A Bonds conjugation

In the remainder of Chapter 16 we examine conjugated dienes, compounds having two double bonds joined by one a bond. Conjugated dienes are also called 1,3-dienes. 1,3-Butadiene (CH2=CH-CH=CH2) is the simplest conjugated diene. 

Isolated dienes have two double bonds separated by two or more a bonds. Conjugated dienes have two double bonds separated by only one a bond. 

All of the carbon atoms are sp hybridized. Each n bond is separated by only one a bond, conjugated... 

Double bonds separated by one a bond = conjugated diene smaller heat of hydrogenation... 

Catalytic hydrogenation of a A -bond Conjugated addition of HCN to C20 ketones I3(i7a) ketones... 

The Huckel method and is one of the earliest and simplest semiempirical methods. A Huckel calculation models only the 7t valence electrons in a planar conjugated hydrocarbon. A parameter is used to describe the interaction between bonded atoms. There are no second atom affects. Huckel calculations do reflect orbital symmetry and qualitatively predict orbital coefficients. Huckel calculations can give crude quantitative information or qualitative insight into conjugated compounds, but are seldom used today. The primary use of Huckel calculations now is as a class exercise because it is a calculation that can be done by hand. 

The electrophilicity of C = C double bonds conjugated with electron withdrawing groupings leads to a -synthons. This is an important example of the vinyiogous principle ... 

As we saw in Chapter 5 dehydrations and dehydrohalogenations are typically regiose lective m the direction that leads to the most stable double bond Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state... 

Both enols have their carbon-carbon double bonds conjugated to a carbonyl group and can form an intramolecular hydrogen bond They are of comparable stability... 

Conjugate acid (Section 1 13) The species formed from a Brpnsted base after it has accepted a proton Conjugate addition (Sections 1010 and 1812) Addition reaction in which the reagent adds to the termini of the con jugated system with migration of the double bond synony mous with 1 4 addition The most common examples include conjugate addition to 1 3 dienes and to a 3 unsaturated car bonyl compounds... 

Sigmatropic rearrangement (Section 24 13) Migration of a a bond from one end of a conjugated tt electron system to the other The Claisen rearrangement is an example... 

A mechanism has been proposed to rationalize the results shown in Figure 23. The relative proportion of the A -pyrazolines obtained by the reduction of pyrazolium salts depends on steric and electronic effects. When all the substituents are alkyl groups, the hydride ion attacks the less hindered carbon atom for example when = Bu only C-5 is attacked. The smaller deuterohydride ion is less sensitive to steric effects and consequently the reaction is less selective (73BSF288). Phenyl substituents, both on the nitrogen atom and on the carbon atoms, direct the hydride attack selectively to one carbon atom and the isolated A -pyrazoline has the C—C double bond conjugated with the phenyl (328 R or R = Ph). Open-chain compounds are always formed during the reduction of pyrazolium salts, becoming predominant in the reduction of amino substituted pyrazoliums. 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Annulene (Section 11.20) Monocyclic hydrocarbon characterized by a completely conjugated system of double bonds. Annulenes may or may not be aromatic. 

Compounds having a cyclic double bond conjugated to an aromatic ring (e.g. indene) undergo oxofluorination, with FCIO3 acting as a 2-centre electrophile ... 

We generally write a for a diagonal element and p for a bonded off-diagonal element in particular uq and Pec for carbon atoms and carbon-carbon conjugated bonds. The physical interpretation is that each of the r-electrons experiences an average field due to the nuclei, the cr-electrons and the remaining TT-electrons. 

Pyrazole and its 3,5-dimethyl and 3,4,5-trimethyl derivatives combined with two moles of dimethyl acetylenedicarboxylate giving products of similar ultraviolet absorption spectra to the parent pyrazoles. These products [e.g., (69)] do not possess the strong broad absorption at ca. 3.20 /u, characteristic of the bonded N—H group which is present in the parent pyrazoles and are formed by two successive Michael addition reactions. In the case of 3,5-dimethylpyra-zole, the initial fumarate (68) has been isolated and possessed a more conjugated type of absorption spectrum to those of the dipyrazolyl-... 

The isoindole-isoindolenine equilibrium has been studied quantitatively only in the case of certain 1-arylisoindoles. Although two structurally different isoindolenines are possible, only that with the carbon-nitrogen double bond conjugated with both aromatic rings was observed. Investigation of the isoindole-isoindolenine ratios for three compounds by NMR and ultraviolet spectroscopy indicated a... 

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

Conjugation (Chapter 14 introduction) A series of overlapping p orbitals, usually in alternating single and multiple bonds. For example, 1,3-butadiene is a conjugated diene, 3-buten-2-one is a conjugated enone, and benzene is a cyclic conjugated triene. 

X-ray crystallographic studies are available on the ethoxy compound 1858 and on N,N-dimethylcyclopent[c]azepin-3-amine(19).59 The former is a fully conjugated, planar, 14k, non-benzenoid aromatic system, while the latter is virtually planar and, on the basis of bond length measurements, is best represented as the dipolar mesomer 19B. 

There is some experimental data from nuclear magnetic resonance5 25 and sonic30 observations which indicate that a bond which would be expected to have a partial double bond character because of conjugation with an adjacent double bond will show a barrier intermediate between single and double bond values. 

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