Photochemistry of alkenes: the short version
The main p,p* absoption of simple alkenes is located at l < 200 nm, thus limiting the usefulness of most conventional light sources.
• carbene leading rearrangements hn p,s Rydberg state
* hn cis-trans isomerizations
p,p* lowest state
12.1 Spectroscopy
• The two lowest absorptions of alkenes involve p,p* and p,3s (Rydberg) states • For ethylene the two transtions are very close and around 170 kcal/mol • For substituted alkenes the Rydberg transition tends to be at lower energies. • For example for cis-2-butene the energies based on the absorption spectra are: • singlet p,p* 164 kcal mol • singlet (Rydberg) 141 kcal/mol • triplet p,p* 97 kcal mol
• The lowest triplet state has p,p* character and energies around 94-99 kcal/mol
12.2 The cis-trans (Z-E) isomerization
R H H R hn
H R H R
S1-cis S1-trans S1-p state
T1-cis T1-trans
T1-p state
So-cis So-trans
12.3 Control of the photostationary state
The photostationary state is a wavelength dependent property [t] ec f ct = [c] e f s t tc
Control of the photostationary ratio • excitation ratio • decay ratio
12.4 Synthetic applications: A convenient way from the more stable (trans) to the less stable (cis) isomer
Example: cinnamic acid
hn H3CO H3CO COOH COOH
cis isomer produced in 74% yield
12.5 Stilbenes: A popular example with photochemists
Lifetimes of the S1 state: 70 ps for trans and 1.4 ps for cis stilbene
12.6 Fluorescence lifetimes controlled by the ability to rotate the double bond
F fluorescence Ph 0.016
Ph 0.03
Ph 0.43
Ph 0.27
12.7 Ketones can promote cis-trans isomerization by two different mechanisms
SENSITIZATION
Energy transfer from a triplet ketone (why not a singlet?) can lead to triplet alkene that then follows the normal isomerization pathway, independent of the ketone sensitizer
12.8 Ketones can promote cis-trans isomerization by a biradical mechanism
Schenck mechanism
Paterno-Buchi biradical
R1 O R1 O R1 hn O + + R2 Ph CH3 Ph CH3 R2 R2 Ph CH3
R1 O minor Ph R2 CH3
12.9 Trans cycloalkenes are highly reactive
Ph
OCH3 Ph CH3OH Ph
H H Ph
Ph 4+2 addition
Followed by hydrogen shift
12.10 Alcohol addition: two distinct possible mechanisms
hn, singlet Ph Ph Ph or triplet CH3OH OCH3
H
hn, singlet only Ph Ph CH OH 3 OCH H 3 H
Direct protonation of the triplet state is spin forbidden
12.11 Sigmatropic shifts: pericyclic reactions
H H H H H H H R 1 H R1 R2 H R2 H
1,5-sigmatropic shift
Questions of antarafacial, suprafacial, etc., for some other opportunity
12.12 Di-p-methane rearrangement Zimmerman rearrangement
one sp3 center
hn
two p systems
Example:
benzophenone sensitizer
12.13 Electrocyclic reactions
disrotatory conrotatory
photochemically thermally allowed allowed
Woodward and Hoffmann rules
12.14 Alkene cyclizations
A B Simple cyclizations open the door for a variety of synthetic routes D C
A B A B A B
D C D C D C
A B C B Aromatic analogue
12.15 The Schenck or ene reaction revisited
H3C CH2 H3C CH3 1O 2 (A) H3C CH3 H C CH 3 3 OOH
O H O H O O
12.16