Rearrangements and Reactive Intermediates [email protected] 1 Rearrangements and Reactive Intermediates Hilary Term 2018 1A Organic Chemistry Handout 1 Me Me 1 7 2 7 2 3 H 3 1 Me HO 6 5 4 Me 4 6 5 Me 8 8 isoborneol camphene http://burton.chem.ox.ac.uk/teaching.html ◼ Polar Rearrangements, Oxford Chemistry Primer no. 5; L. M. Harwood ◼ Organic Chemistry J. Clayden, N. Greeves, S. Warren – Chapters 36-41 ◼ Reactive Intermediates, Oxford Chemistry Primer no. 8; C. J. Moody, G. H. Whitham ◼ Mechanism and Theory in Organic Chemistry, T. H. Lowry, K. S. Richardson ◼ Advanced Organic Chemistry, F. A. Carey, R. A. Sundberg ◼ Modern Physical Organic Chemistry; E. Anslyn, D. Docherty Rearrangements and Reactive Intermediates 2 Synopsis ◼ Carbocations and carbanions NMR spectroscopy and X-ray structures of carbocations; aggregation and pyramidal inversion of carbanions. Reactivity, including SE1, redox, hydride elimination and rearrangements: Wagner–Meerwein, pinacol, semi-pinacol. ◼ Rearrangement of anions and carbocations Orbital theory; Is 3c-2e structure TS or HEI? Stepwise versus concerted rearrangements; non-classical carbocations (carbonium ions), transannular hydride shifts. Carbanions: Favorskii, Ramberg-Bäcklund, Stevens and Wittig rearrangements. ◼ Carbenes Structural features that influence stability. Methods of making them; carbenes versus carbenoids. General classification of the types of reaction that these species undergo. Rearrangements: Wolff, cyclopropanation, C-H insertion. ◼ Rearrangements to electron-deficient nitrogen and oxygen Structure of nitrenes; structural features that influence stability. Methods of making them. Types of reaction: aziridination, C–H insertion. Nitrene versus non-nitrene mechanisms. Rearrangements to electron-deficient nitrogen (Beckmann, Neber, Hoffmann, Curtius, Schmidt, Lossen). Baeyer–Villiger rearrangement. ◼ Introduction to radicals Structure; stability. General types of reaction involving radicals: homolysis, recombination, redox, addition, β-scission, substitution, disproportionation. ◼ Problem class relating to lectures 1–4. ◼ Case studies Elucidating mechanisms of rearrangements. Evidence for currently accepted mechanisms for the Baeyer– Villiger, Beckmann and Favorskii rearrangements. ◼ Problem class relating to lectures 5 and 7. Rearrangements and Reactive Intermediates 3 Types of High Energy Intermediates ◼ Electron Rich Anions reactive towards Carbanion (8 electrons) a) electrophiles ◼ Electron Deficient Cations reactive towards b) acids Two classes of carbocations a) nucleophiles c) oxidising agents b) bases R R • Carbenium ion (6 electrons) c) reducing agents R R • R R R R R R R R ◼ Electron Rich Anions Radical Anion Carbonium ion (8 electrons) H H H H H H e.g. CH5 • Radical cation H H H H H H H H H H H H • ◼ Neutral species • + Radical (7 electrons) H H H H reactive towards H H R a) electrophiles or nucleophiles • b) other high energy agents ◼ Neutral species R R c) oxidising or reducing agents Carbenes (6 electrons) R • R ◼ Neutral species ◼ Neutral species R R • R • • ketenes arynes R R R R R R singlet triplet • O R Rearrangements and Reactive Intermediates 4 Stuctures of Carbocations ◼ Crystal structure of an adamantyl carbocation 110° 1.44 Å 118° 99° 1.53 Å ° δC 38 ppm 1.62 Å 111 δ 29 ppm Me Me C 1.52 Å F5SbFSbF5 adamantane Me ◼ C-C σ to empty p Me Me Me Me Me Me Me Me Me δ 294 ppm F C C-C sp3-sp3 1.54 Å δC 71 ppm 3 2 C-C sp -sp 1.50 Å 2SbF5 δC 90 ppm 2 2 C-C sp -sp 1.46 Å Me Me SO2 Me Me δC 30 ppm C=C 1.34 Å Me Me δC 49 ppm ◼ Bond lengths and bond angles provide evidence of hyperconjugation (T. Laube, Angew. Chem. Int. Ed. 1986, 25, 349). Rearrangements and Reactive Intermediates 5 ◼ Crystal structure of a t-butyl carbocation H H H C-C sp3-sp3 1.54 Å 1.44 Å C-C sp3-sp2 1.50 Å H H C-C sp2-sp2 1.46 Å C=C 1.34 Å H H H H 120° F5SbFSbF5 δC = 335 ppm O F CH3 2SbF5 2SbF5 δC = 94 ppm H3C F H C CH3 3 SO2 H3C CH3 SO2 H3C CH3 δ = 28 ppm CH3 δC = 171 ppm C δ = 47 ppm C δH = 4.35 ppm (adamantyl acid fluoride) δC = 320 ppm F H δ = 13.5 ppm SbF5 H H3C CH3 SO2 H3C CH3 δH = 5 ppm δC = 51.5 ppm ◼ Bond lengths provide evidence of hyperconjugation (T. Laube, J. Am. Chem. Soc. 1993, 115, 7240). Rearrangements and Reactive Intermediates 6 ◼ Hyperconjugation donation of C-H σ-bond (or C-C σ- bond) electrons into empty p orbital empty p-orbital H filled σ C-H orbital CH3 H CH H 3 energy of the bonding electrons reduced system stabilised ◼ greater number of C-H (or C-C) σ-bonds the greater the extent of hyperconjugation and the greater stabilisation tertiary secondary primary ◼ carbenium ion stability therefore goes in the order: R R R R R > > R ◼ conjugation with alkenes, arenes and lone pairs, also stabilises carbenium ions ◼ most carbocations are fleeting reaction intermediates – the triphenylmethyl (trityl) cation persists - crystal structure of trityl cation demonstrates all the phenyl groups are twisted out of plane ◼ Ph3C BF4 is a commercially available crystalline solid HSO4 Ph H2SO4 Ph OH Ph CH2Cl2 δC = 212 ppm B(CN)4 Rearrangements and Reactive Intermediates 7 Structures of Carbanions ◼ generally aggregated in the solid state and in solution ◼ methyllithium is a tetramer (MeLi)4 with CH3 groups sitting above each face of a Li4 tetrahedron — overall a distorted cube ◼ tert-butyllitium is also tetrameric in the solid state (X-ray crystal structures below) Li C C Li C Li Li C t-butyllithium methyllithium idealised arrangement of (t-BuLi)4 (MeLi)4 lithium and carbon atoms (H-atoms removed for clarity) ◼ in coordinating solvents e.g. THF, Et2O most organolithiums become less aggregated and hence more reactive Rearrangements and Reactive Intermediates 8 ◼ stability of carbanions is related to the pKa of their conjugate acids H H H H H H increasing pKa of conjugate acid, 16 24 41 43 44 increasing reactivity, decreasing stability H H H H H H aromatic sp-hybridised conjugated sp2-hybridised sp2-hybridised CH3 CH3CH2 (CH3)2CH (CH3)3C H increasing pKa of conjugate acid, 46 48 50 51 53 increasing reactivity, decreasing stability H H H H Me H Me Me H H Me Me Me sp3-hybridised sp3-hybridised sp3-hybridised sp2-hybridised sp3-hybridised electron donating electron donating electron donating alkyl group alkyl groups alkyl groups Rearrangements and Reactive Intermediates 9 3 ◼ pyramidal inversion is generally fast for sp hybridised carbanions (they are isoelectronic with NH3) and hence chiral carbanions generally undergo rapid racemisation. ‡ fast R'' R'' R R R'' R R' R' R' ◼ vinyl anions and cyclopropyl anions are the exceptions and are generally considered configurationally stable ◼ lithium halogen exchange with alkenyl iodides and bromides is a stereospecific process tBu Li Br Ph Ph R Br Li Ph R Br tBuLi R Br Li R Ph Ph Ph 2 Br Li CO2H ◼ sp hybridisation at transition state for Me BuLi Me CO2 Me pyramidal inversion ◼ ideal 120 ° angles only ca. 60° for cyclopropane Ph Ph Ph Ph Ph Ph ◼ transition state highly strained (S) ‡ retention therefore slow rate of inversion R Rearrangements and Reactive Intermediates 10 Reactions of Carbocations and Carbanions ◼ Generic reaction map of carbocations and carbanions hydride loss reaction with nucleophile reduction E carbocation deprotonation Nu rearrangement X R R H H R R R + 2e - H Nu - H electrophilic ionisation SET substitution + e X - X + e - X X R R R R R • SN1 or E1 - e - e SE1 single electron - H transfer (SET) E - H R E R R R reaction with R electrophile electrophile carbanion deprotonation hydride loss reaction addition Rearrangements and Reactive Intermediates 11 ⊕ ◼ most common reaction of carbanions is reaction with electrophiles (e.g. RLi or RMgBr plus E ) which is amply covered elsewhere ◼ some other reactions are shown below SE1 – Subsitution Electrophilic Unimolecular - formally related to a carbanion as SN1 is to a carbocation ◼ generic mechanism X - X E E R R R SE1 ◼ examples O O O O OH Br Br H Br Ph Ph Ph Ph R R R - N2 ROH N H R N R R O O HO O O P OH, H O Ph P P P H Ph 2 HO Ph + Ph Ph Ph O Ph + heat Ph Rearrangements and Reactive Intermediates 12 ◼ β-hydride elimination from carbanions common for transition metals ◼ reverse reaction is hydrometallation – well known from hydroboration chemistry H H H PdX + HPdX BR2 BR2 ◼ not a common reaction for Grignard reagents or organolithiums; however, β-hydride elimination is a decomposition pathway for organolithiums and tert-butyllithium can act as a source of hydride Li Me Me H + LiH Me Me ◼ redox reactions – Single Electron Transfer - SET MgBr Cl H + Cl + H • H H dimerisation SET of Ph• Cl Cl • Cl SET + • Cl • Rearrangements and Reactive Intermediates 13 ◼ rearrangement of carbocations ◼ the neopentyl system Me Me Me Me Me Me AgNO3, water Me HO + not Me Me I Me Me Me OH Ag H2O then - H - H Me 1,2- shift Me Me ◼ the 1,2 shift is a Wagner- Me Meerwein rearrangement Me Me ◼ as an aside, remember that neopentyl systems, although primary, are unreactive under SN2 conditions as the nucleophile is severely hindered from attacking the necessary carbon atom Me Me ‡ Me Me Me Me Me Me (-) (-) Me Nu Nu H LG Nu LG R'' LG H R' H H ◼ staggered conformation requires nucleophile to approach passed one of the methyl groups Rearrangements and Reactive Intermediates 14 ◼ Wagner-Meerwein rearrangements exemplified Me 1 Me 7 7 2 7 2 1 - H 2 3 H 3 Me 3 1 Me HO 6 5 ◼ overall red bond is broken and 4 4 6 Me Me 5 4 6 5 Me 8 blue bond is formed isoborneol camphene 8 Me 8 H ◼ in general alkyl shifts occur to rotate yield a more stable carbocation Me 1 Me Me Me Me 1 Me Me 1 1 Me 2 2 Me 7 2 7 8 7 2 5 3 3 5 3 6 6 6 best orbital overlap is also H2O 6 ◼ 5 4 5 4 4 rotate 7 Me Me 8 Me 3 important in determining which 8 4 8 group migrates ◼ secondary ◼ tertiary carbocation carbocation Me poor orbital overlap 1 Me Me 1 Me Me 1 Me migration Me Me for migration 2 2 2 7 7 3 3 3 would lead to 1 6 6 7 5 4 5 4 6 4-membered 5 Me Me Me 5 2 8 8 4 ring 8 best orbital overlap H Me 8 for migration (ca.