Oxidation of Olefins by Palladium(II). 15.¹ Oxidation of (R)-(−)-(Z)- and (R)-(+)-(E)-3-Penten-2-ol Using Several Nucleophiles To Give Chiral β-Substituted Ketones. A Method of Finding Modes of Palladation of Olefins Using Chirality Transfer

Abstract

The oxidation of (R)-(−)-(Z)- and (R)-(+)-(E)-3-penten-2-ol with Pd(II) using hydroxyl, methoxyl, acetate, and phenyl as nucleophiles produced β-substituted ketones. The ketone products are optically active, indicating the hydroxyl group was preferentially directing the Pd(II) to one of the two faces of the double bond in forming the initial π-complex. The absolute configuration of the product depends on the absolute configuration of the starting alcohol, the π-complex face to which the nucleophile adds, and the stereochemistry of addition. Because of steric interactions between the two terminal methyl groups, one π-complex is more stable than the other. Phenylpalladium, a reagent which is known to add syn, gave (R)-4-phenyl-2-pentanone, a result that is consistent only with syn addition to the most stable π-complex. With this information, determination of the stereochemistries of addition of the other nucleophiles is possible. In aqueous solution and methanol at low [Cl-], the 4-hydroxy, and 4-methoxy-2-pentanone products had the R configuration, indicating syn addition. These results are consistent with recent stereochemical and kinetic studies. A surprise was the finding of the R configuration, indicating syn addition, for the 4-acetoxy-2-pentanone product in acetic acid solvent. Previous kinetic studies were interpreted in terms of anti addition. Another unexpected result was the reversal of addition to anti at high [Cl-] in acetic acid. As expected, the Z-isomer gave higher degrees of chirality transfer than the E-isomer.

Electrophilic Activation of Alkenes by Platinum(II): So Much More Than a Slow Version of Palladium(II)

“…Only products that resulted from cation formation at the tertiary carbon atom were observed, for example the conversion of b-citronellene into a-thujane in Equation (34). [120] Experiments that employed benzyl alcohol as a cation-trapping nucleophile supported the mechanistic proposal, as d-benzyloxyalkylplatinum species were formed reversibly (Scheme 16).…”

Migratory Insertion of Alkenes into Metal–Oxygen and Metal–Nitrogen Bonds

“…At high chloride ion concentration, products from anti oxypalladation were observed, but at low chloride ion concentration the stereochemical configuration of the observed products indicated that syn oxypalladation, presumably by migratory insertion of an alkene into a metalalkoxo bond, occurred. [34][35][36][37][38][39] Because two sets of products with different relative configurations were observed, the hydroxylation of allylic alcohols must occur by two different mechanisms. Despite these results, the mechanism of the Wacker process was described in leading textbooks as occurring by the attack of a free water molecule on a palladium-bound ethylene.…”

Palladium(II)-catalyzed dicarboxymethylation of chiral allylic alcohols: chirality transfer affording optically active diesters containing three contiguous chiral centers

“…Thus, the hydroxyl group directs the palladium to the face that produces the most stable π-complex 4, which in turn depends on the absolute configuration of the starting allylic alcohol 1. 4,[10][11][12][13] Addition of carbon monoxide to complex 4 followed by insertion of methanol yields the olefin-carbomethoxypalladium intermediate 5, which undergoes insertion of the carbomethoxy group to produce the σ-complex 6. 14,19 Adduct 6 then either undergoes further syn addition of CO to give desired 2, or loses a proton and eliminates palladium yielding an enol which tautomerizes to ketone 3.…”

“…The directing influence of the hydroxyl group has been demonstrated for a number of reactions of chiral allylic alcohols, [1][2][3][4][5][6][7][8][9] and stereoselectivities of over 98%…”