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      Photoelectrochemical process for conversion of stryene dirivetatives. -CHEM GUY  Bookmark   


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Author  Topic:   Photoelectrochemical process for conversion of stryene dirivetatives. 
CHEM GUY
Member   posted 11-17-1999 05:45 PM          
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Okay this is an interesting topic that I feel will open up the door
to direct amination of olefins, but most defintily this article shows
one how to photointitate the formation of a ketone or aldehyde of
olefins, styrene and probably its dirivetives.
From "Topics in Organic Electrochemistry", edited by Albert J. Fry
and Wayne E. Britton, Library of Congress # QD555.5 T66 1986,

"Organic Photoelectrochemistry
...
The term photoelectrochemistry refers to the study of redox reactions
which are intiated or assisted by the absorption of a photon at or
near the surface of an electrode [1]. ...

1.4 Photoelectrochemical Redox Reactivity

The ability of light to bring about enhanced redox activity has long
been recognized. It is, however, only since Fujishima and Honda's
discovery in the early seventies that semiconductor surfaces could be
used to effect difficult redox equilibrium, e.i., to generate hydrogen
gas from water, that the importance of photochemical assistance to
normal electrolytic procedures was recognized [6]. ...
... little work has been reported which describes redox reactivity of
organic subrates. Since the ablity to control redox reactivity will be
significantly influenced by the hetrogeneous nature of the elctrode-
electrolyte interface [8], further studies in this area promise to
provide exciting frontiers for both the control of organic redox
reactivity and for the discovery of new synthetic routes of potiental
interest to organic chemists.

2. Principles of Photoelectrochemistry

Photoelectrochemical events can be initiated either by photoexcition
of the electrode material or by sensitization of the organic substrate
at the surface of a poised electrode. In this article, we will discuss
both types of photoactivation of redox processes.

...

2.2 Band Bending

When a semiconductor is immersed in an electroltye, charge transfer
occurs at the interface because of the difference in the tendency of
the two phase to gain or lose electrons. This flow of electrons can
be attributed to differences in electrochemical potential of the two
phases. When a semicondutor is so immersed, an electrical feild forms
at the surface with a depth ranging form 2 - 500 nm. The bands of the
semiconductor thus bend in the response to the electronic demand of
the redox couple present in the electrolyte, Fig 5.

Band bending will occur until the bulk properties of the elctrode
equilibrate with the redox potential of the elctrolyte, which in turn
is governed by the Nernst equation. ...

An n-type semiconductor, one doped with an electron donor so that
extra elctrons populate the conduction band, forms it's electrical
feild so that electrons flow from the interface toward the bulk of the
semiconductor. Thus photoexcition will cause the hole, or oxidizing
equilivalent, to migrate toward the surface as the photogenerated
electron, promoted by light absorption to the conduction band, moves
toward the bulk of the electrode. As a result of this electronic
motion, n-type semiconductors become highly oxidizing and
photoelectrochemical oxidations of absorbed organic substrates become
conceivable.

... In princilpe, any organic material having an oxidation potential
less positive than the valence of the semiconductor should act as an
electron donor to an excited n-type electrode. Electron donatoion
from a nuetral organic substrate will generate a radical cation, a
reactive species whose chemistry has been extensitively discussed
[10]. ...

... reductions will be assisted by photoexcition of p-type
semiconductors.

... Reduction should occur for any absorbed organic substrate whose
reduction potential lies less negative than the conduction band of the
semiconductor in question, ...

...

3.1.2 Photoelectrocatalytic Cells

Observable chemistry occurs only if the photoinduced electron exchange
is followed by a secondary chemical reaction which renders the redox
process irreversible. ...

...

4.3.1b. Olefin Oxidations. Olefin oxidations have long been studies
electrochemically. That such olefin oxidations can be accomplished on
irraditedsemiconductor electrodes is obvious from the the anodic
photocurrent observed when amperometric measurements between an
irradiated n-type titanium dioxide single crystal and a mettalic
counter electrode are monitored [50]. ... As any organic substrate
possessing an oxidation potential less positive than +2.4 V (vs. SCE),
diphenylethylene (Ep + +1.6 volts), can serve as an electron donor
leading to anodic photocurrent. Single electron transfer from this
substrate would generate a radical cation from which subsequent
reactivity may be anticipated. Photogenerated electrons promated to
the conduction band upon photoexcition with long ultraviolet
wavelengths are availible to effect reduction at the counter
electrode. Exothermic electron transfer from the conduction band to
oxygen, generating superoxide, should be favorable, since O2
reduction occurs at a potential less negative than the conduction band
of TiO2.

If this irradiation is conducted on a semiconductor powder rather than
in a photochemical cell, the olefin radical cation and superoxide
would be generated on sites quite near each other on the
photocatalyst's surface. It is, therefore, not surprizing to find
that oxygen incorpation into hydrocarbon backbone is relatively
efficient.

In fact, irradiating suspensions of titanium dioxide powders in
non-aqeous solvents, for example acetonitrile, containing 0.01 M
concentrations of an oxidizable olefin leads to very efficent
oxidative cleaveage of the carbon-carbon double bond, forming the
carbonyl group. With 1,1-diphenylethylene, Equation 15 [ 50, 51],

Ph-(C=CH2)-Ph --TiO2*/Ch3CN,O2--> Ph-(C=O)-Ph ~100%

oxidative cleavage product is isolated in virtually quanitatively
chemical yeild. At partial conversions, small amounts of expoxides
and ring opened compounds are also formed, Equation 16. ...

Many other olefins can act similarly oxidatively cleaved to carbonyl
groups (table 4) [50]. In general, high chemical yeilds of products
can be isolated. In the one striking exception to this generalization,
semiconductor photocatalysis initiates the polymerization of styrene.
...

Table 4
TiO2 - Photocatalysis of Olefin Oxidative Cleavage (a)

Olefin / Percent conversion / Products (chemical yeilds%) (b)

...

Ph-HC=CH-Ph / 85 / Ph-(C=O)-H (33%) (c)...

Ph-(C=CH2)-CH3 / 50 / Ph-(C=O)-CH3 (100%)

Ph-CH=CH2 / 43 / Ph-(C=O)-H (17%) (d)

...

(a) Irradiation in a Rayonet photochemical reactor equipped with
RPR-balck lights, [wavelength] = 350 +/- 30 nm; room tempature;
irradiation time 6 h.

(b) Yeilds reported are based on dissappearing material.

(c) Yeild of benzaldehyde is based on the expection of 2 mol of
oxidation product derived from 1 mol of stilbene.

(d) Oligomeric products accounted for the remainder of consumed
material.

...

[1]- For reveiws of the princilpes of photoelectrochemistry, see (a)
... J. Photochem., 10, 50, (1979);... Science, 207, 139, (1980);
Electroanalytical Chemistry, 1979, p. 1; Accts. Chem. Res., 12, 303,
(1979); J. Chem Ed., 60, 325, (1983); Faraday Disc., 70, 7, (1970);
Catal. Rev., 3, 1 (1969); J. Chem. Ed., 60, 327 (1983)

[6]- Nature, 238, 37 (1972)

[8]- Photeffects at semiconductor-Electrolyte Interfaces, J. Amer.
Soc. Sympos. Ser. No. 146, American Chemical Soceity, Washington 1981.

[10]- Adv. Phys. Org. 13, 156 (1976).

[50]- J. Am. Chem. Soc. 103, 6757 (1981); J. Photocham. 17, 119 (1981)

[51]- Tetraherdon Lett. 21, 467 (1980)
"

Okay what do you think?


 
Wizard X
Moderator   posted 11-18-1999 07:48 PM          
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"Okay what do you think?", you ask Chem Guy !
I think it is too difficult and impractical for the average bee. I can post endless electrochemical reaction, methods etc, but the reality is most bees can not utilizes this for practical purposes.

Good work anyway!


 
CHEM GUY
Member   posted 11-18-1999 08:04 PM          
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It doesn't seem to difficult. There isn't even a photoelectrochemical cell made for the sited conversion of styrene. The article states:
"In fact, irradiating suspensions of titanium dioxide powders in
non-aqeous solvents, for example acetonitrile, containing 0.01 M
concentrations of an oxidizable olefin leads to very efficent
oxidative cleaveage of the carbon-carbon double bond, forming the
carbonyl group." ; then later in table 4 it lists styrene being converted in low yeilds to a oxidative cleaved aldehyde product. There is no mention of an apparatus, just TiO2 powder in a non-aqeous solvent with the olefin. Sounds simple to me. I'm going to go to the library to get those other references so as to better gauge the importance of this, I will post the results when I can.
As for a better prompt I dug up a question and proposal I directed towards Rev Drone earlier in the month:

These are the articles I looked up in the library concerning the one-step photoamination of safrole as listed on the Rhodium site: Journal of the American Chemical Society, 1973, 95, 4080-4081; Tetrahedron Letters, 1993, 34, 5131-5134; Tetrahedron, 1994, vol. 50, no. 31, 9275-9286; Bulletin of the Chemical Society of Japan, 1998, vol 71, no 7, 1655-1660; and some of the sited references within. From these articles I found out that:

The reaction with just isosafrole and NH3 would NOT take place, as many of the articles almost explictatly stated. But the articles never metions anything about NH4X being used or attempted. The patent after all does list the NH4X as the catalyst for this reaction.

As I understand it, the mechanism of the DCB is to take the electron from the excited singlet state on the isosfrole and usher it to the NH3 where the nucleophilic addition then takes place:

D = isosafrole
* = electron in excited singlet state
(+/-) = the net charge on the ion

D + hv (light of proper frequency) --> D*

D* + DCB (p- or m-) --> D(+)/DCB*(-)

DCB*(-) + NH3 --> NH2*(-)H + DCB

D(+) + NH2*(-)H --> DHNH2 (antimarkinokovian addition)

The DCB only serves as a stablizer of the D(+) and a donator to the NH3. All the incident light is absorbed by the D. Some of the articles list photosensitizers that apparently just stablized the D(-) and prevented a lot of polymerzation and by products. What I was thinking was that maybe a way to get around the DCB route would be electrically.

Follow me here, D is put on a metal plate which is connect to another metal plate by a wire. The D is then irraditated with hv. This produces D*. ? This might create a potential difference (a voltage) across the plates? The other plate is then come into contact with only the gaseous NH3 which picks up the electron. The gaseous NH3 is then allowed to come into contact with the D. The potential problems with this idea are plentiful, but it gives a good starting point for invention.

Problem 1) the plates might not gain a potential voltage across them. But maybe by adding a battery across the plates one could "suck" the electron towards the plate and to the other plate.

Problem 2) The D(-) probably won't be very stable and will tend towards polymerzation, due to free radicals, but as I stated earlier some of the article listed stablizers that increased the yeild with p-DCB because the stabilzer stabilized the D(-) independently.

Problem 3) No electrolytes in the solutions to decrease the resistance. Introductions of electrolytes might fuck things up, but possibly not. After all the oxidation potentials of the substitued-phenylpropenes were very low, about 1.1 V and below

Now with the advent of afore mentioned article I believe that my proposal is sound and only news to be tested...


FreeFlow
Member   posted 11-19-1999 05:15 PM          
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what the hell is going on?????
 
Wizard X
Moderator   posted 11-19-1999 06:52 PM          
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Chem Guy : I think isosafrole converted to an epoxide, or a glycol and then rearranged to MDP2P would be easier.
Acetonitrile, NaBr and two Pt electrodes.

Look at this journal Ref : Journal of Electrochemical Society, March 1964, pg 335.
Topic :
Oxidation of 3,4-Dimethoxypropenylbenzene at a Rotating Pt Electrode.

This will answer may of your oxidation and amination of propenylbenzenes.


Wizard X
Moderator   posted 11-19-1999 06:57 PM          
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"may" should be "many".
This will answer many of your oxidation and amination of propenylbenzenes.


CHEM GUY
Member   posted 11-21-1999 06:59 PM          
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Wizard X,
I like the least amount of steps as possible. Ultimately, I'm trying to hunt down a way to directly aminate the olefin. But for the meantime I'm settleing for a two step shot: Olefin --> Aldehyde ---> Amine .

This is another passage from From "Topics in Organic Electrochemistry", edited by Albert J. Fry and Wayne E. Britton, Library of Congress # QD555.5 T66 1986, concerning this pathway:

"2.4 Powders

A recent extention of these ideas for practical applications of
photoelectrochemistry is found in the use of a 'short-circuited'
photoelectrochemical cell [1a, 17] prepared by the the deposition of
an inert metal (of low over-voltage characteristics) on a powders
semiconductor. ...

In fact, the native powders themselves can sometimes function as active
photocatalysts. A requirement for effective photoelectrochemical
conversions on untreated surfaces is that either the oxidation or
reduction occurs readily on the dark material, thus scavenging one of
the photogenerated charge carriers.

An obvious advantage of such powders is found in their simplicity,
their portiblity, their low cost, and the freedom from the necessity
of extensive electrochemical appariti to observe photochemical redox
reactions. ..."

The article also lists some common semiconductors used: TiO2, SnO2, ZnO, SrTiO3, CdS, CdSe, CdTe, GaAs, GaP, InP

I'm want to find other ways to directly aminate, and I'm looking for any help I can find.


 
CHEM GUY
Member   posted 11-24-1999 05:02 PM          
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This is my proposal for a photo-electro-chemical direct amination of a compound similar to Isosafrole.
Irradite an anode made of a semiconductor (material choice is in above post) with a high pressure Hg lamp. (You can illuminate the back side of the anode, AKA the side not facing the cathode)

Have a metal cathode that is not illuminated.

The electrolyte solution is:
Isosafrole-like compound and liquid NH3, or
Isosafrole-like compound and acetonitrile with NH3 dissolved in it.

Place the electrodes close together, about 1 cm or less. Or perhaps have only the anode in the solution and have only NH3 gas touch the cathode to insure that the olefin won't be reduced.


My assumitions:
1)Now, I'm guessing that the addition is going to be anti-markinikovian.

2)That the current for the photoexcited state of the Isosafrole like compound will be enough to excite the NH3.

3)That this will work.

Please give me info. I would like to have this become a reality...


elias
Member   posted 11-26-1999 09:41 PM          
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From Organic Chemistry, Structure and Function, Third Edition by Vollhardt and Schore.
Focusing in on pages 504-506. Basically describes how electrophilic oxidizing agents are capable of delivering oxygen atoms to the pi bond, thereby producing oxacyclopropanes, vicinal "syn" & "anti" diols, and carbonyl compounds by complete cleavage of the double bond.

Basically the OH group in peroxycarboxylic acids (an example of which would be MMPP->
Magnesium monoperoxyphthalate), contains an electrophilic oxygen. These compounds react with alkenes by adding this oxygen to the double bond to form oxacyclopropanes. So we end up with two products, the one we want, the oxycyclopropane and the other, the carboxylic acid. The reaction usually proceeds at room temperature in an inert solvent, such as chloroform, DCM, or benzene.

The treatment of oxacyclopropanes with water in the presence of catalytic acid or base leads to ring opening to the corresponding vicinal diols.

Sorry for the background but his aim is a good one and maybe this will help more people get in on the discussion.


 
CHEM GUY
Member   posted 12-01-1999 04:10 PM          
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Here is a list of the oxidation potentials of several compounds related to isosafrole. The oxidation of isosafrole is probably very close to these potentials which means that isosafrole is probably below the 1.6 V (see above text) threshold for a semiconductor photo-initiated electrochemical redox reaction.
From: Tetrahedron, 1994, vol. 50, no. 31, 9275-9286

"
2-(R1), 3-(R2), 4-(R3)- C6H2-CH=CH-CH3 ...

1: R1 = OMe; R2 = R3 = H
2: R2 = OMe; R1 = R3 = H
3: R2 = R3 = OMe; R1 = H
4: R3 = OMe; R1 = R2 = H
"
The reduction potentials of these compounds are listed as:
1 = 0.86 V
2 = 1.18 V
3 = 0.82 V
4 = 0.93 V


 
CHEM GUY
Member   posted 12-09-1999 07:20 PM          
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Let's get some's opinoin... liike drone's or wizard X's or Rhodium's,.... etc.
 
Roosteer
unregistered   posted 12-16-1999 11:46 PM           
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I approve and like being surrounded in the thread of ideas  All intertwined and woven together this is some kinda murral ya got there
 
Wizard X
Moderator   posted 12-17-1999 06:56 PM          
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Chem Guy: The only way you will know for certain the oxidation voltage for safrole or isosafrole, is to do it experimentally using Voltammetry or Polarography electrochemical techniques.
 
CHEM GUY
Member   posted 12-18-1999 03:49 PM          
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Wiz,
I agree that the only way to know the oxidation potential is to measure it, but one can make an educated guess knowing the oxidation potential of similar compounds. My only point was to show that the oxidation potential of safrole (and other related compounds) was in the ballpark arena of below 2.4 V. Beacause, " As any organic substrate
possessing an oxidation potential less positive than +2.4 V (vs. SCE),
diphenylethylene (Ep + +1.6 volts), can serve as an electron donor
leading to anodic photocurrent. Single electron transfer from this
substrate would generate a radical cation from which subsequent
reactivity may be anticipated."

That was the only reason for that tid bit of info.


 
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