Photochemistry of alkenes:

16 Pages·1999·0.03 MB·English

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] e c f ct
=
[c] e f
t tc
s
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
H CO H CO
3 3
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
O R1 hn O R1 O R1
+ +
R
Ph CH3 R2 Ph CH 2 Ph CH3 R2
3
R1
O
minor
Ph
CH R2
3
12.9
Trans cycloalkenes are highly reactive
Ph
OCH
3
Ph CH3OH
Ph
H H
Ph
Ph
4+2 addition
Followed by
hydrogen shift
12.10