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Catalytic cycles ancillary ligands

A tentative catalytic cycle for ligand-accelerated Mizoroki-Heck reactions is shown in Scheme 2.14. A pivotal feature of this mechanism is the requirement to have neutral complex 4 with a single ancillary ligand, as related coordinatively saturated species 1 or... [Pg.75]

The mechanism of a typical cross-coupling reaction catalyzed by Pd° or Ni° complexes can be represented by a standard catalytic cycle (Scheme 1, shown for Pd ancillary ligands not given, for simplicity). [Pg.306]

Consequently, to narrow the definition a bit further, we will adhere to Negishi s suggestion and define cross-coupling reactions as those that follow some variation of the mechanism depicted in Scheme 1 (where Mt is a transition metal, L is an ancillary hgand see Ancillary Ligand), and n is the oxidation state of the reduced metal in the catalytic cycle). This mechanism is supported by stoichiometric studies on isolated metal complexes (mostly where Mt = Pd, n = 0, L = triphenylphosphine) thought to be the intermediates in this cycle. [Pg.5641]

Scheme 1 Presumed catalytic cycle for palladium-catalyzed C(sp )—E bond formation (E = NH, OH, and E). L = ancillary ligand(s) Ar—X=(hetero)aryl (pseudo)halide... Scheme 1 Presumed catalytic cycle for palladium-catalyzed C(sp )—E bond formation (E = NH, OH, and E). L = ancillary ligand(s) Ar—X=(hetero)aryl (pseudo)halide...
To this end, monodentate phosphine or bidentate PX (X=P, N, O) ligands have usually been employed as ancillary ligands for transition-metal-catalyzed reactions, with bulky tertiary alkyl phosphines proving particularly effective. Significant advances have been achieved in the use of less active aryl chlorides (bond strength C-Cl>C-Br>C-I) as chemical feedstock [5], with a number of processes mediated by palladium-bulky phosphine systems. This success is often explained by the effect of bulk and electron richness at the metal center along the catalytic cycle depicted in Fig. 1 [6]. [Pg.243]

The use of a flexible substituent (cyclohexyl) has presumably a beneficial role in the reductive-elimination step of the catalytic cycle by increasing the steric pressure on the metal center. Most aryl chlorides including sterically demanding ones can be converted to the corresponding biaryls at room temperature using this flexible ancillary ligand (see Fig. 5) [46]. [Pg.252]

It is important to stress that, in most other transition-metal-catalysed reactions, the control of catalytic processes is achieved through the choice, refinement and adjustment of stable ancillary ligands remaining in the coordination shell throughout all steps of the catalytic cycle. In those areas, the design of ligands is the essence of the art. The Mizoroki-Heck reaction, on the other hand, is very reluctant towards the control of catalytic activity via ancillary ligands. [Pg.54]

As discussed earlier, aU of the reactirais in eqs. (2-6) have been proposed to proceed via Pd catalytic cycles. Thus, it is of great interest and potential relevance to establish the viability of carbon-chalcogen bond-forming reductive eliminatirai from Pd centers. In addition, it is important to understand the effects of steric and electronic factors as well as the influence of ancillary ligands on the mechanisms of reductive elimination from Pd to design improved catalytic transformations. [Pg.64]

The role of the other ligands ("ancillary") is to avoid the precipitation of the metal and to insure a correct stereoelectronic balance (electronic density on the metal center, steric effect, trans effect) allowing all the individual reactions along the catalytic cycle to proceed at a good rate and with a good selectivity. [Pg.354]

A recent noteworthy example is the 14-electron metathesis catalyst 72 reported by Piers, which have no ancillary ligand to dissociate. " As expected, this catalyst initiates extremely rapidly, displaying initial rates of ring-closing metathesis (RCM) better than the bis-pyridine substituted Grubbs catalysts. Furthermore, reaction of the catalyst with 2.2 equivalents of ethylene at — 50°C in CD2CI2 allowed the observation and characterization of the metallocyclobutane 73 formed on expulsion of the vinyl phosphonimn salt (Equation (3.13)). " This represents the first observation of the generally accepted intermediate in the catalytic cycle. [Pg.103]


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