Nucleophile Lewis base

Carbocations are strongly electrophilic (Lewis acids) and react with nucleophiles (Lewis bases)... 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

We said at the beginning of this chapter that two kinds of reactions can happen when a nucleophile/Lewis base reacts with an alkyl halide. The nucleophile can either substitute for the halide by reaction at carbon or cause elimination of HX by reaction at a neighboring hydrogen ... 

Amines may also behave as nucleophiles (Lewis bases). Primary amines are stronger nucleophiles than secondary amines, which, in turn, are stronger nucleophiles than tertiary amines. As nucleophiles, amines attack acid chlorides to form amides. Later in this chapter you see that they re important in the formation of sulfonamides. 

Figure 8.2. Reaction of a carbonyl compound with (a) an electrophile (Lewis acid) and (b) a nucleophile (Lewis base). The n bond is formally broken in the reaction with the nucleophile (b) but not in the reaction with the electrophile (a). Stereoisomeric forms may be generated in either case.

Environmentally relevant nucleophiles (Lewis bases) may be classified according to Pearson s HSAB principle, as hard, soft, or borderline (possessing intermediate hard/soft character) as follows (Larson and Weber, 1994) ... 

Asymmetric organocatalytic Morita-Baylis-Hillman reactions offer synthetically viable alternatives to metal-complex-mediated reactions. The reaction is best mediated with a combination of nucleophilic tertiary amine/phosphine catalysts, and mild Bronsted acid co-catalysts usually, bifunctional chiral catalysts having both nucleophilic Lewis base and Bronsted acid site were seen to be the most efficient. Although many important factors governing the reactions were identified, our present understanding of the basic factors, and the control of reactivity and selectivity remains incomplete. Whilst substrate dependency is still considered to be an important issue, an increasing number of transformations are reaching the standards of current asymmetric reactions. 

Some of the terms associated with acids and bases have evolved specific meanings in organic chemistry. When organic chemists use the term base, they usually mean a proton acceptor (a Brpnsted-Lowry base). Similarly, the term acid usually means a proton donor (a Brpnsted-Lowry acid). When the acid-base reaction involves formation of a bond to some other element (especially carbon), organic chemists refer to the electron donor as a nucleophile (Lewis base) and the electron acceptor as an electrophile (Lewis acid). 

Notice that acetaldehyde acts as the nucleophile (Lewis base) in part (a) and as the electrophile (Lewis acid) in part (b). Like most organic compounds, acetaldehyde is both acidic and basic. It acts as a base if we add a strong enough acid to make it donate electrons or accept a proton. It acts as an acid if the base we add is strong enough to donate an electron pair or abstract a proton. 

Identify nucleophiles (Lewis bases) and electrophiles (Lewis acids), and write equations for Lewis acid-base reactions using curved arrows to show the flow of electrons. 

A carbanion has a trivalent carbon atom that bears a negative charge. There are eight electrons around the carbon atom (three bonds and one lone pair), so it is not electron deficient rather, it is electron rich and a strong nucleophile (Lewis base). A carbanion has the same electronic structure as an amine. Compare the structures of a methyl carbanion and ammonia ... 

This polarization of the carbonyl group contributes to the reactivity of ketones and aldehydes The positively polarized carbon atom acts as an electrophile (Lewis acid), and the negatively polarized oxygen acts as a nucleophile (Lewis base). 

Referring to a mechanistic classification of organocatalysts (Seayad and List 2005), currently the two most prominent classes are Brpnsted acid catalysts and Lewis base catalysts. Within the latter class chiral secondary amines (enamine, iminium, dienamine activation for a short review please refer to List 2006) play an important role and can be considered as—by now—already widely extended mimetics of type I aldolases, whereas acylation catalysts, for example, refer to hydrolases or peptidases (Spivey and McDaid 2007). Thiamine-dependent enzymes, a versatile class of C-C bond forming and destructing biocatalysts (Pohl et al. 2002) with their common catalytically active coenzyme thiamine (vitamin Bi), are understood to be the biomimetic roots ofcar-bene catalysis, a further class of nucleophilic, Lewis base catalysis with increasing importance in the last 5 years. 

Compared to the great variety of Lewis acid catalysts for the catalytic asymmetric aldol reaction the field of nucleophilic (Lewis base) catalysts is less explored. This strategy involves the transient activation of the latent enolate equivalent via Lewis base coordination to the silyl enol ether (Scheme 9) [3], For instance the tri-chlorosilyl enol ether 50 is able to expand its valency at the silicon atom from four to five and six. It reacts with an aldehyde (51), proceeding through a closed Zimmerman-Traxler-like transition state (54), to give 53 after quenching with saturated aqueous NaHCO, [16]. 

For these initiating systems, externally added nucleophiles are necessary to induce controlled/living cationic polymerizations of vinyl ethers [36,64]. Table 2 A lists nucleophiles (Lewis bases) that are effective for such purposes and includes esters (carboxylates and carbonates) [100,101,130-133], ethers (linear and cyclic) [102-104,137-140], methyl-pyridines [140], and phosphines [21,141]. CF3S03H-initiated polymerizations, sulfides are also effective [37,38,134,135]. 

Polar reactions take place between electron-rich reagents (nucleophiles/ Lewis bases) and electron-poor reagents (electrophiles/Lewis acids). These reactions are heterolylic processes and involve species with an even number of electrons. Bonds are made when a nucleophile donates an electron pair to an electrophile bonds are broken when one product leaves with an electron pair. 

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