Dual diradical and cation diradical cyclization mechanisms for silylalkenyl iminium salt. Electron-transfer induced photocyclization reactions

Abstract

Previously, we have described the electron-transfer induced, photoaddition reactions of allylsilane iminium salt systems' exemplified by the additions of the allylsilanes 1 and 2 to 2phenyl-1-pyrrolinium perchlorate (3) to produce the 2-prenylpyrrolidine 4. The regioselectivities of these processes led us to propose mechanisms in which desilylation of intermediate cation radical pairs is followed by radical coupling (Scheme I). Another route in which coupling occurs in the radical cation pair prior to desilylation was ruled out on the basis of the regioselectivity observed in the 1 + 3 reaction.These observations led to the design of several synthetically useful2 photocyclization reactions of silylallyl iminium salt systems (e.g., I in Scheme 11). By analogy to the intermolecular systems, we suggested that the mechanisms for these intramolecular processes involve cyclization of diradical intermediates IV generated by desilylation of diradical cations I1 (path b, Scheme 11). However, the possibility exists for facile carbon-carbon bond formation in diradical cations I1 (path a). Thus, two mechanisms differing in the timing of bond making and breaking appear possible for allylsilane iminium salt photocyclizations. In order to address this issue, a series of regiospecifically deuteriated iminium salts with the isotope labeling pattern shown in I (Scheme 11) were prepared. We anticipated that differentiation between the two pathways could be made based upon the isotope distribution in the methylenepyrrolidines V produced. Cyclization by path a should form the Va isotopomer exclusively while bath b would produce a near3 equal mixture of isotopomers Va and Vb. The results summarized below suggest that both mechanisms are operable in the electron-transfer induced photoprocesses and that the contribution from each is dependent upon the iminium salt structure, substituents and excited-state multiplicity, and the reaction medium.The N-(silyla1lyl)-1-pyrrolinium perchlorates (5-7, R = H) were prepared by AgC104 (MeCN) assisted allylations of the corresponding 1 -pyrrolines4 with trimethylsilylmethallyl iodide 15.536 Photocyclizations of these substances, induced by direct irradiation in MeCN or MeOH, generated the corresponding pyrrolizidines 8-10 in 46-65% yields.' Photocyclizations of 5 and 6 can be triplet sensitized. Irradiation of xanthone solutions (MeCN) of these salts led to production of 8 and 9 (ca. 47%), respectively.* Also, E,Z mixtures of the cyclohexenylidene iminium salts 11 and 12 (R = H), as shown previously,2a produced the spirocyclic amines 13 and 14 upon direct irradiation (MeCN).*

Evidence for Triplet Sensitization in the Visible‐Light‐Induced [2+2] Photocycloaddition of Eniminium Ions

“…Thef ollowing observations (see the Supporting Information) provide evidence for the fact that the rutheniumcatalyzed reaction proceeds via triplet energy transfer and not via electron transfer:a )The ruthenium complex Ru(bpz) 3 -(PF 6 ) 2 (bpz = 2,2'-bipyrazine), which is known to be aw eak reductant in its excited state [E 1/2 (Ru III /Ru II *) [19] = À0.26 V], promoted the [2+ +2] photocycloaddition as efficiently as Ru(bpy) 3 (PF 6 ) 2 .b )Eosin Y( EY), with at riplet energy [20] of E T = 182 kJ mol À1 and an excited-state reduction potential of E 1/2 (EYC + + /EY*) [20] = À1.11 V, did not catalyze the reaction at l = 512 nm. c) Ther eaction was catalyzed by typical triplet sensitizers such as benzil and thioxanthone, [21] albeit upon irradiation at short wavelengths.d)The regioselectivity of the [2+ +2] photocycloaddition can only be explained by at riplet pathway that proceeds via a1 ,4-diradical. Addition of an intermediate radical 11C to 2,3-dimethylbutadiene would lead to the opposite regioisomer.…”

Electron‐transfer Organic and Bioorganic Photochemistry

“…Intramolecular alkylation reactions of l-allyor benzyl-2-phenylpyrrolidines give cyclieation products, i.e. bicyclic pyrrolidines, benzoindolizidines, and benzopyrrolidines (Tu et al, 1987;Lan et al, 1987). The photocyclization of l-benzylpyrroliniutn cation proceeds by way of intramolecular photoelectron transfer from the benzyl group to the pyrrolinium chromophore followed by diradical and diradical-cation couplings (Cho and Mariano, 1988).…”

Triplet Energy Transfer from Ruthenium Complexes to Chiral Eniminium Ions: Enantioselective Synthesis of Cyclobutanecarbaldehydes by [2+2] Photocycloaddition

“…Prior to the Melchiorre studies, the group of Mariano had performed extensive work on the photochemistry of iminium and eniminium ions . Despite the impressive number of important results that they disclosed, the reactivity of the triplet state T 1 of eniminium ions has remained unexplored . Indeed, there was no indication that intersystem crossing (ISC) from the S 1 state would be feasible, and the [2+2] photocycloaddition chemistry of eniminium ions was found to proceed at the singlet hypersurface .…”