Propargylpalladium intermediates

Two reactions of type IV are known. They proceed via the propargylpalladium complexes 13. Formation of alkynes 14 by direct displacement of Pd in 13 with hydride is one reaction. Elimination of Pd—H species from 13 to form the enyne 15 is another reaction which proceeds via the propargylpalladium intermediate 13. 

Tsuji J (2002) Overview of the Palladium-Catalyzed Carbon-Carbon Bond Formation via jr-Allylpalladium and Propargylpalladium Intermediates. In Negishi E, de Mei-jere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 1669... 

Scheme 11.6 Hydiogenolysis of propargylpalladium intermediate 16 to form alkynes.

Tsuji, J. Palladium-catalyzed nucleophilic substitution involving allylpalladium, propargylpalladium, and related derivatives the Tsuji-Trost reaction and related carbon-carbon bond formation reactions overview of the palladium-catalyzed carbon-carbon bond formation viart-allylpalladium and propargylpalladium intermediates, in Handbook of Organopalladium Chemistry for Organic Synthesis (ed. Negishi, E.-L), 2, 1669-1687 (John Wiley Sons, New York, 2002). 

When propargyl carbonates 195 are treated with a Pd catalyst in the absence of other reactants, (S-H elimination from the propargylpalladium intermediates 196 occurs to give conjugated enynes 197. Formation of cumulative 1,2,3-alkatrienes 199 from the allenylpalladiums 198 does not take place. Preparation of the conjugated ene-yne-ene system 201 in high yield from the propargyl carbonate 200 is an example [51]. 

V.2.1.1 Overview of the Palladium-Catalyzed Carbon-Corbon Bond Formation via TT-Allylpalladium and Propargylpalladium Intermediates... 

The Pd(0)-catalyzed reactions of propargylic compounds can be understood by the following mechanistic considerations. The first step in the catalytic reactions is the oxidative addition of a propargylic compound to Pd(0) species to form an inteimediate complex. By this oxidative addition, Pd(0) is oxidized to Pd(II). The intermediate Pd(II) complex undergoes further reactions with other reactants. Complex formation by stoichiometric reactions of propargylic chlorides 2 and 4 with Pd(Ph3P)4 has been studied, and the o-allenylpalladium 3 and the propargylpalladium (or c7-prop-2-ynylpalladium) 5 were isolated as yellow powders (Scheme 11.2) [2]. The allenylpalladium chloride 3 is... 

Pd(0)-catalyzed reactions of various propargyl compounds proceed by the formation of either the cr-allenylpalladiums 1 or the propargylpalladiums (or cr-prop-2-ynylpalladiums) 2 as intermediates. 

In a similar area, Tsutsumi et al. [10] had previously looked at cross-couplings between propargyl electrophiles and organotins that led to both alkynes and aUenes they invoked the intermediacy of tj -propargylpalladium species (with Pd/Phj = 1 1) (Scheme 6.5), as weU as the -propargyl or Tj -aUenyl intermediates (with Pd/Ph3P=l 4). 

Weak base such as cesium carbonate has been utilized in this reaction to generate diazo compounds in situ from tosylhydrazones through the Bamford-Stevens reaction. The reaction is initiated by palladium-promoted decarboxylation of propargylic carbonate to form propargylpalladium complex A, which then tautomerizes to afford allenylpalladium intermediate B. Subsequently, the common carbene formation-migratory insertion-p-hydride elimination occurs to afford various vinylallenes (Fig. 30). 

---