Mel, oxidative addition

As noted above, <7-carbon complexes derived from HCo(CO)4 are of low thermal stability and most of the isolated examples contain phosphine and phosphite ligands. Thus Co(PMe3)4 is readily alkylated by Mel to MeCo(PMc3)4. With excess Mel, oxidative addition with loss of one phosphine to Me2CoI(PMe3)3 is found. Higher alkyls are subject to /3-elimination (see -Elimination). 

In order to accelerate the rate of Mel oxidative addition (and consequently the rate of the overall process), electron-donating ligands need to be used. The IR stretch vibration of the GO coordinated to the metal can be indicative of the electron density on the rhodium center, but steric effects play a key role when relating it to reactivity versus Mel. 

Another family of diphosphines displaying a // 7/7i--coordination mode, SPANphos 63a-d, has also been tested in this reaction.Surprisingly, even if the corresponding frans-[Rhl(CO)63] render systems as active as the Siiss-Fink analogs, when tested in an elementary step, they show no reaction in the oxidative addition of Mel (vide supra). The observed catalytic activity has been attributed to dinuclear species formed under catalytic conditions. The dinuclear compounds [Rh2(/x-Cl)2(CO)263a], 87, react with Mel at 25 °G with a kx value of ca. 0.025 s M and currently they represent the fastest phosphine-based systems reported for methanol carbonylation. According to spectroscopic and GC-MS analysis, the products of the Mel oxidative addition are dinuclear monoacetyl derivatives 88 (Scheme 11). 

The reaction with Mel proceeds in two stages. Initial reaction is oxidative addition to give a rhodium(III) species, isolated as a Mel adduct... 

R, e.g. cyclohexyl, p-tolyl) readily undergo oxidative addition with molecules like Cl2 and Mel to give iridium(III) complexes... 

Insertion of CO into Ti—C bonds has very recently been reported for reaction of CpiTifCHjPhlj with CO and for oxidative additions of Mel and EtI to Cp2Ti(CO)2. See refs. (89a) and (90a), respectively. 

Oxidative addition of methyl iodide to coordinatively unsaturated, low-oxidation state d and [Pg.321]

Photochemical oxidative addition of (303) with I2 Mel or CH2I2 gives dinuclear Ir11 complexes (304).493 The crystal structure of (304), R = I, R = CH2I, is reported. 

In the carbonylation of MeOH in the presence of Rh-exchanged zeolites, the Rhm ions are reduced to Rh1 ions, which lead to Rh-dicarbonyl and Rh-carbonyl-acetyl complexes.29-32 IrY and RhY zeolites catalyze the carbonylation of MeOH in the presence of a Mel promoter. The kinetics have been determined and IR spectra suggested that with the Ir catalyst the ratedetermining step was the addition of MeOH to the active species followed by migration of a Me coordinated to Ir. With the Rh catalyst, oxidative addition of Mel was the rate-determining step.33 A series of EXAFS measurements was made to determine the structural basis for... 

The Ge(TMTAA) complex and the well known Sn(TMTAA) complex undergo facile oxidative addition reactions and reverse ylide formation with Mel and C6F5I because of the reactive M(II) (M = Sn, Ge) lone pair of electrons. In case of the oxidation with Mel it was assumed that, in solution, an ionic-covalent equilibrium exists (equation 48)95. 

As for tin, two potential routes to methyllead compounds might appear to exist (1) Me carbanion attack on lead(II) (e.g. by Me- from MeCoBn) followed by dismutation and (2) oxidative addition by a Me+ carbonium ion from e.g. S adenosylmethionine or Mel etc. 

Fig. 23. Volume profile for the combined oxidative addition and reductive elimination reaction [PdMe2(bpy)] + Mel — [Pd(I)Me3(bpy)] — [Pd(I)Me(bpy)J + C2H6.

Evidence has been presented that iodide salts can promote the oxidative addition of Mel to [Rh(CO)2l2]"> the rate-determining step in the Rh cycle [12]. The precise mechanism of this promotion remains unclear formation of a highly nucleophilic dianion, [Rh(CO)2l3]2 , has been suggested, although there is no direct spectroscopic evidence for its detection. Possible participation of this dianion has been considered in a theoretical study [23]. An alternative nucleophilic dianion, [Rh(CO)2l2(OAc)]2 , has also been proposed [31,32] on the basis that acetate salts (either added or generated in situ via Eq. 7) can promote carbonylation. Iodide salts have also been found to be effective promoters for the anhydrous carbonylation of methyl acetate to acetic anhydride [33]. In the absence of water, the catalyst cannot be maintained in its active form ([Rh(CO)2l2]") by addition of Lil alone, and some H2 is added to the gas feed to reduce the inactive [Rh(CO)2l4]. ... 

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the carbonylation cycle. 

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