Radical Synthesis of Trialkyl , Triaryl , Trisilyl , and Tristannyl Phosphines from P 4 ∗

A reaction scheme has been devised according to 3 RX + 3 Ti(III) + 0.25 P4→ PR3 + 3 XTi(IV) wherein RX = PhBr, CyBr, Me3SiI, or Ph3SnCl with contrasting results in the case of more hindered RX; the scheme accomplishes direct radical functionalization of white phosphorus without intermediacy of PCl3. It is known that P4, white phosphorus, has excellent properties as a trap for carboncentered radicals in solution and under the mild conditions that are typical for organic synthesis. The most prominent example of this was the demonstration that phosphonic acids may be prepared from corresponding carboxylic acids by way of O-acyl derivatives of N-hydroxy-2-thiopyridone (Barton PTOC esters).1 The latter provide carbon centered radicals in an oxygen-initiated chain reaction, and these are consumed upon combination with P4 as the critical P–C bond-forming event; upon oxidative workup, any remaining P–P bonds are cleaved and the phosphonic acid RP(O)(OH)2 is the end product.2 It is also known that P–P bonds other than those in P4 may serve as traps for organic radicals. This has been shown by Sato et al. in a scheme for radical phosphination of organic halides wherein ArX serves as a source of Ar· which in turn attacks Ph2P–PPh2, yielding ArPPh2. ∗We dedicate this work to the memory of Sir Derek Barton. †Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA. E-mail: [email protected]