Cycloadditions thermally allowed

The prediction of the Woodward-Hofrmann rules that thermal concerted cycloadditions are allowed for combinations in which 4 -1- 2 7c electrons are involved has stimulated the search for combinations with 10 and larger numbers of participating electrons. An example of a [6 -1- 4] cycloaddition is the reaction of tropone with 2,5-dimethyl-3,4-diphenylcyclopentadienone ... 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

H-Azepines 1 undergo a temperature-dependent dimerization process. At low temperatures a kinetically controlled, thermally allowed [6 + 4] 7t-cycloaddition takes place to give the un-symmetrical e.w-adducts, e.g. 2.231-248-249 At higher temperatures (100-200°C) the symmetrical, thermodynamically favored [6 + 6] rc-adducts, e.g. 3, are produced. These [6 + 6] adducts probably arise by a radical process, since a concerted [6 + 6] tt-cycloaddition is forbidden on orbital symmetry grounds, as is a thermal [l,3]-sigmatropic C2 —CIO shift of the unsym-metrical [6 + 4] 7t-dimer. 

In a photochemical cycloaddition, one component is electronically excited as a consequence of the promotion of one electron from the HOMO to the LUMO. The HOMO -LUMO of the component in the excited state interact with the HOMO-LUMO orbitals of the other component in the ground state. These interactions are bonding in [2+2] cycloadditions, giving an intermediate called exciplex, but are antibonding at one end in the [,i4j + 2j] Diels-Alder reaction (Scheme 1.17) therefore this type of cycloaddition cannot be concerted and any stereospecificity can be lost. According to the Woodward-Hoffmann rules [65], a concerted Diels-Alder reaction is thermally allowed but photochemically forbidden. 

The rule may then be stated A thermal pericyclic reaction involving a Hiickel system is allowed only if the total number of electrons is 4n + 2. A thermal pericyclic reaction involving a Mobius system is allowed only if the total number of electrons is 4n. For photochemical reactions these rules are reversed. Since both the 2 + 4 and 2 + 2 cycloadditions are Hiickel systems, the Mdbius-Hiickel method predicts that the 2 + 4 reaction, with 6 electrons, is thermally allowed, but the 2 + 2 reaction is not. One the other hand, the 2 + 2 reaction is allowed photochemically, while the 2 + 4 reaction is forbidden. 

Application of the same procedures to other ring closures shows that 4 + 4 and 2 + 6 ring closures and openings require photochemical induction while the 4 + 6 and 2 + 8 reactions can take place only thermally (see 15-52). In general, cycloaddition reactions allowed thermally are those with 4n + 2 electrons, while those allowed photochemically have 4n electrons. 

The suprafacial thermal addition of an allylic cation to a diene (a [3 -f- 4] cycloaddition) is allowed by the Woodward-Hoflfmann rales (this reaction would be expected to follow the same rules as the Diels-Alder reaction ). Such cyclo-... 

A full development of the rate law for the bimolecular reaction of MDI to yield carbodiimide and CO indicates that the reaction should truly be 2nd-order in MDI. This would be observed experimentally under conditions in which MDI is at limiting concentrations. This is not the case for these experimements MDI is present in considerable excess (usually 5.5-6 g of MDI (4.7-5.1 ml) are used in an 8.8 ml vessel). So at least at the early stages of reaction, the carbon dioxide evolution would be expected to display pseudo-zero order kinetics. As the amount of MDI is depleted, then 2nd-order kinetics should be observed. In fact, the asymptotic portion of the 225 C Isotherm can be fitted to a 2nd-order rate law. This kinetic analysis is consistent with a more detailed mechanism for the decomposition, in which 2 molecules of MDI form a cyclic intermediate through a thermally allowed [2+2] cycloaddition, which is formed at steady state concentrations and may then decompose to carbodiimide and carbon dioxide. Isocyanates and other related compounds have been reported to participate in [2 + 2] and [4 + 2] cycloaddition reactions (8.91. 

For the synthesis of estradiol methyl ether 4-319, the cydobutene derivative 4-317 was heated to give the orthoquinonedimethane 4-318 which cydized in an intramolecular Diels-Alder reaction [109]. The thermally permitted, conrotatory elec-trocyclic ring-opening of benzocyclobutenes [110] with subsequent intramolecular cycloaddition also allowed the formation of numerous complex frameworks (Scheme 4.70). 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

The [S2N]+ cation is an important reagent in S-N chemistry,63 especially in thermally allowed cycloaddition reactions with organic nitriles and alkynes, which give quantitative yields of heterocyclic cations (Scheme 3). It is conveniently prepared by reaction of S3N2C12 with AsF5 and S8 in liquid S02.63b The [SNS]+ cation is linear with S-N bond distances in the range indicating a bond order of two, i.e., S=N+=S. 

The well-known Diels-Alder reaction [95,104-106] is a standard method for forming substituted cyclohexenes through the thermally allowed 4s + 2s cycloaddition of alkenes and dienes. In particular, the reaction between ethene and 1,3-butadiene to yield cyclohexene is the prototype of a Diels-Alder reaction (Scheme 28.4). It is now well recognized that this reaction takes place via a synchronous and concerted mechanism through an aromatic boatlike TS [105]. 

To apply the rule we first draw the orbital picture of the reactants and show a geometrically feasible way to achieve overlap. Then the (4q + 2) suprafacial electrons and 4r antarafacial electrons of the components is counted. If the total is an odd number, the reaction is thermally allowed. Let us take the hypothetical cycloaddition of ethene to give cyclobutane. 

Concerted 2 + 2 + 2 cycloadditions are thermally allowed as n2s + n2s + n2s or n2s + n2a + 7i2a processes. The termolecular collisions necessary for these cycloadditions are very unlikely and the only examples known are those where at least two of the component n bonds are held together in one reactant. 

Thermally allowed [6 + 4] cycloadditions offer the attractive features of high stereoselectivity and rapid increase of molecular complexity. The limiting feature of many higher-order processes, however, is a lack of periselectivity that translates directly into the relatively low chemical yields of the desired cycloadducts. 

The thermally allowed [8 + 2] cycloaddition reactions may be considered as the 10tt analogs of the Diels-Alder reaction in which the diene component has been replaced by a tetraene component. Like trienes in the [6 + 4] cycloaddition reactions, the 87t tetraenes must satisfy certain requirements concerning geometry in order to be able to participate in an [8 + 2] cycloaddition. For example, tetraenes 518 and 519 can undergo an [8 + 2] cycloaddition, whereas an [8 + 2] cycloaddition with 520 is virtually impossible. Due to its fixed -system, 519 is more reactive in cycloaddition reactions than 518 and is therefore more often encountered in the literature. [8 + 2] Cycloadditions have been applied only... 

Density functional theory and MC-SCF calculations have been applied to a number of pericyclic reactions including cycloadditions and electrocyclizations. It has been established that the transition states of thermally allowed electrocyclic reactions are aromatic. Apparently they not only have highly delocalized structures and large resonance stabilizations, but also strongly enhanced magnetic susceptibilities and show appreciable nucleus-independent chemical-shift values. 

The classical 4 + 2 Diels-Alder reaction involves the thermally allowed cycloaddition of an electron-rich (nucleophilic) diene a=b-c=d with an electron-deficient (electrophilic) dienophile e=f. In the polar cycloaddition reactions considered here, the a=b-c=d system bears a positive charge and is so obviously ill-suited for a nucleophilic role that the first examples of polar cycloaddition appeared inexplicable in terms of cycloaddition theory then current. In 1962 Sauer and Wiest demonstrated the existence of a Diels-Alder reaction with inverse electron demand in which the electronic roles of a=b-c=d and e=f are exchanged, with the former becoming the electrophile and the latter... 

Having established the validity of the Huckel analysis of these molecules, it is useful to extend the method to the study of the cycloaddition reaction of the RjPSjNj molecules with norbomadiene. Thus the additidn of a 2e-olefin to the R PS Nj system can be regarded as a novel example of a thermally allowed 8 + 2 reaction. As in the addition of olefins to the preference for S,S-addition over... 

Dimerization of lff-azepines is an extensively studied phenomenon and involves a temperature dependent cycloaddition process. At low (0°C for 1 R = Me) or moderate (130 °C for 1 R = C02R or CN) temperatures a kinetically controlled, thermally allowed [6 + 4] dimerization to the exo -adduct (73) takes place, accompanied by a small amount (<10%) of symmetrical dimer (74). The latter are thermodynamically favored and become the major products (83%) when the Iff-azepines are heated briefly at 200 °C. The symmetrical dimers probably arise by a non-concerted diradical pathway since their formation from the parent azepines by a concerted [6+6]tt cycloaddition, or from dimer (73) by a 1,3-sigmatropic C-2, C-10 shift are forbidden on orbital symmetry grounds. Dimerization is subject to steric restraint and is inhibited by 2-, 4- and 7-substituents. In such cases thermolysis of the lif-azepine brings about aromatization to the correspondingly substituted JV-arylurethane (69JA3616). 

A key step in one route to the synthesis of hexamethyl Dewar benzene is the cycloaddition of 2-butyne to tetramethylcyclobutadiene (stabilized by A1 cation). Using the parent compounds (no methyls), develop a Woodward-Hoffmann orbital correlation diagram for the reaction and determine whether the reaction is thermally allowed. 

The mechanism of the reaction has generally been discussed in terms of a thermally allowed concerted 1,3-dipoIar cycloaddition process, in which control is realized by interaction between the highest occupied molecular orbital (HOMO) of the dipole (diazoalkane) and the lowest unoccupied molecular orbital (LUMO) of the dipolarophile (alkyne).76 In some cases unequal bond formation has been indicated in the transition state, giving a degree of charge separation. Compelling evidence has also been presented for a two-step diradical mechanism for the cycloaddition77 but this issue has yet to be resolved. 

The Diels-Alder reaction is one of the most important carbon-carbon bond forming reactions,521 522 which is particularly useful in the synthesis of natural products. Examples of practical significance of the cycloaddition of hydrocarbons, however, are also known. Discovered in 1928 by Diels and Alder,523 it is a reaction between a conjugated diene and a dienophile (alkene, alkyne) to form a six-membered carbo-cyclic ring. The Diels-Alder reaction is a reversible, thermally allowed pericyclic transformation or, according to the Woodward-Hoffmann nomenclature,524 a [4 + 2]-cycloaddition. The prototype reaction is the transformation between 1,3-butadiene and ethylene to give cyclohexene ... 

Much less information is available about [2 + 2]-cycloadditions. These allow the formation of cyclobutane derivatives in the reaction between two alkenes, or that of cyclobutenes from alkenes and alkynes. The reaction can be achieved thermally via biradical intermediates,543 by photoreaction,544 and there are also examples for transition-metal-catalyzed transformations. An excellent example is a ruthenium-catalyzed reaction between norbomenes and alkynes to form cyclobutenes with exo structure ... 

Although thermally allowed, the [2 + 4] cycloaddition of thiophene with nitrile oxides leads to low yields of products, even when thiophene itself is used as solvent (77JCS(P2)706, 78T3545). The yield is improved if the dimerization of the nitrile oxide is retarded. Both mono- and bis-adducts have been isolated. The most important feature of the cycloaddition, however, is the regiospecificity (Scheme 77). In contrast, benzo[6]thiophene gave a mixture of both possible regioisomers, although not in equal amounts. 

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