Eliminations and Additions

The predicted stereochemistry of additions and eliminations, (144), of X and Y at the ends of a conjugated it chain is given in Table 8 (Fukui and Fujimoto, 1966b). The orbital overlap in these allowed processes is indicated in Fig. 21. It must be admitted that many of the 

In connection with a-ir exchanges of equations (94) and (96), we noted that certain additions or eliminations were forbidden. A typical orbital symmetry diagram for the forbidden four-center bromine addition to an alkene is given in Fig. 22. Apparently similar concerted additions are allowed, e.g. hydrogen halides in equation (97), because favorable orbital overlap is available. 

As indicated previously, the application of the selection rules of Table 8 or Fig. 22 requires discrimination. First, a check of LVMO of the atoms undergoing attack is always worthwhile (Coulson and Streitwieser, 

In the HMO if of styrene, the coefficients at the a and / carbon atoms are ca. 0-4 and 0-6 respectively. In the HMO ifi7 of acenaphthylene, the coefficients at carbon atoms 1 and 2 are ca. — 032 and 0-32. Therefore, the directive effect for addition is syn in styrene and anti in acenaphthylene Second, MO calculations may be helpful in new or different systems. With respect to eliminations, Fukui and Fujimoto (1965) used frontier electron theory to provide reactivity indices for two model 

Although carbene type insertions have been discussed as displacements and their reactions with alkenes could be treated as cycloadditions, both could just as easily fit into this section. Typical of the latter is the addition (145). Apart from a primary syn stereospecificity, there is a 

Metal fragments may add to some existing E-M compounds to give higher nuclearity complexes. If a lone pair of electrons is present on E, especially for the lighter E atoms, which are more basic, the metal fragment 

The clusters Co3(CO)9E (E = P, As) are an interesting example showing the basicity of the main group element. For these molecules the discrete trimetallic compound is only observed for E = As, and then it is only short-lived. These molecules undergo a spontaneous trimerization to give [Co3(CO)8E]3 in which three clusters each have an E atom donating to a 

Co of an adjacent metal triangle (213).126 The tetrahedral complex can be trapped by ML fragments as shown in Eq. (92).126 

In complexes containing a planar E unit, it is common for the addition of metal fragments to give a multidecker complex [Eqs. (93)-(96)441,454,479,485]. A metal fragment may attach to several E atoms to give a more complicated cluster core as in Eqs. (97) and (98), 494 or may insert 

2 (triphos)CoP3 + 6CuBr (triphos)CoP3[Cu6Br6]P3Co(triphos) (93) 

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On the other hand, the halohydrin (chloro and bromo) 908 is converted into a ketone via oxidative addition and //-elimination in boiling benzene with catalysis by Pd(OAc)2 and tri(o-tolyl)phosphine in the presence of K2C03[765,766],... 

The stereochemistry of the most fundamental reaction types such as addition, substitution, and elimination are described by terms which specify the stereochemical relationship between the reactants and products. Addition and elimination reactions are classified as syn or anti, depending on whether the covalent bonds which are made or broken are on the same face or opposite faces of the plane of the double bond. 

Addition and elimination processes are the reverse of one another in a formal sense. There is also a close mechanistic relationship between the two reactions, and in many systems reaction can occur in either direction. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that are related as an addition-elimination pair. 

When the addition and elimination reactions are mechanically reversible, they proceed by identical mechanistic paths but in opposite directions. In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same in the reverse as in the forward direction. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as... 

The mechanistic pattern established by study of hydration and alcohol addition reactions of ketones and aldehydes is followed in a number of other reactions of carbonyl compounds. Reactions at carbonyl centers usually involve a series of addition and elimination steps proceeding through tetrahedral intermediates. These steps can be either acid-catalyzed or base-catalyzed. The rate and products of the reaction are determined by the reactivity of these tetrahedral intermediates. 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

Write a detailed mechanism for this condensation using only the molecules whose models are provided. Treat all proton transfers, nucleophilic additions, and elimination reactions as separate steps, and use curved arrows to show electron movement. Which of these steps do you think will be favorable Unfavorable Why ... 

Just as the chemistry of alkenes is dominated by addition reactions, the preparation of alkenes is dominated by elimination reactions. Additions and eliminations are, in many respects, two sides of the same coin. That is, an addition reaction might involve the addition of HBr or H20 to an alkene to form an alkyl halide or alcohol, whereas an elimination reaction might involve the loss of HBr or H20 from an alkyl halide or alcohol to form an alkene. 

Volume 8 Volume 9 Volume 10 Volume 12 Volume 13 Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5. POLYMERISATION REACTIONS (3 volumes)... 

This chapter deals with (1) the transformation of the sulfone functionality into other functional groups by nucleophilic substitution reaction, and (2) the addition and elimination reaction of a,/i-unsaturated sulfones. Particular attention will be paid to recent uses of sulfones in organic syntheses1. 

The aminotin compounds react with aldehydes by addition and elimination, to give enamines (219), but some ketones, by acidolysis, give tin enolates, e.g. (220),... 

The steps are the same as in the addition-elimination mechanism, but in reverse order. Evidence for this sequence is as follows (1) The reaction does not proceed without ethoxide ion, and the rate is dependent on the concentration of this ion and not on that of ArS. (2) Under the same reaction conditions, chloroacetylene gave 83 and 80. (3) Compound 83, treated with ArS, gave no reaction but, when EtO was added, 80 was obtained. It is interesting that the elimination-addition mechanism has even been shown to occur in five- and six-membered cyclic systems, where triple bonds are greatly strained. Note that both the addition-elimination and elimination-addition sequences, as shown above, lead to overall retention of configuration, since in each case both addition and elimination are anti. 

It has been tentatively suggested that one mechanism underlies the Willgerodt reaction and the Kindler modification of it. A labile intermediate is first formed which has a carbon—carbon bond in the side chain. The scheme is indicated below It postulates a series of steps involving the addition of ammonia or amine (R = H or alkyl), elimination of water, re-addition and elimination of ammonia or amine until the unsaturation appears at the end of the chain then an irreversible oxidation between sulphur and the nitrogen compound may occur to produce a thioamide. 

The detailed mechanisms of such reactions have been shown to involve addition and elimination of phosphine ligands. The efficiency of individual reactions can often be improved by careful choice of added ligands. 

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