Phosphaalkynes, cyclooligomerization

Finally, the cyclooligomerization of phosphaalkynes (e.g., - 16) [10, 28] in the coordination sphere of a metal should be mentioned. Zirconium complexes, which will be discussed in detail below, dominate the chemistry of phosphaalkyne cyclotetramers. 

In spite of their sterically demanding substituents, the phosphaalkynes chibit an enormous potential for cycloaddition reactions. Even though no intermediates can yet be detected in their thermal cyclooligomerization reactions, the obtained product palette allows the assumption of head-to-head and head-to-tail dimerizations, i.e. [2 + 2]-initiating reactions. 

In addition to the above-mentioned aluminum halides (Section 6.5.3.1), triorganoaluminum reagents are also able to initiate cyclooligomerization processes of the phosphaalkyne 9 a. In these reactions, aluminum is incorporated directly into the cage system. The element combination phosphorus, carbon, aluminum in a polycyclic system was previously unknown. The product palette is highly influenced by the choice of the solvent and the substituents at the metal center. 

When sterically more demanding trialkylaluminum reagents (R = i-butyl or 2-phenyl-propyl) are allowed to react with 9 a in diethyl ether or n-hexane, the cyclooligomerization process ends at the 1 3 compounds 82 (Scheme 6-21) [65J. The compounds have the structure of a triphosphametallahomobenzvalene, as has been confirmed for the case of R = CH2—CHPh —CH3 by X-ray crystallography. A plausible mechanistic rationale for the product formation involves a two-fold 1,2-addition of the triorganoaluminum to two equivalents of phosphaalkyne 9a (- 84). This is followed by a crossed [2 + 2]-cycloaddition... 

Wettling T, Schneider J et al (1989) Tetra-tcrt-butyltetraphosphacubane the first thermal cyclooligomerization of a phosphaalkyne. Angew Chem Int Ed Engl 28 1013—1014... 

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