Evidence for concerted processes in the course of the homoallenylic transposition

Article abstract of DOI:10.1016/j.tet.2013.02.048

The hydrolysis of β-allenic tosylates produces mainly 2-methylenecyclobutanols resulting from a homoallenylic participation along with isomeric 2-methylenecyclobutanol minor products coming from a 1234-1243 rearrangement. Structures of various cyclopropylvinyl carbocations involved in the course of the hydrolysis have been determined by computational studies. For acyclic tosylates, the hydrolysis of one β-deuterated allenic tosylate confirmed the nucleophilic attack on the corresponding nonclassical carbonium ion before its evolution to a more stable cyclopropylvinyl carbocation. In the case of one cyclic β-allenic tosylate, the structure of the products has been checked by the use of deuterated isotopomer.

Full text of DOI:10.1016/j.tet.2013.02.048

Tetrahedron 69 (2013) 3225e3233
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journal homepage: www.elsevier.com/locate/tet
Evidence for concerted processes in the course of the homoallenylic
transposition
*
Chahinez Aouf, Nicolas Galy, Maurice Santelli
ꢀ
Universite d’Aix-Marseille & CNRS, Institut de Chimie Radicalaire, UMR 7273, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 October 2012
Received in revised form 11 February 2013
Accepted 18 February 2013
Available online 26 February 2013
The hydrolysis of b-allenic tosylates produces mainly 2-methylenecyclobutanols resulting from a ho-
moallenylic participation along with isomeric 2-methylenecyclobutanol minor products coming from
a 1234e1243 rearrangement. Structures of various cyclopropylvinyl carbocations involved in the course
of the hydrolysis have been determined by computational studies.
For acyclic tosylates, the hydrolysis of one
b-deuterated allenic tosylate confirmed the nucleophilic attack
on the corresponding nonclassical carbonium ion before its evolution to a more stable cyclopropylvinyl
Dedicated to the memory of Professor A.
Guillemonat (1909e1999) who introduced
allene chemistry at the University of
Marseille
carbocation. In the case of one cyclic
the use of deuterated isotopomer.
b
-allenic tosylate, the structure of the products has been checked by
Ó 2013 Published by Elsevier Ltd.
Keywords:
Nonclassical ions
Homoallenyl participation
Cyclobutanes
Tosylate hydrolysis
van der Waals interactions
1. Introduction
1234 to 1243 rearrangement.12c In contrast, when R¼H, the
homoallenylic participation affords cyclopropylketones (Scheme 1).
The interconversion reactions of cyclobutyl, cyclopropylcarbinyl
and allylcarbinyl derivatives1 began to gain increasing importance,
especially after the precursor studies of P. Bruylants in 1928,2 which
dealt with the cyclisation of homoallylic chloride to give cyclo-
propylcarbinol and the conversion of cyclobutylamine into cyclo-
propylcarbinol as described by N. J. Demjanow in the early 19073
(Demjanow rearrangement).4 This ionic path constitutes a major
synthetic route to cyclopropane and sometimes cyclobutane de-
rivatives in organic chemistry5 and in biology.6 In fact, cyclopro-
pane derivatives are immensely favoured over cyclobutane
derivatives according to the theory of ring-closing kinetics.7
Since 1964,8,9 evidence from stereochemistry10 and reaction
kinetics11 suggested the existence of an important interaction be-
R
R
3 1
R2
R1
R
R1
H2O
3 2
1
4
HO
HO
2
+
3
OTs
1
R1
2
R2
4
4
R2
H
R = H
R1
O
R2
Scheme 1. Hydrolysis of b-allenic tosylates (co-solvent: acetone or dioxane).
This topic appears as a part of the problem of the formation of
nonclassical carbocations17,18 in the course of the polyolefin cycli-
sations,19 and particularly, the fascinating problem of the bio-
synthesis of terpenes20 and steroids.21
Our purpose is to report on the structure of carbocationic in-
termediates which are involved in the hydrolysis of some particular
tween a carbenium ion centre and a
the hydrolysis and acetolysis of
results have been observed during the solvolysis of
lates15 and from the nitrous deamination of -allenic amines.16
Interestingly, when the -allenyl linkage is substituted by an
b
-allenyl group in the course of
-allenic tosylates.12e14 Similar
-allenic tosy-
b
b
b
b
b
-allenic tosylates.
alkyl group (RsH), the solvolysis reactions produce mainly the
cyclobutyl derivatives along by minor products, resulting from
For example, the two tosylates 1b and 2b afforded the same
products, in different proportions, through hydrolysis or acetolysis
(Table 1).8,12c As previously demonstrated, the reaction occurred
with inversion of configuration at the functional carbon atom.10,12d
* Corresponding author. E-mail address: m.santelli@univ-amu.fr (M. Santelli).
0040-4020/$ e see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tet.2013.02.048

3226
C. Aouf et al. / Tetrahedron 69 (2013) 3225e3233
Table 1
Distribution products of the solvolysis of tosylates 1b and 2b
Me
Me
Me
O
Me Me
Me
OR
OR
O
Me
Me
1a : R = H
1b : R = Ts
2a : R = H
2b : R = Ts
Me
3E
Me
3Z
MeMe
Me
Me
Me
Me
Me
HO
HO
HO
HO
Me
Me
Me
OH
OH
4
E
4
Z
5E
5Z
6
7
Tosylate, reaction
1a
3Z
4E
4Z
5E
5Z
6
7
[Products]/[rearr-products]
Hydrolysis:a 1b
Acetolysis:b 1b
Hydrolysis:a 2b
Acetolysis:b 2b
8.2
7.5
2.6
2.4
4
0
9.6
0
13.5
12.4
25.5
22.2
16
8.1
33.7
14.5
33
15
4
6.3
19
1.5
9.6
S5þ7/S4þ6¼1.62
S5þ7/S4þ6¼1.68
S4þ6/S5þ7¼2.93
S4þ6/S5þ7¼2.10
24.5
13.4
14.5
14.4
7.4
7.7
14
6.2
29
Watereacetone, v/v¼90:10, CaCO3c¼5.10ꢀ3t¼55 ꢁC.
a
b
Acetic acid and sodium acetate, (t¼80 ꢁC) followed by treatment with LiAlH4.
Up to one third of the products possess a rearranged skeleton (4
and 6 from 1b5 and 7 from 2b).
methylenecyclobutanols 5Z and 4Z (rearranged) from 1-
cyclopropylvinyl cation 8Z or corresponding nonclassical ion
(Scheme 2b). These cycle enlargements are induced by the neo-
pentylic structure of carbocations 8E and 8Z.
Hydrolysis of primary tosylate 10b mainly afforded methyl-
enecyclobutanol 11 and, as a result, the reaction has preparative
importance (Scheme 3).
Two possible mechanisms have been postulated to rationalise
the inversion of configuration at the functional carbon atom10,12d
and the apparent exchange of the positions of the substituents:
the formation of 1-cyclopropylvinyl cations,22 or/and the formation
of nonclassical carbocations with a bridged structure. These hy-
potheses are illustrated in Scheme 2a. Indeed, one conformation of
tosylate 1b led to methylenecyclobutanols 5E and 4E (rearranged)
through 1-cyclopropylvinyl cation 8E or/and nonclassical carboca-
tion 9E. A similar route from another conformation of 1b gave
Me
Me
H2O
HO
OR
+
10b
10a
CaCO3
10a : R = H
10b : R = Ts
a)
25%
11
, 75%
OTs
Me
H
Me
H
1
1
H
H
H
Me
H
H
1
Scheme 3. Hydrolysis of the tosylate 10b (wateredioxane, 75:25).
2
2
2
H
H
Me
Me
Me
3
3
4
3
(+)
(+)
4
The ring-opening of 1-cyclopropylvinyl cations to the corre-
sponding homoallenyl derivatives and the reverse reaction are
well-known. Recently, Baines and co-workers have demonstrated
4
H2O
5
5
5
H
H
H
H
H
H
that an
a-cyclopropylvinyl cation 12 generated by protonation of
1b
8E
trans-1-ethynyl-2-phenylcyclopropane underwent a selective ring-
Me
H
Me
3
opening towards the phenyl substituent leading to the b-allenic
1
Me
H
2
1
H
H
alcohol (Scheme 4a).23,24 Clearly, the benzyl cation is more stable
HO
Me
H
1
4
H
H
4E
H
than the cyclopropylvinyl cation 12.
5
2
3
Me
H
2
(+)
Me
(+)
3
4
Me
Me
H2O
H
4
3
1
5
2
a)
H
H2O
9E
5
HO
H
4
(+)
H
H
(+)
H
(+)
H
H
5
Ph
Ph
Ph
5
H2O
E
12
OH
b)
H
Me
H
1
2
Ph
3
TsO
1
H
Me
H
Me
H
Me
H
H
4
OH
5Z
1
2
b)
H
+
5
H
3
4
2
AcONa
AcOH
H
3
4
Me
(+)
(+)
Me
OBs
(+)
H
Me
3
OH
H
2
1
5
13
5
H
OAc
H
H
4
H
1b
8Z
Me
5
77%
4Z
Scheme 4. Ring-opening of 1-cyclopropylvinyl cations 12 and 13.
Scheme 2. Solvolysis pathways of the tosylate 1b.

C. Aouf et al. / Tetrahedron 69 (2013) 3225e3233
3227
In 1967, Bly and Koock observed that the acetolysis of neo-
pentylic -allenic brosylates occurred without formation of cyclised
products but with enhanced rates compared to the corresponding
saturated brosylates (Scheme 4b).25 Their hypothesis that the 1-
(2,2-dimethylcyclopropyl)vinyl cation 13 was unstable with respect
nonclassical ion, one would expect that the C3eC4eC5 angles to
deviate significantly from 180ꢁ. In fact, in the various structures,
values are higher than 178ꢁ.
b
Therefore, the calculated cyclopropylvinyl cation 15 has
a bisected form and consequently, the WagnereMeerwein rear-
rangement leading to cyclobutanol 11 will involve the two bonds
C1eC3 and C2eC3 with equal facility. To check this statement, the
labelled tosylate 10b(2d) has been hydrolysed (80 ꢁC, water-
eacetonitrile, 90e10% v/v) and the deuterium distribution in the
cyclobutanol 11(d) (75% yield) determined by NMR spectrum
analysis of the (Z)-methylenic proton coupling constant12e and the
13C NMR of the C(2) carbon atom (Scheme 5b, and in
Supplementary data, Figs. 1e4). From cation 15(d) and because of
symmetry (neglecting a possible very minor isotope effect), there is
an equal probability of forming the two rearranged products 11(4d)
and 11(3d) (Scheme 5a). The 33% ‘excess’ of unrearranged meth-
ylenecyclobutanol 11(4d) suggests that the cyclopropylvinyl cation
15(d) is not the sole product-forming ion and that an alternate
pathway exists.
to the acyclic tertiary b-allenyl cation is in accordance with the
Baines’ experiments.
2. Results and discussion
All calculations were performed with the Gaussian 09, revision
A02, suites of program.26 Structures of the carbocationic in-
termediates were calculated at the B3LYP,27 B3PW9128 and MP229
levels using the 6-311Gþþ(d,p), or 6-311Gþþ(3df,3pd) basis sets.
A vibrational analysis was performed at the same level of theory in
order to determine the zero-point vibrational energy (ZPE) and to
characterise each stationary point as a minimum (Nimag¼0). The
total energies,
DE were evaluated at 0 K and the Gibbs free energy
DG, at 298.15 K. All calculations were performed using water as
solvent (Polarisable Continuum Model) (calculated geometries by
using ideal gas are given in the Supplementary data, Tables 1e4).
The stabilisation of carbocations induced by the presence of water
is from 47 to 52 kcal/mol. Structures calculated in water are more
compact than those calculated in ideal gas (see, Supplementary
data, pages 7e9).
a)
Me
Me
Me
D
HO
HO
+
+
10a(2d)
OTs
D
D
10b(2d)
11(4d)
11(3d)
67:33
2.1. Solvolysis of acyclic b-allenic tosylates
OTs
H
1
The calculated geometries, employing various levels of theory,
have shown that the parent 1-cyclopropylvinyl cation 14 resembled
to the bisected cyclopropylcarbinyl cation with a large C1eC3 (or
C2eC3) bond length and a short C1eC2 one (Fig. 1a and in
Supplementary data, Table 1).30
H
H
1 H
11(4d)
11(3d)
33%
33%
H
2
2
H
3
4
D
67%
Me
Me
D
3
4
(+)
5
5
H
(a)
H
H
H
H
−
−
1
−
C1 C3 = C2 C3:
C1 C2:
H
10b(2d)
(+)
15(d)
a, 1.623; b, 1.607; c, 1.618;
d, 1.603; e, 1.622
a, 1.432; b, 1.432 ; c, 1.428
d, 1.429; e, 1.435
H
3
H
2
H
−
−
C3 C4:
C4 C5:
H
H
3
1
H
2
(+)
4
a, 1.342; b, 1.344; c, 1.339;
d, 1.340; e, 1.343
a, 1.282; b, 1.280; c, 1.278
d, 1.277; e, 1.294
5
D
Me
H
11(4d)
33%
14
H
4
−
−
C1 C4 = C2 C4: a, 2.529; b, 2.512; c, 2.522; d, 2.504; e, 2.506
H2O
5
H
(b)
16
H
H
−
−
C1 C3 = C2 C3:
1
−
H
C1 C2:
a, 1.632; b, 1.617; c, 1.628;
d, 1.612; e, 1.630; f, 1.622
a, 1.433; b, 1.433; c, 1.429;
d, 1.429; e, 1.436; f, 1.430
b)
11(3d) 11(4d)
H
3
H
2
Me
+
;
13C NMR, 100 MHz, ppm, (CDCl3: 77.16 ppm):
35.017
C3−C4:
a, 1.345; b, 1.347; c, 1.342;
d, 1.344; e, 1.345; f, 1.339
C4−C5:
a, 1.283; b, 1.281; c, 1.279;
d, 1.278; e, 1.296; f, 1.289
(+)
4
25.695
77.097
HO
25.724
Me
35.256
t, J = 21.2 Hz
D
Me
5
77.02, 77.0
HO
H
15
H
157.765
157.844
−
−
C1 C4 = C2 C4: a, 2.483; b, 2.466; c, 2.477; d, 2.460; e, 2.468; f, 2.453
H
D
H
23.542
11(4d)
Methods: a, B3LYP/6-311G++(d,p); b, B3PW91/6-311G++(d,p);
c, B3LYP/6-311G++(3df,3pd); ; d, B3PW91/6-311G++(3df,3pd);
e, MP2/6-311G++(d,p); f, MP2/6-311G++(3df,3pd)
23.354
H
H
t, J = 21.1 Hz
11(3d)
Scheme 5. Hydrolysis of the labelled tosylate 10b(2d).
Fig. 1. Bond lengths and angles of the optimised structures of 1-cyclopropylvinyl
cations 14, and 15 calculated according to the levels of theory aef (bond lengths are
in angstroms) (longest bonds are in red, and shortest bonds in blue) (solvent: water).
An explanation is the rapid solvent capture of the carbonium ion
16 before its complete isomerisation into more stable cyclo-
propylvinyl cation 15(d). Thus, the transition state under solvolytic
conditions can involve an unstable carbocation because, according
to Winstein, ‘there are good reasons to expect carbon bridging lags
behind CeX elongation at the transition state’.31
Concerning the solvolysis of 10b, the structure of the corre-
sponding carbocationic intermediate 15 is very similar to that of 14,
the major difference is the shorter C1eC4 bond distance (Fig. 1b
and in Supplementary data, Table 1). Interestingly, for a bridged

3228
C. Aouf et al. / Tetrahedron 69 (2013) 3225e3233
–
–
–
–
Previous results have confirmed this statement. Hydrolysis of
C1 C3:
C1 C2:
C1 C2:
C1 C3:
the silylated tosylate 17b led mainly to the trimethylsilylethy-
a, 1.808
b, 1.756
c, 1 .739
d, 1.736
a, 1.440
b, 1.444
c, 1.452
d, 1.456
a, 1.433
b, 1.432
c, 1.440
d, 1.438
a, 1.806
b, 1.742
c, 1.732
d, 1.736
Me
H
1
H
1
H
Me
H
nylcyclopropane 18.32 In this case, the vinyl cation 20, stabilised by
the b
-trimethylsilyl substituent,33 (Fig. 2), release a proton before
its capture by the solvent. The cyclobutanol 11 could result from the
nucleophilic attack to the bridge cation 19 (Scheme 6).
H
H
3
2
2
Me
Me
3
–
–
–
C2 C3:
C2 C3:
–
C3 C4:
C3 C4:
(+)
(+)
4
4
a, 1.334
b, 1.338
c, 1 .345
d, 1.342
a, 1.560
b, 1.547
c, 1.559
d, 1.535
a, 1.572
b, 1.567
c, 1.582
d, 1.561
a, 1.330
b, 1.334
c, 1.340
d, 1.335
–
C1–C3:
C1 C2:
5
5
H
H
a, 1.602; b, 1.592; c, 1.596;
d, 1.586; e, 1.590
a, 1.450
b, 1.449
c, 1.449
d, 1.448
e, 1.464
H
H
H
1
H
Angles,
8
E
8
Z
C2–C3:
a, 1.586; b, 1.573; c, 1.579;
d, 1.566; e, 1.566
c,
–
–
C4 C5 H : 123.8°
–
–
C4 C5: a, 1.285; b, 1.283;
c, 1.300; d, 1.294
C4 C5: a, 1.284; b, 1.282;
c, 1.299; d, 1.293
–
–
C4 C5 Si : 122.2°
H
3
H
2
Me
–
–
Si C5 H : 113.9°
(+)
–
–
–
C4 C5:
C1 C4: a, 2.453; b, 2.389;
C1 C4: a, 2.525; b, 2.483;
–
C3 C4:
4
e,
C4–C5–H : 133.3°
C4–C5–Si : 111.2°
Si–C5–H : 115.5°
a, 1.266
b, 1.265
c, 1.260
d, 1.259
e, 1.264
a, 1.369; b, 1.370; c, 1.371;
d, 1.372; e, 1.381
c, 2.380; d, 2.289
c, 2.471; d, 2.420
5
C2–C4: a, 2.484; b, 2.473;
c, 2.479; d, 2.457
C2–C4: a, 2.497; b, 2.489;
c, 2.499; d, 2.488
H
Me3Si
–
C5 Si:
a, 1.963; b, 1.958; c, 1.982;
20
Methods: a, B3LYP/6-311G++(3df,3pd); b, B3PW91/6-311++(3df,3pd);
c, MP2/6-311G++(d,p); d, MP2/6-311G++(3df,3pd)
d, 1.975; e, 2.002
C1–C4: a, 2.474; b, 2.455; c, 2.472; d, 2.453; e, 2.455
C2–C4: a, 2.497; b, 2.486; c, 2.497; d, 2.487; e, 2.491
Fig. 3. Bond lengths of the optimised structures of 1-cyclopropylvinyl cations 8E, and
8Z calculated according to the levels of theory aed (bond lengths are in angstroms)
(longest bonds are in red, and shortest bonds in blue) (solvent: water).
Methods: a, B3LYP/6-311G++(d,p); b, B3PW91/6-311G++(d,p);
c, B3LYP/6-311G++(3df,3pd); ; d, B3PW91/6-311G++(3df,3pd);
e, MP2/6-311G++(d,p).
increasing of the C1eC4 distance (and also the C1eC3eC4 angle,
B3LYP/6-311Gþþ(3df,3pd): 8E, 101.6; 8Z, 106.2; MP2/6-
311Gþþ(3df,3pd): 8E, 100.2; 8Z, 106.4ꢁ). These differences of
structure can explain the greater formation of methyl-
enecyclobutanols E (4E5E) with respect to Z ones (4Z5Z).
Fig. 2. Bond lengths and angles of the optimised structures of 1-cyclopropylvinyl
cation 20 calculated according to the levels of theory aee (bond lengths are in ang-
stroms) (longest bonds are in red, and shortest bonds in blue) (solvent: water).
Concerning the optimised structures 8, the double bond char-
acter of the C1eC2 bond can be represented by considering this 1-
cyclopropylvinyl cation as a p-complex of an allenyl cation and an
alkene (for similar considerations in cyclopropylcarbinyl cations,
see Ref. 34)(Fig. 4a). The relative sizes of component atomic orbitals
of the LUMO are considered in Fig 4c. Clearly, the most electrophilic
carbon atoms are C1 and C4 (Fig. 4b).
Me
Me
Me3Si
H2O
18
, 62%
OR
CaCO3
Me3Si
11
+
, 34% +
17a, 4%
17a : R = H
17b : R = Ts
(+)
H
1
H
(a)
(b)
(c)
(+)
H
H
2
Me
11
0.697
1
Me
H
H
Me
1
.
.
H
3
.
.
.
4
.
.
0.088
.
.
.
.
H
3
5
H
H2O
.
2
.
2
H
.
.
.
.
Me
H
–
Me
0.495
(+)
.
Me3Si
.
19
3
.
0.584
.
17b
4
4
–
0.390
H
H
H
1
1
H
5
H
5
H
H
2
2
H
H
H
H
H
Me
Me
(+)
3
3
+
H
(+)
Fig. 4. (a), Structure, (b), atom polarisability tensor-based charges and (c), LUMO (27th
MO) of the cyclopropylvinyl cation 8E (B3LYP/6-311Gþþ(3df,3pd) level of theory,
solvent: water).
4
4
5
5
H
Me3Si
SiMe3
20
18
Scheme 6. Hydrolysis of the trimethylsilyl substituted tosylate 17b.
The trend of the calculated structures using the B3LYP/6-
311Gþþ(3df,3pd) is similar to MP2/6-311Gþþ(3df,3pd) level of
theory. According to previous works, it appears that MP2 often
tends to overestimate the stability of ‘nonclassical’ structures (MP2
is generally more appropriate than DFT for describing noncovalent
interactions that are more dependent on dispersion interactions).35
In fact, MP2 calculations predicted a stronger bridging in 8 than
B3LYP calculations, as evidenced by the smaller crucial bond
lengths C1eC3, C1eC4 and C2eC4 (Fig. 3).
Fig. 3 shows the optimised geometry of the dimethylcyclopro-
pylvinyl cations 8E and 8Z. The added methyl group induced a dis-
symmetrisation of the cyclopropyl subunit with an elongation of
the C1eC3 bond in comparison to the cyclopropylvinyl cation 15.
The calculated structure 8E, corresponds to a compromise between
the classical 1-cyclopropylvinyl cation 8E, and the bridged structure
9E.
Concerning the nitrous deamination36 of
b-allenic amines per-
The less stable Z-isomer 8Z (8E/8Z, dDGw0.32 kcal/mol, see in
Supplementary data, Table 2) is perturbed by nonbonded in-
teractions between the methyl and vinyl groups which induced an
formed in protic and aprotic media, on seven cases studies, only
one, the threo amine 26, gave rise to a homoallenyl participation (in
aprotic media) and even with a better yield of 81% than the

C. Aouf et al. / Tetrahedron 69 (2013) 3225e3233
3229
hydrolysis of the corresponding tosylate 22 (69%) (Scheme 7).16
Interestingly, in both cases, the participation occurred with in-
version of configuration at the functional carbon atom.
The structure of the unexpected cyclobutanol 30 was established
on the basis of two dimensional NMR spectra (HMQC and NOESY).
Instead of solvolysis in 25% dioxaneewater mixture used pre-
viously, we chose acetonitrileewater mixtures which appeared to
be ones of the less complicated aqueous binary mixtures. Indeed,
acetonitrile competes successfully with waterewater interactions
and destroys the lattice structure.38 Remarkably, hydrolysis of 27b
in watereacetonitrile mixture led quantitatively to a mixture of
only four alcohols, 29, 30, 31 and 33 (Table 2). We note the absence
of 27a, 28a and 32. Therefore, the participation reaction is the sole
possible process. The high ionising power Y of the mixture water-
eacetonitrile 90e10 (as given by the GrunwaldeWinstein equa-
tion)39 of this mixture (Y¼3.03, extrapolated from data for
watereacetonitrile 20e80 to 80e2038b), also shows that the par-
ticipation reactions were favoured by increasing solvent polar-
ity.12e,40 From 28b, hydrolysis in watereacetonitrile mixture
(90e10%, v/v) led to 28a, 19%; 32, 15%; 34, 12%; 35, 42%; 36, 8% and
37, 4%.
Me
Me
Me
Me Me
Me
Me
Me
Me
Me
Me
HO
HO
+
+
Me
X
O
23
24
25
21, X = OH
22, X = OTs; CaCO3; H2O-Dioxane; 80 °C: 21, 31%; 23, 34%;
24, 25%; 25, 10%
26, X = NH2; n-BuONO; AcOH-CHCl3; 50 °C: 21, 19%; 23, 32%;
24, 22%; 25, 27%
Me
Me
(+)
H2N
H
Me
Me
n-BuONO
AcOH
N2
Me
Me
23–25
H
CHCl3
H
H
Table 2
Hydrolysis products of 27b and 27b(d) (c¼0.03 mol/L, 80 ꢁC)
26
H
H
H
H
Solvent
Y
29 (%)
30 (%)
31 (%)
33 (%)
Scheme 7. Nitrous deamination of amine threo 26 and hydrolysis of the corresponding
tosylate 22.
H2OeAcetonitrile
90:10% v/v
3.03
30
28
18
24
H2OeAcetonitrile
50:50% v/v
1.50
25
26
21
28
We propose that the different behaviour of b-allenic amines can
be explained by the tendency of 26 to adopt a conformation in
which the reactive sites are close.
29(d)
30(d)
31(d)
33(d)
H2OeAcetonitrile
90:10% v/v
3.03
28
34
14.5
23.5
Optimisation by molecular mechanics followed by calculation at
the B3LYP/6-311Gþþ(d,p) level of theory, shown that for the amine
26, in the more stable conformation
(
D
E¼ꢀ368.684416 au)
The corresponding diastereoisomeric 1-cyclopropylvinyl cations
38 and 39 show different optimised structures (Fig. 6) (38/39,
dDG¼3.08e3.15 kcal/mol, see in Supplementary data, Table 3). If
C1eC2 bonds have similar bond lengths, nonbonded interactions
induced by the presence of the methyl group strongly modified the
C1eC3 and C2eC3 bond lengths. For the less stable cation 38,
(Scheme 7), the allenyl moiety was in the rear part of the functional
carbon atom allowing a rearward attack on diazonium ion with
minor conformational changes. The participation should be made
easier according to the principle of least motion,37 which states that
those elementary reactions will be favoured, involving the least
change in atomic position and electronic configuration.
2.2. Solvolysis of cyclic b-allenic tosylates
–
C1–C3: a, 1.754
b, 1.709
Me
C1 C2: a, 1.433
1
H
b, 1.433
c, 1 .438
Then, we investigated the case of tosylates 27b and 28b the
solvolysis of which gave complicated product mixtures (Fig. 5).12g
c, 1.704
H
2
–
–
C3 C4: a, 1.330
b, 1.333
C2 C3: a, 1.622
3
(+)
b, 1.615
c, 1 .637
4
–
C4 C5: a, 1.285
c, 1 .339
Me
Me
8
b, 1.283
c, 1.301
5
H
Me
Me
OH
HO
H
H
H
H
38
6
OR
OR
H
–
–
C1 C4: a, 2.503; b, 2.473; c, 2.482
C2 C4: a, 2.513; b, 2.500; c, 2.503
27a : R = H
27b : R = Ts
Me
28a : R = H
28b : R = Ts
29
30
–
–
C1 C3: a, 1.597
H
3
C1 C2: a, 1.433
1
Me
H
b, 1.584
c, 1.584
b, 1.435
c, 1 .443
Me
Me
2
OH
–
–
OH
C3 C4: a, 1.331
b, 1.334
C2 C3: a, 1.773
(+)
b, 1.734
c, 1.746
Me
OH
4
OH
31
–
a, 1.285
b, 1.284
c, 1.302
C4 C5:
c, 1 .342
5
H
H
32
33
34
39
–
–
C1 C4: a, 2.520; b, 2.508; c, 2.509
C2 C4: a, 2.483; b, 2.437; c, 2.423
O
O
Me
HO
Me
Methods: a, B3LYP/6-311G++(3df,3pd); b, B3PW91/6-311G++(3df,3pd);
c, MP2/6-311G++(d,p)
Me
37
Fig. 6. Bond lengths of the optimised structures of 1-cyclopropylvinyl cations 38, and
39 calculated according to the level of theory of calculation aec (bond lengths are in
angstroms) (longest bonds are in red, and shortest bonds in blue) (solvent: water).
35
36
Fig. 5. Hydrolysis products from tosylates 27b and 28b.

3230
C. Aouf et al. / Tetrahedron 69 (2013) 3225e3233
interactions between the methyl and cyclohexane groups elon-
gated the C1eC3 bond. However, the significant C2eC3 elongation
in 39 is due to the interactions between the methyl and the vinylic
cation. As for the precedent cases, MP2 calculations led to stronger
bridged structures. Values for atom polarisability tensor-based
charges (APT charges)41 are also different particularly for C1
(highest value for 38) and C2 (highest value for 39) (Supplementary
data, Fig. 9).
Hydrolysis products resulting from 27b and 28b are different.
This result is the consequence of the inversion of configuration at
the functional carbon atom of these tosylates, which induces the
difference of structure of the corresponding vinyl cations 38 and 39,
groups. Then, the resulting planar cyclopropyl cation intermediate
43 opens according to a disrotatory process in a direction dictated
by steric preferences to give the Z-methylallyl cation 44 precursor
of alcohols 31 and 33.45,46 During the optimisation at the B3LYP/6-
311Gþþ(d,p) level, the geometry of cyclopropyl cation 43 con-
verged to the more stable isomer 44.
Hydrolysis of deuterated tosylate 27b(d) confirmed the hydride
migration in the course of the formation of 30 from 38 (Scheme 9).
In the proton NMR spectra of unlabelled 30, signals of the meth-
ylenic group at 4.76 and 4.72 ppm were triplet (4J¼2.72 Hz, cou-
pling with H6 and H8, Fig. 5). For deuterated 30(d), similar signals
at 4.74 and 4.70 ppm were doublet (coupling with H8, 4J¼2.51 and
2.84 Hz, respectively). Moreover, the multiplet at 2.72 ppm (H6 and
H8) for 30 became a qdd corresponding to only H8 in 30(d). For the
labelled alcohol 29(d), the signal of the proton H7 was a quartet
instead of a doubled quartet for unlabelled alcohol 29. The signal at
4.32 ppm had disappeared for 31(d) (as the signal at 74.4 ppm in
the 13C NMR spectra) and for 33(d) the signal at 5.84 ppm was
missing (as the signal at 124.6 ppm in the 13C NMR spectra). The
proportion of 30 was more important in the labelled products
(Table 2). However, in the course of a transannular 1,5-hydride shift,
no primary isotope effect has been observed when hydrogen atom
known to migrate was replaced by deuterium atom.47
respectively (the free b-allenyl cation 40 is not formed). The fact
that different compounds are produced strongly suggests a series of
stepwise rearrangements, where each step is probably concerted.
From 38, the major product is the methylenecyclobutanol 29 aris-
ing from a WagnereMeerwein rearrangement involving the C1eC3
bond. A concerted route joining a nucleophilic attack and the 1,2-
shift can explain the diastereoselectivity and rule out the in-
tervention of the free cyclobutyl cation 41 (Scheme 8a).42 The for-
mation of the second methylenecyclobutanol 30 implied the same
ring enlargement followed by a 1,2-hydride ion migration (‘b-
hop’)43 to give the convex tertiary cation 42. The nucleophilic attack
occurred on the most accessible face (Scheme 8b).44 We note that
the formation of major products 29 and 30 resulted from the mi-
gration of the longest C1eC3 bond of the cyclopropane moiety of
38. The most striking products 31 and 33 resulted from a Wag-
nereMeerwein rearrangement with migration of the C3eC6 bond
(Scheme 8c). This exceptional migration seems due to relief of
nonbonded interactions between the methyl and cyclohexyl
7
8
Me
Me
HO
D
+
+
D
OH
Me
D
29(d)
30(d)
OTs
27b(d)
Me
a)
Me
1
H
H
Me
OTs
H
Me
1
H
OH
Me
2
1
2
OH
D
2 H
3
HO
H
D
4
3
5
5
3
4
H
(+)
4
31(d)
33(d)
O
H
29
?
Scheme 9. Hydrolysis of labelled tosylate 27b(d).
5
H
38
Me
1
4
H
H
H
H
5
The relative stability of the vinyl cation 39, arising from the
ionisation of tosylate 28b, with respect to 38 (Table 3) can explain
that it is the sole cyclopropylvinyl cation trapped by water to give
corresponding cyclopropyl ketone 36. Calculations have shown that
for 39, the longest bond of the cyclopropane was C2eC3 (Fig. 6)
and, pleasantly, the major product, 35, arose from its migration
(Scheme 10). The concerted process is evidenced by the instability
of the nonplanar twisted methylenecyclobutyl cation 46. It is likely
that the discrete allylic cation 46 does not actually exist.
27b
2
4
3
(+)
H
5
3
1
(+)
Me
40
2
41
b)
Me
1
H
Me
1
Me
H
2 H
(+)
1
(+)
2
2
H
3
OH
5
4
3H
4
Table 3
4
3H
5
Relative Gibbs free energies of various isomeric carbocations in comparison with
5
H
H
b-allenyl cation 40
38
42
30
c)
Cation Level of theory dDG (kcal/mol) Level of theory dDG (kcal/mol)
Me
1
40
38
39
41
42
44
45
46
a
a
a
a
a
a
a
a
0
b
b
b
b
b
b
b
b
0
H
2 H
H
2
2
H
(+)
ꢀ7.1
ꢀ12.4
ꢀ15.3
ꢀ15.3
ꢀ19.4
ꢀ26.2
ꢀ22.9
ꢀ13.3
H
ꢀ10.2
ꢀ12.8
ꢀ8.8
H
3
3
3
1
6
Me
1
(+)
(+)
6
6
4
5
4
5
4
Me
ꢀ26.5
ꢀ24.5
ꢀ3.5
5
H
H
X
38
43
44
31
Level of theory, a: B3LYP/6-311Gþþ(3df,3pd); b: MP2/6-311Gþþ(d,p) (solvent:
2
H
(+)
water).
33
3
Me
1
The comparative energies relative to the b-allenyl carbocation
6
4
5
H
40 and of the various cationic intermediates probably involved in
the solvolysis of 27b and 28b are given in Table 3. We note that 1-
cyclopropylvinyl cations 38 and particularly 39 are more stable
45
Scheme 8. Hydrolysis pathways for the tosylate 27b (in red longest bond).

C. Aouf et al. / Tetrahedron 69 (2013) 3225e3233
3231
showed that it is not a minimum on the potential energy surface
and spontaneously it rearranged into the vinyl cation 52.
H
1
Me
2 H
OTs
Me
H
1
3
In the optimised geometry of vinyl cation 52, the longest bond of
the cyclopropane is C1eC3 (Fig. 7), and the important proportion of
allenic alcohol 47a after hydrolysis can be explained by opening of
the cyclopropane to give 55 and then 47a, or a concerted opening
with the nucleophilic attack to C1 (Scheme 11). Contrary to the
precedent cases, the 2-methylenecyclobutyl cation 54 is less stable
than its precursor, the corresponding 1-cyclopropylvinyl cation 52,
by 1.85 kcal/mol (B3LYP/6-311þþG(3df,3pd)) and 5.09 kcal/mol
(MP2/6-311þþG(d,p)) (Supplementary data, Table 4).
Me
H
3
1
5
2 H
2 H
H2O
35
4
5
3
4
H
4
(+)
O
H
?
5
H
H
H
H
H
1
39
Me
28b
2 H
3
H
1
Me
(+)
2 H
4
C2–C3:
–
–
C1 C2:
5
–
C1 C3:
C1 C2:
3
46
a, 1.577
b, 1.564
c, 1.567
H
a, 1.465
b, 1.460
c, 1.473
2
a, 1.434
b, 1.436
c, 1.449
a, 1.585
b, 1.578
c, 1 .582
H
4
O
5
36
2
H
1
1
Scheme 10. Hydrolysis pathways for the tosylate 28b (longest bond is in red).
C1–C3:
4
–
C3–C4:
C2 C4:
3
5
(+)
H
(+)
a, 1.532
b, 1.527
c, 1 .533
a, 1.786
b, 1.740
c, 1.746
a, 1.329
b, 1.333
c, 1.342
3
4
than their corresponding opened isomer 40. In fact, the allenyl
54
5
–
C3 C4 :
a, 1.426
52
linkage shows a high experimental standard enthalpy of formation
H
H
(3-methyl-1,2-butadiene,
methyl-1,3-butadiene,
high chemical reactivity.
D
Hfꢁ¼30.88 kcal/mol; isomeric 2-
b, 1.425
c, 1.424
–
C4 C5: a, 1.285; b, 1.283; c, 1.301
D
Hfꢁ¼18.11 kcal/mol)48 which explains its
–
C1 C4: a, 2.571; b, 2.512; c,2.477
–
C2 C4: a, 2.502; b, 2.493; c, 2.498
The case of tosylates of 3-vinylidenecycloalkanols and particu-
larly 47b is interesting because strained bicyclic cyclobutanols 50
and 51 are obtained in moderate yields (Scheme 11)(H2OeTHF
70:30, 60 ꢁC, 47a, 20%; 48, 8.6%; 49, 15%; 50, 12.6%; 51, 32.4%).49
Methods: a, B3LYP/6-311G++(3df,3pd); b, B3PW91/6-311G++(3df,3pd);
c, MP2/6-311G++(d,p)
Fig. 7. Bond lengths of the optimised structures of 1-cyclopropylvinyl cations 52, and
54. Bond lengths are in angstroms (longest bonds are in red, and shortest bonds in
blue).
Me
47a +
Me
+
O
O
OH
OH
+
+
3. Crystallographic study of tosylate 27b
OR
The structure of tosylate 27b was unambiguously confirmed by
single crystal X-ray analysis (Figs. 8 and 9). A careful examination of
its geometry showed that the CeO bond was almost antiperiplanar
to the C3eC4 bond and that an attractive force brought closer the
functional carbon atom and the allene linkage. The internuclear
distance between the functional carbon atom and the central car-
47a R
H
:
:
=
=
48
49
50
51
47b
R
Ts
2
H
H
?
1
3
O
H
2
(+)
1
(+)
2
4
1
3
3
OH
ꢁ
4
5
4
4
5
bon atom of the allenyl linkage (2.830 A) is close to the van der
Calcul.
5
H
Waals minimum. As there are no intermolecular interactions
(measured on the basis of the sum of the van der Waals radii
H
53
52
50
ꢁ
þ0.2 A) between the allene moiety and neighbouring molecules,
1
we can assume that this deformation (a>b, c<d) is intrinsic to our
3
TsO
2
2
compound.
OH
2
3
1
1
5
4
3
O
H
(+)
51
47b
5
H
4
5
H
α = 2.830 Å
?
a = 124.5 °
52
H
H
b = 122.7 °
c = 110.9 °
d = 114.9 °
e = 179.3 °
β
γ
= 2.552 Å
= 2.634 Å
?
1
(+)
.
e
.
3
.
2
.
.
.
Calculations
.
.
4
.
(+)
.
. α
λ = 1.495 Å
.
. H
β . .
.
5
Me
λ
2
.
3
1
.
a
b
.
54
O
S
...................... .
4
Tol
2
6
5
4
.
.
c
. γ
7
Me
5
.
.
3
H
O
H
O
.
d
55
O
H
1
.
H
.
(+)
H
.
.
3
47a
.
.
4
5
2
.
OTs
.
H
8
.
.
H
1
B3LYP/6-311G++(d,p)
X-Ray
D1234 = 165.4 °
2345 = –3.8 °
52
H
H
164.5 °
1.4 °
Scheme 11. Hydrolysis pathways for the b-allenic tosylate 47b.
D
–
–
D7238
=
66.9 °
67.9 °
OTs
Even more than previously, the formation of these compounds
and particularly 50 results from concerted processes. The corre-
sponding methylenecyclobutyl cation 53 is very strained (twisted).
Our DFT studies of the cation 53 at the B3LYP/6-311þþ(d,p) level
HOMO–1
Fig. 8. Geometry of the crystallised tosylate 27b and calculated conformation at the
B3LYP/6-311Gþþ(d,p) level of theory.

3232
C. Aouf et al. / Tetrahedron 69 (2013) 3225e3233
OH
OH
47a
51
= –387.238246 au
= –387.244889 au
Δ
G
Δ
G
= 4.2 kcal/mol
δΔ
G
Fig. 10. Relative stability of
a precursor b-allenyl alcohol and the corresponding
methylenecyclobutanol product (B3LYP/6-311Gþþ(3df,3pd) level of theory; solvent:
water).
The constraint structure of some products excludes their for-
mation through bicyclic carbocations. Results are in accordance
with a concerted process with a remote nucleophilic attack by the
allenic linkage followed by the ionisation step of the tosylate.
Therefore, the chirality is preserved in the course of the hydrolysis.
Fig. 9. ORTEP diagram of tosylate 27b.
This stabilising interaction may include (1) attractive Coulombic
interactions of bond dipoles; (2) delocalisation of valence electrons
of the CeC double bonds into the LUMO orbital of the CeO bond;
(3) stabilising van der Waals interactions between the two func-
tional groups.50 Intramolecular van der Waals attraction induced
the preference for a ‘crowded’ syn conformation of 1,n-di-alkyl-
substituted aromatic or hetero-aromatic rings.51 More recently, the
preferential formation of spiroacetals with a folded structure rela-
tive to an extended structure has been explained by the presence of
van der Waals attractive interactions.52 But, these interactions
sometimes referred to hydrophobic bonding in biochemical liter-
ature,53 result from nonbonded interactions between alkyl groups.
In the case of 27b, the attractive interaction occurred between polar
functionalities and the conformation of the tosylate group is in
accordance with an electron delocalisation which anticipates the
homoallenylic participation. We note that the future ionised bond
Acknowledgements
ꢀ
ˇ N.G. is grateful to the CNRS and the Region Provence-Alpes-
Cote d’Azur for a grant. This work has been financially supported
ꢂ
by the CNRS and the Ministere de l’Education Nationale. We thank
ˇ
ꢀ
Dr. M. Giorgi (Spectropole, Universite d’Aix-Marseille) for the
X-ray structure determination, R. Rosas for NMR spectroscopic
studies and Prof. D. Siri for his aid. We are indebted to
ꢀ
Dr. B. Vacher (Pierre Fabre Medicaments, Castres, Fr) for helpful
comments.
Supplementary data
Table 1: Calculated energies for geometry-optimised conforma-
tions of 1-cyclopropylvinyl cations 14, 15 and 20. Table 2: Calculated
energies for geometry-optimised configurations of 8E, and 8Z.
Table 3: Calculated energies for geometry-optimised configurations
of 38, and 39. Table 4: Calculated energies for geometry-optimised
configurations of various carbocations involved in the solvolysis of
27b, 28b and 47b. Figs. 1e4: Calculated structure of various carbo-
cations in the ideal gas. Figs. 5e8: 13C NMR of deuterated methyl-
enecyclobutanols 11(3d) and 11(4d). Fig. 9: Atom polarisability
tensor-based charges of 38 and 39. X-ray crystallographic data
concerning 27b. Experimental section: preparation of tosylates and
1H and 13C NMR data of (E)-2-ethylidene-1-ethynylcyclohexanol, 2-
ꢁ
distance CeO (1.495 A) is superior to the corresponding mean bond
distances in some secondary 2-alkyl tosylates. A survey of the
Cambridge Crystallographic Database revealed that the mean dis-
ꢁ
tance is 1.461ꢂ0.016 A (n¼12).
A correlation between the lengths of CeO bonds and the rates at
which the same bond is broken heterolytically in solution has been
proposed by Kirby: the longer the bond, the faster it breaks.54
Calculations of the structure of 27b at the B3LYP/6-311Gþþ(d,p)
led to the same ground-state conformation that in solid state
(27b:
D
E¼ꢀ1285.081072 au). The largest coefficients of the High
Occupied MO (HOMO) are at the C4 and C5 [HOMO, 82nd MO,
(E¼ꢀ0.24845 au), Satom. coef.(j2sj,j2pj): O1, 0.011; C2, 0.011; C4,
0.248; C5, 0.247; C6, 0.077] and for the HOMO-1 at the C5 and C6
[HOMO-1, 81st MO, (E¼ꢀ0.26461 au), Satom. coef.(j2sj,j2pj): O1,
0.076; C2, 0.053; C4, 0.112; C5, 0.227; C6, 0.362]. We note that an
electron transfer occurred from the C5eC6 double bond towards
the CeO bond.
(1-acetoxyethyl)-1-ethynylcyclohexene,
2-(1-hydroxyethyl)-1-
ethynylcyclo-hexene, 4E, 4Z, 10a, 10a(2d), 10b, 10b(2d), 11, 11(3d),
11(4d), 27a, 27b, 27b(d), 28a, 28b, 29, 29(d), 30, 30(d), 31, 31(d), 32,
33, 33(d), 34, 35, 37, 47a, 47b, 49, 50, 51. Supplementary data related
to this article can be found online at http://dx.doi.org/10.1016/
j.tet.2013.02.048.
References and notes
4. Conclusions
1. Roberts, J. D.; Mazur, R. H. J. Am. Chem. Soc. 1951, 73, 2509e2520.
2. Bruylants, P.; Dewael, A. Bull. Sci. Acad. R. Belg. 1928, 5 (14), 140e153.
3. Demjanow, N. J. Chem. Ber. 1907, 40, 4961e4963 see also, pp 43e46 and pp
4393e4397.
The driving force of these solvolysis reactions is the strained
energy of the allenyl linkage which afforded the cyclobutyl
framework in high yields. So, methylenecyclobutanols or bicyclic
methylenecyclobutanols can be obtained by this way in good yields
from easily prepared
bicyclo[3.2.0]heptanes scaffold can be obtained from
cohol 47a via the hydrolysis of its tosylate ester 47b (Fig. 10).
Calculations have shown that the main carbocationic in-
termediates are the 1-cyclopropylvinyl cations. But, experimental
results observed from the hydrolysis of 10b(2d) show that the
solvent can react during the cyclopropylvinyl cation formation from
4. Vogel, P. Carbocation Chemistry; Elsevier: Amsterdam, 1985; p 325.
5. (a) Richey, H. G., Jr. In Carbonium Ions, Olah, G. A.; Schleyer, P. v. R. Eds.;
WileyeInterscience: New York, NY, Vol. III, pp. 1201e1294. (b) Wiberg, K. B.;
Hess, B. A., Jr.; Ashe, A. J., III. In Carbonium ions; Olah, G. A.; Schleyer, P. v. R.,
Eds.; Wiley–Interscience: New York, NY, 1972; Vol. III, pp. 1295–1341.
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