α‐Mercurio‐Substituted Phosphorus Ylides: A Simple Route to Vinylmercury Compounds

The synthesis of novel α‐mercurio‐substituted phosphorus ylides, Ph 3 PCR[HgN(SiMe 3 ) 2 ] 3a – d (R = Me, Et, i Pr, Ph) is easily accomplished by mixing equivalent amounts of a phosphorus ylide 1a – d with Hg[N(SiMe 3 ) 2 ] 2 in an inert solvent. These organometallic compound are completely characterized including an X‐ray analysis of 3a . Their reaction with benzaldehyde yields vinylmercury amides ( E )/(Z)‐PhHCCR[HgN(SiMe 3 ) 2 ] 4a – d . In reactions involving 3a – c the stereochemistry [favored formation of ( E )‐ 4a (≈;72—83%) and (Z)‐ 4b , c (≈60—73%)] in the absence of a lithium salt is contrary to the stereochemistry of the established Wittig olefination reaction with non‐stabilized phosphorus ylides like 1a – d . From the reported results, the preliminary conclusion is drawn that the stereochemistry is mainly controlled by steric interactions. Only in one case a remarkable salt effect is observed. When 3c reacts with benzaldehyde in the presence of LiBr and THF as solvent up to 95% of ( E )‐ 4c is formed. The semistabilized ylide 3d gives under all conditions studied a 50:50 mixture of ( E )/(Z)‐ 4d . The vinylmercury amides 4a , c are cleaved by hydrous NaOH to yield divinyl‐mercury compounds ( E , E )‐(PhHCCR) 2 Hg and (Z,Z)‐(PhHCCR) 2 Hg (R = Me: 7a ; R = i Pr: 7c ). The compound 4d is cleanly cleaved by NaBH 4 in hydrous 3 M NaOH to yield a 50:50 mixture of cis ‐ and trans ‐stilbene ( 8 ).