Violations of the Woodward-Hoffmann

The products (101) are presumably formed in an orbital symmetry allowed (2 + 2 + 2) 7t concerted cyclo-addition. The formation of the compounds (99) and (100) are more difficult to rationalise unambiguously. Clearly, if an intermediate such as (102) had more than a very transient existence, one would expect to isolate products derived from carbonium ion rearrangements. That no rearranged products have been isolated might be used as an argument for the violation of the Woodward-Hoffmann rules. "There are none " 68>. 

In the third class of thermally allowed [2 + 2] cycloadditions, a very electron rich alkene is allowed to react with a very electron poor diene. These reactions almost certainly proceed stepwise through a zwitterionic or diradical intermediate, so they are not pericyclic reactions, and no violation of the Woodward-Hoffmann rules occurs. [2 + 2] Cycloadditions that proceed by a stepwise mechanism are nonstereospecific. 

The mechanism of the thermal [2k + 2k] cyclodimerization is therefore not a violation of the Woodward-HofFmann rules for pericyclic reactions wherein concerted thermal [2k + 2k] cyclodimerizations are disallowed suprafacially, but is instead a radical-mediated step-wise ring formation. The resulting hexafiuorocyclobutane ring has either a cis-1,2 or a trans-1,2 conformation. The geometry of these two linkages and the effect that they have on the polymer behavior are suspected to be quite dissimilar (vide infra). 

The state-symmetry correlation also indicates that electrocyclic radical interconversion favors a conrotatory path from the first excited state and a disrotatory path from the second excited state. Because of the proximity of the energy levels and the violations of the noncrossing rule, it is probable that the excited state process will not be highly stereoselective. The same detailed considerations must be applied to the five-atom five-electron system and yield the results given in Table 1. Differences between the stereochemical predictions of Table 1 and those of others (Woodward and Hoffmann, 1965a Fukui and Fujimoto, 1966b Zimmerman, 1966) tend to be limited to the excited-state reactions of odd-atom radicals. 

Although [1,2] rearrangements to an electron-rich center are forbidden by the Woodward-Hoffmann rules (Tables 5, 8) there appear to be both real and apparent violations. The following desilylation goes with retention at carbon (Brook et al., 1967) large Hammett p values (3-4 to 4 14) for substituents in Ar indicate a high degree of carbanionic... 

This is a key difference. The Woodward-Hoffmann rules (Chapters 35 and 36) were deduced from theory, and examples were gredually discovered that fitted them. They cannot be violated a reaction that disobeys the Woodward-Hoffmann rules is getting around them by following a different mechanism. Baldwin s rules were formulated by making observations of reactions that do, or do not, work. 

If a reaction is concerted by the enthalpic criterion described earher, then the Woodward-Hoffmann Rules will apply and will invariably predict the reaction stereopathway ( Violations... There are none... Nor can violations be expected... -Woodward, 1970). However, observation of allowed stereochemistry does not prove... 

The complete omission of specific material on allene cycloadditions in the Woodward and Hoffmann review article 142> emphasizes the ticklish theoretical situation there may be no violations to the principle of maximum bonding 142>, but sure applications of the theory in its present early stage of development are more easily made for some types of reaction than for others. 

To see where false rigor can get us in trouble, let s first examine another use of symmetry. Near the end of the book The Conservation of Orbital Symmetry, in which Woodward and Hoffmann described their rules for understanding pericyclic reactions, the authors included a section entitled "Violations" (Woodward Hoffman, 1970). The first, single-sentence paragraph proclaims "There are none "... 

Initially, the most obvious mechanism for olefin metathesis was a land of molecular square dance in which two different olefin molecules join to form a cyclobutane ring and then change partners to form two new olefin molecules. While this thermal reaction is Woodward-Hoffmann forbidden, transition metals were initially perceived to allow violations of these rules. However, no cyclobutanes were detected in olefin metathesis reactions, nor did cyclobutanes produce olefins when placed into metathesis reaction mixtures. The breakthrough came in 1971 when Yves Chauvin (1930- ), at the French Petroleum Institute, made the concepmal link between the Phillips Petroleum reaction discovered in 1964 and metallocarbenes isolated in the same year by Ernest Otto Fischer (see chapter 7). Other important discoveries were made by Michael F. Lappert (1928- ) at Sussex, Charles P. Casey (1942- ) at Wisconsin, and especially Thomas J. Katz (1936- ) at Columbia. The mechanism involves formation of a metallocyclobutane (see the accompanying figure), from reaction of a metallocarbene ( M=CR 2 ) with an olefin (R2C=CR2), that splits into a new olefin (R 2C=CR2) and a new metallocarbene ( M=CR2 ). 

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