There is no possibility of chelation control in this case, so the addition of hydride to the carbonyl will follow the Felkin-Anh model. Thus, the conformation with the carbonyl group located between the largest (Ph) and second largest (Me) groups on the adjacent stereocentre is considered as shown below. Hydride will then attack the C=O at an angle of 109o, from the face bearing the small (H) substituent. This produces the 2R,3S stereoisomer of the product. A 3D representation of the transition state illustrating the difference between the two faces of the C=O bond is also shown below. The reaction is stereoselective since predominantly one stereoisomer of the product is obtained, and is stereospecific since the enantiomer of the starting material would give predominantly the enantiomer of the product as shown below.
There is no possibility of chelation control in this case, so the addition of a butyl group to the carbonyl will follow the Felkin-Anh model. Thus, the conformation with the carbonyl group located between the largest (SiMe3) and second largest (Et) groups on the adjacent stereocentre is considered as shown below. The butyl anion will then attack the C=O at an angle of 109o, from the face bearing the small (H) substituent. This produces the SS stereoisomer of the product. A 3D representation of the transition state illustrating the difference between the two faces of the C=O bond is also shown below. The reaction is stereoselective since predominantly one stereoisomer of the product is obtained, and is stereospecific since the enantiomer of the starting material would give predominantly the enantiomer of the product as shown below.
This is an example of an aldol reaction which proceeds via a chelated transition state as shown below. The aldol reaction is reversible, and the predominant isomer of the product will be that formed from the transition state in which all of the substituents on the six-membered ring are in equatorial positions. This gives the racemic u-diastereomer of the product as shown below. The reaction is stereoselective since one of two possible diastereomeric products (the l and u isomers) is formed predominantly. However, the reaction is not stereospecific since both cis-trans isomers of the starting material will give the same isomer of the product since the reaction is reversible (see answer to the last part of this question).
This is an example of an aldol reaction which proceeds via a chelated
transition state as shown below. The aldol reaction is reversible, and
the predominant isomer of the product will be that formed from the transition
state in which the largest substituents on the six-membered ring are in
equatorial positions. This gives the racemic l-diastereomer of the
product as shown below. The reaction is stereoselective since one of two
possible diastereomeric products (the l and u isomers) is
formed predominantly. However, the reaction is not stereospecific since
both cis-trans isomers of the starting material will give the same isomer
of the product since the reaction is reversible. Also, in this case the
other stereoisomer of the starting material does not exist since it is
not possible to have a trans-alkene within a six-membered ring.
This is nucleophilic addition of a methyl group to the carbonyl bond. In this case however, there is a group attached to the stereocentre (the OBn group) which is capable of forming a chelate with either the lithium or copper ion. Thus, the reaction will follow chelation control and the methyl anion will attack the less hindered face of the chelate giving predominantly the RR stereoisomer of the product as shown below. The reaction is stereoselective since predominantly one stereoisomer of the product is obtained, and is stereospecific since the enantiomer of the starting material would give predominantly the enantiomer of the product as shown below.
This is nucleophilic addition of an allyl group to the carbonyl bond. In this case however, there is a group attached to the stereocentre (the NBn2 group) which is capable of forming a chelate with the tin ion. Thus, the reaction will follow chelation control and the allylsilane will attack the less hindered face of the chelate giving predominantly the SS stereoisomer of the product as shown below. The reaction is stereoselective since predominantly one stereoisomer of the product is obtained, and is stereospecific since the enantiomer of the starting material would give predominantly the enantiomer of the product as shown below.
This is an example of an aldol reaction which proceeds via a chelated transition state as shown below. The aldol reaction is reversible, and the predominant isomer of the product will be that formed from the transition state in which all of the substituents on the six-membered ring are in equatorial positions. This gives the racemic u-diastereomer of the product as shown below. The reaction is stereoselective since one of two possible diastereomeric products (the l and u isomers) is formed predominantly. However, the reaction is not stereospecific since both cis-trans isomers of the starting material will give the same isomer of the product since the reaction is reversible (see answer to part three of this question).
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