Dual reaction behaviour of an in situ generated nitrilium phosphane ylide
complex towards carbon–sulfur p-systems
Rainer Streubel* and Christoph Neumann
Institut fu¨r Anorganische und Analytische Chemie der Technischen Universita¨t Braunschweig, Postfach 3329,
D-38106 Braunschweig, Germany. E-mail: r.streubel@tu-bs.de
Received (in Basel, Switzerland) 15th December 1998, Accepted 8th February 1999
[W]
S
R
[3 + 2] Cycloaddition of in situ generated nitrilium
phosphane ylide complex 4b with phenylisothiocyanate
P
N
3
yielded N-piperidino-substituted D -1,3,2-thiazaphospho-
C
C
lene complex 5 regioselectively, whereas with benzyl N,N-
PhN
pip
dimethyl dithiocarbamate ester the thiaphosphirane 9 is
obtained; these reactions, a 1,3-dipolar cycloaddition and a
transylidation, shed first light on the reactivity of a nitrilium
phosphane ylide complex towards different C,S p-systems.
5
+ PhN
C
3
S
[W]
R
[W]
+
N
i,ii
–
P
PhC
P
75 °C
C
N
R
Betaines I,1 II,1b,2 III, IV3 and V4 with a phosphorus atom in
the central 1,3-dipole skeleton are of increasing interest in
phosphorus and heterocyclic chemistry (Scheme 1).5 Recently,
we gained strong evidence for the transient formation of
nitrilium phosphane ylide complexes (V) by employing dialkyl
cyanamides and different trapping reagents such as dimethyl
acetylene dicarboxylate6 or nitriles.7 Although we gained some
evidence that transiently formed 2H-azaphosphirene com-
plexes7 and, under some circumstances, transition states of the
2:1 donor–acceptor adduct-type, with nitriles as donors and a
terminal phosphanediyl complex as acceptor,8 may be involved
in transylidation processes, those reactions are not completely
understood. To shed more light on this process and to exploit
synthetically the transylidation methodology, we have now
started to investigate the reaction behaviour of transiently
formed nitrilium phosphane ylide complexes towards different
C,S p-systems such as isothiocyanates and dithiocarbamates.
4a
Ph
+ pipCN
1
–PhCN
(2)
[W]
+
N
–
P
pipC
[W] =W(CO)5
R = CH(SiMe3)2
R
4b
pip = NCH2(CH2)3CH2
– pipCN
+ PhH2CS(Me2N)C
S
6
R
[W]
R
P
P
[W]
Me2N
Me2N
+
S
–
P
O
PhH2CS(Me2N)C
C
S
C
S
–[W]
R
PhCH2S
PhCH2S
7
9
8
Scheme 2 Reagents and conditions: i, 1 mmol of 1 was treated with 6 mmol
of phenylisothiocyanate and 2 mmol 1-piperidinonitrile in 2 ml of toluene
at 75 °C for 1.5 h. Work-up by column chromatography at low temperature
and crystallization from pentane afforded 5 as a yellow solid (28%, mp.
98 °C); ii, 1 mmol of 1 was treated with 5 mmol of benzyl N,N-
dimethyldithiocarbamate ester in 3 ml of toluene at 75 °C for 2 h.Work-up
by column chromatography at low temperature afforded 9 as a pale yellow
oil (13%).
[M]
[M]
[M]
[M]
[M]
–P
R
R
R
–P
R
–P
R
R
–P
R
–P
+N
R
+O
C
+N
C
+S
C
+N
N
C
R
R
R
R
R
R
ring-cleavage of 4a in toluene,4 we repeated the reaction of 1
and 6 in toluene in the absence of 2 and obtained, once more,
thiaphosphirane 9 as the only phosphorus-containing product.
This result also supports the assumption of complex 7 as a
highly reactive intermediate in this reaction course.
R
I
II
III
IV
V
Scheme 1 1,3-Dipole complexes ([M] = metal complex fragment, R
denotes ubiquitous organic substituents).
Thermal ring-opening of the 2H-azaphosphirene complex 19
in toluene in the presence of 2 equiv. of 1-piperidinonitrile 2 and
The compositions of the D -1,3,2-thiazaphospholene com-
3
plex 5 and the thiaphosphirane 9 are confirmed by elemental
analyses and mass spectrometry;† the structural formulation is
based on their characteristic NMR spectral data† in solution.
The connectivity of the heterocyclic ring atoms of complex 5
was also confirmed by X-ray structure analysis, although the
refinement was unsatisfactory because of heavily disordered
substituents at the five-membered ring system; therefore, the
structure will not be further discussed here.
3
2 equiv. of phenylisothiocyanate 3 furnished the D -1,3,2-thi-
azaphospholene complex 5, regioselectively; neither regio-
isomers nor isomers resulting from a cycloaddition reaction of
intermediately formed 4b with the C,N p-system of 3 have been
observed. Employment of benzyl N,N-dimethyl dithiocarba-
mate ester 6 under the same reaction conditions gave the
thiaphosphirane 9 exclusively; neither five-membered hetero-
cycles, formed by reaction of 4b with 6, nor the corresponding
thiaphosphirane complex 8 could be detected spectroscopically
(Scheme 2). It is remarkable that this reaction did not change,
even when benzonitrile was employed as solvent. Therefore, the
formation of both heterocycles is explained by reactions of the
in situ generated nitrilium phosphane ylide complex 4b, which
leads either to 5 via [3 + 2] cycloaddition or to 9 via an
intersystem-transylidation-type reaction giving 1,3-dipole com-
plex 7, which undergoes ring-closure to 8 and subsequent
decomplexation to give 9 as the final product. Because a
transient formation of terminal phosphanediyl complex
[(OC)5WPCH(SiMe3)2] can be achieved via thermally induced
The phosphorus nucleus of 5 displays a resonance at d 105.8,
3
which is significantly high-field shifted compared to D -
1,3,2-oxazaphospholene complexes (d 190–2056), with a mark-
edly decreased phosphorus–tungsten coupling constant of 287.1
Hz (cf. 300–306 Hz6). The carbon atom resonances of the
heterocycle appear at d 157.7 and 160.9 with phosphorus–
carbon coupling constants of 9.2 and 12.8 Hz, respectively.
3
Similarly to D -1,3,2-oxazaphospholene complexes, these car-
bon resonances display small carbon–phosphorus coupling
constants; this seems to be a characteristic phenomenon for such
heterocyclic ring systems. The phosphorus and the ring carbon
atom of the thiaphosphirane 9 display resonances at low field
Chem. Commun., 1999, 499–500
499
Streubel, Rainer
