Photochemistry of alkenes: PDF
Preview Photochemistry of alkenes:
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
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