Comparison of homogeneous and heterogeneous Ga(III) catalysis in the cycloisomerization of 1,6-enynes

Figures & data

Figure 1. Cycloisomerization of 1,6-enynes catalyzed by GaCl₃ (40).

Table 1. Cycloisomerization of 1,6-enynes catalyzed by GaBr₃.

			Cyclized products, yields
Entry^a	Solvent	T°C	1b	1c	1d
1^b	PhCH3	16	–	–	–
2	PhCH3	25	7%	–	–
3^c	PhCH3	40	15%	22%	–
4^c	PhCH3	80	12%	12%	60%
5^c	PhCH3	Reflux	Degradation
6	DCE	80	12%	9%	65%
7^d	DCE	80	–	–	–

^a Reaction conditions: 1a (0.5 mmol) in anhydrous and degassed solvent (1 ml) in the presence of 0.025 mmol GaBr₃ for 24 h. DCE = dichloroethane; ^bConversion 70%; ^cSmall amounts of hydroarylation product was formed (GC-MS); and ^dControl experiment without catalyst, 80% recovery of unchanged starting material.

Table 2. Cycloisomerization of various 1,6-enynes catalyzed by GaBr₃.

Entry^a	Substrate	Time	T°C	Products, yields
1^b		24	80
2		10	50
3	2a	10	80	2b, 13%	2b′, 8%
4^c	2a	24	RT	2b, 12%	2b′, 15%
5		1.5	RT	Degradation^d
6		24	50	Degradation
7		24	RT
8		24	40	–^f
9		1	80
10		24	80	–^h

^a Reaction conditions: substrate (0.5 mmol) in anhydrous and degassed DCE (1 ml) in the presence of 0.025 mmol GaBr₃; Conversion 100% unless entry 2:72% and 7:42%; E = COOMe, E′ = COOEt; ^bTaken from Table 1; ^cPerformed in toluene; ^dDegradation similarly occurred at 80°C and 50°C; ^eConversion 42%; ^f0% conversion, and 60% recovery of unchanged starting material; ^gBesides 7b and 7d, propargyl tosylamine was formed in 21% yield presumably by deprenylation; and ^h84% recovery of unchanged starting material.

Figure 2. Mechanistic rationale accounting for the formation of 1d and 5f.

Table 3. Cycloisomerization of enyne 1a catalyzed by Ga-MMT: solvents screening.

						Selectivity
Entry^a	Solvent	T°C	Time (h)	Conversion (%)	Yield (%)	1b	1c
1	PhCH3	40	24	15	–	–	–
2	CH3CN	40	24	12	–	–	–
3	DCM	Reflux	24	38	–	–	–
4	Et2O	Reflux	40	30	–	–	–
5	DCE	60	25^b	65	11	55	45
6	DCE	80	96	100	46	40	60
7^c	DCE	80	24	0	–	–	–
8	CH3OH	Reflux	24	20	–	–	–
9	CH3NO2	80^d	32	<5	–	–	–

^a Reaction conditions: 1a (0.5 mmol) in anhydrous and degassed solvent (1 ml) in the presence of 5 mol% Ga; ^bReaction stopped when no evolution could be detected; ^cControl with MMT free of Ga; and ^dEven at 100°C, the conversion remained below 5%.

Table 4. Cycloisomerization of 1,6-enynes 1a–5a catalyzed by Ga-MMT.

Entry^a	Substrate	Time (h)^b	Conversion (%)	Yield (%)	Selectivity b/c/d^c
1		96	100	46	40/60/0
2^d	1a	23	100	91	42/42/16
3		24	0	0	–
4		24	74	74	72/28/0
5		8	100	78	Mixture

^a Reaction conditions: Substrate (0.5 mmol) in anhydrous and degassed DCE (1 ml) at 80°C in the presence of 5 mol% Ga(III), E = COOMe; ^bReactions stopped when no evolution could be detected by GC/FID; ^cSelectivity determined by ¹H-NMR on the crude product; and ^dRealized with 30 mol% of Ga(III).

Chatani, N.; Inoue, H.; Kotsuma, T.; Murai, S. J. Am. Chem. Soc. 2002, 124, (35), 10294–10295. 10.1021/ja0274554

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