Rearrangement Pd -catalyzed

The bisphosphane BPA was selected as a ligand for the Pd atom because of the high enantioselectivities which had been recorded in the allylation of sulfinate ions with racemic allylic carbonates (vide supra). Besides the variations of the carbon skeleton, the substituent on the S atom of the racemic allylic sulfinates was varied in order to see whether both aryl and alkyl sulfinates are amenable to a highly selective rearrangement. 

---

Pd-catalyzed isomerization of ynones to furans has been an active area of research over the last decade. Huang et al. described a Pd-catalyzed rearrangement of a,P-acetylenic ketones to furans in moderate yield [102], For example, Pd(dba)2 promoted the isomerization of alkyne 124 to a putative allenyl ketone intermediate 125, which subsequently cyclized to the corresponding furan 126. 

Utimoto et al. synthesized substituted furans using a Pd-catalyzed rearrangement of easily accessible P/y-acetylenic ketones [103]. One plausible pathway is illustrated here using the transformation of p,y-acetylenic ketone 127 to 2,5-disubstituted furan 128. Enolization of 127 is followed by an intramolecular oxypalladation of the resulting enol 129 to form furylpalladium(II) species 130, which is subsequently treated with acid to give furan 128. 

The Pd-catalyzed rearrangement of the acyclic S-t-butylsulfinates roc-llaa and rac-llba proceeded quantitatively at room temperature and gave the allylic sulfones 4aa and 4ba with high enantioselectivities in high yields (Scheme 2.1.4.14 and Table 2.1.4.11, entries 1 and 2) [17]. 

The Pd-catalyzed allylic alkylation of sulfinate ions, thiols, and thiocarboxylate ions with racemic cyclic and acyclic allylic esters in the presence of bisphosphane BPA generally provides for an efficient asymmetric synthesis of allylic sulfones, sulfides, and thioesters. The Pd-catalyzed rearrangements of allylic sulfinates and allylic O-thiocarbamates, both of which proceed very efficiently in the presence of BPA, are attractive alternative ways to the asymmetric synthesis of allylic sulfones and allyUc thioesters also starting from the corresponding racemic alcohols. 

The Pd-catalyzed rearrangements most probably follow an ionization-substitution pathway with the intermediate formation of a jc-allyl-Pd complex. The Pd-catalyzed allylic alkylation is generally accompanied by a highly selective kinetic and synthetically useful resolution of the racemic allylic acetates and carbonates. 

In another study, Bienayme [65] obtained retinal in three steps from P-ionone, involving a Pd-catalyzed rearrangement of a mixed carbonate, derived from ethynyl-retro-ionol. 

Thus, the (3-ionone was smoothly deconjugated and ethynylated to give ethynyl-retro-ionol as a mixture of E/Z stereoisomers. Formation of the carbonate and its Pd-catalyzed rearrangement produced straightforward a mixture of aldehydes and a allene compound. After silica-gel chromatography, the allenic-aldehyde was conjugated with a catalytic amount of HBr in acetone. Retinal was obtained as a mixture of E and Z isomers (75/25), which could be converted into the all E isomer by simple equilibration, Fig. (33). 

Transition metal mediated cross couplings of epoxides have remained relatively unexplored, with only a few examples of this potentially useful reaction reported in the literature. A recent report details the Suzuki-Miyaura cross-coupling of epoxides <07JOC3253>. The reaction of aryl epoxides with arylboronic acids under Suzuki-Miyaura coupling conditions provides the coupled product in good yields. Careful monitoring of the reaction is essential to avoid Pd-catalyzed rearrangement of the epoxides. 

Scheme 19 Highly enantioselective Pd-catalyzed rearrangement of allylic sulfinates to allylic sulfones...

The ketenimine (93) reacts only at the C=N bond to give pyrroles and pyrrolines with methylene migration (equations 102,103). Carbon (hoxide codimerizes smoothly with methylenecyclopropanes to yield lactones with alkene isomerization (equations 104, lOS). The Pd -catalyzed rearrangement of y-butyrolactone (94) to butenolide (95) indicates that this CO2 insertion reaction is reversible. Hiis is fur-ter supported by the observation that lactone (94) can serve as a TMM precursor in the codimerization with aldehydes and electron-deficient alkenes (equations 106, 107). No cycloaddition to ketenes or simple ketone carbonyls has ever been reported. 

Epoxides of a,p-unsaturated ketones can undergo a ring opening/arylation reaction under Heck conditions <05JOC4720>. Epoxide 62 undergoes an initial Pd-catalyzed rearrangement to a 1,2-cyclohexanedione which then undergoes a Heck arylation reaction to... 

Rearrangements. Allylic sulfoximines undergo Pd-catalyzed rearrangement to give the tosylamides. The reaction furnishes optically active amine derivatives from chiral sulfoximines. 

Linton and Kozlowski have installed quaternary centers at oxindole C3 in enantioselective fashion via the Pd-catalyzed rearrangement of 2-allyloxy indoles (Scheme 2) [16]. For example, indole 7 underwent an enantioselective Meerwein-Eschenmoser-Claisen rearrangement in the presence of Pd(SbF6)2 and the chiral phosphinooxazoline ligand 8 to afford oxindole 9 in 89% yield and 89% ee. A two-point coordination of the chiral palladium catalyst to the C3 carbonyl and C2 oxygen (6-membered coordination system) has been proposed to rationalize the enantioselectivity of the transformation. Modest to good enantioselectivities were also observed for a series of bisphosphine chiral ligands. 

The allenyl alcohol without the iodophenyl moiety 62 alone undergoes the Pd-catalyzed rearrangement-ring expansion to give the isoquinolinedione 63 [11]. 

Allenyl a,/3-unsaturated isoprenoid aldehydes can be prepared in high yields by the rearrangement of propargyl enol carbonates (Scheme 28). The dienyl propargyl carbonate was prepared from /3-ionone and subjected to Pd-catalyzed rearrangement. The allenyl alde-... 

For some years, we have been investigating the Pd-catalyzed rearrangement of chiral silylated vinyl epoxides. Depending on the substituents attached to the silyl group and the ligands borne by the palladium, a very fast, totally stereoselective 1,2-carbon to carbon transfer of the silyl group was observed. AppUcation to the enantioselective preparation of a,)8-unsaturated lactones has been developed (Scheme 4). 

Involvement of Pd-induced formation of dipolar intermediates has also been imph-cated in other Pd-catalyzed rearrangements. The dipolar intermediates in these reactions often contain carbocationic centers, which can participate in skeletal rearrangements. Those involving net oxidation are presented in Sects. V.3 and VIII.4, while those that lead to net isomerization are discnssed in Sect. IX.2.1.2. 

In principle, there can be many other Pd-catalyzed rearrangement reactions. At present, however, the overall scope of Pd-catalyzed rearrangement reactions is still rather limited. In one sense, lack of skeletal rearrangements in some Pd-catalyzed reactions, such as Wacker processes, where cationic species may be implicated as transient species, is considered to be a very desirable feature of organopalladium chemistry. On the other... 

Scheme 52 Pd-catalyzed rearrangement of 2-benzylidenecylopropyl-carbinols and 2-benzylidenecylopropyl-ketones...

---