Brønsted acid-catalyzed asymmetric allylation and propargylation of aldehydes
A method synthesizing homoallylic or homopropargylic alcohols was developed to react aldehydes with allyl boronates, such as allylboronic acid pinacol ester, or allenylborates in the presence of a catalytic amount of a chiral binaphthyl-derived chiral phosphoric acid. The method showed enhanced enantiocontrol and chemical yield, which increased with lower temperatures. A large series of aldehydes were tested under these catalytic conditions and wide successful substrate scope was found, including aryl, heteroaryl, aromatic aldehydes, heteroaryl aldehydes, α,β-unsaturated aldehydes and aliphatic aldehydes, and alkyl aldehydes. Likewise, the use of crotyl boronates (E and Z) were successfully reacted with aryl aldehydes under the conditions to allow for highly enantio- and diasteo-selective crotylation.
1. A method of synthesizing homoallyl or homopropargyl alcohols, comprising the steps of:
obtaining an aldehyde, where the aldehyde does not possess a protecting group;
obtaining an allylboronate or allenylboronate, where the allylboronate or allenylboronate does not possess a protecting group;
lowering the temperature of the aldehyde and the allylboronate or allenylboronate to between about 0° C. and about −30° C.;
reacting the aldehyde with the allylboronate or allenylborate in the presence of a chiral acid catalyst, wherein the chiral acid catalyst has the formula
where R═SiPh3, 4-(b-Nath)-Ph, 9-anthryl, 4-(a-Naph)-Ph, (2,4,6-i-Pr)-Ph, or (2,5-CF3)-Ph; and
wherein the reaction forms a homoallyl alcohol.
2. The method of claim 1 , wherein the aldehyde is selected from the group consisting of aromatic aldehydes, heteroaryl aldehydes, α,β-unsaturated aldehydes and aliphatic aldehydes.
3. The method of claim 1 , further comprising reacting the aldehyde and allylboronate in a solvent, wherein the solvent is toluene, m-xylene, benzene, methylene chloride, ether, or DCM.
4. The method of claim 3 , wherein the solvent is toluene.
5. The method of claim 1 , wherein the chiral acid catalyst is (R)-3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphate.
6. The method of claim 1 , wherein the temperature of the reactants is 0° C. or −30° C.
7. The method of claim 1 , wherein the chiral acid catalyst is at a catalyst loading amount of between about 5 mol %, and about 1 mol %.
8. The method of claim 7 , wherein the chiral acid catalyst is at a catalyst loading amount of 5 mol %, 2.5 mol %, or 1 mol %.
9. The method of claim 1 , further comprising adding a 4 Å molecular sieve and at a catalyst loading amount of 20 mol %.
10. The method of claim 9 , further comprising lowering the temperature of the reactants to between about 0° C. and about −20° C.
11. The method of claim 9 , wherein the temperature of the reactants is 0° C. or −20° C.
12. The method of claim 2 , wherein the aldehydes are heteroaryl, α,β-unsaturated, or aliphatic.
13. The method of claim 7 , wherein the chiral acid catalyst is at a catalyst loading amount of 2.5 mol % or 1 mol %.