The Allyl System

2 The Allyl System. Another conjugated system we shall need later on is that of the allyl cation (24), allyl radical (25), and allyl anion (26). These three reactive intermediates all have the same orbitals, but different numbers 

It is very important to realize that having conjugation may make a molecule thermodynamically more stable than an unconjugated one, for the reason we 

We would not say that two p orbitals are conjugated—they just make up a double bond—so just how many p orbitals do we need before something can be described as conjugated Tt should be clear that in butadiene the double bonds are conjugated—here we have four p orbitals. 

Is it possible to have three p orbitals interacting How can we get an isolated p orbital-after all, we can t have half a double bond. Let us look for a moment at allyl bromide (prop-2-enyl bromide or 1-bromoprop-2-ene). Carbon 1 in this compound has got four atoms attached to it (a carbon, two hydrogens, and a bromine atom) so it is tetrahedral (or sp hybridized). 

Bromine is more electronegative than carbon and so the C-Br bond is polarized towards the bromine. If this bond were to break completely, the bromine would keep both electrons from the C-Br bond to become bromide ion, Br , leaving behind an organic cation. The end carbon would now only have three groups attached and so it becomes trigonal (sp hybridized). This leaves a vacant p orbital that we can combine with the n bond to give a new molecular orbital for the allyl system. 

As more orbitals combine it becomes more difficult to represent the molecular orbitals convincingly. We shall often, from now on, simply use the atomic orbitals to represent the moiecular orbitals. 

Rather than trying to combine the p orbital with the n bond, it is easier for us to consider how three p orbitals combine after all, we thought of the tc bond as a combination of two p orbitals. Since we are combining three atomic orbitals (the three 2p orbitals on carbon) we shall get three molecular orbitals. The lowest-energy orbital will have them all combining in-phase. This is a bonding orbital since all the interactions are bonding. 

We could work out the orbitals of the allyl anion by combining this p orbital with a readymade jc bond, but instead this time we will start with the three separate p atomic orbitals and combine them to get three molecular orbitals. At first we are not concerned about where the electrons are—we are just building up the molecular orbitals. 

We can summarize all this information in a molecular orbital energy level diagram, and at the same time put the electrons into the orbitals. We need four electrons—two from the alk-ene n bond and two more for the anion (these were the two in the C—H bond, and they are still there because only a proton, H+, was removed). The four electrons go into the lowest two orbitals, /i and j/2/ leaving /j vacant. Notice too that the energy of two of the electrons is lower than it would have been if they had remained in unconjugated p orbitals conjugation lowers the energy of filled orbitals and makes compounds more stable. 

We have ignored all the molecular orbitals from the a framework because the bonding a orbitals are considerably lower in energy than the molecular orbitals of the tt system and the vacant antibonding a orbitals are much higher in energy than the K antibonding molecular orbital. 

Delocalization of the allyl anion, and the localization of the negative charge mainly on the end carbons, is clear from its NMR spectrum as well. In Chapter 3 we explained that NMR gives us a good measure of the amount of electron density around a C atom—the extent to which it is deshielded and therefore exposed to the applied magnetic field. If you need reminding about the terminology, theory, and practice of NMR, turn back now to Chapter 3, pp. 52-53. 

The two p orbitals of ethylene are described as being conjugated with each other in making the % bond. To make longer conjugated systems we add one p orbital at a time to the % bond to make successively the allyl system, butadiene, the pentadienyl system and so on. We continue to separate completely the a framework (using the 2s, 2px and 2py orbitals on carbon with the Is orbitals on hydrogen) from the % system made up from the 2pz orbitals. 

The members of the allyl system are reactive intermediates, and there are three of them the allyl cation 1.1, the allyl radical 1.2 and the allyl anion 1.3. They have the same orbitals, but different numbers of electrons. 

The cation, radical and anion have the same a framework 1.4, with fourteen bonding molecular orbitals filled with 28 electrons made by mixing the Is orbitals of the five hydrogen atoms either with the 2s, 2px and 2py orbitals of the three carbon atoms or with the sp2 hybrids. The allyl systems are bent not linear, but we shall treat the % system as linear to simplify the discussion. 

The n system is made up from the three pz orbitals on the carbon atoms. The linear combination of these orbitals takes the form of Equation 1.9, with three terms, creating a pattern of three molecular orbitals, tpi, ip2 and 3. In the allyl cation there are two electrons left to go into the n system after filling the a framework (and in the radical, three, and in the anion, four). 

The lowest-energy orbital, ipi, has bonding across the whole conjugated system, with the electrons concentrated in the middle. The next orbital up in energy p2 must have a node in the middle of the conjugated system occupied by an atom and not by a bond. Having a node in the middle means having a zero coefficient c2 on C-2, and 

The interaction of atomic orbitals giving rise to molecular orbitals is the simplest type of conjugation. Thus in ethylene the two p orbitals can be described as being conjugated with each other to make the n bond. The simplest extension to make longer conjugated systems is to add one p orbital at a time to the n bond to make successively the n components of the allyl system with three carbon atoms, of butadiene with four, of the pentadienyl system with five, and so on. Hiickel theory applies, because in each case we separate completely the n system from the a framework, and we can continue to use the electron-in-the-box model. 

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Let us follow the first few iterations for the allyl system by hand calculations. We subtract the matrix xl from the HMO matrix to obtain the matrix we wish to diagonalize, just as we did with ethylene. With the rotation block in the upper left conrer of the R matrix (we are attacking an and aai), we wish to find... 

Draw bond order and free valency index diagrams for the butadienyl system. Write a counter into program MOBAS to detemiine how many iterations are executed in solving for the allyl system. The number is not the same for all computers or operating systems. Change the convergence criterion (statement 300) to several different values and determine the number of iterations for each. 

That no degenerate molecular orbitals arose in the above examples is a result of the fact that the C2v point group to which H2O and the allyl system belong (and certainly the... 

The ligand effect seems to depend on the substrates. Treatment of the prostaglandin precursor 73 with Pd(Ph3P)4 produces only the 0-allylated product 74. The use of dppe effects a [1,3] rearrangement to produce the cyclopen ta-none 75(55]. Usually a five-membered ring, rather than seven-membered, is predominantly formed. The exceptionally exclusive formation of seven-membered ring compound 77 from 76 is explained by the inductive effect of an oxygen adjacent to the allyl system in the intermediate complex[56]. 

No 0-allylation is observed in formation of the six-membered ring compound 79 by intramolecular allylation of the /3-keto ester 78(15,57]. Intramolecular allylation is useful for lactone fonnation. On the other hand, exclusive formation of the eight-membered ring lactone 81 from 80 may be in part derived from the preference for the nucleophile to attack the less substituted terminus of the allyl system[58]. 

Electron delocalization m allylic carbocations can be indicated using a dashed line to show the sharing of a pair of rr electrons by the three carbons The structural formula IS completed by placing a positive charge above the dashed line or by adding partial pos itive charges to the carbons at the end of the allylic system... 

The carbocations formed as intermediates when allylic halides undergo Stvfl reactions have their positive charge shared by the two end carbons of the allylic system and may be attacked by nucleophiles at either site Products may be formed with the same pattern of bonds as the starting allylic halide or with allylic rearrangement... 

Substituted allylic halides give mixtures of products resulting from bond formation at both C-1 and C-3 of the allylic system, with the product ratio favoring the product formed by reaction at the less substituted site. The portion of the product formed by reaction at C-1 in allylic systems may result from direct substitution, but it has also been suggested that a... 

Complexes 79 show several types of chemical reactions (87CCR229). Nucleophilic addition may proceed at the C2 and S atoms. In excess potassium cyanide, 79 (R = R = R" = R = H) forms mainly the allyl sulfide complex 82 (R = H, Nu = CN) (84JA2901). The reaction of sodium methylate, phenyl-, and 2-thienyllithium with 79 (R = R = r" = R = H) follows the same route. The fragment consisting of three coplanar carbon atoms is described as the allyl system over which the Tr-electron density is delocalized. The sulfur atom may participate in delocalization to some extent. Complex 82 (R = H, Nu = CN) may be proto-nated by hydrochloric acid to yield the product where the 2-cyanothiophene has been converted into 2,3-dihydro-2-cyanothiophene. The initial thiophene complex 79 (R = R = r" = R = H) reacts reversibly with tri-n-butylphosphine followed by the formation of 82 [R = H, Nu = P(n-Bu)3]. Less basic phosphines, such as methyldiphenylphosphine, add with much greater difficulty. The reaction of 79 (r2 = r3 = r4 = r5 = h) with the hydride anion [BH4, HFe(CO)4, HW(CO)J] followed by the formation of 82 (R = Nu, H) has also been studied in detail. When the hydride anion originates from HFe(CO)4, the process is complicated by the formation of side products 83 and 84. The 2-methylthiophene complex 79... 

Allylsilanes in which the silyl group is at the more substituted end of the allyl system have been prepared by a reaction sequence involving the conjugate addition of silylcuprates to a, jS-unsat-urated esters followed by reduction and dehydration via selenoxide elimination38. 

Alkylation occurs predominantly or exclusively at the more substituted end of the allylic system regardless of the nucleophile. The steric course of the reactions is the same as that observed with palladium88 and molybdenum89 catalysts. 

X = Br, in 50% aqueous ethanol. The observed solvent w =. 44 value for the allenyl system is comparable to the. 455 m value of the allylic system. No products were observed, as neither the expected propargyl alcohol nor acrolein was stable under the reaction conditions. In analogy with the solvolysis of trisubstituted haloallenes (203, 204) these results were interpreted in terms of an SnI mechanism and ionization to an allenyl cation. However, an alternative mechanism involving the unsaturated carbene, C=C=C , cannot be completely ruled out in the case of the parent system. Such a mechanism has been unambiguously established by a number of investigators (206-209) for the solvolysis of R2C=C=CHX or HC C—C(R)2X in aqueous solvents in the presence of a variety of bases. 

The tertiary amines 303 and the acid chlorides 304 (X = Cl) initially formed acylammonium salts 305, which underwent a von Braun type degradation by an attack of the nucleophilic chloride ion at the allyl system to give allyl chlorides 306/307 and carboxylic acid amide functions. 

Scheme 7.4 illustrates some of the important synthetic reactions in which organolithium reagents act as nucleophiles. The range of reactions includes S/v2-(ype alkylation (Entries 1 to 3), epoxide ring opening (Entry 4), and formation of alcohols by additions to aldehydes and ketones (Entries 5 to 10). Note that in Entry 2, alkylation takes place mainly at the 7-carbon of the allylic system. The ratio favoring 7-alkylation... 

Allylic acetates and phosphates can be readily carbonylated.248 Carbonylation usually occurs at the less-substituted end of the allylic system and with inversion of configuration in cyclic systems. 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

The energy level diagram including electron populations for the allyl radical, cation, and anion can be shown as illustrated in Figure 5.16. The orbital diagram and energy levels for the allyl system is shown in Figure 5.17. 

TABLE 8. Calculated energies (AE) for barrier to rotation in the allyl systems and charge differences (Ac/) for the CH2 groups"... 

In the examples presented so far, only two enantiomeric products have been possible in each case, since the substrates have all contained identical substituents on the Cl and C3 positions. However, a more complex situation occurs when the allyl system is unsymmetrically-substituted, as in 43a or 43b (Scheme 12).1161 Here, nucleophilic addition to the corresponding ri3-allyl intermediate 44 may afford an achiral, linear product 45, in addition to the pair of enantiomeric branched products 46. 

Allylmetallic reagents The ally] anions obtained by reductive metallation of ally I phenyl sulfides with lithium l-(dimethy amino)naphthalenide (LDMAN, 10, 244) react with a, 3-enals to give mixtures of 1,2-adducts. The regioselectivity can be controlled by the metal counterion. Thus the allyllithium or the allyltitanium compound obtained from either 1 or 2 reacts with crotonaldehyde at the secondary terminus of the allylic system to give mainly the adduct 3. In contrast the allylcerium compound reacts at the primary terminus to form 4 as the major adduct. 

Thus the l.v orbital of hydrogen or any other species will simultaneously interact with C-l or C-5 of the allyl system. 

So a [1, 5] or larger rearrangements suprafacial shift is symmetry allowed but a [1, 3] shift would be structurally prohibited, because geometrically it will not be feasible. This would require the hydrogen to migrate to the opposite side of the allyl system and this is sterically difficult. 

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