Photochemistry of 1-hydroxy-2-indanones: an alternative route to photoenols

Abstract

Re ce i ved: Ja nu ary 8, 1997 A ir ra di a ção de 1-hidroxi-1,3-difenil-2-indanona leva à for ma ção de dois in ter me diá ri os atri bu í dos aos enóis (E,E e Z,E). O enol Z,E tem vida cur ta e seu pos sí vel modo de de ca i me n to re sul ta na for ma ção de pro du tos, den tre os qua is a or to-benzilbenzofenona é o prin ci pal. O enol E,E tem ma i or es ta bi li da de e apre sen ta um tem po de vida > 100 µs em me ta nol. Seu de ca i men to re sul ta na for ma ção de uma mis tu ra com ple xa de pro du tos, onde 10-fenillantrona e 10-hidroxi-10-fenilantrona são os prin ci pa is. A fo tó li se por pul so de la ser da 1-hidroxi-1,3,3-trifenil-2-indanona mos tra a for ma ção de dois pos sí ve is fo to e nóis Z ( τ = 180 ns) e E ( τ = 30 µs). O modo de de ca i men to des tes enóis é a for ma ção de pro du tos, re sul tan do numa mis tu ra com ple xa.Irra di a ti on of 1-hydroxy-1,3-diphenyl-2-indanone ( 1) le ads to the for ma ti on of two in terme di a tes as sig ned to the enols (E,E and Z,E). The Z,E-enol is short li ved, and its pos si ble de cay mode in vol ves pro duct for ma ti on from which ort ho-benzylbenzophenone is the main pro duct. The lon ger li ved E,E-enol shows a li fe ti me of > 100 µs in met ha nol, and its de cay re sults in the for ma ti on of a com plex mix tu re of pro ducts, with 10-phenylanthrone and 10-hydroxy-10-phenylanthrone be ing the main ones. La ser flash pho toly sis of 1-hydroxy-1,3,3-triphenyl-2-indanone ( 2) shows the for ma ti on of the two pos si ble pho to e nols Z ( τ = 180 ns) and E ( τ = 30 µs). The de cay mode for the se enols is pro duct for ma ti on, re sul ting in a com plex mix tu re of pro ducts.Key words: pho to e nols, 1-hydroxy-2-indanones, la ser flash pho toly sis Intro duc ti onThe pho to che mi cal de com po si ti on of 2-indanones has been used ex ten si vely as a sour ce of ort ho -xylyle nes. This re ac ti on in vol ves the in da no ne tri plet ex ci ted sta te and proce eds through a bi ra di cal for med by α-cle a va ge from this ke to ne. After loss of car bon mo no xi de, a se cond bi ra di cal is for med, which is in fact the xylyle ne tri plet ex ci ted sta te. De cay of this bi ra di cal re sults in ort ho -xylyle ne for ma ti on, which ul ti ma tely gi ves pro ducts through eit her an in tra-or an in ter mo le cu lar path way (see Sche me 1) 1 .Pho to e nols can be re gar ded as hydroxy subs ti tu ted ortho -xylyle nes. They are usu ally for med upon pho toly sis of aro ma tic ke to nes con ta i ning ort ho -alkyl groups. Thus, keto ne ex ci ta ti on le ads to a mix tu re of tri plet ex ci ted sta tes (due to the exis ten ce of dif fe rent ground sta te con for mers for the ke to ne) which then de cay to a ro ta ti o nally equi li brated 1,4-biradical. This bi ra di cal le ads to the for ma ti on of a mix tu re of enols. Each com po nent of this "enol mix tu re" shows very si mi lar spec tros co pic cha rac te ris tics, but qui te dif fe rent ki ne tic be ha vi or 2-4 . Alter na ti vely, pho to e nols can also be for med via the ther mal ope ...