Bidentate group 13 Lewis acids with ortho-phenylene and peri-naphthalenediyl backbones
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The chemistry of polydentate Lewis acids1-3 is still in its infancy but is rapidly expanding to a number of areas including molecular and anion recognition, catalysis and crystal engineering. Conceptually, polydentate Lewis acids are often regarded as the charge reverse analogs of polydentate Lewis bases and are thus expected to act as polydentate ligands for electron-rich substrates.4 This analogy was recognized almost four decades ago by Shriver and Biallas who showed that methoxide anions are effectively chelated by 1,2-bis(difluoroboryl)ethane (1), a charge reverse analog of ethylenediamine (Scheme 1).5 Following this seminal contribution, a great deal of effort has been devoted to the synthesis and study of such polydentate Lewis acids. An important part of the compounds that have been investigated consists of polyfunctional organostannanes1 and organomercurials.6-11 Despite the soft Lewis acidity of tin and mercury, these compounds exhibit remarkable properties and have been used as receptors for anions as well as for small electron-rich molecules. For example, while the 1,2-distannylbenzene 212 or the mercuraborand 313 readily complex chloride anions, trinuclear mercury derivatives such as trimeric perfluoro-ortho-phenylene 4 can be used as receptors for organic substrates including acetone (Scheme 2).14 Aiming at more powerful Lewis acids, a great deal of effort has been devoted to the preparation of polydentate Lewis acids that contain hard Lewis acidic elements of group 13.15,16 Despite their greater sensitivity toward hydrolysis and oxidation, several polydentate group 13 derivatives have been successfully synthesized and investigated. An important aspect of this research resides in the choice of the backbone that serves to hold the Lewis acidic site. In order to insure that the Lewis acidic sites remain available, it is important to choose a backbone that does not contain accessible Lewis basic sites such as oxygen or nitrogen atoms which could neutralize the Lewis acidic centers by intramolecular Lewis adduct formation or by through-bond -electron donation. Moreover, as in the chemistry of polydentate Lewis bases, the structure of the backbone should serve to dictate the spatial orientation of the Lewis acidic centers as well as the overall rigidity or flexibility of the target polydentate Lewis acid. Since the occurrence of cooperative effects is entropically impaired with flexible systems, the preorganization of the Lewis acidic sites in a rigid molecular edifice is often preferred. Recent achievements in this area have centered on derivatives of ortho-substituted benzenes and peri-substituted naphthalenes. In this chapter, we will review the chemistry of bidentate Lewis acids containing two group 13 elements linked by an ortho-phenylene15,16 or a peri-naphthalenediyl backbone. This contribution follows an earlier review concerned with peri-naphthalenediyl group 13 derivatives.17. 2005 Elsevier Inc. All rights reserved.
