Option II: Chiral + Achiral = Optically Active Diastereomers

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1 Option II: Chiral + Achiral = Optically Active Diastereomers What about additions to chiral alkenes? The previous examples were reactions done on achiral alkenes. What is the difference when an alkene is reacting that has a pre-existing chiral center? 3 C 3 C Chiral alkenes inherently do not have a plane of symmetry. Because chiral alkenes are asymmetric, one face of the alkenes would be more accessible by the reagent than the other face, so the product mixture that forms will NOT be 50:50. 3 C 3 C + Chiral + Achiral = Optically active product(s) 3 C (R) What is the relationship between the two products that forms when adds to the alkene above? The old chiral center remains unchanged in the process. The new chiral center is a mixture (albeit not 50:50). The result is unequal amounts of (2R, 3S) and (2S, 3S) which are a mixture of optically active diastereomers! Determine the stereochemical outcome in the following reactions: A. Addition of 2 to an alkene 2, Pd/C Chiral or Achiral Alkene? Like the reaction with OsO 4, this reaction generates two new chiral centers. 2 2 = 4 possible stereoisomers that could be drawn but there will not be 4 products because this reaction can only occur as a SYN addition.

2 2, Pd/C + With two chiral carbons and a plane of symmetry: Meso Compound! B. What about: 2, Pd/C Chiral or Achiral Alkene? Again, this reaction generates two new chiral centers but only 2 possible stereoisomer products, due to SYN addition. 2, Pd/C + Note that there is no central plane of symmetry. The hydrogen atoms are already aligned but not the methyl and ethyl groups. If you rotate to match the methyl and ethyl groups, the hydrogen atoms are no longer aligned: Although there are two chiral carbons, there isn t a plane of symmetry: Racemic mixture! C. Oxymercuration 1. g(oac) 2, 2 O 2. NaB 4 O

3 Chiral or achiral alkene? Chiral + achiral = optically active diastereomers (R) O O One old chiral center. ow many new chiral centers are created? One. Relationship? One chiral center the same, one a mirror image Diastereomers D. Addition of 2 to an alkene 2 This reaction generates two new chiral centers. 2 2 = 4 but there will not be 4 possible stereoisomers, because this reaction can only occur as an ANTI addition. 2 + There is no plane of symmetry because the alkene has two different groups attached at each end (left: methyl, right: ethyl). Although there are two chiral carbons, there isn t a plane of symmetry: Racemic mixture! E. Addition of 2 to an alkene 2

4 Chiral or Achiral Alkene? Again, this reaction generates two new chiral centers but there will not be 4 possible stereoisomer products, because this reaction can only occur as an ANTI addition. 2 + Notice how the alkene was symmetrical and in the products, the methyl groups are already aligned symmetrically but one bromine atom faces the top and the other faces the bottom. Is there a plane of symmetry? No. Although there are two chiral carbons, there isn t a plane of symmetry thus this is a Racemic mixture! F. One more time: Addition of 2 to an alkene 2 Only two possible products, because this reaction can only occur as an ANTI addition. 2 + What s the relationship? Is there a plane of symmetry? No groups are lined up to show any mirror plane. Rotate that second molecule around on its center C-C bond and see how the methyl groups line up:

5 3 C C C C 3 3 C C 3 Is there a plane of symmetry? Yes! With two chiral carbons and a plane of symmetry: Meso Compound! G. ydroboration-oxidation: 1. B 3 2. NaO, 2 O 2 O O ow many chiral centers are created? Only one the other center ISN T chiral Relationship? Racemic Mixture. Draw the product(s). Show stereochemistry. 1. OsO 4 2. NaSO 3, 2 O Chiral or achiral alkene? Chiral + achiral = optically active diastereomers O O O O ow many new chiral centers are created? Two new ones Still have one OLD one

6 Relationship? The original chiral center stayed the same, the two new ones are mirror images Partly the same, partly mirror images (Diastereomers) I. Oxymercuration: 1. g(oac) 2, 2 O 2. NaB 4 1. g(oac) 2, 2 O O 2. NaB 4 ow many chiral centers are created? None! ACIRAL molecule J. Oxymercuration 1. g(oac) 2, 2 O 2. NaB 4 O O ow many new chiral centers are created? Only one Relationship? Racemic Mixture! K. alohydrin Reaction: 2 2 O

7 O O ow many chiral centers are created? Two Relationship? Racemic Mixture!