Pd–Phosphine Precatalysts for Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 114

Figure 3.17 Crystal structure of dtbpfPdCl 2 . 8

The L 2 PdCl 2 (L ¼ trialkylphosphine) precatalysts can be assumed to follow
the same pathway of activation to the Pd(0) species as discussed in the cases
where L is a more traditional triarylphosphine ligand. In the case of
bidentate phosphine ligands, the bite angle of the phosphine has been
suggested to be of great importance for the cross-coupling reaction. The
electron-donating t-Bu groups in the dtbpf ligand are proposed to facilitate
the oxidative addition, whereas the large P-Pd-P angle (104.21)issaidto
promote the final

reductive elimination step in the catalytic cycle

(Figure 3.17).
The importance of the bite angle has been debated, however, since
Kawatsura and Hartwig noted that ketone arylation catalyzed by a Pd(dba) 2 /
dtbpf system likely proceeds via a Pd(II) intermediate containing the ligand
in a monodentate fashion (Scheme 3.24, A; see also Chapter 2). 90 This also
relates to the fact that the proposed catalytically active species, in cases with
bulky phosphine ligands, is LPd(0) and not L 2 Pd(0).
a-Arylation reactions catalyzed by the preformed catalyst dtbpfPdCl 2 ,
however, seem to proceed via a different pathway. In a catalytic reaction
monitored by 31 P NMR spectroscopy, none of the monocoordinated phos-
phine intermediate could be observed. 8 Instead, the NMR data indicated
that the binding mode in the resting state of the catalyst [dtbpfPd(0)] is
bidentate (Scheme 3.24, B). This demonstrates how the reaction mechanism
depends not only on the ligands and type of aryl halide, but also on the type
of cross-coupling reaction.