Chemistry II (Organic)
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1 Chemistry II (rganic) eteroaromatic Chemistry LECTURE 8 (Iso)quinolines & diazines: properties, syntheses & reactivity Alan C. Spivey a.c.spivey@imperial.ac.uk Mar 0
2 Format & scope of lecture 8 pyrrole furan S thiophene Quinolines & isoquinolines: structure & properties syntheses reactivity Diazines: pyrimidines, pyrazines & pyridazines: structure & properties syntheses reactivity indole pyridine quinoline 3 imidazole diazoles pyrazole 4 isoquinoline pyrimidine pyrazine diazines pyridazine
3 Quinolines & Isoquinolines Importance atural products: quinine (antimalarial) Pharmaceuticals: 9 9 QUILIE chloroquine (anitmalarial) Et QUILIE quinidine QUILIE Ph cyanine dye (control of pinworm infection) papaverine (opium alkaloid vasodilator) QUILIIUM SALT F ISQUILIE ciprofloxacin (antibiotic) C morphine (analgesic) QUILE tetrahydroisquilies C Et quinapril (ACE inhibitor for high blood pressure) lycorine (anti-tumour) tetrahydroisquilie Chiral catalysts: Sharpless Angew. Chem. Int. Ed. 00, 4, 04 (DI): (DQD) PAL (in AD-mix for Sharpless AD) QUILIES
4 Quinolines & Isoquinolines Structure and Properties Quinoline: colourless liquid, bp 37 C; isoquinoline: colourless plates, mp 6 C Bond lengths and MR chemical shifts as expected for aromatic systems: bond lengths: MR: 8.00 ppm 7.50 ppm.39 Å.38 Å.45 Å.35 Å cf. ave C-C.48 Å 7.6 ppm 8.45 ppm.44 Å.39 Å ave C=C.34 Å 8.8 ppm.33 Å.34 Å ave C-.45 Å 9.5 ppm quinoline isoquinoline quinoline isoquinoline Resonance energies: quinoline = kjmol - (cf. 5 kjmol - naphthalene) Electron density: for both systems the pyridinyl ring is electron deficient (cf. ~pyridine); the benzenoid ring is slightly electron deficient relative to benzene itself: electron neutral (cf. benzene) quinoline electron deficient (cf. pyridine) electron neutral (cf. benzene) isoquinoline electron deficient (cf. pyridine) both quinoline and isoquinoline are: reactive towards electrophiic substitution (S E Ar) in the benzenoid ring reactive towards nucleophilic subnstitution (S Ar) in the pyridinyl ring Basic: both systems have pk a s similar to pyridine (5.): quinoline: pk a = 4.9 isoquinoline: pk a = 5. isoquinoline (pk a 5.)
5 Quinolines & Isoquinolines Syntheses Quinolines: Doebner-von Miller: enone with aniline then in situ oxidation: via apparent,4-addition of aniline group to enone then cyclodehydration then dehydrogenation (oxidation) by the imine formed between the enone and aniline in a side reaction (Tetrahydro)isoquinolines: Pictet-Spengler: -phenethylamine with aldehyde (intramolecular Mannich) (Dihydro)isoquinolines: Bischler-apieralski: -phenethylamine with acid chloride R R P R P P R R R
6 Quinolines & Isoquinolines Reactivity Electrophilic substitution: via addition-elimination (S E Ar) in the benzenoid ring (i.e. more electron rich ring) reactivity: reactive towards many electrophiles (E + ); <benzene but >pyridine >> ~ > > relative rates ~0-6 ~0-5 ~0 - regioselectivity: substitution at C5 (& C8 for quinolines) predominate via most stable Wheland intermediates: quinoline E isoquinoline E e.g. nitration: (E + = + ) E E 5 quinoline isoquinoline c. 3 c. S 4 5 C 5 [43%] [80%] [47%] [0%]
7 Quinolines & Isoquinolines Reactivity cont. ucleophilic substitution: via addition-elimination (S Ar) reactivity: reactive towards nucleophilies (u - ) provided leaving group is situated at appropriate carbon regioselectivity: reactive at positions for which the isenheimer type intermediates have negative charge stabilised on the electronegative nitrogen [ leaving group (LG) can be but, Br, etc. more facile]: quinoline: C4 > C i.e. as for pyridine isoquinoline: C > C3 quinolines LG 4 > u u LG isoquinolines u LG > 3 LG u e.g. the Chichibabin reaction: (u - = -, LG = ) quinolines ) K, liq. 3 ) + isoquinolines
8 Quinolines & Isoquinolines Reactivity cont. tallation: deprotonation by strong bases ortho to the is difficult due to competing addition reactions but can be achieved using e.g. highly basic and non-nucleophilic zincates: Li Zn t Bu t Bu Et, rt Li I I [93%] tallation at benzylic positions: deprotonation at benzylic positions that give enaminate anions (i.e. C4 > C for quinoline; C > C3 for isoquinoline) are facile (i.e. as for pyridine): ) aet, Et ) (C Et) Et ) aet, Et ) PhC Ph
9 Diazines: Pyrimidines, Pyridazines & Pyrazines Importance atural products: Pharmaceuticals: PYRIMIDIE PYRIMIDIE PYRAZIE C Et S orotic acid thiamine - vitamin B aspergillic acid (biosynthetic intermediate (essential vitamin) (fungal antibiotic) for natural pyrimidines) F AZT (anti-aids) PYRIMIDIE trimethoprim (antibacterial) PYRIMIDIE minoxidil (anihypertensive) PYRIMIDIE--oxide Agrochemicals: PYRIMIDIE C S Bu t PYRIDIZIE t Bu PYRIDIZIE SEt P PYRAZIE bensulfuronmethyl (herbicide) pyridaben (herbicide) maleic hydrazide (plant growth inhibitor) thionazin (soil insecticide)
10 Diazines Bonding, Structure & Properties Diazines can be considered as related to pyridine but containing an additional in place of one C group: pyrimidine 3 pyrazine 4 pyridazine in all cases the new is pyridine-like, i.e. this contributes just electron to the aromatic p-system and has a basic lone pair in the sp orbital in the plane of the ring: pyrimidine pyrazine pyridazine All three diazines are significantly less basic than pyridine: pyrimidine (pk a.3) pyrazine (pk a 0.4) pyridazine (pk a.3)
11 Diazines Structure and Properties Pyrimidine: colourless prisms, mp C Pyridazine: colourless liquid, bp 08 C Pyrazine: colourless prisms, mp 57 C Bond lengths and MR chemical shifts as expected for aromatic systems: bond lengths:.40 Å.35 Å.34 Å pyrimidine.37 Å.39 Å.39 Å.34 Å.34 Å.33 Å pyrazine pyridazine cf. ave C-C.48 Å ave C=C.34 Å ave C-.45 Å MR: 7.4 ppm 8.8 ppm 9.3 ppm pyrimidine 7.5 ppm 9. ppm 8.6 ppm pyrazine pyridazine Resonance energies: all three systems have lower resonance energies than pyridine (7 kjmol - ) susceible to nucleophilic addition reactions Electron density: all three systems are highly electron deficient (cf. ~pyridine) unreactive towards electrophiic substitution (S E Ar) reactive towards nucleophilic substitution (S Ar)
12 Diazines Syntheses Pyrimidines: Pinner:,3-dicarbonyl with amidine R Et Et Et Et Et Pyrazines: dimerisation of a-aminoketone/aldehyde then aerial oxidation: R C, Pd/C air R Pyridazines: Paal-Knorr :,4-dicarbonyl with hydrazine B. hydroxyl 'leaving group' in,4-dicarbonyl obviates oxidation
13 Diazines Reactivity Electrophilic addition at : formation of -oxides as for pyridine; more susceible to S Ar (and S E Ar) than the parent diazines: Electrophilic substitution: via addition-elimination (S E Ar) all diazines are highly electron deficient very unreactive towards S E Ar electron donating substituents and/or -oxides (see above) required to allow reaction even at C5 of pyrimidine: 3x electron releasing i.e. 'activating' groups c. 3 /Ac 5 0 C [85%] B. no reaction on pyrimidine itself regioselectivity: via most stable Wheland intermediate
14 Diazines Reactivity ucleophilic substitution: via addition-elimination (S Ar) all diazines are highly electron deficient very reactive towards S Ar (>pyridines) all halodiazines exce 5-halopyrimidines react readily with nucleophilies: X X 4 4 X > > > > > > X 3 X X C4 & C pyrimidines C4 & C3 pyridazines C pyrazines X tallation: all diazines can be metalated ortho to by LiTMP (pyrimidine at C4 not C): pyrazine Li Li I I [44%] pyrazine--oxide Bu Bu Li Bu Li Bu I Bu I Bu [73%]
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