Stereochemistry of oxidative addition

Accordingly, the rate of oxidative addition of H2 is relatively insensitive to electronic factors, notably the electron-donating power of L, whereas the rates of oxidative addition of organic halides increase markedly with the electron donating ability of L and (in view of the polar transition state) with solvent polarity. The different stereochemistries of oxidative addition, i.e., cis for H2 vs. traits for RX, also reflect this mechanistic distinction. The influence of variation of the metal on the rate of oxidative addition is less well documented and understood, but available data suggest the reactivity sequence, Ir(I)(5d ) > Co(I)(3d ) > Rh(I)(4d )l... 

The stereochemistry of oxidative addition to the palladium(II) palladacycle was studied by Lautens using an enantioenriched secondary alkyl halide (Scheme 9) [32], From alkyl halide 23, product 24 was obtained, showing a net inversion of stereochemistry [33-35], Previous work by Stille showed that reductive elimination from palladium(IV) occurs with retention of stereochemistry [36], suggesting that oxidative addition occurs with an inversion of stereochemistry. This corresponds with the generally accepted SN2 mechanism for the reaction of palladium(O) with alkyl halides [37, 38],... 

Scheme 1.10. Stereochemistry of oxidative addition of aUylic compounds to L M to give allyl complexes and nucleophilic attack on the -aUyl ligand.

Stereochemistry of oxidative addition of allylic acetate to a Pd(0) complex was unequivocally confirmed by using optically active substrate (Scheme 3.22) [42], The oxidative addition resulted in the formation of enantiomerically pure r/ -allylpalladium(II) complex showing inversion of configuration. Further treatment of the isolated r) -allylpalladium(II) complex with sodium dimethyl malonate led to the alkylation with inversion of configuration. Consistently, the catalytic reaction gave net retention product. This is a direct evidence for the stereochemistry of oxidative addition and alkylation. 

Platinum(n) to Platinumljw). The stereochemistry of oxidative addition to the platinum(n) complexes [PtMeaCLL)] has been determined for LL = the bidentate arsenic ligands (9) and (10). The oxidative addition of methyl halides gives the 1,2,3-or /ac-trimethylplatinum(iv) compound (11) it is not possible to choose between cis and traits addition from this observation. 

In the carbohydrate chemistry arena, the Tsuji-Trost reaction has been applied to construct N-glycosidic bonds [53]. In the presence of Pd2(dba>3, the reaction of 2,3-unsaturated hexopyranoside 68 and imidazole afforded N-glycopyranoside 69 regiospecifically at the anomeric center with retention of configuration. In terms of the stereochemistry, the oxidative addition of allylic substrate 68 to Pd(0) formed the jc-allyl complex with inversion of configuration, then nucleophilic attack by imidazole proceeded with another inversion of the configuration. Therefore, the overall stereochemical outcome is retention of configuration. 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

The stereochemistry of the addition has been recognized on the basis of structural features and chemical behavior of the complexes 9, produced in selected reactions of j with 1-ethoxy-1-oxo-2-phospholene 8. Neutralization of these complexes gave exclusively cis-3-substituted phospholane oxides J 0 while alkylation resulted in stereoselective formation of cis-3, cis- -disubstituted phospholane oxides JJ. The progress of the latter reaction was strongly dependent on the size of the alkyl groups involved. 

In this bicyclic case the palladium and methoxyl groups are trans to each other 1X>. A cis stereochemistry would have been expected on the basis of the ethylene oxidation mechanism. Trans-addition, however, is unusually favorable in the bicyclic examples. Although addition to the exo positions is generally strongly preferred, it cannot occur here if the favorable chelating effect of the second double bond is to be obtained. As a result, only the solvent methanol can attack from the exo side. The endo cis adduct has not been prepared and it conceivably could rearrange to the trans isomer even if it were formed initially. Clearly, more work needs to be done on the stereochemistry of the addition reactions. 

Though the stereochemistry for oxidative addition of allylic halide had been rather ambiguous until 1990s, it was found to vary depending on the reaction conditions. Cyclic allylic carboxylates were mainly employed to obtain the stereochemical information for the oxidative addition of allylic electrophiles in general (cf. Section 3.3). As shown in Eq. 3.3 the oxidative addition of the trans allylic chloride to Pd2(dba)3 in a polar solvent such as acetonitrile or DMSO dominantly gives the cis product (trans/cis = 3/97 in DMSO), suggesting conventional Sn2 ... 

Since the total reaction is basically initiated by generation of an organic radical, the reaction rate is closely related to the stability of the resulting radical. This type of oxidative addition therefore takes place rapidly for tertiary alkyl halides and quite slowly for tosylate regardless of steric repulsion. The stereochemistry of the O -carbon is completely lost. When the radical ion pair is not stable enough in the cage, the radical escapes to the solution leading to radical chain process... 

The use of threo and erythro alkyl isomers can be extended and in some cases the preparative aspects improved by the use of other groups on the a-carbon besides H and D. For example, RR, SS) and RS, SR)-C6H5(CHF)(CHD)Br have been used in studies of oxidative addition. Although a secondary carbon is introduced, 5/ )-Ph(CHF)-(CHBrC02Et) has also been used successfully. Both of these systems, however, have the potential of interpretational problems arising if apparent retention is observed. Cis- and trans-substituted cyclohexyl groups provide an opportunity to utilize well-known coupling patterns for the study of stereochemistry.Thus, the oxidative addition of cyclohexyl bromides to [ py) dmg)2Co l)] proceeds with inversion. ... 

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