Addition Lewis base

Summary of the Relationship between Diastereoselectivity and the Transition Structure. In this section we considered simple diastereoselection in aldol reactions of ketone enolates. Numerous observations on the reactions of enolates of ketones and related compounds are consistent with the general concept of a chairlike TS.35 These reactions show a consistent E - anti Z - syn relationship. Noncyclic TSs have more variable diastereoselectivity. The prediction or interpretation of the specific ratio of syn and anti product from any given reaction requires assessment of several variables (1) What is the stereochemical composition of the enolate (2) Does the Lewis acid promote tight coordination with both the carbonyl and enolate oxygen atoms and thereby favor a cyclic TS (3) Does the TS have a chairlike conformation (4) Are there additional Lewis base coordination sites in either reactant that can lead to reaction through a chelated TS Another factor comes into play if either the aldehyde or the enolate, or both, are chiral. In that case, facial selectivity becomes an issue and this is considered in Section 2.1.5. 

Leighton has combined this concept of strained silacycles [62-66] with the asymmetric allylation chemistry in a series of pubhcations [60, 67-70], Leighton s aUyhc silacyclopentane 29 [67] (Scheme 20) works equally for allylation of aromatic and ahphatic aldehydes in the absence of additional Lewis bases (promoter activator) or Lewis acids with high yield and enantioselectivity. The mechanism of... 

When these materials are treated with additional Lewis base, the following reactions are observed, giving the same products obtained from the direct reaction of diborane(6) with ammonia and trimethylamine ... 

The X-ray structural characterization of a series of mono-, di-, and poly-lanthanidocene aluminohydride and alane complexes by B.M. Bulychev and G.L. Soloveichik revealed a variety of coordination modes of the [A1H4] , [AIH3] and [A1H2]+ moieties [273-281]. Routinely, these complexes were synthesized by the reaction of a lanthanidocene chloride with LiAlH4 or A1H3. The nuclearity and All x coordination mode depend on the presence of additional Lewis bases, such as THF, diethyl ether, or triethylamine (Chart 7). Diene polymerization based on lanthanidocene aluminohydrides has not been reported. 

Although the Kyoto group initially called the additives Lewis bases [64] and the Akron group electron donors [106], herein we call them collectively nucleophiles, which we believe is a more general term. 

Reaction of the titanium-aluminum reagent (3) is more complex. If no additional Lewis base is employed, the carbonyl compound takes that function. Interaction with the aluminum atom is required to... 

The presence of additional Lewis base sites within the molecule can result in the formation of chelates with SnCU or TiCU, which can lead to 1,2- or 1,3-asymmetric induction with the appropriate substitution at the C-2 or C-3 center. NMR studies have provided a basis for explaining the levels of diastereofacial selectivity observed... 

Donor-r acceptor (D-rA) interactions. The combination of donor and acceptor sites raises the possibility of D A interactions (intramolecular and intermolecular). In most systems, such D A interactions are prevented by the steric demand of the substituents (steric frustration) and/or the nature of the organic linker (geometric frustration). Noticeable exceptions are illustrated in Scheme 1 (i) the Cj-bridged phosphine-alane 3 adopts a head-to-tail dimeric structure and is usually reacted in the presence of an additional Lewis base to displace the A1 interactions " (ii) the di- and tri-phosphine boranes 7 and 8 equilibrate in solution between open and closed forms which were unambiguously identified in the solid state by X-ray diffraction analyses (iii) the pyridine-borane 2 exists in solution as a mixture of closed monomeric form and head-to-tail dimeric structure, but N B interactions are readily cleaved upon coordination to Ru. ... 

Lewis base is called the internal Lewis base. This is followed by addition of Group I-in metal-alkyl component with an additional Lewis base. The base that is added the second time may be the same or a different one from that used in the original milling. In either case it is called the external Lewis base. It is not unconunon to use an ester as the internal base and an oiganosilane compound, like phenyltriethoxysilane, as the external one. 

There has been an outburst of research interest in the structures, physical properties, and chemistry of the group 2 metal aryloxides. This is particularly true for the elements strontium and barium where work has been stimulated by the possible use of metal aryloxides as precursors (either via sol-gel or MOCVD processes) for the formation of binary and ternary oxides containing these metals.2 Synthetic procedures are based on either the halide (Be) alkyl derivatives (Mg, Grignard derivatives, etc.) or the actual metallic element (Ca, Sr, Ba). Structural studies (Tables 6.14-6.18) show for the smaller elements Be, Mg, and Ca that monomeric and dimeric structural motifs dominate, with rarer examples of trinuclear clusters, e.g. [Ca3(OPh)5(HMPA)6][OPh.2HOPh].2 2 in the case of strontium and barium a more extensive cluster chemistry has been developed for small aryloxide ligands, while monomeric units with terminal aryloxides can be formed with bulky ligands and sufficient additional Lewis bases, e.g. [Ba(OC6H2Bu2-2,6-Me-4)2(THF)3].2 ... 

It is to be noted that alkoxides of more electropositive metals - alkaline, alkaline earth, rare earth (RE), and Zr(IV), Hf(IV), and Sn(IV) - often form quite stable complexes with protic ligands, which are also Lewis bases (alcohok, amines). Their formation is supported by both complexation of the metal atom with this additional Lewis base function, which is apparently weaker than the alkoxide one, and formation of the hydrogen bonds with active hydrogen atoms of the additional ligands [61]. No stable complexes are formed with stronger but aprotic nucleophiles such as trialkylamines, R3N [62]. Very often, the alcohol solvates have a bit lower solubility, but are easily purified by recrystallization, which makes them attractive as commercial products. When the pure alkoxides are highly... 

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. 

---