A simple regiospecific route to otherwise problematic substituted tetracenes is described. The diverse cores (E)-1,2-Ar CH (HOCH )C=C(CH OH)I (Ar =Ph, 4-MePh, 4-MeOPh, 4-FPh) and (E)-1,2-I(HOCH )C=C(CH OH)I, accessed from ultra-low cost HOCH C≡CCH OH at multi-gram scales, allow the synthesis of diol libraries (E)-1,2-Ar CH (HOCH )C=C(CH OH)CH Ar (Ar =Ph, 4-MePh, 4-iPrPh, 4-MeOPh, 4-FPh, 4-BrPh, 4-biphenyl, 4-styryl; 14 examples) by efficient Negishi coupling. Copper-catalysed aerobic oxidation cleanly provides dialdehydes (E)-1,2-Ar CH (CHO)C=C(CHO)CH Ar , which in many cases undergo titanium(IV) chloride-induced double Bradsher closure, providing a convenient method for the synthesis of regiochemically and analytically pure tetracenes (12 examples). The sequence is typically chromatography-free, scalable, efficient and technically simple to carry out.
This communication details the asymmetric metallophosphite-catalyzed 1,3-silylacylation of nitrones (eq 1). This reaction provides access to enantiomerically enriched N-aryl α-amino ketones and, to the best of our knowledge, constitutes the first example of direct C-acylation of nitrones.(1).α-Amino ketones are useful building blocks in organic chemistry. 1 The addition of stoichiometric acyl anion equivalents to imines represents a useful synthetic method that introduces this versatile functional group. 2-5 In a seminal advance, Murry and Frantz described thiazolium carbene-catalyzed aza-benzoin additions between aldehydes and acyl imines generated in situ from tosylamides. 6 In 2005, Miller and co-workers developed an asymmetric variant using a thiazolylalanine-derived catalyst and electron-deficient aryl aldehydes as the acyl donors. 7 Good enantioselectivities were obtained (~75-85%), which increased to 98% for many products upon recrystallization, albeit at the expense of yield. The enantiomeric excess of the products was found to be dependent upon reaction time as racemization occurred under the basic reaction conditions (excess R 3 N).Our laboratory has recently developed metallophosphites 8,9 as a new family of umpolung catalysts for the enantioselective C-acylation of aldehydes and alkenes. 10-12 We were interested in testing the notion that these catalysts could be employed for the asymmetric Cacylation of C=N π bonds as well. We hypothesized that strong nucleophilicity and low basicity of metallophosphites could allow us to develop an asymmetric acylation that would include electron-neutral and electron-rich substrates.A potential complication with the use of imines in the projected application was the appreciable endothermicity of the requisite turnover-enabling [1,4]-O→N silyl transfer (Scheme 1, 4 → 5). 13 We hypothesized that nitrones 14 could be superior azomethine electrophiles due to the reestablishment of thermoneutrality via a [1,5]-O→O silyl transfer (6 → 7). Indeed, in the examination of a range of imine and imine-derived electrophiles as coupling partners with acyl silanes, productive coupling was only observed with nitrone electrophiles (Figure 1). An Naryl moiety was optimal and the N-o-methoxyphenyl (N-OMP) derivative 19 was initially selected as a substrate for optimization.
Thermoelectric properties (conductivity and Seebeck coefficient) are in screening for new tetrathiotetracene lead materials. Iodine doping reveals iPr,Me-TTT as optimal.
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