Hydro-de-metallation

Hydrolysis of Organometallic Compounds Hydro-de-metallation or Demetallation... 

Mechanistic hypotheses play an important role in developing new catalytic and selective heterofunctionalizations of alkenes. Two basic reaction cycles for metal-catalyzed hydroalkoxylation (and hydration, for R = H) of alkenes can be postulated (Scheme 2). One pathway leads to Markovnikov products via activation of the nucleophile, oxy-metallation, and protonolysis (hydro-de-metallation) (Scheme 2a). Alternatively to the inner sphere syn-oxymetallation depicted in Scheme 2a, external anti-attack of the nucleophUe to coordinated olefin is plausible. The oxidation state of the metal remains constant in this cycle. The alternative hydrometallation pathway (Scheme 2b) proceeds via oxidative addition of the H-OR bond, hydrometallation of the olefin, and reductive elimination to the anti-Markovnikov addition product [3,4]. 

While several metal-catalysed approaches to solve the task of generating fl tz-l,2-diols have been developed, this method remains remarkable, since it represents the first small molecule-catalysed, catalytic version of this transformation. The starting materials do not need activation or protection, and the reaction can be performed under standard conditions without further precautions. The selectivity of the reaction potentially results from a hydro Q acetone enamine-initiated transition state (5). More recently, Enders et al. applied a related method to the organocatalytic synthesis of sialic acid precursors (Scheme 5.6). Protected pyruvic aldehyde (6) was reacted with several aldehydes, forming the desired aldols (7) in moderate yields, but good selectivities (31-51%, 90-92% de, 73-99% ee). The conditions were optimised to limit detrimental side reaetions sueh as Mannich elimination or formation of the aeetal self-aldolisation produet. While generally easily applicable and robust, the method laeks effieiency in one key parameter, its reaction time. This limitation, combined with moderate yields unfortunately prevents a scale-up to or beyond the pilot plant. 

It is the values of mass transfer coefficients which change on studying the metal extraction process under different hydro-dynamic conditions, even more so in a different equipment. As shown by Perez de Ortiz et al. (2), the assumption that the rate... 

The previously unreported complex, Tp Zn(K -BH4) (Tp =hydro-tris(3,5-dimethylpyrazolyl)-borate anion), has been characterized by Des-rochers et al. by the use of IR and NMR spectroscopies including measurements of Vhb couplings which allowed the authors to compare it with analogous closed shell borohydride complexes. All measurements have confirmed the persistent but fluxional zinc-borohydride coordination in solution on the NMR timescale. The 1 1 1 1 quartet assigned to coordinated borohydride in the proton spectrum (Vhb = 83Hz) has confirmed the equivalence of all hydrogen atoms because of rapid interconversion of the protons coordinated and not coordinated to the metal. 

---