Pericyclic Reaction Paths

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

Enantiomerically enriched l-(diisopropylaminocarbonyloxy)allyllithium derivatives (Section 1.3.3.3.1.2.) add to carbonyl compounds with syn-l,3-chirality transfer21, giving good evidence for a pericyclic transition state in the main reaction path (Section 1.3.3.1.). However, since the simple diastereoselectivity and the degree of chirality transfer are low, for synthetic purposes a metal exchange with titanium reagents or trialkyltin halides (Section D.1.3.3.3.8.2.3.) is recommended. 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

In reality, the reaction could not be persuaded to go exactly as shown in Scheme 21.1, because the Cl—C3 bond would certainly break at very nearly the same rate as Cl—C2. In the experiments actually conducted by Baldwin et al., this problem was resolved by deuterium labeling both C2 and C3—creahng diastereomericaUy pure, but achiral molecules. Even then, there remained a large number of technical difficulties, which in the end the researchers were able to overcome. Their results indicated that the four stereochemical courses for the reaction run at 300 °C were sr 23%, si 40%, ar 13%, and ai 24%. These numbers do not ht the expectations from either mechanism. Clearly, the Woodward-Hoffmann forbidden and allowed products are formed in nearly equal amounts ([sr] + [ai] =47% [si] + [ar] = 53%)— hardly what one would expect for a pericyclic reaction. On the other hand, the stereochemical paths do not show the pairwise equalities expected from the stepwise mechanism. 

In a pericyclic reaction, the pathway predicted by the selection rules is the one that allows maximum orbital overlap along the reaction pathway, including the transition state. Maximum orbital overlap corresponds to the path of minimum energy and is achieved if the orbitals involved are similar in energy and if the symmetry of the orbitals is maintained throughout the reaction path. 

The antiaromatic geometry found along the concerted path of ground-state-forbidden pericyclic reactions, which is topologically equivalent to an antiaromatic Hiickel [4n]annulene or MObius [An + 2]annulene, is a particularly interesting type of biradicaloid geometry. (Cf. Section 4.4.) Other biradicaloid geometries and combinations of those mentioned are equally possible. 

The global term pericyclic funnel will be used to refer to the funnel or funnels in the S surface that occur at the critically heterosymmetric biradicaloid geometries reached near the halfway point along the path of a thermally forbidden pericyclic reaction, and the minima in S, that are encountered along one-dimensional cuts along reaction paths that miss the conical intersections (in particular, those along high-symmetry paths, which pass... 

Photodimerization often involves an excimer that can be treated as a su-permolecule. (Cf. Section 6.2.3.) Then, the state correlation diagram for the singlet process (Figure 7.27a) ordinarily calls for a two-step return from S, to So along the concerted reaction path. First, an excimer intermediate E is formed. Second, a thermally activated step takes the system to the diagonally distorted pericyclic funnel P" (cf. Section 4.4.1), and the return to So that follows is essentially immediate. The reaction will be stereospecific and concerted in the sense that the new bonds form in concert. However, it will not be concerted in the other sense of the word, in that it involves an intermediate E. ... 

Like all reactions, pericyclic reactions are reversible in principle (even though they may be irreversible in practice). The forward and reverse reactions always go through the same transition state. As an analogy, if you wanted to travel from Lexington, Ky., to Richmond, Va., you would choose the path that went through the lowest gap in the Appalachian mountains. If you wanted to go from Richmond back to Lexington, you would choose the same route, only in reverse. The path you chose would not depend on which direction you were traveling. Reactions obey the same principle. 

The single-parameter X-model is now extended to a parametric description of complex reactions with an arbitrary number of reaction parameters. Let p( 3) be the number of reaction partn s (reactants, products or intermediates) the reaction lattice is then isomorphic to the lattice Pip + 1) 2 with a diagram of a higher dimensional cube (6.32). Accordin y, the dynamic sublattice is isomorphic to P(p) = 2 and thus contains at least one element of the non-roechanistic dimension A (see Ch. "Generalized reaction lattice"). Ck>nsequently, the choice of the reaction path is no longer unique - in contrast to the sin e-parameter X model for pericyclic reactions with a well defined reaction path (via an aromatic or antiaromatic transition state.). The formal algebraic description of... 

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