Triplet- vs. singlet-state imposed photochemistry. The role of substituent effects on the photo-Fries and photodissociation reaction of triphenylmethyl silanes

Abstract

The photochemistry of three structurally very similar triphenylmethylsilanes 1, 2, 3 [p-X-C(6)H(4)-CPh(2)-SiMe(3): X = PhCO, 1; H, ; Ph(OCH(2)CH(2)O)C, 3] is described by means of 248 and 308 nm nanosecond laser flash photolysis (ns-LFP), femtosecond LFP, EPR spectroscopy, emission spectroscopy (fluorescence, phosphorescence), ns-pulse radiolysis (ns-PR), photoproduct analysis studies in MeCN, and X-ray crystallographic analysis of the two key-compounds 1 and 2. The photochemical behavior of 1, 2 and 3 is discussed and compared with that of a fourth one, 4, bearing on the p-position an amino group (X = Me(2)N) and whose detailed photochemistry we reported earlier (J. Org. Chem., 2000, 65, 4274-4280). Silane 1 undergoes on irradiation with 248 and 308 nm laser light a fast photodissociation of the C-Si bond giving the p-(benzoyl)triphenylmethyl radical (1*) with a rate constant of k(diss)= 3 x 10(7) s(-1). The formation of 1* is a one-quantum process and takes place via the carbonyl triplet excited state with high quantum yield (Phi(rad)= 0.9); the intervention of the triplet state is clearly demonstrated through the phosphorescence spectrum and quenching experiments with ferrocene (k(q)= 9.3 x 10(9) M(-1) s(-1)), Et(3)N (1.1 x 10(9) M(-1) s(-1)), and styrene (3.1 x 10(9) M(-1) s(-1)) giving quenching rate constants very similar to those of benzophenone. For comparative reasons radical 1* was generated independently from p-(benzoyl)triphenylmethyl bromide via pulse radiolysis in THF and its absorption coefficient at lambda(max)= 340 nm was determined ([epsilon]= 27770 M(-1) cm(-1)). We found thus that the p-PhCO-derivative 1 behaves similar to the p-Me(2)N one (the latter giving the p-(dimethylamino)triphenylmethyl radical with Phi(rad)= 0.9), irrespective of their completely different ground state electronic properties. In contrast, compounds 2, 3 that bear only the aromatic chromophore give by laser or lamp irradiation both, (i) radical products [Ph(3)C* and p-Ph(OCH(2)CH(2)O)C-C(6)H(4)-C(*)Ph(2), respectively] after dissociation of the central C-Si bond (Phi(rad)= 0.16), and (ii) persistent photo-Fries rearrangement products (of the type of 5-methylidene-6-trimethylsilyl-1,3-cyclohexadiene) absorbing at 300-450 nm and arising from a 1,3-shift of the SiMe(3) group from the benzylic to the ortho-position of the aromatic ring (Phi approximately 0.85 for 2). Using fs-LFP on 2 we showed that the S(1) state recorded at 100 fs after the pulse decays on a time scale of 500 fs giving Ph(3)C* through C-Si bond dissociation. In a second step and within the next 10 ps trityl radicals either escape from the solvent cage (the quantum yield of Ph(3)C* formation Phi(rad)= 0.16 was measured with ns-LFP), or undergo in-cage recombination to photo-Fries products. Thus, singlet excited states (S(1)) of the aromatic organosilanes (2, 3) prefer photo-Fries rearrangement products, while triplet excited states (1, 4) favor free radicals. Both reactions proceed via a common primary photodissociation step (C-Si bond homolysis) and diffe...

On the Route to the Photogeneration of Heteroaryl Cations. The Case of Halothiophenes

“…On the contrary, iodothiophenes 1d , e underwent a photohomolytic C–X bond cleavage (paths b, c), a thermodynamically viable process (Figure ). Thus, a triplet radical pair [ 14 • X • ] 3 was generated , rather than cation 3 14 + . Diffusional separation of the radical center formed a solvated radical pair [ 14 • X • ] solv (path d) .…”

Evidence for the Specific Species of Benzyltriethoxysilane Derivatives with a High-Coordination Silicon Atom

“…They clarified that the radical pair produced by the C−Si bond homolysis plays a crucial role to give the products. 14 According to their consideration, a remarkable solvent effect would not be observed for the photolysis of trimethylsilyltriphenylmethane. We preliminarily studied the transient absorption spectra and two-step laser induced fluorescence spectra of trimethylsilyltriphenylmethane in c-Hex and in MeOH.…”

“…34−37 Thus, our results support the primary photochemical processes proposed by Zarkadis et al for trimethylsilyltriphenylmethane. 14 These primary photochemical processes of trimethylsilyltriphenylmethane are similar to those of phenylacetate, where the radical pair produced by the homolysis of the C−O bond plays an essential role for the formation of 1,3-and 1,5-acyl rearranged intermediates. 38 Zarkadis et al pointed out that there is quite a large difference in the primary photochemical process between trimethylsilyltriphenylmethane and trimethylsilyldiphenylmethane.…”

“…Zarkadis et al studied the photochemistry of trimethylsilyltriphenylmethane derivatives in acetonitrile by means of nano-and femto-second flash photolysis, electronparamagnetic resonance spectroscopy, emission spectroscopy, and product analysis studies. 14 They clarified that their photochemistry proceeds via a common photodissociation step (C−Si bond homolysis), and the multiplicity of the resulting geminate radical pair plays a crucial role in determining the reaction: singlet species prefer the 1,3photorearrangement products (photo-Fries products), and the triplet species favor free radicals. They also pointed out that the substituents, benzophenone and anilino groups, accelerate the intersystem crossing transition.…”

Μετάθεση photo-Fries διχρωμοφορικών βεζυλοπυριτικών παραγώγων και φλουορενυλοπυριτικών συστημάτων

“…120,16,124,42,126,53,127,30,139,79,145,32. 86,120,21,124,43,126,63,127,13,127,16,127,37,128,23,139,60,139,95,145,54. χρησιμοποιήθηκε φασματοσκοπία 1 H-NMR.…”