PHOSPHA-WITTIG REACTION IN THE SYNTHESIS OF 1-PHOSPHADIENES AND (ETA-4-1-PHOSPHADIENE)TRICARBONYLIRON COMPLEXES FROM ALPHA,BETA-UNSATURATED ALDEHYDES

The reaction of the “phospha-Wittig” reagents [(RO)2P(O)–P-R]W(CO)5 with α,β-unsaturated aldehydes yields the corresponding 1-phosphadiene P-W(CO)5 complexes. These unstable species either undergo a conrotatory ring closure to give stable 1,2-dihydrophosphete P-W(CO)5 complexes or are trapped by conjugated dienes to give the [2 + 4] cycloadducts via their P=C double bond, which acts as a dienophile. The reaction of the same “phospha-Wittig” reagents with the η2-(C=C)Fe(CO)4 complexes of α,β-unsaturated aldehydes first gives the η1-P-W(CO)5, η2-(P=C)Fe(CO)4 complexes of 1-phosphadienes, which then lose CO upon heating to yield the corresponding η4-Fe(CO)3 complexes. The structure of one such η4 complex has been investigated by X-ray analysis. It shows a strictly planar phosphadiene unit. The Fe–PC3 plane distance is 1.6672 (7) Å, close to the normal values for (η4-diene)iron complexes. The tungsten atom is on the other side of this plane at 1.8528 (2) Å, and the substituent at phosphorus is almost coplanar with the PC3 unit. © 1990, American Chemical Society. All rights reserved.