FriendlyFinger
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Trans, cis, or both, Beta-Methylstyrene?
Hi there,
Got some p1p and turned it into phenylpropanol with NaBH4 then dehydrated with dry KHSO4, distilling the propenylbenzene at the same time. Bath
tempretures up to 210 deg C. Then Washing then fractional vacuum distillation.
What I want to know is. Do I get trans, cis, or both. Also will anything but trans screw up the performic oxidation? Because I've had a lot of
problems here.
Thanks FF
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Sergei_Eisenstein
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You'll end up with a cis/trans mixture, though you may have more from one isomer than from the other. Epoxidation of alkenes with peracids is a
syn addition, and the cis/trans orientation is not of much importance regarding the feasibility of the epoxidation.
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Kinetic
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Asarone analogue isomers
Hi FF
Nice to see yet another bee sigining up here.
You will get a mixture of isomers, the predominant isomer being the trans- (E) alkene. This is because the E alkene, and
the transition state leading to this isomer, is more stable than the alternative cis- (Z) alkene and it's corresponding
transiton state (for steric reasons).
Hence the stereoselective elimination will favour the E-alkene as product.
Checking a paper I have on the dehydration of 1-(2,4,5-trimethoxyphenyl)-1-propanol to asarone using anhydrous CuSO4, the authors report the product
is a 93:7 mixture of E:Z isomers. I imagine that the ratio will be similar in your case. A representative procedure from this paper
is as follows:
Synthesis of 2-((E)Prop-1-enyl)-1,3,4-trimethoxybenzene (5).
[...] 1.8 g of pure 1-(2,3,6-trimethoxyphenyl)-1-propanol was obtained. The alcohol and anhydrous CuSO4 (1.50 g, 9.3 mmol) in 50 mL of dry toluene
were refluxed for 2 h. The inorganic salt was filtered off and the filtrate was evaporated to dryness. The residue was purified by column
chromatography (carbon tetrachloride-ethyl acetate mixture, 20:1, v/v) to give compound 5 (1.4 g).
Reference: [J. Med. Chem., 43, 3671-3676 (2000)
Since you have predominantly trans-beta-methylstyrene, it is likely to be another factor leadng to your faliure to oxidise it to the glycol
(and other similar products). Maybe you could try a different procedure, such as oxidation to the epoxide followed by isomerisation with LiI (http://rhodium.moppy.net/www.rhodium.ws/chemistry/epoxide.ht... for the oxidation and http://rhodium.moppy.net/www.rhodium.ws/chemistry/phenylacet... for the rearrangement).
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FriendlyFinger
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Very refreshing info
Kinetic, thanks for that nice info and reference, I'll look into it. I got paranoid about the quality of my propenylbenzene when I read a
purification proceedure where they distill it from powdered NaOH under nitrogen.
I wouldn't say that the performic was a complete failure. In my experence, the modified performic using Na2CO3 converted more of the
propenylbenzene into gylcol etc than the performic with NaHCO3, and the best yield of glycol etc was about 50%.
Rearranging that to P2P with tosic and toluene was very good, about 84%. Trying to rearrange with 15% H2SO4 with or without MeOH has been a horible
experience for me. My impression at the hive with this subject was that performic oxidation of Trans-beta-Methylstyrene and the standard rearrangement
to P2P was not really an issue .
You mention that;
"Since you have predominantly trans-beta-methylstyrene, it is likely to be another factor leadng to your faliure to oxidise it to the glycol (and
other similar products). " What do you mean by that?
Thanks, FF]
[Edited on 29-3-2005 by FriendlyFinger]
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Kinetic
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Propenylbenzene
Propenylbenzene is pretty stable, and I wouldn't worry about if your only concern is the inert atmosphere during distillation.
My comment was made with regard to your initial question: | Quote: |
Also will anything but trans screw up the performic oxidation?
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I simply wanted to point out that, even if anything but trans-propenylbenzene did screw up the
performic oxidation (unlikely, as Sergei_Eisenstein points out), then this still wouldn't account for your problems as you have predominantly the
trans isomer, which is known to work. It was not meant to imply that for some reason 'anything but trans' would screw up the
product.
I am interested to hear more on your success with the rearrangement using tosic acid. I too have had problems rearranging a related epoxide
(1-(5-indanyl)-2-methyloxirane) using sulfuric acid in methanol. It apparently worked reasonably well, but an NMR of the distilled ketone is pretty
ugly. I never tried reductive amination as I'm not particularly interested in the product.
Though there are some other variables to consider, the method using LiI gives a far cleaner product, essentially pure by NMR. Because of severe
bumping contaminating my recieving flask during distillation I don't have an accurate isolated yield for the latter. I think it would be pretty
good if I were to try it again though; better than the 63% I mentioned in a recent post in, towards the end of the following thread:
http://67.15.145.24/~sciencem/talk/viewthread.php?tid=3006
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