Sigmatropic reactions 8-electron

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

Sigmatropic reaction (Section 30.8) A pericyclic reaction that involves the migration of a group from one end of a tt electron system to the other. 

The [2,3]-Wittig Rearrangement is a [2,3]-sigmatropic reaction, a thermal isomerization that proceeds through a six-electron, five-membered cyclic transition state. A general scheme for [2,3]-sigmatropic reactions is given here ... 

Sigmatropic reactions encompass a vast number of synthetically useful variants in terms of both the atom pair involved (X, Y) and the electronic state (Y anions, non-bonding electron pairs, ylides). 

This intuitive parallel can be best demonstrated by the example of electrocye-lic reactions for which the values of the similarity indices for conrotatory and disrotatory reactions systematically differ in such a way that a higher index or, in other words, a lower electron reorganisation is observed for reactions which are allowed by the Woodward-Hoffmann rules. In contrast to electrocyclic reactions for which the parallel between the Woodward-Hoffmann rules and the least motion principle is entirely straightforward, the situation is more complex for cycloadditions and sigmatropic reactions where the values of similarity indices for alternative reaction mechanisms are equal so that the discrimination between allowed and forbidden reactions becomes impossible. The origin of this insufficiency was analysed in subsequent studies [46,47] in which we demonstrated that the primary cause lies in the restricted information content of the index rRP. In order to overcome this certain limitation, a solution was proposed based on the use of the so-called second-order similarity index gRP [46]. This... 

The antara-antara reaction is allowed if the upper lobe of C, has the same sign as the lower lobe of C , i.e. whenever the corresponding supra-supra reaction is permitted. A similar analysis shows that the thermal supra-antara (or antara-supra) reaction will be allowed when one of the pairs (Q, C() and (C/, C.) has coefficients of the same sign and the other has coefficients which are opposed. This occurs when (z + j) = An. To summarize thermal supra-supra or antara-antara sigmatropic reactions are allowed for systems having An+2 electrons and supra-antara or antara-supra reactions for systems having An electrons. 

This [1,7] sigmatropic reaction proceeds with antarafacial geometry because four electron pairs are involved in the rearrangement. 

The simplest sigmatropic reaction, 1,2-shift (2-electron system), in carbocations is the well-known 1,2-alkyl shift (Schemes 2.9 and 2.10). This shift can be concerted Wagner-Meerwein rearrangement (see section 2.1.3) and suprafacial in carbocations. The 1,2-methyl shift involves three carbons held together by a three-centre two-electron bond at the transition state, representing the smallest and simple system (Scheme 8.14). 

Sigmatropic reactions can be designated by the same scheme we have used for IT electron reactions, i.e., [tt/ -I- ], etc. However, sigmatropic reactions... 

For sigmatropic reactions, as for electrocyclic reactions and cycloadditions, the course of reaction can be predicted by counting the number of electrons involved and applying the selection rules. A comprehensive rationalization of all the stereochemical aspects of these reactions requires application of the frontier orbital or orbital symmetry approaches, and, at this point, we will content ourselves with pointing out the salient features of the more common reactions of this class. 

For a sigmatropic reaction, the number of electrons involved is the number of tt electrons plus the pair of electrons in the migrating [Pg.369]

Therefore, if we derive or remember one rule for a pericyclic reaction, then any time an MO phase change is added the rule will reverse. Two reversals cancel each other. For example, 4n face to face (supra-supra) cycloadditions are not thermally allowed. If we add two electrons, we fill the next highest MO, which has a phase reversal. This means An+2 cycloadditions are thermally favored. Thermal electrocyclic reactions of 4n species go conrotatory, whereas thermal 4n+2 electrocyclic reactions go disrotatory. Thermal sigmatropic reactions of 4n species go supra-inversion or antara-retention. Count arrows to tell whether the pericyclic reaction is 4n or 4n + 2. Phase reversals occur between retention/inversion at the migrating center, between antarafacial/suprafacial migration, with 4n vs. 4n+2 electrons, and between thermal and photochemically excited species. 

Now we consider sigmatropic reactions. Woodward and Hoffmann defined a sigmatropic reaction of order [i,j as the migration of a a bond flanked by one or more n electron systems, to a new position removed by t — 1 and / — 1 atoms from the original bonded sites in an uncatalyzed intramolecular process 7>. 

An alternative analysis of sigmatropic reactions involves drawing the basis set atomic orbitals and classifying the resulting system as Htickel or Mobius in character. When this classification has been done, the electrons involved in the process are counted to determine if the TS is aromatic or antiaromatic. The conclusions reached are the same as for the frontier orbital approach. The suprafacial 1,3-shift of hydrogen is forbidden but the suprafacial 1,5-shift is allowed. Analysis of a 1,7-shift of hydrogen shows that the antarafacial shift is allowed. This analysis is illustrated in Figure 10.31. These conclusions based on orbital symmetry considerations are supported by HF/6-31G calculations, which conclude that 1,5-shifts should be suprafacial, whereas... 

A cycloaddition reaction is actually a type of pericydic reaction, but the term peri-cyclic includes other types of reactions. The textbook definition of pericydic is a reaction whose transition state has a cyclic structure (i.e., the electrons are flowing in a closed loop). In addition to cycloaddition reactions (which exchange two Jt-bonds for two o-bonds, or vice versa), pericydic reactions include sigmatropic reactions, electro-cyclic reactions, and cheletropic reactions (and a few others which we ll ignore). 

As is the case for other pericyclic reactions, the selection rules for a thermal [i, ] sigmatropic reaction are reversed for the photochemical reaction. If irradiation of a 1,5-hexadiene produces the electronically excited state of one and only one of the two allyl components, then the HOMO of one component is (/f3, and the HOMO of ihe other component is suprafacial-suprafacial reaction (Figure 11.46) is forbidden (as is the antarafacial-antar-afacial pathway), but the antarafacial-suprafacial and suprafacial-antarafacial pathways are allowed (Figure 11.47). Analysis of higher sigmatropic reactions shows that the selection rules also reverse with the addition of a carbon-carbon double bond to either of the n systems. Thus, the [3,5] sigmatropic reaction is thermally allowed to be suprafacial-antarafacial or antarafacial-suprafacial and photochemically allowed to be suprafacial- suprafacial or antarafacial-antarafacial. Two of these reaction modes are illustrated in Figure 11.48. 

Sigmatropic reactions can be treated successfully by PMO method, and similar conclusions are arrived at as by other approaches (Figure 3.6). For instance, [1,3] suprafacial shift of hydrogen occurs via a transition state with zero node and four electrons (antiaromatic) and thus is a photochemically allowed... 

Another broad class of [1 j] sigmatropic reactions is represented, e.g., Iqr the groiq) of [1,2] rearrangements involving the migration to both electron deficient and anionic centers. 

As can be seen from this Table, the original insufficiency of the index r is indeed remedied by the index g p. This result is very interesting since if we realize that the primary source of the increased information content of the index g p is the partial inclusion of electron correlation, then the discrimination between the allowed and forbidden reactions in these cases seems to suggest that certain delicateness of cycloadditions and sigmatropic reactions, which both belong to the class of the so-called multibond reactions [99], can be apparently related to the greater sensitivity of these reactions to the effects of electron correlation. This conclusion, together with the systematic analysis of the role of correlation effects in pericyclic reactivity will be discussed in more details in chapter 8. 

The electronic 7t system of ruthenium vinylidenes can participate in pericyclic reactions such as electrocyclizations, cycloadditions, and sigmatropic reactions to afford a variety of interesting poly(hetero)cyclic products. 

Of all the above mentioned reactions, (i) and (ii) reactions are known as sigmatropic reactions. A sigmatropic reaction involves the migration of a bond adjacent to one more 7c-electron systems, to a new position in the uncatalysed intramolecular reaction . The (ii) reaction is a thermal reaction and known as cope-rearrangement. 

Sigmatropic reactions are of order [ 1,/] or of [i,/]. The values (1 +/) or (i + /) give the numbers of interacting atomic nuclei. When these values are even, then they are also equal to the numbers of electrons delocalized in the transition states of the pericyclic changes. However, when (1 +/) or (i +/) is odd, the sigmatropic change will have cationic or anionic character, and the participant electrons will then be one less (cationic) or one greater (anionic) than these values. 

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

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