1,4,2-Diazaphospholidine-3,5-diones and related compounds: A lecture on unpredictability in catalysis

Article abstract of DOI:10.1002/chem.200900039

1,4,2-diazaphospholidine-3,5-diones and related compounds and unpredictability in catalysis was studied. The five-membered ring species 4 6, which are all readily accessible by the McCormack reaction, optionally with subsequent hydrogenation, were selected for this study. The use of saturated species of type 4 (1-organyl-phospholanes) as catalysts led to a complete reverse in the selectivity of the reaction, in favor of the formation of the isocyanate trimers. Owing to the high activity of the corresponding P-oxides in carbodiimidization reactions, it can be assumed that the findings can be attributed to partial air oxidation of the trivalent phosphorus species applied. The symmetrical species 6, in total contrast to the other two members of the series, do not exhibit any catalytic activity with respect to isocyanates and instead react cleanly with two equivalents of isocyanate to effect the extrusion of 1,3-diene.

Full text of DOI:10.1002/chem.200900039

DOI: 10.1002/chem.200900039
1,4,2-Diazaphospholidine-3,5-diones and Related Compounds: A Lecture on
Unpredictability in Catalysis
Frank U. Richter*[a]
This article is dedicated to Prof. Dietmar Seyferth on the occasion of his 80th birthday.
Trivalent phosphorus compounds have long been used as
catalysts in isocyanate chemistry.[1] The major products of
such syntheses are mixtures of isocyanate ’trimers’ (1 and 2,
see below) and ’dimers’ (3).[1–3] Dimer 3 is the major prod-
reaction,[6] optionally with subsequent hydrogenation (4),
have been selected for this study.
Surprisingly, the use of saturated species of type 4 (1-or-
ganyl-phospholanes) as catalysts[7] led to a complete reverse
in the selectivity of the reaction, in favor of the formation of
the isocyanate trimers, 1 and 2. This is in contrast to earlier
findings,[8] in which compounds of type 4–6 were claimed to
uct, especially at low isocyanate conversion. The composi-
tion of such mixtures is strongly dependent on the substrate,
reaction conditions, and catalyst type. It was recently estab-
lished that increasing the steric hindrance about the P atom
considerably increases the uretdione selectivity of the phos-
phane catalyst in reactions with aliphatic isocyanates, such
as the industrially important hexamethylene diisocyanate,[4]
and a mechanism for this type of reaction has been pro-
posed.[5]
In the course of this investigation the question arose,
whether the migration of the phosphorus atom into the ring
of a carbocycle would exert an influence on the selectivity
of the reactions. The five-membered ring species 4–6 (see
below), which are all readily accessible by the McCormack
be versatile carbodiimidization catalysts. Owing to the high
activity of the corresponding P-oxides in carbodiimidization
reactions,[9] it can be assumed that the findings mentioned in
Ref. [8] can be attributed to partial air oxidation of the tri-
valent phosphorus species applied.
Compared to the saturated phosphacycles 4, which are
potent catalysts for isocyanate trimerization reactions,[7] the
2-unsaturated species 5 are considerably less active, but ex-
hibit a very similar selectivity. The symmetrical species 6, in
total contrast to the other two members of the series, do not
exhibit any catalytic activity with respect to isocyanates and
instead react cleanly with two equivalents of isocyanate to
effect the extrusion of 1,3-diene (Scheme 1).
To our knowledge, neither this type of reaction nor the re-
sultant P-containing products, 1,4,2-diazaphospholidine-3,5-
diones 7, have been reported to date.[10] There are also only
two preparative examples of the extrusion of a 1,3-diene
from an unsaturated phosphacycle, such as 6.[10a,b] In both
cases, oxygen-containing nucleophiles (diethyl peroxide and
methanol, respectively) induce the fragmentation.
[a] F. U. Richter
Bayer MaterialScience, 51368 Leverkusen, (Germany)
Fax : (+49)214-3050708
E-mail: frank.richter@bayerbms.com
Primary aliphatic isocyanates react smoothly without for-
mation of P-containing side products (Scheme 1). The reac-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900039.
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Chem. Eur. J. 2009, 15, 5200 – 5202

COMMUNICATION
phosphanes. This finding may help to afford a better under-
standing of the driving forces underlying the catalytic cycle
in isocyanate oligomerization reactions; reversible adduct
formation involving two or three isocyanate units, ring clo-
sure with formation of 1–3, and simultaneous extrusion of
catalyst or, in this case, 1,3-diene (Scheme 2).
The new type of reaction is not limited to isocyanates.
Other heterocumulenes, isothiocyanates and carbodiimides,
also form the appropriate phosphacycles; 1,4,2-diazaphos-
pholidine-3,5-dithiones 9 and 1,4,2-diazaphospholidine-3,5-
Scheme 1. Synthesis of 1,4,2-diazaphospholidine-3,5-diones 7 and concur-
rent extrusion of 1,3-dienes.
tivity of the phospholene moiety
with a given isocyanate increas-
es in the order 6a<6b<6c.
Longer alkyl or aromatic sub-
stituents at the P atom are toler-
ated.
Depending on reaction condi-
tions, minor amounts of isocya-
nate oligomers, especially carbo-
diimides and trimers, may be
formed. This is especially true if
6 is contaminated with 5 and/or
the corresponding phospholene
oxides. However, the reaction
can be used to obtain pure 5
from mixtures with the symmet-
rical isomer, if desired.
Scheme 2. Mechanistic proposal for isocyanate reactions of trivalent P compounds. ꢀ=trialkylphosphane
P
(R3P) or 4–6.
diimines 10, respectively (see below). It must be noted at
this point that the situation in the aromatic series (PhNCO,
The reaction proceeds much more slowly with secondary
isocyanates, cyclohexyl isocyanate reacting even slower than
2-propyl isocyanate, and does not proceed to a detectable
degree with tert-butyl isocyanate.
PhNCS, PhNCNPh) is not yet fully understood.
On treating excess 2-propylisocyanate with 1-methyl-3-
phospholene 6a in the presence of methylisocyanate, the
major reaction product was 1-(2-propyl)-2,4-dimethyl-1,4,2-
diazaphospholidine-3,5-dione 8,
alongside the N,N’-dimethyl
derivative 7 (R1 =R4 =Me; see
the Supporting Information).
The formation of 8 is indica-
tive of a stepwise mechanism
The new species exhibit the typical reactivity for trivalent
P compounds and readily form P-oxides, P-sulfides, and
quaternary onium salts.
for this reaction. The first step
is undoubtedly the formation of
The identity of the new compounds was unambiguously
ascertained by multinuclear NMR spectroscopy, high-resolu-
tion mass spectrometry and X-ray crystal structure determi-
nation (see the Supporting Information).
As the title compounds are air-sensitive, the P-sulfides 11
(1,2,4-trimethyl-1,4,2-diazaphospholidine-3,5-dione 2-sulfide)
and 12 (1,2,4-trimethyl-1,4,2-diazaphospholidine-3,5-dithione
2-sulfide) were chosen as suitable compounds for X-ray dif-
fraction investigations (Figure 1). Compound 11 was shown
to incorporate two slightly different molecules in the asym-
metric unit, 11a and 11b (the latter is omitted from Figure 1
for clarity).
an adduct, 6–MeNCO, which is, unlike 6 itself, reactive
enough to attack 2-propyl isocyanate in the second step. As
it is highly unlikely that the P atom in the 6–MeNCO
attacks the second isocyanate (in this case 2-propyl isocya-
nate) the mechanism of this new reaction differs considera-
bly from that proposed for the retro-McCormack reactions
discussed in Ref. [10a,b], in which the formation of phos-
phorane-type activated species involving both oxygen nucle-
ophiles in
(Scheme 2).
a
P-centered intermediate is proposed
Notably, P-phenyl phospholenes also react cleanly in
accordance with Scheme 1. This is surprising as no catalytic
activity towards alkyl isocyanates is shown by aryl dialkyl
The N-C-N-C-units in the five-membered rings of 11 and
12 are almost planar (torsion angles: 11a: À3.0(3)8;
11b: 0.3(3)8; 12: À1,15(6)8) with the P atom in a distorted
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F. U. Richter
[1] A. W. Hofmann, Chem. Ber. 1870, 3, 765.
[2] K.-H. Slotta, R. Tschesche, Chem. Ber. 1927, 60, 295.
[3] D. Liebsch, W. Altner, R. Kress (Bayer AG), DE-A 1670720, 1966.
[4] F. U. Richter, R. Halpaap (Bayer AG), EP-A 1982979, 2007.
[5] Z. Pusztai, G. Vlꢁd, A. Bodor, I. T. Horvꢁth, H. J. Laas, R. Halpaap,
F. U. Richter, Angew. Chem. 2006, 118, 113–116; Angew. Chem. Int.
Ed. 2006, 45, 107–110.
[6] K. Dimroth, in Comprehensive Heterocyclic Chemistry, Vol. 1, Per-
gamon, Oxford, 1983, pp. 514–518. and references therein.
[7] F. U. Richter, R. Halpaap (Bayer AG) EP 08004769.9, 2008 (submit-
ted).
[8] W. J. Balon (E. I. du Pont de Nemours and Company), US 2853 518,
1956 .
[9] T. W. Campbell, J. J. Verbanc (E. I. du Pont de Nemours and Com-
pany), US 2853 473, 1956.
Figure 1. Molecular structures of 11a and 12. Thermal ellipsoids are set
[10] a) C. D. Hall, J. D. Bramblett, F. F. S. Lin, J. Am. Chem. Soc. 1972,
94, 9264–9266; D. B. Denney, D. Z. Denney, C. D. Hall, K. L. Marsi,
J. Am. Chem. Soc. 1972, 94, 245–249; G. Scott, P. J. Hammond,
C. D. Hall, J. D. Bramblett, J. Chem. Soc. Perkin Trans. 2 1977, 882–
888; b) K. A. Mesch, L. D. Quin, Tetrahedron Lett. 1980, 21, 4791–
4794; c) For a non-preparative approach by photolysis, see: H. To-
mioka, S. Nakamura, T. Ohi, Y. Izawa, Bull. Chem. Soc. Jpn. 1976,
49, 3707–3708; d) Compared to the wealth of triazoles, PN2-hetero-
cycles, especially those with a P-N-C-N-C-sequence in the ring, are
rare and most of them contain the P atom in the PV oxidation state,
usually as P(O)OR. For example, see: R. Merten, C. Weber, Chem.
Ber. 1969, 102, 2143 (see also 1970 US 3551527) G. H. Birum (Mon-
santo), US 3980618, 1976. A. N. Pudovik, E. S. Batyeva, E. N. Ofit-
serov, O. G. Sinyashin, N. V. Ivasyuk, Izv. Akad. Nauk SSSR Ser.
Khim. 1977, 26, 1177–1180. I. V. Konovalova, R. D. Gareev, L. A.
Burnaeva, M. V. Cherkina, A. I. Khayarov, A. N. Pudovik, Zh.
Obshch. Khim. 1980, 50, 1165–1168. V. N. Kozhushko, N. K. Mikhai-
lyuchenko, E. B. Silina, V. V. Doroshenko, V. A. Shokol, Zh.
Obshch. Khim. 1987, 57, 1092–1097, and references therein. A. N.
Cherkolov, A. Y. Aksinenko, O. V. Korenchenko, V. B. Sokolov,
E. A. Fokin, I. V. Martynov, Zh. Obshch. Khim. 1990, 60, 1739–
1745. T. C. Kim, M.-R. Mazieres, R. Wolf, M. Sanchez, Tetrahedron
Lett. 1990, 31, 4459–4462. M. A. Pudovik, L. K. Kibardina, R. K. Al’
myanova, R. M. Kamalov, A. N. Pudovik, Izv. Akad. Nauk Ser.
Khim. 1996, 1857–1859.
at 50% probability.
tetrahedral environment with a maximum deviation from
tetrahedral geometry in the N2-P-C1 bond angle
(11a: 90.16(11)8, 11b: 90.40(11)8; 12: 89.81(6)8).
The chemistry of the new compounds will be the topic of
further investigations. In spite of the interesting complexa-
tion behavior of structurally related 2H-1,2,4-diazaphos-
ACHTUNGTRENNUNG
pholes reported by Streubel and co-workers,[11] they may
find applications as ligands in transition metal chemistry.
Acknowledgements
Support by Dres. H.-Chr. Weiss (X-ray analysis), J. Wesener (mass spec-
trometry), both of Currenta GmbH, K. Naumann and H.-D. Block, both
former Bayer AG, and N. Weferling, former Hçchst/Clariant AG, for
helpful discussions on phospholene chemistry is gratefully acknowledged.
[11] R. Streubel, H. Wilkens, F. Ruthe, P. G. Jones, Tetrahedron 2000, 56,
21–26 and references therein.
Keywords: dienes
·
isocyanates
·
organocatalysis
·
Received: January 7, 2009
phosphanes · phosphorus heterocycles
Published online: April 3, 2009
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Chem. Eur. J. 2009, 15, 5200 – 5202