Impact of Steric Blocking on Diastereoselective C−H Activation of Ethylbenzene by Cationic Platinum(II) Complexes with 7-Azaindolyl Derivative Ligands

Ethylbenzene C−H activation reactions using the cationic compounds [Pt(BAB)(CH3)(L)]+ and [Pt(BAM)(CH3)(L)]+ generated in situ by the reaction of [(Et2O)2H][BAr‘4] with the parent molecules Pt(BAB)(CH3)2 (1) and Pt(BAM)(CH3)2 (2), respectively, have been examined (BAB = 1,2-bis(N-7-azaindolyl)benzene, BAM = bis(N-7-azaindolyl)methane). The impact of the steric blockage of the Pt
axial coordination site by the BAB and BAM ligands on the regio- and diastereoselectivity of the
ethylbenzene C−H activation has been investigated. For both Pt complexes the benzylic C−H activation
products were found to be thermodynamically favored, with the BAB complex showing a higher
regioselectivity. In addition, both complexes display distinct diastereoselectivity in the formation of the
η1-benzylic products [Pt(BAB)(CH3CN)(CH(Me)Ph)][BAr‘4] (3) and [Pt(BAM)(CH3CN)(CH(Me)Ph)][BAr‘4] (4), with the BAB complex showing a much higher diastereoselectivity. The structures of the
diastereomers have been established by single-crystal X-ray diffraction and 2D NOESY NMR analyses.
The two η3 complexes [Pt(BAB)(η3-CH(Me)Ph)][BAr‘4] (5) and [Pt(BAM)(η3-CH(Me)Ph)][BAr‘4] (6)
have been characterized, and the structure of 5 has been determined by single-crystal X-ray diffraction
analysis. The η3 complexes have been found to exist in two isomeric forms, with one isomer being
dominant. NMR experiments established that the η3 complexes 5 and 6 can be converted quantitatively
to the corresponding η1 complexes 3 and 4, respectively, with the retention of the isomer (or diastereomer)
ratio. The results of this investigation are consistent with the η3-benzylic complex being the reaction
intermediate in the formation of the η1-benzylic complex. The asymmetric blocking of the Pt axial
coordination site by the BAB and BAM ligands was found to be responsible for the observed
diastereoselectivity in ethylbenzene C−H activation.