Table 1. Screening of the Redox Mediators for the Reoxidation of Pd-Catalyzed Tryptophan Coupling in Air.

entry	additive/ligand	yielda after 3 h (%)	yielda after 24 h (%)
1	none	14	20
2b	none	23	60
3	1,4-benzoquinone	8	44
4	2,5-di-t-butyl-1,4-benzoquinone	16	46
5	pyrroloquinoline quinone (1)	2	3
6	riboflavin (vitamin B2) (2)	19	39
7	ascorbic acid (3)	n.d.c	2
8	uric acid (4)	7	11
9d	CuCl2	5	8
10d	Cu(OAc)2	11	25
11	MnCl2·4H2O	7	12
12	FeCl2·4H2O	50	66
13	FeII(phthalocyanine) (5)	n.d.	1
14	FeIII(phthalocyanine) chloride	15	25
15	bipyridine	1	3
16	6,6′-dimethylbipyridine	7	11
17	phenanthroline	1	3
18	neocuproine (6)	8	14
19	1,10-phenanthroline-5,6-dione	3	9
20	Ac-Ile-OH	17	49
21	Boc-Val-OH	9	21
22	4,5-diazafluoren-9-one (7)	35	92
23	4,5-diazafluoren-9-one + FeCl2·4H2O	23	48
24	4,5-diazafluoren-9-one + 2,5-di-t-butyl-1,4-benzoquinone	23	67
25	4,5-diazafluoren-9-one + Ac-Ile-OH	38	79
26	4,5-diazafluoren-9-one + Boc-Val-OH	21	66

^aYields determined via HPLC with the corresponding reactant and product calibration curves.

^bReaction performed in enriched oxygen environment (1 atm).

^cNot detected.

^dN-Arylated side product was detected.