Pivaloylmetals ( t Bu‐COM: M=Li, MgX, K) as Equilibrium Components

Short‐lived pivaloylmetals, (H 3 C) 3 C‐COM, were established as the reactive intermediates arising through thermal heterolytic expulsion of O=C t Bu 2 from the overcrowded metal alkoxides t BuC(=O)‐C(‐OM) t Bu 2 (M=MgX, Li, K). In all three cases, this fission step is counteracted by a faster return process, as shown through the trapping of t Bu‐COM by O=C( t Bu)‐C(CD 3 ) 3 with formation of the deuterated starting alkoxides. If generated in the absence of trapping agents, all three t Bu‐COM species “dimerize” to give the enediolates MO‐C( t Bu)=C( t Bu)‐OM along with O=C t Bu 2 (2 equiv). A common‐component rate depression by surplus O=C t Bu 2 proves the existence of some free t Bu‐COM (separated from O=C t Bu 2 ); but companion intermediates with the traits of an undissociated complex such as t Bu‐COM & O=C t Bu 2 had to be postulated. The slow fission step generating t Bu‐COMgX in THF levels the overall rates of dimerization, ketone addition, and deuterium incorporation. Formed by much faster fission steps, both t Bu‐COLi and t Bu‐COK add very rapidly to ketones and dimerize somewhat slower (but still fairly fast, as shown through trapping of the emerging O=C t Bu 2 by H 3 CLi or PhCH 2 K, respectively). At first sight surprisingly, the rapid fission, return, and dimerization steps combine to very slow overall decay rates of the precursor Li and K alkoxides in the absence of trapping agents: A detailed study revealed that the fast fission step, generating t Bu‐COLi in THF, is followed by a kinetic partitioning that is heavily biased toward return and against the product‐forming dimerization. Both t Bu‐COLi and t Bu‐COK form t Bu‐CH=O with HN(SiMe 3 ) 3 , but only t Bu‐COK is basic enough for being protonated by the precursor acyloin t BuC(=O)‐C(‐OH) t Bu 2 .