Cationic Au(I)-Catalyzed Cycloisomerization of Aromatic Enynes for the Synthesis of Substituted Naphthalenes

Abstract

A cationic Au(I) complex catalyzed the cycloisomerization of aromatic enynes that possess a substituent on their alkyne terminus. Cyclization of the 6-endo-dig type proceeded dominantly to give 1,3-di- and 1,2,3-trisubstituted naphthalenes.

Key words

cycloisomerization - catalysis - enynes - gold - indenes - naphthalenes

References and Notes

• Pioneering works of cyclization triggered by alkyne activation using Au catalyst:
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• 15 (1E)-1-(Iodomethylene)-3-methyl-1H-indene (3m): yellow oil. IR (neat): 1110, 1079, 1019 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 2.16 (s, 3 H), 6.35 (s, 1 H), 7.14-7.29 (m, 4 H), 7.39 (d, 1 H, J = 7.2 Hz). ¹³C NMR (100 MHz, CDCl₃): δ = 13.5, 79.1, 119.1, 125.1, 125.7, 126.5, 127.8, 135.6, 144.3, 145.9, 150.0. HRMS (FAB): m/z calcd for C₁₁H₉I [M⁺]: 267.9749; found: 267.9747

9

Using AuCl₃ (1 mol%) as a catalyst in toluene (see ref. 4), the cycloisomerization of 1a did not proceed at r.t. and it did at 100 °C for 24 h to give 2a yet in ca. 30% yield. Moreover, AuCl(PPh₃) is more stable and easy to handle than hygroscopic AuCl₃.

10

Cationic platinum, which was prepared from PtCl₂ (1 mol%) and AgOTf (2.5 mol%), did not work as a catalyst even in refluxed CH₂Cl₂.

11

Typical Experimental Procedure (Table 2).
AuCl(PPh₃) (1.2 mg, 0.0024 mmol) was placed in a flask and a CH₂Cl₂ solution (2.5 mL) of aromatic enyne (0.25 mmol) was added. To the resulting mixture was added AgOTf (0.8 mg, 0.0030 mmol) and the mixture was stirred at r.t. for 1-2 h. After completion of the reaction, the mixture was quenched with H₂O and extracted with CH₂Cl₂ three times. The combined extracts were washed with brine and dried over MgSO₄. The solvent was removed under reduced pressure and the crude products were purified by thin-layer chromatography.

12

(1-Methylnaphthalen-3-yl)methanol (2f): yellow oil. IR (neat): 3350, 872, 773, 748 cm^-1. ¹H NMR (600 MHz, CDCl₃): δ = 1.92 (br s, 1 H), 2.68 (s, 3 H), 4.79 (s, 2 H), 7.31 (s, 1 H), 7.47-7.52 (m, 2 H), 7.64 (s, 1 H), 7.81-7.82 (m, 1 H), 7.96-7.97 (m, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 19.4, 65.4, 123.7, 123.9, 125.6, 125.7, 125.8, 128.4, 132.0, 133.4, 134.7, 137.8. HRMS (FAB): m/z calcd for C₁₂H₁₂O [M⁺]: 172.0888; found: 172.0891.

13

The stereochemistry of alkene moiety was determined by NOE observation (Figure [1] ).

Figure 1

14

In the presence of AuCl₃ (5 mol%) as a catalyst, almost no cycloisomerization of 1j proceeded even at 80 °C in toluene (see ref. 5).

16

During the purification of the products, some vinyl iodide (E)-3m was isomerized to (Z)-3m.