Cycloadditions, thermal photochemical

Thus, the frontier-orbital and Hiickel-Mobius methods (and the correlation-diagram method as well) lead to the same conclusions thermal 2 + 4 cycloadditions and photochemical 2 + 2 cycloadditions (and the reverse ring openings) are allowed, while photochemical 2 + 4 and thermal 2 + 2 ring closings (and openings) are forbidden. 

Cycloaddition reactions involving thermal/photochemical/catalytic decomposition of iodonium ylides are applicable to oxazole derivatives... 

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

The most common way to prepare silacyclopropanes is by [2+1] cycloaddition of silylenes to alkynes. The corresponding silylenes can be prepared thermally, photochemically, by metal reduction or catalyzed by a metal. 

Fig. 12.4 shows two possible ways for this to happen the HOMO of the diene can combine with the LUMO of the dienophile or the LUMO of the diene can combine with the HOMO of the dienophile. The thermal reaction with this six-electron transition state is allowed, but the corresponding photochemical mechanism is forbidden. More generally, the Woodward-Hoffmann rule for concerted cycloaddition reactions can be stated If the number of electrons in the transition state equals An [An + 2], then thephotochemical [thermal] reaction will be allowed, but the thermal [photochemical] reaction will be forbidden. 

Thermally, dithiins may react as dienophiles in [2 -f 4] cycloadditions <86SR123>. Photochemically, they afford [2 -I- 2] cyclodimerization products in the absence of oxygen in its presence, they are degraded to carbon dioxide and hydrogen sulfide among other photooxidation products <82TL2651 >. 

Photolysis of 3-buten-l-ol nitrite affords no cyclized products (Cy5/Cy4) neither does 5-hexen-l-ol nitrite (Cy6/Cy7). The same result is obtained on peroxydisulphate oxidation of 5-hexen-l-ol. In the Cy6/Cy7 case an important competitive pathway is probably 1,5-intramolecular ally lie hydrogen abstraction and, indeed, esr spin trapping by nitrosodurene " provides evidence of this. Cyclization in the Cy6/Cy7 case was considered to explain the reaction products of tetrahalogeno-o-benzoquinones with 2,3-dimethylbut-2-ene but was discarded in favor of a direct cycloaddition process on the basis of spin trapping and deuteration experiments. As discussed before, cyclization in the Cy3/Cy4 case must be difficult to observe because of the high j5-scission rate of oxyranylalkyl radicals. Nevertheless, this pathway has been used recently to explain the formation of diepoxides in the thermal-, photochemical-, or ferrous-salt-induced decomposition of unsaturated cyclic peroxides. In view of the multistep scheme involved this conclusion must await further confirmation. 

Some cycloaddition reactions of more than six ji electron systems have been reported. Thermal suprafacial [its+ s] [ s + s] [its + 7Cs]" y lo ditions are forbidden according to Woodward-Hofifmann rules. These cycloadditions are photochemically allowed processes. Thermal antarafacial [itj + 7c" a] addition is possible, but is rare. The following examples are illustrative for [4+4]- and [6+61-cycloadditions ... 

Allenes and cumulenes in general are more reactive than alkenes in undergoing cycloaddition reactions with isolated non-activated double bonds. The stereochemistry of the major cycloadducts can be predicted from the most stable diradicals formed upon initial bonding of the sp carbon to the allene. The [2-1-2] cycloaddition reaction of allenes to olefins proceeds thermally, photochemically and at room temperature, using a Lewis acid catalyst. The mechanism of these reactions are considered to be stepwise processes, with radical or ionic intermediates. 

Cycloaddition Reactions. Furan is known to undergo various t) es of cycloaddition reactions. It functions as an ene and participates in [2 + 2] and [2 + 3] cycloadditions under photochemical and thermal conditions, respectively. With an embedded cyclic conjugated diene system in it, furan also functions as a diene that undergoes various [4 + 2] and [4 + 3] cycloaddition reactions. ... 

The reaction of two alkene molecules to give a cyclobutane ring has four ti electrons in the transition state. It is an example of a 4 7t system. Since each molecule contributes two ti electrons to the transition state, it is a [2 + 2] cycloaddition. A [2+2] cycloaddition occurs photochemically, not thermally. 

Five-membered heterocycles have been obtained by the thermal dipolar cycloaddition of photochemically generated 1,3-dipoles to dipolarophiles. 

Reactions.—The chemical behaviour of thioketones has mainly been investigated in four directions reaction with organometallic compounds, reaction with nucleophiles, thermal cycloadditions, and photochemical reactions. 

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

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. 

According to the Woodw ard-Hofmann rules the concerted thermal [2n + 2n] cycloaddition reaction of alkenes 1 in a suprafacial manner is symmetry-forbidden, and is observed in special cases only. In contrast the photochemical [2n + 2n cycloaddition is symmetry-allowed, and is a useful method for the synthesis of cyclobutane derivatives 2. 

An explanation for the finding that concerted [4 -I- 2] cycloadditions take place thermally, while concerted [2 + 2] cycloadditions occur under photochemical conditions, is given through the principle of conservation of orbital symmetry. According to the Woodw ard-Hofmann rules derived thereof, a concerted, pericyclic [4 -I- 2] cycloaddition reaction from the ground state is symmetry-allowed. 

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

In contrast with the [4 + 2]- --electron Diels-Alder reaction, the [2 + 2 thermal cycloaddition between two alkenes does not occur. Only the photochemical [2 + 2] cycloaddition takes place to yield cyclobutane products. 

In contrast with the thermal process, photochemical [2 + 2] cycloadditions me observed. Irradiation of an alkene with UV light excites an electron from i /, the ground-slate HOMO, to which becomes the excited-slate HOMO. Interaction between the excited-state HOMO of one alkene and the LUMO of the second alkene allows a photochemical [2 + 2j cycloaddition reaction to occur by a suprafacial pathway (Figure 30.10b). 

Figure 30.10 (a) Interaction of a ground-state HOMO and a ground-state LUMO in a potential [2 - 2] cycloaddition does not occur thermally because the antarafacial geometry is too strained, (b) Interaction of an excited-state HOMO and a ground-state LUMO in a photochemical [2 r 2] cycloaddition reaction is less strained, however, and occurs with suprafacial geometry. 

---