New models for the study of the racemization mechanism of carbodiimides. Synthesis and structure (X-ray crystallography and 1H NMR) of cyclic carbodiimides

Article abstract of DOI:10.1021/jo951789c

The crystal and molecular structure of carbodiimides 2 (5,6,18,19-tetradehydro-5,12,13,18,25,26-hexahydrotetrabenzo[d,h,m,q][1,3,10,12] tetraazacyclooctadecine) and 3 (8,10,22,24-tetraazapentacyclo-[23.3.1.^3,7.1^11,15.1 ^17,21]dotriaconta-1(29),3,5,7(32),8,9,11,13,15(31),17,19,21(30),22, 23,25,27-hexadecaene) have been determined. The activation barriers for the racemization of carbodiimides 1 (6,7-dihydrodibenzo[d,h][1,3]diazonine), 2, and 3 have been determined. While 1 presents a relatively high barrier (17.4 kcal mol^-1), 2 and 3 have very low activation barriers (between 5 and 7 kcal mol^-1). We tentatively conclude that open-chain and large-ring carbodiimides racemize by nitrogen inversion or trans-rotation while medium-size cyclic carbodiimides racemize by cis-rotation.

Full text of DOI:10.1021/jo951789c

J . Org. Chem. 1996, 61, 4289-4299
4289
New Mod els for th e Stu d y of th e Ra cem iza tion Mech a n ism of
Ca r bod iim id es. Syn th esis a n d Str u ctu r e (X-r a y Cr ysta llogr a p h y
1
a n d H NMR) of Cyclic Ca r bod iim id es
Pedro Molina,* Mateo Alajar´ın,* and Pilar Sa´nchez-Andrada
Departamento de Quı´mica Orga´nica, Facultad de Quı´mica, Universidad de Murcia,
Campus de Espinardo, E-30071 Murcia, Spain
J uan Server Carrio´ and Mart´ın Mart´ınez-Ripoll*
Departamento de Cristalografı´a, Instituto de Quı´mica-Fı´sica ‘Rocasolano’, CSIC, Serrano, 119,
E-28006 Madrid, Spain
J . Edgar Anderson*
Chemistry Department, Christopher Ingold Laboratories, University College London, 20 Gordon Street,
London, WC1H 0AJ , England
Mar´ıa Luisa J imeno and J ose´ Elguero*
Instituto de Quı´mica Me´dica, CSIC, J uan de la Cierva 3, E-28006 Madrid, Spain
Received October 2, 1995X
The crystal and molecular structure of carbodiimides 2 (5,6,18,19-tetradehydro-5,12,13,18,25,26-
hexahydrotetrabenzo[d,h,m,q][1,3,10,12]tetraazacyclooctadecine) and 3 (8,10,22,24-tetraazapentacyclo-
[23.3.1.13,7.111,15.117,21]dotriaconta-1(29),3,5,7(32),8,9,11,13,15(31),17,19,21(30),22,23,25,27-hexa-
decaene) have been determined. The activation barriers for the racemization of carbodiimides 1
(6,7-dihydrodibenzo[d,h][1,3]diazonine), 2, and 3 have been determined. While 1 presents a
relatively high barrier (17.4 kcal mol-1), 2 and 3 have very low activation barriers (between 5 and
7 kcal mol-1). We tentatively conclude that open-chain and large-ring carbodiimides racemize by
nitrogen inversion or trans-rotation while medium-size cyclic carbodiimides racemize by cis-rotation.
Cyclic carbodiimides have been the subject of theoreti-
cal and spectroscopic studies in an attempt to examine
the configurational stability of nitrogen in the NdCdN
linkage.1-4 The problem is related to the “inversion” vs
“rotation” mechanisms in imines, especially N-phe-
nylimines (Schiff bases).5 The following summary of
what is known of the isomerization mechanism of car-
bodiimides is based on the work of Hiatt,1 Gordon,6 Anet,7
Damrauer,2 and Firl (ketenimine III):8 Most results, as
those obtained for structures I-III, concern nine-
membered rings closely related to our compound 1.
(i) Open-chain carbodiimides have an energy barrier
the barrier, so in light of the above discussion, this
suggest that with five flexible CH2-CH2 bonds, the ring
places no constraint on the change of configuration of the
carbodiimide.
(iv) A contrast is presented by another nine-membered
ring carbodiimide, the compound I. From the description
1
of its H-NMR spectrum given by Hiatt (diastereotopic
chemical shifts not reported, only J gem ) 15.4 Hz),1 the
interconversion is slow at room temperature, and reason-
able assumptions linked to this observation suggest the
barrier is at least 15.0 kcal mol-1. It is already well-
known that in such bridged biphenyls the barrier is
dominated by the biphenyl bond rotation, although the
bridge may contribute somewhat to the barrier.9
of racemization of 6.7 ( 0.2 kcal mol-1 7
.
(ii) There are three possible mechanisms for the
isomerization:6 inversion, cis-rotation, and trans-rotation.
According to MP4/6-311G** calculations for the parent
carbodiimide (HNdCdNH), the corresponding barriers
are 10.6, 8.0, and 7.9 kcal mol-1 respectively.
(iii) A nine-membered ring cyclic carbodiimide (1,3-
diazacyclonona-1,2-diene, II) has the same barrier, 6.7
( 0.2 kcal mol-1 2
The ring appears to add nothing to
.
X Abstract published in Advance ACS Abstracts, J une 1, 1996.
(1) Hiatt, R. R.; Shaio, M.-J .; Georges, F. J . Org. Chem. 1979, 44,
3265.
(2) Damrauer, R.; Soucy, D.; Winkler, P.; Eby, S. J . Org. Chem. 1980,
45, 1315.
(3) Richter, R.; Tucker, B.; Ulrich, M. J . Org. Chem. 1983, 48, 1694.
(4) J ohnson, R. P. Chem. Rev. 1989, 89, 1111.
(5) J ennings, W. B.; Wilson, V. E. Trigonal Nitrogen in Acyclic
Organonitrogen Stereodynamics, Lambert, J . B.; Takeuchi, Y., Eds.;
VCH Press: New York, NY, 1992; Chapter 6.
(6) Nguyen, M. T.; Riggs, N. V.; Radom, L.; Winnerwisser, M.;
Winnerwisser, B. P.; Birk, M. Chem. Phys. 1988, 122, 305.
(7) Anet, F. A. L.; J ochims, J . C.; Bradley, C. H. J . Am. Chem. Soc.
1970, 92, 2557.
In spite of the implications of this last example,
Damrauer2 has suggested that cyclic compounds might
give an indication of the relative importance of nitrogen
(8) Firl, J .; Schink, K.; Kosbahn, W. Chem. Lett. 1981, 527.
(9) Sutherland, I. O.; Ramsay, M. J . V. Tetrahedron 1965, 21, 3401.
S0022-3263(95)01789-0 CCC: $12.00 © 1996 American Chemical Society

4290 J . Org. Chem., Vol. 61, No. 13, 1996
Molina et al.
Sch em e 1
inversion and rotation about a CdN bond for isomeriza-
tion of carbodiimides. This interaction is attractive, as
the diagram below shows for an n-membered ring.
Locating atoms Cn, N1, C2, and N3 in the plane of the
paper, one configuration has C4 in front of the plane as
shown by C4A. The interconversion process takes C4 to
position C4B either by nitrogen inversion via position
C4N, or by rotation about a CdN bond through a
transition state C4cis where C4 and Cn are cis in the
same plane. The ground-state distance Cn-C4A or Cn-
C4B is greater than Cn-C4cis but less than Cn-C4N.
The chain of the ring, which has to bridge positions C4
and Cn should have an optimum conformation which will
hopefully have a marked preference for one of these
interconversion pathways, one of which stretches the
chain and the other shortens it. In particular, the link
between Cn and C4 excludes the trans-rotation mecha-
nism for medium-size cyclic carbodiimides.
(v) For ketenimines the barrier for the open-chain
compound is 14.1 kcal mol-1, while for compound III (R
) benzyl) 19.3 kcal mol-1, δ∆G‡ ) 5.2 kcal mol-1 8
Firl
.
considered both the inversion (δ∆G‡ > 0) and the cis-
rotation (δ∆G‡ < 0) mechanisms, concluding that nitro-
gen inversion mechanism is preferred in ketenimines.
Recently we reported the preparation of a series of
cyclic carbodiimides from bis(iminophosphoranes),10,11
among them compounds 1-3.
On the other hand, cyclic bis(carbodiimides) such as 2
and 3 may be too large (18- and 20-membered rings) to
observe fluxionality by NMR. Thus the objectives of the
present work are as follows: (i) to prepare and determine,
1
by H NMR, the conformation of nine-membered cyclic
carbodiimides (derivatives of 1) bearing substituents at
the ethane bridge; and (ii) to study, by dynamic 1H NMR,
the fluxionality of cyclic carbodiimides 1-3 and to
determine their X-ray crystal structures.
Resu lts a n d Discu ssion
Ch em istr y. The previously unreported bis(imino-
phosphoranes) 9, 14, and 18 were prepared by Staudinger
reaction of the corresponding bis(azides) with triphen-
ylphosphane. The bis(azide) 8 was prepared from stil-
bene derivative 4, available by condensation of the
o-nitrophenylacetic acid with o-nitrobenzaldehyde in 78%
yield, by conversion of the carboxylic group into a methyl
group, reduction of both nitro- and carbon-carbon
double-bond functionalities and finally diazotization/
azidation (Scheme 1). The isomeric bis(azides) 13 and
17 were prepared from 10a by the sequence: reduction
of the nitro groups, diazotization/azidation, and further
epoxidation . Bis(iminophosphorane) 21 was also pre-
pared by a similar sequence, starting from 10b (Scheme
2).
6,7-Dihydrodibenzo[d,h][1,3]diazonine 1 is a nine-
membered ring cyclic carbodiimide, for which molecular
mechanics and semiempirical calculations (AM1) led to
the same conformation for the fully optimized geometry.11
This system is quite rigid and there should be a high
barrier to any interconversion (nitrogen and/or ring
inversion). Keeping this is mind, the presence of a chiral
center in the ethane bridge would give rise to diastere-
oisomeric cyclic carbodiimides capable of being separated.
Such a compound would be of interest in the study of
the mechanism of racemization of this heterocumulene
functionality.
Conversion of bis(iminophosphoranes) into cyclic car-
bodiimides was achieved by three methods: (i) reaction
with 2 equiv of Boc2O in the presence of 1 equiv of DMAP
(10) Molina, P.; Alajar´ın, M.; Sa´nchez-Andrada, P. Tetrahedron Lett.
1993, 34, 5155.
(11) Molina, P.; Alajar´ın, M.; Sa´nchez-Andrada, P.; Elguero, J .;
J imeno, M. L. J . Org. Chem. 1994, 59, 7306.

Racemization Mechanism of Carbodiimides
J . Org. Chem., Vol. 61, No. 13, 1996 4291
Sch em e 2
a sealed tube (method B); and (iii) reaction with an excess
of carbon disulfide (20 equiv) in benzene at reflux
temperature and further addition of 1 equiv of starting
bis(iminophosphorane) (method C).
Bis(iminophosphorane) 9 was converted into the nine-
membered cyclic carbodiimide 22, and the method of
choice was found to be method B. However, the bis-
(iminophosphorane) 14 led to the cyclic carbodiimide 23
as the only reaction product in modest yields, method A
being that which afforded the higher yield (21%). It is
not easy to explain why the cyclic carbodiimide 24 was
not formed in this reaction, because molecular modeling
of this compound reveals that the torsion angle between
the two hydrogen atoms in the oxirane ring is similar to
the one observed in the closely related carbodiimide 1.
The bis(iminophosphorane) 18 provided a 1:1 mixture of
two isomeric cyclic bis(carbodiimides) 25a and 25b, from
which only one was isolated in pure state by fractional
crystallization. Finally, the cyclic bis(carbodiimide) 26
was obtained as a crystalline solid in modest yields from
the bis(iminophosphorane) 21, and no traces of the
expected cyclic carbodiimide 27 could be detected in the
crude product (Scheme 3).
X-r a y Cr ysta llogr a p h y. Cyclic carbodiimides de-
scribed in this and the previous paper11 were used to
obtain suitable crystals. After many attempts, only those
of compounds 2 (5,6,18,19-tetradehydro-5,12,13,18,25,
26-h exa h ydr ot et r a ben zo[d ,h ,m ,q][1,3,10,12]t et r a -
azacyclooctadecine) and 3 (8,10,22,24-tetraazapentacyclo-
[23.3.1.13,7.111,15.117,21]dotriaconta-1(29),3,5,7(32),8,9,11,
13,15(31),17,19,21(30),22,23,25,27-hexadecaene) were of
good quality.
The final X-ray models of compounds 2 and 3, drawn
with ORTEP (see also ref 28),12 are shown in Figures 1
and 2. In both cases, the molecules are arranged about
crystallographic inversion centers. The unit cell of 3
contains two crystallographically independent molecules,
thus the crystallographic asymmetric part is formed by
two independent half-molecules, which show only slight
differences in the torsional angles. In the carbodiimide
moiety, the averaged N-C bond length is 1.21 Å (com-
pound 2 and molecules 1 and 2 of compound 3). The
valence angles at the carbodiimide C atoms are respec-
tively 166.8(3)° for compound 2 and 167.8(4) and 168.7(4)°
for molecules 1 and 2 of compound 3. The torsional
angles around N×01-C×14 are 134(2)° for both molecules
of compound 3, not far away from the mean value of 134°
(range 112-142°) obtained from other carbodiimides
(MXPCIM, NIPCIM, PMCBIM, TOCDIM, VOJ PEP)13
while the corresponding torsional angle around N1-C14
in compound 2 is 156(1)°, showing greater angular
deformation. The geometry of the NdCdN part com-
pares well with the calculated geometry of HNdCdNH:
d(NdC) ) 1.21-1.23 Å, NCN angle ) 168-170°.14 No
voids are found in these crystal structures, the total
packing coefficient being 0.69 for both compounds.15 We
have summarized in Table 1 some geometrical charac-
teristics of compounds 2 and 3 and the five retrieved
structures from the CSD. We will use the same descrip-
(12) J ohnson, C. K. ORTEP, Report ORNL-3974, Oak Ridge Na-
tional Laboratory, TN, 1965.
(13) Allen, F. H.; Davies, J . E.; Galloy, J . J .; J ohnson, O.; Kennard,
O.; Macrae, C. F.; Mitchell, E. M.; Smith, J . M.; Watson, D. G.; J . Chem.
Info. Comput. Sci. 1991, 31, 187.
(14) Bertra´n, J .; Oliva, A.; J ose, J .; Duran, M.; Molina, P.; Alajar´ın,
M.; Lo´pez-Leonardo, C.; Elguero, J . J . Chem. Soc., Perkin Trans. 2
1992, 299.
(15) Cano, F. H.; Mart´ınez-Ripoll, M. J . Mol. Struct. (THEOCHEM)
1992, 258, 139.
in dichloromethane at room temperature (method A); (ii)
reaction with carbon dioxide in dry benzene at 70 °C in

4292 J . Org. Chem., Vol. 61, No. 13, 1996
Molina et al.
Sch em e 3
F igu r e 1. Compound 2 showing the atomic numbering used.
F igu r e 2. Compound 3 showing the atomic numbering used
(x ) 1, 2 for molecules 1 and 2).
Ta ble 1. X-r a y Geom etr ies of Ca r bod iim id es
angle, deg
compd
dihedral θ
CdNsPh1
CdNsPh2
NdCdN
2
3
98
57
95
114
95
88
135
130
129
130
125
127
128
136
131
130
134
126
128
130
166.8
168.2
169.0
170.0
170.0
170.0
168.0
MXPCIM
NIPCIM
PMCBIM
TOCDIM
VOJ PEP
92
tors for the semiempirical calculations (see below). The
angles CdNsPh and NdCdN are similar but the small
“dihedral” angle θ (PhsN...NsPh) in compound 3 (57°)
shows that its ring strain is larger than that of compound
2 (θ ) 98°).
1H NMR Sp ectr oscop y. Compound 22 shows only a
methyl signal and a complex system of protons at 300
MHz, but all signals are narrow. Since compound 22 can
exist in two conformations 22a and 22b (Scheme 4), this
implies either a rigid conformation (22a or 22b) or a rapid

Racemization Mechanism of Carbodiimides
J . Org. Chem., Vol. 61, No. 13, 1996 4293
1
F igu r e 3. Experimental (a) and calculated (b) H NMR spectra of compound 22.
Sch em e 4
methyl “in”, results from replacing H2 by Me. Thus, in
the case of 22a a J (H2,H3) ≈1.5 Hz is expected while for
22b a J (H1,H4) ≈12 Hz is expected.
Since the replacement of the proton by a methyl group
slightly modifies the conformation of 1 as well as the
constant term in the Karplus equation, it is possible that
the values estimated for 22 are slightly erroneous. An
alternative approach is to minimize the geometry of both
isomers (using the AM1 Hamiltonian like in the case of
1)11 and to calculate the 3J (H,H) coupling constants using
the Karplus relationship. The compound with the methyl
“out” 22a is calculated to be more stable than its isomer
22b by 4.4 kcal mol-1, the dihedral angles [and the
corresponding vicinal coupling constants between square
brackets] are as follows: 22a H2,H4 ) 144.32° [8.45 Hz];
H2,H3 ) -101.22° [1.50 Hz]; 22b H1,H4 ) 11.17° [10.07
Hz]; H1,H3 ) 123.58° [4.91 Hz].
Ta ble 2. 1H-NMR Da ta (δ, p p m ; J , Hz) of
Diben zod ia zon in es 1 a n d 22
atom
δ
fragment
J (1H-1H)
Compound 1 (acetone, 298 K)
H1 (H4)
H2 (H3)
3.129
3.200
H1,H2 (H3,H4)
H1,H3 (H2,H4)
H1,H4
-15.56
7.58
11.97
1.52
H2,H3
Compound 22 (CDCl3, 298 K)
The experimental values (Table 1) are 8.33 and 1.05
Hz, so there is no doubt that the structure of the methyl
derivative is 22a . Since in the case of compound 1 no
modification of the spectrum was observed until 183 K,
the previous conclusion of a rigid system is the only
possibility.11 The calculation of the spectrum was a little
complicated due to the accidental coincidence of the
chemical shifts of H2 and H3 (Table 1). We have
observed in another context (∆2-pyrazolines) that the
replacement of a proton by a methyl group in an ethane
Me1
H2
H3
1.445
3.582
3.583
2.372
Me1,H2
Me1,H3
Me1,H4
H2,H3
H2,H4
H3,H4
6.43
0
0
H4
1.05
8.33
-15.27
ring plus nitrogen inversion. In our previous paper11 we
stated that compound 1 exists at room temperature in a
rigid conformation; we have now the possibility to confirm
or reject this conclusion.
1
fragment results in a deceptively simple H-NMR spec-
Figure 3 shows the experimental and calculated 1H-
NMR spectrum of compound 22, and Table 2 reports the
data for the ethane fragment of compounds 111 and 22.
Small couplings between these protons and aromatic
protons11 have been neglected in the case of compound
22. Structure 22a , methyl “out”, results formally from
replacing H1 in compound 1 by Me while structure 22b,
trum due to accidental coincidence of the geminal proton
and the proton gauche to the methyl group (the proton
cis to the methyl group is shifted upfield by the anisot-
ropy of the methyl single bonds).16
(16) Elguero, J .; Fruchier, A. J . Chem. Res. 1990 (S) 200; (M) 1501.

4294 J . Org. Chem., Vol. 61, No. 13, 1996
Molina et al.
The 13C NMR spectrum of compound 1 shows a signal
at 32.6 ppm (CDCl3); in compound 22a there are three
signals in the saturated part of the spectrum (CDCl3):
18.5 (Me), 35.0 (CH2) and 42.7 (CH). No signals corre-
sponding to 22b were observed. The assignment of these
spectra is represented below:
shift of prochiral protons the ring inversion barrier is at
most 7 kcal mol-1, but in view of the greater broadening
observed for 3, probably less than this, indeed somewhat
less than the 6.2 kcal mol-1 which was estimated for 3.
The experimental observation is of geminal protons
changing from being diastereotopic on the NMR time
scale at “low” temperature to being enantiotopic at “high”
temperature, so we think these molecules processes are
taking place which interconvert chiral structures.
A
simple equation allows us to calculate a barrier to this
process for each molecule, but the temperatures at which
changes take place and thus the barriers are markedly
higher for 1 than for 2 and 3, and we want to discuss
what may be involved in these processes.
In acyclic dialkyl-substituted carbodiimides it is rea-
sonable to assume that the energy necessary to change
the configuration of the carbodiimide determines the
barrier, the two alkyl groups rearranging their conforma-
tion as may be necessary at some late stage, without
affecting the barrier size. With cyclic derivatives, this
may once again be the case, but there is another pos-
sibility, that the rearrangement of the ring conformation
may be constrained, and so may contribute to or even
dominate the barrier that is measured.
It is plausible that if the chain joining the two ends of
the carbodiimide is flexible, as is the case when the chain
has many bonds about which rotation takes place easily,
the change in configuration at the carbodiimide deter-
mines the measured barrier size, and the barriers are
then small, around 6-8 kcal mol-1. If the chain has few
bonds about which rotation takes place easily (or has one
bond with a high barrier about which rotation must take
place), then the conformation requirements of the ring
overall may determine the measured barrier size.
Replacement of a CH2sCH2 bond in a medium-size
ring with an inflexible CHdCH bond reduces the con-
formational mobility of the ring, and if that double bond
is incorporated as a benzene ring the effect is even
greater. Many examples of this can be extracted from
tabulations of ring-inversion barriers in medium-size
rings such as that given by Anet and Anet.18 It is thus
reasonable to suggest that in 1 and I, the observed
barrier owes less to carbodiimide isomerization (although
this has to take place to render protons equivalent on
the NMR time scale) than to their dibenzannelated ring
structure.
Probably, open-chain carbodiimides and large-size
rings like 2 and 3 would racemize by the inversion or
trans-rotation mechanisms with barriers between 6 and
8 kcal mol-1 while medium-size cyclic carbodiimides, 1
and I, would racemize by the cis-rotation mechanism. To
have a numerical estimation of ring-size effects on the
racemization of diarylcarbodiimides we have carried out
a series of AM1 calculations in which compound 1 is
compared with diphenylcarbodiimide IV (Table 3). Al-
though we have already used succesfully the AM1
Hamiltonian for describing carbodiimides,11,14 we have
checked the validity of these calculations in the case of
carbodiimide itself V. As we have reported in the
introduction, this molecule has been subjected to very
high level calculations.6 The results gathered in Table
3 confirm the validity of the AM1 Hamiltonian both in
terms of energy and geometry. Note that the geometry
of IV (ground state) is close to the average geometry of
Since in open-chain carbodiimides, the central NdCdN
carbon atom appears at ∼132 ppm,17 the shift to ∼142
ppm in compounds 1 and 22a probably reflects change
of hybridization due to angular strain.
Dyn a m ic 1H NMR Sp ectr oscop y. Since the meth-
ylene proton signal of 1 appears as an AA′BB′ system at
room temperature, interconversion of enantiomeric con-
formations must be slow on the NMR time scale. On
raising the temperature of a [2H8]toluene solution of 1,
lines began to broaden, and eventually coalesce at 369
K. Making the simplifying assumption that this can be
treated as the coalescence of an AB-quartet (J AB ) -15.56
Hz,11 νA ) 747.33 Hz and νB ) 907.20, ν0δ ) 159.87 Hz),
the values for the racemization of carbodiimide 1, kc )
365.1 s-1 and ∆Gc‡ ) 17.4 kcal mol-1 both at 369 K, were
obtained. Compound 1 is dramatically more rigid than
open-chain carbodiimides, but this barrier to intercon-
version is too low to allow resolution of chiral structures.
The spectrum of compound 22 contrast in behavior
with that of 1 as it consists of a single set of signals (the
minor isomer has to be present in a very minor amount
since a 100 multiplication of the intensity does not show
another methyl doublet) which does not change signifi-
cantly over the temperatures range -90 °C to +80 °C.
By analogy with 1, conformational processes should be
slow on the NMR time scale in 22, so following the
discussion given above, the observation of a single set of
signals indicates that one conformation, presumably 22a
with methyl “out” (and its enantiomer) is much more
stable than the other 22b with methyl “in”.
Compounds 2 and 3 have much more flexible struc-
tures. Compound 3 was studied at 400 MHz using vinyl
bromide/CD2Cl2 4:1 approximately as solvent. The CH2
signal broadens in comparison with a reference peak from
about 166 K downward and is very broad at 136 K, but
the solution froze at 134 K, twice. The coalescence
temperature is expected to be 131 K at the lowest.
Assuming a reasonable chemical shift, the rate constant
might be as much as 400 s-1 at 131 K and the lower limit
for the barrier (k ) 400 s-1 at 131 K) should be 5.9 kcal
mol-1. A reasonable estimate for the barrier is 6.2 ( 0.3
kcal mol-1. Compound 2 was studied at 400 MHz in a
mixture of CHF2Cl/CHFCl2/CD2Cl2, ca. 3:1:1. At 147 K
the CH2 proton signal is broad (10 Hz) compared with
the solvent (3 Hz); at 133 K crystallization of the solute
appeared to have taken place and no spectrum was seen.
On the basis of reasonable assumptions for the relative
(18) Anet, F. A. L.; Anet, R. Conformational Processes in Rings. In
Dynamic Nuclear Magnetic Resonance Spectroscopy; J ackman, L. M.,
Cotton, F. A., Eds.; Academic Press: New York, 1985; Chapter 14.
(17) Heinisch, G.; Matuszczak, B.; Pu¨rstinger, G.; Rakowitz, D. J .
Heterocycl. Chem. 1995, 32, 13.

Racemization Mechanism of Carbodiimides
compound
J . Org. Chem., Vol. 61, No. 13, 1996 4295
a
Ta ble 3. AM1 ca lcu la tion s (En th a lp y Va lu es in k ca l m ol-1
)
ground state
(inversion)‡
(cis-rotation)‡
(trans-rotation)‡
1
dihedral angle θ, deg
CdNsPh1 angle, deg
CdNsPh2 angle, deg
NdCdN angle, deg
δ∆H
74.2
121.2
121.2
173.8
0b
19.3
180
113.6
161.2
27.8
0
180
----
----
----
----
125.5
125.5
163.4
8.4
IV
dihedral angle θ, deg
CdNsPh1 angle, deg
CdNsPh2 angle, deg
NdCdN angle, deg
δ∆H
102.2
125.8
125.8
166.6
0c
172.9
180
122.5
171.3
10.5
0
180
135.4
135.4
154.9
10.0
136.0
136.0
180.0
7.1
1-IV
δδ∆H
0
17.3
-1.6
----
V
dihedral angle θ, deg
CdNsH1 angle, deg
CdNsH2 angle, deg
NdCdN angle, deg
δ∆H
100.0
120.1
120.1
167.4
0d
2.6
180
118.4
169.6
11.5
0
180
131.1
131.1
156.3
10.6
131.4
131.4
180.0
7.6
Ve
dihedral angle θ, deg
CdNsH1 angle, deg
CdNsH2 angle, deg
NdCdN angle, deg
δ∆H
89.6
119.7
119.7
170.8
0
0
0
180
172.0
118.6
173.4
10.6
131.2
131.2
168.5
8.0
130.9
130.9
180.0
7.9
b
d
aAngles imposed (constrains) are in bold. ∆H ) 105.53 kcal mol-1
.
c ∆H ) 106.64 kcal mol-1
.
∆H ) 41.41 kcal mol-1 e MP4/6-
.
311G**.6
the five open-chain diarylcarbodiimides found in the CSD
(Table 1: 97°, 129°, 129°, 169°).
group. To this aim ring strain is not an automatic
solution, since in some forms it makes racemization more
difficult but in others makes it easier.
Although quite different in shape (2 is an 18-membered
ring and 3 is a 20-membered ring) these large systems
have similar flexibility.
Exp er im en ta l Section
Gen er a l Meth od s. 1H and 13C NMR spectra were recorded
on a Varian Unity 300 apparatus (Murcia) (1H at 299.95 MHz
and 13C at 74.43 MHz), and chemical shifts are expressed in
parts per million (δ) relative to tetramethylsilane (error in
temperature measurements (0.1 °C). The 1H NMR spectra
of compound 22 were recorded at 30 °C on a Varian Unity-
500 spectrometer (Madrid) operating at 499.84 MHz, using
CDCl3 as solvent. The spectra were acquired using 3967.1 Hz
spectral width, digitized with 32K data points with a pulse
width of 7 µs (90° flip angle). Digital resolution for the real
part was (0.25 Hz. Gaussian multiplication was used prior
to Fourier transform. The 1H-NMR iterative analysis of the
spectrum was performed using the PANIC program.19 Vari-
able-temperature experiments on compounds 2 and 3 were
carried out on a Varian VXR 400 spectrometer (London)
operating at 400 MHz.
In the case of compound 1, the trans-rotation is a
meaningless process since the NdCdN and CH2sCH2
groups must crossover. Of the remaining two processes,
the cis-rotation mechanism is preferred although the
calculated barrier (8.4 kcal mol -1) is lower than the
experimental one (17.4 kcal mol-1). For the open-chain
compound IV, the values are in excellent agreement with
the calculations (both AM1 and MP4) for carbodiimide
itself V, showing that the hydrogen atoms can be replaced
by phenyl groups without significant changes in the
relative energies. Assuming that some errors will cancel
when comparing the energies involved in compounds 1
and IV (δδ∆H), it appears that a relatively short chain
joining the two ends of the diphenylcarbodiimide facili-
tates the cis-rotation (by about 2 kcal mol-1), increases
considerably the energy of the inversion process (by about
17 kcal mol-1), and renders impossible the trans-rotation
mechanism.
Cr ysta l Str u ctu r e of Com p ou n d s 2 a n d 3. A summary
of the crystallographic data is given in Table 4. The structures
were solved using SIR92.20 Slight secondary extinction effects21
were corrected during the last cycles of least-squares refine-
ment.
Com p u ta tion a l Ca lcu la tion s. The AM1 Hamiltonian26
was used within its original formalism. In all cases, the
Con clu d in g Rem a r k s
Assuming that the activation barrier determined for
1 (17.4 kcal mol-1) is similar for 22 it can be concluded
that if there were a mixture of 22a and 22b present in
solution, two methyl signals and two ABC(X3) systems
should have been observed at room temperature in 1H-
NMR, but the clear preference for methyl “out” 22a over
methyl “in” 22b prevent this observation. Probably, two
groups of similar size on the same sp3 carbon would be
necessary to observe both isomers (for instance, CH3 and
CD3). Moreover, the barrier is too low for enantiomers
to be separated (separation requires ∆G‡ > 30 kcal
mol-1), so there is little hope in obtaining a chiral
carbodiimide whose chirality lies on the carbodiimide
(19) PANIC 86, Bruker Program Library, Germany.
(20) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, G.; Guagliardi, A.; Polidori, G. SIR92. J . Appl. Crystallogr. 1995,
in preparation.
(21) Larson, A. C. Acta Crystallogr. 1967, 23, 664.
(22) Stewart, J . M.; Machin, P. A.; Dickinson, C. W.; Ammon, H.
L.; Heck, H.; Flack, H. The X-Ray 80 System; Computer Science Center,
Univ. of Maryland: College Park, MD, 1985.
(23) Mart´ınez-Ripoll, M.; Cano, F. H. PESOS. Computer program
for the automatic evaluation of l.s. weights; Instituto ‘Rocasolano’, CSIC;
Madrid, Spain, 1975.
(24) Nardelli, M. Comput. Chem. 1983, 7, 95.
(25) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, 1974; Vol. IV, p 72.
(26) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P.
J . Am. Chem. Soc. 1977, 99, 4899; 4907 (AMPAC V2.1, QCPE program
No. 506).

4296 J . Org. Chem., Vol. 61, No. 13, 1996
Molina et al.
Ta ble 4. Exp er im en ta l Da ta a n d Str u ctu r e Refin em en t P r oced u r es
(2)
crystal data
(3)
formula
crystal habit
crystal size (mm)
symmetry
C30H24N4
C28H20N4
colorless prisms
colorless prisms
0.17 × 0.23 × 0.60
monoclinic P21/n
LS fit from 63 refl (θ < 30°)
14.587(2), 5.4087(4),15.082(2)
107.00(1)
1137.9(2), 2
1.29, 440.5, 464
5.9
0.18 × 0.27 × 0.30
monoclinic P21/a
LS fit from 54 refl (θ < 28°)
9.601(1), 10.642(1), 21.272(1)
95.08(1)
2164.9(3), 4
1.27, 412.49, 864
5.9
unit cell determination
unit cell dimen. (a,b,c), Å
(â), deg
packing: V (Å3), Z
Dc (g cm-3), M, F(000)
(cm-1
)
experimental data
technique:
four circle diffractometer, SEIFERT XRD-3000S
bisecting geometry
graphite-oriented monochromator: CuKR (λ ) 1.5418 Å)
ω/2θ scans, scan width (deg): 1.5 + 0.15 tan θ
detector apertures 2 × 2°
θmax ) 60°
θmax ) 65°
no. of reflections:
measured
independent
1761
1691
3845
3515
observed
range of hkl
max sin θ/λ
value of Rint
968 [2σ(I) criterion]
-16 16, 0 6, 0 16
0.562
1923 [2σ(I) criterion]
-11 11, 0 13, 0 25
0.590
0.004
0.004
standard reflections
solution and refinement
solution
2 reflections every 100 reflections; no variation
direct methods
refinement
least squares on Fobs, full matrix
parameters:
no. of variables
degrees of freedom
ratio of freedom
H atoms
maximun final shift/error
w-scheme
191
777
5.1
350
573
5.5
difference synthesis
0.003 (U13 of C15)
difference synthesis
0.004 (x of H213)
empirical as to give no trends in w∆2F vs Fo or sin θ/λ
max thermal value
final ∆F peak
0.082 (U22 of C11)
0.088 (U22 of N201)
0.22 e/Å3)
0.12 e/Å3
extinction coefficient21
S, unit weight standard deviation
final R and Rw
0.0019(1)
1.16
0.041, 0.046
0.004(1)
0.83
0.036, 0.035
computer and programs
scattering factors
anomalous dispersion
VAX6410, SIR92,20 XRAY76,22 PESOS,23 PARST24
Int. Tables for X-Ray Crystallography25
Int. Tables for X-Ray Crystallography25
PRECISE keyword was used and full geometry optimization
was carried out (with the Fletcher-Powell algorithm) with the
only geometry constraints showed in Table 3. It was verified
(FORCE command) that the ground states (compounds 1, IV,
and V) and some transition states (1, cis-rotation; IV, cis- and
trans-rotations; V, cis- and trans-rotations) correspond to none
and one imaginary frequencies. The inversion values (com-
pounds 1, IV, and V) correspond to two imaginary frequencies
(transition states of second order) and could be visualized as
the transition state between the cis- and trans-rotations. It
has been reported6 that the transition states of carbodiimide
itself V have one or two imaginary frequencies depending on
the level of the calculations.
of 2-nitrobenzaldehyde or 2-nitrophenylacetaldehyde, potas-
sium carbonate (5.07 g, 40 mmol), and a few crystals of
dibenzo-18-crown-6. The mixture was stirred at room tem-
perature under nitrogen for 16 h. The precipitated solid was
separated by filtration, and the filtrate was concentrated to
dryness. The resultant solid was washed with ethanol and
air-dried to give 2,2′-dinitrostilbene 10a , as a mixture of E and
Z isomers (1:3) as revealed by 1H-NMR, in 95% yield or purified
by column chromatography (silica gel, dichloromethane) to give
1,3-bis(2-nitrophenyl)propene 10b in 65% yield as a mixture
1
of E and Z isomers (1:1.7) as revealed by H-NMR. 10a : IR
1
(Nujol) 1604, 1572, 1515, 1349 cm-1; H-NMR (CDCl3) δ 7.02
(dd, 2H(Z), J ) 7.5, 1.8 Hz), 7.10 (s, 2H(Z)), 7.29 (td, 2H(Z), J
) 7.3, 1.6 Hz), 7.37 (td, 2H(Z), J ) 7.3, 1.6 Hz), 8.07 (dd, 2H(Z),
2-Nitr op h en yla ceta ld eh yd e. To a solution of 2-nitro-
phenethyl alcohol (2 g, 12 mmol) in 50 mL of dry dichlo-
romethane was added PCC (4.6 g, 20 mmol). The resultant
mixture was well-stirred at room temperature for 3 h and
filtered over MgSO4. The filtrate was concentrated to dryness
under reduced pressure and the residual material was purified
by column chromatography (silica gel, dichloromethane) to give
2-nitrophenylacetaldehyde as a viscous oil in 71% yield: IR
J
) 7.6, 1.6 Hz); 13C-NMR (CDCl3) δ 124.7(Z), 128.5(Z),
128.9(Z), 132.5(Z), 132.6(Z), 133.2(Z), 148.3(Z); mass spectrum
(relative intensity) 270 (M+, 20), 165 (57), 135 (65), 92 (100).
Anal. Calcd for C14H10N2O4: C, 62.22; H, 3.73; N, 10.37.
Found: C, 62.49; H, 3.48; N, 10.13. 10b: IR (Nujol) 1609,
1530, 1351, 745 cm-1; 1H-NMR (CDCl3) δ 3.73 (dd, 1.2H(Z), J
) 7.3, 1.6 Hz), 3.89 (dd, 0.8H(E), J ) 6.8, 1.3 Hz), 5.97 (dt,
0.6H(Z), J ) 11.5, 7.3 Hz), 6.36 (dt, 0.4H(E), J ) 15.7, 6.8 Hz),
6.91-6.97 (m, 0.6H(Z)), 7.29-7.64 (m, 3.6H(Z) + 3.2H(E)),
7.88-7.93 (m, 0.6H(Z)), 7.97 (dd, 0.4H(E), J ) 8.1, 1.3 Hz),
8.07 (dd, 0.6H(Z) J ) 8.1, 1.3 Hz); 13C-NMR (CDCl3) δ 31.4,
36.5, 124.5, 124.7, 124.8, 124.9, 127.5, 127.7, 127.8, 128.0,
128.2, 128.4, 128.8, 129.6, 131.3, 131.6, 132.1, 132.2, 132.3,
132.5, 132.8, 133.1, 133.2, 133.3, 133.4, 134.4, 134.7, 148.2,
149.1 one methine carbon was not observed; mass spectrum
(relative intensity) 220 (100), 189 (52), 132 (64), 92 (94). Anal.
Calcd for C15H12N2O4: C, 63.38; H, 4.25; N, 9.85. Found: C,
63.01; H, 4.42; N, 10.01.
1
(Nujol) 1725, 1532, 1352, 718 cm-1; H-NMR (CDCl3) δ 4.13
(d, 2H, J ) 0.9 Hz), 7.32 (dd, 1H, J ) 7.6, 1.3 Hz), 7.49 (dd,
1H, J ) 7.9, 1.3 Hz), 7.62 (td, 1H, J ) 7.5, 1.3 Hz), 8.11 (dd,
1H, J ) 0.9 Hz), 9.82 (t, 1H, J ) 0.9 Hz); 13C-NMR (CDCl3) δ
48.4, 125.2, 128.4, 128.8, 133.5, 133.8, 148.3, 196.5; mass
spectrum (relative intensity) 165 (M+, 15), 149 (57), 136 (94),
120 (100). Anal. Calcd for C8H7NO3: C, 58.18; H, 4.27; N,
8.48. Found: C, 58.39; H, 4.08; N, 8.25.
P r ep a r a tion of Din itr od er iva tives 10a a n d 10b. To a
solution of (2-nitrobenzyl)triphenylphosphonium bromide (pre-
pared in the usual way from 2-nitrobenzyl bromide) (15 g, 30
mmol) in 100 mL of dry dicloromethane were added 30 mmol

Racemization Mechanism of Carbodiimides
J . Org. Chem., Vol. 61, No. 13, 1996 4297
(E)-2,2′-Din itr ostilben e (11). To a solution of the mixture
of E and Z isomers (1:3) of 2,2′-dinitrostilbene (10a , 1g, 3.7
mmol) in 30 mL of dry benzene was added thiophenol (0.21 g,
1.9 mmol). The resultant mixture was heated at reflux
temperature and azoisobutyronitrile (0.81 g, 4.8 mmol) was
added in three portions over a period of 5 h. The reaction
mixture was stirred at this temperature overnight. After
cooling, the solvent was removed under reduced pressure and
the residual material was treated with ethanol, and the
precipitated solid was filtered and air-dried to give (E)-2,2′-
dinitrostilbene in 80% yield: mp 176-178 °C, yellow prisms
(dichloromethane); IR (Nujol) 1517, 1447, 1356, 747 cm-1; 1H-
NMR (CDCl3) δ 7.48 (td, 2H, J ) 7.7, 1.5 Hz), 7.57 (s, 2H),
7.67 (td, 2H, J ) 7.6, 1.3 Hz), 7.82 (dd, 2H, J ) 7.9, 1.5 Hz),
8.04 (dd, 2H, J ) 8.1, 1.3 Hz); 13C-NMR (CDCl3) δ 125.0, 128.9,
129.0, 129.2, 132.6, 133.7, 147.9; mass spectrum (relative
intensity) 270 (M+, 9), 151 (53), 135 (69), 92 (100). Anal. Calcd
for C14H10N2O4: C, 62.22; H, 3.73; N, 10.37. Found: C, 62.56;
H, 3.59; N, 10.60.
was stirred at 0 °C for 1 h. A solution of sodium azide (2.1g,
32 mmol) in 15 mL of water was then added dropwise, and
the reaction mixture was allowed to warm at room tempera-
ture. The resultant mixture was extracted with ether (3 ×
60 mL), and the combined organic layers were washed with
water (75 mL) and dried over MgSO4. The solvent was
removed under reduced pressure and the residual material
was purified by column chromatography to give the corre-
sponding diazide:
8: (Silica gel, n-hexane/diethyl ether 7:3); 95% yield; pale
1
yellow oil; IR (Nujol) 2127, 1586, 1491, 1294 cm-1; H-NMR
(CDCl3) δ 1.18 (d, 3H, J ) 6.9 Hz), 2.79 (d, 2H, J ) 7.6 Hz),
3.35-3.49 (m, 1H), 6.96-7.29 (m, 8H); 13C-NMR (CDCl3) δ
20.2, 34.1, 38.5, 118.0, 118.1, 124.4, 124.9, 127.1, 127.5, 127.6,
131.2, 132.1, 137.6, 138.0, 138.4; mass spectrum (relative
intensity) 278 (M+, 43), 221 (78), 207 (67), 77 (100). Anal.
Calcd for C15H14N6: C, 64.73; H, 5.07; N, 30.20. Found: C,
64.53; H, 5.22; N, 30.01.
12: (Silica gel, n-hexane/diethylether 7:3); 94% yield; mp
142-144 °C, pale yellow prisms ; IR (Nujol) 2137, 1490, 1453,
P r ep a r a tion of (E)- a n d (Z)-2,2′-d ia m in ostilben e (15).
To a well stirred mixture of (Z)-2,2′-dinitrostilbene (10a , E/Z
1:3) or (E)-2,2′-dinitrostilbene (11, 13 mmol) in 85 mL of
ethanol were added 85 mL of glacial acetic acid and reduced
iron powder (14.5 g, 260 mmol). The reaction mixture was
heated at reflux temperature for 3 h. After cooling, the
mixture was poured into water (200 mL) and neutralized with
Na2CO3. The resultant mixture was extracted with ether (3
× 250 mL) and the combined organic layers were washed with
water (250 mL) and dried over MgSO4. The solvent was
removed under reduced presure and the resultant solid was
recrystalized from chloroform/n-hexane to give respectively
Z-15 (E/Z 1:3) in 90% yield or E-15 in 92% yield. Z-15: IR
1
1310 cm-1; H-NMR (CDCl3) δ 7.09-7.17 (m, 4H), 7.23-7.34
(m, 4H), 7.36 (dd, 2H, J ) 7.5, 1.5 Hz); 13C-NMR (CDCl3) δ
118.6, 124.5, 125.0, 126.7, 129.0, 129.1, 137.5; mass spectrum
(relative intensity) 262 (M+, 6), 205 (38), 204 (100), 178 (41).
Anal. Calcd for C14H10N6: C, 64.11; H, 3.84; N, 32.04.
Found: C, 63.89; H, 3.62; N, 31.79.
16: (Silica gel, petroleum ether/diethyl ether 7:3); 96% yield
1
(E/Z 1:3) as revealed by H-NMR; mp 106-108 °C; IR (Nujol)
2130, 1478, 1307, 1285 cm-1; 1H-NMR (CDCl3) δ 6.68 (s, 2H),
6.86 (td, 2H, J ) 7.5, 1.1 Hz), 7.03 (dd, 2H, J ) 7.6, 1.4 Hz),
7.13 (dd, 2H, J ) 8.1, 1.1 Hz), 7.23 (td, 2H, J ) 7.6, 1.5 Hz);
13C-NMR (CDCl3) δ 118.4, 124.4, 126.8, 128.7, 128.8, 130.4,
138.2; mass spectrum (relative intensity) 262 (M+, 33), 206
(42), 205 (100), 179 (24). Anal. Calcd for C14H10N6: C, 64.11;
H, 3.84; N, 32.04. Found: C, 64.35; H, 4.03; N, 32.17.
1
(Nujol) 1613, 1491, 1455, 751 cm-1; H-NMR (CDCl3) δ 3.63
(s, 4H), 6.53-6.63 (m, 6H), 6.97-7.05 (m, 4H); 13C-NMR
(CDCl3) δ 115.8, 118.3, 122.6, 127.1, 128.5, 129.6, 143.8; mass
spectrum (relative intensity) 211 (M+ + 1, 21), 210 (M+, 100),
209 (72), 193 (55). Anal. Calcd for C14H14N2: C, 79.97; H,
6.71; N, 13.32. Found: C, 80.12; H, 6.98;N, 13.05. E-15: Mp
179-181 °C, pale yellow prisms (chloroform); IR (Nujol) 1623,
1575, 1501, 759 cm-1; 1H-NMR (CDCl3) δ 3.59 (s, 4H), 6.71 (d,
2H, J ) 8.1 Hz), 6.80 (t, 2H, J ) 7.4 Hz), 7.02 (s, 2H), 7.10 (t,
2H, J ) 7.4 Hz), 7.39 (d, 2H, J ) 7.6 Hz); 13C-NMR (CDCl3) δ
116.2, 119.3, 124.2, 126.0, 127.3, 128.7, 144.0; mass spectrum
(relative intensity) 210 (M+, 100), 209 (52), 193 (62), 118 (48).
Anal. Calcd for C14H14N2: C, 79.97; H, 6.71; N, 13.32.
Found: C, 80.09; H, 6.91; N, 13.44.
20: (Silica gel, n-hexane/diethyl ether 7:3); 80% yield; yellow
oil; IR (Nujol) 2122, 1491, 1289, 748 cm-1; 1H-NMR (CDCl3) δ
1.78-1.88 (m, 2H), 2.60 (t, 4H, J ) 7.8 Hz), 7.04 (td, 2H, J )
7.4, 1.3 Hz), 7.10 (dd, 2H, J ) 8.1, 1.0 Hz), 7.15 (dd, 2H, J )
7.6, 1.3 Hz), 7.21 (td, 2H, J ) 7.5, 1.6 Hz); 13C-NMR (CDCl3)
δ 30.7, 31.1, 118.1, 124.7, 127.3, 130.4, 133.7, 138.0; mass
spectrum (relative intensity) 278 (M+, 19), 193 (55), 118 (100),
77 (50). Anal. Calcd for C15H14N6: C, 64.73; H, 5.07; N, 30.20.
Found: C, 64.97; H, 5.32; N, 30.04.
P r ep a r a tion of cis- a n d tr a n s-2,2′-Dia zid ostilben e Ox-
id es (Ep oxid es 13 a n d 17). To a solution cooled at 0 °C of
(E)-2,2′-diazidostilbene (12) or (Z)-2,2′-diazidostilbene (16, E/Z
1:3) (2 g, 7 mmol) in 50 mL of dry dichloromethane was added
m-CPBA (50-60%) (1.97 g). The resultant mixture was stirred
at 0 °C for 3 h. A second portion of m-CPBA (1.97 g) was then
added. The reaction mixture was allowed to warm at room
temperature and stirred overnight. A saturated aqueous
solution of NaHCO3 (30 mL) was added. The organic layer
was separated, washed with water, and dried over MgSO4. The
solvent was removed under reduced pressure, and the residual
material was purified by column chromatography to give
respectively trans-2,2′-diazidostilbene oxide (13, silica gel,
dichloromethane) or cis-2,2′-diazidostilbene oxide (17, silica
gel, n-hexane/dichloromethane 4:1).
P r ep a r a tion of th e Dia m in es 7 An d 19. To a solution
of the appropriate dinitro compound 6 or 10b (7.04 mmol) in
50 mL of ethanol was added 0.2 g of Pd on charcoal (10%),
and the reaction mixture was stirred at room temperature
under hydrogen for 7 or 3 h, respectively. Then, the mixture
was filtered over celite. The filtrate was concentrated to
dryness under reduced pressure and the resultant materials
were chromatographed to give the corresponding diamine:
7: (Silica gel, diethyl ether) 89% yield; yellow oil; IR (Nujol)
1
3357, 1623, 1501, 1454 cm-1; H-NMR (CDCl3) δ 1.27 (d, 3H,
J ) 6.8 Hz), 2.66 (dd, 1H, J ) 13.9, 8.0 Hz), 2.89 (dd, 1H, J )
13.9, 5.9 Hz), 3.18-3.29 (m, 1H), 4.87 (s, broad, 4H), 6.72-
6.80 (m, 3H), 6.89 (td, 1H, J ) 7.3, 1.1 Hz), 6.95-7.04 (m, 3H),
7.23 (dd, 1H, J ) 7.6, 1.5 Hz); 13C-NMR (CDCl3) δ 20.0, 32.4,
40.2, 116.1, 116.6, 118.9, 119.6, 122.5, 126.0, 126.8, 127.3,
131.1, 132.1, 143.6, 144.3 ; mass spectrum (relative intensity)
226 (M+, 12), 121 (10), 120 (100), 106 (20). Anal. Calcd for
C15H18N2: C, 79.81; H, 8.02; N, 12.38. Found: C, 80.03; H,
7.69; N, 12.57.
13: 72% yield; mp 124-126 °C, yellow prisms; IR (Nujol)
1
2129, 1499, 1298, 752 cm-1; H-NMR (CDCl3) δ 3.94 (s, 2H),
7.05-7.16 (m, 4H), 7.21-7.33 (m, 4H); 13C-NMR (CDCl3) δ
58.1, 116.0, 125.1, 125.7, 128.3, 129.3, 138.7; mass spectrum
(relative intensity) 220 (16), 193 (85), 91 (82), 75 (100). Anal.
Calcd for C14H10N6O: C, 60.43; H, 3.62; N, 30.20. Found: C,
60.13; H, 3.99; N, 30.03.
19: (Silica gel, n-hexane/ethyl acetate 1:1) 90% yield; mp
215-217 °C, colorless prisms; IR (Nujol) 1572, 1542, 1495, 744
cm-1 1H-NMR (CDCl3) δ 1.75-1.95 (m, 2H), 2.67-2.74 (m,
;
17: 42% yield; mp 117-119 °C, yellow prisms; IR (Nujol)
1
2130, 1493, 1296, 764 cm-1; H-NMR (CDCl3) δ 4.42 (s, 2H),
4H), 4.25 (s, broad, 4H), 7.06-7.23 (m, 8H); 13C-NMR (CDCl3)
δ 30.4, 30.5, 121.1, 124.8, 127.0, 130.1, 132.9, 134.8 ; mass
spectrum (relative intensity) 226 (M+, 61), 120 (100), 107 (56),
106 (85). Anal. Calcd for C15H18N2: C, 79.81; H, 8.02; N,
12.38. Found: C, 79.99; H, 8.30; N, 12.14.
6.88-7.01 (m, 4H), 7.12-7.25 (m, 4H); 13C-NMR (CDCl3) δ
57.0, 117.6, 124.0, 125.6, 128.2, 128.8, 138.3; mass spectrum
(relative intensity) 221 (38), 194 (100), 193 (59), 92 (25). Anal.
Calcd for C14H10N6O: C, 60.43; H, 3.62; N, 30.20. Found: C,
60.16; H, 3.35; N, 29.98.
P r ep a r a tion of Dia zid es 8, 12, 16, a n d 20. To a cooled
(0 °C) solution of the appropriate diamine (7.5 mmol) in 2 N
HCl (52 mL) was added dropwise a solution of sodium nitrite
(1.3 g, 19 mmol) in 10 mL of water. The resultant solution
P r ep a r a tion of Bis(im in op h osp h or a n es) 9, 14, 18, a n d
21. To a solution of the appropriate diazide 8, 13, 17, or 20 (3
mmol) in 30 mL of dry diethyl ether was added triphenylphos-

4298 J . Org. Chem., Vol. 61, No. 13, 1996
Molina et al.
phane (1.6 g, 6 mmol). The resultant mixture was stirred at
room temperature for 6 h. The precipitated solid was collected
by filtration and air-dried to give the corresponding bis-
(iminophosphorane).
(CDCl3) δ 2.65 (s, 1H), 4.47 (s, 2H), 7.01-7.04 (m, 1H), 7.13
(s, 1H), 7.17 (dd, 1H, J ) 7.6, 1.6 Hz), 7.24-7.29 (m, 2H), 7.34-
7.41 (m, 1H), 7.42-7.48 (m, 1H), 7.91-7.97 (m. 2H); 13C-NMR
(CDCl3) δ 66.5, 124.2, 124.4, 124.6, 128.0, 128.8, 131.5, 132.2,
132.4, 132.9, 133.4, 133.6, 141.9, 148.1, 148.7; mass spectrum
(relative intensity) 283 (1), 269 (1), 135 (75), 104 (87), 92 (100).
Anal. Calcd for C15H12N2O5: C, 60.00; H, 4.03; N, 9.33.
Found: C, 59.76; H, 4.21; N, 9.65.
9: 82% yield; mp 214-216 °C, pale yellow needles (diethyl
ether); IR (Nujol) 1487, 1444, 1352, 1113 cm-1 1H-NMR
;
(CDCl3) δ 1.32 (d, 3H, J ) 7.1 Hz), 3.33 (d, 2H, J ) 7.4 Hz),
4.21-4.32 (m, 1H), 6.43-6.49 (m, 2H), 6.54 (td, 1H, J ) 7.4,
0.8 Hz), 6.62-6.76 (m, 3H), 7.27-7.42 (m, 20 H), 7.69-7.82
(m, 12H); 13C-NMR (CDCl3) δ 20.3, 34.9, 38.7, 117.0, 117.4,
(E)-2,3-Bis(2-n itr op h en yl)p r op en yl Meth a n esu lfon a te.
To a solution of (E)-2,3-bis(2-nitrophenyl)propenol (5, 0.31g,
1 mmol) and triethylamine (0.16 g, 1.55 mmol) in 10 mL of
dry THF at 0 °C under nitrogen was added dropwise a solution
of methanesulfonyl chloride (0.18 g, 1.55 mmol). The reaction
mixture was stirred and allowed to warm up at room temper-
ature. After 3 h the precipitated triethylammoniun chloride
was separated by filtration. The filtrate was concentrated to
dryness under reduced pressure, and the resultant material
was purified by column chromatography (silica gel, diethyl
ether/n-hexane 4:1) to give (E)-2,3-bis(2-nitrophenyl)propenyl
methanesulfonate in 97% yield: mp 106-108 °C, colorless
prisms (diethyl ether/n-hexane); IR (Nujol) 1528, 1340, 1178,
950 cm-1; 1H-NMR (CDCl3) δ 3.04 (s, 3H), 5.11 (s, 2H), 6.98-
7.03 (m, 1H), 7.16-7.21 (m, 1H), 7.27 (s, 1H), 7.30-7.35 (m,
2H), 7.43-7.49 (m, 2H), 7.99-8.07 (m, 2H); 13C-NMR (CDCl3)
δ 37.9, 72.2, 124.7, 124.9, 128.9, 129.6, 129.8, 131.2, 131.3,
131.6, 133.0, 133.4, 133.9, 134.8, 147.8, 148.4; mass spectrum
(relative intensity) 203 (1), 206 (42), 135 (54), 134 (100). Anal.
Calcd for C16H14N2O7S: C, 50.79; H, 3.73; N, 7.40. Found: C,
50.89; H, 3.55; N, 7.57.
3
3
120.9 (d, J C-P ) 10.0 Hz), 121.12 (d, J C-P ) 10.1 Hz), 125.1,
4
3
125.3, 126.4 (d, J C-P ) 2.1 Hz), 128.4 (d, J C-P ) 11.7 Hz),
3
4
128.4 (d, J C-P ) 12.1 Hz), 129.9 (d, J C-P ) 1.8 Hz), 131.3 (d,
4J C-P ) 2.7 Hz, two carbons), 131.9 (d, J C-P ) 99.1 Hz, two
1
carbons), 132.6 (d, 2J C-P ) 9.5 Hz, two carbons), 137.0 (d, 3J C-P
) 22.0 Hz), 142.9 (d, J C-P ) 22.0 Hz), 148.6, 149.4; 31P-NMR
3
δ (CDCl3) -1.63, -0.87; mass spectrum (relative intensity) 487
(1), 183 (26), 86 (49), 84 (100). Anal. Calcd for C51H44N2P2:
C, 82.02 H, 5.94; N, 3.75. Found: C, 81.68; H, 5.67; N, 3.57.
14: 95% yield; mp 176-178 °C, white needles (diethyl
ether); IR (Nujol) 1480, 1437, 1114, 722 cm-1; 1H-NMR (CDCl3)
δ 4.79 (s, 2H), 6.43 (d, 2H, J ) 7.6 Hz), 6.73 (t, 2H, J ) 7.3
Hz), 6.88 (td, 2H, J ) 7.5, 1.5 Hz), 7.01-7.11 (m, 10H), 7.21-
7.47 (m, 10H), 7.54-7.65 (m, 12H); 13C-NMR (CDCl3) δ 61.9,
117.2, 120.3 (d, 3J C-P ) 9.6 Hz),124.0 (d, 4J C-P ) 2.7 Hz), 127.0,
3
4
128.3 (d, J C-P ) 12.1 Hz), 131.2 (d, J C-P ) 2.9 Hz), 131.3 (d,
1J C-P ) 100.0 Hz), 132.5 (d, J C-P ) 9.6 Hz), 132.8 (d, J C-P
)
2
3
21.1 Hz), 149.7; 31P-NMR δ (CDCl3) -1.83; mass spectrum
(relative intensity) 469 (29), 468 (100), 467 (27), 183 (31). Anal.
Calcd for C50H40N2OP2: C, 80.41; H, 5.40; N, 3.75. Found: C,
80.14; H, 5.18; N, 4.02.
1,2-Bis(2-n itr op h en yl)p r op en e (6). To a solution of (E)-
2,3-bis(2-nitrophenyl)propenyl methanesulfonate (0.57 g, 1.51
mmol) in 23 mL of HMPA and 5.7 mL of water was added
NaBH4 (0.23g, 6.03 mmol), and the reaction mixture was
stirred for 15 min; afterward water (25 mL) was added, and
the resultant mixture was stirred for 10 min and extracted
with diethyl ether (50 mL), and the organic layer was
separated, washed with water (2 × 40 mL) and dried over
MgSO4. The solvent was removed under reduced pressure and
the resultant material was chromatographed (silica gel, diethyl
ether/n-hexane 3:2) to give 6 in 75% yield as a mixture of E
18: 80% yield; mp 122-124 °C dec; yellow needles (diethyl
ether); IR (Nujol) 1597, 1448, 1334, 1110 cm-1 1H-NMR
;
(CDCl3) δ 5.30 (s, 2H), 6.24 (t, 2H, J ) 7.8 Hz), 6.43 (t, 2H, J
) 7.3 Hz), 6.67 (td, 2H, J ) 7.5, 1.4 Hz), 7.09-7.33 (m, 20H),
7.56-7.66 (m, 12H); 13C-NMR (CDCl3) δ 59.7, 116.1, 120.3 (d,
3J C-P ) 11.2 Hz),126.9, 127.9 (d, 4J C-P ) 2.4 Hz), 128.4 (d, 3J C-P
3
4
) 12.0 Hz), 129.4 (d, J C-P ) 21.1 Hz), 131.2 (d, J C-P ) 2.8
Hz), 131.3 (d, 1J C-P ) 99.1 Hz), 132.6 (d, 2J C-P ) 9.6 Hz), 150.5;
31P-NMR δ (CDCl3) 0.47; mass spectrum (relative intensity)
468 (4), 206 (47), 205 (35), 183 (100). Anal. Calcd for C50H40N2-
OP2: C, 80.41; H, 5.40; N, 3.75. Found: C, 80.75; H, 5.14; N,
3.42.
1
and Z isomers (4:1) as revealed by H-NMR: IR (Nujol) 1613,
1581, 1527, 1347 cm-1; 1H-NMR (CDCl3) δ 2.00 (d, 0.6H(Z), J
) 1.8 Hz), 2.19 (d, 2.4H(E), J ) 1.6 Hz), 6.67-6.69 (d, 0.2H(Z),
J ) 1.8 Hz), 6.84-6.86 (m, 0.8H(E), J ) 1.6 Hz), 7.00-7.97
(m, 6.4H(E) + 1.2H(Z)), 7.99 (dd, 0.2H(Z), J ) 8.1, 1.3 Hz),
8.10 (dd, 0.2H(Z), J ) 8.4, 1.3 Hz); 13C-NMR (CDCl3) δ 20.6(Z),
24.7(E), 124.1(E), 124.4(E), 124.5(E), 124.6(Z), 125.5(Z), 127.6(E),
128.1(Z), 128.2(E), 130.8(Z), 131.5(E), 131.8(E), 132.1(Z),
132.6(Z), 132.7(E), 132.9(E), 133.2(Z), 133.3(Z), 133.6(E),
137.1(E), 137.5(Z), 138.5(E), 139.7(Z), 147.8(Z), 148.0(Z),
148.1(E), 148.2(E), two methine carbons were not observed;
mass spectrum (relative intensity) 241 (8), 136 (52), 135 (100),
104 (72). Anal. Calcd for C15H12N2O4: C, 63.38; H, 4.25; N,
9.85. Found: C, 63.55; H, 4.08; N, 10.01.
21: 87% yield; mp 198-200 °C, colorless prisms (diethyl
ether); IR (Nujol) 1482, 1452, 1348, 1111 cm-1 1H-NMR
;
(CDCl3) δ 2.09-2.19 (m, 2H), 3.06 (t, 4H, J ) 7.7 Hz), 6.46 (d,
2H, J ) 7.6 Hz), 6.56 (d, 2H, J ) 7.3 Hz), 6.72 (td, 2H, J )
7.6, 1.6 Hz), 7.04-7.09 (m, 2H), 7.31-7.46 (m, 18H), 7.72-
7.79 (m, 12H) ; 13C-NMR (CDCl3) δ 30.0, 33.6, 117.2, 121.0 (d,
3
3J C-P ) 9.6 Hz), 121.5, 128.5 (d, J C-P ) 11.6 Hz), 129.3 (d,
4
1
4J C-P ) 2.0 Hz), 131.4 (d, J C-P ) 3.0 Hz), 131.9 (d, J C-P
)
99.2 Hz), 132.6 (d, 2J C-P ) 9.6 Hz), 137.6 (d, 3J C-P ) 21.6 Hz),
148.5; 31P-NMR δ (CDCl3) -0.85; mass spectrum (relative
intensity) 380 (21), 183 (97), 120 (100), 108 (51). Anal. Calcd
for C51H44N2P2: C, 82.02; H, 5.94; N, 3.75. Found: C, 81.77;
H, 5.67; N, 3.99.
P r ep a r a tion of Bis(isoth iocya n a tes). To a solution of
the appropriate bis(iminophosphorane) 9, 14, 18, or 21 (1.04
mmol) in 30 mL of dry benzene was added an excess of carbon
disulfide (3 mL), and the resultant mixture was stirred under
nitrogen at reflux temperature for 4 h. After cooling the
solvent was removed under reduced pressure, and the crude
material was chromatographed to give the corresponding bis-
(isothiocyanate).
(E)-2,3-Bis(2-n itr op h en yl)p r op en ol (5). To a stirred
solution of NaBH4 (0.79 g, 21 mmol) in 25 mL of dry
1,2-dimethoxyethane at 0 °C under nitrogen was added
dropwise titanium tetrachloride (0.76 mL, 7 mmol). The
resultant mixture was stirred 10 min and then a solution of
(E)-2,3-bis(2-nitrophenyl)propenoic acid27 in 7 mL of dry 1,2-
dimethoxyethane was added dropwise. The resultant mixture
was stirred overnight. The reaction mixture was cooled at 0
°C, and water was added slowly and then the whole mixture
was extracted with diethyl ether. The combined organic layer
was washed with water and dried over MgSO4. The solvent
was removed under reduced pressure, and the resultant
material was purified by column chromatography (silica gel,
diethyl ether/n-hexane 7:3) to give 5 as a yellow oil in 55%
2,2′-P r op ylen e-1,1′-bis(p h en yl isoth iocya n a te): (Silica
gel, n-hexane/dichloromethane 1:1); 97% yield; colorless oil; IR
1
(Nujol) 2100, 1491, 1448, 759 cm-1; H-NMR (CDCl3) δ 1.34
(d, 3H, J ) 7.0 Hz), 2.84 (dd, 1H, J ) 13.4, 8.1 Hz), 3.01 (dd,
1H, J ) 13.4, 6.9 Hz), 3.35-3.53 (m, 1H), 6.00-7.35 (m, 8H);
13C-NMR (CDCl3) δ 20.2, 36.1, 40.0, 126.6, 126.8, 127.0, 127.2,
127.3, 127.4, 127.7, 129.4, 130.2, 130.6, 134.9, 135.2, 136.1,
141.3; mass spectrum (relative intensity) 310 (M+, 45), 162
(100), 128 (98), 77 (21). Anal. Calcd for C17H14N2S2: C, 65.78;
H, 4.55; N, 9.02. Found: C, 65.95; H, 4.34; N, 9.17.
yield: IR (Nujol) 3405, 1542, 1523, 1353 cm-1 1H-NMR
;
(27) Pailer, M.; Schleppnick, A.; Meller, A. Monatsh. Chem. 1958,
89, 211.
(28) The author has deposited atomic coordinates for 2 and 3 with
the Cambridge Crystallographic Data Centre. The coordinates can be
obtained, on request, from the Director, Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.
2,2′-(tr a n s-Oxir a n e-2,3-d iyl)-1,1′-bis(p h en yl isoth iocy-
a n a te): (Silica gel, n-hexane/dichloromethane 1:1) 52%; mp
121-123 °C, colorless prisms; IR (Nujol) 2111, 1485, 1458, 755
cm-1; 1H-NMR (CDCl3) δ 4.08 (s, 2H), 7.28-7.39 (m, 8H); 13C-
NMR (CDCl3) δ 58.8, 125.5, 126.8, 127.8, 129.4, 130.6, 132.4,

Racemization Mechanism of Carbodiimides
J . Org. Chem., Vol. 61, No. 13, 1996 4299
138.6; mass spectrum (relative intensity) 310 (M+, 2), 179 (47),
146 (62), 120 (100). Anal. Calcd for C16H10N2OS2: C, 61.91;
H, 3.25; N, 9.03. Found: C, 62.07; H, 3.06; N, 8.87.
HRMS (EI) M+ 234.1158, theor 234.1157. Anal. Calcd for
C16H14N2: C, 82.02; H, 6.02; N, 11.96. Found: C, 81.88; H,
5.85; N, 12.06.
2,2′-(cis-Oxir a n e-2,3-d iyl)-1,1′-bis(p h en yl isoth iocya n -
a te): (Silica gel, n-hexane/dichloromethane 1:1) 55% yield; mp
73-75 °C, colorless prisms; IR (Nujol) 2074, 1603, 1385, 754
cm-1; 1H-NMR (CDCl3) δ 4.59 (s, 2H), 7.03-7.17 (m, 8H); 13C-
NMR (CDCl3) δ 57.3, 126.5, 126.6, 127.8, 128.9, 129.8, 130.3,
138.6; mass spectrum (relative intensity) 310 (M+, 4), 179 (54),
146 (60), 120 (100). Anal. Calcd for C16H10N2OS2: C, 61.91;
H, 3.25; N, 9.03. Found: C, 61.75; H, 3.12; N, 9.21.
23: (Silica gel, petroleum ether/dichloromethane 1:1) 21%
yield (method A); mp 226-228 °C, colorless prisms (chloro-
form); IR (Nujol) 2159, 1484, 1454, 757 cm-1; 1H-NMR (CDCl3)
δ 4.04 (s, 4H), 6.93-6,96 (m, 4H), 7.05-7.11 (m, 8H), 7.30-
7.33 (m, 4H); 13C-NMR (CDCl3) δ 59.5, 124.2, 125.0, 125.8,
129.1, 131.3, 135.2, 136.8; HRMS (EI) M+ 468.1593, theor
468.1586. Anal. Calcd for C30H20N4O2: C, 76.91; H, 4.30; N,
11.96. Found: C, 77.07; H, 4.54; N, 11.75.
2,2′-Tr im eth ylen e-1,1′-bis(ph en yl isoth iocyan ate): (Silica
gel, n-hexane/dichloromethane 4:1) 77% yield; colorless oil; IR
25 (Diastereoisomer more soluble in dichloromethane, re-
maining in solution): (Silica gel, n-hexane/diethyl ether 7:3)
1
(Nujol) 2095, 1491, 1454 cm-1; H-NMR (CDCl3) δ 1.87-1.98
1
10% yield; IR (Nujol) 2153, 1602, 1581, 1448 cm-1; H-NMR
(m, 2H), 2.77 (t, 4H, J ) 7.7 Hz), 7.17-7.25 (m, 8H); 13C-NMR
(CDCl3) δ 30.6, 32.0, 126.5, 127.3, 127.6, 130.1, 135.6, 137.1,
138.4; mass spectrum (relative intensity) 312 (M+ + 2, 8), 311
(M+ + 1, 17), 310 (M+, 90), 252 (100). Anal. Calcd for
C17H14N2S2: C, 65.78; H, 4.55; N, 9.02. Found: C, 65.86; H,
4.60; N, 9.18.
P r ep a r a tion of Bis(ca r bod iim id es) 22, 23, 25, a n d 26.
Meth od A: To a solution of the appropriate bis(iminophos-
phorane) 9, 14, 18, or 21 (0.69 mmol) in 20 mL of dry
dichloromethane was added Boc2O (0.3 g, 1.39 mmol), except
for the bis(iminophosphorane) 9 which requires 5 equiv of
Boc2O, and DMAP (0.083 g, 0.69 mmol). The resultant mixture
was stirred under nitrogen at room temperature for 12 h. The
solvent was removed under reduced pressure and the resultant
material was chromatographed to give the corresponding bis-
(carbodiimide).
Meth od B: To a solution of the appropriate bis(iminophos-
phorane) 9, 14, 18, or 21 (0.33 mmol) in 20 mL of dry benzene
was added an excess of solid carbon dioxide. The mixture was
heated in a glass sealed tube at 70 °C for 16 h. After cooling,
the solvent was removed under reduced pressure, and the
remaining material was chromatographed to give the corre-
sponding bis(carbodiimide).
Meth od C: To a solution of the appropriate bis(isothiocy-
anate) (0.7 mmol) in 30 mL of dry benzene was added the
corresponding bis(iminophosphorane) (0.7 mmol). The reaction
mixture was stirred at reflux temperature under nitrogen for
24 h. After cooling, the solvent was removed under reduced
pressure and the residue was chromatographed to give the
corresponding carbodiimide.
(CDCl3) δ 4.63 (s, 4H), 6.92-7.22 (m, 16H); 13C-NMR (CDCl3)
δ 57.6, 123.9, 124.6, 127.9, 128.2, 128.6, 130.7, 136.6; Anal.
Calcd for C30H20N4O2: C, 76.91; H, 4.30; N, 11.96. Found: C,
77.03; H, 4.47; N, 12.01.
25 (Diastereoisomer crystallized from dichloromethane):
(Silica gel, n-hexane/diethyl ether 7:3) 10% yield; mp 248-
250 °C, colorless prisms (dichoromethane); IR (Nujol) 2147,
1
1492, 1455, 751 cm-1; H-NMR (CDCl3) δ 4.61 (s, 4H), 6.92-
7.00 (m, 8H), 7.10 (td, 4H, J ) 7.6, 1.6), 7.18 (dd, 4H, J ) 7.6,
1.6 Hz); 13C-NMR (CDCl3) δ 57.7, 124.1, 124.7, 128.0, 128.4,
128.5, 133.2, 136.8; HRMS (EI) M+ 468.1574, theor 468.1586.
Anal. Calcd for C30H20N4O2: C, 76.91; H, 4.30; N, 11.96.
Found: C, 76.79; H, 4.22; N, 11.78.
26: (Silica gel, n-hexane/dichloromethane 7:3) 40% yield;
mp 199-201 °C, colorless prisms (chloroform/n-hexane); IR
(Nujol) 2143, 1581, 1491, 1384 cm-1; 1H-NMR (CDCl3) δ 1.88-
199 (m, 4H), 2.79 (t, 8H, J ) 8.0 Hz), 7.03-7.20 (m, 16H); 13C-
NMR (CDCl3) δ 32.3, 33.2, 125.2, 125.4, 127.1, 130.2, 133.7,
136.9, 137.3; HRMS (EI) M+ 468.2316, theor 468.2314. Anal.
Calcd for C32H28N4: C, 82.02; H, 6.02; N, 11.96. Found: C,
82.14; H, 6.23;N, 12.05.
Ack n ow led gm en t . One of us (J .S.C., permanent
address: Facultad de Farmacia, Universidad de Valen-
cia, Spain) thanks the Conselleria de Educacio´n y
Ciencia (Generalitat Valenciana) for a research fellow-
ship to work at the Instituto “Rocasolano”, CSIC. We
gratefully acknowledge the financial support of the
Direccio´n General de Investigacio´n Cient´ıfica y Te´cnica
(project number PB92-0984). One of us (P.S.A.) also
thanks to the Consejer´ıa de Educacio´n de la Comunidad
Auto´noma de Murcia for a studentship. We thank Dr.
Ibon Alkorta (IQM, CSIC) for helpful discussions con-
cerning the AM1 calculations.
22: (Silica gel, n-hexane/dichloromethane 3:2) 98% yield
(method B); mp 89-91 °C, colorless prisms (n-hexane); IR
1
(Nujol) 2121, 1590, 1454, 759 cm-1; H-NMR (CDCl3) δ 1.44
(d, 3H, J ) 6.4 Hz), 2.37 (dd, 1H, J ) 15.2, 8.3 Hz), 3.50-3.65
(m, 2H), 6.98-7.27 (m, 7H), 7.35 (dd, 1H, J ) 7.2, 1.9 Hz);
13C-NMR (CDCl3) δ 18.5, 35.0, 42.7, 123.4, 123.5, 125.0, 125.1,
125.8, 127.5, 128.1, 130.6, 134.2, 138.4, 138.7, 139.2, 142.8;
J O951789C