Further characterization of mitsunobu-type intermediates in the reaction of dialkyl azodicarboxylates with P(III) compounds

Article abstract of DOI:10.1021/jo051997x

Structural characterization of compounds analogous to the proposed intermediates in the Mitsunobu esterification process is achieved by the combined use of NMR spectroscopy and X-ray diffractometric studies. The results show that compounds (t-BuNH)P(μ-N-t-Bu)₂P[(N-t-Bu)(N-(CO ₂R)-N(H)(CO₂R))] [R = Et (11), i-Pr (12)], obtained by treating [(t-Bu-NH)P-μ-N-t-Bu]₂ (10) with diethylazodicarboxylate (DEAD) or diisopropylazodicarboxylate (DIAD), respectively, have a structure with the NH proton residing between the two nitrogen atoms ((P)N(t-Bu) and (P)N-N(CO₂Et)); this is the tautomeric form of the expected betaine (t-BuNH)P(μ-N-t-Bu)₂P⁺[(NH-t-Bu)(N-(CO ₂R)-N-(CO₂R)]. Treatment of ClP(μ-N-t-Bu) ₂P[(N-t-Bu){N-(CO₂-i-Pr)-N(H)(CO₂-i-Pr)] (6) with 2,6-dicholorophenol affords (2,6-Cl₂-C₆H ₃-O)P-(μ-N-t-Bu)₂P⁺[(NH-t-Bu){N[(CO ₂i-Pr)(HNCO₂i-Pr)]}](Cl^-)(2,6-Cl ₂-C₆H₃-OH) (14) that has a structure similar to that of (CF₃CH₂O)P(μ-N-t-Bu)₂P ⁺[(NH-t-Bu){N[(CO₂i-Pr)(HNCO₂i-Pr)]}](Cl ^-) (13), but with an additional hydrogen bonded phenol. Both of these have the protonated betaine structure analogous to that of Ph₃P ⁺N(CO₂R)NH(CO₂R)(R′CO₂) ^- (2) proposed in the Mitsunobu esterification. Two other compounds, (ArO)P(μ-N-t-Bu)₂P⁺(NH-t-Bu){N(CO₂i-Pr) (HNCO₂i-Pr)}(Cl^-) [Ar = 2,6-Me₂C ₆H₃O- (15) and 2-Me-6-t-Bu-C₆H₃-O- (16)], are also prepared by the same route. Although NMR tube reactions of 11 or 12 with tetrachlorocatechol, catechol, 2,2′-biphenol, and phenol revealed significant changes in the ³¹P NMR spectra, attempted isolation of these products was not successful. On the basis of ³¹P NMR spectra, the phosphonium salt structure (t-BuNH)P(μ-N-t-Bu)₂P ⁺[(HN-t-Bu){N-(CO₂R)-N(H)(CO₂R)]-(ArO ^-) is proposed for these. The weakly acidic propan-2-ol or water did not react with 11 or 12, Treatment of 12 with carboxylic acids/p-toluenesulfonic acid gave the products (t-BuNH)P(μ-N-t-Bu)₂P⁺[(HN-t- Bu){N-(CO₂-i-Pr)-N(H)(CO₂-i-Pr)](ArCO₂-) [Ar = Ph (18), 4-Cl-C₆H₄CH₂ (19), 4-Br-C ₆H₄ (20), 4-NO₂-C₆H₄ (21)] and (t-BuNH)P(μ-N-t-Bu)₂P⁺|(HN-t-Bu){N-(CO ₂-i-Pr)-N(H)(CO₂-i-Pr)](4-CH₃-C ₆H₄SO₃^-) (22) that have essentially the same structure as 2. Compound 18 has additional stabilization by hydrogen bonding, as revealed by X-ray structure determination. Finally it is shown that the in situ generated (t-BuNH)P(μ-N-t-Bu)₂P⁺[(HN-t-Bu) {N-(CO₂Et)-N(H)(CO₂Et)](4-NO₂-C ₆H₄CO₂^-) can also effect Mitsunobu esterification. A comparison of the Ph₃P-DIAD system with the analogous synthetically useful Ph₃P-dimethyl acetylenedicarboxylate (DMAD) system is made.

Full text of DOI:10.1021/jo051997x

Further Characterization of Mitsunobu-Type Intermediates in the
Reaction of Dialkyl Azodicarboxylates with P(III) Compounds
K. C. Kumara Swamy,* K. Praveen Kumar, and N. N. Bhuvan Kumar
School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, A. P., India
kckssc@uohyd.ernet.in
ReceiVed September 23, 2005
Structural characterization of compounds analogous to the proposed intermediates in the Mitsunobu
esterification process is achieved by the combined use of NMR spectroscopy and X-ray diffractometric
studies. The results show that compounds (t-BuNH)P(µ-N-t-Bu)2P[(N-t-Bu)(N-(CO2R)-N(H)(CO2R))]
[R ) Et (11), i-Pr (12)], obtained by treating [(t-Bu-NH)P-µ-N-t-Bu]2 (10) with diethylazodicarboxylate
(DEAD) or diisopropylazodicarboxylate (DIAD), respectively, have a structure with the NH proton residing
between the two nitrogen atoms ((P)N(t-Bu) and (P)N-N(CO2Et)); this is the tautomeric form of the
expected betaine (t-BuNH)P(µ-N-t-Bu)2P+[(NH-t-Bu)(N-(CO2R)-N-(CO2R)]. Treatment of ClP(µ-N-t-
Bu)2P[(N-t-Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)] (6) with 2,6-dicholorophenol affords (2,6-Cl2-C6H3-O)P-
(µ-N-t-Bu)2P+[(NH-t-Bu){N[(CO2i-Pr)(HNCO2i-Pr)]}](Cl-)(2,6-Cl2-C6H3-OH) (14) that has a structure
similar to that of (CF3CH2O)P(µ-N-t-Bu)2P+[(NH-t-Bu){N[(CO2i-Pr)(HNCO2i-Pr)]}](Cl-) (13), but with
an additional hydrogen bonded phenol. Both of these have the protonated betaine structure analogous to
that of Ph3P+N(CO2R)NH(CO2R)(R′CO2)- (2) proposed in the Mitsunobu esterification. Two other
compounds, (ArO)P(µ-N-t-Bu)2P+(NH-t-Bu){N(CO2i-Pr)(HNCO2i-Pr)}(Cl-) [Ar ) 2,6-Me2C6H3O- (15)
and 2-Me-6-t-Bu-C6H3-O- (16)], are also prepared by the same route. Although NMR tube reactions of
11 or 12 with tetrachlorocatechol, catechol, 2,2′-biphenol, and phenol revealed significant changes in the
31P NMR spectra, attempted isolation of these products was not successful. On the basis of 31P NMR
spectra, the phosphonium salt structure (t-BuNH)P(µ-N-t-Bu)2P+[(HN-t-Bu){N-(CO2R)-N(H)(CO2R)]-
(ArO-) is proposed for these. The weakly acidic propan-2-ol or water did not react with 11 or 12. Treatment
of 12 with carboxylic acids/ p-toluenesulfonic acid gave the products (t-BuNH)P(µ-N-t-Bu)2P+[(HN-t-
-
Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)](ArCO2 ) [Ar ) Ph (18), 4-Cl-C6H4CH2 (19), 4-Br-C6H4 (20), 4-NO2-
C6H4 (21)] and (t-BuNH)P(µ-N-t-Bu)2P+[(HN-t-Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)](4-CH3-C6H4SO3-) (22)
that have essentially the same structure as 2. Compound 18 has additional stabilization by hydrogen
bonding, as revealed by X-ray structure determination. Finally it is shown that the in situ generated
-
(t-BuNH)P(µ-N-t-Bu)2P+[(HN-t-Bu){N-(CO2Et)-N(H)(CO2Et)](4-NO2-C6H4CO2 ) can also effect Mit-
sunobu esterification. A comparison of the Ph3P-DIAD system with the analogous synthetically useful
Ph3P-dimethyl acetylenedicarboxylate (DMAD) system is made.
Introduction
an acid (with inversion of configuration for asymmetric alco-
hols), known as the Mitsunobu reaction, has proven useful in a
The triphenylphosphine/diethyl azodicarboxylate (DEAD)- or
diisopropyl azodicarboxylate (DIAD)-mediated esterification of
* To whom correspondence should be addressed. Fax: (+91)-40-23012460.
10.1021/jo051997x CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/06/2006
1002
J. Org. Chem. 2006, 71, 1002-1008

Characterization of Mitsunobu-Type Intermediates
wide variety of synthetic applications.1 Mechanistic discernment
of this reaction with respect to the initial redox chemistry has
received substantial documentation.2 Several other key features
of this important reaction have been investigated by various
groups.2-5 An elegant DFT study in which different pathways
are analyzed was recently conducted by Anders and co-workers.6
The reaction is thought to proceed through the following steps
(Scheme 1):1a (a) addition of phosphorus(III) compound to
DEAD/ DIAD to lead to the Morrison-Brunn-Huisgen inter-
mediate 1, (b) protonation of 1 to lead to 2, (c) formation of
the alkoxy phosphonium salt 3, and (d) SN2 displacement of
the phosphonium carboxylate 3 to give R′COOR′′.
SCHEME 1
The Morrison-Brunn-Huisgen (MBH) betaine 1 is the
intermediate in the first step of this reaction, and its involvement
has been previously substantiated by multinuclear NMR and
FT-IR but not by X-ray diffraction.2 Formation of 1 occurs
through radical cations, and it was noted that the intensity of
the EPR signal in the reaction of DIAD with P(III) compounds
varies depending upon the substituents.2d Thus the nature of
the intermediates and hence the product distribution could also
vary depending upon the P(III) precursor.2,7-9 In the formation
of 2-oxazolidones from CO2 and ethanolamine using a Mit-
sunobu protocol, the use of triphenylphosphine vs tributylphos-
phine did indeed afford different isomers (eq 1).7
(1) Selected references: (a) Mitsunobu, O. Synthesis 1981, 1 (review).
(b) Hughes, D. L. Org. React. 1992, 42, 335 (review). (c) Hughes, D. L.
Org. Prep. Proced. Int. 1996, 28, 127 (review). (d) Barrero, A. F.; Alvarez-
Manzaneda, E. J.; Chahboun, R. Tetrahedron Lett. 2000, 41, 1959. (e)
Weissman, S. A.; Rossen, K.; Reider, P. J. Org. Lett. 2001, 3, 2513. (f)
Liu, P.; Jacobson, E. N. J. Am. Chem Soc. 2001, 123, 10772. (g) Snider, B.
B.; Song, F. Org. Lett. 2001, 3, 1817. (h) Paterson, I.; Savi, C. D.; Tudge,
M. Org. Lett. 2001, 3, 213. (i) Appendino, G.; Minassi, A.; Daddario, N.;
Bianchi, F.; Tron, G. C. Org. Lett. 2002, 4, 3839. (j) Chandrasekhar, S.;
Kulkarni, G. Tetrahedron: Asymmetry 2002, 13, 615. (k) Xu, J. Tetrahe-
dron: Asymmetry 2002, 13, 1129. (l) Crimmins, M. T.; Stanton, M. G.;
Allwein, S. P. J. Am. Chem. Soc. 2002, 124, 5958. (m) Kan, T.; Fujiwara,
A.; Kobayashi, H.; Fukuyama, T. Tetrahedron 2002, 58, 6267. (n)
Markowicz, M. W.; Dembinski, R. Org. Lett. 2002, 4, 3785. (o) Ahn, C.;
Deshong, P. J. Org. Chem. 2002, 67, 1754. (p) Lan, P.; Porco, J. A., Jr.;
South, M. S.; Parlow, J. J. J. Comb. Chem. 2003, 5, 660. (q) Dembinski,
R. Eur. J. Org. Chem. 2004, 2763 (review). (r) Dembinski, R. In Handbook
of Fluorous Chemistry; Gladysz, J. A., Curran, D. P., Horva´th, I. T., Eds.;
VCH: Wienheim, 2004; Chapter 10.3, pp 190-202 (review). (s) Nair, V.;
Biju, A. T.; Abhilash, K. G.; Menon, R. S.; Suresh, E. Org. Lett. 2005, 7,
2121 (use of Mitsunobu protocol in the reactions of diketones). (t) Harned,
A. M.; He, H. S.; Toy, P. H.; Flynn, D. L.; Hanson, P. R. J. Am. Chem.
Soc. 2005, 127, 52.
Direct reaction of phosphoramidite (Me2N)P(2,2′-(OC10H6)2)
with DIAD/ DEAD led to the pentacoordinate phosphoranes
(Me2N)P[2,2′-(OC10H6)2][N(CO2R)NC(OR)O] that could be
utilized for the kinetic resolution of alcohols in an asymmetric
Mitsunobu reaction (cf. eq 2).8 Thus, even when the initial
reaction of the P(III) compound with DEAD/ DIAD gives a
product other than a betaine of type 1, the esterification takes
place smoothly.8,9
(2) (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.; Jenkins, I. D. J.
Org. Chem. 1989, 54, 3045. (d) Camp, D.; Hanson, G. R.; Jenkins, I. D. J.
Org. Chem. 1995, 60, 2977. (e) Hughes, D. L.; Reamer, R. A. J. Org. Chem.
1996, 61, 2967. (f) Hughes, D. L.; Reamer, R. A.; Bergan, J. J.; Grabowski,
E. J. J. J. Am. Chem. Soc. 1988, 110, 6487. (g) Watanabe, T.; Gridnev, I.
D.; Imamoto, T. Chirality 2000, 12, 346. (i) Fokina, N. A.; Kornilov, A.
M.; Kukhar, V. P. J. Fluorine Chem. 2001, 111, 69. (h) Li, Z.; Zhou, Z.;
Wang, L.; Zhou, Q.; Tang, C. Tetrahedron: Asymmetry 2002, 13, 145. (i)
Ahn, C.; Correla, R.; DeShong, P. J. Org. Chem. 2002, 67, 1751. (j)
McNulty, J.; Capretta, A.; Laritchev, V.; Dyck, J.; Robertson, A. J. Angew.
Chem., Int. Ed. 2003, 42, 4051 (a comparative study with Bu3P-dibenzoyl
peroxide system).
(3) (a) Michalski, J.; Skowronska, A.; Bodalski, R. Phosphorus-31 NMR
Spectroscopy in Stereochemical Analysis; Verkade, J. G., Quin, L. D., Eds.;
VCH: Deerfield Beach, Florida, 1987; Chapter 8, pp 255-296. (b) Pautard-
Cooper A.; Evans, S. A., Jr. J. Org. Chem. 1989, 54, 2485.
(4) (a) Nikam, S. S.; Kornberg, B. E.; Rafferty, M. F. J. Org. Chem.
1997, 62, 3754. (d) Shull, B. K.; Sakai, T.; Nichols, J. B.; Koreeda, M. J.
Org. Chem. 1997, 62, 8294. (c) Saylik, D.; Horvath, M. J.; Elmes, P. S.;
Jackson, W. R.; Lovel, C. G.; Moody, K. J. Org. Chem. 1999, 64, 3940.
(d) Persky, R.; Albeck, A. J. Org. Chem. 2000, 65, 377. (e) Barrett, A. G.
M.; Roberts, R. S.; Schro¨der, J. Org. Lett. 2000, 2, 2999. (f) Racero, J. C.;
Mac´ıas-Sa´nchez, A. J.; Herna´ndez-Gala´n, R.; Hitchcock, P. B.; Hanson, J.
R.; Collado, I. G. J. Org. Chem. 2000, 65, 7786. (g) Charette, A. B.; Janes,
M. K.; Boezio, A. A. J. Org. Chem. 2001, 66, 2178.
(5) For macrolactonization using Mitsunobu conditions, see: (a) Smith,
A. B., III; Ott, G. R. J. Am. Chem. Soc. 1998, 120, 3935. (b) Paterson, I.;
Savi, C. D.; Tudge, M. Org. Lett. 2001, 3, 213. (c) Kan, T.; Fujiwara, A.;
Kobayashi, H.; Fukuyama, T. Tetrahedron 2002, 58, 6267. (d) Shen, R.;
Lin, C. T.; Bowman, E. J.; Porco, J. A., Jr. Org. Lett. 2002, 4, 3103. (e)
Shen, R.; Lin, C. T.; Porco, J. A., Jr. J. Am. Chem. Soc. 2002, 124, 5650.
(f) Ghosh, A. K.; Wang, Y.; Kim, J. T. J. Org. Chem. 2001, 66, 8973. (g)
Crimmins, M. T.; Stanton, M. G.; Allwein, S. P. J. Am. Chem. Soc. 2002,
124, 5958.
The betaine 1 formed in the first step can react with either
carboxylic acids or alcohols to give phosphonium carboxylate
salts (2) or phosphoranes (4 and 5, Scheme 2), respectively.2
Thus the order of addition of the acid and the alcohol to betaine
(7) Kodaka, M.; Tomohiro, T.; Okuno, H. (Y). J. Chem. Soc., Chem.
Commun. 1993, 81.
(8) (a) Hulst, R.; van Basten, A.; Fitzpatrick, K.; Kellogg, R. M. J. Chem.
Soc., Perkin Trans. 1 1995, 2961. (b) Li, Z.; Zhou, Z.; Wang, L.; Zhou,
Q.; Tang, C. Tetrahedron Asymmetry 2002, 13, 145.
(9) Castro, J. L.; Matassa, V. G.; Ball, R. G. J. Org. Chem. 1994, 59,
2289.
(6) Schenk, S.; Weston, J.; Anders, E. J. Am. Chem. Soc. 2005, 127,
12566. Alternate pathways involving phosphoranes of type Ph3P(OR)2 are
shown in Scheme 1 of this article.
J. Org. Chem, Vol. 71, No. 3, 2006 1003

Swamy et al.
SCHEME 2
SCHEME 3
whose structures are close to those of 1, 2, and 4′ by using
cyclodiphosphazane precursors. These results shed light on some
of the intricacies in the mechanism of Mitsunobu reaction. Some
common features among Ph3P-DIAD, Ph3P-DMAD, and related
systems are highlighted at the end. We also show that the
compounds (of type 2) thus obtained with our precursors can
effect esterification.
1 is important and implies a potential duality of the mechanism
in that the alkoxyphosphonium salt [Ph3P+(OR”)](R′CO2-) (3)
can be formed via either 2 or phosphorane 5. Furthermore, the
stability of intermediate 2 formed by the reaction of acid
R′COOH and the betaine 1 can be enhanced by hydrogen
bonding.2 In the next step, the reaction of 2 with 1 molar equiv
of R”OH leads to 3 (Scheme 1); the latter can also be formed
from 5 and R′CO2H (cf. Scheme 2). It is possible that some of
these various intermediates can be stabilized by a suitable choice
of the P(III) precursor.
The above observations prompted us to look into the behavior
of DIAD/ DEAD toward P(III) compounds other than Ph3P.10
Thus, we have previously characterized the first stage products
6-9 by starting with the precursors ClP(N-t-Bu)2PNH-t-Bu, S(6-
t-Bu-4-Me-C6H2O)2P-NH-t-Bu, CH2(6-t-Bu-4-Me-C6H2O)2P-
NCO, and CH2(6-t-Bu-4-Me-C6H2O)2]PPh, respectively.10b,11
Compounds 6 and 7 are the tautomeric forms of the corre-
sponding betaines, with N(CO2R)-H and CdO forming a
hydrogen-bonded dimer (cf. 6′). Compound 8, despite having
a structure different from the betaine 1, does participate in the
Mitsunobu coupling between ethanol and benzoic acid, sug-
gesting that the five-membered heterocycle is in equilibrium
with its betaine form.
Results and Discussion
(A) Compounds 11 and 12. The reaction of the cyclodiphos-
phazane [t-BuNHP-µ-N-t-Bu]2 (10) with DEAD or DIAD
readily affords the respective addition compounds 11 and 12
(Scheme 3).10 A naive extrapolation of the previously published
structure 6 to the amino compounds 11 and 12 (cf. Scheme 3)
suggested a hydrogen-bonded dimer through the syn-oriented
N-H and CdO groups (cf. 6′ above).10b However, the X-ray
structure (Figure S1) of 11 is not consistent with the dimer
structure due to the following observations:
(a) The P(2)-N(3) (numbering is shown on the structural
drawing in Scheme 3) distance of 1.533(1) Å is much longer
than the corresponding distance of 1.488(3) Å in 6.10b
(b) The N(5)-H and CdO(3) are transoidal in 11, whereas
they are cisoidal in 6. The structure of 11 is well refined, and
the N(5)-H could be readily located by a difference Fourier
map. In this structure, the N(5)-H proton is closer to N(3)
(N(5)-H(N5) 0.83(2), H(N5)‚‚‚N(3) 2.22(2), N5‚‚‚N(3) 2.670-
(1), N(5)-H(N5)‚‚‚N(3) 114.7°). Thus, unlike 6, compound 11
does not form a hydrogen-bonded dimer. This feature suggests
the hydrogen bonding is not the controlling factor for tautomeric
phosphinimine structure (of the betaine form) observed in these
compounds (6 or 11).
(c) The sum (Σ) of the bond angles at N(5) is 336.7°, and
hence this nitrogen is quite far from planarity (i.e., more
pyramidal); the corresponding nitrogen in 6 is essentially planar
(ΣN ) 357.8°).
All of these observations point to a greater contribution from
the phosphonium structure 11′.
(B) Reaction of 6, 11, and 12 with Phenols/Alcohols. In a
previous paper, we briefly alluded to compound 13, obtained
by the reaction of 6 with trifluoroethanol (better X-ray data is
now available; see Figure S2 in Supporting Information).10b
Further work using 2,6-dichlorophenol has revealed that in such
(10) (a) Satish Kumar, N.; Kommana, P.; Vittal, J. J.; Kumara Swamy,
K. C. J. Org. Chem. 2002, 67, 6653. (b) Satish Kumar, N.; Praveen Kumar,
K.; Pavan Kumar, K. V. P.; Kommana, P.; Vittal, J. J.; Kumara Swamy,
K. C. J. Org. Chem. 2004, 69, 1881.
(11) Compounds of type 9 are interesting examples of pentacoordinate
phosphoranes with “reversed apicophilicity”. This aspect has been discussed
by us as well others elsewhere. See ref 9 and (a) Timosheva, N. V.;
Chandrasekaran, A.; Prakasha, T. K.; Day, R. O.; Holmes, R. R. Inorg.
Chem. 1996, 35, 6552. (b) Kojima, S.; Sugino, M.; Matsukawa, S.;
Nakamoto, M.; Akiba, K.-y. J. Am. Chem. Soc. 2002, 124, 7674. (c)
Kommana, P.; Satish Kumar, N.; Vittal, J. J.; Jayasree, E. G.; Jemmis, E.
D.; Kumara Swamy, K. C. Org. Lett. 2004, 6, 145.
In continuation of our previous studies, we present herein
further data, including X-ray structures, pertaining to species
1004 J. Org. Chem., Vol. 71, No. 3, 2006

Characterization of Mitsunobu-Type Intermediates
reactions more complex species such as 14 can also be formed.
Figure S3 shows the structure of 14 with the hydrogen bond
parameters. Considering the chloride ion as the equivalent of
the carboxylate anion, this structure suggests that prior to the
formation of phosphonium salts 3 from 2 (Scheme 1), the
alcohol is perhaps held by hydrogen bonding with the carboxy-
late (RCO2-) ion. In the 2,2,2-trifluoroethoxy compound 13,
the chloride ion is hydrogen bonded only to the cyclophosp-
hazane NH atoms.12 While it is true that in the Mitsunobu
reaction a carboxylate ion (in place of the chloride present here)
is involved, the structures of 13 and 14, obtained essentially
from the same type of reaction, point toward an additional
feature not explicit in Scheme 1. That is, hydrogen bonding
between the carboxylate anion and the alcohol, and not just the
substituents on phosphorus, is involved in bringing about the
esterification. Both 13 and 14 are phosphonium salts with the
hydrazine residue connected to phosphorus and are comparable
to intermediates of type 2. The chloride anion engages itself in
hydrogen bonding as does a carboxylate (see below for
discussion on the structure of a compound with a carboxylate
anion). The anion in 13 and 14 has at least two hydrogen bonded
partners: (a) -NH proton of hydrazine end and (b) NH(t-Bu)
proton or/and the phenolic OH proton. The second partner in
our structure is likely to be replaced by the alcoholic -OH in
the normal Mitsunobu reaction.13
ppm for each of the phosphorus atoms) in solution between
trifluoroethanol and phenols with 11 and 12 (cf. Figure 1). Based
on the chemical shifts observed, it is unlikely that these species
have pentacoordinate phosphorane structures of the type of 4
and 5.14 The structure 17 is proposed for these new compounds.
Use of the less acidic 2,6-dimethyl-phenol and 2-methyl-6-
tert-butyl-phenol resulted in the isolation of compounds 15 and
16 for which we assign a structure analogous to that for 13.
Our initial attempts to isolate a product from the correspond-
ing reaction of 11 and 12 with trifluoroethanol or catechol were
unsuccessful, and the starting compounds 11 and 12 crystallized
out. However, 31P NMR monitoring of the reaction revealed
that there was a significant interaction (∆δ in the range 20-25
It is possible that the hydrogen bonding pattern could vary
depending upon the alcohol/ phenol, but the local environment
close to the tetracoordinate phosphorus is the same.15 These
structures are close to the ionic forms 4′ of the phosphoranes 4
displayed in Scheme 2. The less acidic propan-2-ol (or water),
by contrast, showed little or no interaction (cf. Table 1; entries
(12) An interesting point related to the two structures, as regards
phosphorus chemistry, is that the P-NH-t-Bu group and the P-OR (Ar)
groups are cis in 10, whereas they are trans in 11. This has some relevance
to cyclodiphosphazane chemistry but is not elaborated here. For cis-trans
isomerization in cyclodiphosph(III)azanes, see: (a) Reddy, V. S.; Krish-
namurthy, S. S.; Nethaji, M. J. Organomet. Chem. 1992, 438, 99. (b) Reddy,
V. S.; Krishnamurthy, S. S.; Nethaji, M. J. Chem. Soc., Dalton Trans. 1994,
2661.
(13) This statement is corroborated by the observation of additional
partners for hydrogen bonding in the structures of following compounds
(phenol in B and water in C) (Bhuvan Kumar, N. N.; Manab Chakravarty;
Kumara Swamy K. C., submitted for publication).
(14) Compounds of type 5 were isolated by several workers. See ref 3e
and (a) Bone, S. A.; Trippett, S. J. Chem. Soc., Perkin Trans. 1 1976, 156.
(b) von Itzstein, M.; Jenkins, I. D. J. Chem. Soc.Perkin Trans. 1 1986,
437.
(15) Repeated attempts to crystallize the product, by cooling to 5 °C
gave small amount of a crystalline material with the composition, [t-Bu-
HNP(µ-N-t-Bu)2P(NH-t-Bu){N-(CO2i-Pr)-N(H)(CO2i-Pr)}]2(1,2-OC6Cl4-
OH) [(1,2-OC6Cl4-OH) (t-BuNH3‚1,2-OC6Cl4OH)] (A), probably by partial
decomposition. At room temperature, there appeared to be extensive
deformation of the crystallinity. The data quality was not good, but by
comparison of the bond parameters to structure of 18, it does support the
formulation 17 for these products. An ORTEP picture (Figure S5) with
bond parameters around phosphorus is available as Supporting Information.
J. Org. Chem, Vol. 71, No. 3, 2006 1005

Swamy et al.
SCHEME 4
alkoxyphosphonium ions of type 33b and (b) Hughes and co-
workers in connection with alcohol activation.2b The latter, in
which hydrogen bonded HCOO-‚‚‚HCOOH was considered
responsible for the rate diminution in esterification when the
amount of acid is higher, is relevant to the present study.
However, as mentioned above, it is possible that one oxygen
of the carboxylate anion is hydrogen bonded to hydrazine
residue and the other to alcohol during the formation of the
alkoxyphosphonium ion. More structural studies are required
to ascertain whether this happens or not. One factor responsible
for the isolation of the stable acid-addition product using our
substrates is likely to be the additional hydrogen bonding
possibility as shown by the X-ray structure, but nevertheless
these compounds clearly demonstrate the existence of the second
stage intermediates of type 2 (cf. Scheme 1) proposed in the
Mitsunobu reaction. It is also noted that the 31P NMR chemical
shifts of 18-22 are in the same region as those for 17 (Table
1), lending credence to the structure assigned to 17.
FIGURE 1. 31P NMR spectra of (a) 12 and (b) immediately after the
addition of 1,2-tetrachlorocatechol to 12.
TABLE 1. NMR Data for the Products from the Addition of
Alcohols/Phenols to 11 or 12
entry
phenol/alcohol/water
product 31P NMR (δ)
Although a different P(III) substrate in our reaction was used
in these experiments, it is noteworthy that the esterification can
be conducted, albeit at higher temperatures, by the addition of
alcohol to the in situ generated 23 (δ(P) 3.8 and 82.9; DEAD
analogue of 21) to lead to the ester 24 (40% isolated yield)
(Scheme 4). This leaves room for further expansion of Mit-
sunobu chemistry.
1
2
3
4
5
6
7
8
none
tetrachlorocatechol
catechol
2,2′-biphenol
phenol
2,2,2-trifluoroethanol
propan-2-ol
water
-28.0 ( 0.5, 68.9 ( 0.5
-1.0 ( 0.5, 81.2 ( 0.5
-2.9 ( 0.5, 80.2 ( 0.5
-2.7 ( 0.5, 80.6 ( 0.5
-9.7 ( 0.5, 76.1 ( 0.5
0.5 ( 0.5, 81.2 ( 0.5
-25.9 ( 0.5, 69.4 ( 0.5
-26.6 ( 0.5, 69.2 ( 0.5
Summary and Outlook
7 and 8). This observation shows that the alternative pathway
shown in Scheme 2 may not occur when acid is present.
(C) Reaction of 12 with Carboxylic Acids. The reaction of
12 with acids straightforwardly leads to compounds 18-22 (eq
3), which are essentially intermediates of type 2. After repeated
This paper reports the isolation and structural characteriza-
tion of compounds analogous to some of the proposed inter-
mediates in the Mitsunobu esterification reaction. The X-ray
structure of 11 shows that subtle changes in the substituents on
the phosphine affect the nature of the first stage intermediate
(betaines of type 1). It is shown that the alcohol-added
intermediates (4) are much less stable than the acid-added
intermediates (2). Whereas the more acidic phenols (or trifluo-
roethanol) interact with the betaine 1 (or its tautomer in our
case) more strongly, the less acidic propan-2-ol (or water) shows
little or no interaction. We have also provided the first structural
precedent for the acid-addition intermediate of type 2 through
the isolation and characterization of product 18. Although the
present work is directed toward structural aspects of Mitsunobu-
type intermediates, one can note the similarity of this Ph3P-
DIAD system to the Ph3P-DMAD system. In the latter combi-
nation, Ph3P is used as a catalyst to activate the electron-deficient
alkyne and very interesting applications have emerged in the
past few years.16 An analogous intermediate (e.g., 25) is
proposed in these systems.17 The cyclodiphosphazanes also react
attempts, we were successful in crystallizing 18 whose X-ray
structure (Figure S4) clearly shows stabilization by hydrogen
bonding that was alluded to before. This interaction holds the
carboxylate with the phosphorus substrate: one of the carbox-
ylate oxygen atoms (O(6), see the drawing in eq 3 for
numbering) is connected to the NH-t-Bu proton, whereas the
other [O(5)] is connected to N-NH(COOR) proton. Hydrogen
bonding had been invoked earlier by (a) Evans and co-workers
in the reaction of phosphoranes of type 5 with alcohols to give
(16) (a) Lu, X.; Zhang, C.; Xu, Z. Acc. Chem. Res. 2001, 34, 535. (b)
Nair, V.; Rajesh, C.; Vinod, A. U.; Bindu, S.; Sreekanth, A. R.; Mathen, J.
S.; Balagopal, L. Acc. Chem. Res. 2003, 36, 899.
(17) For other types of products from the reaction of P(III) compounds
with electron deficient alkynes, see: Kumaraswamy, S.; Kommana, P.;
Satish Kumar, N.; Kumara Swamy, K C. Chem. Commun. 2002, 40.
1006 J. Org. Chem., Vol. 71, No. 3, 2006

Characterization of Mitsunobu-Type Intermediates
1
(cm-1): 3090, 1750, 1235, 1182, 1065. H NMR: δ 1.23-2.64
with DMAD (cf. species 26).18 It is interesting to note that in
the synthetically useful reactions of pyridines or disulfides with
DMAD also similar structures are proposed but characterization
of such species is still to be undertaken.19
(many lines, 39 H), 2.43 (s, 6 H), 4.78-5.09 (m, 2 H), 6.92-7.60
(m, 3 H), 7.60 (d, J ≈ 9.0 Hz, 1 H), 9.95 (s, 1 H). 13C NMR: δ
21.5, 21.8, 21.9, 22.0, 30.7 (d, J ≈ 4.5 Hz), 31.0 (∼t, J ≈ 4.5 Hz),
56.2, 57.0, 70.3, 73.5, 119.8, 124.1, 128.4, 130.0, 152.6 (d, J )
20.2 Hz), 156.3. 31P NMR: δ 10.7, 119.5 (d each J ≈ 11 Hz).
Anal. Calcd for C28H52ClN5O5P2: C, 52.86; H, 8.20; N, 11.00.
Found: C, 52.82; H, 8.25; N, 11.04.
Compound 16. This compound was prepared in a manner similar
to that for 14 using 2-methyl-6-tert-butyl-phenol (0.48 g, 2.92
mmol) and 6 (1.50 g, 2.92 mmol). Yield: 1.60 g (81%). Mp: 156-
158 °C. IR (cm-1): 3138, 1757, 1305, 1229, 1186, 1074. 1H
NMR: δ 1.25-1.55 (many lines, 48 H), 2.23 (s, 3 H), 4.78-5.09
(br, 2 H), 6.72-7.60 (many lines, ca. 4 H). 10.4 (br, 1 H). 13C
NMR: δ 20.8, 21.5, 21.7, 21.8, 22.0, 29.6, 30.5, 30.8, 31.0, 31.1,
31.2, 34.4, 34.9, 56.3, 56.4, 57.3, 70.4, 73.3, 115.8, 116.6, 116.8,
127.0, 127.2, 129.3, 132.8, 135.5, 140.0, 150.5 (d, J ) 19.2 Hz),
155.3. 31P NMR: δ 9.4, 114.6 (d each, J ) 10.4 Hz). Anal. Calcd
for C31H58ClN5O5P2: C, 54.90; H, 8.62; N, 10.33. Found: C, 54.92;
H, 8.75; N, 10.39. The sample was not very stable; two additional
Experimental Section
2
doublets at δ -9.3 and 6.8 with a J value of 40.0 Hz, probably
due to the oxidation of the P(III) moiety, were observed in the
reaction mixture.
The precursor [(t-Bu-NH)P-µ-N-t-Bu]2 (10)20 was prepared by
literature procedures and was crystallized prior to use. Compounds
ClP(µ-N-t-Bu)2P[(N-t-Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)] (6), (t-
BuNH)P(µ-N-t-Bu)2P[(N-t-Bu){N(CO2R)-N(H)(CO2R] [R ) Et
(11), i-Pr (12)], and (CF3CH2O)P(µ-N-t-Bu)2P+[(NH-t-Bu){N[(CO2i-
Pr)(HNCO2i-Pr)]}](Cl-) (13) were prepared as described before.10b,21
Details of X-ray structure determination22 and crystal data are
available in Supporting Information.
Reaction of 11 or 12 with Phenols, Alcohols, or Water. These
reactions were conducted in an NMR tube by adding an equimolar
quantity of tetrachlorocatechol, catechol, 2,2′-biphenol, phenol,
2,2,2-trifluoroethanol, propan-2-ol or water to 11 or 12 in an NMR
tube in CDCl3 solution. Although there was significant shift in the
phosphorus resonances except in the case of propan-2-ol (even when
two mole equivalents were used), we were unable to isolate the
products; the starting material (11 or 12) could be recovered in the
reaction with biphenol/ trifluoroethanol. Details of the spectral
changes are given in Table 1. Repeated attempts in the reaction of
11 using tetrachlorocatechol gave a crystalline material (A) with
the structure at the phosphorus analogous to those proposed here
(X-ray evidence, see Supporting Material), but with additional
tetrachlorocatechol residues.15
Synthesis of (t-BuNH)P(µ-N-t-Bu)2P+[(HN-t-Bu){N-(CO2-i-
Pr)-N(H)(CO2-i-Pr)](ArCO2-)[Ar ) Ph (18), 4-Cl-C6H4CH2
(19), 4-Br-C6H4 (20), 4-NO2-C6H4 (21)] and (t-BuNH)P(µ-N-t-
Bu)2P+[(HN-t-Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)](4-CH3-
C6H4SO3-) (22). Compound 18. Benzoic acid (0.25 g, 2.04 mmol)
was added all at once to a stirred solution of 12 (1.12 g, 2.04 mmol)
(preparation in situ also works well) in toluene (20 mL), the mixture
was stirred for 2 h and concentrated to ∼2 mL, and heptane (2
mL) was added to the residue. Crystals of 18 were obtained at 5
°C after 2 d. Yield: 1.23 g (90%). Mp: 86-88 °C. IR (cm-1):
3382, 1728, 1372, 1248, 1080. 1H NMR: δ 1.14, 1.20, 1.25, 1.30
(4 d, J ) 6.2 Hz, 12 H), 1.34 (d, 9 H, J ≈ 3 Hz), 1.41, 1.50 and
1.62 (s each, 27 H), 3.19 (d, J ) 4.3 Hz, 1 H), 4.84 and 4.99 (2 m,
2 H), 7.10-7.44 (m, 3 H), 8.09 (m 2 H), 9.50 (br, 2 H). 13C NMR:
δ 21.7, 21.9, 22.0, 30.9, 31.2, 32.5, 32.6, 52.6 (d, J ) 9.0 Hz),
54.8, 55.4, 55.8, 69.4, 72.3, 125.3, 127.3, 128.2, 129.6, 129.9, 137.4,
153.1 (d, J ) 20.2 Hz), 155.9, 171.6. 31P NMR: δ 1.1, 81.0 (J <
5.0 Hz). Anal. Calcd for C31H58N6O6P2: C, 55.36; H, 8.63; N, 12.50.
Found: C, 55.42; H, 8.72; N, 12.61.
Compounds
(2,6-Cl2-C6H3-O)P(µ-N-t-Bu)2P+[(NH-t-Bu)-
(14),
{N[(CO2-i-Pr)(HNCO2i-Pr)]}](Cl-)(2,6-Cl2-C6H3-OH)
(ArO)P(µ-N-t-Bu)2P+(NH-t-Bu){N(CO2-i-Pr)(HNCO2i-Pr)}-
(Cl-) [Ar ) 2,6-Me2C6H3O- (15); 2-Me-6-t-Bu-C6H3O- (16)].
Compound 14. 2,6-Dichlorophenol (0.54 g, 3.31 mmol) in toluene
(10 mL) was added dropwise to a stirred solution of 6 (1.70 g,
3.31 mmol) in toluene (20 mL), the mixture was stirred for 24 h
and concentrated to 2 mL, and heptane (2 mL) was added to the
residue. Crystals of 14 were obtained at 5 °C after 2 d. Yield: 1.08
g (50% based on phosphazane). Mp: 140-142 °C. IR (cm-1): 3428
1
(vw), 3083, 1742, 1229, 1190, 1092. H NMR: δ 1.23 (br d, J )
6.1 Hz, 6 H), 1.30-1.55 (3 lines, 15 H), 1.68 (br s, 18 H), 4.75-
5.10 (m, 2 H), 6.92-7.60 (m, ∼8 H), 11.26 (s, 1 H). 13C NMR: δ
21.3, 21.5, 21.8, 21.9, 30.4, 30.9, 31.5 and 31.6 (merged d and t),
56.6 (d, J ) 9.0 Hz), 57.4, 70.4, 73.3, 120.7, 125.2, 126.3, 128.1,
130.0, 152.6 (d, J ) 20.2 Hz), 155.6. 31P NMR: δ 10.9, 115.9.
The J value is < 5.0 Hz and hence slightly broadened signals instead
of well-separated doublets are observed. Anal. Calcd for C32H50-
Cl5N5O6P2: C, 45.75, H, 5.96; N, 8.34. Found: C, 45.89; H, 6.01;
N, 8.42.
Compound 15. This compound was prepared in a manner similar
to that for 14 using 2,6-dimethylphenol (0.30 g, 2.30 mmol) and 6
(1.18 g, 2.30 mmol). Yield: 1.11 g (75%). Mp: 138-140 °C. IR
(18) In this reaction, apart from the broad peaks at δ(P) 72.0, -24.9,
and -35.2 (major, combined intensity ca. 80%), sharp signals at δ(P) 90.1,
31.8, and 30.1 (combined intensity ca. 15%) were also observed in the 31
P
NMR. The major peaks are close to (t-BuNH)P(µ-N-t-Bu)2P(dN-t-Bu)-
CHdCH(CO2Me)] (δ(P) -25.7, 70.0) (X-ray) from the reaction of 10 with
methyl propiolate. The results are discussed in connection with the utility
of Ph3P-DMAD system in another paper quoted in ref 13 above.
(19) (a) Nair, V.; Sreekanth, A.; Vinod, A. U. Org Lett. 2001, 3, 3495.
(b) Li, C.-Qun; Shi, M. Org. Lett. 2003, 5, 4273. (c) Islamia, M. R.;
Mollazehia, F.; Badieib, A.; Sheibania H. ARKIVOC 2005, XV, 25.
(20) Bulloch, G.; Keat, R.; Thompson, D. G. J. Chem. Soc., Dalton Trans.
1977, 99.
Compound 19. DIAD (0.52 g, 2.56 mmol) was added dropwise
to a stirred solution of 10 (0.90 g, 2.56 mmol) in toluene (20 mL),
and the mixture was stirred for 30 min at room temperature. To
this, (4-Cl-C6H4CH2COOH) was added all at once, the mixture was
stirred for 24 h at room temperature and concentrated to 2 mL,
and heptane (2 mL) was added. Crystals of 19 were obtained at 5
°C after ca. 24 h. Yield: 1.70 g (92%). Mp: 80-82 °C. IR (cm-1):
1
(21) Praveen Kumar, K.; Chakravarty, M.; Kumara Swamy, K. C. Z.
Anorg. Allg. Chem. 2004, 630, 2063.
3382, 1726, 1379, 1246, 1084. H NMR: δ 1.17-1.28 (merged
4 d, J ≈ 6.2 Hz, 12 H), 1.29, 1.32, 1.42 and 1.47 (s each, 36 H),
3.15 (br, 1 H), 3.47 (s, 2 H), 4.78-5.13 (m, 2 H), 7.10-7.30 (m,
4 H), 9.98 (br, 2 H). 13C NMR: δ 21.6, 21.8, 22.0, 30.8, 30.9,
31.4, 32.4, 32.5, 44.6, 52.3 (d, J ) 15.0 Hz), 55.2, 55.4, 69.1, 71.9,
125.9, 127.7, 128.1, 128.9, 130.8, 131.0, 137.2, 153.1 (d, J ) 20.2
(22) Programs used: (a) Sheldrick, G. M. SADABS, Siemens Area
Detector Absorption Correction; University of Go¨ttingen: Germany, 1996.
(b) Sheldrick, G. M. SHELX-97, A package for structure solution and
refinement; University of Go¨ttingen: Germany, 1997. (c) Sheldrick, G. M.
SHELXLTL+, 1991.
J. Org. Chem, Vol. 71, No. 3, 2006 1007

Swamy et al.
Hz), 155.6, 176.3. 31P NMR: δ 0.1 (br), 80.4 (br). Anal. Calcd for
C32H59ClN6O6P2: C, 53.29; H, 8.24; N, 11.66. Found: C, 53.39;
H, 8.14; N, 11.37.
(CDCl3): δ 2.3, 82.2. Anal. Calcd for C31H60N6O7P2S: C, 51.52;
H, 8.31; N, 11.63. Found: C, 51.45; H, 8.25; N, 11.46.
Esterification Reaction Using in Situ Formed 23 and Ethanol
Leading to Ester 24. DEAD (0.58 g, 3.44 mmol) was added
dropwise to a stirred solution of 10 (1.20 g, 3.44 mmol) in toluene
(20 mL) and the mixture stirred for 30 min at room temperature.
To this 4-NO2-C6H4COOH was added all at once, and the mixture
stirred for 2 h at room temperature. To this mixture was added
ethanol (0.24 g, 5.16 mmol), and the contents were heated at 80
°C (no reaction at room temperature; 31P NMR δ 3.8 and 82.9 (90%;
ascribed to 23 by comparing with the data for compound 21)) for
12 h. Solvent was removed, dichloromethane (20 mL) added to
the residue and insoluble material was filtered off. Evaporation of
the solvent gave a gummy material that was chromatographed over
silica gel (hexane followed by ethyl acetate/hexane 1:10) to afford
the ester (4-NO2-C6H4CO2Et) (24). Yield: 0.26 g (40%). Mp: 54-
58 °C; lit. mp 55-59 °C23. We also attempted the reaction utilizing
21, but side reactions appear to occur, inhibiting the esterification
at the high temperatures used.
Compound 20. Procedure was the same as that for 19 using
similar molar quantities. Yield: 90%. Mp: 84-86 °C. IR (cm-1):
3383, 1725, 1368, 1304, 1219, 1080. 1H NMR: δ 1.14, 1.21, 1.26
and 1.31 (4 d, J ) 6.0 Hz, 12 H), 1.34, 1.43, 1.51 and 1.64 (4 s,
36 H), 3.05 (d, J ) 6.0 Hz, 1 H), 4.82 and 4.98 (2 m, 2 H), 7.35
and 7.98 (2 d, J ≈ 12.0 Hz, 4 H). The NH proton signals were too
broad. 13C NMR: δ 21.6, 21.8, 21.9, 22.0, 30.9, 31.5, 32.4, 32.6,
52.5 (d, J ) 17.0 Hz), 55.7 (d, J ) 7.0 Hz), 55.2, 55.4, 55.6, 69.4,
72.2, 123.9, 130.3, 131.2, 137.7, 153.2 (d, J ) 20.5 Hz), 155.7,
171.1. 31P NMR: δ 2.1, 81.5. Anal. Calcd for C31H57BrN6O6P2:
C, 49.53; H, 7.59; N, 11.18. Found: C, 49.46; H, 7.76; N, 11.09.
Compound 21. Procedure was the same as that for 19 using the
similar molar quantities. Yield: 90%. Mp: 118-120 °C. IR (cm-1):
3370, 1734, 1227, 1082, 1030. 1H NMR: δ 1.14, 1.21, 1.28 and
1.34 (4 d, J ) 6.0 Hz, 12 H), 1.45, 1.49, 1.53 and 1.68 (4 s, 36 H),
3.20 (d, J ) 6.0 Hz, 1 H), 4.78 and 4.90 (2 m, 2 H), 8.20 (AB qrt,
4 H). The NH proton signals were broad. 13C NMR: δ 21.6, 21.7,
21.8, 22.0, 30.9 (t, J ) 4.5 Hz), 31.3 (d, J ) 3.7 Hz), 32.5 (d, J )
10.2 Hz), 52.6 (d, J ) 18.0 Hz), 55.8 (d, J ) 7.3 Hz), 55.7, 69.5,
72.3, 124.1, 128.2, 129.0, 130.4, 131.3, 137.2, 153.1 (d, J ) 20.2
Hz), 155.7, 171.0. 31P NMR: δ 2.9, 82.0. Anal. Calcd for
C31H57N7O8P2: C, 51.87; H, 8.00; N, 13.66. Found: C, 52.01; H,
7.94; N, 13.89.
Acknowledgment. This work was supported by the Depart-
ment of Science and Technology (DST), New Delhi. We also
thank (i) the Council of Scientific and Industrial Research, New
Delhi for fellowships to N.N.B.K., (ii) UGC (New Delhi) UPE
program for equipment, and (iii) DST (New Delhi) for Single
Crystal Diffractometer facilities.
Compound 22. Procedure was the same as that for 19 using
Supporting Information Available: Further X-ray structural
information including plots for 11, 13 (improved bond parameters),
14, 18, and A; 13C NMR spectrum of 19; and CIF files for 11, 13,
14, and 18. This material is available free of charge via the Internet
at http://pubs.acs.org.
similar molar quantities. Yield: 91%. Mp: 176-178 °C. IR (cm-1):
1
3372, 3152, 1740, 1302, 1167, 1080. H NMR: δ 1.08-1.21
(many lines, 48 H), 2.32 (s, 3 H), 3.21 (d, J ) 4.0 Hz, 1 H), 4.00-
5.10 (m, 2 H), 6.66 (d, J ≈ 13.2 Hz, 1 H), 7.10 and 7.85 (d each,
4 H, J ) 7.0 Hz), 10.71 (br, 1 H). 13C NMR: δ 21.2, 21.5, 21.7,
21.9, 30.8, 30.9, 31.1, 31.2, 32.3, 32.5, 52.8 (d, J ) 17.0 Hz), 55.3
(d, J ) 7.0 Hz), 55.6 (d, J ) 7.0 Hz), 56.2, 69.5, 72.3, 126.4,
128.1, 138.1, 144.0, 152.6 (d, J ) 20.2 Hz), 156.5. 31P NMR
JO051997X
(23) This compound is available from Aldrich (cat. no. 15, 595-0).
1008 J. Org. Chem., Vol. 71, No. 3, 2006