Prominent Ligand Types in Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 66

R R

R

PR 2
Pd

R

P

P Pd

Pd

R' 2

N M

R' R'

Biaryl Monophosphines
Strong P-Pd interaction
Weak C-Pd interaction

P,N-Ligands
Strong P-Pd interaction
Moderate N-Pd interaction

Bisphosphines
Two Strong P-Pd interactions

Important DalPhos Ligands:

P(1-Ad) 2

P(1-Ad) 2

N

NMe 2

O

Me-DalPhos Mor-DalPhos

Figure 2.60 Pd-ligand interactions in biarylphosphines, P,N ligands and bisphos-
phines (top) and commonly used DalPhos ligands (bottom).

R 1

R 1

N N

N N

R

R

R 2

R 1

R 2 R 1

IMe R = Me

R 1 = i -Pr, R 2

(S)IPr = H

ItBu
ICy
IAd

R = t -Bu
R = cyclohexyl
R = 1-adamantyl

R 1 = R 2

(S)IMes = Me

Figure 2.61 Commonly employed N-heterocyclic carbene ligands in cross-coupling
reactions.

monoarylation of acetone. 197a A detailed discussion of this class of ligands
and their applications is presented in Chapter 5. 198

2.3.5 N-Heterocyclic Carbene (NHC) Ligands
N-Heterocyclic carbenes, first isolated by Arduengo et al. in 1991, 199 have
been introduced as an alternative to phosphines as supporting ligands in
several transition metal-catalyzed reactions. 4d The first applications in
Pd-catalyzed coupling processes were Heck reactions, SMC reactions and
alkynylations reported by Herrmann. 4,122c Since these early reports, the area
of Pd-catalyzed cross-coupling reactions utilizing NHCs as supporting lig-
ands has grown immensely. 4 The work of the groups of Nolan, 4a,c-e Glorius 4b
and Organ 200 has taken the area significantly forward. The high activity of
NHCs can be traced to their electronic and steric properties. NHCs are
known to be stronger s-donors than trialkylphosphines and, by some
measures, even more sterically demanding. 4b The most common NHC lig-
ands used in cross-coupling reactions are depicted in Figure 2.61.