The synthesis of a variety of phosphaalkenes has received great interest due to their potential as ligands in transition-metal catalysed transformations or for the synthesis of π-conjugated molecules and materials. One attractive method for their preparation is the phospha-Wittig reaction. Despite its similarities to the Wittig reaction, it is much less well-developed and still restricted to the conversion of aldehydes, when metal-free reagents are used. Due to structural similarities, phosphonio-phosphanides 1[OTf] can be regarded as P-imidazoliumyl-substituted phospha-Wittig reagents. Consequently, when reacted with aldehydes, cationic phosphaalkenes are formed intermediary as monitored by means of 31P NMR spectroscopy. Owing to their high reactivity, they readily undergo cycloaddition reactions with dipolarophiles giving cationic phospha-heterocycles such as 2[OTf]2. Replacing aldehydes with thiocarbonyls instead gives access to a variety of isolable and differently functionalized cationic phosphaalkenes or phosphanides 3[OTf]. In addition, phosphonio-phosphanides 1[OTf] show a surprisingly complex reaction with CS2 that can be controlled to yield various cationic products, including phosphanylidene thiolates 4[OTf]. The details, the scope of the reactions and selected follow-up chemistry are discussed.

Phospha-Wittig type reaction for the synthesis of imidazoliumyl-substituted phosphaalkenes and phosphanides

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