D. A. Evans An Introduction to Frontier Molecular Orbital Theory-1 Chem 206 http://www.courses.fas.harvardedu/-chem206/ ■ Problem of the Day The molecule illustrated below can react through either Path A or Path B to i form salt 1 or salt 2. In both instances the carbonyl oxygen functions as the Chemistry 206 nucleophile in an intramolecular alkylation. What is the preferred reaction pat for the transformation in question Advanced Organic Chemist Lecture number 1 Path B Introduction to FMo Theory ■ Your Answer General Bonding considerations The H2 Molecule Revisited (Again!) a Donor& Acceptor Properties of Bonding Antibonding States L Hyperconjugation and"Negative"Hyperconjugation Anomeric and related Effects Reading Assignment for week A. Carey& Sundberg: Part A; Chapter 1 B. Fleming, Chapter 1&2 C Fukui, Acc. Chem. Res. 1971, 4, 57 D. O.J. Curnow J Chem. Ed. 1998. 75. 910 E J 1. Brauman. Science. 2002. 295 2245 Matthew d shair Wednesday, eptember 18, 2002
D. A. Evans Chem 206 Matthew D. Shair Wednesday, September 18, 2002 http://www.courses.fas.harvard.edu/~chem206/ ■ Reading Assignment for week: A. Carey & Sundberg: Part A; Chapter 1 B. Fleming, Chapter 1 & 2 C. Fukui,Acc. Chem. Res. 1971, 4, 57. D. O. J. Curnow, J. Chem. Ed. 1998, 75, 910. E. J. I. Brauman, Science, 2002, 295, 2245. Chemistry 206 Advanced Organic Chemistry Lecture Number 1 Introduction to FMO Theory ■ General Bonding Considerations ■ The H2 Molecule Revisited (Again!) ■ Donor & Acceptor Properties of Bonding & Antibonding States ■ Hyperconjugation and "Negative" Hyperconjugation ■ Anomeric and Related Effects An Introduction to Frontier Molecular Orbital Theory-1 ■ Problem of the Day The molecule illustrated below can react through either Path A or Path B to form salt 1 or salt 2 . In both instances the carbonyl oxygen functions as the nucleophile in an intramolecular alkylation. What is the preferred reaction path for the transformation in question? + + Br – Br – 1 2 Path A Path B Br N H O Br O O Br N O H O N O H Br ■ Your Answer
D. A. Evans An Introduction to Frontier Molecular Orbital Theory-1 Chem 206 Universal Effects Governing Chemical Reactions Stereoelectronic Effects There are three ■ Steric Effects Geometrical constraints placed upon ground and transition states by orbital overlap considerations Nonbonding interactions (Van der Waals repulsion) between substituents within a molecule or between reacting molecules Fukui postulate for reaction During the course of chemical reactions, the interaction of the highest filled(HOMO) and lowest unfilled(antibonding) J I Brauman, Science, 2002, 295, 2245 molecular orbital (LUMO)in reacting species is very important to the stabilization of the transition structure General Reaction Types Radical reactions(-10%):A·+B·—A-B Electronic Effects(Inductive Effects) Polar Reactions(90%): A(+ B(+)-A--B rate decreases as r becomes more electronegative Lewis Base Lewis Acid Inductive Effects: Through-bond polarization FMO concepts extend the donor-acceptor paradigm to Field Effects: Through-space polarization non-obvious families of reactions Your thoughts on this transformation ■ Examples to consider H2+2Li()→2LH R3Sio Lewis acid EtO2 C\ CH3-1+ Mg(O)- CH3-MgBr OSiR3 Danishefsky, JOC 1991, 56, 38 diastereoselection >94.6
RO H O C Br Me R R SN2 C R R Me Br Me2CuLi O OSiR3 R3SiO EtO C Me R R RO H O Me H C R R Me Nu RO H O H Me OSiR3 OSiR3 EtO2C H2 CH3–I A(:) A· B(+) B· 2 LiH CH3–MgBr A B A B D. A. Evans An Introduction to Frontier Molecular Orbital Theory-1 Chem 206 + Br:– minor major Br: – Nu: Nonbonding interactions (Van der Waals repulsion) between substituents within a molecule or between reacting molecules ■ Steric Effects Universal Effects Governing Chemical Reactions There are three: ■ Electronic Effects (Inductive Effects): + SN1 rate decreases as R becomes more electronegative Inductive Effects: Through-bond polarization Field Effects: Through-space polarization Danishefsky, JOC 1991, 56, 387 Lewis acid diastereoselection >94 : 6 Your thoughts on this transformation "During the course of chemical reactions, the interaction of the highest filled (HOMO) and lowest unfilled (antibonding) molecular orbital (LUMO) in reacting species is very important to the stabilization of the transition structure." Geometrical constraints placed upon ground and transition states by orbital overlap considerations. ■ Stereoelectronic Effects Fukui Postulate for reactions: ■ General Reaction Types Radical Reactions (~10%): + Polar Reactions (~90%): + Lewis Base Lewis Acid FMO concepts extend the donor-acceptor paradigm to non-obvious families of reactions ■ Examples to consider + 2 Li(0) + Mg(0) J. I. Brauman, Science, 2002, 295, 2245
D. A. Evans The H2 Molecular Orbitals Antibonds Chem 206 The H2 Molecule(again!!) Linear Combination of Atomic Orbitals(LCAO): Orbital Coefficients Let's combine two hydrogen atoms to form the hydrogen molecule ■ Rule two Mathematically, linear combinations of the 2 atomic 1s states create two new orbitals, one is bonding, and one antibonding Each MO is constructed by taking a linear combination of the individual atomic orbitals(AO) Bonding mo Rule one: a linear combination of n atomic states will create n Mos Antibonding MO 0*=C 1V+C2Y2 1s ■ Rule three(c12+(c2 The squares of the C-values are a measure of the electron population AE in neighborhood of atoms in question I Rule Four: bonding(C-+ antibonding(C1=1 In lCao method, both wave functions must each contribute one net orbital Let's now add the two electrons to the new mo, one from each h atom Consider the pi-bond of a C=o function: In the ground state pi-C-0 o*(antibonding is polarized toward Oxygen. Note(Rule 4) that the antibonding MO is polarized in the opposite direction 1s G(bonal Note that△E1 is greater than△E2.Why
D. A. Evans Chem 206 The H2 Molecule (again!!) Let's combine two hydrogen atoms to form the hydrogen molecule. Mathematically, linear combinations of the 2 atomic 1s states create two new orbitals, one is bonding, and one antibonding: Energy 1s 1s s* (antibonding) ■ Rule one: A linear combination of n atomic states will create n MOs. DE DE Let's now add the two electrons to the new MO, one from each H atom: Note that DE1 is greater than DE2 . Why? s (bonding) s (bonding) DE2 DE1 s* (antibonding) 1s 1s y2 y2 y1 y1 Energy s = C1y1 + C2y2 Linear Combination of Atomic Orbitals (LCAO): Orbital Coefficients Each MO is constructed by taking a linear combination of the individual atomic orbitals (AO): Bonding MO Antibonding MO C* s* =C*1y1– 2y2 The coefficients, C1 and C2, represent the contribution of each AO. ■ Rule Three: (C1 )2 + (C2 )2 = 1 antibonding(C*1 ) = 1 2 bonding(C + 1 )2 ■ Rule Four: Energy p* (antibonding) p (bonding) Consider the pi -bond of a C=O function: In the ground state pi-C–O is polarized toward Oxygen. Note (Rule 4) that the antibonding MO is polarized in the opposite direction. C C O C O The H2 Molecular Orbitals & Antibonds The squares of the C-values are a measure of the electron population in neighborhood of atoms in question In LCAO method, both wave functions must each contribute one net orbital ■ Rule Two: H H H H O
D. A. Evans Bonding generalizations Chem 206 I Bond strengths(Bond dissociation energies)are composed of a Orbital orientation strongly affects the strength of the resulting bond covalent contribution (o Ecow and an ionic contribution( Eionid For g Bonds Bond Energy(BDE)=o Covalent+ 8 Ionic( Fleming, page 27) A○ OBo teoA○B When one compares bond strengths between C-C and C-x, where X is some other element such as O, N, F, Si, or S, keep in mind that covalent and ionic contributions vary independently. Hence, the mapping of trends is not a trivial exerci A-B Useful generalizations on covalent bonding This is a simple notion with very important consequences. It surfaces in OVerlap between orbitals of comparable energy is more effective the delocalized bonding which occurs in the competing anti(favored) than overlap between orbitals of differing energy syn (disfavored)E2 elimination reactions. Review this situation For example, consider elements in Group IV, Carbon and Silicon. We An anti orientation of filled and unfilled orbitals leads to better overlap know that C-C bonds are considerably stronger by Ca 20 kcal mol This is a corrollary to the preceding generalization than c-si bonds There are two common situatio ⊙⊙→(t(⑥( Case-1: Anti Nonbonding electron pair&C-x bond 土 X G'C-X lone pa 00 LUMO HOMO LUMO C-SP CSP. 土 cC-C H3C-CH3 BDE =88 kcal/mol H3C-SiH3 BDE -70 kcal/mol Case-2: Two anti sigma bonds Bond length =1.534 A Bond length= 1.87A This trend is even more dramatic with pi-bonds F"C-X C-C= 65 kcal/mol C-si= 36 kcal/mol Si-si= 23 kcal/mol 你8m 0 LUMO a Weak bonds will have corresponding low-lying antibonds Formation of a weak bond will lead to a corresponding low-lying antibonding orbital. Such structures are reactive as both nucleophiles electrophile
A B A A C A C A Y C X A C X X X ·· lone pair HOMO s* C–X LUMO s* C–X LUMO lone pair HOMO C C C C C Si C-SP3 C-SP3 C-SP3 C Si Si-SP3 Y C C X A B A B Y C C B X D. A. Evans Bonding Generalizations Chem 206 ■ Weak bonds will have corresponding low-lying antibonds. p C–C = 65 kcal/mol p C–Si = 36 kcal/mol p Si–Si = 23 kcal/mol This trend is even more dramatic with pi-bonds: s* C–Si s* C–C s C–Si s C–C Bond length = 1.534 Å Bond length = 1.87 Å H3C–CH3 BDE = 88 kcal/mol H3C–SiH3 BDE ~ 70 kcal/mol Useful generalizations on covalent bonding When one compares bond strengths between C–C and C–X, where X is some other element such as O, N, F, Si, or S, keep in mind that covalent and ionic contributions vary independently. Hence, the mapping of trends is not a trivial exercise. Bond Energy (BDE) = Ecovalent + Eionic (Fleming, page 27) ■ Bond strengths (Bond dissociation energies) are composed of a covalent contribution ( Ecov) and an ionic contribution ( Eionic). better than For example, consider elements in Group IV, Carbon and Silicon. We know that C-C bonds are considerably stronger by Ca. 20 kcal mol-1 than C-Si bonds. ■ Overlap between orbitals of comparable energy is more effective than overlap between orbitals of differing energy. Formation of a weak bond will lead to a corresponding low-lying antibonding orbital. Such structures are reactive as both nucleophiles & electrophiles Better than Better than Case-2: Two anti sigma bonds s C–Y HOMO s* C–X LUMO s* C–X LUMO Case-1: Anti Nonbonding electron pair & C–X bond ■ An anti orientation of filled and unfilled orbitals leads to better overlap. This is a corrollary to the preceding generalization. There are two common situations. Better than For p Bonds: For s Bonds: ■ Orbital orientation strongly affects the strength of the resulting bond. Better than This is a simple notion with very important consequences. It surfaces in the delocalized bonding which occurs in the competing anti (favored) syn (disfavored) E2 elimination reactions. Review this situation. s C–Y HOMO
D. A. Evans Donor-Acceptor Properties of Bonding and Antibonding States Chem 206 Donor Acceptor Properties of C-c&C-O Bonds Hierarchy of Donor Acceptor States Consider the energy level diagrams for both bonding& antibonding Following trends are made on the basis of comparing the bonding and orbitals for C-c and c-o bonds antibonding states for the molecule CH3-x where X=C, N,o,F,&H gc-C I o-bonding States: (C-x) c。 C-SP 上csp土 CH3-NH2 decreasing o-donor capacity C-C poorest donor gC-O a The greater electronegativity of oxygen lowers both the bonding g-anti-bonding States:(C-X) antibonding C-O states. Hence oc-c is a better donor orbital than gc-o CHH Io'C-O is a better acceptor orbital than gC-c CH3CH3 CH3-NH2 Donor Acceptor Properties of CsP3-CsP3& CsP3-CsP2 Bonds CH3-OH CH3-F Increasing o'-acceptor capacity best acceptor C-SE The following are trends for the energy levels of nonbonding states of several common molecules. Trend was established by photoelectron spectroscopy C-SP2 Nonbonding States gC-C W 中p卅 a The greater electronegativity of Csp? lowers both the bonding antibonding C-C states. Hence Io CSP3-CsP3 is a better donor orbital than g CsP3-CSP2 do'CSP3-CsP2 is a better acceptor orbital than oCSP3-CSP3 decreasing donor capacity poorest donor
D. A. Evans Donor-Acceptor Properties of Bonding and Antibonding States Chem 206 ■ s *CSP3-CSP2 is a better acceptor orbital than s *CSP3-CSP3 C-SP3 C-SP3 s* C–C s C–C C-SP3 s C–C s* C–C C-SP2 Donor Acceptor Properties of CSP3-CSP3 & CSP3-CSP2 Bonds ■ The greater electronegativity of CSP2 lowers both the bonding & antibonding C–C states. Hence: ■ s CSP3-CSP3 is a better donor orbital than s CSP3-CSP2 ■ s *C–O is a better acceptor orbital than s *C–C ■ s C–C is a better donor orbital than s C–O ■ The greater electronegativity of oxygen lowers both the bonding & antibonding C-O states. Hence: Consider the energy level diagrams for both bonding & antibonding orbitals for C–C and C–O bonds. Donor Acceptor Properties of C-C & C-O Bonds O-SP3 s* C-O s C-O C-SP3 s C-C s* C-C better donor better acceptor decreasing donor capacity Nonbonding States poorest donor The following are trends for the energy levels of nonbonding states of several common molecules. Trend was established by photoelectron spectroscopy. best acceptor poorest donor Increasing -acceptor capacity s-anti-bonding States: (C–X) s-bonding States: (C–X) decreasing -donor capacity Following trends are made on the basis of comparing the bonding and antibonding states for the molecule CH3–X where X = C, N, O, F, & H. Hierarchy of Donor & Acceptor States CH3–CH3 CH3–H CH3–NH2 CH3–OH CH3–F CH3–H CH3–CH3 CH3–NH2 CH3–OH CH3–F HCl: H2O: H3N: H2S: H3P: