Optimization of reaction conditions^a.

Entry	Variation from “standard conditions”	4aab (%)
1	No change	97 (95)
2	No [Ru(p-cymene)Cl2]2	Trace
3	No base	Trace
4	Ru(PPh3)3Cl2 used instead	96
5	Ru(PPh3)3Cl2 used instead	96
6	[RuCl(p-cymene)(dppe)]Cl used instead	71
7	Cp*Ru(cod)Cl used instead	24
8	Ru(Cp)(PPh3)2Cl used instead	14
9	Ni(cod)2 used instead	N.D.
10	[Pd(allyl)Cl]2 used instead	N.D.
11	Dppe instead of dmpe	96
12	Dppb instead of dmpe	84
13	Dppf instead of dmpe	66
14	KOH instead of tBuOLi	92
15	K3PO4 instead of tBuOLi	66
16	K2CO3 instead of tBuOLi	12
17	DBU instead of tBuOLi	10

^aReaction conditions: benzaldehyde 1a (0.4 mmol), hydrazine (0.48 mmol), para-vinyl pyridine 3a (0.2 mmol), [Ru(p-cymene)Cl₂]₂ (2.5 mol%), ligand (5 mol%) and base (0.1 mmol) in THF (0.5 mL) at 80 °C for 12 h under N₂ unless otherwise noted. Note that phenyl hydrazone 2a was generated in situ from benzaldehyde and hydrazine without isolation.

^bThe yield of 4aa was determined by ¹H NMR using mesitylene as an internal standard and based on 3a (isolated yield of 4aa in parentheses). N.D. = not detected.