The reaction of [Pd­(η3-allyl)­ClL] (L = AsPh3, PPh3) with [M­(CCR)] n (M = Cu, Ag; R = nBu, Ph) leads to transmetalation of the alkynyl group from M to Pd. However, the group 11 metal stays η2-bound to the new Pd–alkynyl fragment and heterometallic Pd–M complexes are formed with different nuclearities depending on M: [{Pd­(η3-allyl)­(alkynyl)­L}­CuCl]2 (3, 4) or [{Pd­(η3-allyl)­(alkynyl)­L}2AgCl] (5, 6). The M-containing fragment can be eliminated to give the actual transmetalation complex [Pd­(η3-allyl)­(alkynyl)­L] by adding an excess of arsine or phosphine, whereas amines do not have this effect. Allyl–alkynyl reductive elimination is a slow process; therefore, complexes 3–6 cleanly decompose by dimerization (homocoupling) of the alkynyl group. In the decomposition process reversible alkynyl transmetalation from Pd to Cu has been observed.

A characterful carbene: The high electrophilicity of a genuine palladium alkoxycarbene, obtained by transmetalation, is evident in its reactivity. Nucleophilic attack on two electrophilic centers (red) is observed. Alkyl abstraction and addition to the carbene carbon by different nucleophiles occur. This palladium(II) alkoxycarbene also undergoes comproportionation with palladium(0) to give an unprecedented palladium(I) dimeric carbene (see scheme).

The palladium-catalyzed reaction of alkynyltin compounds with allylic chlorides leads to a 2:2 coupling to give trienynes by regio-and stereoselective formation of three new C À C bonds. The reaction can be applied to different alkynyl and allylic fragments, providing a wide range of trienynes with different substitution patterns in very good yields. They can be prepared in a green way using recyclable polymeric tin alkynyls.

The migratory insertion reaction of a carbene into a palladium−acyl bond has been observed both for monoaminocarbenes and for methoxycarbenes. The acyl derivative [PdCl(COMe){C(NEt 2 )-Ph}(PPh 3 )] undergoes an acyl−carbene coupling, leading to the enolate-type complex [PdCl{C(COMe)(NEt 2 )Ph}(PPh 3 )] 2 (2). This complex decomposes either by reductive elimination to give the iminium salt or by protonation of the enolate to give a ketoamine. In a similar fashion, the reaction of [Pd 2 (μ-Cl) 2 (COMe) 2 (SMe 2 ) 2 ] with [W(CO) 5 {C(OMe)Ph}] leads to an undetected palladium enolate that, after protonation, is stabilized by coordination to palladium ([PdCl 2 {(OH)MeCCPh(OMe}(SMe 2 )], 8). The reaction of [Pd 2 (μ-Cl) 2 Me 2 (SMe 2 ) 2 ] with [W(CO) 5 {C(OMe)-Ph}] leads to the migratory insertion of the carbene into the Pd− methyl group to give an alkyl palladium complex. The transfer of CO from tungsten, followed by insertion into the Pd−Me group, also occurs. This leads to the formation of a Pd−COMe group, which also undergoes migratory insertion of the carbene fragment. These results support that migratory insertion is a key C−C coupling step, as proposed for the new Pd-catalyzed transformations that use carbene precursors.

The reaction of [W(CO) 5 {C(NEt 2 )Ph}] (3) with [PdClR(SMe 2 )] 2 (R ) Me, Ph) occurs with transfer of carbene and carbonyl groups to give [PdCl{C(O)R}{C(NEt 2 )Ph}] 2 (R ) Me, 4; R ) Ph, 5). When the reaction is monitored for R ) Me, only [PdCl(COMe)(SMe 2 )] 2 and the final carbene 4 are observed, suggesting that the transfer and insertion of the carbonyl group are faster than the carbene transmetalation. Although CO insertion into M-X bonds is thermodynamically excluded in many systems (e.g., in M-halogen and M-C 6 F 5 bonds), this study warns of the fact that CO is easily available when [M(CO) 5 (carbene)] complexes are used as carbene sources, whether for stoichiometric or for catalytic reactions. This CO could react with the intermediates or the products in these reactions.