Au(I)-catalyzed cycloaddition pathways of non-terminal propargyl substrates

Figures & data

Scheme 1. Gold(I)-nitrone-catalyzed [2 + 2+2] cyclotrimerization of diarylpropargyl acetal 1.

Scheme 2. Synthesis of chiral nitrone ligands 6a and 6b.

Scheme 3. In situ generation of Au(I)–nitrone complexes (I–6a,b,c) by ligand exchange of JohnPhosAu(I)(ACN)SbF₆ complex I with nitrones 6a–c.

Scheme 4. Coordination of nitrone 6a with JohnPhosAu(ACN)SbF₆ I, shown as an equilibrium between [complex I + 6a] and [complex I–6a + “free” ACN].

Figure 1. ¹H NMR study of nitrones 6a and 6b coordination with JohnPhosAu(ACN)SbF₆ I (1:1 in CDCl₃).

Figure 3. ¹H NMR coordination study of nitrone 6a with Au(I) complex I with different I:6a ratios.

Figure 2. Crystal structure (X-ray) of Au(I)–nitrone complex I–6a. Selected bond lengths (Å) and angles (°) in gold(I)-nitrone complexes I–6a, I–6c and I–6d are given.

Table 1. Changes in ¹H NMR peak shifts, Δδ¹H_coord^a (ppm), of mixtures with decreasing I:6a ratio.

Entry	Signal observed	δ (ppm)	I:6a ratio:
			Δδ^1Hcoord^a
			1:1	1:2	1:5	1:10
1	6a: H^a	8.17	0.07	0.05	0.04	0.01
2	I; ACN	2.39	0.08	0.13	0.19	0.25 (δ 2.14)
3	I: t-Bu (d)	1.40	0	0	−0.01	−0.01
4	I–6a (2× d)	1.40	0.003^b	0.005^b	0.007^b	0.009^b

^a Δδ¹H_coord = δ¹H_complex – δ¹H_ligand.

^b Δδ¹H_diastereomer = δ¹H_isomer 1 – δ¹H_isomer 2.

Table 2. Gold(I)-catalyzed [2 + 2 + 2] cyclotrimerisation of propargyl acetal 1 to trimer 2.

Entry	Au(I) complex^a	Time	Yield	2a^b:2b:2c
1	I	2–24 h	–^[Citation8]	–
2	I–6a	18 h	40%	55:25:20
3	I–6b	18 h	73%	45:35:20
4	I–6c^[Citation10]	18 h	66%	45:25:30
5	I–6c^c^[Citation8]	2 h	74%	n.r.^d

^a Complex product mixture.

^b 2a = trans,trans,cis; main product.

^c (JohnPhosAuCl)AgNTf₂)–6c^[8].

^d n.r.: ratio not reported.

Table 3. Gold(I)-catalyzed dimerization of propargl acetal 10.

Au(I) complex	Time	Yield
I	2 h	11, 25%^a
I–6c	24 h	12, 30%^a

^a Unstable.

Table 4. Cycloaddition reactions of (a) diene 14 with phenylbenzaldimine^[18^] and (b) aryl-alkyl-propargyl acetals 10 and 10′ with imines 15a–d.

			Yield
Entry	Acetal	Imine	16	17b	18	19
1	10	15a	16a, 33%	21%	–	–
2	10	15b	16b, 30%	–	18%	–
3	10	15c	–	–	–	29%
4	10	15d	Full conv. of 10, no prod. isolated
5	10′	15a	16a′ <15%	–	–

Scheme 5. Proposed pathways for (a) gold-catalyzed generation of allene 13 and diene 14 intermediates from propargyl acetal 10; (b) dimerization of diene 14 with allene 13 to give open dimer 11 and (c) dimerization of two units of diene 14 to give cyclic dimer 12.

Scheme 6. Proposed mechanisms for formation of products 16–19.

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