Introduction
(R )- and (S )-3,3′-bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diylhydrogenphosphates,
also known as (R )-TRIP and (S )-TRIP are chiral phosphoric acids (CPA’s) derived from BINOL. They are available
from commercial sources, and they can be prepared through a three-step sequence that
starts with a Kumada coupling between compound 1 and two equivalents of 2,4,6-(triisopropoxy)phenylmagnesiumchloride. This reaction
is followed by a deprotection and a high-yielding phosphorylation step (Scheme [1a ]).[1 ]
Since their first application in enantioselective organocatalytic hydrogenations by
the List Group, in 2005,[2 ] these catalysts have been applied in numerous enantioselective transformations,
including strategies involving cooperative catalysis.[3 ]
Scheme 1 General synthetic sequence for the preparation of (R )- and (S )-TRIP
Table 1 Use of (R )-TRIP and (S )-TRIP – Very Recent Applications
(A) Early in 2013, List and co-workers reported an asymmetric protonation of silyl
ketene imines (SKI’s) catalyzed by (S )-TRIP or STRIP (a spiroderivative of TRIP).[4 ] During the catalyst screening both of these showed high efficiency. This transformation
has no precedents in literature and showed to be a mild and straightforward strategy
to access α-branched nitriles with high enantiopurity.
(B) Faber, Orthaber and co-workers reported an asymmetric allylation reaction between
a zinc(II)-allylbutyrolactone species and (hetero)aromatic aldehydes using TRIP as
catalyst.[5 ] DFT studies showed that a complex ion-pair involving TRIP, Zn2+ and substrates is formed prior to enantioenriched β-substituted α-methylenebutyrolactone
formation. Although high enantioselectivities had been reached, a two-step total synthesis
of natural product (S )-(–)-hydroxymatairesinol was also performed in order to demonstrate the applicability
of the methodology.
(C) In order to expand the applicability of isochromenyliums in enantioselective transformations,
an asymmetric [4+2] annulation between 2-hydroxystyrenes and isochromenyliums prepared
in situ from 2-alkynylbenzaldehyes or 1-(2-alkynylphenyl)ketones was developed by
Yao and co-workers.[6 ] Among the catalytic conditions investigated, the combination of Pd(OAc)2 with (S )-TRIP gave the best results. This cooperative catalytic system showed to be applicable
for a broad spectrum of substrates. Good to excellent enantioselectivities were achieved.
(D) Still in 2013, an asymmetric synthesis of cyclic trifluoromethyldihydroquinazolines
via a TRIP-catalyzed aza-Friedel–Crafts reaction between indoles and cyclic N -acylketimines was developed by Ma and co-workers.[7 ] This work was based on a previous report in which aryl-imines generated in situ
from hemi-acetals were used as electrophiles.[8a ] In 2011, Bolm and co-workers had also reported another example using trifluoropyruvate
derived N -Boc-imines as electrophiles.[8b ]
(E) Recently, the desymmetrization of pro-chiral diesters by an intramolecular transesterification
catalyzed by TRIP was disclosed and had its scope explored by Petersen and co-workers.[9 ] The process showed to be scalable and robust, leading to the preparation of several
enantioenriched lactones with all-carbon chiral quaternary centers, which showed to
be useful small building blocks.
(F) Organocatalysed transfer hydrogenation of heteroaromatic compounds has been widely
investigated over the last years. Recently, Zhou and co-workers reported the use of
TRIP on the asymmetric hydrogenation of 2-aryl-quinolone-3-amines and 3-(trifluoromethyl)quinolones
with up to 99% and 98% ee, respectively.[10a ]
[b ]
(G) Taking advantage of a highly favoured heterodimerization of carboxylic acids with
TRIP, List and co-workers investigated the desymmetrization of meso -aziridines and the kinetic resolution of terminal aziridines using this catalyst.[11 ] The catalytic system proved useful for the conversion of cyclic and acyclic aziridines
into O-protected amino alcohols with high yields and enantioselectivities. This was
the first report of a CPA’s catalyzed reaction using carboxylic acids as nucleophiles
instead of only as additives, which has opened new perspectives in the field.
