Diverse Modes of Reactivity of Dialkyl Azodicarboxylates with P(III) Compounds: Synthesis, Structure, and Reactivity of Products Other than the Morrison-Brunn-Huisgen Intermediate in A Mitsunobu-Type Reaction

Article abstract of DOI:10.1021/jo035634d

The reactivity of diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD) with P(III) compounds bearing oxygen or nitrogen substituents is explored. Compounds with structures quite different from that of Morrison-Brunn-Huisgen intermediate R′<

Full text of DOI:10.1021/jo035634d

Diver se Mod es of Rea ctivity of Dia lk yl Azod ica r boxyla tes w ith
P (III) Com p ou n d s: Syn th esis, Str u ctu r e, a n d Rea ctivity of
P r od u cts Oth er th a n th e Mor r ison -Br u n n -Hu isgen In ter m ed ia te
in a Mitsu n obu -Typ e Rea ction
N. Satish Kumar,† K. Praveen Kumar,† K. V. P. Pavan Kumar,† Praveen Kommana,†
J agadese J . Vittal,‡ and K. C. Kumara Swamy*,†
School of Chemistry, University of Hyderabad, Hyderabad 500 046, A.P., India, and Department of
Chemistry, National University of Singapore, Science Drive 3, Singapore 117543, Singapore
kckssc@uohyd.ernet.in
Received November 7, 2003
The reactivity of diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD) with P(III)
compounds bearing oxygen or nitrogen substituents is explored. Compounds with structures quite
different from that of Morrison-Brunn-Huisgen intermediate R′3P+N(CO2R)N-(CO2R) (1), observed
in the Mitsunobu reaction, have been established by using X-ray crystallography and NMR
spectroscopy. Thus reactions with X(6-t-Bu-4-Me-C6H2O)2P-NH-t-Bu [X ) S (8), CH2 (9)] or XP(µ-
N-t-Bu)2P-NH-t-Bu [X ) Cl (14) or NH-t-Bu (15)] and DEAD/DIAD lead to phosphinimine-
carbamate-type of products X{6-t-Bu-4-Me-C6H2O}2P{N-t-Bu}{N(CO2R)NH(CO2R)} [X ) S, R )
Et (16); X ) CH2, R ) Et (17); X ) CH2, R ) i-Pr (18)] or XP(µ-N-t-Bu)2P(N-t-Bu){N-(CO2-i-Pr)-
N(H)(CO2-i-Pr) [X ) Cl (19), NH-t-Bu (20)]. Treatment of 19 with 2,2,2-trifluoroethanol afforded
the product [(CF3CH2O)P(µ-N-t-Bu)2P+(NH-t-Bu){N(CO2-i-Pr)(HNCO2-i-Pr)}][Cl-] (21) whose struc-
ture is close to one of the intermediates proposed in the Mitsunobu reaction. The isocyanate CH2-
(6-t-Bu-4-Me-C6H2O)2P-NCO (10) underwent 1,3-(P,C) cycloaddition with DEAD/DIAD to lead to
CH2(6-t-Bu-4-Me-C6H2O)2P{N(CO2R)N(CO2R)-C(O)-N} [R ) Et (22), i-Pr (23)]. Reaction of 22-23
with 1,1′-bi-2-naphthol or catechol leads to novel tetracoordinate CH2(6-t-Bu-4-Me-C6H2O)2P(2,2′-
OC10H6-C10H6-OH){NC(O)-(CO2R)NH(CO2R)} [R ) Et (24), i-Pr (25)] or pentacoordinate CH2(6-t-
Bu-4-Me-C6H2O)2P(1,2-O2C6H4){NHC(O)-N(CO2R)NH(CO2R)} [R ) Et (26), i-Pr (27)] compounds
in which the original NCO residue is retained; this mode of reactivity is quite different from that
observed for the MBH betaine 1. In 27, the nitrogen, rather than the oxygen, occupies an apical
position of the trigonal bipyramidal phosphorus violating the commonly assumed preference rules
for apicophilicity. It is shown that the previously reported azide derivative 3, obtained from the
reaction of 11 with DIAD, undergoes a Curtius-type rearrangement to lead to the fused
cyclodiphosphazane [{CH2(6-t-Bu-4-Me-C6H2O)2}P{OC(O-i-Pr)NN(CO2-i-Pr)N}]2 (28); this compound
is in equilibrium with its monomeric form in solution at >300 K. Finally, reaction of S(6-t-Bu-4-
Me-C6H2O)2P(OPh) (13) with DIAD gave the hexacoordinate compound S{6-t-Bu-4-Me-C6H2O}2P-
(OPh){N(CO2-i-Pr)NC(O-i-Pr)O} (30) with an intramolecular SfP bond. X-ray crystallographic
evidence for compounds 16, 19, 21, 22, 25, 27, 28, and 30 has been provided.
In tr od u ction
A combination of the redox couple of a triaryl- or
trialkylphosphine and dialkyl azodicarboxylate
a
[ROOCNdNCOOR, R ) Et (DEAD), i-Pr (DIAD)] is the
key to the success of the Mitsunobu reaction in a wide
range of synthetic applications including esterification
(eq 1).1 Although the Morrison-Brunn-Huisgen [MBH]
betaine 1 is the proven intermediate in the first step of
this reaction, the P-O bonded tetracoordinate species of
type 2 was also proposed as another possible intermedi-
ate in the earlier literature.2 Formation of the species 1
has been previously established by multinuclear NMR
and FTIR, but not by X-ray.3
Formation of the betaine 1 occurs through radical
cations of type RO2C-N-(P+Ph3)-N•-CO2R as shown by
EPR spectroscopy.2c Additionally, it is noted that treat-
ment of DIAD with tributylphosphine gives a much
weaker EPR signal relative to that with triphenylphos-
phine, while a much more intense signal could be
detected in the reaction of DIAD with tris(dimethylami-
† University of Hyderabad.
‡ National University of Singapore.
10.1021/jo035634d CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/14/2004
1880
J . Org. Chem. 2004, 69, 1880-1889

Dialkyl Azodicarboxylates
no)phosphine. These features suggest that the nature of
the intermediates in the Mitsunobu reaction could also
vary depending upon the P(III) precursor. Indeed it has
been observed that in the formation of 2-oxazolidones
from CO2 and ethanolamines with a Mitsunobu protocol,
the use of triphenylphosphine and tributylphosphine
afforded different isomers.1c As regards the esterification
with triphenylphosphine/DIAD, it is reported that when
the acid is added last, or when a large excess of azodi-
carboxylate and triphenylphosphine are used, radicals
are certainly generated prior to the formation of the
betaine 1.2c
A different facet for the reaction of P(III) compounds
with DIAD is illustrated by the structural characteriza-
tion of novel pentacoordinate phosphoranes 3-6 showing
the “reversed apicophilicity” phenomenon.4-6 It can be
noted that the [4+1] cycloaddition compounds 3-6
(1) Selected references: (a) Mitsunobu, O. Synthesis 1981, 1. (b)
Hughes, D. L. Org. React. 1992, 42, 335. (c) Kodaka, M.; Tomohiro, T.;
Okuno, H. (Y) J . Chem. Soc., Chem. Commun. 1993, 81. (d) Dodge, J .
A.; J ones, S. A. Rec. Res. Dev. Org. Chem. 1997, 1, 301. (e) Nikam, S.
S.; Kornberg, B. E.; Rafferty, M. F. J . Org. Chem. 1997, 62, 3754. (f)
Saylik, D.; Horvath, M. J .; Elmes, P. S.; J ackson, W. R.; Lovel, C. G.;
Moody, K. J . Org. Chem. 1999, 64, 3940. (g) Barrero, A. F.; Alvarez-
Manzaneda, E. J .; Chahboun, R. Tetrahedron Lett. 2000, 41, 1959. (h)
Weissman, S. A.; Rossen, K.; Reider, P. J . Org. Lett. 2001, 3, 2513. (i)
Liu, P.; J acobson, E. N. J . Am. Chem Soc. 2001, 123, 10772. (j) Snider,
B. B.; Song, F. Org. Lett. 2001, 3, 1817. (k) Paterson, I.; Savi, C. D.;
Tudge, M. Org. Lett. 2001, 3, 213. (l) Appendino, G.; Minassi, A.;
Daddario, N.; Bianchi, F.; Tron, G. C. Org. Lett. 2002, 4, 3839. (m)
Chandrasekhar, S.; Kulkarni, G. Tetrahedron: Asymmetry 2002, 13,
615. (n) Xu, J . Tetrahedron: Asymmetry 2002, 13, 1129. (o) Crimmins,
M. T.; Stanton, M. G.; Allwein, S. P. J . Am. Chem. Soc. 2002, 124,
5958. (p) Kan, T.; Fujiwara, A.; Kobayashi, H.; Fukuyama, T. Tetra-
hedron 2002, 58, 6267. (q) Shen, R.; Lin, C. T.; Bowman, E. J .; Bowman,
B. J .; Porco, J . A., J r. Org. Lett. 2002, 4, 3103. (r) Ahn, C.; Deshong, P.
J . Org. Chem. 2002, 67, 1754. (s) Lan, P.; Porco, J . A., J r.; South, M.
S.; Parlow, J . J . J . Comb. Chem. 2003, 5, 660.
involve both the N- and O-additions shown in 1 and 2,
suggesting that other modes of reactivity are possible
when the electronic environment at phosphorus is al-
tered. Direct reaction of phosphoramidites (Me2N)P(2,2′-
O2-(C10H6)2) with DIAD also afforded the pentacoordinate
phosphorane (Me2N)P[2,2′-O2-(C10H6)2][N(CO2-i-Pr)NC-
(O-i-Pr)O] [31P NMR: δ -36.7] that could be utilized for
the kinetic resolution of alcohols in an asymmetric
Mitsunobu reaction, although the details of the mecha-
nism are still not clear.5d With use of a similar protocol,
enantioselective reaction of racemic secondary alcohols
with phthalimide in the presence of (Me2N)P(2,2′-O2-
(C10H6)2) could be effected resulting in unreacted, enan-
tiomerically enriched, alcohols (eq 2).5e These results also
(2) (a) Brunn, E.; Huisgen, R. Angew. Chem., Int. Ed. Engl. 1969,
8, 513. (b) Huisgen, R.; Knorr, R.; Mo¨bius, L.; Szeimies, G. Chem. Ber.
1965, 98, 4014. (c) Camp, D.; Hanson, G. R.; J enkins, I. D. J . Org.
Chem. 1995, 60, 2977.
(3) (a) Varasi, M.; Walker, K. A. M.; Maddox, M. L. J . Org. Chem.
1987, 52, 4235. (b) Hughes, D. L.; Reamer, R. A.; Bergan, J . J .;
Grabowski, E. J . J . J . Am. Chem. Soc. 1988, 110, 6487. (c) Camp, D.;
J enkins, I. D. J . Org. Chem. 1989, 54, 3045. (d) Camp, D.; Hanson, G.
R.; J enkins, I. D. J . Org. Chem. 1995, 60, 2977. (e) Hughes, D. L.;
Reamer, R. A. J . Org. Chem. 1996, 61, 2967. (f) Ahn, C.; Correla, R.;
deShong, P. J . Org. Chem. 2002, 67, 1751.
(4) Satish Kumar, N.; Kommana, P.; Vittal, J . J .; Kumara Swamy,
K. C. J . Org. Chem. 2002, 67, 6653.
(5) For earlier literature on the formation of pentacoordinate
compounds from the reactions of P(III) species with DEAD/DIAD,
see: (a) Arbuzov, B. A.; Polezhaeva, N. A.; Vinogradova, V. S. Izv. Akad.
Nauk SSSR, Ser. Khim. 1968, 2525. (b) Gonclaves, H.; Domroy, J . R.;
Chapleur, Y.; Castro, B.; Faudet, H.; Burgada, R. Phosphorus Sulfur
1980, 147. (c) Majoral, J . P.; Kraemer, R.; Gando M’Pondo, T.; Navech,
J . Tetrahedron Lett. 1980, 21, 1307. (d) Hulst, R.; van Basten, A.;
Fitzpatrick, K.; Kellogg, R. M. J . Chem. Soc., Perkin Trans. 1 1995,
2961. (e) Li, Z.; Zhou, Z.; Wang, L.; Zhou, Q.; Tang, C. Tetrahedron:
Asymmetry 2002, 13, 145.
(6) For other interesting examples with “reversed apicophilicity”,
see: (a) Timosheva, N. V.; Prakasha, T. K.; Chandrasekaran, A.; Day,
R. O.; Holmes, R. R. Inorg. Chem. 1995, 34, 4525. (b) Prakasha, T. K.;
Chandrasekaran, A.; Day, R. O.; Holmes, R. R. Inorg. Chem. 1995,
34, 1243. (c) Timosheva, N. V.; Chandrasekaran, A.; Prakasha, T. K.;
Day, R. O.; Holmes, R. R. Inorg. Chem. 1996, 35, 6552. (d) Kojima, S.;
Kajiyama, K.; Nakamoto, M.; Akiba, K.-y. J . Am. Chem. Soc. 1996,
118, 12866. (e) Kajiyama, K.; Yoshimune, M.; Nakamoto, M.; Mat-
sukawa, S.; Kojima, S.; Akiba, K.-y. Org. Lett. 2001, 3, 1873. (f)
Kobayashi, J .; Goto, K.; Kawashima, T. The Ninth International
Symposium on Inorganic Ring Systems; Saarbru¨cken, Germany, J uly
23-28, 2000; Abstract No. P-115. (g) Kumaraswamy, S.; Muthiah, C.;
Kumara Swamy, K. C. J . Am. Chem. Soc. 2000, 122, 964. (h)
Kommana, P.; Kumaraswamy, S.; Vittal, J . J .; Kumara Swamy, K. C.
Inorg. Chem. 2002, 41, 2356. (i) Kojima, S.; Sugino, M.; Matsukawa,
S.; Nakamoto, M.; Akiba, K.-y. J . Am. Chem. Soc. 2002, 124, 7674. (j)
Kommana, P.; Satish Kumar, N.; Vittal, J . J .; J ayasree, E. G.; J emmis,
E. D.; Kumara Swamy, K. C. Org. Lett. 2004, 6, 145.
suggest that even when the initial products of the P(III)
compound with DEAD/DIAD are different from a betaine
of type 1, Mitsunobu reaction takes place smoothly, thus
leaving room to explore other P(III) compounds for
specific reactions.
The order of addition of the acid and the alcohol to
betaine 1 in the Mitsunobu esterification has a profound
effect on the reaction pathway, implying potential duality
of the mechanism.7 Thus different types of phosphorus
intermediates could be involved depending upon the
order of addition. The intermediate from the reaction
between the acid R′COOH and the betaine 1 (obtained
from PPh3) is a species of type {RO2CN-(P+Ph3)-NH-
CO2R)}(R′CO2-) (7);3 the stability of this species may be
(7) Paudard-Cooper, A.; Evans, S. A., J r. J . Org. Chem. 1989, 54,
2485.
J . Org. Chem, Vol. 69, No. 6, 2004 1881

Kumar et al.
(i) First examples of imino-phosphorus compounds (X-
ray evidence) 16-20,12 that have a structure halfway
between the classical MBH betaine 1 and protonated
betaine in the Mitsunobu reaction. In addition to posing
a unique structural problem, a low-temperature 31P NMR
study on compounds 16 and 17 reveals an unprecedented
solution state behavior wherein at least four isomeric
phosphoranes are present. While compound 16 is unre-
active toward 2,2,2-trifluoroethanol, compound 19 forms
the salt 21 that has a structure (X-ray) analogous to the
intermediate 7 proposed in the Mitsunobu esterification
process.3
enhanced by hydrogen bonding. Oxaphosphorane
intermediates, Ph3P(OR′′)2, are formed by the reaction
of either 1 or 7 with alcohols. Reaction of 7 with 1 mol
equiv of R′′OH leads to the phosphonium salt
[Ph3P+(OR′′)](R′CO2-); the latter can also be formed from
Ph3P(OR′′)2 and R′CO2H.7 It is possible that some of these
intermediates can be stabilized by a suitable choice of
the P(III) precursor. This, together with the fact that
dialkyl azodicarboxylates ROOCNdNCOOR are electron-
poor and hence could participate in other types of
cycloadditions [e.g. Diels-Alder, dipolar],8 has prompted
us to look into their behavior toward P(III) compounds
possessing reactive functional groups. We also felt that
in this process we may be able to isolate compounds of
type 7. Finally, it was also our intention to probe the
reactivity of products of types other than 1 that could
generate new chemistry.
As P(III) precursors for our studies, we have chosen
compounds 8-15 (Chart 1). In the original Mitsunobu
reaction, triphenylphosphine that contains three carbons
attached to phosphorus is utilized. The phosphorus atom
in the P(III) compounds used in the present study is
bonded to oxygen and/or nitrogen atoms, thus rendering
it electronically different from that in Ph3P.9 The precur-
sors 14-15 have two reactive P(III) centers that facilitate
a comparison of their reactivity. All these P(III) com-
pounds can be readily prepared and are inexpen-
sive.6g-h,10,11 More importantly, they possess a sterically
encumbered phosphorus, a feature that could facilitate
isolation and structural characterization of the products
and study of their reactivity. Since it is shown (cf. eq 2)
that Mitsunobu esterification can be effected even with
the intermediacy of species other than the betaine 1, we
believe that the data presented here can lead to the
extension of related organic methodology.
(ii) X-ray structural proof for a previously unknown
mode of dipolar cycloaddition involving the P(III) isocy-
anate 10 and DEAD/DIAD to lead to 22 or 23.13 Addition
of 1,1′-bi-2-naphthol or catechol to 22-23 proceeds by an
unusual pathway in which the DEAD/DIAD residue is
retained, leading to the respective tetra- and pentacoor-
dinate compounds 24-25 and 26-27. In the previously
reported general reaction of betaine 1 with diols leading
to pentacoordinate phosphorus compounds, the DEAD/
DIAD residue was cleaved.14 Compounds 25 and 27 are
characterized by X-ray structure determination.
CHART 1
In this paper, we highlight the following points (all
based on the reaction of P(III) compounds with DEAD/
DIAD).
(8) Weinreb, S. M. In Comprehensive Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Paquette, L. A., Vol. Ed.; Pergamon: Oxford,
UK, 1996; Vol. 5, Chapter 4.2, pp 401-449.
(9) Reaction of CH2(6-t-Bu-4-Me-C6H2O)2P(t-Bu) with DIAD afforded
the oxidized product CH2(6-t-Bu-4-Me-C6H2O)2P(O)(t-Bu) [mp 262-
264 °C, δ(P) 27.2] in 72% isolated yield. No other phosphorus product
was identified in this case.
(iii) A Curtius-type rearrangement of 3 (obtained from
the reaction of 11 with DIAD) leading to 28 (X-ray) that
is dynamic, with respect to its monomeric form, in
solution.
1882 J . Org. Chem., Vol. 69, No. 6, 2004

Dialkyl Azodicarboxylates
SCHEME 1
(iv) Isolation of penta- and hexacoordinate phospho-
ranes 29-30, the latter with an SfP coordination (X-
ray), from the reaction of 12-13 with DIAD. This
provides a synthetic route to a new class of hexacoordi-
nate phosphorus compounds.
pentacoordinate methylamino compound 5.4 There is also
a fairly strong band at 1211 cm-1 ascribable to ν(PdN).
For the question as to why the X-ray structure of -NHMe
derivative 5 shows pentacoordination, whereas the -NH-
t-Bu compound 16 shows tetracoordination, the factors
responsible could be the bulkiness of the t-Bu group
resulting in a very strong PdN bond. This together with
the hydrogen bonding involving the -NH-C(O) group is
the likely driving force for the stability of the phosphin-
imine-carbamate form compared to the betaine form [cf.
Scheme 1]. It is important, however, to note that com-
pounds 16-18 can be considered to be tautomeric forms
of the corresponding betaines Y{6-t-Bu-4-Me-C6H2O}2P+-
{NH-t-Bu}{N(CO2R)N-(CO2R)} [Y ) CH2, S].
These four points will be discussed in the same order
below.
Resu lts a n d Discu ssion
(i) Im in o Com p ou n d s 16-20 a n d th e Sa lt 21. The
reaction of 8-9 with DEAD or DIAD in toluene leads to
compounds 16-18, most likely via betaines analogous to
1 [Scheme 1]. The X-ray structure of 16 (see the Sup-
porting Information for the drawing) clearly shows (i) a
strong PdN(t-Bu) bond [P-N(3) 1.464(4) Å]15 and (ii) the
carbamate-type linkage -NH-C(O)OR that is a hydrogen-
bonded dimer through the NH and the CdO moieties.
There is no significant SfP interaction in 16. The IR
(KBr) spectra of these compounds showed two ν(NH)
bands [for 16 at 3262, 3159 cm-1] consistent with the
carbamate-phosphinimine structure;16 this is different
from a single ν(NH) band at 3383 cm-1 observed for the
However, the solution and the solid-state 31P NMR
spectra of 16 [Figure 1] appear to be inconsistent with
the X-ray structure. The δ(P) value of -56.3 [C6D5CD3,
298 K, sharpens at higher temperatures] for 16 is clearly
in the pentacoordinate region (cf. compounds 3-6)4 and
quite upfield to the tetracoordinate region [cf. compounds
24-25 and 31]. The solid-state 31P NMR signal [δ -61.1]
(10) Sherlock, D. J .; Chandrasekaran, A.; Day, R. O.; Holmes, R. R.
J . Am. Chem. Soc. 1997, 119, 1317.
(11) For the synthesis of cyclodiphosphazane derivatives 14-15,
see: J efferson, R.; Nixon, J . F.; Painter, T. M.; Keat, R.; Stobbs, L. J .
Chem. Soc., Dalton Trans. 1973, 1414.
is also in the pentacoordinate region.6g-h,18 Rather as-
tonishingly, at least four different signals could be clearly
identified at 255 K [cf. Figure 1]; upon warming to 298
K, the original spectrum was obtained. On the basis of
previously available data,4,18 these signals can be ascribed
to pentacoordinate isomers A-D [Chart 2, the extra low
intensity peak at 273 K is perhaps due to a conforma-
(12) We have also utilized (morpholino)P(N-t-Bu)2P(NH-t-Bu) wherein
the chlorine in 14 is replaced by a morpholino group. Interestingly,
we have obtained the analogous compound (morpholino)P(µ-N-t-Bu)2P-
(N-t-Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)} (X-ray) in which only the -NH-
t-Bu end has reacted. Details are available from the authors.
(13) The corresponding sulfur compound S(6-t-Bu-4-Me-C6H2O)2P-
{N(CO2-i-Pr)N(CO2-i-Pr)-C(O)-N} [mp 58-60 °C; IR (KBr) 2268, 1711,
1660 cm-1
Me-C6H2O}2PNCO
Kumara Swamy, K. C. Unpublished results.
;
31P NMR δ 23.6] can also be prepared with S{6-t-Bu-4-
[
31P NMR δ 129.4]: Pavan Kumar, K. V. P.;
(14) Bone, S. A.; Trippett, S. J . Chem. Soc., Perkin Trans. 1 1976,
156.
(15) The PdN bond length in Ph3PdNC6H4Br is 1.57 Å. See:
Corbridge, D. E. C. PhosphorussAn Outline of its Chemistry, Biochem-
istry and Technology, 4th ed.; Elsevier: Amsterdam, The Netherlands,
1990; p 47.
(16) Generally it is expected that amides show two or more bands
for the NH-C(O) group due to hydrogen-bonded oligomers; we believe
that our compounds also behave similarly. See: Williams, D. H.;
Fleming, I. Spectroscopic Methods in Organic Chemistry, 4th ed.; Tata
McGraw-Hill: New Delhi, India, 1987; pp 60-61 (Figures 2.13a-c).
(17) It is expected that the solution to solid-state difference in δ(P)
will be ∼4 ppm [see: Holmes, R. R.; Prakasha, T. K.; Pastor, S. D. In
Phosphorus-31 NMR Spectral Properties in Compound Characteriza-
tion and Structural Analysis; Quin, L. D., Verkade, J . G., Eds.; VCH:
New York, 1994; p 27], but in 16 this difference is slightly higher.
(18) (a) Said, M. A.; Pu¨lm, M.; Herbst-Irmer, R.; Kumara Swamy,
K. C. J . Am. Chem. Soc. 1996, 118, 9841. (b) Said, M. A.; Pu¨lm, M.;
Herbst-Irmer, R.; Kumara Swamy, K. C. Inorg. Chem. 1997, 36, 2044.
(c) Kumaraswamy, S.; Kommana, P.; Satish Kumar, N.; Kumara
Swamy, K C. J . Chem. Soc., Chem. Commun. 2002, 40.
J . Org. Chem, Vol. 69, No. 6, 2004 1883

Kumar et al.
NH-t-Bu compounds with DEAD/DIAD, the imine-
carbamate type of structure is favored.
Compound 19 exhibits broad (solid as well as solution)
signals in the 31P NMR in tri-, tetra-, and pentacoordinate
regions [major peaks in C6D5CD3: 138.6 and 10.5 with
2J (PP) < 5.0 Hz; 133.6 and -34.6 with 2J (PP) of ∼16.0
Hz]. There was no significant improvement in the quality
of the solution spectra at lower temperatures and the
compound could be recovered after removal of solvent
from the solution. Thus for 19, we think that all the three
forms, betaine, imine-carbamate, and pentacoordinate
structures, are present in solution. Compound 20, how-
ever, exhibited relatively sharp signals [CDCl3: δ 68.9
for P(III) and -28.9 for the tetra-/pentacoordinate P (see
2
above for discussion), J (PP) < 5.0 Hz].
When compound 19 is treated with 1 mol equiv of 2,2,2-
trifluoroethanol, the P(III)-Cl end reacts and the proton
from the liberated HCl adds to the nitrogen at the
PdN-t-Bu end to afford 21 (eq 3). This compound can be
F IGURE 1. Solution (VT) and solid-state (5 kHz) 31P NMR
spectra for compound 16; ssb refers to spinning sidebands. The
spinning sidebands were verified by recording the solid-state
spectrum at 7 kHz also.
CHART 2
considered to be a protonated form of the betaine (CF3-
CH2O)P(µ-N-t-Bu)2P+(NH-t-Bu){N-(CO2-i-Pr)-N-(CO2-i-
Pr)}. An X-ray structural analysis (see the Supporting
Information) clearly reveals the hydrogen-bonded chlo-
ride. This kind of species [cf. structure 7] is one of the
intermediates proposed in the Mitsunobu reaction.
Compound 20 did not react with 2,2,2-trifluoroethanol
as shown by 31P NMR. However, when benzoic acid was
added to a solution of 20 prepared in situ, the 31P NMR
of the solid obtained (after removal of all solvent)
exhibited two major peaks (∼95%) at δ 80.7 and 1.1
(2J (PP) < 7.0 Hz) that are quite different from that of
20, but clearly in the tri- and tetracoordinate region. The
1H NMR spectrum is also different from that of 20 and
agrees with structure E, which is essentially the type of
intermediate [cf. 7 and 21] proposed in the Mitsunobu
reaction. So far, we have not succeeded in obtaining the
single crystals of this material.
tional isomer involving the eight-membered ring6g,h]. The
bulky -NH-t-Bu group may not favor significant hexa-
coordination via the SfP bond; this assumption is
consistent with a similar spectral pattern observed for
17 (see the Supporting Information for details) also. To
our knowledge, this is the first ever observation of four
distinct isomeric phosphoranes in solution.
It is likely that the formation of cyclophosphazane
compounds XP(µ-N-t-Bu)2P(N-t-Bu){N-(COO-i-Pr)-N(H)-
(COO-i-Pr)} [X ) Cl (19), NH-t-Bu (20); see above for
drawings] from their precursors 14-15 also occurs
through a betaine in a manner similar to that for 16. The
electron-rich P-NH-t-Bu end of 14 is the reactive center
and the P-Cl end remains intact during the reaction.
The X-ray structure of 19 (see the Supporting Informa-
tion) shows (i) a very short PdN bond [P-N 1.488(3) Å]
and (ii) a hydrogen-bonded carbamate NH-C(O)OR dimer;
these features are analogous to that observed for 16. The
IR spectrum shows a strong band at 1202 cm-1 ascribable
to ν(PdN). Thus it appears that in reactions of P(III)-
(ii) Cycloa d d ition Com p ou n d s 22-23 a n d Th eir
Rea ction s w ith Ca tech ol a n d 2,2′-Bin a p h th ol To
Lea d to Ad d ition Com p ou n d s 24-27. In the case of
the P(III) isocyanate, CH2(6-t-Bu-4-Me-C6H2O)2P-NCO
(10), the reaction with DEAD/DIAD takes an entirely
different turn with the formation of the cyclic products
22-23, presumably via betaine (eq 4) in a stepwise
pathway.19 The structure of 22 is proven by X-ray
crystallography.20 The formal PdN bond in this com-
1884 J . Org. Chem., Vol. 69, No. 6, 2004

Dialkyl Azodicarboxylates
SCHEME 2
pound [P-N 1.564(4) Å] is significantly longer than that
in 16 or 19, suggesting some phosphonium character in
it. Despite having a structure different from that of the
betaine 1, compound 23 does participate in the Mitsunobu
coupling between ethanol and benzoic acid, suggesting
that the five-membered heterocycle is in equilibrium with
the betaine.
Previously, we have characterized the pentacoordinate
isothiocyanate compound 4, obtained from analogous
reaction of CH2(6-t-Bu-4-Me-C6H2O)2P-NCS with DIAD;4
the difference in the reactivity of this isothiocyanate from
the isocyanate 10 probably stems from a better charge
separation at the CdO end in the latter compared to the
CdS end in the former. Reaction of 10 with DEAD/DIAD
affording the heterocycles 22-23 can, however, be com-
pared to the 1,3-(P,C) cycloaddition of 10 with MeO2CCt
CCO2Me (DMAD, a dipolarophile) to form the heterocycle
32.18c
also falls into the category of pentacoordinate compounds
with reversed apicophilicity that was alluded to in the
Introduction.
(iii) Cu r tiu s-Typ e Rea r r a n gem en t of th e Azid e
P r od u ct 3 To Lea d to 28. The behavior of the P(III)
azide 11 compared to the isocyanate 10 toward DEAD/
DIAD is different, although the azide (-N3) group is
isoelectronic to isocyanate (NdCdO). Isolation of the
pentacoordinate azide CH2(6-t-Bu-4-Me-C6H2O)2P(N3)-
{N(CO2-i-Pr)N-C(O-i-Pr)-O} (3) in the reaction of 11 with
DIAD has been reported earlier.4 However, during the
process of recrystallization of 3, it was noticed that a
second type of crystals (28, rectangular blocks) with
morphology much different from 3 (needles) formed at 5
°C over a period of several days. The same compound is
also formed when a toluene solution of 3 is heated. The
X-ray structure of 28 showed it to be a fused cyclodiphos-
phazane derivative (Scheme 3) that is inconsistent with
the 31P NMR spectrum of 28 [C6D5CD3, δ(P) -6.7] at room
temperature. A variable-temperature 31P NMR study [see
the Supporting Information] reveals that there is a
dynamic equilibrium between the monomer (28′)21 and
In an earlier work, Trippett and co-workers used the
betaine 1 (R ) Et, R′ ) Ph) as a precursor to various
pentacoordinate phosphoranes [cf. Scheme 2a].14 In these
reactions, DEAD becomes the hydrazine derivative EtO2-
CNHNHCO2Et and the pentacoordinate compound Ph3P-
[1,2-O2C6H4] is formed. Later, this method was utilized
for probing the mechanism of the Mitsunobu reaction.7
In contrast to these, compounds 22-23 undergo a two-
step addition depending on the diol. First the P-N single
bond is cleaved and then addition across the PdN
(double) bond takes place. When 1,1′-bi-2-naphthol is
used, the reaction stops at the first stage to lead to
tetracoordinate compounds 24 and 25. When catechol is
used, the addition across the PdN bond also takes place
to lead to the pentacoordinate compounds 26-27. These
reactions are shown in Scheme 2b. The structures of 25
and 27 have been confirmed by X-ray crystallography.
The PdN bond in 25 [P-N(1) 1.546(3) Å] is slightly
shorter than that in 22, but is still significantly longer
than those in 16 or 19. In the pentacoordinate compound
27 the newly formed -NHC(O)N(CO2-i-Pr)NH(CO2-i-Pr)
and a catecholate oxygen are at the apical positions of
trigonal-bipyramidal phosphorus. Thus, this compound
SCHEME 3
(19) Most of the cycloadditions are assumed to take place in a
concerted fashion. However, there are examples in which the cycload-
dition takes place stepwise. See: Huisgen, R.; Mloston, G.; Langhals,
E. J . Org. Chem. 1986, 51, 4087.
(20) Previously a weak interaction between the â-nitrogen with
isocyanate carbon was envisaged for compound 23 (ref 4), but no proof
could be given.
J . Org. Chem, Vol. 69, No. 6, 2004 1885

Kumar et al.
the dimer (28) with the former predominating at higher
temperatures. Formation of 28′ (and hence 28) must
occur through N2 elimination from 3 followed by rear-
rangement; this is analogous to the formation of isocy-
anates from acid azides via Curtius rearrangement. Thus
the identification of the monomer 28′ in addition to the
structural characterization of 28 feature gives firm
evidence for Curtius-type rearrangement involving pen-
tacoordinate phosphorus.22
(iv) F or m a tion of P en ta coor d in a te P r od u ct 29
a n d Hexa coor d in a te P r od u ct 30. In the precursors
CH2(6-t-Bu-4-Me-C6H2O)2P(OCH2CH2NMe2) (12) and S(6-
t-Bu-4-Me-C6H2O)2P(OPh) (13) the phosphorus has three
oxygen atoms around it. In addition, there is a possibility
of NfP or SfP coordination that could stabilize hyper-
valent phosphorus. Treatment of 12 with DIAD afforded
only the pentacoordinate compound 29 [31P NMR, δ(P)
-59.6, -66.7 (1:5)]; it is likely that steric factors have
prevented the formation of the additional NfP coordi-
nate bond. Hexacoordination, however, is realized in the
product S(6-t-Bu-4-Me-C6H2O)2P(OPh)[N(CO2-i-Pr)-N-
C(O-i-Pr)O] [30: δ(P) -67.0] obtained by starting with
13. The SfP distance of 2.794(1) Å is in the range
expected for such hexa-coordinate compounds.10,23 Thus
a new range of hexacoordinate phosphoranes can be
realized with these reactions.
Exp er im en ta l Section
The precursors CH2{6-t-Bu-4-Me-C6H2O}2PCl,6h S{6-t-Bu-
4-Me-C6H2O}2PCl,10 and XP(µ-N-t-Bu)2PNH-t-Bu [X ) Cl (14,
bp 140 °C/3 mm)11 and NH-t-Bu (15)11] were prepared by
literature procedures and sublimed/crystallized prior to use.
The P(III) precursors S{6-t-Bu-4-Me-C6H2O}2PX [X ) NH-t-
Bu (8), OPh (13)] or CH2{6-t-Bu-4-Me-C6H2O}2PX [X ) NH-
t-Bu (9), NCO (10),18c N3 (11),6g OCH2CH2NMe2 (12)] were
prepared by the reaction of the chloro compound with the
appropriate reagent from the following: (i) tert-butylamine,
(ii) phenol + triethylamine, (iii) sodium cyanate, (iv) sodium
azide, or (v) 2-(dimethylamino)ethanol.
S{6-t -Bu -4-Me -C6H 2O}2P (N-t -Bu ){N(CO2E t )NH (CO2-
Et)} (16). To a stirred solution of 8 [δ(P) 138.2] (1.23 g, 2.68
mmol) in toluene (15 mL) was added diethyl azodicarobxylate
(DEAD) (0.46 g, 2.68 mmol) dropwise over a period of 10 min.
After being stirred for 24 h, the solution was concentrated (ca.
4 mL) and heptane (ca. 1 mL) was added to obtain 16 as a
crystalline solid. Yield: 1.35 g (80%). Mp: 170-171 °C. IR
(KBr, cm-1): 3262, 3159, 2965, 2361 (w), 1719, 1422, 1287,
1211, 1098. The two bands at 3262 and 3159 cm-1 are
indicative of an amide -HNC(O) linkage. IR (CHCl3): 3416,
3250 (vw), 3019, 1736, 1426, 1285, 1215, 1096 cm-1. H NMR
1
(CDCl3): δ 1.11 (d, J ∼ 2.0 Hz, 9 H), 1.24 and 1.46 (2 s, 18 H),
1.32 (t, J ) 7.0 Hz, 6 H), 2.25 and 2.32 (2 s, 6 H), 4.30 (qrt, J
) 7.0 Hz, 4 H), 6.45 (br, 1 H), 7.08-7.31 (m, 4 H). 13C NMR
(CDCl3): δ 14.7, 20.5, 20.8, 30.1, 31.3, 33.5 (d, J ) 10.2 Hz),
34.4, 35.9, 51.2 (br), 61.9, 62.8, 120.0, 128.0, 128.5, 130.3, 130.8,
133.1, 133.7, 135.8, 148.0, 155.0 (d, J ∼ 12.0 Hz), 155.9. 31P
NMR (CDCl3): δ -56.2 (br). 31P NMR (solid-state, center band
determined by recording the spectra at 5 and at 7 kHz): δ
-61.1. Anal. Calcd for C32H48N3O6PS: C, 60.67; H, 7.58; N,
6.64. Found: C, 60.42; H, 7.46; N, 6.59. This compound did
not react with 2,2,2-trifluoroethanol at room temperature [31P
NMR].
Su m m a r y
What we have shown here is that the naive-looking
reaction of DEAD/DIAD with P(III) compounds, the key
to the enormous synthetic utility of the Mitsunobu
reaction, leads not just to the MBH betaine 1, but has
the potential to open up new frontiers. When electrone-
gative substituents are present on P(III) precursors, one
of the preferred pathways is the formation of pentacoor-
dinate derivatives; if a suitable donor site in the substit-
uents is available, hexacoordination is also possible.
When reactive functionalities (NCO, N3, etc.) are present
on phosphorus, other pathways including cycloaddition
are possible. The molecular formulas of the pentacoor-
dinate (and hexacoordinate) compounds [3-6, 29-30] or
cycloaddition compounds [22-23] are the same as that
of the corresponding betaines, but additional covalent
bonding [O-P or N-C] makes them structurally different
from the betaines. Finally, if a tert-butylamino group is
available on the P(III) precursor, the phosphinimine-
carbamate compounds [X-ray; e.g. 16-19] are favored.
These products are useful in stabilizing some of the
intermediates proposed in the Mitsunobu reaction [e.g.
21]. While compounds 16-19 can be considered to be the
tautomeric forms of the corresponding betaines, com-
pound 21 is the protonated form of the betaine. Cur-
rently, we are exploring this aspect further.
CH2(6-t-Bu -4-Me-C6H2O)2P (N-t-Bu ){N(CO2Et)NH(CO2-
Et)} (17). DEAD (0.17 g, 1.0 mmol) was added in one lot to a
solution of 9 [mp 116-118 °C, δ(P) 141.9] (0.44 g, 1.0 mmol)
in dry toluene (10 mL) and the mixture was stirred for 72 h
at room temperature. Removal of all the solvent afforded 17
as a white solid. Crystallization was done with a toluene-
heptane mixture [4:3; 0.48 g (76%)]. Mp: 96-98 °C. IR (KBr,
cm-1): 3258, 3164, 1719, 1437, 1345, 1279, 1206, 1134, 1062,
933. IR (CHCl3, cm-1): 3407, 3299, 2967, 1738, 1443, 1219.
1H NMR (CDCl3): δ 0.97, 1.32, and 1.45 (3 br s, 33 H), 2.24
and 2.35 (2 s, 6 H), 3.75 (br m, 1 H), 4.17-4.28 (m, 3 H), 4.45
(qrt, J ∼ 4.5 Hz, 2 H), 6.50 (br s, 1 H), 6.93-7.10 (m, 4 H). 13
C
NMR (CDCl3, a complex spectrum; for reasons see 31P NMR
data): δ 13.5, 14.0, 14.7, 20.8, 20.9, 30.6, 31.3, 31.8, 33.7, 33.8,
34.4, 34.9, 35.4, 36.1, 50.3, 50.5, 61.9, 63.1, 65.3, 126.7, 127.0,
128.9, 129.2, 130.0, 132.6, 132.8, 133.4, 140.2, 140.4, 147.0,
147.2, 154.5, 154.9, 155.6. 31P NMR (C6D5CD3): 298 K δ -56.3
(br), 242 K δ -48.0, -50.3 (major), -53.4, -54.2. 31P NMR
(solid state, taken at 5 and 7 kHz to determine the center
peak): δ -50.2 (>93%), -2.5 (unassigned, minor). Anal. Calcd
for C33H50N3O6P: C, 64.36; H, 8.18; N, 6.82. Found: C, 64.55;
H, 8.13; N, 6.90.
CH2(6-t-Bu -4-Me-C6H2O)2P (N-t-Bu ){N(CO2-i-P r )NH(CO2-
i-P r )} (18). This compound was prepared similarly by using
the same molar quantities as the ethyl compound in a yield of
87%. Mp: 174-178 °C. IR (KBr, cm-1): 3346, 1713, 1694, 1601,
1468, 1389, 1302, 1204, 1105, 1028, 949, 916. IR (CHCl3, cm-1):
(21) The monomeric structure 28′ is consistent with the 31P NMR
chemical shifts for similar phosphinimines [e.g. δ(P) -8.3 for (EtO)3Pd
NSiMe3]. See: Tebby, J . C. In Phosphorus-31 NMR Spectroscopy in
Stereochemical Analysis; Verkade, J . G., Quin, L. D., Eds.; VCH:
Deerfield Beach, FL, 1987; pp 1-60.
(22) In the only other reported example in the literature, the
structure of a similar dimeric species was assigned based on NMR and
the mass spectrum; the monomeric species of type 28′ could not be
detected. See: Baceiredo, A.; Bertrand, G.; Majoral, J .-P.; Wermuth,
U.; Schmutzler, R. J . Am. Chem. Soc. 1984, 106, 7065.
(23) Holmes, R. R.; Prakasha, T. K.; Day, R. O. Inorg. Chem. 1993,
32, 4360.
1
3418, 3281, 2926, 1744, 1460. H NMR (CDCl3): δ 1.32 (d, J
) 6.2 Hz, 6 H), 1.38 (s, 18 H), 1.42, 1.45, and 1.49 (3 lines, 15
H), 2.33 and 2.35 (2 s, 6 H), 3.42 (dd, J ) 2.7 and 16.0 Hz, 1
H), 5.17 (m, 3 H), 5.32 (s, 2 H), 7.07-7.20 (m, 4 H). 13C NMR
(CDCl3): δ 20.9, 21.1, 21.6, 22.0, 22.6, 30.9, 33.3 (d, J ) 20.7
Hz), 34.9, 35.3, 35.9, 70.0, 53.4, 74.8, 126.7, 127.1, 128.8, 129.4,
129.5, 133.0, 133.3, 134.6, 134.7, 140.0, 140.2, 147.2, 148.4,
149.1, 153.3. 31P NMR (CDCl3): δ -57.6 (br). Anal. Calcd for
C
35H54N3O6P: C, 65.93; H, 8.29; N, 6.40. Found: C, 66.02; H,
8.35; N, 6.48.
1886 J . Org. Chem., Vol. 69, No. 6, 2004

Dialkyl Azodicarboxylates
ClP (µ-N-t-Bu )2P (N-t-Bu ){N-(COO-i-P r )-N(H )(COO-i-
P r )} (19). DIAD (0.59 g, 2.92 mmol) in toluene (10 mL) was
added dropwise to a stirred solution of 14 (0.91 g, 2.92 mmol)
in toluene (20 mL). The mixture was stirred for 24 h at room
temperature and concentrated to 2 mL, heptane (2 mL) was
added to the residue, and crystals of 19 were obtained at 5 °C
after ca. 24 h. Yield: 1.23 g (78%). Mp: 118-120 °C. IR (KBr,
cm-1): 3368, 3221, 1759, 1711, 1306, 1202, 899. 1H NMR
(CDCl3): δ 1.26-1.56 (many lines, 39 H), 4.92-5.18 (m, 2 H).
13C NMR (CDCl3): δ 21.7, 27.8, 28.1, 30.8, 31.2, 51.8, 52.1,
52.5, 56.1, 57.0, 69.5, 70.1, 70.4, 128.1, 128.9, 153.1, 155.5. The
spectra taken in toluene-d8 were also too complicated to make
a detailed assignment. 31P NMR (CDCl3): δ -38.0, 11.8, 133.6,
140.1. Anal. Calcd for C20H42ClN5O4P2: C, 46.74; H, 8.23; N,
13.63. Found: C, 46.65; H, 8.32; N, 13.88.
δ 1.13 and 1.27 (2 t, J ∼ 6.0 Hz, 6 H), 1.45 (s, 18 H), 2.25 (s,
6 H), 3.48 (d, J ) 15.8 Hz, 1 H), 4.14 (br m, 4 H), 5.61 (br s, 1
H), 6.54 (br s, 1 H), 6.77-8.14 (m, 16 H). 13C NMR (CDCl3): δ
14.0, 14.3, 20.9, 30.2, 30.5, 34.4, 34.7, 61.8, 62.2, 63.0, 111.8,
114.2, 117.9, 119.2, 120.1, 123.1, 124.5, 125.3, 125.9, 126.1,
126.2, 126.9, 127.1, 127.2, 127.4, 127.9, 128.2, 128.3, 129.0,
129.1, 129.4, 129.9, 131.0, 131.6, 133.7, 133.9, 134.6, 135.0,
140.4, 140.6, 147.6, 152.8, 155.4. 31P NMR (CDCl3): δ -7.4.
Anal. Calcd for C50H54N3O9P: C, 68.89; H, 6.20; N, 4.82.
Found: C, 68.63; H, 6.10; N, 4.62.
CH2(6-t-Bu -4-Me-C6H2O)2P (2,2′-OC10H6-C10H6-OH){NC-
(O)-(CO2-i-P r )NH(CO2-i-P r )} (25). Racemic 1,1′-bi-2-naph-
thol (0.29 g, 1.0 mmol) was added in one lot to a solution of 23
[mp: 170-173 °C4] (0.61 g, 0.99 mmol) in dry THF (10 mL)
and the mixture was stirred for 72 h at room temperature.
Crystallization was done with a toluene-heptane mixture (4:
3, ca. 5 mL). Yield: 0.68 g (76%). Mp: 145-148 °C. IR (cm-1):
3378, 1732, 1682, 1470, 1385, 1198 (vs), 1101, 1005. The strong
band at 1198 cm-1 is most likely the ν(PdN) stretch. 1H NMR
(CDCl3): δ 1.12, 1.16 (2 d + br s, 21 H), 1.46 (s, 9 H), 2.25 (s,
6 H), 3.50 (d, J ) 14.5 Hz, 1 H), 4.83-4.95 (m, 2 H), 6.40 (br,
1 H), 6.67-8.11 (m, 16 H). 13C NMR (CDCl3): δ 20.7, 21.3,
21.5, 21.8, 30.1, 30.4, 34.2, 34.6, 69.4, 70.7, 114.9, 118.6, 120.2,
122.9, 124.4, 125.2, 125.6, 126.0, 126.8, 127.1, 127.7, 128.1,
128.9, 129.0, 129.6, 130.0, 130.5, 131.5, 133.6, 134.7, 137.7,
140.2, 146.8, 147.5, 152.4, 152.8, 154.8. 31P NMR (CDCl3): δ
-7.6. Anal. Calcd (after drying in a vacuum for 2 h) for
(t-Bu NH )P (µ-N-t-Bu )2P (N-t-Bu ){N-(COO-i-P r )-N(H )-
(COO-i-P r )} (20). The procedure was the same as that for 19
with use of 15 [mp 142-144 °C; δ(P) 89.4] (2.26 g, 7.46 mmol)
and DIAD (1.42 g, 7.46 mmol) to yield 20 [3.03 g, 74%]. Mp:
104-106 °C. IR (cm-1): 3380, 3192, 1761, 1720, 1665, 1391,
1308, 1260. 1H NMR (CDCl3): δ 1.22 and 1.24 (2 d, J ∼ 6.0
Hz, 12 H), 1.28, 1.32, 1.34, 1.38 (4 s, 36 H), 2.70 (d, J ) 10.0
Hz, 2 H), 4.89-5.00 (m, 2 H). 13C NMR (CDCl3): δ 21.8, 22.1,
22.2, 30.9 (t, J ) 5.0 Hz), 31.0 (t, J ) 5.0 Hz), 32.7 (d, J ) 9.0
Hz), 34.4 (d, J ) 9.0 Hz), 51.1, 51.5 (t or dd, J ) 3.0 Hz), 53.0
(d, J ) 9.0 Hz), 68.8, 69.4, 153.3, (d, J ) 16.0 Hz), 157.7. 31P
NMR (CDCl3): δ -28.9, 68.9 (2J PNP e 7.0 Hz). Anal. Calcd for
C24H52N6O4P2: C, 52.35; H, 9.51; N, 15.26. Found: C, 52.25;
H, 9.56; N, 15.41. This compound did not react with 2,2,2-
trifluoroethanol.
[(C F 3C H 2O )P (µ-N -t -B u )2P +(N H -t -B u ){N (C O 2-i -P r )-
(HNCO2-i-P r )}][Cl-] (21). Trifluoroethanol (0.10 g, 0.97 mmol)
was added dropwise to a stirred solution of 19 (0.50 g, 0.97
mmol) in toluene (20 mL), the mixture was stirred for 2 d then
concentrated to 2 mL, and heptane (2 mL) was added to the
residue. Crystals of 21 were obtained at 5 °C after ca. 2 d.
Yield: 0.40 g (62%). Mp: 171-173 °C. IR (cm-1): 3086, 1757.
1H NMR (CDCl3): δ 1.22 and 1.40 (2 d, J ∼ 6.0 Hz, 12 H),
1.28 (s, 9 H), 1.52 (br s, 18 H), 4.25-4.50 (m), 4.89-5.15 (m,
2 H). 13C NMR (CDCl3): δ 21.5, 21.9, 27.7, 30.6, 30.8, 31.0,
55.6, 56.1, 57.5, 58.0, 69.8, 70.5, 73.4, 156.2, 157.0. 31P NMR
(CDCl3): δ 11.7, 114.4 (J ∼ 16.0 Hz). Anal. Calcd for C22H44N5O5-
ClF3P2: C, 43.08; H, 7.23; N, 11.43. Found: C, 43.12; H, 7.26;
N, 11.55.
C
52H58N3O9P: C, 69.31; H, 6.45; N, 4.66. Found: C, 69.05; H,
6.42; N, 4.58.
CH2(6-t-Bu -4-Me-C6H2O)2P (1,2-O2C6H4){NHC(O)-N(COO-
Et)NH(COOEt)} (26). The procedure was similar to that for
25 with 22 and catechol (0.11 g, 1.0 mmol). Removal of all the
solvent afforded 26 as a white solid. Yield: 0.67 g (82%). Mp:
1
70-72 °C. IR (cm-1): 3295, 1755, 1490, 1120, 1054. H NMR
(CDCl3): δ 1.17 (s, 18 H), 1.29 and 1.38 (2 t, J ∼ 6.0 Hz, 6 H),
2.38 (s, 6 H), 3.58 (d, J ) 14.0 Hz, 1 H), 4.26 (br m, 4 H), 5.33
(br, 1H), 6.11 (br s, 1 H), 6.74-7.23 (m, 8 H). 13C NMR
(CDCl3): δ 14.2, 14.4, 21.1, 30.3, 33.7, 34.5, 62.4, 63.9, 109.2
(d, J ) 13.2 Hz), 110.4 (d, J ) 16.9 Hz), 115.2, 119.7, 120.4,
123.3, 125.7, 126.4, 128.3, 129.1, 133.6, 133.8, 139.0, 139.2,
142.0, 144.4, 148.7, 149.0, 151.8, 155.9 (d, J ) 6.9 Hz). 31P
NMR (CDCl3): δ -59.2. Anal. Calcd for C36H46N3O8P: C, 63.60;
H, 6.82; N, 6.18. Found: C, 63.46; H, 6.83; N, 5.98.
CH2(6-t-Bu -4-Me-C6H2O)2P (1,2-O2C6H4){NHC(O)-N(CO2-
i-P r )NH(CO2-i-P r )} (27). The procedure was similar to that
for 25 with catechol (0.11 g, 1.0 mmol). Removal of all the
solvent afforded 27 as a white solid. This was crystallized with
toluene (ca. 4 mL). Yield: 0.58 g (80%). Mp: 118-120 °C. IR
(cm-1): 3290, 1752, 1491, 1375, 1258, 1101, 1034. 1H NMR
(CDCl3): δ 1.10 and 1.12 (2 s, 18 H), 1.27 (d, J ) 6.0 Hz, 6 H),
1.39 (d, J ∼ 6.0 Hz, 6 H), 2.33 (s, 6 H), 3.55 (d, J ) 13.6 Hz,
1 H), 4.95-5.18 (m, 3 H), 6.10 (br s, 1 H), 6.64-7.28 (m, 8 H),
9.98 (d, J ∼ 16.0 Hz, 1 H). 13C NMR (CDCl3): δ 20.9, 21.3,
21.7 (3 s, ArCH3 + CH(CH3)2), 30.2 (s, C(CH3)3), 33.5 (s,
C(CH3)3), 34.3 (s, ArCH2), 69.9, 71.9 (2 s, OCHMe2), 108.9 (d,
J ∼ 8 Hz), 110.1 (d, J ∼ 8 Hz), 119.4, 123.0, 125.2, 126.2, 128.1,
128.9, 133.3, 133.7, 138.9, 148.5, 148.8, 151.4, 155.1 (d, J )
15.3 Hz). 31P NMR (CDCl3): δ -59.3. Anal. Calcd (after drying
in a vacuum for 2 h) for C38H50N3O9P: C, 63.05; H, 6.96; N,
5.80. Found: C, 63.15; H, 6.92; N, 5.87.
CH 2(6-t-Bu -4-Me-C6H 2O)2P {N(CO2E t )N(CO2E t )-C(O)-
N} (22 a s CH2Cl2 solva te). Diethyl azodicarboxylate (1.74 g,
10.0 mmol) was added in one lot to a solution of 10 [mp 124-
126 °C] (4.11 g, 9.99 mmol) in dry toluene (20 mL) and the
mixture was stirred for 72 h at room temperature. After
removing the solvent, compound 22 was obtained as a white
solid. This was crystallized (as CH2Cl2 solvate) with a dichlo-
romethane-hexane mixture (3:4). Yield: 4.97 g (85%). Mp:
116-120 °C. IR (cm-1): 2266, 1699, 1601, 1443, 1381, 1314,
1206, 1123, 961. Both bands at 1206 and 1314 cm-1 are strong,
and hence the assignment of ν(PdN) stretch is difficult. 1H
NMR (CDCl3): δ 1.25-1.43 (many lines, 24 H), 2.29 (s, 6 H),
3.71 (d, J ) 16.6 Hz, 1 H), 4.27-4.57 (m, 5 H), 5.30 (s, 2 H),
7.01-7.10 (m, 4 H). 13C NMR (CDCl3): δ 14.0, 14.3, 20.9, 30.6,
34.7, 53.4, 64.4, 64.6, 128.0, 128.9, 130.0, 135.9, 139.7, 139.9,
147.2, 147.5, 152.5. 31P NMR (CDCl3): δ 26.6. Anal. Calcd
(after drying in a vacuum for 2 h) for C30H40N3O7P: C, 61.52;
H, 6.88; N, 7.17. Found: C, 61.48; H, 6.80; N, 7.20.
[{CH 2(6-t -Bu -4-Me -C6H 2O)2}P {OC(O-i-P r )NN(CO2-i-
P r )N}]2 (28). Compound 3 (3.24 g, 5.28 mmol) was dissolved
in CH2Cl2 (4 mL) and CH3CN (∼4 mL) was added and left for
crystallization. The first batch of crystals (of 3) was removed
and the mother liquor was left aside. After ∼30 d crystals of
28 were obtained. The same compound was formed upon
heating a toluene solution, but other products were present
and hence a pure compound could not be obtained by this
route. Yield: 0.82 g (27%). Mp: 203-205 °C. 1H NMR: δ 0.80
(d, J ) 6.0 Hz, 12 H), 1.38 (s, 36 H), 1.41 (d, J ) 6.0 Hz, 12
H), 2.29 (s, 12 H), 3.70 (br, 2 H), 4.15 (br, 2 H), 4.65-4.80 and
Compound 23 was prepared as described before.4
CH2(6-t-Bu -4-Me-C6H2O)2P (2,2′-OC10H6-C10H6-OH){NC-
(O)-(CO2Et)NH(CO2Et)} (24). Racemic 1,1′-bi-2-naphthol
(0.29 g, 1.00 mmol) was added in one lot to a solution of 22
(0.59 g, 1.00 mmol) in dry THF (10 mL) and the mixture was
stirred for 72 h at room temperature. Removal of all the solvent
afforded 24 as a white solid. Yield: 0.77 g (89%). Mp: 120-
123 °C. IR (cm-1): 3409, 3368, 1765, 1726, 1680, 1593, 1516,
1439, 1354, 1302, 1204, 1132, 1100, 1007. 1H NMR (CDCl3):
J . Org. Chem, Vol. 69, No. 6, 2004 1887

Kumar et al.
4.95-5.20 (m each, 4 H), 7.05 and 7.20 (2 s, 8 H). Variable-
Com p ou n d 21: C22H45ClF3N5O5P2, M ) 614.02, monoclinic,
space group P21/c, a ) 18.2506(18) Å, b ) 9.7929(16) Å, c )
18.982(2) Å, â ) 109.54(4)°, V ) 3197.2(7) Å3, Z ) 4, F ) 1.276
g cm-3, F(000) ) 1304, µ ) 0.275 mm-1; data/restraints/
parameters 5628/0/357; S(all data) ) 1.572; R indices (I > 2σ-
(I)) R1 ) 0.1114, wR2 (all data) ) 0.2830; max/min residual
electron density (e Å-3) 0.680/-0.710 [note: since the data
quality is not good, the bond parameters are not used for
accurate comparison].
Com pou n d 22‚CH2Cl2: C31H42Cl2N3O7P, M ) 670.55, orthor-
hombic, space group Pcab, a ) 15.1196(18) Å, b ) 17.9591-
(16) Å, c ) 25.632(2) Å, V ) 6959.9(12) Å3, Z ) 8, F ) 1.280 g
cm-3, F000 ) 2832, µ ) 0.280 mm-1; data/restraints/parameters
6087/0/415; S(all data) ) 1.102; R indices (I > 2σ(I)) R1 )
0.0614, wR2 (all data) ) 0.2077; max/min residual electron
density (e Å-3) 0.341/-0.383.
1
temperature H NMR (after evacuation, C6D5CD3) at selected
temperatures: 243 K, δ (broad signals) 1.16-1.49 (br) and
1.73, 1.79 (2 br s) [24 and 36 H], 2.30 (br s, 12 H), 3.78 (br, 2
H), 4.53 (br, 2 H), 5.40 and 5.85 (m each, 4 H), 7.21 and 7.27
(2 s, 8 H); 305 K, δ 1.25 (d, J ) 6.2 Hz, 6 H), 1.35 (d, J ) 6.2
Hz, 6 H), 1.73 (s, 18 H), 2.28-2.33 (m, 6 H), 3.79 (br, 1 H),
4.25 (br, 1 H), 5.16 and 5.83 (m each, 2 H), 7.20, 7.21, 7.24,
and 7.32 (4 s, 4 H). Low-intensity peaks (∼10%) at ca. 1.60
and 2.20 ppm, probably due to the dimer, were also seen. 13C
NMR (C6D5CD3): 21.4, 21.6, 22.0, 22.1, 30.3, 31.0, 32.0, 34.8,
35.0, 37.8, 68.6, 70.0, 72.0, 72.8, 74.6, 124.5, 125.0, 125.2, 125.4,
126.0, 126.4, 132.4, 133.2, 133.5, 134.3, 135.0, 137.3, 139.3,
140.3, 141.1, 146.8, 151.0, 155.1 [this spectrum was recorded
at 294 K where both monomer and dimer are present in
solution and hence include many lines]. 31P NMR (C6D5CD3):
δ at 243 K -74.5; at 294 K -6.5, -74.7 (∼2:1); at 305 K -6.5
[also see the Supporting Information, Figure S10]. 31P NMR
(CDCl3): δ 15.8 (∼ 95%), -6.7 (∼ 5%). The peak at δ 15.8 ppm
in CDCl3 may perhaps be due to the isomeric phosphonium
salt. The 31P NMR spectrum (C6D5CD3) recorded immediately
after heating compound 28 under neat conditions at 120 °C
exhibited two peaks at δ -6.5 (70%) and -10.3 (30%). Anal.
Calcd for C62H88N6O12P2: C, 63.57; H, 7.57; N, 7.17. Found:
C, 63.71; H, 7.61; N, 7.20.
Com p ou n d 25‚3/2C6H5CH3: C62.5H70N3O9P, M ) 1038.19,
triclinic, space group P1h, a ) 12.4123(7) Å, b ) 16.2723(10) Å,
c ) 17.5862(10) Å, R ) 63.7960(10)°, â ) 70.9040(10)°, γ )
79.2730(10)°, V ) 3007.9(3) Å3, Z ) 2, F ) 1.146 g cm-3, F000
) 1106, µ ) 0.101 mm-1; data/restraints/parameters 10548/
19/684; S(all data) ) 1.098; R indices (I > 2σ(I)) R1 ) 0.0759,
wR2 (all data) ) 0.2170; max/min residual electron density (e
Å-3) 0.631/-0.505.
Com p ou n d 27‚3/2C6H5CH3: C97H124N6O18P2, M ) 1723.96,
triclinic, space group P1h, a ) 9.6724(8) Å, b ) 11.9518(10) Å,
c ) 22.0645(19) Å, R ) 80.273(2)°, â ) 86.782(2)°, γ ) 73.459-
(2)°, V ) 2409.9(4) Å3, Z ) 1, F ) 1.188 g cm-3, F000 ) 922, µ
) 0.113 mm-1; data/restraints/parameters 8496/14/581; S(all
data) ) 1.019; R indices (I > 2σ(I)) R1 ) 0.0691, wR2 (all data)
) 0.1325; max/min residual electron density (e Å-3) 0.445/-
0.384.
Com p ou n d 28: C62H88N6O12P2, M ) 1171.32, monoclinic,
space group P2/n, a ) 17.050(2) Å, b ) 10.0336(12) Å, c )
19.816(2) Å, â ) 92.216(2)°, V ) 3387.5(7) Å3, Z ) 2, F ) 1.148
g cm-3, F000 ) 1256, µ ) 0.124 mm-1; data/restraints/
parameters 5951/44/370; S(all data) ) 1.069; R indices (I >
2σ(I)) R1 ) 0.0816, wR2 (all data) ) 0.2662; max/min residual
electron density (e Å-3) 0.666/-0.550.
Com p ou n d 30: C36H47N2O7PS, M ) 682.79, monoclinic,
space group P21/c, a ) 20.7843(12) Å, b ) 10.6218(6) Å, c )
16.7361(10) Å, â ) 99.050(10)°, V ) 3648.8(4) Å3, Z ) 4, F )
1.243 g cm-3, F000 ) 1456, µ ) 0.181 mm-1; data/restraints/
parameters 6433/0/436; S(all data) ) 1.008; R indices (I > 2σ-
(I)) R1 ) 0.0534, wR2 (all data) ) 0.1632; max/min residual
electron density (e Å-3) 0.486/-0.533.
Ad d ition a l Com m en ts on th e Bon d Len gth s a n d Bon d
An gles in th e Str u ctu r es Rep or ted Her e. The PdN(t-Bu)
bond in 19 is extremely short (similar to that in 16); even in
21, the P-NH(t-Bu) bond [1.585(7) Å] is shorter than those
normally observed for P-N single bonds. The O-P-O bond
angle in the eight-membered ring at the X{6-t-Bu-4-Me-
C6H2O}2P [X ) CH2, S] varies between 96.8(2)° and 117.5(2)°
and thus allows easy geometry adjustment at phosphorus
depending upon the other substituents. In compound 30,
instead of the eight-membered ring as in 16, two fused five-
membered rings are generated because of sulfur coordination
to phosphorus. This in turn makes the corresponding O-P-O
bond angle approach the expected 90° in a distorted octahedral
geometry. Interestingly, the N-P-N angles (in the four-
membered cyclodiphosphazane ring) at the phosphorus bearing
the DIAD residue in the tetracoordinate compounds 19 and
21 are 83.2(2)° and 85.3(3)°, respectively, whereas it is 75.7-
(2)° in the pentacoordinate compound 28. This difference is
perhaps due to higher number of substituents on phosphorus
in 28 (relative to 19 or 21) forcing the N-P-N angle at the
cyclophosphazane ring to contract.
CH 2{6-t-Bu -4-Me-C6H 2O}2P (OCH 2CH 2NMe2){N(CO2-i-
P r )NC(O-i-P r )O} (29). DIAD (0.20 g, 1.0 mmol) was added
in one lot to a solution of CH2(6-t-Bu-4-Me-C6H2O)2POCH2-
CH2NMe2 (12) [δ(P) 129.0] (0.46 g, 1.0 mmol) in dry toluene
(10 mL) and the mixture was stirred for 72 h at room
temperature. After removal of all the solvent, 29 was obtained
as white solid. Yield: 0.46 g (70%). Mp: 198-202 °C. IR (cm-1):
1
1682, 1597, 1510, 1259, 1172, 1109, 1020. H NMR (CDCl3):
δ 1.35 (br, 30 H), 2.20 and 2.30 (2 s, 12 H), 2.55 (t, J ∼ 6.0 Hz,
2 H), 3.45 (d, J ∼ 14.0 Hz, 1 H), 4.35 (m, 3 H), 5.0 (br m, 2 H),
6.95-7.25 (m, 4 H). 13C NMR (CDCl3): δ 21.0, 22.0, 31.1, 34.1,
35.0, 45.6, 70.5, 72.7, 127.8, 128.8, 132.8, 135.2, 141.1, 144.7,
153.1, 153.5, 155.0. 31P NMR (CDCl3): δ -59.6, -66.7 (1:5).
Anal. Calcd for C35H54N3O7P: C, 63.70; H, 8.25; N, 6.37.
Found: C, 63.61; H, 7.98; N, 6.20.
S{6-t -Bu -4-Me -C6H 2O}2P (OP h ){N(CO2-i-P r )NC(O-i-
P r )O} (30). To a solution of 13 [δ(P) 136.1] (0.84 g, 1.75 mmol)
in toluene (15 mL) was added diisopropylazodicarboxylate
(DIAD) (0.35 g, 1.75 mmol) dropwise over a period of 10 min.
After being stirred for 1 d, the solution was concentrated to
ca. 3 mL and heptane (0.5 mL) was added. Crystals of 30 (0.96
g, 80%) were obtained at 5 °C after a day. These were used
for X-ray structure determination. Mp: 179-181 °C. IR (KBr,
cm-1): 2961, 1721, 1672, 1593, 1437, 1408, 1277, 1209, 1107,
1049. 1H NMR: δ 1.01, 1.05, 1.07, and 1.43 (d each, 12 H, J ∼
6.2 Hz), 1.42 and 1.47 (2 s, 18 H), 2.21 and 2.31 (2 s, 6 H),
4.54 and 4.85 (2 m, 2 H), 7.07-7.40 (m, 9 H). 13C NMR: δ
20.8, 21.5, 21.8, 21.9, 29.6, 29.9, 34.8, 34.9, 69.5, 72.5, 118.4,
118.6, 122.2, 122.3, 124.1, 124.3, 129.0, 129.6, 130.0, 130.7,
131.1, 131.4, 131.6, 137.8, 138.0, 138.7, 138.9, 151.8, 152.1,
152.4, 152.6, 153.6, 153.8. 31P NMR (CDCl3): δ -67.0. Anal.
Calcd for C36H47N2O7PS: C, 63.35; H, 6.93; N, 4.10. Found:
C, 63.66; H, 7.13; N, 4.29.
Cr ysta l Da ta . Com p ou n d 16: C32H48N3O6PS, M ) 633.76,
triclinic, space group P1h, a ) 10.681(9) Å, b ) 10.761(3) Å, c
) 17.409(4), R ) 74.34(7)°, â ) 72.41(5)°, γ ) 71.50(11)°, V )
1775.1(16) Å3, Z ) 2, F ) 1.186 g cm-3, F000 ) 680, µ ) 0.180
mm-1; data/restraints/parameters 6237/0/405; S(all data) )
1.009; R indices (I > 2σ(I)) R1 ) 0.0629, wR2 (all data) )
0.2057; max/min residual electron density (e Å-3) 0.481/-0.343.
Com p ou n d 19: C20H42ClN5O4P2, M ) 513.98, triclinic,
space group P1h, a ) 11.439(2) Å, b ) 14.028(3) Å, c ) 19.909-
(2) Å, R ) 76.23(4)°, â ) 75.65(4)°, γ ) 80.66(2)°, V ) 2987.9-
Ack n ow led gm en t. This work was supported by the
Department of Science and Technology (DST), New
Delhi. We also thank the Council of Scientific and
Industrial Research, New Delhi for fellowships to P.K.
(9) Å3, Z ) 4, F ) 1.143 g cm-3, F000 ) 1104, µ ) 0.265 mm-1
;
data/restraints/parameters 10535/0/585; S(all data) ) 1.044;
R indices (I > 2σ(I)) R1 ) 0.0605, wR2 (all data) ) 0.2143;
max/min residual electron density (e Å-3) 0.526/-0.287.
1888 J . Org. Chem., Vol. 69, No. 6, 2004

Dialkyl Azodicarboxylates
ORTEP drawings for 16, 19, 21, 22, 25, 27, 28, and 30;
variable-temperature 31P NMR data for 17 (including solid
state) and 28; 31P NMR data with structural drawings. This
material is available free of charge via the Internet at
http://pubs.acs.org.
and N.S.K., Dr. Sudha Kumaraswamy for useful sug-
gestions, DST (New Delhi) and NUS (Singapore) for
Single Crystal Diffractometer facilities, and SIF, Indian
Institute of Science (Bangalore) for solid-state 31P NMR.
Su p p or tin g In for m a tion Ava ila ble: X-ray structural
information for 16, 19, 21, 22, 25, 27, 28, and 30 as a CIF file;
J O035634D
J . Org. Chem, Vol. 69, No. 6, 2004 1889