexes. Angew Chem Int Ed Engl 18 549-550... 

Besides rhodium catalysts, palladium complex also can catalyze the addition of aryltrialkoxysilanes to a,(3-unsaturated carbonyl compounds (ketones, aldehydes) and nitroalkenes (Scheme 60).146 The addition of equimolar amounts of SbCl3 and tetrabutylammonium fluoride (TBAF) was necessary for this reaction to proceed smoothly. The arylpalladium complex, generated by the transmetallation from a putative hypercoordinate silicon compound, was considered to be the catalytically active species. 

An acyl-palladium complex might undergo a series of follow up reactions. Subsequent transmetalation and reductive elimination lead to the formation of a carbonyl compound. This process is also coined carbonylative coupling, referring to the cross-coupling reaction, which would take place in the absence of carbon monoxide under similar conditions (for more details see Chapter 2.4.). 

Subsequent to the first PCP-pincer study, Johansson used NMR spectroscopy to investigate the reactivity of the PCP-Pd complexes in transmetallation reactions with organotin derivatives. Somewhat surprisingly, only those palladium complexes with electron-withdrawing groups and less sterically demanding substituents could react efficiently with tributylallyltin [50]. 

Transition-metal-catalysed reactions of glycals with carbon nucleophiles provide an important route to C-glycosides.24 These reactions may proceed by two different pathways depending on the oxidation state of the transition metal used and the nature of the carbon nucleophile (Scheme 3.5c). For example,25 transmetallation of Pd(OAc)2 with pyrimi-dinyl mercuric acetate results in the formation of a pyrimidinyl palladium complex, which adds to the double bond of glycal derivatives. The resulting intermediate can undergo different transformations depending on the reaction conditions. 

The reactions may also be carried out under an atmosphere of carbon monoxide, CO (Scheme 10.22), when the usual catalytic cycle occurs. CO inserts easily into the palladium complex Ar-Pd -X. The aryl ligand migrates on to the carbonyl group to form a metal-acyl species, X-Pd - C(0)Ar. A transmetallation-reductive elimination sequence follows, forming the ketone and regenerating the Pd catalyst. 

Now the coupling can take place on the palladium atom producing the product and Pd(0), which can insert oxidatively in the C-Cl bond. Transmetallation now sets up a sustainable cycle of reactions. It is better to have an atmosphere of carbon monoxide because the acyl palladium complex can give off CO and leave a PdPh ct-complex. The atmosphere of CO reverses this reaction. 

The method was made considerably more general by inclusion of a catalytic amount of a palladium(O) complex during addition of the organometallic. Alkenylcopper reagents actually react relatively slowly with acid halides but, in a fashion analogous to other alkenyl metal species (see Section 1.13.4), they may be readily transmetallated to form an acylpalladium(II) complex which then undergoes reductive elimination to the product (Scheme 29)." A further discussion of acylation mediated by palladium complexes is included in Section 1.13.4. Interestingly, a,P-unsaturated acid chlorides react under these conditions to form divinyl ketones. 

Miyaura, N. Cross-coupling reaction of organoboron compounds via base-assisted transmetalation to palladium " complexes. J. 

Conversion of norbornadiene to nortricyclenes can also be achieved with organomercurials. This reaction proceeds via transmetalation to form a c-bonded organopalladium system which inserts norbornadiene. Thus an enrfo-cw-addition is observed. Conversions of this type were observed in the phenylation of dichloro(t -norbornadiene)palladium either with diphenyl-mercury or, more conveniently, with sodium tetraphenylborate to give di-/r-chlorobis(2 5,6-t -c ffo-3-phenylbicyclo[2.2.1]hept-e /o-2-yl)palladium (30) via cis addition.By contrast, the analogous platinum complex did not give a phenylation product. The palladium complex 30 underwent reversible ring closure to a nortricyclenyl complex 31 with pyridine. ... 

If the electrophile is a vinyl triflate, it is essential to add LiCl to the reaction so that the chloride may displace triflate from the palladium o-complex. Transmetallation takes place with chloride on palladium but not with triflate. This famous example illustrates the similar regioselectivity of enol triflate formation from ketones to that of silyl enol ether formation discussed in chapter 3. Kinetic conditions give the less 198 and thermodynamic conditions the more highly substituted 195 triflate. 

Palladium-catalyzed cross couplings of organofluorosilanes with vinyl [37] and aryl [38] halides and triflates are known, where the activation of the Si—C bond by fluoride ion plays an essential role in the coupling. The pentacoordinated fluoroorganic silanes promote the transmetallation to the aryl halide-palladium complexes strongly. At least one fluorine... 

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