Alkene, photocycloaddition substituted

Reaction Wave function Alkene, photocycloaddition, 366. 420-23 addition to benzene. 420 substituted, 432 Alkyl amines, tertiary. 466 Alkyl aryl ketones, 399, 402 Alkylethylenes, 420 Alkylidenecyclopropene. 57-58 N-Alkylimine, 375 Alkyl iodide, 471 Alkyl methyl ketones. 383-84 Alkyl radical, 380 Allene. 416 Ally radical. 102, 460 Allyl resonance, 461... 

Whereas the cycloaddition of arylazirines with simple alkenes produces A -pyrrolines, a rearranged isomer can be formed when the alkene and the azirine moieties are suitably arranged in the same molecule. This type of intramolecular photocycloaddition was first detected using 2-vinyl-substituted azirines (75JA4682). Irradiation of azirine (54) in benzene afforded a 2,3-disubstituted pyrrole (55), while thermolysis gave a 2,5-disubstituted pyrrole (56). Photolysis of azirine (57) proceeded similarly and gave 1,2-diphenylimidazole (58) as the exclusive photoproduct. This stands in marked contrast to the thermal reaction of (57) which afforded 1,3-diphenylpyrazole (59) as the only product. 

Reports of [ 2 + 2] cycloaddition of nitrogen containing heterocycles to alkenes are so numerous that attention can be drawn here to only a few. Recent examples include the acetone-sensitized photoaddition of 4-oxazolin-2-one (248) to ethylene to give the cis-adduct 249,203 the photocycloadditions of N-substituted imidazoles to acrylonitrile204 and of N-methyl-4-hydroxy-quinol-2-one to cyclohexene,205 and the photoaddition of pentafluoro-pyridine to ethylene to give the 1 1- and 1 2-adducts 250 and 251,... 

Photocycloaddition of thiones to alkenes is the most popular and fruitful method for the preparation of the thietane system. In analogy to the formation of the oxetanes by cycloaddition of the electronic excited ( ,tc ) carbonyls, thietanes can be expected to arise photochemically from aromatic thioketones and substituted olefins as well as 1,2- and 1,3-dienes. ° Thiobenzophenone serves as a source of a sulfur atom and, because of its blue color, which disappears on photocycloaddition, permits exact control over the reaction time. A mixture of thiobenzophenone and a-phellandrene must be irradiated for 70 hr before the blue color disappears (Eq. 2) and... 

Photocycloaddition of an alkene to the thione group of 13 gave the thietane 14 which was stable at room temperature, but on refluxing 14 in toluene an iminothietane 15 and/or a 2-substituted benzoxazole were obtained by rearrangement processes <99JCS(P1)1151>. 

As mentioned at the beginning of this section, intermolecular [2 + 2] photocycloadditions quite often afford product mixtures, depending on the alkene used as a ground state partner. A complete overview of such reactions described in the last twenty years would be far beyond the scope of this section and therefore attention will be directed to examples where the symmetric substitution pattern of the alkene allows for the formation of a specific cyclobutane derivative. 

Previously, a variety of (2 + 2) photocycloaddition of alkenes to naphthalene rings have been reported as discussed in the previous subsections. Very recently, Yokoyama and Mizuno have reported the highly regioselective and stereoselective (2 + 2) photocycloaddition between 4-substituted 1-cyanonapha-thalenes (148) and 1-substituted 3-methyl-2-butenes (149) assisted by hydrogenbonding in the ground states as shown in Scheme 48 [226],... 

Gilbert and his co-workers reported that the photocycloaddition of 1-cyanonaphthalenes (260 and 262) gave 261 and 263, independently [288] (Scheme 72). In addition, they prepared and photoirradiated a variety of compounds that connected naphthalene and alkenes by methylene, ether, and ester tethers at various lengths [289], Among them, the reaction of 264 gave the best result, whereas the 2-substituted derivative (265) did not give any product. 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

Stable spirocyclic aminothietanes 25 (R = Me or H), obtained by photocycloaddition iV-alkoxy- and iV-aryloxycar-bonylbenzoxazole-2-thiones with alkenes upon reflux in toluene, were transformed into iminothietanes 26 (path a) and/or 2-substituted benzoxazole 27 (path b) (Scheme 2) < 1999J(P1) 1151, 2002HCA2383>. 

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]. 

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

The [2 + 2]-photocycloaddition chemistry of a,(3-unsaturated lactones has been widely explored. The factors governing regio- and simple diastereoselectivity are similar to what has been discussed in enone photochemistry (substrate class Al, Section 6.2). The HT product is the predominant product in the reaction with electron-rich alkenes [84]. A stereogenic center in the y-position of ot,P-unsaturated y-lactones (butenolides) can serve as a valuable control element to achieve facial diastereoselectivity [85, 86]. The selectivity is most pronounced if the lactone is substituted in the a- and/or P-position. The readily available chiral 2(5H)-furanones 79 and 82 have been successfully employed in natural product total syntheses (Scheme 6.30). In both cases, the intermediate photocycloaddition product with 1,2-dichloroethylene was reductively converted into a cyclobutene. In the first reaction sequence, the two-step procedure resulted diastereoselectively (d.r. = 88/12) in product 80, which was separated from the minor diastereoisomer (9%). Direct excitation (Hg lamp, quartz) in acetonitrile solution was superior to sensitized irradiation (Hg lamp, Pyrex) in acetone, the former providing the photocycloaddition products in 89% yield, the latter in only 45%. Cyclobutene 80 was further converted into the monoterpenoid pheromone (+)-lineatin (81) [87]. In the second reaction... 

The direct irradiation of the parent coumarin in the presence of alkenes results only in an inefficient photodimerization and [2 + 2]-photocycloaddition. Lewis acid coordination appears to increase the singlet state lifetime, and leads to improved yields in the stereospecific [2 + 2]-photocycloaddition [95]. Alternatively, triplet sensitization can be employed to facilitate a [2 + 2]-photocycloaddition. Yields of intramolecular [2 + 2]-photocycloadditions remain, however, even with electron-rich alkenes in the medium range at best. The preference for HT addition and for formation of the exo-product is in line with mechanistic considerations discussed earlier for other triplet [2 + 2]-photocycloadditions [96, 97]. Substituted coumarins were found to react more efficiently than the parent compound, even under conditions of direct irradiation. 3-Substituted coumarins, for example, 3-methoxy-carbonylcoumarin [98], are most useful and have been exploited extensively. The reaction of 3-ethoxycarbonylcoumarin (100) with 3-methyl-l-butene yielded cleanly the cyclobutane 101 (Scheme 6.36) with a pronounced preference for the exo-product (d.r. = 91/9). Product 101 underwent a ring-opening/ring-closure sequence upon treatment with dimethylsulfoxonium methylide to generate a tetrahydrodibenzofur-an, which was further converted into the natural product ( )-linderol A (102) [99]. 

The regioselectivity of the Paterno-Biichi reaction with acyclic enol ethers is substantially higher than with the corresponding unsymmetrically alkyl-substituted olefins. This effect was used for the synthesis of a variety of 3-alkoxyoxetanes and a series of derivatives [55]. The diastereoisomeric cis-and tnms-l-methoxy-l-butenes were used as substrates for the investigation of the spin state influence on reactivity, regio- and stereoselectivity [56]. The use of trimethylsilyloxyethene 62 as electron rich alkene is advantageous and several 1,3-anhydroapiitol derivatives such as 63 could be synthesized via photocycloaddition with l,3-diacetoxy-2-propanone 61 (Sch. 17) [57]. 

In the preparative application of [2 + 2]-photocycloadditions of cyclic enones to (substituted) alkenes, two factors concerning product formation are of decisive relevance, namely the regioselectivity and the (overall) rate of conversion. Regarding the regioselectivity in the addition to mono- and 1,1-disubstituted alkenes, Corey had shown that the preferred addition mode of cyclohex-2-enone to isobutene or 1,1-dimethoxyethylene was the one leading to—both cis- and trans-fused—bicyclo[4.2.0]octan-2-ones with the substituents on C(7) [8]. In contrast, in the reaction with acrylonitrile, the alternate orientation was observed to occur preferentially. Similar results were also reported by Cantrell for the photocycloaddition of 3-methyl-cyclohex-2-enone to differently substituted alkenes [14]. No significant differences in the overall rates of product formation for the different alkenes were observed in these studies. In order to explain these observed... 

Only few reports deal with a para photocycloaddition as the major reaction path. Recently, however, several cinnamide derivatives like 21 were efficiently transformed into the corresponding para adducts 22 (Sch. 6) [37]. Yields higher than 90% could be achieved. The para photocycloaddition is also observed with naphthalene derivatives like 1-acetylnaphthalene 23 and captodative enamino nitriles as 24 [38]. Other captodative substituted alkenes [39] as well as the fluorinated uracil derivative 26 [40] are transformed in the same way. Especially in the cases of 21 and 23, the... 

The Paterno-BUchi Reaction. One well-known class of photocycloadditions is the Paterno-Buchi reaction in which aldehydes or ketones combine with alkenes to give oxetanes. The excited state of the ketone is 11-71, and it is the orbitals of this state which interact with the ground-state orbitals of the alkene. The orientation usually observed for C- and X-substituted alkenes is shown for benzophenone 8.15 and 2-methylpropene 8.16. 

However, the photocycloaddition of ketones to Z-substituted alkenes does not fit the explanation based on the relative stability of the diradicals. Irradiation of a solution of acrylonitrile 8.22 in acetone 8.21 gives the adduct 8.23, together with dimers of acrylonitrile. This regioselectivity is consistent with a frontier orbital argument. 

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