Hf-centered Lewis acid

This chapter describes the use of Hf-centered Lewis acids in organic synthesis. Although the last two decades have witnessed an explosive growth of the use of titanium-centered Lewis acids [la] and then their zireonium eounterparts [lb], the corresponding status of the hafnium derivatives remains limited, presumably because of less ready availability, and also the well-known similarity to Zr because of the lanthanide contraction [2]. Increasing data on the uniqueness of hafnium-centered Lewis acids are, however, becoming available. 

In 1989 the synthesis and isolation of the first compounds with stable Xe—C bond were reported by Naumann and Tyrra285 and Frohn and Jakobs1068 by the introduction of the organic group through nucleophilic substitution called xenodeborylation [Eq. (4.256)]. The boron as Lewis acid center polarizes the Xe—F bond and thereby allows it to overcome the low electrophilicity of the Xe center to form the fluoroar-ylxenonium fluoroborates 366. When the reaction is carried out in anhydrous HF, all C6F5 groups of the borane are transferred to xenon [Eq. (4.256)].1069... 

Another stereochemically interesting feature was observed in the synthesis of the gilvocarcins [17]. The al preference is not obvious for the furanoside-series, and indeed the outcome depends heavily on the metal center of the Lewis acid—a-selec-tivity is obtained with Hf, /3-selectivity with Sn (Eq. 7). Although the difference might reflect the behavior of the eoordinated species, many factors inhibit understanding of the true origin of the stereoselectivity. 

Gosh and Kydd [520] reported on an infrared study of the effect of a mild treatment with hydrofluoric add, HF, on the acidity of H-ZSM-5. A decrease of the concentration of Bronsted add centers and an increase of the Lewis site density was observed. Due to results of parallel TPD experiments with pyridine as a probe, the authors attributed to these Lewis acid sites a higher strength than to the remaining Bronsted add centers. Severe HF treatment removed all L-sites. 

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