Photochemical cycloaddition of two

Now let us return to our discussion of the conical intersection structure for the [2+2] photochemical cycloaddition of two ethylenes and photochemical di-Jt-methane rearrangement. They are both similar to the 4 orbital 4 electron model just discussed, except that we have p and p overlaps rather than Is orbital overlaps. In Figure 9.5 it is clear that the conical intersection geometry is associated with T = 0 in Eq. 9.2b. Thus (inspecting Figure 9.5) we can deduce that... 

The photochemical cycloaddition of two different alkenes leads to multiply substituted cyclobutanes, and allows for a general access to this class of compounds. More specifically, ever since Ciamician observed the light-induced isomerization of carvone (1 —> 2) (Scheme 6.1) in 1908 [1], the inter- and intramolecular reaction between an a,(3-unsaturated carbonyl compound and an alkene has become the most intensively studied and most widely used class of [2 + 2]-photocycloaddition reactions [2-9]. 

Fig. 10. Correlation Diagram for Photochemical Cycloaddition of two Ethylenic Components...

ANALYTIC REPRESENTATION Of A CONICAL INTERSECTION IN THE PHOTOCHEMICAL CYCLOADDITION OF TWO ETHYLENE MOLECULES... 

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. 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5),... 

Cycloaddition of alkenes to the benzene ring does not occur when both molecules are in their ground electronic states. The reaction can only be brought about by photoexcitation of either of the two addends. Three types of photochemical cycloaddition of alkenes to benzene and its derivatives are presently known. Ortho photocycloaddition, also referred to as 1,2-photocycloaddition or [2 + 2] photocycloaddition, leads to bicyclo[4.2.0]octa-2,4-dienes. Meta photocycloaddition, also referred to as 1,3-photocycloaddition or [2 + 3] photocycloaddition, gives triyclo[3.3.0.02 8]oct-3-enes, also named l,2,2a,2b,4a,4b-hexahydrocyclo-propa[crf]pcn(alcnes. Para photocycloaddition, also referred to as 1,4-photocy-cloaddition or [2 + 4] photocycloaddition, results in bicyclo[2.2.2]octa-2,5-... 

Photochemical cycloaddition of 2-cyanofuran with 2-alkoxy-3-cyanopyridine results in the formation of [4+4] photoadducts 228 and 229. The latter compound is seen to arise through the intermediate 230 (Scheme 40) <2004TL4437>. Mechanistic studies show that the photoadditions proceed from the singlet-excited state of the pyridine. The preference for the formation of 228 over 229 is explained by the two heteraromatics approaching each other such as to avoid proximity of their electronegative heteroatoms. 

When ultraviolet light rather than heat is used to induce pericyclic reactions, our predictions generally must be reversed. For example, the [2 + 2] cycloaddition of two ethylenes is photochemically allowed. When a photon with the correct energy... 

An additional example is a two-step approach to the preparation of 1,5-cyclo-decadiene. By a photochemical cycloaddition of a substituted cyclobutene and 2-cyclohexenone, a strained tricyclo[4.4.0.02,5]decane system is generated. Thermolysis of the tricycle gave 1,5-cyclodecadiene [52]. 

Photochemical cycloaddition of these two compounds is claimed to give the single diastereoisomer shown. The chemists who did this work claim that the stereochemistry of the adduct is simply provided by its conversion into a lactone on reduction. Comment on the validity of this deduction and explain the stereochemistry of the cycloaddition. 

Photochemical cycloaddition of 2-arylisoindoline-l-thiones 8 to electron-poor alkenes 9, such as methacrylonitrile, 2-butene-2-nitrile and methyl methacrylate, affords the corresponding tricyclic isoindolines 10 as two diastereomers143. 

The photochemical [2 + 2] cycloaddition reaction of alkenes, the light-induced cycloaddition of two carbon-carbon ir-bonds to produce a cyclobutane, is a potentially highly useful reaction in organic synthesis since two new carbon-carbon bonds are formed and a maximum of four new stereocenters are introduced in the process. Ciamician reported the first example in 1908 when he observed the formation of carvone camphor (2) on prolonged exposure of carvone (1) to Italian sunlight (equation 1). ... 

An azulene (36) having two intramolecularly crossing ethynyl groups was obtained together with its isomer (37) on irradiation of o-diethynylbenzene (35) and no further photochemical reaction was observed . On the other hand, o-bisphenyl-ethynylbenzcne (38) gave an azulenophenalene system, verdene (39), on irradiation . The mechanism of the formation of 39 was considered as a result of further cycloaddition of two triple bonds crossed in an intermediate azulene derivative (40). This photochemical reaction is reminiscent of the Biichi reaction in which diphenyl-acetylene yields an azulene system. 

According to the Woodward-Hoffmann rules, the concerted cycloaddition of two olefins to afford a cyclobutane is allowed photochemically as + 2J reaction and thermally as the [,2s + reaction. The different modes of addition give rise to products with different stereochemical structures as indicated in Figure 7.26. If the reaction does not follow a concerted pathway... 

Photochemical 67T-6tt cycloaddition of two benzene rings, in principle, produces benzene dimers having two 1,3-dienes units. However, as expected, the dimers are unstable and revert back to benzene rings easily. Prinzbach and coworkers found that two benzene rings, locked in face-to-face relationship, undergo 67r-67r photocycloaddition on irradiation... 

In contrast to the thermal [4 -i- 2] Diels-Alder reaction, the 12 + 2] cycloaddition of two alkenes to yield a cyclobutane can only be observed photochemically. The explanation follows from orbital-symmetry arguments. 

The photochemical cycloaddition of ethene to the bis-butenolides (20) has been examined in an attempt to establish the influence of the ether-protecting groups of the diol system. Generally only two adducts are formed as can be seen from the results shown for the appropriate structures. The most effective ether protecting group is the trimethylsilyl function and here the facial selectivity yields predominantly the anti,anti adduct (21). With the unprotected systems (20, R = H), there is virtually no selectivity and in this case the three adducts (21), (22) and (23) are formed. Irradiation of the butenolides (20a) and (20b) in the absence of ethene leads to intramolecular hydrogen abstraction (a Norrish Type II process) with the formation of the products (24a) and (24b) in 79% and 76%, respectively. 

Photochemical cycloadditions of homodienes are extremely rare. An unusual photocycloaddition of the 3-OXO-4,6-diene steroid 28 to 2,3-dimethylbutadiene has been reported.The reaction has been described as a [(27H- 27t) -t- (2n + 2a)] cycloaddition, but can also be explained as a sequence of two independent processes, a [(2n + 2n) + (2n + 2a)] and a [27c- -27t] addition (both photochemically allowed). 

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