Sequential Catalytic Reactions for the Synthesis of Benzofulvenes Using an Iridium Complex with Dual Function

Abstract

The cationic iridium complex ([Ir(cod)₂]OTf + rac-­BINAP) efficiently catalyzed a sequential process of ortho-C-H bond functionalization, cyclization and dehydration, leading to a concise preparation of 1-methylene indene (benzofulvene) derivatives. The iridium complex operated as a catalyst in the ortho-C-H bond alkenylation of aryl ketones with alkynes and as a Lewis acid catalyst in the cyclization of the alkenylated product and the subsequent dehydration.

catalysis - cyclization - C-H bond functionalization - ­indenes - iridium

References and Notes

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10

Typical experimental procedure (Table  [¹] , entry 1):
[Ir(cod)₂]OTf (10.9 mg, 20 µmol), rac-BINAP (12.6 mg, 20 µmol), and diphenylacetylene (1a; 37.7 mg, 0.21 mmol) were placed in an oven-dried Schlenk tube, which was then evacuated and backfilled with argon (× 3). To the reaction vessel were added 2-methoxyacetophenone (2b; 60.1 mg, 0.40 mmol) and PhCl (0.2 mL, pretreated by argon bubbling for 30 s) before the solution was stirred at 135 ˚C for 24 h. The resultant mixture was cooled to room temperature and filtered through a silica pad. After evaporation of the solvent, the crude products were purified by thin-layer chromatog-raphy (hexane-EtOAc, 10:1) to yield analytically pure product 3ab (62.4 mg, 95%). Yellow solid; mp 95 ˚C. ^¹H NMR (400 MHz): δ = 3.98 (s, 3 H), 5.84 (d, J = 1.5 Hz, 1 H), 6.73 (d, J = 1.5 Hz, 1 H), 6.86 (d, J = 8.3 Hz, 1 H), 7.01 (d, J = 7.6 Hz, 1 H), 7.18-7.28 (m, 11 H); ^¹³C NMR (100 MHz): δ = 55.3, 109.0, 113.5, 119.8, 121.5, 126.8, 127.2, 127.9, 128.1, 129.0, 129.6, 130.9, 134.6, 134.7, 138.2, 140.7, 144.6, 146.5, 156.2; IR (KBr): 1599, 1442, 1260, 1083, 700 cm^-¹; HRMS (FAB⁺): m/z calcd for C₂₃H₁₈O: 310.1358; found: 310.1351.

11 Similar regioselectivity was observed in the Ru-catalyzed ortho-C-H bond addition of meta-substituted acetophenones to vinylsilanes, see: Kakiuchi F. Murai S. Acc. Chem. Res.  2002,  35:  826 

12

The geometry was determined by NOESY measurement.

13 The relatively bulky phenyl group was situated at the external position against the aromatic ring of the aryl ketone. The same regioselectivity was also reported in the Ru-catalyzed directed C-H alkenylation of aryl ketones with unsymmetrical alkynes, see: Kakiuchi F. Yamamoto Y. Chatani N. Murai S. Chem. Lett.  1995,  681 

14 TsOH-catalyzed cyclization of ortho-alkenylated aromatic ketones for the synthesis of benzofulvenes, see: Harris PWR. Woodgate PD. Synth. Commun.  1997,  27:  4195 

The cyclization of arylimines and alkynes using a rhenium catalyst afforded aminoindenes. The authors proposed intramolecular 1,2-addition of the rhenium-carbon bond to the imine moiety in the reaction mechanism, see:

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