P-02-C1 angle is less acute than in epo~ides.[~l The most interesting feature of this structure is that it keeps the "memory" of its phosphaalkene precursor. The sum of the W-P-C6, W-P-C1, and Cl-P-C6 angles (357.3') is close to 360°, as in the parent phosphaalkene complex.r3b1 The plane of the oxaphosphirane ring is almost orthogonal to the WPCl C6 mean plane [90.3(0.2)"]. The phosphorus atom appears to retain its initial sp2 hybridization. Even the P-C1 bond [1.778(6) A] is shorter than expected for a normal P-C single bond. The structure of 3 also shows that the epoxidation of the P = C double bond takes place with retention of the initial trans stereochemistry of 1 [C3-P-CI-C6 dihedral angle 161.3( )"]. However, with the less hindered phosphaalkene complex 2, this retention is not complete. Indeed, the epoxidation of 2 mainly leads to 4.[*] The 'H-NMR spectrum of 4, like that of 3 shows no H-C1-P coupling.r41 Thus, 4 probably has the same trans stereochemistry as 3. However, besides 4 (h31P = +2.9 in C,D,), another compound 5 (h31P = +6.2) is produced in minor amounts (ca. 10%). This compound is probably the cis isomer corresponding to 4. Finally, the main decomposition path of the oxaphosphiranes 3 and 4 in the mass spectrometer involves splitting of the ring into the terminal phosphinidene complex [Mes-P=W(CO),] and the aldehydes RCHO. This process is exactly the reverse of the first described synthesis of this ring."]
Experimental
A solution of m-chloroperbenzoic acid (1.9 g, 1.1 x lo-, mol) in THF (10 mL) was added dropwise to a solution of the phosphaalkene complex 1 (5.3 g, 1 x lo-, mol) in THF (30 mL). The reaction was slightly exothermic. After evaporation of T H E the residue was extracted with hexane. The extract after removal of hexane was recrystallized from pentane/CH,CN. 5.0 g of 3 (-86%). colorless crystals: m.p. 68-69 "C. The same procedure with 2 gave 4 in 40% yield after recrystallization; m.p. 61 -64°C (along with ca 10% of the cis isomer).