of C10-BridgePHOS oxides possessing different substituted groups on the diphenyl phosphine system were synthesized and tested as organocatalysts in the allylation of aldehydes with allyltrichlorosilane, providing chiral homoallylic alcohols. These types of organocatalysts showed high catalytic activity and only 2 mol% catalyst loading was required to induce short reaction times. Under optimal reaction
Asymmetricallylation of aromatic aldehydes 1 with allyltrichlorosilane (2) can be catalyzed by new terpene-derived bipyridine N,N′-dioxides 12–15 and an axially chiral biisoquinoline dioxide 17b with good enantioselectivities. Dioxides have been found to be more reactive catalysts than their monooxide counterparts.
Allylation of aromatic aldehydes 1a-m with allyl- and crotyl-trichlorosilanes 2- 4, catalyzed by the chiral N-oxide QUINOX (9), has been found to exhibit a significant dependence on the electronics of the aldehyde, with p-(trifluoromethyl)benzaldehyde 1g and its p-methoxy counterpart 1h affording the corresponding homoallylic alcohols 6g, h in 96 and 16% ee, respectively, at -40 degrees C. The kinetic
Chiral dinitrones were synthesized by the condensation of a C-2-symmetrical chiral dihydroxylamine with various aldehydes. The electronic and steric properties of the dinitrones can be modified by changing the aldehyde component. The activity of dinitrones as Lewis base catalysts was examined for the asymmetric allylation of aldehydes with allyltrichlorosilanes. Using DMPU as an additive in chloroform, the reaction proceeded at room temperature to afford allylated products in good yields and good enantioselectivities. (C) 2010 Elsevier Ltd. All rights reserved.
Mechanistic Dichotomy in the Asymmetric Allylation of Aldehydes with Allyltrichlorosilanes Catalyzed by Chiral Pyridine <i>N</i>
-Oxides
Detailed kinetic and computational investigation of the enantio‐ and diastereoselective allylation of aldehydes 1 with allyltrichlorosilanes 5, employing the pyridine N‐oxides METHOX (9) and QUINOX (10) as chiral organocatalysts, indicate that the reaction can proceed through a dissociative (cationic) or associative (neutral) mechanism: METHOX apparently favors a pentacoordinate cationic transition