Heck reaction, palladium chemistry phosphines

Studies regarding the nature of the catalytically active species for NHC complexes in Heck-type reactions have focused on the Mizorvki—Heck reaction and have consistently revealed a palladium(O) species as the active catalyst. The induction period is shortened upon addition of a reducing agent,and postulated intermediates of the reaction were isolated and characterized as well as employed in stoichiometric and catalytic reactions. Theoretical studies using DPT calculations showed the mechanism for NHC complexes to most likely he in agreement with phosphine chemistry. ... 

Regiocontrol is a major challenge in intermolecular Mizoroki-Heck reactions, and usually results in a mixture of Unear and branched products (cf. Section 1.2.2.3 on directed Mizoroki-Heck chemistry). Both, Cabri et al. [14] and Hayashi et al. [15] showed that the nature of the ancillary Ugand L has a pronounced effect on the regioselectivity when palladium is Ugated by bidentate phosphines (Scheme... 

In the quest for additional active catalysts for the Mizoroki-Heck reaction, the advent of N-heterocychc carbene (NHC) ligands was warmly welcomed [32], Transition metal-transition metal-phosphine complexes, and have therefore attracted considerable interest as competitive alternatives in Mizoroki-Heck chemistry, which requires high reaction temperatures. Since the seminal application of NHC ligands in Mizoroki-Heck arylations by Herrmann et al. [33], several research groups have introduced novel palladium catalyst-NHC ligand combinations. These were tested and assessed in standard couplings of simple iodo- or bro-moarenes 60 and activated acceptors such as acrylates 61 or styrene (63) [32], and a selection of impressive examples is summarized in Scheme 7.14. 

Besides halides and triflates, other electrophiles can be applied to Heck reactions. The first classical alternative was diazonium salts. Reactions proceed in the absence of phosphine (partly due to the fact that phosphines result in uncontrolled decomposition of the diazonium salt). The Heck reaction using these species can be useful in cases when mild conditions are required. Alternatively, iodonium salts behave in a similar manner to diazonium salts and show better tolerance to bases. " The reactions take place at ambient temperature and so are once again most useful in situations when mild conditions are required. Some main group metallie eompounds such as lead(IV) and thallium(III) have also been shown to undergo Heck-type chemistry and can be useful in specific cases. Of particular interest is the fact that acid chlorides and anhydrides can be employed in Heck chemistry, the use which was pioneered by Blaser and Spencer in 1982. " The process involves oxidative addition of palladium into the C-X bond followed by decarbonylation to yield the intermediate ArPdX species, de Vries has exploited this reaction, demonstrating the use of benzoic anhydride (105) as an effective arylating agent. ... 

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