User:Benjah-bmm27/degree/3/VKA
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Orbitals, VKA
[edit]"Orbitals in Organic Chemistry"
- Neighbouring group participation in SN2 reactions: faster with allyl, benzyl and α-carbonyl halides due to stabilisation of the carbon 2p orbital in the transition state
- Stereochemistry of E2 eliminations (requirement that acidic hydrogen and leaving group are antiperiplanar) arises due orbital geometry
- Order of stability of carbocations (3° > 2° > 1° > methyl) is due to hyperconjugation of C-H σ bonds with empty 2pz orbital of the cationic trigonal planar carbon
- Grob fragmentation
- C-C σ bond donates its electron pair into a C-OMs σ* orbital, breaking the C-C and C-OMs σ bonds
- OH lone pair donates its electrons into C-C σ*, forming C=O π bond and breaking the C-C σ bond
- See also Eschenmoser fragmentation and this 2010 review: Synthetic Applications of the Carbonyl Generating Grob Fragmentation
- Orbital effects on conformation
- C-H σ → C-H σ* hyperconjugation as a contributory factor in the preference of ethane for a staggered conformation
- C-H σ → C-F σ* hyperconjugation is much better than C-F σ → C-F σ*, so 1,2-difluoroethane prefers a gauche conformation. This is called the gauche effect.
- Anomeric effect — pyrans
- Important in carbohydrate conformation
- Esters prefer the conformation that allows the non-carbonyl oxygen to donate a lone pair into the carbonyl C-O σ* orbital
- Lactones normally cannot access this conformation, so they are more reactive
- Diels-Alder reaction (regio- and stereoselectivity)
- Cyclopentadiene > furan (often reacts reversibly because aromaticity is lost) > pyrrole (poor diene for DA)
- Acyclic dienes (e.g. 1,3-butadiene) need to adopt the s-cis conformation to react, although the equilibrium lies heavily in favour of s-trans
- Isoprene reacts better as its methyl group sterically clashes with a methylene H in both s-trans and s-cis conformations, pushing the equilibrium further towards s-cis than usual
- 4-methylpenta-1,3-diene reacts very poorly as its s-cis conformation is heavily sterically crowded
- 3-methylenecyclohex-1-ene does not react at all, as it is locked in the s-trans conformation
- Dienophiles with electron withdrawing groups work best (lowest LUMO)
- Maleic anhydride and maleate esters are excellent, methyl acrylate very good, 1,3-butadiene OK
- Dienes with electron donating groups work best (highest HOMO)
- Danishefsky's diene is exceptionally good
- Woodward–Hoffmann rules
- for an allowed thermal cycloaddition process, (4n+2) π electrons are required
- [2+2] cycloadditions are symmetry forbidden, fail thermally, proceed photochemically
- Endo rule
- Lewis acid catalysis (e.g. SnCl4) lowers LUMO of dienophile, increasing rate and selectivity
- Nitrone, ozone and nitrile oxide [3+2] cycloadditions
- Nitrone + alkene → isoxazolidine
- Ozonolysis: first step is O3 + alkene → molozonide
- Nitrile oxide + alkene → isoxazoline
- Sigma (σ bonds), tropic (movement)
- The main class is [3,3] sigmatropic rearrangements
- Cope rearrangement (all carbon in the cyclic TS)
- Claisen rearrangement (oxygen in the cyclic TS)
- oxy-Cope rearrangement (oxygen present, but outside the all-carbon cyclic TS)
Baldwin's rules for ring closure
[edit]Exo-tet and exo-trig
[edit]- All exo-tet cyclisations are favoured
- All exo-trig cyclisations are favoured
Endo-tet and endo-trig
[edit]- 5- and 6-endo-tet cyclisations are disfavoured
- 3-, 4-, and 5-endo-trig cyclisations are disfavoured; 6- and 7-endo-trig are favoured
Dig
[edit]- All endo-dig cyclisations are favoured
- 3- and 4-exo-dig are disfavoured; 5-, 6- and 7-exo-dig are favoured