[a confused chiral girl]

hi,

I have a question about the major product that is formed from anti-addition of Cl2 to an alkene, as in the diagram below.

I know that it cannot be a meso compound with configuration 2R-3S because the Cl2 adds on the opposite direction.
How do I know whether it is  (2R,3R)-dichlorobutane or (2S,3S)-dichlorobutane though?

thanks!

[english]

You know that halogen additions result in cyclic halonium ions.  This means the halogen can only add in a concerted reaction.

Once you have the cyclic ion, an anti addition must occur because not only are the halogens relatively large compared to C, but the cyclic nature of the ion (resembling cyclopropane with a susbsituted C for X) blocks the attempt of your nucleophile to approach on the same side of the molecule as your halogen.

When you visualize this, it will help if you draw the alkene like below:



The bond created from the added nucleophile will therefore just be a solid line, with the other groups retaining their relative configurations, also shown below (using bromine as X):



The configurational name will derive from relative priorities of the bonded groups to both sp³ C atoms.  A meso will occur if you can mentally reflect one asymmetric center over the other (both asymmetric centers are bonded to the same unique groups; the molecule then has mirror symmetry).

I don't know if I've helped or rambled.   

(note: the easiest way to detect meso compounds is to focus on the bonded groups to both asymmetric carbons, and their relative configurations [wedges or dashes].  If both centers have the same groups, each with the same wedge-dash relativity, then it's a meso). 

[PRIYA1022]

Anti addition of Cl- can occur at both the carbons( 50:50 chance) of the cyclic chloronium ion. So the product in your case would be a racemic mixture.( 1:1)
By the way, I know about a cyclic bromonium ion, but is a cyclic chloronium ion ever detected or represented in the mechanisms?

[english]

is a cyclic chloronium ion ever detected or represented in the mechanisms?

Br and Cl are about the same. 

Iodine is....funny.  And fluorine is dangerous. 

[PRIYA1022]

Had you taken a trans-2-butene, the product would be meso.

thanks g_english.

[a confused chiral girl]

yes G_EngLish!

your rambling has always helped...you type whatever you are thinking and the reasoning actually  pieces together as I read the posts.

so after reading all your rambling  I decided that is it both (2R,3R)-dichlorobutane or (2S,3S)-dichlorobutane, am I correct? 

[english]

Right; they're enantiomers.

[sjb]

You know that halogen additions result in cyclic halonium ions.  This means the halogen can only add in a concerted reaction.

But do they do this all the time?    See e.g. http://dx.doi.org/10.1021/ol070673n - annoyingly I've just finished a temporary contract and no longer have access to mainstream literature .

Are the conclusions in this paper valid for a wide range of alkenes, or just special ones - can't tell much from the supplementary info.

S

[english]

You know that halogen additions result in cyclic halonium ions.  This means the halogen can only add in a concerted reaction.

But do they do this all the time?    See e.g. http://dx.doi.org/10.1021/ol070673n - annoyingly I've just finished a temporary contract and no longer have access to mainstream literature .

Are the conclusions in this paper valid for a wide range of alkenes, or just special ones - can't tell much from the supplementary info.

S

According to the paper,

The observed 13C NMR isotope shifts are consistent with a combination of large equilibrium shifts and small upfield intrinsic shifts. The presence of equilibrium shifts in both halonium ions indicates that these ions are not closed 1,2-bridged structures. Rather, they are best represented by equilibria of -halocarbenium ions.

I've never heard of this before.  It seems kind of strange in that normally one would think that the carbocation would be destabilized by the "loose" bromine atom.  I guess hyperconjugation gives a more significant stabilizing effect.

It does seem valid for this to happen if the resulting carbocation is reasonably stable (e.g. benzylic, allylic cations, or even 3° alkly cations).


I've attached the image for such a mechanism below just for clarification.

[bluemonster]

but do they do this all the time?  Huh  See e.g. http://dx.doi.org/10.1021/ol070673n - annoyingly I've just finished a temporary contract and no longer have access to mainstream literature Angry.

Are the conclusions in this paper valid for a wide range of alkenes, or just special ones - can't tell much from the supplementary info

sorry, but I dont think so ! In this paper, only study  about tetramethylethylenechloronium and tetramethylethylenebromonium ions, you  shouldn't envelop for a wide range of alkenes !
I think, for structure of intermediates of electrophilic addition reactions alkenes, each case has a evident experiment separately !

Two of the principal points at issue in the description of the mechanism for a given reaction are:
1.   Is there a discrete positively charged intermediate, or is the addition concerted !?
2.   If there is a positively charged intermediate, is it a carbocation or a bridged halonium ion ?
   For brominations, anti addition is preferred for alkenes that do not have substituent groups that sould strongly stabilize a carbocation intermediate. When the alkene is conjugated with an aryl group, the extent of syn addition becomes greater, and syn addition can become the dominant pathway.
   Chlorination is not as stereospecific as bromination, but it tends to follow the same pattern.
 

   A freely rotating open carbocation would be expected to give both the syn and anti addition products. If the principal intermediate were an ion pair that collapsed faster than rotation about the C-C bond, syn addition could predominate.
 

Whether a bridged intermediate or a carbocation is involved in bromination depends primarily on the stability of the potential cation. Aliphatic systems normally go through the bridged intermediate, but styrenes are borderline cases. When the phenyl ring has electron – releasing sutstituents, there is sufficient stabilization to permit carbocation formation, whereas electron-attracting groupos favor the bridged intermediate. As a result, styrenes with electron-attracting substituents give a higher proportion of the anti addition product.
   Substituent effects on addition reactions of stilbenes also give insight into the role of bridged in versus nonbridged carbocation intermediates. The compounds react with second-order kinetics in protic solvent. In aprotic solvents, stilbene give clean anti addition, but 4,4’-dimethoxystilbene give a mixture of the syn and anti addition products, indicating a carbocation intermediate. In nucleophilic solvents, solvent competes with bormide, but anti stereoselectivity is still observed, except in the case of stilbenes with donor substituents.
   Chlorine would be expected to be a somewhat poorer bridging group than bromine because it is less polarizable and more resistant to becoming positively charged

     Additional, you can read more about the electrophilic reaction between alkenes with peroxy-acid to form epoxide ...

and:
 

I guess hyperconjugation gives a more significant stabilizing effect.

yes, I agree with u !

Best regard !