Lewis basicity towards

The order of decreasing Lewis basicity toward trimethylaluminum is thiane, thiolane, diethyl (or dimethyl) sulfide, thietane and 2-methylthiirane (67IC1461). Thietane can be protonated at -60 °C with fluorosulfonic acid in antimony pentafluoride-liquid sulfur dioxide the NMR spectrum shows bands at 8 7.40 (S—H) and 3.20-4.40 p.p.m. (C—H) (71JOC1121). 

Soft metals such as Ag+, Cu+ and MeHg+ coordinate to Co SR" complexes and uncoordinated thioethers with about equal ease. Thus it seems that Co" and R" " have similar effects on the binding power of RS , reducing it by about 10 (Table 90 equation 167). This property has implications for electron transfer since ligation of coordinated thiol to a second metal centre is probably the first step in inner-sphere electron transfer. Data given in Table 90 demonstrate that Co" —SR" retains some Lewis basicity towards soft metals. These studies have utilized [Co(R-—S)(en)2] systems (R—S = cyst, tga) but the metals Ru"" and Co" also form very stable... 

Measurements of gas phase dissociation constants have shown that the Lewis basicity towards hard reference acids such as BMc3, BF3 or BCI3 falls down Group V in the order MejN > Me P > MejAs > Me3Sb ( MejBi). Towards the softer acid BH3, however, the order is Me3 > MejN > MejxAs. Orders of basicity are strongly influenced by the nature of the acceptor. This is especially evident from the chemistry of transition elements, which often form stable complexes with phosphines and arsines for which there are no amine analogues (Chapter 5). Phosphines and arsines are, in the classification of Pearson, typical soft bases which form their most stable complexes with soft acids such as polarizable transition metal and heavy post transition metal acceptors. 

The arene groups in (r/ -arcnc)tricarbonylchromium complexes are typically electron poor and display poor reactivity toward electrophiles. In the case of mercuration reactions, this lack of reactivity can be overcome by attachment of Lewis-basic substituents to the arene ring. For example, in the case of 75a-c, the presence of a pyridyl, oxazolyl, or methyl-A,A-dimcthylami no group promotes ortho-mercuration, leading to the formation of the bimetallic complexes 76a-c (Equation (28)). 07... 

The mercuration of ferrocenylimines with Hg(OAc)2 has been studied. - 6 Mercuration occurs selectively at the a-position relative to the imine group to afford compounds 86a-i (Scheme gy107,108 The regioselectivity of these reactions points to the directing role of the Lewis-basic imine functionality. Similar factors probably play a role in the formation of the ferrocenylketone and ferrocenylaldehyde derivatives 87a-f and 87g-j, respectively. These derivatives readily react with amines to afford the corresponding imines (Scheme 9). Presumably, the Lewis-acidic mercury center of the monomercurated ferrocenylketones and ferrocenylaldehydes activates the carbonyl functionality toward nucleophilic attack by the amine. 

To uncover new reaction pathways towards unprecedented target materials, the understanding of structure formation principles is important. One of the lead structural principles in lithium organic chemistry is the /X3-capping of the metalated carbon atom Ca to a lithium triangle. This motif can further aggregate to form deltahedra. The tetrahedra and octahedra can either be free of solvent or be coordinated by Lewis-basic donor molecules... 

An investigation of keto-enol tautomerism for perfluorinated keto-enol systems was undertaken. N-methylpyrrolidone (NMP) catalyzes equilibration of the keto and enol forms, but if used in more than trace amounts, it drives the equilibrium strongly toward enol because of hydrogen bonding to the amide. The enol is much more thermodynamically stable than its ketone, and it was found that in mildly Lewis basic solvents, such as ether, THE, acetonitrile, and NMP, the enohzation equilibrium lies too far right to allow detection of ketone (Correa et al., 1994). 

Whereas THF and DME were suitable solvents, Lewis basic additives such as HMPA, DMSO or TMEDA hampered the reaction.82 These results point towards an intramolecular assistance of the zinc coordinated by the nitrogen atom. A related effect was evidenced in the additions of Grignard reagents to allylic alcohols83-87. Two mechanisms were proposed for the allylzincation of homoallylic amines involving formation of a complex with the... 

Boron heterocycles bearing a hydroxy group on the boron atom are acidic. Boric, boronic and borinic acids are Lewis acids towards hydroxide ion. It was suggested that six-membered 5-hydroxy compounds behaved as Brdnsted acids. The experimental evidence for this was the similarity between the UV spectra of the hydroxy compounds in neutral and basic solution. The rationale for this fact was the maintained aromaticity which was supposed to favour the formation of, for example, ion (163) over ion (164). This suggestion was at first supported by nB NMR spectroscopy, which differentiates between anions of Lewis-acidic... 

Just as there can be secondary IEs on acidity and basicity, namely, on proton-transfer equilibria, so also can there be secondary IEs on Lewis acidity and basicity. All the examples though have been deuterium IEs, perhaps because heavy-atom effects are too small to measure reliably except when they are primary. Deuteration can affect not only basicity toward protonation but also basicity toward a Lewis acid. Less closely analogous but also possible is the effect of deuteration on the electrophilicity of a Lewis acid. 

The bonding is sometimes described as synergistic, implying that the donation M— CO enhances the Lewis basicity of CO towards M. Carbon monoxide tends to stabilise low oxidation states of the d block elements, since its bonding requires the presence of filled nd orbitals on M. 

Scheme 11.1 also summarizes other impressive examples of the performance of the CBS method [1-8]. Several excellent reviews on the CBS method have appeared recently [1, 2], and no detailed discussion of the development of the process or substrate scope shall be presented in this review. Please note, however, that the oxazaborolidine-catalyzed borane reduction of ketones is a prime example of bi-functional catalysis [2, 9] - as shown in Scheme 11.2, the current mechanistic picture involves simultaneous binding of both the ketone and the borane to the Lewis-acidic (boron) and Lewis-basic (nitrogen) sites of the catalyst A. In the resulting ternary complex B, the reaction partners are synergistically activated toward hydride transfer. 

The solvating ability of solvents depends not only on their general polarity, which is a non-specific property, but in a large part to their ability to interact in a specific manner with the solute. This may take place by the donation of a nonbonding pair of electrons from a donor atom of the solvent towards the formation of a coordinate bond with the solute, therefore exhibiting Lewis basicity, or the acceptance of such a pair from a solute, an exhibition of Lewis acidity of a protic or protogenic solvent towards the formation of a hydrogen bond between it and... 

The ability of a solvent to donate a pair of electrons of one of its donor atoms towards the formation of a coordinate bond with an acceptor atom of a solute is a measure of its Lewis basicity. Several methods have been proposed over the years to express this donor ability or donicity, but only few of them have proved to be viable and of any real usefulness. 

Positively-charged fragments such as [ML,]+, CH , and H + are all strong electrophiles ( superelectrophiles in the extreme sense (13)) towards the Lewis basic H2, but transition metals can uniquely stabilize H2 and other cr-bond coordination by back donation from d-orbitals that main group analogues cannot do. This bonding is then remarkably analogous (14) to the Dewar-Chatt- Duncanson model (15) for ji-complexes (6). 

The mono-alkoxy product from (+)-ethyl lactate and diisopropylsilane (le) is shown to be more reactive than la and If (Scheme 31). We can only speculate at this time that there may be some intramolecular interactions between the Lewis basic carbonyl oxygen of the (+)-ethyl lactate and the silicon atom, which may either, activate the silane towards oxidative addition to the metal catalyst or increase the rate at which the silane-metal complex is attacked by the nucleophilic alcohol. 

The Lewis acidity of these materials can be varied by varying the relative amount of organic salt and AICI3 with a molar excess of AICI3 they are Lewis acidic with a molar excess of the organic salt they are Lewis basic. To yield neutral liquids it is necessary to buffer the 50 50 mol% mixture with NaCl, because minimum variations from the equimolar composition would shift them towards basic or acidic compositions [1]. 

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