Photothermally Induced Bergman Cyclization of Metalloenediynes via Near-Infrared Ligand-to-Metal Charge-Transfer Excitation

Abstract

Reaction of 1,2-bis(tert-butyldimethylsilyloxy)-4,5-diiodobenzene with 2 equiv of phenylacetylene followed by deprotection with KF/HBr yields the catechol-enediyne ligand 4,5-bis(phenylethynyl)benzene-1,2-diol (CatED, 1). Metathesis of VO(SALIMH)ACAC.CH(3)OH (2) with 1 and subsequent air oxidation yields (4,5-bis(phenylethynyl)-1,2-dihydroxyphenyl)[4-(2-(salicylideneamino)ethyl)imidazolyl]oxovanadium(V).CH(3)OH [VO(SALIMH)CatED], (3), in 85%. The thermal Bergman cyclization temperature for 3 is very high (246 degrees C), which is expected for a rigid, benzannulated enediyne motif. The electronic spectrum of 3 exhibits two strong ligand-to-metal charge transfer (LMCT) transitions centered at 584 nm (epsilon = 6063 M(-)(1) cm(-)(1)) and 1028 nm (epsilon = 8098 M(-)(1) cm(-)(1)). These transitions derive from CatED-to-V(V) ligand-to-metal charge transfer, the assignment of which is verified by resonance enhancement of several CatED vibrational modes in the Raman spectra obtained with lambda = 785 vs lambda = 457.9 nm under low power and/or temperature conditions. At elevated temperatures (113-323 K) and powers (2-5 mW), excitation of 3 in the solid state with lambda = 785 nm leads to generation of a black, sparingly soluble, fluorescent product that exhibits weak vibrational features in the 580-600, 1200-1350, and 1450-1600 cm(-)(1) regions, indicative of V-O (CatED) and aromatic ring units. The C=C ring modes correspond well with the vibrational characteristics of poly(p-phenylene) and derivatives thereof. Additionally, materials generated in both the solid-state thermal and photothermal reactions of 3 demonstrate the formation of high molecular weight species ranging from 5000 to 274 000. On the basis of these data and the literature precedent for formation of poly(p-phenylene) via thermolysis of simple enediynes, the reaction poses a unique approach for photoinitiating Bergman cyclization with long-wavelength excitation, as well as the generation of polymeric products.

Synthesis and reactivity of anthracenyl-substituted arenediynes

“…It has been shown that the cyclization reaction in enediynes is associated with an exothermic peak in DSC. 6,[22][23][24][25] Thermogravimetric analyses (TGA) (Supporting information Fig. S1) and DSC (Fig.…”

Unique Metal‐Diyne, ‐Enyne, and ‐Enediyne Complexes: Part of the Remarkably Diverse World of Metal‐Alkyne Chemistry

“…34). Bergman cyclization (576) of simple metalloenediynes have been reported (577,578). The first of these involves direct copper-pyridine charge-transfer photoexcitation (cf.…”

“…In a slightly different approach to photochemical Bergman cyclization by CT excitation, the development of a long-wavelength, enediyne LMCT transition was used to thermally heat the metalloenediyne complex by nonradiative decay. The choice of V(V) coupled with a catechol-enediyne motif (Scheme 97) generates a strong LMCT transition (e $8000 M À1 cm À1 ) at 1032 nm that is used to photochemically (l ¼ 785 or 1064 nm) generate sufficient thermal energy to create Bergman and polymerized product (578). The photoreaction is monitored by Raman spectroscopy that shows luminescence from the photoproducts that builds during the timecourse of the photolysis.…”

Bergman Cyclization in Polymer Chemistry and Material Science

“…Besides thermal Bergman cyclization, Kraft et al elaborated a distinctive photo-initiated route. [11] Excitation of metalloenediynes in solid state with l ¼ 785 nm diode laser led to metallo-poly (p-phenylene) as a black material insoluble in many common solvents. The polymeric materials possessed M w ¼ 5 600 and M n ¼ 4 170 corresponding with an average of 9-10 monomeric units.…”