Lewis acid—base reactions electrophilic addition

The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions. In Step [1], the alkene is the Lewis base that donates an electron pair to H-Br, the Lewis acid, while in Step [2], Br is the Lewis base that donates an electron pair to the carbocation, tbe Lewis acid. 

In this context, an avalanche of studies were devoted to acid-base reactions in their broadest sense (i.e., the Lewis picture), also involving complexation reactions, to the typical organic reactions of addition, substitution, and elimination types, involving nucleophilic and electrophilic reagents including the case of radicalar reactions and excited states (for a review see Ref. [11]) in which our group has... 

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. 

Several Lewis acid-base interactions between alkali metal cations and heteroatom-containing molecules are indispensable in the promotion of reactions involved in critically important and fundamental transformations—deprotonation with lithium amides at the a-hydrogens of carbonyl or imino compounds and the addition of organolithium compounds to such electrophilic substrates. Because it is impossible to cover the multitude of these and other closely related subjects, this chapter describes only briefly general aspects of current interest. 

Electrophilic Addition This can be considered as an acid—base reaction, where the reagent acts as an acid, whether a protic one (e.g., hydriodic acid or iodine monochloride) or a Lewis acid (e.g., molecular iodine, which can be polarized by electrophilic solvents or catalysts), and the double-bond acts as abase (Argentini, 1982). 

This book chapter is limited to Lewis acid-mediated reactions, and does not discuss the important field of Lewis base-mediated allylations, nor does it describe the reactions of allylsilanes with other electrophiles such as epoxides, imines, and allyl-X (X = -Cl, -OR, -OAc). The SaJcurai reaction has been covered under different forms in reviews focusing on The Stereochemistry of the Sakurai reaction , Intramolecular Addition Reactions of Allylic and Propargylic Silanes ," Selective Reactions Using Allylic Metals , Catalytic Enantioselective Addition of Allylic Organometallic Reagents to Aldehydes and Ketones , and Modem Carbonyl Chemistry . ... 

Although surface organometallic chemistry is still in its infancy, there are already several examples of surface reactions leading to well-defined surface complexes (Table l-I). It appears that these reactions obey the same principles as those encountered in molecular chemistry nucleophilic attack at the ligands, electrophilic attack of the metal-carbon bond, oxidative addition, Lewis acid-base adduct formation, redox reactions, disproportionation, and the cooperative effect of dual acid-base sites in an insertion reaction. 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

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. 

The mechanism through which catalytic metal carbene reactions occur is outlined in Scheme 2. With dirhodium(II) catalysts the open axial coordination site on each rhodium serves as the Lewis acid center that undergoes electrophilic addition to the diazo compound. Lewis bases that can occupy the axial coor-... 

Ethylene is the template for olefin reactions, but ethylene itself is rather unreactive, undergoing electrophilic attack by moderately strong Lewis acids. Nucleophilic attack on the bond even by the strongest Lewis bases has not been reported. The following sequence involves intramolecular addition of a carbanion to an unactivated olefin [111, 112]. The reaction is undoubtedly facilitated by active participation of the lithium cation as a Lewis acid [113]. 

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