Oxidative addition Polar

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

The general utility of the oxidative addition of functionalized organic halides to zinc was demonstrated by the formation of organozinc iodides 28 from protected (3- and 7-amino acids (Scheme 26).73 The organozinc iodides prepared in this manner were neither sufficiently stable nor sufficiently reactive in THF, but excellent yields were obtained in more polar aprotic solvents, such as DMF and DMSO. 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

For the related [CpIr(PH3)(CH3)]+ system, four basis sets were used. Basis set one (BS1) is the same as the ones described above for Ir and P, but the C and H are described as D95. Basis set two (BS2) is the Stuttgart relativistic, small core ECP basis set (49) augmented with a polarization function for Ir, and Dunning s correlation consistent double-zeta basis set with polarization function (50) for P, C and H. Basis set three (BS3) is the same as BS1 except the d-orbital of Ir was described by further splitting into triple-zeta (111) from a previous double-zeta (21) description and augmented with a f-polarization function (51). Basis set four (BS4) is the same as BS2 for Ir, P, and most of the C and H, but the C and H atoms involved in the oxidative addition were described with Dunning s correlation consistent triple-zeta basis set with polarization. 

To summarize, both chloropyrazines and chloroquinoxalines are sufficiently activated to serve as viable substrates for palladium chemistry under standard conditions. In contrast to chlorobenzene, the inductive effect of the two nitrogen atoms polarizes the C—N bonds. Therefore, oxidative additions of both chloropyrazines and chloroquinoxalines to Pd(0) occur readily. One exception is 2-chloropyrazine A-oxide, which does not behave as a simple chloropyrazine. All Pd-catalyzed reactions with 2-chloropyrazine A-oxide failed, presumably because the nitrogen atom no longer possesses the electronegativity required for activation. 

The effect of solvent polarity on the rate of the individual steps was also deduced from a comparison of the kinetics determined by IR. It was concluded that, comparing MeOH/Mel (80 20 v/v) with CH2Q2/MCI (80 20 v/v), the overall increase in rate of reaction of [Rh(CO)2l2] with Mel to give [Rh(C(0)Me)( CO)I] included contributions due to enhancement of the forward rates of both oxidative addition (ca. 50%) and migratory insertion (ca. 100%). 

A polar solvent like DMF is known to accelerate the oxidative addition of a C — I bond by low-valent metals, which is thought to be the preceding step for the formation of 2-iodovinylidene intermediates 77. Scheme 6.27 illustrates one application of this solvent-dependent chemoselectivity, that l-iodo-naphthen-2-ols 79 and 6-iodo-7-oxa-benzocydoheptenes 80 can be selectively produced in DMF and benzene using the same substrate. 

A widely used route for the synthesis of Grignard reagents is the oxidative addition of magnesium metal to organic halides in a polar, aprotic solvent like THE or diethyl ether (equation 1). 

The entries into transition metal catalysis discussed so far, required the presence of a specific bond (a polar carbon-heteroatom bond for oxidative addition or a carbon-carbon multiple bond for coordination-addition processes) that was sacrificed during the process. If we were able to use selected carbon-hydrogen bonds as sacrificial bonds, then we could not only save a lot of trouble in the preparation of starting materials but we would also provide environmentally benign alternatives to several existing processes. In spite of the progress made in this field the number of such transformations is still scarce compared to the aforementioned reactions. 

The above criteria apply in the case of isolated hydroxyl groups but when additional polar substituents are placed in the vicinity of the substrate hydroxyl the oxidation rate can be expected to change. Allylic hydroxyls are generally oxidized more rapidly than their saturated counterparts. Burstein and Rin-gold have studied the chromic acid oxidation of steroidal allylic alcohols in some detail and have found that the quasi-equatorial 3)3-isomer is oxidized more... 

If X—Y is an electrophilic polar molecule such as CH,I, oxidative addition reactions tend to proceed by SN2 mechanisms involving two-electron transfer (Eq. 15.99) or via radical, one-electron transfer mechanisms (Eq. 15.100). 

As expected for a nucleophilic silicon(II) compound, 82 does not react with triethylsilane, but with the electrophilic trichlorosilane to form the corresponding unsymmetrical disilane (equation 69)189. Other polar and non-polar substrates react in a similar fashion to give the corresponding oxidative addition products (equations 69188... 

A number of different polar and nonpolar covalent bonds are capable of undergoing the oxidative addition to M( ). The widely known substrates are C—X (X = halogen and pseudohalogen). Most frequently observed is the oxidative addition of organic halides of sp2 carbons, and the rate of addition decreases in the order C—I > C—Br >> C—Cl >>> C—F. Alkenyl halides, aryl halides, pseudohalides, acyl halides and sulfonyl halides undergo oxidative addition (eq. 2.1). 

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