Photochemical reactivity of α‐phenyl β,γ‐enones. Singlet 1,3‐acyl shift, triplet aromatic di‐π‐methane (DPM) rearrangement and triplet aryl‐carbonyl bridging

Abstract

Abstract. The photochemistry of the series of a-phenyl P,y-enones 6-10 has been studied under conditions of both direct (A 300 nm) and triplet-sensitized irradiation with the aim of determining the reactivity patterns of these "mu1ti"-chromophoric systems. Upon direct irradiation, the reactants exhibit the typical photoreactions of P,y-enones, viz. the 1,3-acyl shift, affording the corresponding (E)-and (Z)-5-phenyl-4-hexen-3-ones, decarbonylation of the radicals formed by a-cleavage and recombination of the resulting alkyl radicals and, in addition, a new type of reaction from the triplet-excited state yielding small amounts of the corresponding acetophenones. The acetophenones are thought to be formed by initial P-bridging between the carbonyl and the phenyl group, followed by extrusion of the C,H, fragment from the 1,4-or 1,3-0xa-diradical. Upon sensitized irradiation, the o-methoxy-and p-cyano-substituted reactants 9 and 10 exhibit the di-n-methane rearrangement, leading to mixtures of the corresponding cis-and trans-1 -acetyl--1 -methyl-2-phenylcyclopropanes, with quantum yields of 0.03 and 0.10, respectively. The formation of the 1,3-AS and decarbonylation products illustrates the occurrence of a-cleavage, whereas the acetophenones and di-n-methane products are formed via initial aryl-carbonyl and aryl-vinyl bridging, respectively. The inability of the other a-phenyl P,y-enones to undergo photocyclopropanation is discussed in terms of excitation-energy partition. In the o-methoxy-and p-cyano-substituted reactants, the excitation energy may be predominantly concentrated in the aryl moiety in contrast to the other two systems in which the excitation energy may be mainly localized on the P,y-enone moiety. Subsequent triplet-energy dissipation by a free-rotor mechanism would then account for the stability of these systems. This was observed for (E)-7 which, upon triplet sensitization, affords only the (Z) isomer, whereas this process is degenerate for the other substrates studied.

Reações de alquinilação eletrofílica de cetonas promovidas por reagentes de iodo hipervalente

“…Preparação de 3-fenil-2-butanona. C-RMN com os dados da literatura 33. Observou-se no espectro RMN de1 H sinais na região 1,38-1,4 ppm que foram atribuídos aos hidrogênios da metila (C4), em 2,0 ppm que foram atribuídos aos hidrogênios do grupo CH 3 (C1), na região entre 3,7-3,8 ppm correspondente ao hidrogênio na cadeia linear e na região dos aromáticos entre 7,2 e 7,4 ppm (Anexo 15).…”

“…-pentanona (10) foi obtida através da mesma metodologia apresentada no item anterior (3.2.2) 33 , a única diferença no halogenoalcano usado no primeiro caso era iodeto de metila e para este composto utilizou-se iodeto de etila (Esquema 20). O mecanismo proposto para esta reação é análogo ao apresentado no Esquema 19, substituíndo-se apenas o iodeto de metila por iodeto de etila.Para descrever o mecanismo de formação deste substrato seguiu-se o descrito na literatura 33. Uma reação similar ao de substratos anteriores, com a diferença de que o grupo carbonila da cetona é em outra posição.…”