The reaction of allenes with phosphorus(III) compounds bearing a P-NH-(t-Bu) group: isolation of both enantiomers in crystalline form from an achiral system

Article abstract of DOI:10.1016/j.tetlet.2008.09.153

Structural characterization of the tautomeric forms of the zwitterions proposed in the phosphine catalyzed transformations of electron-deficient allenes by utilizing a cyclodiphosphazane with a P-NH-t-Bu group and Ph₂P(NH-t-Bu) is described. Spontaneous resolution of the products (R and S enantiomers of the crystals) via crystallization is highlighted.

Full text of DOI:10.1016/j.tetlet.2008.09.153

Tetrahedron Letters 49 (2008) 7135–7138
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Tetrahedron Letters
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The reaction of allenes with phosphorus(III) compounds bearing
a P-NH-(t-Bu) group: isolation of both enantiomers in crystalline
form from an achiral system
*
N. N. Bhuvan Kumar, K. C. Kumara Swamy
School of Chemistry, University of Hyderabad, Hyderabad 500 046, AP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Structural characterization of the tautomeric forms of the zwitterions proposed in the phosphine cata-
lyzed transformations of electron-deficient allenes by utilizing a cyclodiphosphazane with a P-NH-t-Bu
group and Ph2P(NH-t-Bu) is described. Spontaneous resolution of the products (R and S enantiomers of
the crystals) via crystallization is highlighted.
Received 7 August 2008
Revised 23 September 2008
Accepted 26 September 2008
Available online 1 October 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Phosphine activated reactions of electron deficient alkenes/
alkynes have found diverse and useful applications for a variety
of organic transformations.1,2 The species Ph3P+C(CO2Me)@CÀ(CO2-
Me) (1) is proposed as an intermediate in the reaction utilizing the
Ph3P–DMAD combination (cf. Scheme 1). This combination also
leads to many other products,3 and to our knowledge, no structural
proof for 1 is available. Phosphines also initiate the polymerization
of acrylonitrile, wherein the zwitterion R3P+CH2CHÀCN (2) is the
likely initiator.4,5 In the umpolung nucleophilic addition to the es-
ter allene H2C@C@C(H)CO2Me, a species of type 3 is involved as an
intermediate; it is possible that an analogous species is involved in
the reaction using the isomeric compound H3CC„CCO2Me.1a In
Ts
N
MeO2C
R
OMe
O
CO2Me
_
Ph3P
R
MeO2C
CO2Me
R3P
(a)
(b)
+
N
PPh3
MeO2C
_
(1)
Ts
_
CH2=CHCN
+
+
( CH CH)
CH2=CHCN
R3P
CH2CHCN
R3PCH2CHCN
2
n
(2)
CN
Polymer
H
H
C
H
PhCH2OH
HOAc (20 mol%)
C
C
PhCH2O
(c)
CO2Me
or
Ph3P (5 mol%)
CO2Me
H3C
CO2Me
+
Ph3P
CO2Me
(3)
Scheme 1.
* Corresponding author. Tel.: +91 40 23134856; fax: +91 40 23012460.
E-mail address: kckssc@uohyd.ernet.in (K. C. Kumara Swamy).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.153

7136
N. N. Bhuvan Kumar, K. C. Kumara Swamy / Tetrahedron Letters 49 (2008) 7135–7138
COOMe
t
-Bu
COOMe
+
S
+
_
.
COOMe
COOMe
.
N
N
N
.
_
.
P
P
H2N
S
NH-t-Bu
Bu-t-HN
t
-Bu
H2N
4
6
5
CO2Me
H
MeO
H
MeO2C
β
C
α
β
C
α
t
-Bu
t
-Bu
C
C
N
N
.
.
N
N
.
.
Ph2PNH-
t-Bu
t-Bu
P
P
P
P
N
H
N
NH-t-Bu
NH-
t
-Bu
t
7
-Bu
O
t
-Bu
t
-Bu
9
8
earlier reports on heterocyclic syntheses utilizing pyridine–DMAD
or disulfide–DMAD, the involvement of zwitterions 4 and 5 is pro-
posed.5 One of our interests in this connection is to characterize
species such as 1–5 and find utility for them later. Toward this
end, we used the PIII compound [t-BuNHPN(t-Bu)]2 (6, cyclophosp-
hazane heterocycle) which has a PNH(t-Bu) group.6 Tautomeric
forms of the expected zwitterions (e.g., 7) or novel heterocycles
(e.g., 8) were thus synthesized. The choice of this substrate was
dictated by our desire to obtain crystalline materials that will be
amenable for conclusive X-ray structural studies. As a continuation
of this study, we have explored the reaction of phosphorylated
allenes (allenylphosphonates) with 6 and extended it in one case
to Ph2P(NH-t-Bu) (9). A comparison is made to the reactions with
alkynes and acrylates. An interesting case of spontaneous resolu-
tion via crystallization is also presented.
Scheme 2 shows the results obtained from the reaction of differ-
ent allenes with the phosphazane 6.7,8 It is clear from these reac-
tions that the PIII end of the cyclophosphazane reacts with the b-
carbon of the allene. The difference between the proposed interme-
diate 3 (Scheme 1) and compounds 11a–c is that in the latter, the
O
O
CH2
O
O
O
P
t-Bu
H
H
6
P
O
N
.
.
C
C
β
C
P
P
α
γ
Toluene
1 d, rt
N
R
R
N
NH-t-Bu
t-Bu
t-Bu
R = H (10a)
R = Me (10b)
R = Ph (10c)
R = H [11a, δ(P): 71.7, 23.5, -16.5; 85%]
R = Me [11b, δ(P): 71.0, 27.6, -17.9; 90%]
R = Ph [11c, δ(P): 70.5, 20.4, -19.4; 90%, X-ray]
CH2
t-Bu
EtO2C
6
H
H
EtO2C
γ
C
α
C
β
N
.
.
C
C
P
P
H2
N
Toluene, 1 d, rt
H
N
NH-t-Bu
t-Bu
t-Bu
12
[13, δ(P): 71.3, -20.2, X-ray]
+
CH3
P
EtO2C
H
t-Bu
C
C
N
N
.
.
P
N
NH-t-Bu
t-Bu
t-Bu
[14, δ(P): 70.5, -20.7, X-ray]
Ph
O
CH2
O
O
O
-t-Bu
NH
(9)
P
O
O
H
P
Ph
P
Ph
Ph
C
C
C
P
Toluene, 1 d, rt
Ph
Ph
H
N
t-Bu
10c
[15, δ(P): 19.3, -7.7; 80%, X-ray]
Scheme 2.

N. N. Bhuvan Kumar, K. C. Kumara Swamy / Tetrahedron Letters 49 (2008) 7135–7138
7137
proton of the NH-t-Bu group from 6 has migrated to the
a
-carbon.
The 13C NMR spectra of compounds 11a–c show the
a-carbon in
the expected aliphatic region [d 24.9–41.8] with a 1J(P–C) value of
127 Hz. In compound 11c, the ipso-carbons of the four CMe3 groups
exhibit separate signals, which could indicate that resolution of the
enantiomers may be possible.
In contrast to the above, in the reaction using the ester allene
12, we obtained the rearranged product 14 by keeping the initially
formed normal product 13 in solution over a period of 2 d (for com-
plete conversion).9 Thus the allenylphosphonates and ester allene
differ marginally as regard the nature of the final product. We have
also shown in Scheme 2 that the aminophosphine Ph2P(NH-t-Bu) 9
leads to a product similar to that from 6 which suggests that the
reaction is more general for compounds with a P-NH-t-Bu group.
Confirmatory structural proof for 11c (R and S forms) and 13-14
is given by single crystal X-ray crystallography (Figs. 1–3).10,11
It should be noted that in compounds 11c and 15, a chiral center
is generated at C(22) in each case. Interestingly, in both of these
cases, the compounds crystallized in the chiral space groups, P-
na21 and P212121, respectively. The absolute configurations at the
chiral center as suggested by checkcif are S and R, respectively. In
the case of 11c, we checked the structures of several crystals and
were able to obtain the structures of both the S and the R enantio-
mers (Fig. 4). Although a priori identification of the crystals was not
feasible because of similar morphology of the crystals, this feature
suggests that there is spontaneous resolution upon crystalliza-
Figure 3. ORTEP diagram (probability ellipsoid 20%) of compound 14 [P(2)–C(17)
1.831(2), P(2)–N(1) 1.672(2), P(2)–N(2) 1.678(2), P(2)–N(3) 1.518(2), P(1)–N(1)
1.739(4), P(1)–N(2) 1.738(2), P(1)–N(4) 1.652(2) Å].
Figure 4. The R (left) and S (right) configurations at C(22) in the two different
crystals of 11c.
tion.12 The CD spectra (Fig. 5) of the crystals showed expected fea-
tures in the UV region, but in solution we were not able to obtain
significant optical rotation. The enantiomeric forms of crystals
were detected using the CD spectrum as an indicator. We are
exploring this aspect further.
In summary, we have reported structural proof for the attack of
a PIII center at the b-carbon of allenes. The NH proton of the P-NH-
t-Bu group migrates to the
a- or c-carbon of the allene resulting in
the formation of
a
phosphinimine.13 Spontaneous resolution
through crystallization has been observed in two cases (11c and
15), wherein a chiral center is generated during the reaction. In
the case of 11c, both the (R) and (S) enantiomers have been char-
acterized via crystallization in the absence of any chiral agent.
Figure 1. ORTEP diagram (probability ellipsoid 20%) of (S)-11c [P(1)–C(23)
1.826(3), P(1)–N(1) 1.677(3), P(1)–N(2) 1.688(3), P(1)–N(3) 1.543(3), P(2)–N(1)
1.751(3), P(2)–N(2) 1.742(3) P(2)–N(4) 1.661(3) Å].
1.5
S form
R form
1.0
0.5
0.0
-0.5
-1.0
300
325
350
375
400
Figure 2. ORTEP diagram (probability ellipsoid 20%) of compound 13 [P(1)–C(17)
1.806(6), P(1)–N(1) 1.683(4), P(1)–N(2) 1.686(4), P(1)–N(3) 1.525(4), P(2)–N(1)
1.735(4), P(2)–N(2) 1.724(4), P(2)–N(4) 1.649(5) Å].
Wavelength (nm)
Figure 5. CD spectra of the R and S forms of crystals of 11c.

7138
N. N. Bhuvan Kumar, K. C. Kumara Swamy / Tetrahedron Letters 49 (2008) 7135–7138
successful in isolating 13 in ca 95% purity and fortunately some crystals
suitable for X-ray structure analysis could be obtained. This solution, upon
keeping for 2 d, gave 14 which could be purified. Compound 13: Yield (see
above): 0.35 g (80%, purity ca. 95%, remainder was 14). 1H NMR (400 MHz,
CDCl3): d 1.27 (t, 2J(H-H) ꢀ 7.2 Hz, 3H, CH2CH3), 1.33 and 1.38 (2s, 36H, t-Bu-H),
2.75 (br, 1H, NH), 3.39 (br, 2H, PCCH2), 4.14 (q, 3J(H-H) = 7.1 Hz, 2H, CH2CH3),
5.55 (br, 1H, @CHAHB, cis to P) 5.70 (d, 1H, 3J(P-H) = 49.8 Hz, @CHAHB, trans to
P). 13C NMR (100 MHz, CDCl3): d 14.2 (s, CH2CH3), 31.4, 31.45, 31.47, 31.50, 31.54
and 31.6 (many lines, C(CH3)3), 32.9 (dd, 3J(P-C) = 3.6 Hz and 9.6 Hz, C(CH3)3),
34.3 (d, 3J(P-C) = 12.7 Hz, PCCH2), 34.5 (d, 3J(P-C) = 11.8 Hz, C(CH3)2), 51.6, 51.7,
52.3, and 52.4 (4s, C(CH3)3), 59.8 (s, OCH2CH3), 126.4 (s, PC@CH2), 139.6 (d,
1J(P-C) = 137.5 Hz, PC), 163.8 (s, COOEt). Detailed assignment was difficult
because, during the time of recording, signals due to 14 also appeared. 31P NMR
(160 MHz, CDCl3): d -20.2, 71.3. X-ray structure was determined for the crystals
thus obtained. Compound 14: Yield (see above); mp 100–103 °C. IR (KBr,
cmÀ1): 3329, 2965, 2874, 1703, 1615, 1460, 1372, 1316, 1190, 1047, 990. 1H
NMR (400 MHz, CDCl3): d 1.25 (t, 2J(H-H) ꢀ 7.2 Hz, 3H, CH2CH3), 1.32, 1.33, 1.36
and 1.38 (4s, 36H, t-Bu-H), 2.00 (br d, 3J(P-H) = 16.0 Hz, 3H, PCCH3), 2.87 (d,
2J(P-H) = 7.2 Hz, 1H, NH), 4.17 (q, 3J(H-H) = 7.2 Hz, 2H, CH2CH3), 7.16 (br, 1H,
PCCH). 13C NMR (100 MHz, CDCl3): d 14.3 (merged s, PCCH3 + CH2CH3), 31.6
(dd ? t, 3J(P-C) ꢀ 5.0 Hz, two of C(CH3)3), 33.0 (d, 3J(P-C) = 10.0 Hz, C(CH3)2),
34.5 (d, 3J(P-C) = 12.0 Hz, C(CH3)3), 51.3 (d, 2J(P-C) = 15.0 Hz, C(CH3)3), 51.7 (d,
2J(P-C) = 7.0 Hz, C(CH3)3), 52.3 (d, 2J(P-C) = 8.0 Hz, two of C(CH3)3), 59.9 (s,
OCH2CH3), 130.1 (s, PC@CH), 154.2 (d, 1J(P-C) = 190.0 Hz, PC), 166.7 (d, 3J(P-
C) = 26.0 Hz, CO2Et). 31P NMR (160 MHz, CDCl3): d À20.7, 70.5. LC–MS: m/z 461
[M+1]+. X-ray structure was determined for this compound. Anal. Calcd for
C22H46N4O2P2: C, 57.37; H, 10.07; N, 12.16. Found: C, 57.46; H, 10.05; N, 12.08.
The P@N center in these compounds is attacked readily by acids
and even by carbon dioxide/moisture leading to new ionic species.
Acknowledgments
We thank DST (New Delhi) for financial support and for the Sin-
gle Crystal X-ray diffractometer facility at the University of Hyder-
abad, and UGC (New Delhi) for equipment under UPE and CAS
programs. NNBK thanks CSIR for a fellowship.
Supplementary data
Characterization data for compounds 11a, 11b and 15, an OR-
TEP drawing of 15 and crystal data (CIF files). Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2008.09.153.
References and notes
1. (a) Lu, X.; Zhang, C.; Xu, Z. Acc. Chem. Res. 2001, 34, 535–544 (phosphine
catalysis); (b) Valentine, D. H., Jr.; Hillhouse, J. H. Synthesis 2003, 317–334; (c)
Nair, V.; Rajesh, C.; Vinod, A. U.; Bindu, S.; Sreekanth, A. R.; Mathen, J. S.;
Balagopal, L. Acc. Chem. Res. 2003, 36, 899–907 (multicomponent reactions
involving phosphines).
2. Selected references: (a) Luis, A. L.; Krische, M. J. Synthesis 2004, 2579–2585; (b)
Hanédanian, M.; Loreau, O.; Taran, F.; Mioskowski, C. Tetrahedron Lett. 2004, 45,
7035–7038; (c) Kuroda, H.; Hanaki, E.; Izawa, H.; Kano, M.; Itahashi, H.
Tetrahedron 2004, 60, 1913–1920; (d) Xu, Li-W.; Xia, Chun-C. Tetrahedron Lett.
2004, 45, 4507–4510; (e) Lu, C.; Lu, X. Tetrahedron 2004, 60, 6575–6579; (f)
Koech, P. K.; Krische, M. J. J. Am. Chem. Soc. 2004, 126, 5350–5351; (g) Gimbert,
C.; Lumbierres, M.; Marchi, C.; Moreno-Mañas, M.; Sebastián, R. M.; Vallribera,
A. Tetrahedron 2005, 61, 8598–8605.
10. X-ray data were collected on a Bruker AXS SMART diffractometer using Mo-K
a
(k = 0.71073 Å) radiation. The structures were solved and refined by standard
methods.11 In some cases, the terminal carbon atoms of the t-butyl groups
showed high thermals; this was particularly so in the case of (R)-11c and 13,
where the quality of data was not great.
Crystal data: (S)-11 c C30H55N4O3P3, M = 612.69, orthorhombic, space group
Pna2(1), a = 19.9847(19), b = 17.5360(16), c = 10.1393(10) Å, V = 3553.3(6) Å3,
Z = 4,
(I > 2
l , data/restraints/parameters: 5460/1/379, R indices
= 0.201 mmÀ1
(I)): R1 = 0.0491, wR2 (all data) = 0.1201. Flack parameter: À0.05(10).
r
CCDC 697359. (R)-11c C30H55N4O3P3, M = 612.69, orthorhombic, space group
Pna2(1), a = 19.966(4), b = 17.548(4), c = 10.131(2) Å, V = 3549.4(12) Å3, Z = 4,
= 0.201 mmÀ1, data/restraints/parameters: 6231/1/379, R indices (I > 2
r(I)):
3. (a) Tebby, J. C.; Wilson, I. F.; Griffiths, D. V. J. Chem. Soc., Perkin Trans. 1 1979,
2133–2135 and references cited therein; (b) Aitken, R. A.; Costello, S. J.; Slawin,
A. M. Z.; Wilson, N. J. Eur. J. Org. Chem. 2003, 623–625.
l
R1 = 0.0561, wR2 (all data) = 0.1332. Flack parameter: 0.02(11). CCDC 697360.
Compound 13: C22H46N4O2P2, M = 460.57, monoclinic, space group P2(1)/c,
4. Nuyken, O. Polymers of acrylic acid, methacrylic acid, maleic acid and their
derivatives. In Handbook of Polymer Synthesis, 2nd ed.; Kricheldorf, H. R.,
Nuyken, O., Swift, G., Eds.; Marcel Dekker: New York, 2005, pp 241–332.
5. (a) Nair, V.; Sreekanth, A. R.; Vinod, A. U. Org. Lett. 2001, 3, 3495–3497; (b) Li,
C.-Q.; Shi, M. Org. Lett. 2003, 5, 4273–4276; (c) Ding, H.; Ma, C.; Yang, Y.; Wang,
Y. Org. Lett. 2005, 7, 2125–2127; (d) Islami, M. R.; Mollazehi, F.; Badiei, A.;
Sheibani, H. ARKIVOC 2005, XV, 25–29.
6. Bhuvan Kumar, N.; Manab Chakravarty, N.; Kumara Swamy, K. C. New J. Chem.
2006, 30, 1614–1620.
7. Representative procedure for 11c: Allenylphosphonate 10c (0.20 g, 0.78 mmol)
a = 9.9107(12),
V = 2800.7(6) Å3, Z = 4,
287, R indices (I > 2 (I)): R1 = 0.0817, wR2 (all data) = 0.2355. CCDC 697361.
b = 36.162(4),
c = 8.5923(10) Å,
b = 114.562
(2)°,
l
= 0.178 mmÀ1, data/restraints/parameters: 4921/1/
r
Compound 14: C22H46N4O2P2, M = 460.57, monoclinic, space group P2(1)/n,
a = 9.5893(8), b = 18.8492(15), c = 15.7060(12) Å, b = 98.2660(10)°, V =
2809.4(4) Å3, Z = 4,
l
= 0.177 mmÀ1, data/restraints/parameters: 4959/0/289,
R
indices (I > 2r(I)): R1 = 0.0573, wR2 (all data) = 0.1681. CCDC 697362.
Compund 15: C30H37NO3P2, M = 521.55, orthorhombic, space group
P2(1)2(1)2(1), a = 9.068(2), b = 11.551(3), c = 27.114(6) Å, V = 2839.9(11) Å3,
Z = 4,
(I > 2
CCDC 697363.
l , data/restraints/parameters: 5598/0/330, R indices
= 0.184 mmÀ1
(I)): R1 = 0.0396, wR2 (all data) = 0.1015. Flack parameter: À0.03(8).
and cyclodiphosphazane
6 (0.27 g, 0.78 mmol) were dissolved in toluene
r
(4 mL), and the mixture was stirred for 1 d. The solution was concentrated in
vacuo (to ca. 1.5 cm3) and cooled for 1 d at À4 °C to obtain large crystals of 11c.
Yield: 0.42 g (90%); mp 152–154 °C. IR (KBr, cmÀ1): 3310, 2964, 2888, 1599,
1476, 1456, 1366, 1292, 1221, 1063, 1007. 1H NMR (400 MHz, CDCl3): d 0.96,
1.12, 1.25, and 1.37 (4s, 42H, C(CH3)2+t-Bu-H), 2.67 (d, 2J(P-H) = 7.6 Hz, 1H,
NH), 3.79–4.33 (m, 4H, OCH2), 5.78 (dd, 3J(P-H) = 22.8 Hz, 4J(P-H) = 4.0 Hz, 1H,
@CHAHB, cis to P), 6.04 (dd, 3J(P-H) = 14.4 Hz, 2J(P-H) = 18.0 Hz, 1H, PCH), 6.80
(d, 3J(P-H) = 50.4 Hz, 1H, @CHAHB, trans to P), 7.18–7.56 (m, 5H, Ar–H). 13C NMR
(100 MHz, CDCl3): d 21.0 and 22.0 (2s, C(CH3)2), 31.0 (dd ? t, 3J(P-C) ꢀ 4.9 Hz,
C(CH3)3), 31.3 (dd ? t, 3J(P-C) ꢀ 4.7 Hz, C(CH3)3), 32.4 (d, 3J(P-C) = 6.5 Hz,
C(CH3)2), 32.9 (d, 3J(P-C) = 10.0 Hz, C(CH3)3), 34.6 (d, 3J(P-C) = 12.0 Hz,
C(CH3)3), 41.8 (dd, 1J(P-C) = 125.5 Hz, 2J(P-C) = 4.8 Hz, P(O)C(Ph)), 51.4 (d,
2J(P-C) = 14.7 Hz, C(CH3)3), 52.2 (d, 2J(P-C) = 8.4 Hz, C(CH3)3), 52.3 (d, 2J(P-
C) = 10.0 Hz, C(CH3)3), 52.6 (d, 2J(P-C) = 8.2 Hz, C(CH3)3), 76.5 and 76.6 (2 s,
OCH2), 127.0, 127.1, 128.2, 128.3, 130.2,130.3, 136.2 (d, 2J(P-C) = 5.5 Hz,
PC@CH2), 143.4 (d, 1J(P-C) = 162.9 Hz, PC@CH2). 31P NMR (160 MHz, CDCl3): d
À19.4 (dd, 3J(P-P) = 34.8 Hz, 2J(P-P) ꢀ 6.6 Hz), 20.4 (d, 3J(P-P) = 34.8 Hz), 70.5
(d, 2J(P-P) ꢀ 6.6 Hz). LC–MS: m/z 613 [M+1]+. X-ray structure was determined
for this compound. The enantiomeric forms were detected using the CD spectra
as an indicator.
11. (a) Sheldrick, G. M.. SADABS Siemens Area Detector Absorption Correction;
University of Göttingen: Germany, 1996; (b) Sheldrick, G. M. SHELXTL NT Crystal
Structure Analysis Package, Bruker AXS, Analytical X-ray System, WI, USA, 1999,
version 5.10.
12. (a) Pérez-Garcíaa, L.; Amabilino, D. B. Chem. Soc. Rev. 2007, 36, 941–967; (b)
Jayanty, S.; Radhakrishnan, T. P. Chem. Eur. J. 2004, 10, 2661–2667. this paper
refers to conformationally chiral molecules) and references 7–9 cited therein;
(c) Raghavaiah, P.; Supriya, S.; Das, S. K. Chem. Commun. 2006, 2762–2764.
13. We also have corroborative evidence for the regiochemistry in the formation of
a species such as 2 by the high yield isolation of compounds of type I from the
reaction of unsymmetrical alkenes H2C@CHR [R = electron withdrawing group]
with 6. Full characterization data are available from us.
R
t-Bu
R
H
CH2 CH2
N
6
.
N
.
P
P
8. (a) Allene preparation: Satish Kumar, N. Studies on the Synthesis, Reactivity and
Utility of Cyclic Phosphorus(III) Compounds and Organophosphonates, Ph.D. thesis,
University of Hyderabad: India, 2004.; (b) Guillemin, J. C.; Savignac, P.; Denis, J.
M. Inorg. Chem. 1991, 30, 2170; (c) Lang, R. W.; Hansen, H.-J. Organic Syntheses,
Coll. Vol. 7, p. 232.
Toluene
rt
N
NH-t-Bu
t-Bu
t-Bu
(I)
9. In a procedure similar to that for 11c, compounds 6 (0.33 g, 0.95 mmol) and 12
(0.13 g, 1.14 mmol) were reacted. However, unlike 11c, isomeric products 13-
R = CO2Me [δ(P): 72.3, -6.4; 87%]
R = CO2Et [δ(P): 72.8, -6.2; 90%]
R = CO2-t-Bu [δ(P): 72.1, -3.4; 87%]
R = SO2Et [δ(P): 72.8, -10.4; 82%]
14 in which the PNH proton migrated to the
a or c carbon of the original allene
were obtained. Although further purification proved to be difficult, the
crystalline material obtained initially contained 13, predominantly. We were