Magnesium-Promoted Selectively Reduction of Unprotected Indoles and Quinoxalines

doi:10.6023/cjoc202508021

Abstract

This study present a transfer hydrogenation method employing ammonia borane (H₃N•BH₃) as the hydrogen source and inexpensive magnesium as the catalyst for the selective reduction of unprotected indoles, quinoxalines, benzoxazole, benzothiazole, quinolines, and their derivatives, resulting the corresponding alicyclic heterocyclic compounds with desirable yields. This catalytic system is applicable to gram-scale syntheses and demonstrates compatibility with various functional groups, including fluorine, chlorine, bromine, trifluoromethyl, and hydroxyl. Deuterium labeling experiments show that the BH₃ counterpart of NH₃•BH₃ served as the hydride source, while the NH₃ counterpart of ammonia borane acted as a proton source. It offers a novel approach for the preparation of partially saturated heterocyclic derivatives.

Key words: reduction, magnesium, indoles, quinoxalines, ammonia borane

Cite this article

Nana Wei, Wanzhen Guo, Xing Lu, Zhiqiang Ren, Haojie Ma, Yuqi Zhang, Jijiang Wang, Bo Han. Magnesium-Promoted Selectively Reduction of Unprotected Indoles and Quinoxalines[J]. Chinese Journal of Organic Chemistry, 2026, 46(3): 1027-1038.

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Entry	Catalyst (mol%)	AB/equiv.	Solvent	Temp./℃	2a/%
1	MgCl₂ (10)	2.0	Toluene	80	49
2	MgSO₄ (10)	2.0	Toluene	80	52
3	Mg(OTf)₂ (10)	2.0	Toluene	80	40
4	MgI₂ (10)	2.0	Toluene	80	37
5	Mg (50)	2.0	Toluene	80	75
6	Zn (50)	2.0	Toluene	80	50
7	Al (50)	2.0	Toluene	80	40
8	Mn (50)	2.0	Toluene	80	45
9	Mg (50)	2.0	Et₂O	80	24
10	Mg (50)	2.0	THF	80	nd^b
11	Mg (50)	2.0	CH₂Cl₂	80	43
12	Mg (50)	2.0	CH₃CN	80	Trace
13	Mg (50)	2.0	Toluene	100	77
14	Mg (50)	2.0	Toluene	120	78
15	Mg (50)	3.0	Toluene	80	85
16	Mg (50)	4.0	Toluene	80	83
17^c	Mg (50)	3.0	Toluene	80	71
18	Mg (40)	3.0	Toluene	80	80
19	Mg (30)	3.0	Toluene	80	77
20	—	3.0	Toluene	80	14

Entry	Catalyst (mol%)	AB/equiv.	Solvent	Temp./℃	2a/%
1	MgCl₂ (10)	2.0	Toluene	80	49
2	MgSO₄ (10)	2.0	Toluene	80	52
3	Mg(OTf)₂ (10)	2.0	Toluene	80	40
4	MgI₂ (10)	2.0	Toluene	80	37
5	Mg (50)	2.0	Toluene	80	75
6	Zn (50)	2.0	Toluene	80	50
7	Al (50)	2.0	Toluene	80	40
8	Mn (50)	2.0	Toluene	80	45
9	Mg (50)	2.0	Et₂O	80	24
10	Mg (50)	2.0	THF	80	nd^b
11	Mg (50)	2.0	CH₂Cl₂	80	43
12	Mg (50)	2.0	CH₃CN	80	Trace
13	Mg (50)	2.0	Toluene	100	77
14	Mg (50)	2.0	Toluene	120	78
15	Mg (50)	3.0	Toluene	80	85
16	Mg (50)	4.0	Toluene	80	83
17^c	Mg (50)	3.0	Toluene	80	71
18	Mg (40)	3.0	Toluene	80	80
19	Mg (30)	3.0	Toluene	80	77
20	—	3.0	Toluene	80	14

镁促进未保护的吲哚及喹喔啉衍生物的选择性还原