Hydroarylation, of norbomene

The catalytic arylation of SEM-protected azoles (SEM = [2-(trimethylsilyl) ethoxy]methyl) with monocarbene-Pd(ll) complexes [72], and the reductive Heck or hydroarylation of norbomenes with NHC-phosphine chelating ligands... 

Ligands of type 1 have also led to interesting results in kinetic resolutions during Cu-catalyzed reactions of dialkyIzinc with cycloalkene oxides (eq. (1)) [18] and cyclohexenones [19]. On the other hand, monophosphonites bearing a fused 1,4-dioxane ring behave moderately in the Rh-catalyzed hydrosilylation of ketones (up to ca. 56 % ee) [20] cf. Section 2.6 Finally, a phosphapallada-cyclic complex has been reported as an exceptionally fast catalyst for the hydroarylation of norbomene (TONs up to 10 °) but with low ees (< 25 %) [21]. 

Cyclopropanone acetals with a quaternary carbon atom in chiral form can be established by addition of bisoxazoline-ligated allylzinc reagents to the cyclopropenes.The t-BuLi/(-)-sparteine combination favors Br-Li exchange and also promotes enantioselection in the intramolecular addition of aryllithium to an o-alkenyl side chain (e.g., indoline synthe-j.ijj) I28.I29 Moderate asymmetric induction is shown in the hydroarylation of norbomene in the presence of 63. 

Insertion of aUcynes into aromatic C-H bonds has been achieved by iridium complexes. Shibata and coworkers found that the cationic complex [Ir(COD)2]BF4 catalyzes the hydroarylation of internal alkynes with aryl ketones in the presence of BINAP (24) [111]. The reaction selectively produces ort/to-substituted alkenated-aryl products. Styrene and norbomene were also found to undergo hydroarylation under similar condition. [Cp IrCl2]2 catalyzes aromatization of benzoic acid with two equivalents of internal alkyne to form naphthalene derivatives via decarboxylation in the presence of Ag2C03 as an oxidant (25) [112]. 

Several other examples of palladium catalyzed asymmetric hydroarylations and hydroalkeny-lations of norbomene have been reported with up to 93% ee71,12. The same reaction type was used in stereoselective synthesis of epibatidine 1, a non-opiate analgesic alkaloid73. 

Alkene Hydroarylation. The enantioselective addition of aryl iodides to norbomene has been reported using a palladium(II) complex of (5, iS )-CHIRAPHOS. The reaction of norbomadiene with 4-methoxyiodobenzene proceeded with 30% ee (eq 5). Enantioselectivities were dependent upon phosphine structure (see (+)-trans-(2S,3S)-Bis(diphenylphosphino)bicyclo[2.2.1 ]hept-5-ene). 

The present section summarizes reductive Mizoroki-Heck-type arylations-that is, palladium-catalyzed hydroarylation reactions of alkenes-which are essentially limited to (hetero-)norbomenes (Section 7.3.2). It should be noted however, that only a handful of remarkable examples are known that are not based on the bicyclo[2.2.1]heptane framework (see Section 7.3.3). 

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