Anomalous Staudinger reaction at intramolecular frustrated P-B Lewis pair frameworks

Article abstract of DOI:10.1039/c2cc36782c

The FLP-mesityl azide addition products 5, formed by FLP-addition to the terminal azide nitrogen atom, undergo N-N bond cleavage in an unusual variant of the Staudinger reaction upon thermolysis or photolysis to give an internally borane stabilized [P]NH

Full text of DOI:10.1039/c2cc36782c

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COMMUNICATION
Anomalous Staudinger reaction at intramolecular frustrated P–B Lewis
pair frameworksw
¨
Annika Stute, Lukas Heletta, Roland Frohlich,z Constantin G. Daniliuc,z Gerald Kehr and
Gerhard Erker*
Received 18th September 2012, Accepted 15th October 2012
DOI: 10.1039/c2cc36782c
The FLP-mesityl azide addition products 5, formed by FLP-
addition to the terminal azide nitrogen atom, undergo N–N bond
cleavage in an unusual variant of the Staudinger reaction upon
thermolysis or photolysis to give an internally borane stabilized
to yield the intramolecular vicinal frustrated Lewis pair 4.
Compound 4 is an example of an electronically controlled FLP
without any appreciable interaction between its Lewis acid and
Lewis base components.9c,10 It features a 11B NMR signal at
d = 67.8, typical of a strongly Lewis acidic planar-tricoordinate
boron center (31P NMR of 4: d = ꢀ32.5).
Q
[P] NH phosphinimine and a dimethylindazole derivative.
Compound 4, in situ generated in n-pentane, reacted rapidly
with mesityl azide. After 1 h at room temperature the product 5a
had precipitated and was isolated as a white solid in ca. 60%
yield. The X-ray crystal structure analysis of 5a revealed that the
FLP 4 has undergone 1,1-addition to the terminal nitrogen atom
of the mesityl azide reagent to form a five-membered heterocycle
(see Scheme 2).11 Both the P1–N1 and B1–N1 bonds in 5a are
rather long (see Table 1). The N–N–N bond lengths are alter-
nating. The N2QN3 double bond is trans-disubstituted (dihedral
angle N1–N2–N3–C51 174.81) (see Fig. 1). The heterocyclic ring
is non-planar, it shows a typical cyclopentane-like conformation.
In solution compound 5a features heteroatom NMR signals
at d = 18.1 (31P) and d = ꢀ5.9 (11B). Due to the chiral center
inside the five-membered heterocycle we have observed the
19F NMR signals of pairs of diastereotopic C6F5 substituents
at both boron and phosphorus (for details see the ESIw).
H. Staudinger reported in 1919 about the formation of phos-
phinimines (iminophosphoranes) by treatment of tertiary phos-
phanes with organic azides.1 Since then the ‘‘Staudinger reaction’’
has been a valuable synthetic method for the preparation of
phosphinimines and of various follow up products.2 It has been a
useful method for the synthesis of amines from azides,3 but more
importantly it has provided an invaluable tool in chemical biology
where it is used for the preparation of bio-conjugates under very
mild ‘‘biotic’’ conditions (‘‘Staudinger ligation’’).4
The phosphinimines 2 are formed in the Staudinger reaction
by transfer of an N–R unit from the N3–R azide reagent 1 to
the phosphane with thermodynamically favorable elimination of
dinitrogen (see Scheme 1).5 We have now found an anomalous
course of the Staudinger reaction6 where the N2 elimination is
avoided by formation of an indazole derivative to give an
unsubstituted Ar2RPQNH phosphinimine. This remarkable
alternative to the ‘‘normal’’ Staudinger reaction has been
observed eventually by reacting some intramolecular vicinal
frustrated phosphane–borane Lewis pairs7,8 with mesityl azide.9
For this study we reacted bis(pentafluorophenyl)-2-propenyl-
phosphane (3) with Piers’ borane [HB(C6F5)2]. This resulted in a
clean hydroboration reaction with anti-Markovnikov orientation
Scheme 1
Organisch-Chemisches Institut, Universitat Munster, Corrensstraße 40,
¨
¨
D-48149 Munster, Germany. E-mail: erker@uni-muenster.de
¨
w Electronic supplementary information (ESI) available: Text and
figures giving further experimental and spectroscopic details and CIF
files giving crystallographic data for 5a, 5b, 6a, 6b and 7ꢁB(C6F5)3.
CCDC: 902225–902229. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2cc36782c
z X-ray crystal structure analysis.
Scheme 2
c
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Table 1 Selected structural parameters of compounds 5 and 6a
phosphinimine character12 of the product 6a. In solution,
compound 6a exhibits a 31P NMR resonance at d = 36.6
and an 11B NMR signal at d = ꢀ4.8, typical of tetracoordina-
tion at boron. Compound 6a also shows the typical 19F NMR
signals of two pairs of diastereotopic C6F5 substituents at the
heterocyclic core structure.
5a
6a
5b
6b
Compound
P1–N1
B1–N1
N1–N2
N2–N3
P1–N1–B1
C1–P1–N1
C2–B1–N1
P1–C1–C2–B1
1.651(2)
1.594(2)
1.378(2)
1.255(2)
114.9(1)
97.0(1)
98.9(1)
47.0
1.583(3)
1.586(5)
—
1.687(2)
1.600(4)
1.368(3)
1.255(3)
116.2(2)
95.1(1)
99.1(2)
43.5
1.609(2)
1.564(3)
—
—
—
The sublimed co-product (see above) was isolated and
identified spectroscopically as the dimethylindazole derivative
7 (for details see the ESIw).13 This was confirmed by an X-ray
crystal structure analysis of its adduct with the Lewis acid
B(C6F5)3, which was isolated in 80% yield and characterized
spectroscopically (see the ESIw) and by X-ray diffraction
(see Fig. 3, N2–B1: 1.599(3) A).
115.4(3)
100.2(2)
100.4(3)
36.5
116.2(2)
96.2(1)
100.5(2)
44.9
a
Bond lengths in A, angles in degrees.
A similar overall reaction was observed starting from the
FLP 8 and mesityl azide. Compound 8 contains a markedly
more nucleophilic phosphane Lewis base. It is a weakly
interacting intramolecular vicinal P–B FLP, as we had pre-
viously shown.8a It reacted with mesityl azide in n-pentane
(1.5 h, r.t.) to give the 1,1-addition product 5b, isolated in 68%
yield (see Scheme 3). The X-ray crystal structure analysis of 5b
showed a similar five-membered heterocyclic structure as
found for 5a (for details see Table 1 and the ESIw). In solution,
compound 5b features an 11B NMR resonance at d = ꢀ5.9
and a 31P NMR signal at d = 54.6.
Compound 5b was slowly decomposed upon UV irradiation
(HPK 125, Pyrex filter) in a dichloromethane solution at
ambient temperature. After 4 d the reaction was complete and
we isolated the corresponding FLPNH product 6b in 87% yield.
Fig. 1 A view of the molecular structure of compound 5a (thermal
ellipsoids are shown with 30% probability).
The FLP–mesityl azide adduct 5a was then thermolyzed in
the solid at 150 1C for 1 h. During that period of time a white
solid had sublimed which was collected (see below). The
remaining residue was worked up chromatographically to give
the five-membered heterocyclic FLPNH product 6a in 75%
yield. (6a was also obtained by photolysis of 5a in dichloro-
methane, for further details see the ESIw). The compound was
identified by X-ray diffraction (see Fig. 2 and Table 1).
The structure features a five-membered heterocyclic core
that contains an unsubstituted NH functionality. The P1–N1
bond in 6a is markedly shorter than the corresponding P–N
linkage in its precursor 5a, indicating a substantial [P]QNH
Fig. 3 A view of the molecular structure of the 7ꢁB(C6F5)3 adduct
(thermal ellipsoids are shown with 30% probability).
Fig. 2 Molecular structure of compound 6a (thermal ellipsoids are
shown with 30% probability).
Scheme 3
c
11740 Chem. Commun., 2012, 48, 11739–11741
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Scheme 4
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(thermolysis of 5b also gave 6b + 7, but not as cleanly, see the
ESIw). Compound 6b shows a single set of 19F NMR features
of the pair of C6F5 substituents at boron and the 1H/13C NMR
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[P]QNH 1H NMR signal occurs at d = 2.80 (d) with a 2JPH
=
¨
12.1 Hz coupling constant. Compound 6b shows the core hetero-
nuclei NMR resonances at d = ꢀ4.6 (11B) and d = 50.7 (31P),
respectively. The compound was characterized by X-ray diffrac-
tion (for details see the ESIw and Table 1).
Although a detailed mechanistic picture of this unusual
FLP variant of the Staudinger reaction will require additional
experimental evidence, the observed reactions may be ratio-
nalized by the pathway that is schematically depicted in
Scheme 4. This would involve initial heterolysis of the N–N
single bond in the azide adducts 5 to generate the ion pairs 9.14
Deprotonation at the benzylic position of the mesityl substi-
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phosphinimines15 6 and open a viable pathway for the for-
mation of the indazole derivative 7 (see Scheme 4).
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The reaction of the two examples of the intramolecular
vicinal FLPs 4 and 8 with mesityl azide followed by thermo-
lysis or photolysis, respectively, has opened pathways to an
unusual variant of the Staudinger reaction. This indicates the
power that the active Lewis acid–Lewis base combination, as it
is characteristic of frustrated Lewis pair chemistry, has to
influence reactivity and to lead to new chemical reactions.16
Financial support from the European Research Council is
gratefully acknowledged.
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1 H. Staudinger and J. Meyer, Helv. Chim. Acta, 1919, 2, 635–646.
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c
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Chem. Commun., 2012, 48, 11739–11741 11741