Allenylpalladium complex

The extraordinary power of the Ishikura palladium-catalyzed couplings of indolylborates is revealed by the several examples shown below [148,152,154,155]. The carbonylation version is discussed in Section 3.6. The formation of allenylindoles 139 vis-d-vis alkynylindoles 141 apparently depends on the equilibrium between an allenylpalladium complex and a propargylpalladium complex, and SN2-like attack on the latter by 136 to give 141 is favored by the Ph3P-ligated Pd complex. 

In 1985, Tsuji s group carried out a Pd-catalyzed reaction of propargyl carbonate with methyl acetoacetate as a soft carbonucleophile under neutral conditions to afford 4,5-dihydrofuran 109 [89-91]. The resulting unstable 109 readily isomerized to furan 110 under acidic conditions. In addition, they also reported formation of disubstituted furan 112 via a Pd-catalyzed heteroannulation of hydroxy propargylic carbonate 111 [92], Presumably, an allenylpalladium complex (c/. 114) was the key intermediate. 

Some synthetically important allenylmetallics, such as allenylzinc and allenylin-dium reagents, are prepared from allenylpalladium intermediates. These reactions are discussed in appropriate sections of this chapter. This section covers the reactions of allenylpalladium compounds without further transmetallation. Allenylpalladium complexes can be prepared from propargylic halides, acetates, carbonates, mesylates, alcohols and certain alkynes [83-87], The allenylpalladium compound prepared from 3-chloro-3-methyl-l-butyne has been isolated and characterized spectroscopically (Eq. 9.106) [83], It was found to couple with organozinc chlorides to produce homologated allenes quantitatively (Eq. 9.107). 

Chiral allenylpalladium complexes have been prepared from non-racemic propargylic chlorides and Pd2(dba)3-PPh3 [88]. Although nominally stable, these complexes were found to racemize under certain conditions (Eq. 9.108). Reagents prepared under oxygen-free conditions racemize more slowly than those prepared in... 

Substituted propargylic carbonates react with terminal acetylenes in the presence of a catalytic amount of Pd(PPh3)4 and Cul to produce Sonogashira-type cross-coupling products (Eq. 9.114) [84]. Presumably, the reaction proceeds through an allenylpalladium complex. Addition of a salt, such as KBr, increased the yield of the coupling product. Only tetrasubstituted allenes could be obtained by this procedure. 

Sulfonic amides of alkynylaziridines can also be converted to allenylindium reagents by oxidative transmetallation of transient allenylpalladium complexes. When the reaction is perfomed in the presence of an aldehyde the expected adducts are produced stereoselectively (Eq. 9.150) [120],... 

When the propargyl phosphate 225 was heated in the presence of Pd2(DBA)3-CHCl3 and sodium acetate in THF, the central sp carbon atom of the (j-allenylpalladium complex, formed from the propargyl alcohol ester and Pd(0), was attacked by the lactam nitrogen atom to yield the carbapenam 226 with an exocyclic methylene group <2001TL4869>. 

Scheme 11.9 Allenylpalladium complex formation from 2-(l-alkynyl)oxiranes. 

The formation of (31) was favoured in most reactions tried, presumably due to the directing influence of the hydroxyl group present as the lithium anion. Palladium catalysis finds yet another fruitful application in the conversion of acetylenic carbonates to allenic products. The allenylpalladium complexes... 

The Pd-carbon bond in the allenylpalladium complex A has reactivity similar to that of bond formed by the oxidative addition of alkenyl haUdes to Pd(0) in the Heck reaction.t lP Smooth insertion of alkenes into the allenylpalladium complexes A and subsequent elimination of /3-hydrogen affords 1,2,4-trienes (Scheme 2). 

Attack of Soft Carbon Nucleophiles on sp Carbon of Allenylpalladium Complexes... 

A soft carbon nucleophile attacks the sp carbon of the allenylpalladium complex A to give a TT-aUylpalladium complex. Another soft carbon nucleophile attacks the Tr-aUylpal-ladium complex thus formed to afford an alkene, into which the same nucleophiles are doubly introduced (Schemes 7 and The reaction with /S-keto esters provides... 

The Pd(II) species on an allenylpalladium complex A has ample Lewis acidity to activate a triple bond, on which a suitably arranged carboxylate anion attacks intramolecularly. Reductive elimination of Pd(0) species completes the formation of exo-enol lactones (Scheme The Pd-catalyzed coupling of the propargylic acetates with 4-pentynoic acids proceeds very smoothly in the presence of potassium bromide using tris(2-furyOphosphine (TFP) as the ligand (Scheme 10). 

Substitution reactions of 7r-allylpalladium complexes with heteroatom nucleophiles have been well documented. In sharp contrast, the reactions of allenyl and/or propargylpalla-dium complexes with heteroatom nucleophiles are very rare, and only one reaction has been recorded so far that provides propargylic amines through Sn2 type substitution reactions of allenylpalladium complexes with aromatic amines. 

TT-Allylpalladium complexes have long been recognized as an allyl cation equivalent and widely utilized for the allylation of a variety of hard and stabilized carbon nucleophiles. Both allenylpalladium and vinylpalladium complexes are chemically neutral, or very feebly nucleophilic, if any. Under conditions described below, however, these rr-allyl-, vinyl-, and allenylpalladium complexes alter their reactivity in mode and behave as nucleophilic species. "... 

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