Lewis acids coordination bonds

The mechanism of conjugate addition reactions probably involves an initial complex between the cuprate and enone.51 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species, which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.52 Solvent molecules also affect the reactivity of the complex.53 The mechanism can be outlined as occurring in three steps. 

In Chapter 9, the hard-soft acid-base principle was discussed, and numerous applications of the principle were presented. This principle is also of enormous importance in coordination chemistry. First-row transition metals in high oxidation states have the characteristics of hard Lewis acids (small size and high charge). Consequently, ions such as Cr3+, Fe3+, and Co3+ are hard Lewis acids that bond best to hard Lewis bases. When presented with the opportunity to bond to NH3 or PR3, these metal ions bond better to NH3, which is the harder base. On the other hand, Cd2+ bonds better to PR3 because of the more favorable soft acid-soft base interaction. 

The metalloporphyrin-initiated polymerizations are accelerated by the presence of steri-cally hindered Lewis acids [Inoue, 2000 Sugimoto and Inoue, 1999]. The Lewis acid coordinates with the oxygen of monomer to weaken the C— O bond and facilitate nucleophilic attack. The Lewis acid must be sterically hindered to prevent its reaction with the propagating center attached to the prophyrin structure. Thus, aluminm ortho-substituted phenolates such as methylaluminum bis(2,6-di-/-butyl-4-methylphenolate) accelerate the polymerization by factors of 102-103 or higher. Less sterically hindered Lewis acids, including the aluminum phenolates without ortho substituents, are much less effective. 

Transition metal complexes act as templates that regulate organic reactions that occur in the coordination sphere (4). Ligands are often activated or stabilized by participation of metal d orbitals, where the central metals are electronically amphoteric in contrast to the main group elements, which normally act as Lewis acids. The bonding scheme of an olefin-transition metal complex is illustrated in Scheme 2. The olefin 7T electrons are donated to a vacant metal orbital to make a a-type bond the metal d elections are back-donated to olefin anti-bonding orbitals with the same symmetry to form a ir-type bond. In this way, the olefin is activated by formal electron promotion from the tt to tt orbital, as... 

Allyltriorganosilanes react with activated C-N double bonds such as iminium salts and Lewis acid-coordinated imines at the y-position to give homoallylamines.14,118 For example, in the presence of BF3, iV-acylimines generated in situ by the reaction of aldehydes or acetals with carbamates are efficiently allylated with allyltrimethylsilanes (Equation (26)).119,119a,12° The use of homochiral crotylsilanes such as 20 leads to highly diastereo- and enantioselective synthesis of homoallylamines (Equation (27)). a Allenylation of the iV-acylimines can be performed with propargylsi lanes. 

When atoms possess an incomplete outer shell (e.g., nonpaired electrons), yet their net charge is zero, attraction between such atoms takes place because of their strong tendency to complete their outer electron orbital shell by sharing their unpaired electrons. This gives rise to a covalent bond. One example of a covalent bond is the bimolecular chlorine gas (Cl2) (Fig. 1.1). Covalent bonding is a characteristic of some nonmetals or metalloids (bimolecular molecules), but may also arise between any two atoms when one of the atoms shares its outer-shell electron pair (Lewis base) with a second atom that has an empty outer shell (Lewis acid). Such bonds are known as coordinated covalent bonds or polar covalent bonds. They are commonly weaker than the covalent bond of two atoms which share each other s unpaired outer-shell electrons (e.g., F2 and 02). Coordinated covalent bonds often involve organometallic complexes. 

Two theoretical studies on this topic have been reported (79,80) which indicate that when the Lewis acid coordinates to the carbonyl oxygen, it activates that group toward alkyl migration by withdrawal of electron density. This lowers the energies of both bonding and antibonding orbitals. 

Lewis acids prefer to lie syn to the smaller substituent of the carbonyl, e.g. sy/t to H for aldehydes, anti to —OR for simple alkyl esters. In a,p-unsaturated systems, Lewis acid coordination syn to the double bond favors the s-trans conformation, but in two crys structures, where coordination anti to the alkene occurs, s-cis complexes are observed. " Finally, chelation with titanium and tin occurs readily and yields stable, crystalline complexes. 

Coordinate covalent bonds involve the unequal sharing of an electron pair by two atoms, with both electrons (originally) coming from the same atom. The electron pair donor is the ligand, or Lewis base, whereas the acceptor is the central atom (because it frequently can accept more than one pair of electrons), or Lewis acid. These bonds are important in all interactions between transition metals and organic ligands (e.g., Fe + in hemoglobin and the cytochromes). 

He also pointed out 12 possible N—H- -F—C hydrogen bonds [2]. The hydrogen bond donors (N—H) among these could roughly be classified into four N—H of amides (6) (Scheme 4.5), Lewis acid coordinated amine (8), aniline (10), and ammonium (9). Howard et al. showed another hydrazine proton donor (7) [23]. All these have acidic amino protons and thus could protonate negatively charged organic fluorine atoms. 

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