dormouse (Member)
04-19-00 16:20
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Author  Topic:   oxidative amination revisted: could this be it? 
rev drone
Member   posted 02-16-99 04:54 PM          
Now people are familiar with my heart-felt ramblings on photocatalyzed oxidative aminations of propenylbenzenes (a vastly underrepresented category of very useful chemistry, I feel), but perhaps you'll be even more surprised than I was when I discovered this: the vast
rev drone
Member   posted 02-16-99 06:02 PM          
The Hive is acting wierd! It cut me off mid-post (normally when it does that, it just chews up whatever I was working on, but this is the first time its decided to do something strange like that.)
Anyways: oxidative amination, the addition of amines across carbon-carbon double bonds. A thousand and one practical household uses... if it can be done easily enough.

As I was saying, photocatalyzed oxidative amination is high-yielding, but people seem to have a disdain for the exotic catalysts.

There are some glimmers of hope. In all cases, something comes along, and pulls some oelctron density off, either by hook or by crook. When it does this, it leave the amine wide open for attack. Some of this can be seen in the following reaction:

Reaction ID 4860649
Reactant BRN 1209228 formaldehyde
3541409 N-methyl-hydroxylamine; hydrochloride
5735136 C12H17N
Product BRN 7919086 C13H22N2

Reaction Details

Reaction Classification Preparation
Reagent 1.) K2CO3; 2.) NiCl2-NaBH4
Other conditions 1.) CHCl3, H2O, 60 deg C, 6 d
Note 1 Multistep reaction
Ref. 1 6086106; Journal; Bell, Kathryn E.; Coogan, Michael P.; Gravestock, Michael B.; Knignt, David W.; Thornton, Steven R.; TELEAY; Tetrahedron Lett.; EN; 38; 49; 1997; 8545-8548;

As you can see, its quite something. This isn't entirely oxidative amination, but its a good step in the right direction (not to mention that it uses some of my favorite reagents.)

Another of this genre that is worth noting is:

Reaction ID 1767860
Reactant BRN 136380 5-allyl-benzo<1,3>dioxole
Product BRN 150196 2-benzo<1,3>dioxol-5-yl-1-methyl-ethylamine
150252 3-benzo<1,3>dioxol-5-yl-propylamine

Reaction Details

Reaction Classification Preparation
Reagent 1.) BH3*THF, 2.) aq. NH4OH, NaOCl
Other conditions 1.) THF, 0 deg C to r.t., 1 h, 2.) 0 deg C, 15 min
Note 1 Yield given. Multistep reaction. Yields of byproduct given
Ref. 1 5643971; Journal; Kabalka, George W.; Sastry, Kunda A. R.; McCollum, Gary W.; Yoshioka, Hiroaki; JOCEAH; J.Org.Chem.; EN; 46; 21; 1981; 4296-4298;

This one I have to admit I haven't looked up yet, but if its half as good as it looks, this would really be something, and the use of a lewis acid (with safrole, no less!) to catalyze things is quite interesting.

Regardless, the point that is most important that I want you to take into your minds is that these oxidative aminations are just that, oxidations. Why shouldn't the same transition metal catalysts that encourage the oxidation of alkenes into ketones, epoxides, and diols do the same thing for electron-rich methylamine?

Finally, I also wanted to add one more example of photo-catalyzed oxidative amination:


Reaction ID 4089573
Reactant BRN 605259 isopropylamine
1859179 2-methyl-trans(?)-stilbene
Product BRN 7132626 C18H23N
7133324 C18H23N

Reaction Details 1 of 2

Reaction Classification Preparation
Reagent 1,4-dicyanobenzene
Other conditions Irradiation
Note 1 Title compound not separated from byproducts
Ref. 1 5949362; Journal; Lewis, Frederick D.; Bassani, Dario M.; Burch, Eric L.; Cohen, Bliss E.; Engleman, Jeffrey A.; et al.; JACSAT; J.Amer.Chem.Soc.; EN; 117; 2; 1995; 660-669;

Reaction Details 2 of 2

Reaction Classification Preparation
Reagent 1,4-dicyanobenzene
Other conditions Irradiation
Note 1 Title compound not separated from byproducts
Ref. 1 5949362; Journal; Lewis, Frederick D.; Bassani, Dario M.; Burch, Eric L.; Cohen, Bliss E.; Engleman, Jeffrey A.; et al.; JACSAT; J.Amer.Chem.Soc.; EN; 117; 2; 1995; 660-669;

-the good reverend drone

Member   posted 02-18-99 01:37 PM          
Are you sure that the hydroboration / amination will take place at the right C atom? The addition is sterically controlled and the difference between phenyl and methyl may not be high enough.
rev drone
Member   posted 02-18-99 03:45 PM          
The addition is very selective. Plenty of examples already exist of this very thing.
There is a huge difference in a methyl and a substituted aryl group. The size difference *is* big, but steric hindrance is not what I believe necessarily is the sole reason for the selective addition. The better reason, I believe, is that the aromatic group can stabilize an ionic intermediate better than a methyl. In any case, plenty of examples exist, showing exactly the type of addition we want, and this is how mechanisticly I'd rationalize it. Imagine the two scenarios:

1. The amine attacks the double bond, and covalently bonds to the benzylic position, leaving the aliphatic (and less stable) 2-position ionicly charged.

2. The amine attacks the double bond, and covalently bonds to the 2-position, leaving the benzyliuc carbon ioncily charged (which is a far more stable intermediate.)

I'd be very surprised if a synthetically significant amount of benzylicly aminated byproduct was produced.

-the good reverend drone

Member   posted 02-19-99 09:58 AM          
Why can't I post here?
Member   posted 02-19-99 10:09 AM          
What happened?
I tried to post here yesterday and I didn't work. I gave it a shot today and it worked! I didn't even need the special password, just my regular password is fine!

NEway, I checked out JOC 46: 4296-8('81). It looks very nice and the reagents are pretty common.

Thus, safrole is hydroborated and oxidised with bleach in the presence of one mole ammonia to give 1-(3,4-methylenedioxyphenyl)-3-propylamine in a 96% yield! If instead of safrole isosafrole is used, a good yield of MDA can be expected.

Unfortunately this ain't the whole story. When safrole is hydroborated, three moles of safrole react with one mole borane to form to the trialkylborane (R3B). When oxidising this borane in the presence of ammonia, only 2 of 3 alkylgroups will react and 1 alkylgroup will stay bonded to the borane. Thus the maximum yield from 3 moles of safrole is 2 moles of product. This is where the 96% yield is based on, 96% of 2 moles, so actually it's more like 60-65% yield, based on the amount of safrole you started out with.

Too bad. Lr/

Member   posted 02-19-99 02:28 PM          
Below I give you results from hydroborations / hydroperoxide oxidations to alcohols. The hydroboration is the directing step, the oxidation only replaces the added boronhydride with onother group. So we may transfer these results to our hydroboration / amination reaction. As you can see, the alpha substitution is preferred. The reason is that the hydroboration is indeed sterically controlled (but electronic factors also play a role as you can see for the effect of different phenyl substituents). The yields are regularily over 80%, so the reactions Rhodium mentioned seem not to appear with these substituted styrenes and (trans-) propenylbenzene. Rhodium, do you have the references?
The values are in the following order:
alpha = 1-position (= bad), beta = 2-position (= good), (total yield of alcohol)

J. Org. Chem. 50, 3039 1985 H. Kwart:
cis-propenylbenzene: 89%, 12% (60%)
trans-propenylbenzene: 93%, 7%

Synthesis 1029 1982 C. Camacho:
propenylbenzene: 91%, 9% (81% - 85%)
styrene: 19%, 81% (82% - 84%)

J. Am. Chem. Soc. 82, 4708 1960 H. C. Brown
styrene: 20%, 80% (91% - 94%)
p-MeO-styrene: 9%, 91%
p-Me-styrene: 18%, 82%
p-Cl-styrene: 35%, 65%

J. Am. Chem. Soc. 88, 5851 1966 H. C. Brown:
styrene: 19%, 81% (87% - 93%)
m-NO2-styrene: 37%, 63% (87% - 93%)

o-MeO-styrene: 14%, 86% (87% - 93%)
m-MeO-styrene: 19%, 81% (87% - 93%)
p-MeO-styrene: 7%, 93% (87% - 93%)

o-Cl-styrene: 26%, 74% (87% - 93%)
m-Cl-styrene: 30%, 70% (87% - 93%)
p-Cl-styrene: 27%, 73% (87% - 93%)

o-CF3-styrene: 38%, 62% (87% - 93%)
m-CF3-styrene: 32%, 68% (87% - 93%)
p-CF3-styrene: 34%, 66% (87% - 93%)

Member   posted 02-19-99 02:46 PM          
But if we use sterically hindered boranes (like disiamylborane) the world may look totally different. For good reviews on hydroborations have a look into:
Borane Reagents, A. Pelter, K. Smith, H. C. Brown, Academic press 1988
Hydroboration, H. C. Brown, W. A. Benjamin, Inc., 1962
Organic Synthesis Via Boranes, H. C. Brown, John Wiley & Sons, 1975

Member   posted 02-19-99 04:31 PM          
Ber. 98, 904 1965 T. Kauffmann: Addition of hydrazine to alkenes yielding alkylhydrazines; propenylbenzene 63%, anethol 71%, isoeugenol 59% (92% raw yield). These may me reduced to amines and reductively alkylated to methylamines...
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