Lewis bases adducts formation

EVANS provides a summary of the reactions of organometallics with oxide surfaces that lead to well-defined surface species including mononuclear and polynuclear complexes and monometallic and bimetallic particles. These surface reactions are described by the same principles encountered in molecular chemistry the reaction classes include nucleophilic attack at the ligands, electrophilic attack at the metal-carbon bond, oxidative addition, Lewis base adduct formation, redox reactions, etc. The synthesis of well-defined reactive sites on surfaces by these organometallic routes will facilitate the study of elementary steps in surface chemistry. 

The electrophilic center is sometimes generated from the Lewis base by formation of the adduct, and the reaction proceeds by migration of a boron substituent. 

Lewis base adducts, 25 64, 68-69 metal exchange reactions, 25 57 NMR spectra, 25 93-95 pyrolysis, 25 107 silicide formation, 25 110 tetracarbonylsilyl hydride reaction with isoprene, 25 75 reductive elimination, 25 81 site, formation, ribonucleotide reductase, 43 372-375... 

The ability of the boron atom of 59 to engage in a donor-acceptor interaction was illustrated with DMAP and DABCO (DABCO = diazabi-cyclo-[2.2.2]-octane) that readily formed the corresponding Lewis adducts. Interestingly, a similar behavior was retained after coordination of the phosphorus atom to palladium. The formation of the Lewis base adducts 66a and 66b of complex 65 (Scheme 38) was supported by solid-state 31P and nB CP/MAS-NMR spectroscopy (<5 1 B = 5-6 ppm), although the occurrence of decomposition and/or dissociation processes impeded spectroscopic characterization in solution and recrystallization to obtain X-ray quality crystals. Compounds 66a and 66b substantiate the ability of ambiphilic compounds to engage concomitantly into the coordination of donor and acceptor moieties. Such a dual behavior opens interesting perspectives for the preparation of metallo-polymers and multimetallic complexes. 

Scheme 19).53 Refluxing of 56 in THF led to the formation of dimeric [Cy7Si7012Ga]2 (57) as a structural analog of the corresponding boron and aluminum dimers 23 and 33, respectively. The reactivity of 58 also resembles that of the aluminum species 33 in that the dimeric structure could readily be split by triphenylphosphine oxide to form the neutral Lewis base adduct Cy7Si7012Ga(0PPh3) (58). 

Methyltrioxorhenium (MTO) has proven to he a useful oxidant for a number of reactions. A problem with the use of MTO in epoxidation is the acidity of the reagent, which leads to diol formation. Several different methods have previously been reported in an attempt to solve this problem. The use of microeneapsulated Lewis base adducts of MTO appears to be a good solution <05T1069>. Another modification of MTO is (PPhjljlReCNCS) ] with H Oj as an oxidant <05TL339>. 

The unique reactivity of the above system with H2 appears to arise from the unquenched Lewis basicity and acidity of the respective donor P and the acceptor B centers. This inference prompted questions about the nature and reactivity of other phosphine-borane systems and, more broadly, of Lewis acid/base combinations. Is it necessary to have a link between the donor and acceptor sites Could similar H2 activation arise from combinations of donors and acceptors in which steric encumbrance frustrates Lewis acid-base adduct formation If indeed such frustrated Lewis pairs could be uncovered, could one exploit them for the activation of small molecules and applications in catalysis ... 

The most important drawback of the MTO-catalyzed process is the concomitant formation of diok instead of the desired epoxides, especially in the case of more sensitive substrates [30]. It was quickly detected that the use of Lewis base adducts of MTO significantly decreases the formation of diols due to the reduced Lewis acidity of the catalyst system. However, while the selectivity increases, the conversion decreases [30-32]. It was found that biphasic systems (water phase/organic phase) and addition of a significant excess of pyridine as Lewis base not only hamper the formation of diols but also increase the reaction velocity in comparison to MTO as catalyst precursor [33, 34]. 

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