Oxidative addition of allyl halides

This type of addition takes place in the case of complexes in which the metal possesses a low oxidation state. Coordinatively saturated complexes (18e) or compounds with a coordination number lower than the maximum (e.g., 16c) may be utilized  

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Chlorination of the Cp Ru(amidinate) complexes is readily achieved by treatment with CHCI3, while oxidative addition of allylic halides results in formation of cationic Ti-allyl ruthenium(IV) species (Scheme 243). °... 

Similar to the formation of allylmagnesium chloride (25), the oxidative addition of allyl halides to transition metal complexes generates allylmetal complexes 26. However, in the latter case, a 7i-bond is formed by the donation of 7i-electrons of the double bond, and resonance of the n-allvl and 7i-allyl bonds in 26 generates the 7i-allyl complex 27 or (/ -allyl complex. The carbon-carbon bond in the 7i-allyl complexes has the same distance as that in benzene. Allyl Grignard reagent 25 is prepared by the reaction of allyl halide with Mg metal. However, the 7i-allyl complexes of transition metals are prepared by the oxidative addition of not only allylic halides, but also esters of allylic alcohols (carboxylates, carbonates, phosphates), allyl aryl ethers and allyl nitro compounds. Typically, the 7i-allylpalladium complex 28 is formed by the oxidative addition of allyl acetate to Pd(0) complex. 

The sequence is compatible with substitution on the allyl group . The anion LII can be obtained in situ by hydroxide deprotonation of HCo(CO)4 . The intermediate f -allyl complex LIII loses CO below RT to yield the > -complex LIV. Although the loss of CO can be monitored , isolated yields of LIV (66% ) and its simple derivatives are rarely stated. Triphenylphosphine can replace one carbonyl group in LIV affording LV . If a CO pressure is applied during the oxidative addition of allyl halides, the intermediate i -allyl complex inserts CO instead of forming a 7t-complex . 

Thus oxidative addition of allylic halides to (PPhj)4Pt in benzene yields ( -allyl)PtCl(PPh3)2 . If one of the phosphines is oxidized in acetone with HjOj, the neutral >/ -allyl complex XCVII is formed ... 

Oxidative addition of allylic halides to Pt(AsPh3)4 results in formation of the neutral (tj -allyOPtClfAsPhj) ... 

As mentioned above, the oxidative addition of allyl halides, carboxylates, ethers, and sulfides to Ru(0) complexes is a powerful tool for synthesizing jt-allylrutheni-um(II) complexes (Eqs. 5.7 and 5.8) [15, 16]. 

Other examples include the synthesis of jt-allylruthenium(IV) complexes by the oxidative addition of allylic halides to (CsR5)RuL2X (R = H, Me L= CO, PPhs) (Eq. 5.13) [22, 23]. 

An X-ray structure determination was carried out on (C5Me5)RuBr2(Jt-C3H5), and showed a pseudo-piano-stool structure with two Br atoms and two terminal carbons for the endo-Jt-allyl ligand located at the basal positions. A crystal mirror plane bisects the pentamethylcyclopentadienyl and jt-allyl ligands. The oxidative addition of allylic halides to (C5R5)Ru(CO)2X is reversible, since the reductive elimination of... 

A similar oxidative addition of allyl halides to zerovalent ruthenium complex, RuJj/ -cod)( 7 -cot), in the presence of PMe3 has been independently reported to give RuX(n-C3Hs)(PR3)3 (X = Br, Cl) (Eq. 5.11). 

Oxidative addition of the carbon-halogen bond is a well-documented reaction for Group 10 transition metal complexes, but it is relatively limited for ruthenium. The example given here involves the reversible oxidative addition of allyl halide to RuCp(CO)2X to produce RuCp(p -allyl)X2 [78]. Oxidative addition of allyl halide to a Ru(0) complex Ru(l,5-COD)(l,3,5-COT) is also reported, but the product yield was poor [79]. Nevertheless, a catalytic Heck-type alkenylation of bromostyrene with methyl acrylate by Ru(l,5-COD)(l,3,5-COT) proceeded smoothly [80]. A cross-coupling reaction of alkenyl halide with Grignard reagents or alkyl lithium also pro-... 

Scheme 5.3.3 Stereochemical course of the oxidative addition of allyl halides with a halogen directly attached to a stereogenic carbon...

The alkoxytitanium propene compound Ti(T] -propene)(OTr)2 (46) [153], which is believed to be generated from Ti(0 Pr)4 and two equivalents of /-PrMgCl, reacts with internal alkynes to give titanium-alkyne compounds Ti(ri-alkyne) (0 Pr)2 (47) in quantitative yield (Scheme 6.9) [154,155]. 46 reacts with carboxylic esters to produce cyclo-propan- 1 -ols in modest yields [ 156,157]. Oxidative addition of allyllic halides or allyllic alcohols to 46 proceeds readily to form allyl titanium compounds 48, whose reaction with aldehyde provides a stereoselective synthesis of homoallylic alcohols [153]. 

Although the reaction mechanism is not clear at present, the intermediate formation of allylbismuth species through the oxidative addition of allylic halide to Bi(0) generated in situ has been proposed [90BCJ1738], The generation of metallic bismuth by the reduction of bismuth chloride with metallic zinc is known [58DOK(122)614]. A plausible catalytic cycle with... 

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 ... 

Similar to the formation of allylmagnesium halide, the oxidative addition of allyl halides to Pd(0) complexes generates allylpalladium complexes 15. However, in the latter case, the zr-bond is formed by the donation of r-electrons of the double bond, and resonance of the cr-allyl and tt-allyl bonds in 15 generates the tt-allyl complex 16 or -allyl complex. The carbon-carbon bond in the tt-allyl... 

The kinetics of the related oxidative addition of allyl halides (allY) to fac-[MX(phen)(CO)3] (M = Mo or W) have not been measured but the product is [MX(773-all)(phen)(CO)2l and Y". This would support the loss of halide to solution from the tj -allyl halide in the oxidation step rather than its transfer directly to the metal atom. 

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