The conjugation of two n bonds

The simplest compound that can have two conjugated 7i bonds is butadiene. As we would now expect, this is a planar compound that can adopt two different conformations by rotating about the single bond. Rotation is somewhat restricted (around 30 kj mol ) but nowhere near as much as in an amide (typically 60-90 kJ moP ). What do the molecular orbitals for the butadiene n system look like The lowest-energy molecular orbital will have all the p orbitals combining in-phase. The next lowest will have one node, and then two, and the highest-energy molecular orbital will have three nodes (that is, all the p orbitals will be out-of-phase). 

Butadiene normally refers to 1,3-butadiene. It is also possible to have 1,2-butadiene which is another example of an allene (p. 157), 

Butadiene has two K bonds and so four electrons in the n system. Which molecular orbitals are these electrons in Since each molecular orbital can hold two electrons, only the two molecular orbitals lowest in energy are filled. Let s have a closer look at these orbitals. In, the iowest-energy bonding orbital, the electrons are spread out over all four carbon atoms (above and below the plane) in one continuous orbital. There is bonding between all the atoms. The other two electrons are in 4 2-orbital has bonding interactions between carbon atoms 1 and 2, and also between 3 and 4 but an interaction between carbons 2 and 3. Overall, in both the occupied Jc orbitals there are 

2 antibonding interactions overall - anti bonding orbital 

Y2 -1 nodal plane 2 bonding interactions 1 antibonding interaction overall - bonding orbital 

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Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

A 7r-bond can react with various active species, such as the electrophile oxene and its isoelec-tronic species (nitrenes and carbenes) and radicals. A 7r-bond can also react with a nucleophile, when it is conjugated with an electron-withdrawing group. In these reactions O, N, or C atom(s) are transferred from the active species to the olefins, forming two tr-bonds, such as C—O, C—N, and C—C, at the expense of the 7r-bond. If the 7r-bond is prochiral, chiral center(s) are... 

A general type of chemical reaction between two compounds, A and B, such that there is a net reduction in bond multiplicity (e.g., addition of a compound across a carbon-carbon double bond such that the product has lost this 77-bond). An example is the hydration of a double bond, such as that observed in the conversion of fumarate to malate by fumarase. Addition reactions can also occur with strained ring structures that, in some respects, resemble double bonds (e.g., cyclopropyl derivatives or certain epoxides). A special case of a hydro-alkenyl addition is the conversion of 2,3-oxidosqualene to dammara-dienol or in the conversion of squalene to lanosterol. Reactions in which new moieties are linked to adjacent atoms (as is the case in the hydration of fumarate) are often referred to as 1,2-addition reactions. If the atoms that contain newly linked moieties are not adjacent (as is often the case with conjugated reactants), then the reaction is often referred to as a l,n-addition reaction in which n is the numbered atom distant from 1 (e.g., 1,4-addition reaction). In general, addition reactions can take place via electrophilic addition, nucleophilic addition, free-radical addition, or via simultaneous or pericycUc addition. 

X-ray crystal structure determination. The lithium atom is -chelate bonded to the monoanionic ligand while a tetrahedral coordination geometry at lithium is reached by the coordination of two additional THF molecules. The presence of considerable conjugation in this part of the molecule is reflected by the almost equal N—Li distances [2.014(4) and 2.000(4) A] and almost equal S—N distances [1.5607(17) and 1.5622(17) A]. The third nitrogen atom acts as a neutral donor to the drmethylzinc molecule [Zn—N 2.1374(17) A] rendering the zinc atom trigonal planar. 

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