Alkenylphosphonium salts

Chapter 8. Nucleophilic Species That Form Carbon-Carbon Bonds 

An interesting modification of this reaction generates a normal Wittig reagent by reaction of 594 with an organocuprate (sec. 8.7.A). Addition of dibutyl cuprate to 594 generated ylid 602. Subsequent reaction with hexanal gave the expected alkene 603 as a 1 4 mixture of (Z/ ) isomers.524 

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The mixed triarylphosphine 787 can be prepared by the reaction of (trimethylsily )dipheny phosphine (786) with aryl halides[647]. Ph3P is converted into the alkenylphosphonium salt 788 by the reaction of alkenyl tri-flates[648]. 

Phosphonium salts can be synthesized by the transition-metal-catalyzed addition reaction of triaryphosphines and acids to unsaturated compounds. The reaction of PPh3, CH3SO3H, and alkynes in the presence of a palladium or rhodium catalyst gave alkenylphosphonium salts. Although Pd(PPh3)4 directed the C-P bond formation at the internal carbon atom of aliphatic 1-alkynes (Markovnikov mode), [RhCl(cod)]2... 

Novel alkenylphosphonium salts were subjected to the Wittig reaction (Scheme 12). Allylic deprotonation took place for phosphonium salts possessing such protons, and the olefination proceeded after double bond migration. In cases where such protons were absent, allene formation was observed. 

During our investigation, PPhs and CH3SO3H were found to add to unactivated alkynes in the presence of a palladium or rhodium complex. The regiochemistry and stereochemistry could be controlled by a judicious selection of the metal catalyst. Alkenylphosphonium salts have various applications in synthetic chemistry,and the present method enables an easy access to the organophosphorous compounds using readily available starting materials. 

Allenes. On activation with (/-PrO)2TiCl2 and (McjSOjNNa the phosphorane (Me2N)jP=CH2 reacts with aldehydes to form alkenylphosphonium salts. Allenes are obtained when such salts are further deprotonated and treated with other aldehydes. 

The alkenylphosphonium salt 108 was prepared by the addition of PPhs at the internal carbon of 1-alkyne in the presence of methanesulfonic acid. After anion exchange with LiPFe, the conjugated diene 109 was obtained by the reaction of 108 with an aldehyde [33]. 

Unconventional transformations were also discovered. The reaction of a phosphonium ylide such as tributyl[(trimethylsilyl)methylene]phos-phorane with enolisable aldehydes did not afford the expected Wittig reaction product. Conversely, alkenylphosphonium salts were obtained in good yields (65-89%, Scheme 3). ... 

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