FULL PAPER
this achievement, more focus was placed on the develop-
ment of intermolecular and easily available FLPs for hydro-
gen activation that relied on the tris(pentafluorophenyl)bor-
ane (1) as the LA component.[8] In parallel with these ef-
forts, highly active FLPs 4b–d with a linked design were re-
ported from the groups of Erker,[4b] Repo, Rieger,[5d] and
Tamm.[5i] Finally, an important advance has been made re-
cently by the Klankermayer group to realize asymmetric
FLP hydrogenation that utilizes a sterically crowded chiral
borane Lewis acid 4e.[9]
though these types of reactivities represent a breakthrough
in main-group chemistry, their enhanced and nonorthogonal
nature obviously limits the synthetic applicability of FLPs in
catalytic hydrogenation.
To minimize or bypass the above undesirable FLP reactiv-
ities upon hydrogenation, our group has recently developed
an FLP hydrogenation catalyst that has a size-exclusion
design for the LA component to control the selectivity and
chemical reactivity of FLPs by purely physical constrain.[18]
Aside from the enhanced functional-group tolerance and or-
thogonal reactivity, we have also showed that improved che-
moselectivity can be achieved in catalytic metal-free hydro-
genations. The aim of the present study is to expand further
our preliminary investigations in several ways. Additional
catalyst with better performance was prepared and tested in
a synthetically challenging hydrogenation: the partial reduc-
tion of quinolines. Then the investigation of the functional-
group tolerance and orthogonality was expanded; the scope
of the aryl halides and styrenes was evaluated. Finally, de-
tailed NMR spectroscopic, DFT, and isotopic labeling stud-
ies were performed to elucidate the underlying chemical
events that take place upon LA–LB association and the cat-
alytic reduction of quinolines.
In addition to the area of FLP hydrogenation, several un-
usual and interesting FLP reactivities were revealed with a
diverse set of substrates (Scheme 2). Thus, the bifunctional
Results and Discussion
Scheme 2. Selected examples of the reactivity of FLPs.
The FLP concept provides unique opportunities to develop
metal-free catalysts for hydrogenations. However, the ap-
plicability of these previous catalysts is limited by two cir-
cumstances. First, the advances in this area have relied on
the utilization of bulky and highly acidic LAs with boron or
alumina center as the FLP component. As a direct conse-
quence of their hard-type nature, there is considerable po-
tential for quenching the LA by common oxygen- and/or ni-
trogen-containing molecules. Therefore, careful structural
fine-tuning of the substrates (e.g., increasing the steric
demand around LB functionality or blocking it by means of
complexation to a LA) was often necessary to avoid catalyst
inhibition and execute the desired catalytic hydrogenation.
Second, the enhanced reactivity of FLPs made them capable
of reacting with a broad variety of functionalities (e.g., ole-
finic and acetylenic bonds), which obviously narrowed the
applicability of FLP hydrogenation. This synthetic limitation
triggered us to develop FLP hydrogenation catalysts that
have a broader range of applications and possible selectivity
in catalytic reduction processes.
and unquenched nature of the FLPs makes them capable of
reacting with alkenes,[10] dienes,[11] acetylenes,[12] cyclopro-
panes,[13] THF,[5f] isocyanate,[14] N2O,[15] CO2,[16] and O2.[17] Al-
Abstract in Hungarian: A frusztrꢀlt Lewis sav-bꢀzis pꢀrok
(FLP) megjelenꢁse paradigma vꢀltꢀst okozott a katalitikus
hidrogꢁnezꢁs terꢂletꢁn, miutꢀn felismertꢁk, hogy ezen kꢂlçn-
leges vegyꢂletek kꢁpesek kivꢀltani az ꢀtmenetifꢁm alapffl kata-
lizꢀtorokat.
A kezdetben kifejlesztett FLP katalizꢀtorok
azonban csak jól megvꢀlasztott modell vegyꢂletek esetꢁben
voltak sikerrel alkalmazhatók, aminek oka az FLP-ok foko-
zott, nem szelektív reaktivitꢀsa volt. Jelen kutatꢀs cꢁlja, hogy
az ffln. „mꢁretkizꢀrꢀs elv” alkalmazꢀsꢀval nçvelje a frusztrꢀlt
Lewis sav-bꢀzis pꢀrok funkciós csoport toleranciꢀjꢀt ꢁs ke-
moszelektivitꢀsꢀt a hidrogꢁnezꢁsi reakciókban. A mꢁret ki-
˝
zꢀrꢀsos tervezꢁs azon az egyszeru felismerꢁsen alapul, hogy a
˝
dihidrogꢁn a leheto legkisebb molekula a folyamatban. Ýgy
˝
annak szelektív aktivꢀlꢀsa megoldható fflgy, hogy az egyideju-
˝
leg jelenlevo reakciópartnerek Lewis savas centrumhoz való
˝
Size exclusion as a catalyst design element in FLP chemis-
hozzꢀfꢁrꢁsꢁt sztꢁrikus tꢁnyezokkel korlꢀtozzuk. Ezen elv
˝
try—introduction of MesBACTHNGUETRN(NUG C6F5)2 (Mes=mesityl): We as-
alapjꢀn tervezett frusztrꢀlt Lewis pꢀrokkal jelentosen sikerꢂlt
sumed that this seemingly paradoxical task—that is, employ-
ing a very strong and hard-type Lewis acid as a hydrogena-
tion catalyst component in the presence of organic substan-
ces that have strong and hard-type Lewis basic sites—can be
solved if one takes into consideration that the hydrogen
molecule is the smallest possible molecule. We envisioned
that FLP with increased shielding of the acidic center would
kiterjeszteni a redukꢀlható szubsztrꢀtok kçrꢁt, megvalósítha-
tóvꢀ vꢀlt pl. a,b-telítettlen iminek, ketonok ꢁs kinolinok kata-
˝
litikus redukciója. Ezen tffllmenoen, NMR mꢁrꢁsekkel, deutꢁ-
riummal izotópjelzett anyagokon vꢁgzett kísꢁrletekkel ꢁs el-
mꢁleti kꢁmiai szꢀmítꢀsokkal sikerꢂlt a hidrogꢁnezꢁs mecha-
nizmusꢀba is bepillantꢀst nyerni.
Chem. Eur. J. 2012, 18, 574 – 585
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
575