Orbital symmetry rules compounds

The orbital symmetry rules also help us to explain, as on pages 1083 and 1433, the unexpected stability of certain compounds. Thus, 102 could, by a thermal [1,3] sigmatropic rearrangement, easily convert to toluene, which of course is far more stable because it has an aromatic sextet. Yet 102 has been prepared and is stable at dry ice temperature and in dilute solutions. ... 

As discussed in Section 10.4 of Part A, concerted suprafacial [2tt + 2tt] cycloadditions are forbidden by orbital symmetry rules. Two types of [2 + 2] cycloadditions are of synthetic value addition reactions of ketenes and photochemical additions. The latter group includes reactions of alkenes, dienes, enones, and carbonyl compounds, and these additions are discussed in the sections that follow. 

Cycloaddition of carbon-carbon double bonds can also occur intramolecularly. Direct irradiation of simple dienes leads to cyclobutanes. This is a singlet-state process and is concerted. The stereochemistry of the cyclobutane can be predicted on the basis of orbital-symmetry rules (Part A, Section 10.1). Nonconjugated dienes can also undergo photochemical cyclization employing mercury or carbonyl compounds as sensitizers. Cyclobutane formation is usually unfavorable with 1,4-dienes because it would result in a very strained ring system. When the alkene units are separated by at least two carbon atoms, cyclization becomes more favorable sterically ... 

The photochemistry of alkenes and dienes has already been examined in part, since these compounds are particularly illustrative of the principles of orbital symmetry control in electrocyclic processes. The orbital symmetry rules for cycloadditions and electrocyclic processes were covered in Section 11.2. Cycloadditions are also considered, from a synthetic viewpoint, in Part B, Chapter 6, Section 6.2. This section will emphasize unimolecular rearrangements of alkenes and dienes. 

Two reviews deal with the relation between homogeneous and heterogeneous catalysis. One of these reviews is of a general physical and theoretical nature, while the other is concerned with specific classes of compounds — alkenes, alkynes, and fats — and concentrates on their hydrogenation and isomerization. Homogeneous catalysis by ruthenium complexes has been reviewed. The application of molecular orbital symmetry rules, to organic as well as to organometallic reaction mechanisms, has been discussed, and a set of rules similar to, but simpler to apply than, the Woodward-Hoffmann rules has been described. ... 

The present state of knowledge concerning homogeneous catalysis of ethylene polymerization by vanadium and by chromium compounds has been reviewed. The relevance of orbital symmetry rules to transition-metal catalysis of [2 + 2] cycloaddition reactions has been discussed. Examples of oligomerization reactions, catalysed by transition-metal complexes, are given in Table 3,105-m... 

Larrabee, R. B., Fluxional Main Group IV Organometallic Compounds, The Implications for Orbital Symmetry Rules, J. Organometal. Chem. 74 [1974] 313/64. 

One attraction of electrocyclic ring closures for use in synthesis is related to fact that they proceed by means of clearly defined mechanisms. In a single step, one can form a ring and control relative or even absolute stereochemistry at multiple sites in a molecule leading to a rapid increase in complexity. Relative stereochemistry is controlled by orbital symmetry rules (Table 19.1), making the prediction of stereochemistry straightforward. The 7t-electron count always refers to the ring-open compound. 

An intramolecular rearrangement of the conjugate acid of the triazene compound to form the oc-complex without an additional molecule of amine would correspond to a thermal [l,3]-sigmatropic rearrangement. However, such a mechanism can be ruled out on the grounds of the antarafacial pathway required from orbital symmetry considerations (Woodward-Hoffmann rules). 

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>. 

This question is the subject of much debate, because the mechanism by which the oxaphosphetane is formed is not entirely understood. One possible explanation relies on rules of Orbital symmetry, which you will meet in Chapters 35 and 36—we need not explain them in detail here but suffice it to say that there is good reason to believe that, if the ylid and carbonyl compound react... 

Photochemically induced [2 + 2] cycloaddition is of extraordinary importance in organic synthesis,as this is a method ideally suited for the preparation of sterically congested compounds. The reaction may occur by a concerted mechanism allowed by rules of orbital symmetry, or, more often, via a biradical pathway. For preparative purposes, the most widely exploited is the enone-alkene photochemical [2 + 2] cycloaddition. This reaction proceeds with high regioselectivity, although its stereoselectivity might be low. The first example of the utilization of this reaction for the synthesis of a natural compound, a-cariophyllene 385, was described by Corey (Scheme 2.129). Adduct 386, formed as a mixture of stereoisomers in high yield from simple precursors, was further transformed via the tricyclic intermediate 387 into the... 

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