Phosphorus Compounds Containing Sterically Demanding Groups

PHOSPHORUS COMPOUNDS CONTAINING STERICALLY DEMANDING GROUPS 

Submitted by ALAN H. COWLEY, NICHOLAS C. NORMAN, and MAREK PAKULSKI  

Stable compounds containing unsupported double bonds between phosphorus and/or arsenic atoms have been isolated recently. This has been achieved by the use of sterically demanding substituents that ofler a high degree of kinetic and thermodynamic stability. The synthesis of representative examples of these compounds is described in Section 47. Herein are described the preparations of the relevant starting materials. 

This procedure is a modification of that published by Pearson et al. A l.S-L two-necked flask equipped with a reflux condenser, magnetic stirrer bar, and surrounded by aluminum foil (to minimize exposure to light) is charged with trimethyl phosphate (Aldrich) (1 L) and dessicator dried (P2Oj) tri-tert-butyl-benzene (Aldrich) (76 g, 0.3 mol). The mixture is then stirred and warmed to 8S°C by means of an oil bath until the hydrocarbon dissolves. The temperature is lowered to 70 °C and bromine (56 g, 18 mL, 0.35 mol) is added rapidly. 

The temperature is then maintained at 65-70 °C and stirring is continued for 30 h. The exclusion of air is not vital however, the reaction must be carried out in moisture-free conditions and a nitrogen atmosphere is advised. After completion of the reaction, the solution is allowed to cool to room temperature, and it is then added to 1 L of water at 0 °C (no further precautions to exclude air are required), causing immediate precipitation of the product. Extraction with 5 x 200-mL portions of hexane, followed by evaporation of the combined fractions affords the product as a white solid, crude yield, 92 g (95%). Further purification can be achieved by double recrystallization from ethanol-2-propanol mixtures at — 20 C, first from 1 L of 5 1, then from 500mL of 1 5, affording white crystals. Yield 54g (55%) mp 172-174°C. HNMR (CsDe) 1.35 (s, 9H, p-t-Bu), 1.58 (s, 18H, o-t-Bu), 7.42 (s, 2H, m-H). 

The temperature is then maintained at 65-70 °C and stirring is continued for 30 h. The exclusion of air is not vital however, the reaction must be carried out in moisture-free conditions and a nitrogen atmosphere is advised. After completion of the reaction, the solution is allowed to cool to room temperature, and it is then added to 1 L of water at 0 (no further precautions 

---

Phosphorus Compounds Containing Sterically Demanding Groups 235... 

PHOSPHORUS COMPOUNDS CONTAINING STERICALLY DEMANDING GROUPS... 

---