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.