Oxidative addition complexes

A significant number of Ir111 complexes arise from the oxidative addition reactions of Ir1 species. Such reactions may proceed via routine addition, whereas some proceed by ligand expulsion in conjunction with oxidative addition. Complexes containing Ir111 have a low-spin d6 electronic configuration, and are usually to be found with an octahedral-based ligand set. 

Significantly, however, if the C02 is introduced into the solution after formation of the oxidative addition complex with acetonitrile, C02 insertion does not proceed. This is positive evidence that the mechanism of insertion in this particular reaction does not involve direct insertion into the metal-carbon bond. 

Many oxidative addition complexes are stable because four and higher coordination inhibits decomposition via de-insertion and elimination. For benzylhalides that are pendant to a polymer main chain, compounds that exhibit T 3-bonding to the benzyl group are possible. The small molecule analog is the T 3-benzylpalladium chloride dimer. This complex is neither electronically nor coordinately saturated and is therefore not inert to ligand addition. 

When the Stille reaction is carried out under a CO atmosphere, the carbonylative coupling proceeds in a manner similar to that described for the Suzuki reaction namely, carbonyl insertion into the Pd-C bond of the oxidative addition complex. Transmetalation, followed by ds-trans-isomerization and reductive elimination, generates the ketone product. " ... 

Apparently this is due to the decomposition of the addition complex to form a carbonyl compound, followed by subsequent reaction of the carbonyl compound with the Grignard reagent. Acetone, for example, is one of the products formed when the propylene oxide addition complex is heated.22... 

Graham has reported that irradiation of (r 6-C6Me6)Os(CO)2 in alkane solution leads to the formation of alkane oxidative addition complexes in competition with C6Me6 loss [97]. Perutz and Werner have also reported the photochemical reaction of (r 6-mesitylene)Os(CO)H2 in methane matrices leading to the formation of the methane activation product (ri6-mesitylene)Os(CO)(CH3)H [98]. 

The discrete steps and related processes of importance to the synthetic chemist, such as catalyst generation, decomposition of oxidative addition complexes, and the key aspects of regiocontrol with regard to insertion in particular, but also double bond migration and elimination, will be discussed in some detail below. 

Arylation of butyl vinyl ether with an arylpalladium chloride complex predominantly delivers the terminally arylated product (as expected if a neutral oxidative addition complex is involved). Internal arylation of the highly polarized vinyl ether dominates with the corresponding iodide precursor, suggesting that in this case the cationic intermediate is more plausible as the key intermediate. ... 

B.iii. Side Products from Aryl M ation in the Oxidative Addition Complex... 

Frequently, a considerable amount of side products, derived from facile aryl-aryl exchange in the oxidative addition complex, is formed in a Heck reaction executed in the presence of phosphine hgandsJ t This process is particularly significant at higher reaction temperatures and with electron-rich aryl halides. Thus, a reaction of 4-bromoanisole with butyl acrylate with Pd(OAc)2/PPh3 as the catalyst system and sodium acetate furnish butyl E-cinnamate in addition to the expected coupling product (Schente... 

Notably it was demonstrated in 1987 that oxidative addition complexes, substituted with strongly electron-withdrawing groups, 02NPh(PPh3)2l, 02NPh(PPh3)2Br, and... 

If the counterion (X) in the oxidative addition complex is iodide or bromide (and no thallium or silver salts are present) the dissociation of one of the phosphorus atoms in the bidentate ligand from the metal is probably attributed to the relatively high trans effect exerted by the halidesJ This reversible displacement facilitates formation of a neutral rr-complex, in which the rr-system of the electron-rich alkene is only weakly polarized. Therefore, after insertion and hydridopalladium halide elimination, a larger fraction of /3-arylated product is formed, since steric factors always favor terminal ary-lation. 

Figure 3.7 (a) Aryl scrambling in the triphenylphosphine-modulated Mizoroki-Heck vinylation of 4-bromoanisole. (b) Aryl migration In the oxidative addition complex. 

On the other hand, the oxidative addition of aliphatic acid chlorides occurs in the absence of alkyne, but the oxidative addition complex could not be isolated due to fast decarbonylation followed by facile /1-hydrogen elimination. The decarbonylation of carboxylic acid was reported with palladium catalysts as well [47-56], In general, the reactions to acid anhydride as the intermediate need relatively high temperatures. 

The formation of an Ni(I) intermediate, instead of a classical Ni(0)/Ni(II) couple, in the silane reduced Ni(cod)2/PCy3-catalysed (cod = 1,5-cyclooctadiene, Cy = cyclohexyl) cleavage of inert C-O bonds is experimentally and theoretically supported by various spectroscopic studies of the reaction mixture. In the absence of silane, the / -hydride elimination from the oxidative addition complexes leads either to Ni(0)-CO or to Ni(0)-CHO complexes. Water deactivates the Ni catalyst by forming Ni-bridged hydroxo species. ... 

Expanding upon Youngs discovery, the group of Kawase developed a Ni-mediated reductive cyclization of 2-bromoarylacetylenes (Scheme 2.10) [40]. Optimized conditions required one equivalent of a Ni(II) complex and excess Zn dust. A variety of substituted dibenzopentalenes 14e-h could be prepared in adequate yields (13-46%), most notably with aryl groups at the 5,10-positions of the pentalene system. The intermediate arylnickel(ll) oxidative addition complex could be isolated in the absence of Zn. Upon heating the Ni complex in toluene, the dibenzopentalene derivative was produced in 83% yield. Terminal alkynes and acetylenecarboxylates gave complex mixtures under the reaction conditions. 

The relative reactivities of the most common aryl halides and pseudohahdes, regarding the oxidative addition, foUow the trend Arl > ArOTf > ArBr > ArCl > ArOTs. However, these trends can be altered selecting an appropriate catalytic system. For instance, Fu and coworkers demonstrated that the combination of Pd(dba)2 and P(t-Bu)3 selectively reacted with ArCl in preference for ArOTf [40]. Potentially, this could be explained by a more stable oxidative addition complex obtained from ArCl compared to the ArOTf, comprising the weakly coordinating triflate counterion, provided that the oxidative addition is indeed reversible in such cases. Moreover, oxidative addition complexes obtained from aryl chlorides are known to dimerize when isolated, which could possibly stabilize the T-shaped triligated palladium(ll) intermediate even further [36]. 

The coordination and subsequent assimilation of the nucleophilic coupling partner to the discrete oxidative addition complex generating a new diorganopalladium(ll) species represent the next step in the catalytic cycle. Depending on the type of the catalytic process and the incoming nucleophile two major pathways are typically considered transmetallation or carbopalladation. In the following a short presentation and description of the two distinct reactions will be given. 

In order to examine the influence of the intermediary cationic nickel(II) complexes on the elementary reactions constituting the catalytic cycle, a DFT study was performed [45]. Ni(DPPP), resembling the catalyticaUy active DPPF derived nickel(O) catalyst, was chosen as the starting model complex in combination with phenyl triflate and ethyl vinyl ether. The bisphosphine nickel(O) complex coordinated to the phenyl triflate prior to the oxidative addition (complex I) was selected as set point. In the following, the free energies are given for the total system relative to this complex, (see Computational Details for further information). 

The catalytic cycle of this reaction has been shown in Scheme 40.5. At first, the Pd(0) catalyst reacts with the aryl bromide to generate the Pd(Ar)Br complex 19 via a simple oxidative addition. Complex 19 reacts with the amino alkene in the presence of NaO Bu base to generate the Pd (Ar)amido complex 21, which after subsequent alkene insertion into the Pd N bond followed by C C bondforming reductive elimination leads to the formation of substimted pyrrolidine derivative 23 along with the regeneration of the Pd(0). 

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