Alkenes

[DrRoyal Pains]

Advertisement - Members don't see this ad

Can someone specify when your adding syn or anti, I am a bit confused. I know you will do so when you have 2 chiral centers, but what then determines syn or anti?

 

[DrRoyal Pains]

Whoops, three membered ring is anti, correct? And 2 is syn?

 

[nondescriptive]

syn and anti are addition reaction to a double bond(or triple). Only a few form a 3 membered ring that forces a syn addition, such as BH3*THF(anti-markovnikov) and Hg(OAc)(mark...) and one or two more I can remember off the top of my head. The rest are anti addition. For alkynes, H2/PD and lindlars catalyst adds syn and forms cis while the other one(cant remember) adds trans.

 

[rmm30]

syn and anti are addition reaction to a double bond(or triple). Only a few form a 3 membered ring that forces a syn addition, such as BH3*THF(anti-markovnikov) and Hg(OAc)(mark...) and one or two more I can remember off the top of my head. The rest are anti addition. For alkynes, H2/PD and lindlars catalyst adds syn and forms cis while the other one(cant remember) adds trans.

This statement isn't accurate. The Alkene rxns that add anti are Brominaton/markovnikov (w. either Methylene Chloride, Water, Or Alcohol sovents). Oxymercuration (Hg(Oac) NaBH4, HOH), Alcoxymeruration (Hg(Oac) NaBH4, ROH). Brominations proceed through 3 membered, positively charged bromonium ring intermediate. Oxymercuration procees via 3 membered, positively charged mercuranium ring intermediate.

Some notable alkene rxns that add syn are Hydroboration / anti mark (BH3THF ROOR OH), Catalytic Hydrogenation (H2, Pt or Pd or Raney Ni), Addition of 2 OH groups to alkene (Cold Dilute KmnO4 or Osmuim Tetroxide and a peroxide)

 

[rmm30]

Can someone specify when your adding syn or anti, I am a bit confused. I know you will do so when you have 2 chiral centers, but what then determines syn or anti?

You just have to know which rxns add syn and which add anti (see last post). There isn't a hard fast rule short of memorization. You should be aware that when you have 2 stereocenters, you don't automatically just add syn or anti. That's not how it works. This is how it works more or less: The ONLY time you care about syn or anti is when you generate 2 chiral centers. For example, if you reacted cyclohexene in Hydroboration, you would be adding H and OH across the dbl bond and not generating any chiral centers. So in this case you dont care about stereoselectivity (syn or anti) you just draw a cycloalkane with an H and OH and your done. A more comprehensive view for any alkene addition follows.

Chiral #Potential
Centers Products
Created
0 1
1 2

2 neither syn or anti 4 (eg H30+ addition)
2 (syn) 2
2 (anti) 2


On a final note; The list says "Potential" Products because when you have 2 chiral centers, there is a possibility of meso.

 

[rmm30]

I don't know how to get the margins right on these posts. They always resize them. It's annoying sorry.

Number of chiral centers / Number of Potential Products: 0 / 1, 1 /2, 2 (no stereoselectivity)/4, 2 (syn) / syn, 2 (anti), 2.

If anybody knows how to successfully do what I tried to do in the last post please be so kind as to reply.

 

[DrRoyal Pains]

Yes, I know about the chiral centers. Thanks for the input all.

You just have to know which rxns add syn and which add anti (see last post). There isn't a hard fast rule short of memorization. You should be aware that when you have 2 stereocenters, you don't automatically just add syn or anti. That's not how it works. This is how it works more or less: The ONLY time you care about syn or anti is when you generate 2 chiral centers. For example, if you reacted cyclohexene in Hydroboration, you would be adding H and OH across the dbl bond and not generating any chiral centers. So in this case you dont care about stereoselectivity (syn or anti) you just draw a cycloalkane with an H and OH and your done. A more comprehensive view for any alkene addition follows.

Chiral #Potential
Centers Products
Created
0 1
1 2

2 neither syn or anti 4 (eg H30+ addition)
2 (syn) 2
2 (anti) 2


On a final note; The list says "Potential" Products because when you have 2 chiral centers, there is a possibility of meso.