Photoaddition reactions of N-benzylglycinates containing α-trimethylsilyl group with dimethyl acetylenedicarboxylate: competitive formation of pyrroles vs. β-enamino esters

Abstract

A study was conducted to gain insight into the preparative potential of photosensitized reactions of acyclic N-benzylglycinates containing an a-trimethylsilyl group with dimethyl acetylenedicarboxylate (DMAD). The photosensitizers employed in the reactions include 9,10-dicyanoanthracene (DCA), 1,4dicyanonaphthalene (DCN), rose bengal (RB) and fullerene C 60 . The results show that photoirradiation of oxygenated solutions containing the photosensitizers, glycinates and dimethyl acetylenedicarboxylate leads to competitive formation of pyrroles and b-enamino-esters. The distributions of pyrrole and benamino-ester products formed in these reactions are highly influenced by the electronic nature of the phenyl ring substituent on the benzylglycinates and the photosensitizer used. These photoaddition reactions take place via mechanistic pathways involving competitive formation of azomethine ylides and secondary amines, generated by a mechanistic routes involving initial SET from the benzylglycinates to photosensitizers.Scheme 1 Photoinduced single electron transfer (SET) reaction pathways opened for amine substrates. Scheme 2 SET-promoted 1,3-dipolar cycloaddition reactions of ethyl 2-(3,4-dihydroisoquinolin-2(1H)-yl)acetate 6 with dipolarophile. Scheme 3 SET-promoted 1,3-dipolar cycloaddition reactions of tertiary N-benzylglycinates 10 (E ¼ H, SiMe 3 ) with fullerene C 60 . 5640 | RSC Adv., 2019, 9, 5639-5648 This journal is a 220 mL of MeCN solution containing glycinate (3.2 mM), acetylene (3.2 mM) and RB (0.32 mM). b Isolated yields. 5642 | RSC Adv., 2019, 9, 5639-5648 This journal is a 220 mL of MeCN or toluene solutions containing N-benzylamines (3.2 mM), acetylene (3.2 mM) and photosensitizer (DCA (0.27 mM), DCN (0.32 mM), RB (0.32 mM) and C 60 (0.16 mM)). b Isolated yields. This journal isScheme 7 Non-regioselective formation of iminium ions 39-41 from non-silyl amine analog 21.This journal is

Photosensitizer-Promoted Photoaddition Reactions of α-Silyl Group Containing N-Alkyl Glycinates to Dimethyl Acetylenedicarboxylate (DMAD)

“…Further studies are underway to accumulate the information about the origin of chemoselectivity. Based on the analysis of photoadducts generated from above photosensitizer-promoted photoaddition reactions occurring between N--trimethylsilyl-N-alkyl glycinates and DMAD as well as the observation made from earlier studies, [21][22][23] feasible mechanistic pathways involved in both pyrroles and enamino-esters formation can be suggested, where in situ generated azomethine ylides and secondary amines are key intermediates. Specifically, as described in Scheme 6, a photochemical process is initiated by a SET process from glycinates 15 to excited state of photosensitizer ( S1 Sens and/or T1 Sens).…”

“…Specifically, as described in Scheme 6, a photochemical process is initiated by a SET process from glycinates 15 to excited state of photosensitizer ( S1 Sens and/or T1 Sens). Taking into redox potentials of glycinates (i.e., oxidation potentials of glycinates, Eox) and photosensitizer (i.e., excited state reduction potentials of photosensitizers, * Ered), 21 the SET process from glycinates to photosensitizers (e.g., DCA, C60 and RB) should be thermodynamically favorable. 25 Since this favorable SET event takes place, as a result, aminium radicals of glycinates 16 and radical anions of photosensitizers (Sens •‾ ) are formed, in which Sens •‾ are subsequently interacted with molecular oxygen (O2), resulting in both ground state sensitizers and radical anions of oxygen (O2 •‾ ).…”

“…The generated iminium ions 17 seems to confront two competitive pathways, in which one of them is HOO ‾ -assisted deprotonation (~ H + ) from -carbon position adjacent to carboxy group to result in azomethine ylide intermediates 18 and the other is HOO ‾ -assisted hydrolysis to lead to the formation of non-silyl containing secondary amines 19. 16,17,21 Noticeably, the extent of competition taking place between these two processes seems to be dependent on structural nature of glycinates and photosensitizers.…”

“…10,13,18,19 In recent study, we found that tertiary amines possessing both -silyl and carboxy group at nitrogen atom could serve as efficient azomethine ylide precusors depending on reaction conditions. 12,20,21 For instance, in the photochemical reactions of N--silyl-Nbenzylglycinates 1 with electron deficient fullerene C60, while irradiation of N2-purged (deoxygenated) solutions containing glycinates 1 and C60 led to production of 1-aminomethyl-1,2-dihydrofullerene 3a via addition of -amino radicals 2a to C60, irradiation of O2-purged solutions containing the same reactants gave rise to the formation of fulleropyrrolidines 3b through 1,3-dipolar cycloaddition reactions of in situ formed azomethine ylides 2b to C60. (Scheme 1a) 20 In addition, in the photosensitizer-mediated cycloaddition reactions of glycinates 1 with dimethyl acetylene dicarboxylate (DMAD), (Scheme 1b) 21 the results showed that two competitive pathways were involved in this reactions, one of which involves azomethine ylide (4a)-forming process (leading to cycloaddition to DMAD to form pyrrole adducts 5a) and the other involves desilylmethylated secondary amine (4b) formation (leading to amination of DMAD to form enaminoesters 5b).…”

Fullerene C60 promoted photochemical hydroamination reactions of an electron deficient alkyne with trimethylsilyl group containing tertiary N-alkylbenzylamines

“…In the studies exploring application of amine SET photochemistry for the synthesis of nitrogen-containing compounds, interesting observation was made in our recent studies aiming at exploring photochemical behaviors of both a-trimethylsilyl and a-alkoxycarbonyl group containing N-benzylamines (i.e., N-a-trimethylsilyl-N-benzylglycinates). 10 The results showed that photoirradiation of N-a-trimethylsilyl-Nbenzylglycinates in the presence of molecular oxygen (O 2 ) and photosensitizer (PS) could convert starting tertiary amines to non-silyl containing secondary amines via hydrolytic cleavage of in situ formed iminium ions. This investigation suggested that a-trimethylsilyl group substituted tertiary amines can participate in photochemical hydroamination reactions with alkynes to produce enamine derivatives predominantly.…”