Donor-bases aggregation

The most frequently used carbanionic lithiating agents, RLi, are MeLi, n-BuLi, s-BuLi and t-BuLi. The RLi compounds are aggregates, the degree of aggregation depending on the bulk of R, concentration and solvent. They are electron-deficient compounds, i.e., electron-pair acceptor acids, which on coordination with electron-pair donor bases, in particular ethers and amines, depolymerize to smaller units e.g., addition of TMED to hexameric n-BuLi in hydrocarbons forms the coordinatively saturated, monomeric n-BuLi-TMED. An exception is MeLi, for which the tetramer persists even in THF or on addition of TMED. 

A further significant factor slowing the development of the chemistry of the heavy alkali metal derivatives has been the high reactivity of the complexes, with frequent attack of the ethers used to break up the aggregates in hopes to achieve increased solubility. In fact, ether cleavage is a common observation and manipulations at very low temperatures are often required to overcome this issue.9 Ether cleavage may also be suppressed by the introduction of nitrogen-based donors such as TMEDA and PMDTA. 

Fluorescence lifetime measurements on the aggregate have shown that the rate constant of the intermolecular energy transfer from the zinc porphyrin unit to the free-base porphyrin unit has been evaluated to be 3.0 x 109 s-1. This value is reasonable from a model in which dendritic donor 6b and acceptor 5a contact each other directly at their exterior surfaces (Scheme 2). Therefore, electrostatic assembly of positively and negatively charged dendrimers provides a promising supramolecular approach to construct photofunctional materials with nanometric precision. 

In addition to the determination of distances at a supramolecular level, RET can be used to demonstrate the mutual approach of a donor and an acceptor at a supramolecular level as a result of aggregation, association, conformational changes, etc. The donor and acceptor molecules are generally covalently linked to molecular, macromolecular or supramolecular species that move toward each other or move away. From the variations in transfer efficiency, information on the spatial relation between donor and acceptors can thus be obtained. Because of its simplicity, the steady-state RET-based method has been used in many diverse situations as shown below5 . 

The complexation of the base with the oxirane in nonpolar solvents drives the fi-deprotonation to occur from the syn face (see above), but also influences its regio- and stereochemistry. During the aggregation step, two conformational structures can be formed depending on which lone pair of the oxygen is used as the donor site. The relative stabilities of these complexes are largely influenced by nonbonded interactions between the base and the syn substituents on the epoxide (Scheme 16). 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

Instead of coordinating mono-hapto bases to the protons of (Ph2SiO)8[Al-0(0H)]4, we have explored the field of di-hapto bases like poly methylene diamines as possible donors. As the molecule (Ph2Si0)8[A10(0H)]4 has pairs of hydroxyl groups directed in the same direction a coordination of H2N(CH2) NH2 could either lead to an intramolecular loop or to intermolecular aggregations of molecules. This field of chemistry is actually very much expanding and we give in this review only a small out-look, essentially with the diamines 1,3-diaminopropane, 1,4-diaminobutane and 1,5-diaminopentane. 

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