The $\ce{TBDPS}$ group (tert-butyldiphenylsilyl) is a very bulky protecting group. A silicon atom is bonded to the oxygen, which already acts as a larger carbon atom. That is then substituted by three alkyl or aryl groups, one of which is the very bulky tert-butyl group. There is a lot of congestion going on next to that oxygen atom, so its ability to coordinate to Lewis acids (including via hydrogen bonds) is severely limited. Therefore, it is mainly the polar Felkin–Anh relationship that dictates the stereochemistry of the attack in the case of a $\ce{TBDPS}$ group.

On the contrary, the benzyloxymethyl (BOM) group $\ce{CH2OBn}$ used in the other example has a primary carbon attached directly to the protected oxygen. Not only is carbon a lot smaller than silicon, but it also features two hydrogen substituents ($\ce{CH2}$) and only one substituent group – an oxygen which itself only bears one other group. Therefore, the BOM-protected oxygen can still coordinate to Lewis acids easily, such as the lithium cation or aluminium as present in the example. Thus, the Cram chelate model is feasible in this case and correctly predicts the observed stereochemistry.

This is quite often the case that some protecting groups allow for chelation while others don’t. Pretty much all the silicon-based protecting groups (those that include a $\ce{O-Si}$ bond) will inhibit chelation due to their size. Typically anything that starts off as $\ce{CH2}$ will allow chelation. A non-conclusive list of common groups of this type would be:

• MOM
• SEM
• Bn
• PMB

The $\ce{TBDPS}$ group (tert-butyldiphenylsilyl) is a very bulky protecting group. A silicon atom is bonded to the oxygen, which already acts as a larger carbon atom. That is then substituted by three alkyl or aryl groups, one of which is the very bulky tert-butyl group. There is a lot of congestion going on next to that oxygen atom, so its ability to coordinate to Lewis acids (including via hydrogen bonds) is severely limited. Therefore, it is mainly the polar Felkin–Anh relationship that dictates the stereochemistry of the attack in the case of a $\ce{TBDPS}$ group.

On the contrary, the benzyloxymethyl (BOM) group $\ce{CH2OBn}$ used in the other example has a primary carbon attached directly to the protected oxygen. Not only is carbon a lot smaller than silicon, but it also features two hydrogen substituents ($\ce{CH2}$) and only one substituent group – an oxygen which itself only bears one other group. Therefore, the BOM-protected oxygen can still coordinate to Lewis acids easily, such as the lithium cation or aluminium as present in the example. Thus, the Cram chelate model is feasible in this case and correctly predicts the observed stereochemistry.

This is quite often the case that some protecting groups allow for chelation while others don’t. Pretty much all the silicon-based protecting groups (those that include a $\ce{O-Si}$ bond) will inhibit chelation due to their size. Typically anything that starts off as $\ce{CH2}$ will allow chelation. A non-exhaustive list of common groups of this type would be:

• MOM
• SEM
• Bn
• PMB

The $\ce{TBS}$ group (tert-butyldimethylsilyl) is a very bulky protecting group. A silicon atom is bonded to the oxygen, which already acts as a larger carbon atom. That is then substituted by three alkyl groups, one of which is the very bulky tert-butyl group. There is a lot of congestion going on next to that oxygen atom, so its ability to be coordinated by Lewis acids (including hydrogen bonds) is severely limited. Therefore, it is mainly the polar Felkin-Anh relationship that dictates the stereochemistry of the attack in the case of a $\ce{TBS}$ group.

On the contrary, the benzyl group $\ce{Bn}$, used in the other example has a primary carbon attached directly to the protected oxygen. Not only is carbon a lot smaller than silicon, but it also features two hydrogen substituents ($\ce{CH2}$) and only one carbon substitution — which again happens to be the not-really-bulky phenyl group. Therefore, the benzyl-protected oxygen can still be coordinated easily by Lewis acids, such as the lithium cation or aluminium as present in the example. Thus, the Cram chelate model is feasible in this case and able to correctly predict stereochemistry.

This is quite often the case that some protecting groups allow for chelation while others don’t. Pretty much all the silicon-based protecting groups (those that include a $\ce{O-Si}$ bond) will inhibit chelation due to their size. Typically anything that starts off as $\ce{CH2}$ will allow chelation. A non-conclusive list of common groups of this type would be:

• MOM
• SEM
• Bn
• PMB

The $\ce{TBDPS}$ group (tert-butyldiphenylsilyl) is a very bulky protecting group. A silicon atom is bonded to the oxygen, which already acts as a larger carbon atom. That is then substituted by three alkyl or aryl groups, one of which is the very bulky tert-butyl group. There is a lot of congestion going on next to that oxygen atom, so its ability to coordinate to Lewis acids (including via hydrogen bonds) is severely limited. Therefore, it is mainly the polar Felkin–Anh relationship that dictates the stereochemistry of the attack in the case of a $\ce{TBDPS}$ group.

On the contrary, the benzyloxymethyl (BOM) group $\ce{CH2OBn}$ used in the other example has a primary carbon attached directly to the protected oxygen. Not only is carbon a lot smaller than silicon, but it also features two hydrogen substituents ($\ce{CH2}$) and only one substituent group – an oxygen which itself only bears one other group. Therefore, the BOM-protected oxygen can still coordinate to Lewis acids easily, such as the lithium cation or aluminium as present in the example. Thus, the Cram chelate model is feasible in this case and correctly predicts the observed stereochemistry.

This is quite often the case that some protecting groups allow for chelation while others don’t. Pretty much all the silicon-based protecting groups (those that include a $\ce{O-Si}$ bond) will inhibit chelation due to their size. Typically anything that starts off as $\ce{CH2}$ will allow chelation. A non-conclusive list of common groups of this type would be:

• MOM
• SEM
• Bn
• PMB