1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Electro, Physical & Theoretical Chemistry Aryl-Aryl Coupling of Salicylic Aldehydes through Oxidative CH-activation in Nickel Salen Derivatives Andrei N. Yankin, Daniil A. Lukyanov,* Evgenii V. Beletskii, Olga Yu. Bakulina, Petr S. Vlasov, and Oleg V. Levin [a] The preparative electrosynthetic procedure for gram-scale preparation of symmetric 4,4’-dihydroxy-3,3’-diformylbiphenyls from salicylic aldehydes via the intermediate formation of ethylene-bridged bis((2-hydroxybenzylidene)imino)nickel (II) complexes is reported for the first time. This procedure represents a practical route to the variety of rare bisphenol derivatives. The electrochemical aryl-aryl coupling via an oxidative CH – activation in NiSalens is exploited for facile and selective C  C bond formation. Introduction The electrochemical synthesis offers green and cheap alter- native to the conventional synthetic procedures as it allows to avoid application of chemical oxidizing or reducing agents, often hazardous or expensive. [1] In particular, oxidative electro- chemical aryl-aryl coupling offers facile, inexpensive and energy effective route to biaryl compounds, commonly obtained via cross-coupling of aryl halides with arylboron or arylstannane compounds. [2] Among biaryl compounds, 4,4’-dihydroxy-3,3’-diformylbi- phenyls find application in material chemistry [3,4] and synthesis of transition metal catalysts. [5,6] The synthetic routes to these compounds are either expensive due to the palladium- catalyzed coupling involved, [7–9] or provide low yields, like Reimer-Tiemann formylation, [10] Duff formylation [5] and magne- sium-catalyzed ortho-formylation approaches. [11] The formation of 4,4’-dihydroxy-5,5’-dimethoxy-3,3’-diformylbiphenyl was also reported in the mechanistic study of electrochemical oxidation of imines of ortho-vanillin [12] but the preparative utilization of this protocol was unexplored. Electrochemical oxidation of nickel complexes of bis-imines of salicylic aldehyde derivatives, referred to as NiSalens, leads to electroactive polymers which are admired for their energy storage performance, [13,14] electrochromic [15] and electrocatalytic [16] properties. Except some specific cases, [17] polymerization of NiSalens occurs through the formation of C  C bonds in the para-positions to the hydroxyl group of the phenyl rings of the ligand, [18] which provides a new route to biphenyl compounds. However, the known investigations of the NiSalens electro- polymerization are focused on the formation of stable thin films of the polymers on the conductive substrates and involve elaborated electrochemical equipment. As a result, polymers are produced in microgram scale and thus are hardly applicable for further processing. To the best of our knowledge, there were no examples for the preparative-scale electropolymeriza- tion of NiSalens, as well as the implementation of this process as a tool for electroorganic synthesis. Herein we report a new protocol for the synthesis of a series of 4,4’-dihydroxy-3,3’-diformylbiphenyls from substituted salicylic aldehydes via the electrooxidative coupling of bis (salicylidenimino)nickel(II) complexes as a key step. Protocol described is simple and may be reproduced in a large scale with standard synthetic equipment. Results and Discussion NiSalen complexes are known to form oligomers upon anodic oxidation with the formation of C  C bond between the positions para- to the hydroxy group (Scheme 1). [19] The [a] Dr. A. N. Yankin, D. A. Lukyanov, E. V. Beletskii, Dr. O. Y. Bakulina, P. S. Vlasov, Prof. Dr. O. V. Levin Institute of Chemistry, Saint Petersburg State University, 17/9 Universi- tetskaya nab., St. Petersburg 199034, Russia E-mail: lda93@yandex.ru Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201902385 Scheme1. Electrochemical oxidative polymerization of salicylic aldehydes 1. Reagents and conditions: a) 1,2-diaminoethane, EtOH, reflux; b) Ni (OAc) 2 *2H 2 O, reflux; c) electrochemical oxidation, LiClO 4 in MeCN; d) H 2 SO 4 (20%). Full Papers DOI: 10.1002/slct.201902385 8886 ChemistrySelect 2019, 4, 8886–8890 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim