Regioselective Pauson-Khand Processes with Allylphosphonates as the Olefinic Partners

Abstract

The ability of allylphosphonates to act as the olefinic partner within the Pauson-Khand reaction has been investigated. It has been found that these substrates not only react efficiently within the developed reaction manifold but also deliver good to excellent levels of olefinic regiocontrol.

Key words

alkyne complexes - allylphosphonates - annulations - Pauson-Khand reaction - regioselectivity

References

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10

General Experimental Procedure.
A 250-mL three-necked round-bottomed flask was flame dried under vacuum and allowed to cool under nitrogen prior to being fitted with a reflux condenser. The entire system was then purged three times with nitrogen gas. The vessel was charged with dry CH₂Cl₂ (5.0 mL), dry MeCN (5.0 mL), and diethyl allylphosphonate (1.0 mmol). The reaction mixture was heated slowly to reflux at which time a solution of the desired cobalt complex (0.5 mmol) in dry CH₂Cl₂ (10.0 mL) was added over 8 h via syringe pump. Following complete addition, heating was continued at reflux for 10 h. After this time, the mixture was concentrated to dryness, dissolved in EtOAc, and filtered through Celite to remove cobalt residues. The filtrate was concentrated and the resultant oil distilled under vacuum to remove any excess alkene starting material. The residue was then purified by silica column to yield the desired regioisomeric cyclopentenones. The ratio of these regioisomers was determined by ¹H NMR. Sample data:
Diethyl (2-oxo-3-phenylcyclopent-3-enyl)methylphos-phonate (3): IR (CH₂Cl₂): ν = 1027, 1054, 1133, 1247, 1447, 1494, 1598, 1704 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 1.34 (t, 3 H, ^³ J _HH = 7.1 Hz), 1.36 (t, 3 H, ^³ J _HH = 7.1 Hz), 1.73 (ddd, 1 H, ^² J _PH = 16.6 Hz, ^² J _HH = 15.5 Hz, ^³ J _HH = 11.7 Hz), 2.54 (ddd, 1 H, ^² J _PH = 18.2 Hz, ^² J _HH = 15.5 Hz, ^³ J _HH = 2.9 Hz), 2.71 (dt, 1 H, ^² J _HH = 19.8 Hz, ^³ J _HH = 2.8 Hz), 2.77-2.87 (m, 1 H), 3.08 (ddd, 1 H, ^² J _HH = 19.8 Hz, ^³ J _HH = 6.8 Hz, ^³ J _HH = 3.2 Hz), 4.08-4.23 (m, 4 H), 7.32-7.42 (m, 3 H), 7.68-7.73 (m, 2 H), 7.83 (dd, 1 H, ^³ J _HH = 3.2 Hz, ^³ J _HH = 2.8 Hz) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 207.4, 157.7, 142.3, 131.4, 128.8, 128.7, 127.2, 62.0, 41.5, 34.0, 27.3, 16.7 ppm. ³¹P NMR (162 MHz, CDCl₃): δ = 30.87 ppm. HRMS (EI): m/z calcd for C₁₆H₂₂O₄P [M⁺ + H]: 309.1250; found: 309.1247. Anal. Calcd for C₁₆H₂₁O₄P: C, 62.33; H, 6.87; P, 10.05. Found: C, 62.21; H, 6.82; P, 9.90.
Diethyl (4-oxo-3-phenylcyclopent-2-enyl)methylphos-phonate (4): ¹H NMR (400 MHz, CDCl₃): δ = 1.35 (t, 3 H, ^³ J _HH = 7.1 Hz), 1.37 (t, 3 H, ^³ J _HH = 7.1 Hz), 1.97 (ddd, 1 H, ^² J _PH = 17.8 Hz, ^² J _HH = 15.2 Hz, ^³ J _HH = 8.3 Hz), 2.05 (ddd, 1 H, ^² J _PH = 18.1 Hz, ^² J _HH = 15.2 Hz, ^³ J _HH = 6.6 Hz), 2.44 (dd, 1 H, ^² J _HH = 19.0 Hz, ^³ J _HH = 2.5 Hz), 2.92 (dd, 1 H, ^² J _HH = 19.0 Hz, ^³ J _HH = 6.7 Hz), 3.26-3.38 (m, 1 H), 4.08-4.19 (m, 4 H) 7.32-7.45 (m, 3 H), 7.68-7.73 (m, 2 H), 7.82 (d, 1 H, ^³ J _HH = 2.6 Hz) ppm. ³¹P NMR (162 MHz, CDCl₃): δ = 29.14 ppm.
Isomer 3 was identified as the major product by its characteristic ¹H NMR resonances at δ = 2.71, 3.08, and 7.83 ppm. The splitting and coupling constants associated with these signals is indicative of structure 3. This can be compared with the corresponding ¹H NMR resonances for compound 4 at δ = 2.44, 2.92, and 7.82 ppm. The ratio of 3:4 in the unseparated mixture was established from the relative integral values of the signals at δ = 2.77-2.87 ppm (in 3) and δ = 3.26-3.38 ppm (in 4). All other regioisomeric ratios were established in a similar fashion.