Transmetalation as a Route to a Cyclic Heteronuclear Bifunctional Lewis Acid Containing Tin and Gallium

Tridentate Lewis acids with phenyl substituted 1,3,5-trisilacyclohexane backbones

A new 1,3,5-trisilacyclohexane scaffold with axially orientated ethynyl groups provides opportunities to create new tridentate Lewis acids. Some examples of these Lewis acids have been synthesized.

Transmetalation of self-assembled, supramolecular complexes

Substituting one metal for another in inorganic and organometallic systems is a proven strategy for synthesizing complex molecules, and in some cases, provides the only route to a particular system. The multivalent nature of the coordination in metal-ligand assemblies lends itself more readily to some types of transmetalation. For instance, a binding site can open up for exchange without greatly effecting the many other interactions holding the structure together. In addition to exchanging the metal and altering the local binding environment, transmetalation in supramolecular systems can also lead to substantial changes in the nature of the secondary and tertiary structure of a larger assembly. In this tutorial review we will cover discrete supramolecular assemblies in which metals are exchanged. First we will address fully formed structures where direct substitution replaces one type of metal for another without changing the overall supramolecular assembly. We will then address systems where the disruptive exchange of one metal for another leads to a larger change in the supramolecular assembly. When possible we have tried to highlight systems that use supramolecular self-assembly in tandem with transmetalation to synthesize new structures not accessible through a more direct approach. At the end of this review, we highlight the use of transmetalation in self-assembled aqueous inorganic clusters and discuss the consequences for material science applications.

Binding Cooperativity of Two Different Lewis Acid Groups at the Edge of Ferrocene

The binding properties of heteronuclear bidentate Lewis acids, in which an organoboron and an organotin moiety are attached adjacent to each other at one of the Cp rings of ferrocene, have been studied. Treatment of [1,2-fc(SnMe2Cl)(BClMe)] (1-Cl) (fc = ferrocenediyl) with one equivalent of pyridine or 4-dimethylaminopyridine (DMAP) resulted in diastereoselective complexation of boron. Adducts 2 and 3 have been studied by multinuclear NMR, and the stereoselectivity of complexation was further confirmed by single crystal X-ray diffraction of 2. The importance of cooperative effects that involve an intramolecular B-ClSn interaction on the diastereoselectivity is evident from comparison with binding studies on the phenyl-substituted analogue [1,2-fc(SnMe2Cl)(BPhMe)] (1-Ph). Complexation of 1-Ph led to diastereomeric mixtures of adducts 4 and 5, respectively, which were identified by multinuclear NMR including NOESY experiments. The solid-state structure of one of the diastereomers of 5 was confirmed by X-ray crystallography. Facile isomerization was found in solution and the barrier of activation was determined by VT NMR studies (4: Delta(#)(298) = 54.9+/-0.4 kJ mol(-1); 5: Delta(#)(298) = 70.3+/-0.1 kJ mol(-1)). Competitive binding of pyridine to 1-Cl and [FcB(Cl)Me] (Fc = ferrocenyl) showed that cooperative effects between tin and boron lead to significant Lewis acidity enhancement. Binding of a second nucleophile in the presence of excess of base occurred also at boron. The novel zwitterionic complexes [1,2-fc(BMe(py)2)(SnMe2Cl2)] (6) and [1,2-fc(BMe(dmap)(2))(SnMe(2)Cl2)] (7) formed, which consist of boronium cation and stannate anion moieties. The structure of 7 in the solid-state was confirmed by X-ray crystallography. Multinuclear NMR data and competition experiments indicate weak binding of chloride to tin in 7 and partial dissociation in solution.

Synthesis of heteronuclear bifunctional Lewis acids by transmetalation of 1,8-bis(trimethylstannyl)naphthalene with BCl3

Reaction of 1,8-bis(trimethylstannyl)naphthalene (1) with an excess of BCl3 at –78°, followed by warming to 0°C, results in the exclusive formation of the novel bifunctional Lewis acid 1-(chlorodimethylstannyl)-8-(dichloroboryl)naphthalene (2), a compound in which a boryl and a stannyl moiety coexist at the peri-positions of a naphthalene core. At elevated temperature compound 2 undergoes a chloride–methyl exchange, which affords 1-(dichloromethylstannyl)-8-(chloromethylboryl)naphthalene (3). Compounds 2 and 3 have been characterized by multi-nuclear NMR spectroscopy. The single crystal X-ray analysis for compound 3 reveals a sterically crowded structure with an essentially trigonal planar boron center, and a tin center pentacoordinated in a [4+1]-fashion. Upon exposure to traces of water, compound 3 is converted into the borinic acid derivative 1-(dichloromethylstannyl)-8-(hydroxymethylboryl)naphthalene (4), which has been characterized by multi-nuclear NMR spectroscopy and single crystal X-ray analysis.Key words: multidentate Lewis acids, organoboranes, organostannanes.