Ancillary ligand

The Mizoroki-Heck reaction involves an immense variety of ancillary ligands, since it has been a matter of common agreement that phosphines play a distinct role different from any other ancillary ligand. Heck himself introduced two major types of catalytic system (1) phosphine-free for aryl iodides [2] (2) using PhsP [6, 7] or (2-tolyl)3P [8] for aryl bromides. This dichotomy has laid the basis for further division of all new protocols into those requiring phosphine ligands (phosphine assisted) and those that are phosphine free (quite often erroneously referred to as ligand free). 

The analysis of hundreds of protocols published so far, however, prompts a conclusion that this dichotomy is rather misleading, as it does not reflect the real differences of catalytic systems. Thus, the analysis of the role of ligands should be more refined. The roles of ancillary ligands can be categorized into four major divisions  

Non-nucleophilic bases (/-Pr2NEt, CyaNEt and proton sponge) are widely used in type 3 and type 4 processes, where the base should perform only as a Brpnsted base and where various side-effects associated with Lewis basicity, hgation and nucleophilicity are considered to be detrimental. 

On the other hand, bases possessing high Lewis basicity might serve as hgands in phosphine-free protocols. For example, tetramethylguanidine (TMG) and 1,4-diazabicyclo [2.2.2]octane (DABCO) were shown to improve yields markedly in phosphine-free reactions of aryl iodides, bromides and activated chlorides, compared with identical system without additive. Representative protocols PdCla (0.1 mol%), TMG, NaOAc, NfT-dimethylacetamide (DMA), 140 °C or Pd(OAc)2 (0.0001-5 mol%), DABCO, K2CO3, dimethylformamide (DMF), 120 °C. 

It should be noted that palladium(II) hydride intermediates, by all evidence, are quite acidic. Consequently, strong bases are not needed for deprotonation, and weak bases (e.g. NaHCOs) do quite well. In fact, it is well known that, in the absence of a base, the Mizoroki-Heck reaction does take place but is rapidly quenched after a few catalytic turnovers. From this, we might assume that it is not the deprotonation of Pd-H itself. 

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Heteroleptic complexes of uranium can be stabilized by the presence of the ancillary ligands however, the chemistry is dominated by methyl and benzyl ligands. Examples of these materials include UR4(dmpe) (R = alkyl, benzyl) and U(benzyl)4MgCl2. The former compounds coordinate "soft" chelating phosphine ligands, a rarity for the hard U(IV) atom. 

Another class of heteroleptic alkyl complexes contains TT-donating ancillary ligands such as RU[N(Si(CH2)3)2]3 (R = 4)- The hydride... 

Modem cross coupling chemistry is heavily dominated by the use of palladium and nickel complexes as the catalysts, which show an impressively wide scope and an excellent compatibility with many functional groups.2 This favorable application profile usually overcompensates the disadvantages resulting from the high price of the palladium precursors, the concerns about the toxicity of nickel salts, the need for ancillary ligands to render the complexes sufficiently active and stable, and the extended reaction times that are necessary in certain cases. 

Jafarpour, Laleh, and Nolan, Steven R, Transition-Metal Systems Bearing a Nucleophilic Carbene Ancillary Ligand from... 

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