Table 1. Optimization of the Reaction Parameters^a.

entry	Cu	ligand	% yield (% conversion)b
1	CuSO4	L1	79 (99)
2	CuSO4	L2	81 (99)
3	CuSO4	L3	66 (99)
4	CuSO4	L4	17 (91)
5	CuSO4	–	5 (30)
6	CuCl2·2H2O	L2	78 (99)
7	CuBr2	L2	79 (99)
8	Cu(OAc)2	L2	78 (99)
9	–	L2	0 (22)c
10d	CuSO4	L2	44 (72)
11e	CuSO4	L2	7 (30)
12f	CuSO4	L2	NR
13g	CuSO4	L2	5 (99)
14h	CuSO4	L2	4 (99)
15i	CuSO4	L2	NR

^aSee the Supporting Information for details. Amino acids are shown in a one-letter code. NR = no reaction.

^bYields were determined by integration of the total ion currents (TICs) from LC–MS analyses of the unpurified reaction mixtures.

^cElimination and diselenide were the only observable products.

^d0.5 mM CuSO₄, 0.5 mM L2, and 0.5 mM boronic acid were used.

^e0.25 mM CuSO₄, 0.25 mM L2, and 0.25 mM boronic acid were used.

^fThe Sec-TNP residue replaced with Ser.

^gThe Sec-TNP residue was replaced with Cys.

^hThe Sec-TNP residue was replaced with Cys-TNP.

ⁱThe Sec-TNP residue was replaced with Met.