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Oxidations to Carbonyl Oxidations at Carbon General references March, Advanced Org Chem, 1992, 1158-1238 Trost, Comp. Org. Syn. 1991, vol 7 Carey and Sundberg, Advanced Org Chem, part B, 615-664 Smith, Org Syn, Chap 3 O OH O O C CH2 CH COH CH3 O H H H H C(-IV) C(-II) C(0) C(+II) C(+IV) isonitrile RNC OH O O O CH3 CH2 CH COR COR R R R R RO C(-III) C(-1) C(+I) C(+III) C(+IV) O alkane RCH2X X=halide Acetal: RCH(OR)2 ester carbamate RCH2M RCH2NR2 RHC NR amide RO S NR2 RCH2SiR3 RCH2SR Masked form: thioester xanthate RCH2PR2 nitrile CH RO SR RCX O OR 3 Ortho ester: RC(OR)3 urea OH O Masked form: H2 H2N NH2 C CH C O OR R R R R R R carbodiimide C C(0) C(II) RN C NR C(-II) OR Ketal: R2C(OR)2 isocyanate aminal: R C(OR)(NR ) Ketene Ketene acetal 2 2 RN C O dithiane: R2C(SR)2 Imine: R2CNR Chromium-based Oxidations Very powerful oxidants Many variations on the theme Reactivity modulated by ligands on Cr (Org. Rxn., 1998, 53, 1-122) Likely pretty toxic H2CrO4 Jones reagent As solution with H2SO4 in acetone/water Very strong and not very selective Mechanism has been studied extensively Predominant mechanism likely to be Cr(VI) --> Cr(IV), taken as prototype for other Cr-based reagents Mechanism: See Westheimer J. Phys. Chem. 1959, 538; Chem Rev. 1949, 419 Involving radical intermediates: Wieberg, JACS, 1974, 1884 Involving Cr(IV) --> Cr(II) Espenson, JACS, 1992, 4205 O Consensus mechanism: HO Cr O O k1 O H O O - OH + HO Cr O + + + Cr O OH HO OH (H2CrO4 pKa = -1) HO Cr O k2 O H Evidence: kH/kD = 6 + + 2 rate = d[Cr(VI)]/dt = (k1[H ] + k2[H ] )[HCrO4][ROH] k1/k2 = 0.04 O 'instantaneous' O O Cr O pyridine O Jones Reagent: Reactivity Jones reagent can promote acid-catalyzed reactions (desilylation, Friedel-Crafts type, olefin migrations, epoxide openings) and diol cleavage, as in the examples below. C8H17 85% AcO AcO OH O OH AcO O CO2H AcO O TL, 1988, 6403 O OTBS Jones Reagent BnO BnO CO2H 88-97% O O CO2CH3 CO2CH3 P. A. Evans, ACIEE, 1999, 3175 Cr-amine reagents review: Org. React. 1998, 1-122 added amine helps moderate reactivity of Cr and buffers reaction mixture Three common reagents: CrO3Py2 (Collins Reagent): hydroscopic, often requires excess, if dry will stop at aldehyde [ClCrO3][PyH] (pyridinium chlorochromate, PCC, Corey's reagent): often oxidant of choice for first attempt, used in cunjuction with mol sieves (to keep dry and prevent clumping) or Celite (usually 1:1 mass ratio for both); Air stable, not too hydroscopic; will stop at aldehyde + (PyH )2Cr2O7 (pyridinium dichromate, PDC): in CH2Cl2, alcohol to aldehyde; more often used in DMF for alcohol to acid Examples CrO3Py2 OH O OH 6 equiv O 84% PCC JOC, 1971, 2035 O 81% O O AcO AcO O JOC, 1992, 3789 CrO3Py2 96% OH O PCC Tet, 1974, 2027 98% HO OH O O BzO CH3 CrO3Py2 O BzO O CH3 CH 67% O 3 CH3 TL, 1985, 3731 O Cr-amine reagents examples, cont. PCC O 70% O OH O De Brabander, OL, 2002, 481 OEt OEt OTBS OTBS H H H H OH PDC OH O O DMF PDC OPv OPv 91% NH NH O O O OH O JOC, 1986, 2717 Tet, 1988, 2149 O O O O O O PDC AcO DMF AcO OH BnO >78% BnO JOC, 1985, 2095 Cr-amine reagents alternative reaction manifolds PCC OH O OH O O PDC PCC, NaOAc 40% H3CO H3CO CO CH CO CH 2 3 2 3 HO Tet, 1980, 3091 JACS, 1987, 3991 CHO [2+2] O OH O Cr O Cr O HO HO O O repeat or repeat McDonald, JACS, 1994, 7921 similar reactivity with Mn: JOC, 2004, 3386; with Re JACS 1997, 6022 O O O Cr O H H OH HO 9% O Cr-mediate synthesis of β,β-disubstituted enones Dauben, JOC, 1977, 682 overall transformation: O R R-M PCC O mechanism O R R R R-M OH O [3,3] Cr O O OCrO3H OH H O no H to eliminate examples O O CHO O MeLi; MeLi; O PCC PCC MgB r 90% 90% 62% O Activated DMSO oxidations General mechanism O R - X + - O O S+ H S R N + + 3 S XY S+ O R O H R activating agent + R R R HO R S (so please keep everything in the R hood!) R an added step with (COCl)2 as activating agent: O HO R Cl + O S + as above - + S S+ O CO2 + CO + Cl O R Cl R Cl- Many activating agents are available. Some of the more common: DCC (Pfitzner-Moffatt oxidation) JACS, 1963, 3027 Ac2O JACS, 1967, 2416 SO3-Pyridine (Perikh-Doering oxidation) JACS, 1967, 5505 Cl2 TL, 1973, 919 SOCl2 Tet, 1978, 1651 (COCl)2 (Swern oxidation) JOC, 1978, 2480 Activated DMSO oxidations Applications positives negatives Very mild conditions Carbodiimide-derived urea hard to remove in Moffatt Reactions are very fast and reliable (use of EDCI partially addresses this problem) Never go to acid Opperationally more involved that some other methods Inexpensive and nontoxic reagents (but still pretty easy) Pummerer rearrangement can interfere (see below) DMS angers the biologists Examples O O O OH O O O O O [O] O N O N + O N 80-90% H3C CH3 H3C i-Pr i-Pr i-Pr [O] Ratio CrO3, Py, rt 73:27 Jones 79:21 PCC 92:8 PDC, Py, TFA 97:3 o SO3-Py, DMSO, Et3N, 0 C 99:1 Evans, JACS, 1984, 1154 Activated DMSO oxidations Applications OH O (ClCO)2, DMSO; Et3N 86% Br Br H H O O OTBDPS O O OTBDPS Falck, OL, 2002, 969 OBn OBn OBn H H H HO (COCl)2, DMSO; O Et3N; NH3 HO O N not isolated Heathcock JACS, 1988, 8734 OTES OMe OTES OMe SO3-Py, DMSO, Et3N 90% HO O OMOM OMOM De Brabander, ACIEE, 2003, 1648 Limitations and Extensions of DMSO-based oxidations: the Pummerer Reaction review: Org. React. 1991, 157 What: R R' S R' O O H How: R OCOR O R activation + R' S R' S R R' S+ R R' S O OX R' usually use Ac2O or TFAA Why: install protecting group O O O DMSO O O H3C Methyl thiomethyl ether Ac2O/AcOH O S = removed with Hg(II) TBSO OH TBSO O TBSO OMTM TL, 2003, 3175 introduce aldehyde (or equivalent): O O O OO OMe OO OO 1.TFAA 1. PhSLi Ph 2. Hg(II), MeOH CH 2. mCPBA MsOH MeO CH 3 S CH3 3 O H H H H H H N O OH N (78%) OH N H C Ts H C Ts 3 H3C Ts 3 Rapoport, JOC, 1985, 325 Limitations and Extensions of DMSO-based oxidations: the Pummerer Reaction, cont. O OCOPh OCOPh CO2Me BzCl CO2Me 4 CO2Me [2,3] + 4 SOPh 4 + (Evans-Mislow O S rearrangement) S -O Ph Li Ph retro E.-M. 1. Ac O, TFAA, OCOPh 2 AcONa, 2,6-lut. O OCOPh O 2. HgCl , CaCO CO2Me 2 3 S CO2Me 4 Ph Corey, TL, 1982, 3463 65% 4 Nitrogen Nucleophile: O SPh S TMSOTf i-Pr2NEt Ph NH O NH2 Kaneko, JACS, 1985, 5490 Carbon Nucleophiles O d.r = 2.7:1 41% S SMe O OX O S+ TsOH MeO N O OMe Δ O N H adjacent carbonyl OMe MeO increases acidity and Perkin 1, 1985, 605 promotes elimination Hypervalent Iodine-based reagents the Dess-Martin reagent AcO O AcO KBrO3/H2SO4 HO Ac O, I I (D.&M. JACS 1991, 7277) I 2 OAc AcOH O O or CO2H Oxone (KHSO + sulfate salts) 5 O (JOC, 1999, 4537) O IBX Dess-Martin periodinane low solubility in most organic (DMP) Advantages of DMP: solvents (but, see below) Commercially available Reliable with complex molecules explosive on impact or >200oC Often need ~1 equiv Effective for 1o or 2o ROH; never go to acid Easy to use (nearly idiot-proof) path a B path a (added base doesn't help) Mechanism: H AcO O O H AcO AcO AcO I RCH2OH I I+ OAc -AcOH OAc path b O -OAc (added acid O O doesn't help) O O O H H O O O O- AcO AcO I I+ path c O O or O O O R tight ion pair O O Hypervalent Iodine-based reagents one equivalent of water was found to accelerate the reaction: Schreiber, JOC, 1994, 7549 More e- donating H H O O than OAc... O O AcO HO I I O vs O ...so acetate is more basic O O O O in practice, often easiest to use CH2Cl2 out of he bottle (not distilled) Ph Ph DMP OH O conditions results Dry CH2Cl2 97%, 14h 1.1 equiv H O 97% 0.5h Other examples: 2 SciFinder search yielded 104,000 hits for DMP O O DMP O 70% O OH O H3CO H3CO Danishefsky, JACS, 1988, 6890 Hypervalent Iodine-based reagents: examples OH O OMe O OMe O OMOM DMP OMOM O 95% O CH3 CH3 De Brabander, JACS, 2002, 3245 OTMS OTMS BnO2C BnO2C CO2Bn CO2Bn DMP CHO O OH 93% O O O O O O O Nicolaou, ACIEE, 1994, 2184 t-Bu t-Bu O O Si t-Bu Si t-Bu O O DMP >95% PMBO PMBO OH O Evans, JACS, 1990, 7001 N O N O OMe OMe O IBX HO I O Insoluble in most organic solvents, but somewhat soluble in DMSO.
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