Methylation catalyzed by S-adenosylmethionine (AdoMet) dependent enzymes is one of the most common reactions occurring in biological systems. While a large number of methyltransferases which catalyze methylation at carbon, instead, presumably by means of b-H elimination. [7] As shown also in Scheme 3, activation of EtOPh by 1 (toluene, 110 8C, 2 h) gave compound 6 (see Supporting Information). It thus appears that the very important steric demands of the Tp x ligand (as compared with the somewhat less bulky cyclopentadienyl ligand, Cp x ) and its rigid nature, which tends to enforce six-fold coordination to the metal center, favors a-H elimination over b-H elimination in the transformations studied here.
In summary, we have shown that the Ir I compound 1 effects the regioselective, double CΓH activation of a variety of ether and amine substrates by allowing their precoordination to the metal center. The formation of Fischer-type carbenes in this cascade activation (cyclometalation-b-elimination (or vice versa)-a-elimination) is made possible by the presence of an adequate hydrogen-accepting leaving group. The Tp x -and Cp x -Ir systems reported herein include O and N heteroatoms. Comparative studies on the facility of these fundamental organometallic reactions (a-and b-eliminations) on closely related Tp x -and Cp x -Ir systems without these heteroatoms that could alter the relative energies of the transition states and intermediates, [17] are presently in progress and will be reported in due course.
Experimental Section
1: To a suspension of [(IrCl(coe) 2 ) 2 ] (175 mg, 0.20 mmol) in CH 2 Cl 2 (6 mL), 2-methylbutadiene (0.3 mL, excess) was added at room temperature to give a colorless solution. A solution of TlTp Ph (253 mg, 0.40 mmol) in CH 2 Cl 2 (6 mL) was then added. Stirring the reaction mixture for 4 h at room temperature resulted in the precipitation of TlCl. After filtration, the clear solution was evaporated to dryness. The crude product (orange powder) was purified via column chromatography (neutral aluminum oxide 90 % activated, eluents PE (petroleumether):Et 2 O 15:1, eluted as a pale violet band). Compound 1 was obtained upon changing the eluents to PE:Et 2 O 4:1 and it eluted as a yellow band. Removing the solvent and drying in vacuum gave pure 1.