Prominent Ligand Types in Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 47

Hartwig 2000 Buchwald/Blackmond 2002

ArN R'

ArN R'

HOR"

Pd(BINAP) 2 NaBr

RNR'
H

Pd(BINAP)

BINAP

BINAP

(BINAP)Pd[N(Ar)R]

Pd(BINAP)

R'

H

ArBr

(BINAP)PdN(R)R'

RNR' NaOR"

HN(R)R'

(BINAP)Pd − Ar (BINAP)Pd(Ar)Br

(BINAP)Pd(Ar)Br

ArBr

NaBr
HOR"

NaOR"
RNR'

H

Figure 2.29 Original mechanisms for aryl amination catalyzed by Pd/BINAP pro-
posed by the groups of Hartwig (left) and Buchwald (right).

Hartwig/Buchwald/Blackmond 2006

Pd(BINAP) 2

ArN R'

ArBr

Pd(BINAP)

N(R)R'

(BINAP)Pd(Ar)Br

(BINAP)Pd − Ar

NaBr
HOR"

NaOR"
RNR'

H

Figure 2.30 Revised mechanism of Pd/BINAP-catalyzed aryl amination.

Pd(BINAP) is generated by ligand dissociation of Pd(BINAP) 2 , which lies off
the catalytic cycle (Figure 2.30).
van Leeuwen and co-workers introduced the use of the chelating ligand
9,9-dimethyl-4,6-bis(diphenylphosphino)xanthene (Xantphos) for the Pd-
catalyzed amination of aryl bromides (Figure 2.31). 117 Xantphos, originally
developed by van Leeuwen's group for Rh-catalyzed hydroformylation pro-
cesses, 118 features a very large bite angle 119 (108.81 for the
(Xantphos)Pd(tetracyanoethylene) complex and 111.71 calculated natural
bite angle). This ligand, along with a family of similar chelating
bisphosphines, was easily synthesized from 9,9-dimethylxanthene via
selective bis-lithiation with sec-butyllithium/TMEDA and then reacted with
diphenylchlorophosphine to give the ligand in 75% yield.