Di-.pi.-methane rearrangement of the naphthobarrelenes. Mechanistic and exploratory organic photochemistry. L

Abstract

1,2-Naphthobarrelene (8) and 2,3-naphthobarrelene (11) were synthesized. Direct irradiation of 1,2-naphthobarrelene (8) in cyclohexane afforded 1,2-naphthocyclooctatetraene (14) as the major product (74%) along with equal amounts of the syn and anti isomers (12 and 13) of naphtho[l,2-c]tricyclo[3.3.0.02i8]octa-3,6-diene (1,2-naphthosemibullvalene) as the minor products (26%), the latter arising from di-tr-methane rearrangements. In contrast, the benzophenone sensitized reaction afforded solely the syn-and anti-1,2-naphthosemibullvalenes (12 and 13). The direct irradiation was shown by deuterium-labeling studies to proceed by naphtho-vinyl bonding of the excited state in the case of the naphthocyclooctatetraene formation. Similarly, the naphthosemibullvalene formation was demonstrated to result primarily from naphtho-vinyl bridging with vinyl-vinyl bridging as a lesser route. In the case of the sensitized reaction, only naphtho-vinyl bonding was encountered. The quantum yield for disappearance of 1.2-naphthobarrelene (8) was 0.49 for both the direct and sensitized reactions. Similar studies on 2,3-naphthobarrelene (11) were carried out. Here direct and sensitized runs led to the same results in terms of products, deuterium distribution, and (within experimental error) the quantum yields. Irradiation of 2,3-naphthobarrelene (11) afforded naphtho[2,3-c]tricyclo[3.3.0.02'8]octa-3,6-diene (2,3-naphthosemibullvalene, 15) by a vinyl-vinyl bonding route and with a quantum efficiency of 0.46. It is concluded that in general the least preferred process for the excited states studied is /3-naphtho-vinyl bridging with vinyl-vinyl bridging intermediate and a-naphtho-vinyl bridging most preferred. The evidence supports a single excited triplet as responsible for the photochemistry in the 2.3-series while the lowest energy triplet is one of two reacting excited states in the 1,2-series. position, r-Description and assignment^B enzo-SBc 2,3-Naphtho-SBc j>ví-1 ,2-Naphtho-SBca«/i-l,2-Naphtho-SBc Multiple!, aromatic 2.67-3.20 2.36-2.93 2.04-2.90 2.18-2.74 ABX quartet,

Synthetic Aspects of the Di-π-methane Rearrangement

“…Just one example 11 is given in the following where benzobarrelene (9) is shown on sensitization to afford benzosemibullvalene (10). However, on direct irradiation, benzocyclooctatetraene (11) is formed by an electrocyclic [2π + 2π] addition of the benzo group to double bond, followed by a retrocycloaddition (Scheme 12).…”

“…In contrast, cyclic dienes tend to be perfectly reactive as triplets, and this can be ascribed to their inability to undergo free rotation in the excited state. One example is that of the barrelene ( 3 ) to semibullvalene ( 4 ) transformation depicted in Scheme and another is that of 2,3-naphthobarrelene ( 7 ) as outlined in Scheme 10 …”

“…When the three bridges are nonequivalent, there is the question of which two π-systems will bond. In the cases of benzobarrelene ( 9 ), 2,3-naphthobarrelene ( 7 ), 2,3-anthracenobarrelene, and related examples, there is a preferential bonding between the two vinyl bridges rather than between the benzo and vinyl bridges. One exception is that of 1,2-naphthobarrelene where α-naphtho−vinyl bridging is preferred .…”

Crystal structure correlations in the photochemistry of dimethyl 9,10-dimethyl-9,10-dihydro-9,10-ethenoanthracene-11,12-dicarboxylate

“…These COTs have been well characterized for barrelene, benzobarrelene and naphthobarrelene derivatives, and support for the proposed intramolecular [2+2] cycloaddition mechanism ( Fig. 1) PHOTOCHEMISTRY OF C22H2004 has been obtained from deuterium-labelling studies (Zimmerman & Grunewald, 1966;Zimmerman, Binkley, Givens & Sherwin, 1967;Zimmerman, Givens & Pagni, 1968;Zimmerman & Bender, 1970). For dibenzobarrelenes, COT photoproducts have been reported in direct irradiations; these COTs were assumed to form via the same mechanisms as in the benzobarrelenes (Fig.…”

Photochemistry of Quinoxalinobarrelenes and (Benzo[g]Quinoxalino)Barrelenes

“…Moreover, quinoxaline reportedly undergoes intersystem crossing with high efficiency of (Ill;", = 0.99).8 The ratios of the products via vinyl-vinyl bridging to those via quinoxalino-vinyl bridging were 71: 29 and 56: 44 for 3 and 4 respectively whereas vinyl-vinyl bridging occurred exclusively for 2,3-naphthobarreIene. 6 The triplet energy of quinoxaline (ET = 60.6 kcal mor 1 )9is near that of naphthalene (ET = 61 kcal mol")." The increase of quinoxalino-vinyl bridging in 3 and 4 is thus mainly attributed to the stabilization effect of nitrogen 12.1112 Th hi h f .…”