Angewandte
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Chemie
Figure 2. Molecular structure of the phenylene-bridged FLP 8a (ther-
mal ellipsoids are shown with 50% probability). Selected bond lengths
[] and angles [deg]: P1–B1 2.203(6), C1–C2 1.402(7), C1–C6 1.398(7),
C2–C3 1.399(8), C4–C5 1.419(7); P1–C1–C2 100.0(4), B1–C2–C1 106.9-
(4).
Figure 1. Molecular structure of 1,3-cyclohexadiene-derived FLP 7a (R:
Ph; thermal ellipsoids are shown with 30% probability). Selected bond
lengths [] and angles [deg]: P1–B1 2.182(3), C1–C2 1.478(3), C1–C6
1.337(3), C2–C3 1.330(3), C4–C5 1.512(4); P1–C1–C2 98.3(2), B1–C2–
C1 104.5(2).
We have subsequently oxidized the cyclohexadiene core
of the compounds 7a,b to the respective aromatic phenylene-
bridged FLPs 8a,b by treatment with the 2,2,6,6-tetramethyl-
piperidine-N-oxyl radical (TEMPO; 2 equivalents).[11] H-
atom abstraction from 7a by the persistent nitroxide radical
was achieved at 608C (2 d) to give 8a, which was isolated in
84% yield. The compound 8a shows typical FLP NMR
features of the weakly interacting P/B Lewis base/Lewis
acid pair at d 5.5 ppm (31P) and d 5.8 ppm (11B), respectively.
The X-ray crystal structure analysis shows the presence of the
newly formed carbocyclic aromatic nucleus with the PMes2/
B(C6F5)2 pair in 1,2-position and the phenyl substituent
attached at carbon atom C4 (see Figure 2).
Usually, the unsaturated vicinal P/B systems 2 show no or
only very low FLP reactivities.[7] The derivative 6b is no
exception. Compound 6b did not react with H2 under our
typical FLP reaction conditions. In contrast, the cyclohexa-
diene-derived systems 7a,b both are very active FLPs despite
the presence of a pair of C(sp2) centers in their vicinal bridge.
Thus, compound 7b (R: tBu) splits dihydrogen rapidly at near
to ambient condition (2 bar H2, RT) to give the zwitterionic
phosphonium/hydrido-borate product 9b, which was isolated
in 79% yield. It shows 11B NMR resonances (243K) at d
À21.8 ppm (d, 1JBH = 90 Hz) and at d À22.6 ppm (d, 1JBH = 75
Hz) and a 2:1 intensity pair of 31P NMR signals (243K) at d
À26.2 ppm (d, JPH = 510 Hz) and À28.1 (d, JPH = 500 Hz).
We assume that the occurrence of a pair of diastereoisomers
in solution is due to the presence of an element of conforma-
tional chirality in addition to the carbon chirality center C4. In
the crystal we observed a single isomer of compound 9b (see
Figure 3). It contains the cyclohexadiene core in its typical
conformation with the tBu substituent in a pseudo-equatorial
position at carbon atom C4 and the PHMes2 and BH(C6F5)2
functionalities attached at the sp2-carbon centers C1 and C2.
Figure 3. A view of the molecular structure of the FLP dihydrogen-
splitting product 9b (R: tBu; thermal ellipsoids are shown with 30%
probability). Selected bond lengths [] and angles [deg]: P1–B1 3.155,
C1–C2 1.499(2), C1–C6 1.351(2), C2–C3 1.343(2), C4–C5 1.530(2); P1–
C1–C2 114.8(1), B1–C2–C1 120.0(2).
Compound 7a is an equally reactive dihydrogen splitting
reagent. In a competition experiment it was shown that 7a
reacts only about six times slower with dihydrogen than the
“parent” saturated vicinal P/B FLP Mes2PCH2CH2B(C6F5)2 1,
which is one of the most active metal-free dihydrogen-
activating systems.[2c,12] The PH/BH product 9a was isolated
in 84% yield (see the Supporting Information for its
characterization by C, H elemental analysis, spectroscopy,
and X-ray diffraction).
1
1
Both the aromatic systems 8a,b behave as active FLPs and
split dihydrogen under mild conditions (e.g. 2 bar H2, CH2Cl2,
Angew. Chem. Int. Ed. 2016, 55, 5526 –5530
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