Center-to-propeller and propeller-to-propeller stereocontrol in a series of macrobicyclic tri-λ5-phosphazenes

Article abstract of DOI:10.1016/j.tetlet.2007.03.087

Center-to-propeller stereocontrol in a family of macrobicyclic, double-propeller shaped tri-λ⁵-triphosphazenes remains constant in the upper tribenzylamine fragment as the size of the pivotal group at the lower tris(phosphane) fragment is gradu

Full text of DOI:10.1016/j.tetlet.2007.03.087

Tetrahedron Letters 48 (2007) 3583–3586
Center-to-propeller and propeller-to-propeller stereocontrol
in a series of macrobicyclic tri-k5-phosphazenes
*
*
´
´
´
´
´
Mateo Alajarın, Carmen Lopez-Leonardo, Jose Berna and Pilar Sanchez-Andrada
Departamento de Quı´mica Orga´nica, Facultad de Quı´mica, Universidad de Murcia, Campus de Espinardo, E-30100 Murcia, Spain
Received 10 January 2007; revised 13 March 2007; accepted 16 March 2007
Available online 21 March 2007
Abstract—Center-to-propeller stereocontrol in a family of macrobicyclic, double-propeller shaped tri-k5-triphosphazenes remains
constant in the upper tribenzylamine fragment as the size of the pivotal group at the lower tris(phosphane) fragment is gradually
increased. In contrast the propeller-to-propeller stereocontrol diminishes as a result of the increasing conformational lability in the
lower hemisphere.
Ó 2007 Elsevier Ltd. All rights reserved.
The self-assembly of tripodal reactants such as tris(3-
azidobenzyl)amines and the commercially available
tris(phosphane) CH3C(CH2PPh2)3 gives intermediate
triphosphazides1 which under mild heating in solution
yield tri-k5-phosphazenes 12 by the well known two-step
Staudinger reaction.3 Macrobicycles 1 so obtained have
been shown to possess double-propeller topology and
the process occurs with total stereoselectivity in favor
of the formation of the species in which both propeller
units, the upper tribenzylamine and the lower tert-pen-
tane fragments, present the same sense of twist (Fig. 1).
In the context of these investigations we have demon-
strated that the sense of twist of the two propeller units
of compounds 1 can be simultaneously controlled by a
single chiral benzylic carbon in one of the arms of the
upper fragment,4 just by replacing one of the benzylic
protons by a methyl group.
That stereocontrol seems to be determined by the
marked preference of the methyl group for occupying
a pseudoaxial position (parallel to the pseudo C3 axis)
instead of the alternative pseudoequatorial position clo-
ser to the internal cavity, and therefore to the aryl
groups. The sense of twist of the arm bearing the a-
methyl group in turn determines those of the other
two arms (Fig. 2).5 This was confirmed by an X-ray
crystal structure determination that showed the CH3
pseudoaxial topology of the R*, M*, M* diastereoiso-
mers, the only obtained isomers.4
R
R
R
N
N
N
Here we show that while this is valid for several tri-k5-
N
N
P
phosphazenes in which the lower fragment bears either
P
P
CH3
= -N=P(Ph)2-
propeller sense in 1
Ar
N
Ar
H
Ar
CH3
Ar
)
1
N
H
H
H
Figure 1. Tri-k5-phosphazenes 1 and schematic representation as
viewed along their C3 axis, showing the two propeller units with the
same sense of twist.
Ar
Ar
(
R*, P*
(
R*, M*
)
CH3 pseudoequatorial
disfavored
CH3 pseudoaxial
favoured
Keywords: Cage compounds; Propeller macrobicycles.
*
Figure 2. The two possible conformations for the upper fragment of
a-methyltribenzylamine of compounds 1.
Corresponding authors. E-mail addresses: alajarin@um.es; melill@
um.es
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.087

3584
M. Alajarı´n et al. / Tetrahedron Letters 48 (2007) 3583–3586
PPh2
PPh2
CH3 at the pivotal carbon atom (as in 1) or a smaller H
H3C
atom, the introduction at that position of larger groups,
i
t
PPh2
3a
PPh2
PPh2
PPh2
3b Rpiv = H 42% ref. 7
such as Et, Pr, and Bu, gradually decreases the effi-
ciency of the propeller-to-propeller induction (from the
upper to the lower propeller) while retaining the cen-
ter-to-propeller total induction at the upper hemisphere.
This is evidenced by the gradually lower diastereoiso-
meric excesses measured in the formed macrobicycles
(R*, M*, M* and R*, M*, P*, major and minor diastereo-
isomers, respectively).
ii)
OH
OH
OH
Cl
Cl
i)
Rpiv
Rpiv
Rpiv
Cl
R
piv = H not isolated
3c
Rpiv = CH3CH2 45% ref. 8
3d Rpiv = (CH3)2CH 71% ref. 9
3e
Rpiv = CH3CH2 not isolated
Rpiv = (CH3)2CH 80%
Rpiv = (CH3)3C 85%
Rpiv = (CH3)3C 42% ref. 9
To this end we prepared a series of a-methyl substituted
triazides 2 in racemic form by using standard methods,4
starting from the commercially available a-methyl-3-
aminobenzyl alcohol and 2-substituted 5-azidobenzyl
alcohols2,6 (Scheme 1).
Scheme 2. Synthesis of tris(phosphanes) 3. Reagents and conditions:
(i) SOCl2, pyridine, 0–115 °C, 6 h; (ii) HPPh2, KOtBu, THF, reflux,
16 h.
R
CH3
N
R
CH3
We also synthesized several tris(phosphanes) 3 of gen-
eral formula RpivC(CH2PPh2)3, analogous of the com-
mercially available 3a (Rpiv = CH3), in which the
pivotal Rpiv group is H,7 CH3CH2,8 (CH3)2CH,9 and
(CH3)3C9 (Scheme 2).
R
N
i), ii)
N
N3
N3
N
N
P
2
P
P
2
Rpiv
Triazides 2 coupled efficiently with triphosphanes 3 to
give, after dinitrogen extrusion, macrobicycles 4 in 13–
86% yields (based on the starting triazides 2) (Scheme
3 and Table 1).10
+
RpivC(CH2PPh2)3
4
3
Scheme 3. Synthesis of tri-k5-phosphazenes 4. Reagents and condi-
tions: (i) Et2O, 25 °C, 3 h; (ii) CDCl3, 60 °C, 24 h.
As presumed, macrobicycles 4a (Rpiv = H) and 4c
(Rpiv = CH3) were obtained as single diastereoisomers
(Table 1, entries 1 and 3) in a similar way to other pre-
viously reported by us (Rpiv = CH3, R = H, Cl, entries 2
and 4)4 and showing comparable spectroscopic data. In
Table 1. a-Methyl substituted tri-k5-phosphazene 4
Entry
Compound
Rpiv
R
de (%)
Yield (%)
1
2
3
4
5
6
7
8
9
4a
4ba
4c
4da
4e
4f
H
CH3
CH3
CH3
CH3CH2
CH3CH2
(CH3)2CH
(CH3)2CH
(CH3)3C
H
H
>98
>98
>98
>98
90
86
52
56
4
61
70
89
63
86
58
55
42
13
CH3
OH
CH3
OH
CH3
NH2
Br
Cl
Br
Cl
Br
Cl
Br
i)
ii), iii)
42%
79%
NH2
R
N3
R
N3
R
vi)
4g
4h
4i
iv)
v)
OH
Cl
I
a Compounds 4b and 4d have been previously described in Ref. 4.
N3
N3
N3
R = H
R = Br
R= Cl
R = H; 86%
R = Br; 44%
R= Cl; 71%
R = H; 91%
R = Br; 74%
R= Cl; 99%
these cases, the sense of twist of their two propeller units
is totally controlled by the chiral benzylic carbon atom,
and it should be the same for both propellers as in the
previously reported examples. That is, 4a and 4c are ob-
tained as the R*, M*, M* diastereoisomers exclusively.
CH3
R
N
The situation changes with the introduction of larger
Rpiv groups. In compounds 4e and 4f (Rpiv = CH3CH2,
entries 5 and 6) minor amounts of a second diastereoiso-
mer were detected in their NMR spectra (de 90 and 86%,
respectively). The proportion of the second diastereoiso-
mer increased notably in 4g and 4h [Rpiv = (CH3)2CH,
entries 7 and 8], whereas in 4i [Rpiv = (CH3)3C, entry
9] the two diastereisomers were present in nearly equal
amount (Table 1). In all these cases, we could not re-
solve the diastereoisomeric mixtures into their compo-
nents by chromatography or fractional crystallization.
N3
N3
2
2a R = H, 71%
2b R = Br, 63%
2c R= Cl, 60%
Scheme 1. Preparation of tribenzylamines 2. Reagents and conditions:
(i) NaNO2, dil HCl, 0 °C, 30 min, then NaN3, 25 °C, 16 h; (ii) TPP,
DEAD, phthalimide, THF, 0–25 °C, 24 h; (iii) N2H4ÆH2O, EtOH,
reflux, 3 h; (iv) SOCl2, CH2Cl2, 0 °C, 3 h; (v) NaI, acetone, 25 °C, 12 h;
(vi) CH3CN, Na2CO3, reflux, 16 h.

M. Alajarı´n et al. / Tetrahedron Letters 48 (2007) 3583–3586
3585
Table 2. Calculated chemical shifts of 5aa and 5ba and experimental
chemical shifts of 4bb
In a first approximation we postulated that the minor
isomers accompanying the expected, major R*, M*, M*
diastereoisomers 4e–i should be those differing in the
sense of twist of the lower propeller, R*, M*, P*, as there
is no apparent reason to expect an alteration of the
asymmetric control of the center over the upper propel-
ler (R* leading to M*), whereas the introduction of large
groups at the pivotal carbon of the lower hemisphere
could alter considerably the conformational preferences
of this moiety.
Compound
1H NMR
13C NMR
CHMeN a-Me CHMeN a-Me (CH2)A (CH2)B
5a (a-Meax
5b (a-Meeq
4b
)
)
3.64
3.28
3.66
1.35 54.79
0.94 69.19
1.22 55.24
8.49 52.47
26.69 56.29
7.35 51.94
54.14
69.37
53.76
a The data for 5a and 5b were obtained by GIAO calculations at the
RHF/6-31G* level.
b Measured in CDCl3 at 300 (1H NMR) and 75 (13C NMR) MHz.
We have found some support for this assumption. The
main spectroscopic differences between the diastereoiso-
meric macrobicycles in each pair 4e–i do not lie in the
signals attributed to the upper propeller, but in those
corresponding to the lower part. For instance,10 whereas
six phosphorus resonances are more or less clearly dis-
tinguished in each mixture of two diastereoisomers,
the resonances of their a-methyl groups at the benzylic
mers 4e–h. For instance, the a-methyl and methine
carbons of diastereoisomeric pair 4h resonate at 8.28
and 55.21 ppm in the major diastereoisomer, and at
8.00 and 57.77 ppm in the minor component of the
mixture.
Thus, the experimental and calculated data of com-
pounds 4 are in accord with the assignation of the R*,
M*, M* and R*, M*, P* relative configurations to the
major and minor components of the mixtures of diaste-
reoisomers 4e–h respectively, which differ only in the
sense of twist of the lower tris(phosphane) propeller
(Fig. 4).
1
carbon in their H NMR spectra appear in a narrow
range of chemical shifts. One would expect more signifi-
cant differences in chemical shifts between pseudoaxial
and pseudoequatorial methyl groups, and this seems to
be not the case.
Moreover, we have calculated the chemical shift of sev-
eral nuclei in a model of the upper part of these tri-k5-
phosphazenes, a-methyltribenzylamine 5 in two confor-
mational minima, those with the Me group in axial and
equatorial positions, 5a and 5b, respectively (Fig. 3) by
means of the GIAO (gauge-independent atomic orbi-
tal)11 computational methodology. The results are sum-
marized in Table 2 and compared with the NMR data of
the upper fragment of macrobicycle 4b measured in
CDCl3.
The results of molecular mechanics calculations12 of the
two diastereoisomers in each pair 4e–h are summarized
in Table 3. In all the cases the R*, M*, M* isomer is
the most stable and the differences in energy with respect
to the R*, M*, P* isomer DE are gradually lower when
the size of the pivotal group Rpiv increases. The same
tendency was observed as far as the experimental diaste-
reoisomeric excesses were concerned.
In summary, in macrobicyclic tri-k5-phosphazenes 4a–e
(Rpiv = H, CH3) the chiral center at the benzylic carbon
controls the sense of twist of the upper tribenzylamine
fragment, which in turn determines that of the lower
tris(phosphane) propeller. The propeller-to-propeller
The data in Table 2 clearly reveal the similarity of the
shifts measured in 4b with those calculated for 5a (a-
Meax) which in turn are considerably far from those of
its conformer 5b (a-Meeq), particularly, but not only,
in the 13C NMR shifts of the a-methyl (8.49 vs 26.69)
and the methine (54.79 vs 69.19) carbons. These data
support the a-Meax conformation of the tribenzylamine
fragment of 4b and also of the rest of tri-k5-phosphaz-
enes here prepared, including the pairs of diastereoiso-
Ph
Ph
P
Ph
Ph
N
R
N
P
R
R
R
N
N
N
N
H
H
Ph
P
P
H
Ph
Ph
CH3
H
Ph
P
H
PhN
Ph
P
Ph
H
Ph
N
N
5
(R*,M*,P*)
(R*,M*,M*)
Figure 4. Major and minor diastereoisomers of compounds 4e–h.
Table 3. Calculated difference of energy for the components of the
pairs (R*, M*, M*)/(R*, M*, P*) of some tri-k5-phosphazenes 4
Tri-k5-phosphazene
R
Rpiv
DE (kcal molꢀ1
)
4c
4e
4g
4i
Br
Br
Br
Br
CH3
25.86
19.45
11.04
1.95
CH3CH2
(CH3)2CH
(CH3)C
5a (α-Meax)
5b (α-Meeq)
Figure 3. Two conformations of a-methyltribenzylamine 5.

3586
M. Alajarı´n et al. / Tetrahedron Letters 48 (2007) 3583–3586
(75.4 MHz, CDCl3): d 8.68 (a-CH3), 27.50 (CH3C),
36.73–37.26 (m, 3CH2P), 38.98 (q, 2J(C,P) = 3.8 Hz;
CH3C), 50.09 (CH2N), 52.57 (CH2N), 55.84 (CH),
112.49 (q), 112.59 (q), 115.32, 123.16 (d, 3J(C,P) = 12.7 Hz;
s-cis-CH@C–N@P), 123.99 (d, 3J(C,P) = 12.2 Hz;
s-cis-CH@C–N@P), 124.45 (d, 3J(C,P) = 12.7 Hz; s-cis-
CH@C–N@P), 125.43 (d, 3J(C,P) = 28.3 Hz; s-trans-
CH@C–N@P), 127.19 (d, 3J(C,P) = 28.6 Hz; s-trans-
CH@C–N@P), 127.29 (d, 3J(C,P) = 28.7 Hz; s-trans-
CH@C–N@P), 128.34–132.80, 140.24 (q), 140.37 (q),
145.06 (q), 150.73 (3 q); 31P{1H} NMR (121.4 MHz,
CDCl3): d 0.44 (s, 1P), 1.36 (s, 1P), 1.54 (s, 1P); IR (Nujol):
m = 1453 (CP), 1118 (NP) cmꢀ1; MS (FAB+): m/z
(%) = 1124 (13) [M++4], 1122 (13) [M++2], 1121 (11)
[M++1], 1120 (6) [M+], 133 (100); C63H57Br2N4P3
(1122.88): Calcd C, 67.39, H, 5.12, N, 4.99. Found: C,
67.50, H, 5.07, N, 5.13.
stereocontrol is gradually less effective as the size of the
i
t
pivotal group increases (Et, Pr, Bu), whereas the cen-
ter-to-propeller stereocontrol remains equally effective.
Large pivotal groups in the lower hemisphere increase
the conformational lability of this moiety.
Acknowledgments
This work was supported by the MEC and FEDER
´
´
(Project CTQ2005-02323/BQU) and Fundacion Sene-
ca-CARM (Project 00458/PI/04). J.B. also thanks the
MEC for a fellowship.
Tri-k5-phosphazenes 4h + 4h0 (diastereoisomers ratio
References and notes
1
3.5:1): Yield: 42%; H NMR (300 MHz, CDCl3): d ꢀ0.41
[d, J(H,H) = 6.9 Hz, 3H; (CH3)A (h)], ꢀ0.29 [d,
J(H,H) = 6.9 Hz, 3H; (CH3)A (h0)], ꢀ0.23 [d, J(H,H) =
6.9 Hz, 3H; (CH3)B (h)], ꢀ0.03 [d, J(H,H) = 6.9 Hz, 3H;
(CH3)B (h0)], 0.93 [m, 2H; CH(CH3)2 (h + h0)], 1.13 [d,
J(H,H) = 6.7 Hz, 3H; a-CH3 (h0)], 1.26 [d, J(H,H) =
6.9 Hz, 3H; a-CH3 (h)], 2.90 [d, J(H,H) = 12.6 Hz, 1H;
CHAHBN (h)], 2.94 [d, J(H,H) = 12.2 Hz, 1H; CHAHBN
(h)], 3.03 [d, J(H,H) = 16.5 Hz, 1H; CHAHBN (h0)], 3.24
[d, J(H,H) = 16.5 Hz, 1H; CHAHBN (h0)], 3.45–4.31 [m,
18H; 4 CHAHBN (h + h0) + 12 CH2P (h + h0) + 2 CHCH3
(h + h0)], 6.25 (br s, 1H; Harom), 6.30 (br s, 2H; Harom), 6.52
(d, J(H,H) = 7.2 Hz, 1H; Harom), 6.97–7.53 [m, 64H; Harom
(h + h0)], 7.96–8.00 [m, 12H; Harom (h + h0)]; 13C{1H}
NMR (75.4 MHz, CDCl3): d = 8.00 [a-CH3 (h0)], 8.28 [a-
CH3 (h)], 19.30 [(CH3)A (h)], 19.72 [(CH3)A (h0)], 20.01
[(CH3)B (h)], 20.08 [(CH3)B (h0)], 35.34 [(CH3)2CH (h)],
35.39 [(CH3)2CH (h0)], 36.60–37.40 [m, 3CH2P (h + h0)],
47.07 [q, 2J(C,P) = 3.1 Hz; (CH3)2CHC (h)], 49.48 [q,
2J(C,P) = 3.6 Hz; (CH3)2CHC (h0)], 49.77 [CH2N (h)],
49.98 [CH2N (h)], 50.38 [CH2N (h0)], 52.51 [CH2N (h0)],
55.21 [CHCH3 (h)], 57.77 [CHCH3 (h0)], 115.02 (h), 115.83
(h0), 122.46 [2 q; (h/h0)], 122.57 [2 q;(h/h0)], 123.27 [d,
3J(C,P) = 12.8 Hz; s-cis-CH@C–N@P (h)], 123.97 [d,
3J(C,P) = 12.2 Hz; s-cis-CH@C–N@P (h)], 124.44 [d,
3J(C,P) = 12.2 Hz; s-cis-CH@C–N@P (h)], 125.54 [d,
3J(C,P) = 27.8 Hz; s-trans-CH@C–N@P (h)], 126.70 [d,
3J(C,P) = 27.3 Hz; s-trans-CH@C–N@P (h)], 126.81 [d,
3J(C,P) = 27.3 Hz; s-trans-CH@C–N@P (h)], 127.84–
133.55 (h + h0), 136.42 [q; (h0)], 136.66 [q; (h0)], 138.05 [q;
(h)], 144.70 [q; (h)], 144.82 [q; (h0)], 149.71 [q; (h0)], 149.93 [2
q; (h)], 150.47 [2 q; (h0)], 150.59 [q; (h)]. Only signals clearly
observed for the minor diastereoisomer are detailed;
31P{1H} NMR (121.4 MHz, CDCl3): d ꢀ0.90 [s, 1P; (h0)],
ꢀ0.07 [s, 1P; (h0)], 0.55 [s, 1P; (h0)], 2.75 [s, 1P; (h)], 4.07 [s,
2P; (h)]; IR (Nujol): m = 1444 (CP), 1114 (NP) cmꢀ1; MS
(FAB+): m/z (%) = 1064 (76) [M++4], 1062 (100) [M++2],
1060 (37) [M+]; C65H61Cl2N4P3 (1062.03): Calcd C, 73.51,
H, 5.79, N, 5.28. Found: C, 73.64, H, 5.82, N, 5.19.
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same experimental procedure used for 3c, as reported in
Ref. 8.
10. Tri-k5-phosphazene 4c: Yield: 89%; mp 259–261 °C (yel-
low prisms from chloroform/n-pentane); 1H NMR
(300 MHz, CDCl3): d ꢀ0.76 (s, 3H; CH3), 1.42 (d,
J(H,H) = 6.8 Hz, 3H; a-CH3), 3.20 (m, 5H;
2
CHAHBN + 3 CHAHBP), 3.56 (d, J(H,H) = 12.1 Hz, 1H;
CHAHBN), 3.69 (q, J(H,H) = 6.8 Hz, 1H; CH), 3.85 (d,
J(H,H) = 12.9 Hz, 1H; CHAHBN), 3.90 (m, 3H;
CHAHBP), 6.32 (d, J(H,H) = 2.8 Hz, 1H; Harom), 6.35
(br s, 1H; Harom), 6.41 (d, J(H,H) = 2.8 Hz, 1H; Harom),
6.66 (d, J(H,H) = 6.6 Hz, 1H; Harom), 6.95 (dd, J(H,H) =
5.8, 2.9 Hz, 1H; Harom), 6.98 (dd, J(H,H) = 5.8, 2.9 Hz,
2H; Harom), 7.10–7.13 (m, 5H; Harom), 7.26–7.45 (m, 22H;
12. MM+ force field as implemented in the HyperChem 6.0
molecular modeling program (Hypercube, Inc; http://
www.hyper.com).
H
arom), 7.79–7.91 (m, 6H; Harom); 13C{1H} NMR