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Organic Electron Donors 1/10/15 Baran Group Meeting Julian Lo Organic Electron Donors 1/10/15 1. Introduction Organic electron donors (OEDs) are neutral, ground state organic molecules that reduce substrates by single electron transfer. Reactions with OEDs thus involve the intermediacy of radicals, which can ultimately end up getting either reduced, converted into nucleophiles, or converted into electrophiles. D or D•+ or 1D•+1 D2+ E+ R– R–E D D•+ X– SET R–X [R–X]•– R• R–H Dr. John A. Muphy Dr. William R. Dolbier, Jr. Dr. Patrice Vanelle SET HAT University of Strathclyde University of Florida Aix-Marseille University R–D+ R–Nuc Several reviews on OEDs as they pertain to organic synthesis have been recently published. D•+ Nuc– D Comprehensive review: Vanelle, Angew. Chem. Int. Ed. 2014, 384 Additional review: Murphy, "Organic Electron Donors." In Encyclopedia of Radicals in Chemistry, Biology, and Materials *Photoinduced electron transfer will be minimally covered, see "Photoinduced Electron Perspective on super electron donors: Murphy, J. Org. Chem. 2014, 3731 Transfer in the Days of Yore," Yan 2014 group meeting for more. Perspective on super electron donors: Murphy, Chem. Commun. 2014, 6073 OEDs are typically electron rich alkenes, although there are some reports of aliphatic amines bearing weak OED properties. Shown below are some of the OEDs discussed along with their corresponding redox potentials (E) and typical parent substrates for a qualitative reference. + Cl Br I N2 MeBr MeI CBr4 –3.5 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0.0 +0.5 1.0 E (V) Me Me N N N N Me N NMe S S N N Me N N 2 2 H N Ph Me Me N N N N Me2N NMe2 S S N N Me Me2N NMe2 Me Me Me Murphy Murphy Murphy TDAE TTF DMBI 1,4-DMP nPr3N E = –1.24 V E = –1.20 V E1 = –0.82 V E1 = –0.78 V E1 = +0.32 V E = +0.33 V E = +0.89 V E = +0.95 V E2 = –0.76 V E2 = –0.61 V E2 = +0.71 V Baran Group Meeting Julian Lo Organic Electron Donors 1/10/15 2. Amines 3. Tetrathiafulvalene (TTF) The reductive cleavage of halides from α-haloketones, esters, and acids using DBMI was initially The first studies on TTF centered on its oxidation (Wudl, Chem. Commun. 1970, 1453). believed to proceed via S 2 displacement with a hydride (Chikashita, J. Org. Chem. 1986, 540). n + + S S Cl2 S S S S Cl2 S S Me O O S S CCl S S S S CCl S +S DMBI N H N,N-dimethylaniline, 4 4 Ph H Ph H benzylamine also facilitate TTF, yellow solid TTF•+, deep purple solid TTF 2+, yellow solid THF, Δ N Ph facilitate similar reactions Br (89%) H TTF facilitiated cyclizations of aryldiazonium salts via a radical-polar crossover mechanism DMBI Me (Murphy, J. Chem. Soc. Perkin Trans. 1 1995, 623). 2 R1 R However, later experiments supported the intermediacy of radicals and led to the proposal of a R1 OH ROH and MeCN could also be SET-initiated pathway (Tanner, J. Org. Chem. 1989, 3842). N + TTF 2 R2 used as nucleophiles, but others Me O acetone (N -, HO-, malonate) were The addition of radical H O 3 N H O 2 unsuitable (Murphy, Chem. initiators/inhibitors H Ph O altered yields ca. ±50% O 1a: R1 = R2 = H 2b: R1 = Me, R2 = H, (73%) Commun. 1997, 1923). N Ph Me Br 1b: R1 = Me, R2 = H 2c: R1 = R2 = Me, (58%) Also: Ph Me 1 2 1c: R = R = Me S S O Me Me Me H O Cl O TTF -N2 2 Ph N N SET •+ H -TTF S S 1 1 Ph Ph Ph R R R2 O DMBI N N Me 3a Me R2 Me Me Me Mech? NaN3, acetone O N O O H Me O -TTF Ph Ph Br N3 N N Ph S – 2 Br Me Ph R1 R Me Me R2 R1 S N +TTF•+ S Br– Me O + S S S S 1,4-DMP was proposed to dehalogenate trichloroacetamides through an analogous pathway O O S S (Ishibashi, Tetrahedron Lett. 2008, 7771). 3a: R1 = R2 = H, (75%) (0.5 equiv) H OAc The presence of heteroatom adjacent to the aryl group was required for termination by OAc 1,4-DMP H Reaction can still Me proceed at 65 °C nucleophilic substitution (Murphy, Chem. Commun. 2000, 627). (neat) Cl N N CCl Cy 3 133 °C N Cl N S S Me DBU effects similar TTF S S (52%) reactivity at rt + O O 1,4-DMP acetone S N + OMe H2O S 2 (48%) (32%) Similarly, solvent quantities of nPr3N could facilitate SET to fragment PhSSPh (Ishibashi, Org. •+ Lett. 2009, 3298). -TTF -MeOH +TTF•+ PhSSPh Me PhSSPh Me -N2 nPr N TsHN nPr N, H O PhS Me Cy 3 3 2 S S TsHN Me S 1 140 °C SPh 140 °C RN + (68%) O (45%) O OMe OMe Baran Group Meeting Julian Lo Organic Electron Donors 1/10/15 This was used in total synthesis of aspidospermine (Murphy, J. Chem. Soc. Perkin Trans. 1 However, the DTDAFs were prone to rapid cleavage if DTDAF•+ trapped an intermediate 1999, 995). radical (Murphy, J. Chem. Soc. Perkin Trans 1 1999, 3637). NHCOCF3 NHCOCF3 NHCOCF3 CO2Me OH O O O CO2Me TTF DTDAF S Me N + N acetone 2 acetone Me CHO N H2O N N H2O (50%) + (45%) H H O N2 Ms Ms Ms Mech? O N N NCOCF3 4. Tetrakis(dimethylamino)ethylene (TDAE) Me TDAE had some illuminating properties (Pruett, J. Am. Chem. Soc. 1950, 3646). Me2N NMe2 N Me N N excess NMe2 H H H F F Me2N NMe2 O H Ms Ms Me2NH O NMe2 -hν aspidospermine O NMe2 Me N NMe F Cl Me2N NMe2 -TDAE 2 2 NMe2 (2 equiv) Radical translocations via a [1,5]-HAT were also demonstrated (Murphy, J. Chem. Soc. Perkin Me N NMe Trans. 1 1995, 1349). "This was a clear, slightly yellow, mobile 2 2 liquid which was strongly luminescent in "A small room can even be dimly lit for over + N2 O Me O Me O Me O Me contact with air." an hour... with about 10 mL of TDAE." TTF H2O N N N N acetone H Can perform similar radical cyclizations to TTF, however, a leaving group typically needs to be H Me H2O H Me H Me H incorporated into the substrate to terminate the reaction since TDAE•+ does not recombine with (85%) radical intermediates (Murphy, Beilstein J. Org. Chem. 2009, 1). Me N + Me Me TTF Me 2 N O N O TTF N O +S N O Br + TDAE Me N TDAE acetone S Me Me H2O Me Me Me S S Me Me N DMF N S O acetone + (74%) O MeOH (53%) (14%) N2 Ms Ms + N2 Bulking up the substitution around the TTF core led to a reduced rate of premature radical -N Br •+ 2 -Br• N trapping with TTF and its derivatives (Murphy, Tetrahedron Lett. 1997, 7635). -TDAE•+ + S O O PhS O PhS O N N H O OED Ms (33%) (60%) + OH + N2 acetone H2O However, TDAE was initially used to dehalogenate polyhalogenated molecules with the more O 4 O 5 O electropositive halogens being easier to remove (Carpenter, J. Org. Chem. 1965, 3082). – Me Me E Me E -Cl S S S S N S Cl CCl2 F3C F3C Cl Cl TDAE TDAE TDAE2+ +H+ S S S S S N BrCCl3 CHCl3 (22%) Me Me E E Cl pentane decane – – Me CF Cl CF Br CCl3 Cl 3 17 h 3 15 min BrCCl3 TTF TMTTF DTDAF (97%) CCl4 (31%) – 4 (19%), 5 (48%) 4 (8%), 5 (67%) 4 (0%), 5 (72%) -BrCCl2 Baran Group Meeting Julian Lo Organic Electron Donors 1/10/15 It has been suggested that TDAE performs two sequential SETs to acceptor substrates (Hetero)aryl difluorochloromethyl ketones could also be added into activated electrophiles to generate anions. such as aryl aldehydes, α-ketoesters, and thiocyanates (Médebielle, Tetrahedron Lett. 2008, Me2N NMe2 589; for more examples, see Dolbier J. Fluorine Chem. 2008, 930). Me2N NMe2 Me2N NMe2 R3C–X SET Ph Ph Me N X NMe F Me N NMe -20 °C 2 2 ~0 °C Me N NMe Me Me F 2 2 2 2 N O O F O CR3 Me N TDAE TDAE•+ PhCHO 2 N N -Me N– N charge transfer complex – TDAE 2 CR3 + X CF2Cl O O Me2N NMe2 DMF O– -HF OH – – -20 °C to rt + CR3 + X SET Me2N NMe2 O CF2Cl O Ph O Ph TDAE 2+ F F (60%) F F – TDAE was used to generate HetCF2 , which could add into aldehydes, ketones (Médebielle, Reductive cleavage of electron-deficient benzyl chlorides leads to adducts with α-halocarbonyl J. Org. Chem. 1998, 5385), pyruvates, and thiocyanates (Médebielle, Synlett 2002, 1541 compounds and other electrophiles (Vanelle, Tetrahedron 2009, 6128). and Tetrahedron Lett. 2001, 3463). NO OEt CN Br O O 2 O S O NO2 TDAE Ph Ph O + N N N O DMF O N TDAE N O Cl + Me O -20 °C to 70 °C O N N CHO DMF N (68%) O CF2Br -20 °C to rt O OH (ca. 55% for both) (60%) F F NMe2 Irraditation could increase yields in certain cases, but it altered the reaction outcome in other Radical intermediates could be intercepted using dihydrofuran as a radical trap. ones (Vanelle, Eur.
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