Mechanisms of Addition

The Lewis acid-mediated addition of electrophiles to allylsilanes has been extensively studied [10 c], In most cases the addition of an electrophile to an allylsilane proceeds via an anti Se process. In the ground state structure, simple allylsilanes are known to prefer the conformation wherein the allylic hydrogen eclipses the double bond. The electrophile can then approach the double bond from the same side as the allylmetal (syn Se) or from the side opposite the allyl- 

The regiochemical and the stereochemical course of electrophilic additions to allylsilanes has been modeled computationally by Hehre [15]. In this study the conformational profile of 2-silylbut-3-ene was determined and three energy minima were observed (Chart 10-1). In the two most stable conformers the C-Si bond is perpendicular to the C-C double bond. Experimental evidence has been obtained (microwave [16a,b], electron diffraction [16c], infrared and Raman [16d]) which is in agreement with the computational results. 

Entry Lewis acid Time, min temp, C. syn, % anti, % Yield, % 

The results obtained from the cyclization of model 5 indicated that the size of the Lewis acid-aldehyde complex influences the selectivity of the reaction. For model system 10 it appears that the steric bulk of the Lewis acid does not play a significant role in determining the stereochemical outcome of the reaction. In model system 10 no external methylene unit exists which could interact with the Lewis acid-aldehyde complex. In fact, the silane is fixed in an anti orientation with respect to the approaching aldehyde (anti Se ). The cyclization of model system 10 with fluoride ion affords primarily the distal product resulting from cyclization through an antiperiplanar transition structure. Thus, the antiperiplanar transition structure is accessible, but is not favored in reactions with the Lewis acids. 

The high selectivity for the synclinal transition structure in these models may also arise from frontier orbital interactions. Anh and Thanh have suggested that the stereochemical outcome of an aldol reaction may be controlled by the overlap between the frontier molecular orbitals [23aJ. This same cycloaddition-like transition structure was first used by Mulzer to explain the high selectivity observed in 

---

FIGURE 6 8 Mechanism of addition of sulfuric acid to propene... 

Electrophile (Section 4 8) A species (ion or compound) that can act as a Lewis acid or electron pair acceptor an elec tron seeker Carbocations are one type of electrophile Electrophilic addition (Section 6 4) Mechanism of addition in which the species that first attacks the multiple bond is an electrophile ( electron seeker )... 

Radicals are employed widely in the polymer industry, where their chain-propagating behavior transforms vinyl monomers into polymers and copolymers. The mechanism of addition polymeri2ation involves all three types of reactions discussed above, ie, initiation, propagation by addition to carbon—carbon double bonds, and termination ... 

Bromination of isoprene using Br2 at —5 ° C in chloroform yields only /n j -l,4-dibromo-2-methyl-2-butene (59). Dry hydrogen chloride reacts with one-third excess of isoprene at —15 ° C to form the 1,2-addition product, 2-chloro-2-methyl-3-butene (60). When an equimolar amount of HCl is used, the principal product is the 1,4-addition product, l-chloro-3-methyl-2-butene (61). The mechanism of addition is essentially all 1,2 with a subsequent isomerization step which is catalyzed by HCl and is responsible for the formation of the 1,4-product (60). The 3,4-product, 3-bromo-2-methyl-1-butene, is obtained by the reaction of isoprene with 50% HBr in the presence of cuprous bromide (59). Isoprene reacts with the reactive halogen of 3-chlorocyclopentene (62). 

Because the bromine adds to the less substituted carbon atom of the double bond, generating the more stable radical intermediate, the regioselectivity of radical-chain hydrobromination is opposite to that of ionic addition. The early work on the radical mechanism of addition of hydrogen bromide was undertaken to understand why Maikow-nikofF s rule was violated under certain circumstances. The cause was found to be conditions that initiated the radical-chain process, such as peroxide impurities or light. 

Electrophilic addition (Section 6.4) Mechanism of addition in which the species that first attacks the multiple bond is an electrophile ( electron seeker ). 

Bhuiyan, A. L. Some Problems Encountered with Degradation Mechanisms of Addition Polymers. Vol. 47, pp. 1 —65. 

The fundamental understanding of the diazonio group in arenediazonium salts, and of its reactivity, electronic structure, and influence on the reactivity of other substituents attached to the arenediazonium system depends mainly on the application of quantitative structure-reactivity relationships to kinetic and equilibrium measurements. These were made with a series of 3- and 4-substituted benzenediazonium salts on the basis of the Hammett equation (Scheme 7-1). We need to discuss the mechanism of addition of a nucleophile to the P-nitrogen atom of an arenediazonium ion, and to answer the question, raised several times in Chapters 5 and 6, why the ratio of (Z)- to ( -additions is so different — from almost 100 1 to 1 100 — depending on the type of nucleophile involved and on the reaction conditions. However, before we do that in Section 7.4, it is necessary to give a short general review of the Hammett equation and to discuss the substituent constants of the diazonio group. 

These reactions serve as a link in understanding selectivity differences between inter- and intramolecular cyclopropanation reactions, and they have been useful in defining the mechanism of addition as a function of catalyst [50,69,70]. 

In investigating the mechanism of addition to a double bond, perhaps the most useful type of information is the stereochemistry of the reaction. The two carbons of the double bond and the four atoms immediately attached to them are all in a plane (p. 8) there are thus three possibilities. Both Y and W may enter from the same side of the plane, in which case the addition is stereospecific and syn they may enter from opposite sides for stereospecific anti addition or the reaction may be nonstereospecific. In order to determine which of these possibilities is occurring in a given reaction, the following type of experiment is often done YW is added to the cis and trans isomers of an alkene of the form ABC=CBA. We may use the cis alkene as an example. If the addition is syn, the product will be the erythro dl pair, because each carbon has a 50% chance of being attacked by Y ... 

Protonation of the enolate ion is chiefly at the oxygen, which is more negative than the carbon, but this produces the enol, which tautomerizes. So, although the net result of the reaction is addition to a carbon-carbon double bond, the mechanism is 1,4 nucleophilic addition to the C=C—C=0 (or similar) system and is thus very similar to the mechanism of addition to carbon-oxygen double and similar bonds (see Chapter 16). When Z is CN or a C=0 group, it is also possible for Y to attack at this carbon, and this reaction sometimes competes. When it happens, it is called 1,2 addition. 1,4 Addition to these substrates is also known as conjugate addition. The Y ion almost never attacks at the 3 position, since the resulting carbanion would have no resonance stabilization " ... 

When we explore the mechanisms of addition reactions, we will see why some reactions proceed through a Markovnikov addition while others proceed through an anti-Markovnikov addition. For now, let s make sure that we are comfortable using the terms. 

A more complete discussion of the mechanism of addition of hydrogen halides to alkenes is given in Chapter 6 of Part A. In particular, the question of whether or not discrete carbocations are involved is considered there. Even when a carbocation is not involved, the regioselectivity of electrophilic addition is the result of attack of the electrophile at the more electron-rich carbon of the double bond. Alkyl substituents increase the electron density of the terminal carbon by hyperconjugation (see Part A, Section 1.1.8). 

John, G.R. and Mane-Maguire, A.P, Kinetics of nucleophilic attack on coordinated organic moieties. Part 7. Mechanism of addition of tertiary phosphines to tricarbonyl(dienyl)iron cations, /. Chem. Soc., Dalton, 873, 1979. 

Mechanism of Additive Effects with Inorganic Salts and Urea. 

A similar mechanism of addition was presumed for the reaction of nitrosobenzene with OH-. How-... 

Finally, the possibility of obtaining 1,2- or 1,4-adducts, depending on reaction conditions, has been interpreted23, in agreement with the accepted mechanism of addition of... 

Section V has revealed a new mechanism of addition of Grignard reagents to carbonyl compounds. The mechanism was thought to be very complex due to aggregation and a competing SET mechanism. No attempts to elucidate the correlation between the... 

It is possible to obtain anti-Markovnikov products when HBr is added to alkenes in the presence of free radical initiators, e.g. hydrogen peroxide (HOOH) or alkyl peroxide (ROOR). The free radical initiators change the mechanism of addition from an electrophilic addition to a free radical addition. This change of mechanism gives rise to the anh-Markovnikov regiochemistry. For example, 2-methyl propene reacts with HBr in the presence of peroxide (ROOR) to form 1-bromo-2-methyl propane, which is an anh-Markovnikov product. Radical additions do not proceed with HCl or HI. 

The light-induced addition elimination reaction of diethyl phosphite to both 1,3,3,4,4-penta-fluorocyclobutene and l-chloro-3,3,4,4-tetrafluorocyclobutene afforded a mixture of diethyl 3,3,4,4-letrafluorocyclobutene-l-phosphonate (11) and tetraethyl 3,3,4,4-tetrafluorocyclobutane-1,2-diphosphonate (12). The postulated mechanism of addition involves phosphonyl radical attack at the cyclobutene moiety.17... 

Buell, McEwen, and Kleinberg have observed that weak acids such as hydrogen azide and acetic acid add readily across the double bond of vinylferrocene (XLI, M = Fe) (8). They have postulated that the mechanism of addition proceeds via intermediate formation of the a -ferrocenylcarbonium ion (X-LII, M = Fe), followed by conversion to the acetate (XLIII, M = Fe). Stabilization of carbonium ions of this type can result from overlap of filled metal orbitals with the vacant p -orbital of the carbonium ion. 

The mechanism of addition of amines to the epoxy ring considered above is part of the general problem of the mechanism of the reaction under study. The main feature of this mechanism is formation of different hetero- and autocomplexes of the starting substances and reaction products characterized by different reactivity5,13,14,33). These complexes may be formed both inter- and intramolecularly. The existence of these complexes has been proved and the thermodynamic parameters in complexing reactions have been determined 5.13,14,29,33-43)... 

Limited data are available on hypofluorous acid, which is a very reactive, explosive reagent.145 Exceptionally among hypohalous acids, it is polarized in the sense H08+-F8. It converts alkenes into fluoroalkanols with regioselectivites opposite to other hypohalous acids [Eq. (6.24)] the large amount of HF present may affect the actual mechanism of addition ... 

---