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Hydrogen Atom Transfer 4/12/14 Baran Group Meeting Julian Lo Hydrogen Atom Transfer 4/12/14 1. Introduction 2. Polarity Reversal Catalysis Hydrogen atom transfer (HAT) is a concerted movement of a proton and an electron (i.e., H•) A useful concept in HAT is radical polarity. Despite being uncharged species, radicals can in a single kinetic step from one group to another. (Mayer, J. Am. Chem. Soc. 2007, 5153) have nucleophilic or electrophilic tendancies. A—H + B• A• + B—H - e– + e– A A A nucleophilic electrophilic By definition, HAT is intimately linked to organic free readical chemistry, with one of the radicals radicals most useful hydrogen atom transfers in organic synthesis being the termination of a carbon A qualitative approach to determining the "philicity" of a radical centered radical with Bu SnH. (Curran, Tetrahedron Lett. 1985, 4991) 3 1. Consider the oxidized (cationic) and reduced (anionic) forms of A• Me Me Me Me 2. Determine which of the forms is more stable Br nBu3SnH H Me AIBN Me 3. Assign the "philicity" of the radical: a. If A+ is more stable, A• is a nucleophilic radical because it wants to lose an e– PhH, 80 °C b. If A– is more stable, A• is an electrophilic radical because it wants to gain an e– Me H Me H Me H Some practice: Me Me Me Me - e– + e– HH HH tBuO tBuO tBuO nBu SnH 3 nucleophilic electrophilic more stable radical radical HAT Quantitative treatments of radical "philicities" have also Me H Me H - e– + e– been developed. (Fisher, ∆9(12)-capnellene tBu tBu tBu Angew. Chem. Int. Ed. more stable nucleophilic electrophilic 2001, 1340; Héberger, J. radical radical Org. Chem. 1998, 8646; De Although nBu SnH is a great hydrogen atom 3 – – Proft, Org. Lett. 2007, 2721) donor (BDE = 78 kcal/mol), it's toxicity is a - e + e Et Si Et Si Et Si well-known problem and it can be difficult to 3 3 3 more stable nucleophilic electrophilic purify nBu3SnX byproducts away from the radical radical desired product. Just like Sn2 reactions, polarities of the reactants should be matched for favorable reactivity. Procedures that are catalytic in tin have been El• + Nuc–H El–H + Nuc• developed. (Fu, J. Org. Chem. 1996, 6751) favored Nuc• + El–H Nuc–H + El• O O These classifactions nBu3SnH represent polar effects (10 mol%) El1• + El2–H El1–H + El2• in the transition states disfavored 1• 2 1 2• Me PhSiH3 (1.2 eq.) Me Nuc + Nuc –H Nuc –H + Nuc Me Me Me AIBN, PhMe, ∆ Me (80%) E.g., trialkyl silanes are typically awful at homolytic reduction of functional groups via HAT. Several reviews have been published on fast Et Si• + Et Si–X + • alternatives to Bu3SnH and other stannane 3 R–X 3 R reducing agents; several of these topics will unfavorable • • not be discussed. (Walton, Angew. Chem. Int. R + Et3Si–H R–H + Et3Si Replace with two steps polarity Ed. 1998, 3072; Studer, Synthesis 2002, 835; Nuc1• Nuc2–H Nuc1 –H Nuc2• that are polarity matched? Chatgilialagou, Chem. Eur. J. 2008, 2310) mismatch Baran Group Meeting Julian Lo Hydrogen Atom Transfer 4/12/14 Adding a catalyst that replaces the polarity mismatched step with two polarity matched ones Me R Me R should yield a net favorable reaction. This known as polarity reversal catalysis (PRC). (tBuO) , hυ R• + R'S–H R–H + R'S• Favorable polar effects in Me H 2 Me H • favorable • the transition states for the O Me N BH Thx O Nuc El–H Nuc–H El H H 3 2 H H two PRCed steps lowers no catalyst gives Me O O complex mixture • + + • the Ea of the overall R'S Et3Si–H R'S–H Et3Si only radical observed El• Nuc–H favorable El–H Nuc• unfavorable transformation An example that can best be rationalized by radical "philicities." Thus, adding a catalytic amount of thiol to Et3SiH reductions of alkyl halides dramatically (Roberts, J. Chem. Res. (S) 1988, 264) improves the yield. (Roberts, J. Chem. Soc. Perkin Trans. 1 1991, 103) R Me (tBuO)2, hυ Et SiH + + (anti)aromaticity of the 3 Me N BH Thx oxidized and reduced forms DLP (2 mol%) Me H 3 2 Me of the radicals can be used Me Br Me no cat. 1 25 tC12H25SH H H to rationalize the outcome cat. (1 mol%) 99% vs 10% 30 1 Br 86% C6H12, ∆ without RSH H Catalyst-controlled enantioselective HAT utilizing a polarity reversal catalyst. However, using R3SiH in Barton-McCombie reactions proceeds very efficiently. (Roberts, Tetrahedron Lett. 2001, 763) (Roberts, J. Chem. Soc., Perkin Trans. 1 1998, 2881) R Me R Ph SiH Me Me 3 R OAc O Me R TBHN H OAc H Ph SiH O O O 3 H 1 (5 mol%) O O AcO O O TBHN O O O O AcO SH O 60 °C SiPh MeS Me 3 1,4-dioxane Me 1 O O Me 60 °C R = Me 84%, 76% ee 90% O Me R = Ph 90%, 95% ee S H In situ formation of polarity reversal catalyst: O • O O 3. Reagents Derived from 1,4-Cyclohexadiene -MeSSiPh3 Ph3Si O C S "Pro-aromaticity" can be used to drive radical chain reactions. MeS SSiPh3 Ph3SiS MeS SSiPh3 (Walton, J. Chem. Soc., Chem. Commun. 1995, 27) -CO H2O Me CO R CN 2 Me polarity Ph3SiH MeSH or Ph SiSH H CN Ph SiS H Ph SiS• 3 reversal cat. 3 3 + polarity reversal cat. competitive processes: (tBuO)2 R Me CO R CO2R Amine-alkylboranes can be used to alter regioselectivity of HAT via PRC. O H H 140 °C 2 (Roberts, J. Chem. Soc., Perkin Trans. 2 1989, 1953) 26–57% -Me 2 H H H tBuO Me N B Me O uncatalyzed O 2 Me3N B Me HAT 3 up to 51% tBuO• Me CN 2 Me Me R R H Me Me Me O Me CO2R • -CO CN Nuc 2 up to 37% O Me3N BH2Thx R catalyzed -PhMe R O Baran Group Meeting Julian Lo Hydrogen Atom Transfer 4/12/14 Replacing Me in 2 with Ph led to a cleaner reaction. III The BDE of H2O was proposed to decrease upon complexation with Ti . (Walton, J. Chem. Soc., Perkin Trans. 1 2002, 304) (Oltra, Angew. Chem. Int. Ed. 2006, 5522) O 2 • R Ph O O no observed loss of Ph tBuO2Bz Cp Cl Cp Cl + Ph Ph Cp 1 3 2 low (~20%) yields of H2O Ti R R H R Ti tAmylOH O Ti Cl Cp O H + Cp O intermolecular additions Cp R1 R3 100 °C 66% H H BDE = 49.4 kcal/mol The "pro-aromaticity" concept can also be applied to generate carbamoyl radicals. calc (Walton, J. Org. Chem. 2004, 5926) And the system was found to be applicable to reductive openings of other epoxides. OTr O N OH Me Me nC H DLP O 10 21 Me cat. RSH Me Bn O Me N PhH, ∆ N AcO AcO Me H H Mech? Bn Me Me Me Me OH nC10H21 O 43% 68% 85% Several other 1,4-cyclohexadiene derived reagents have been developed. Double HAT from the same Ti complex to conventional [H] catalysts allowed for (Walton, Acc. Chem. Res. 2005, 794) alkene and alkyne reduction. (Oltra, Org. Lett. 2007, 2195) R CO H Ph CO H Me TBS Me NHBoc 2 2 Cp Cl H MeO OMe R H H Ti MLn H R Works with Pd/C, 2 H R or Cp O H ML R Pd/Al2O3, Pd(dba)2, n or R R Wilkinson, Lindlar Me CO2Me H R R H alkyl radicals hydroxyformyl Bu3SnH substitute radical radicals transfer hydrosilation hydroamination A similar reductive epoxide opening has been developed that occurs via a catalytic bimetallic system using H2 as the terminal reductant (Gansäuer, J. Am. Chem. Soc. 2008, 6916) 4. HAT from Unlikely Sources Cp2TiCl2 (10 mol%) 0 HO The identification of a trace byproduct led to an interesting discovery. O Mn , Coll•HCl, H2 (4 atm) (Oltra, J. Org. Chem. 2002, 2566) Cl RhCl(PPh ) O OH OH Me 9 3 3 Cl Me Me (5 mol%) Me 9 Proposed mechanism: Me Cp2TiCl O + OH IV IV Me THF Me Me [Ti ] [Ti ] H H H + Collidine Coll•HCl Me O O Me O H O R R R O O O R [RhII] H O 3 4 H IV R [Ti ] [TiIV] Cl R IV H 3 4 [Ti ] 0 R O 0.5 Mn R A similar system employing Me H [RhIII] H anhydrous 97 3 IrCl(CO)(PPh3)2 allows for 0.5 MnCl O with H O (28 eq.) 85 2 IV cyclization of radical 2 15 [TiIII] [Ti ] with D2O (28 eq.) 25 75 (70% D) intermediate onto alkenes Me [RhI] H O R R H2 and alkynes. (Gansäuer, J. Me O Am. Chem. Soc. 2011, 416) O R R Baran Group Meeting Julian Lo Hydrogen Atom Transfer 4/12/14 A curious observation was made en route to the phomoidrides. The authors originally proposed an "ate" complex to be responsible for HAT, but it was not (Wood, J. Am. Chem. Soc. 2005, 12513) observed by 11B NMR—only the methanolysis products were. E Et (Renaud, Chem. Commun. 2010, 803) desired E E C4H9 Et E O OMe E Et O OH C H B H O MeOH 4 9 R3B E O O C4H9 O O B OMe + (MeO)3B O S air, O Me O OH O PhH E not observed! O Et SMe E C H not 4 9 Therefore, the following mechanism was proposed: desired O O H O B R R R H 99% for Me B OH O O R O 3 B O OH Upon extensive deuterium labeling studies, authors found that adventitious H2O was O • OH the H source, and proposed: OH SH2 Me Me H Me Me H2O Me Me -Me For entire process, B O B O B O 4-tert-Butylcatecol was shown to be superior to catechol (J.
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