Advances in C–C and C–X Coupling Using Palladium–NHeterocyclic Carbene (Pd–NHC) Complexes - New Trends in Cross-Coupling: Theory and Applications - page 184

Table 4.20 Activity of the [Pd(NHC)(dvtms)] complexes in Heck coupling.

[Pd (NH C )(dv tm s )]
55a - g (0.1 mol%)

Br COO t Bu

+

CO O t Bu

NaOAc
DM A, 1 20° C

MeOC MeO C

Yield (%) a TOF b Entry [Pd(NHC)(dvtms)] Time (h)

1 55a 3 61 217

2 55b 3 50 287

3 55c 3 59 520

4 55d 3 85 353

5 55e 3 100 1140

6 55f 3 100 512

7 55g 3 64 443

8 [Pd(IMes)dvtms)] 3 14 8

a By GC.
b Approximate turnover frequency (TOF) values calculated at 30 min.

[Pd(IPr)(Im)Cl 2 ](16a)
(1 mol%)

Cl

R'

R + R'

Cs 2 CO 3, TBAB
140°C, air

R

NN

Cl Pd Cl

MeO

N

F

85% 71% 91%

N

[Pd(IPr)(Im)Cl 2 ]
16a

S S

F

16a

67% 53% 51%

Scheme 4.34 Heck coupling of aryl chlorides using [Pd(IPr)(Im)Cl 2 ].

coupling in water. For example, Luo and Lo reported the preparation of the
caffeine-based Pd-NHC complex 56 (Figure 4.29), 122 allowing the coupling of
bromoacetophenone and iodobenzene at 90 1C. Shao and co-workers 123 and
Ma and Lu, 124 on the other hand, prepared NHCs derived from proline to
promote the coupling of acrylic acid, esters and styrenes (57 and 58,
Figure 4.29).
Another significant contribution was reported by Tu and co-workers. 125 A
robust hydrophilic pyridine-bridged bisbenzimidazolylidene-palladium
pincer complex was developed (59), enabling Heck coupling to be performed
in water at very low catalyst loadings. Aryl iodides could be coupled to tert-
butyl acrylate, generally in good yields, using Et 3 N as the base and TBAI as an
additive at 100 1C (Scheme 4.35). Unfortunately, iodonaphthalene and
3-chloropyridine were found to be poor coupling partners.