Diethylamine boron complexes

From a formal point of view this process may be regarded as the [2-f 1] cycloaddition of a transient phosphinidene to the PC triple bond of the electron-rich phosphaalkyne. A little later a different approach to diphosphirenes was devised by Bertrand. When a toluene solution of phosphaalkene Pr NP=C(NPr2 )P(NPt2 )2 90 was treated with 2 equiv of boron trifluoride-diethylamine complex at room temperature, a clean reaction took place to afford the pale yellow crystalline diphosphirenium salt 91 (60% yield) <1991JA8160> <1994JA6146>. 

Amines are good Lewis bases, donating two electrons to electron-deficient atoms to form a dative bond in the expected ate complex (see Chapter 2, Section 2.5). Diethylamine (iV-ethylethanamine, 40) readily reacts with the strong Lewis acid to form ate complex 41. Note the dative bond between nitrogen and aluminum and the direction of the arrow from the electron-rich nitrogen atom to the electron-deficient aluminum atom. Similarly, tertiary amine 42 (iV-ethyl-iV-methylethanamine) reacts with boron trifiuoride (BF3) to form ate complex 43. Amines will be used as Lewis bases several times in this book. 

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