This digression considers reactions sparked by one photon
per coupling. They get the unconcentrated light from lamps or semiconductor light-emitting diodes
, both more mature than lasers of same wavelength:
- ArF, KrCl, KrF, XeI lamps for 193nm, 222nm, 248nm, 253nm;
- AlN diodes for 210nm, AlGaN for 245nm-400nm approximately;
- Hg lamps for 254nm.
from illuminated substituted ethylenes are known. The "Mono-enes" diagram shows that 193nm, 210nm and 222nm, maybe 248nm and 254nm, discriminate enough between the ethylenics to favour the desired product, with a gas pressure adapted to the absorption.
For instance beta-pinene
absorbs 100 to 10,000 times more than pentene and ethylene, so a 1:10 to 1:100 dilution hopefully lets convert its double bond to one cyclobutane more. This is slightly better than hydrogenating it to cis-pinane, as a rocke... oh, you guessed.
show an even better constrast to ethylene on the "poly-enes" diagram. Again, a discriminating wavelength and proper dilution shall excite the diene to react with the ene.
Take as an example the product cyclobutyl-cyclobutane
, which is... a compound that burns well with oxygen and doesn't polymerize in a cooling jacket. I suggested there to make it from butadiene and ethylene:http://www.chemicalforums.com/index.php?topic=50579.msg233808#msg233808
and at least the UV cross-sections support this scenario, the orientations of Lumo and Homo pi as well if textbooks don't cheat. The lamps were affordable to buy and run when I checked.
Better: the cross-section at 193nm of methylbutene C=CC(C)C, taken for want of vinyl-cyclobutane C=CC1CCC1, suggests a one-pot one-lamp
synthesis, where butadiene diluted in ethylene becomes vinyl-cyclobutane and then cyclobutyl-cyclobutane.
I've also put propadiene and isobutene (for want of methylenecyclobutane) on the diagram, if someone wants to try his big luck at spiroheptane...
More exotic: the cited JPL's Pdf suggests on page 4G-24 that tribromomethane can absorb one photon to convert in carbene
CHBr by expelling Br2
when it doesn't split in CHBr2
and Br. The purported photon-to-carbene yield is 0.16 at 267nm and 0.26 at 234nm
- and at 222nm we have strong KrCl lamps.
Such carbenes diluted in alkenes may lead to cyclopropanes without using active metals, where the remaining haloatom is useful or easy to remove. Tribromomethane absorbs 222nm light better than alkenes do (diagram), including beta-pinene (consider also alpha-pinene, carene...); longer waves improve this selectivity.
Avoiding active metals eases the cyclopropanation in big amounts considered therehttp://www.chemicalforums.com/index.php?topic=65186.0
I haven't seen carbenes cited for longer molecules in the Pdf, nor for triiodomethane, and for chlorodibromo neither. If
iodides make carbenes at longer waves than bromides do, the selectivity over alkenes would improve (diagram).
Marc Schaefer, aka Enthalpy