Highly twisted C=C double bonds in 4-methyleneisoxazolones

Article abstract of DOI:10.1071/CH09359

As determined by X-ray crystallography, isoxazol-5(4H)-one derivatives 1219 feature dihedral angles around the exocyclic C4=C6 double bonds of 2690. In the most highly twisted bis-tert-butylamino derivatives 18 and 19, the C4C6 bonds are essentially single bonds. Density functional theory calculations at the B3LYP/6-31G(d) level with inclusion of a simulated solvent field, which helps stabilize zwitterionic structures, are in good agreement with the experimental crystallographic data. A good correlation between bond lengths and calculated π/π*orbital occupation quotients is observed. A good correlation between the twisting angle and the charge separation, measured by the calculated negative charge on the isoxazolone moiety, is also observed. Low barriers to rotation about the twisted exocyclic double bonds C4=C6 in compounds 13, 20, and 21 (G ^? = 1516 kcal mol^-1 (6367 kJ mol ^-1)) were determined by dynamic ¹H NMR coalescence measurements. The rotational barrier for 17 was estimated to be less than 10 kcal mol^-1. The rotational barriers for compounds 10 and 18 were calculated to be ～8 kcal mol^-1. CSIRO 2009.

Full text of DOI:10.1071/CH09359

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2009, 62, 1068–1078
www.publish.csiro.au/journals/ajc
Highly Twisted C=C Double Bonds
in 4-Methyleneisoxazolones
David Kvaskoff,A Paul V. Bernhardt,A Rainer Koch,B and Curt WentrupA,C
ASchool of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, Qld 4072, Australia.
BInstitut für Reine und Angewandte Chemie and Center of Interface Science,
Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany.
CCorresponding author. Email: wentrup@uq.edu.au
As determined by X-ray crystallography, isoxazol-5(4H)-one derivatives 12–19 feature dihedral angles around the exo-
cyclic C4=C6 double bonds of 26–90◦. In the most highly twisted bis-tert-butylamino derivatives 18 and 19, the C4–C6
bonds are essentially single bonds. Density functional theory calculations at the B3LYP/6-31G(d) level with inclusion of
a simulated solvent field, which helps stabilize zwitterionic structures, are in good agreement with the experimental crys-
tallographic data. A good correlation between bond lengths and calculated π/π* orbital occupation quotients is observed.
A good correlation between the twisting angle and the charge separation, measured by the calculated negative charge
on the isoxazolone moiety, is also observed. Low barriers to rotation about the twisted exocyclic double bonds C4=C6
in compounds 13, 20, and 21 (ꢀG‡ = 15–16 kcal mol−1 (63–67 kJ mol−1)) were determined by dynamic 1H NMR coa-
lescence measurements. The rotational barrier for 17 was estimated to be less than 10 kcal mol−1. The rotational barriers
for compounds 10 and 18 were calculated to be ∼8 kcal mol−1
.
Manuscript received: 24 June 2009.
Manuscript accepted: 6 August 2009.
Introduction
A computational analysis of the twisting and rotational barri-
ers in push–pull-substituted model ethylenes of the general for-
mula (X,Y)C=C(CHO)2 using density functional theory (DFT)
has been published.[9] The push–pull effect has been quantified
in terms of the quotient of occupation numbers of the bond-
ing and antibonding orbitals of the twisted C=C bond (π/π*
quotients).[10] A good correlation is observed between these
π/π* quotients and the lengths of the twisted double bonds,[10]
and this was also found to be the case for Meldrum’s acid deriva-
tives of the typesshown in Scheme 1.[4] An analogous correlation
between C≡C bond lengths and π/π* occupation quotients in
push–pull substituted alkynes was published recently.[11]
Isoxazolones 11 are important starting materials for the
synthesis of several interesting molecules, including linear
and cyclic alkynes RC≡CRꢀ, isocyanides RN≡C, fulminic
acid HCNO, and unusual cumulenes such as S=C=C=S,[12]
RN=C=C=C=NRꢀ,[13] andRN=C=C=C=S.[14] Inthepresent
work we have studied the push–pull-substituted 4-methylene-
isoxazolones 12–19 (Scheme 2). The torsional angles were
determined by X-ray crystallography, and the rotational barriers
for 13 and 10–21 by dynamic NMR spectroscopy.The structures
were also examined using DFT.
The rotational barriers around the C=C double bonds in
alkenes substituted with donor and acceptor substituents at the
two termini (‘push–pull’ alkenes 1, D = donor, A = acceptor))
are significantly lower than the habitual 65 kcal mol−1
(1 kcal mol−1 = 4.186 kJ mol−1).[1] Conjugated molecules of
this type may exhibit variable zwitterionic character and twist-
ing around the C=C bond. They are of general interest for their
non-linear optical properties and potential applications.[2]
We have shown previously that derivatives of Meldrum’s
acid with one or two donor substituents (OR, SR, or NRRꢀ)
in position 7 can exhibit dihedral angles about the C=C
bond (or twisting angles) as high as 83◦ (compound 2)
as determined by X-ray crystallography (Scheme 1).[3] The
anionic derivative 3 features an angle of 90◦.[4] Twisting
angles of 15.2◦ and 41.1◦ were measured for the substituted
2-aroylacrylonitriles8and 9, respectively,[5] and53◦ fortheMel-
drum’s acid derivative 10.[3] Angles of 41–89◦ were recorded for
2-imidazolidinylidene-1,3-diketones[6] and N,N-disubstituted
1,1-diamino-2,2-dinitroethylenes.[7]
Several other relevant X-ray crystal structures of push–pull
ethylenes are described in the literature.[3,4,8] Hydrogen bonding
between NH and CO groups stabilize coplanar conformations
in molecules such as 6 and 7,[3] and large substituents on the
adjacent nitrogen atom cause increasingly large dihedral angles
because of steric hindrance.[3,4] The presence of water molecules
in the lattice can act as hydrogen-bond acceptors from the
RNH groups and thus prevent intramolecular hydrogen bond-
ing, thereby changing a structure from nearly planar to nearly
perpendicular (e.g., 2).[3]
Results and Discussion
Compounds 12–19 were prepared from 3-phenylisoxazol-
5(4H)-one using procedures described previously.[13,14] The
X-ray crystal structures of the S,S derivatives 12 and 13
(Scheme 2) are as expected from previous structural determi-
nations of push–pull-substituted Meldrum’s acid derivatives.[3,4]
The most relevant dimensions for 12 and 13 are the lengths of the
© CSIRO 2009
10.1071/CH09359
0004-9425/09/091068

RESEARCH FRONT
Highly Twisted C=C Double Bonds
1069
formal double bonds C4=C6 of 1.369(2) and 1.375(3) Å, respec-
tively, and the twisting angles about these bonds of 26.2(1)◦
and 30.0(2)◦, respectively, each expressed as the dihedral angle
between the C3/C4/C5 and S1/C6/S2 planes.
The structures of 12 and 13 are similar to those of the cor-
responding Meldrum’s acid derivatives (Scheme 1),[3,4] but the
C=C bond lengths are shorter in the isoxazolones. Compound
12 is more twisted (26.2◦) than the related acrylonitrile deriva-
tive 8 (15.2◦, Scheme 1), but is very similar to the Meldrum’s
acid analogue 4 (25◦, Scheme 1).
The crystal structures of compounds 14 (Fig. 2, top left) and
15 (Fig. 2, top right) reveal longer C4=C6 bonds (1.422(5)
and 1.427(3) Å, respectively) and larger twist angles between
the C3/C4/C5 and N3/C6/X (X = S1 or N4) planes (45.1(2)
and 48.1(1)◦, respectively). Similar parameters are also
observed for the other amine-substituted compounds 16 and
17 (Fig. 2, 16: C4=C6 1.420(2) Å, twist angle 42.9(1)◦, 17:
C4=C6 1.433(2) Å, twist angle 48.6(1)◦). The structure of 17
exhibits a twisting angle intermediate of those in the related
acrylonitrile derivative 9 (41.1◦, Scheme 1) and the Mel-
drum’s acid analogue 10 (53◦, Scheme 1). The C4–C6 bond
X
D
D
A
A
X
D
D
A
A
ϩ
ϩ Ϫ
6
R
R
3
Y
Y
4
Ϫ
5
N
N
2
1
O
O
O
O
1
11
i-Pr-S
N
O
t-Bu
O
Me
SMe
Ph 1.42
S
MeS
O
O
Ph
H
H
N
O
O
1.395
Ph
1.38
S
1.48
NHMe
1.37
S
O
30Њ
45Њ
H2O
26Њ
N
O
O
1.473
Me
i-Pr
83Њ
MeS
25Њ
N
N
O
N
N
Ϫ
O
O
O
O
O
O
O
t-Bu
MeS
90Њ
12
13
14
O
2
3
4
Me
N
N
N
Me2N
Me2N
1.42
1.43
H
O
Ph
Ph
N
Ph
NMe2
1.43
NHPh
H
O
49Њ
48Њ
43Њ
Me
N
N
O
O
1.42
3Њ
O
N
O
O
1.41
N
N
N
H
O
O
O
O
O
1.435
30Њ
MeS
O
O
O
O
H
15
16
17
Me2N
46Њ
O
O
5
6
7
H
H
t-Bu
N
t-Bu-N
t-Bu-N
t-Bu
H
1.44
Me
Me
1.47
Ph
1.46
O
Ph
Ph
N
O
O
N
N
52Њ
89Њ
90Њ
t-Bu
Me
S
S
Ar
N
Ar
N
O
O
H
t-Bu
1.50
15.2Њ
1.45
41.1Њ
1.44
53Њ
N
N
N
O
O
O
O
O
O
O
CN
N
CN
N
O
Me
O
Me
18·1/2(C4H8O2)
Me
18
19
8
9
10
Scheme 2. New compounds 12–19 investigated. C4=C6 bond lengths in
Å and torsional angles in degrees. Atom numbering is shown in structure 11.
Scheme 1. C=C bond lengths in Å and torsional angles in degrees.
C10
C8
O5
O1
N2
S1
C5
C5
C9
O1
C10
C11
C7
O5
C8
C6
C3
C4
C14
N2
C4
C12
S2
C3
C7
C11
C14
C13
C12
C9
C6
C18
C15
C16
S1
S2
C13
C17
Fig. 1. ORTEP plots of (left) compound 12, and (right) compound 13, each with 30% probability ellipsoids shown.

RESEARCH FRONT
1070
D. Kvaskoff et al.
O1
N2
C3
O1
O5
N2
C3
O5
C10
C5
C16
C11
C12
C5
C4
C7
C15
C4
C9
C7
C8
C6
C14
C14
C13
N3
C12
C6
N4
S1
C13
C10
N3
C9
C8
C11
C9
C9
C10
C15
N3
C8
C14
N4
C16
C15
N3
C8
C6
C10
C11
C14
C6
N4
C11
C13
C17
C7
C16
C4
C13
C4
C12
C12
C5
C18
C7
C3
C5
O5
O5
C3
C20
N2
C19
O1
O1
N2
C22
O3
C21
C15
C14
C13
C16
C9
C10
O5
C6
C4
O1
N3
C3
C12
C5
C3
C7
N2
O5
C8
N3
N4
C20
N2
C4
C16
C6
C14
C5
C7
C11
C15
O1
C10
C19
C8
C11
C18
C12
C17
C9
C13
Fig. 2. ORTEP drawings of compound 14 (top left), 15 (top right), 16 (centre left), 17 (centre right), 18·1/2(C4H8O2) (bottom left), and 18 (bottom right).
30% probability ellipsoids are displayed.
lengths are also the same within experimental uncertainty
(1.44 0.01 Å).
The bis-tert-butylamino derivative 18 was crystallized in
two forms. When crystallized from 1,4-dioxan, crystals of
18·1/2(C4H8O2) were obtained. As is evident from Fig. 2 (bot-
tom left), the dioxan molecule accepts a hydrogen bond from
N3–H3. In fact, the dioxan molecule is situated at a centre
of inversion, which accounts for the 2:1 ratio of 18:dioxan in
the crystal. The key parameters are C4=C6 1.438(3) Å, with
a corresponding twist angle of 52.5(2)◦. The remaining NH
group (N4–H4) donates a hydrogen bond to an adjacent isoxa-
zolonering(N4–H4···O5ꢀ 2.29 Å, 146◦, symmetryoperation−x,

RESEARCH FRONT
Highly Twisted C=C Double Bonds
1071
C11
C12
C10
C13
C18
C4
C20
C19
C21
C22
N3
C6
C8
N4
N3
C7
C3
C5
N2
C9
O5
O5
C14
O1
N4
O1
N2
C15
C17
C16
N2
Fig. 4. ORTEP drawing of compound 19. 30% probability ellipsoids are
displayed.
O1
O5
N3
bond. In this case, the bulky tert-butyl groups enforce a twisted
conformation to avoid clashes with the phenyl ring and car-
bonyl O-atom. Both tert-butyl groups are rotationally disordered
about their connecting C10–N3 and C14–N4 bonds and the two
contributors to each disordered group were refined with comple-
mentary occupancies. Only one contributor is shown in Fig. 4
for clarity.
Fig. 3. H-bonding in the structures of 18·1/2(C4H8O2) (top, dioxane
molecule omitted) and 18 (bottom).
−y, −z − 1) resulting in hydrogen bonded isoxazolone dimers
(Fig. 3).
The dimethyl analogue (15, Fig. 2 centre right) is the clos-
est analogue and the more compact methyl groups attached at
the 1- and 3-positions of the imidazolidine ring allow a smaller
twist angle (48.1◦) and significantly higher bond order for C4–
C6 (1.433(2) Å).The structure of compound 19 also has obvious
parallels with that of compound 18 (Fig. 3 (bottom), solvent free
form) where the conformations of the two molecules are virtu-
ally identical and the tert-butyl groups occupy space above and
below the isoxazolone ring. Unlike the more flexible compound
18, the rigid imidazolidine ring cannot accommodate alternative
orientations of the tert-butyl groups, such as that seen in the
dioxan adduct of 18 (Fig. 3, top), so the structure is locked into
a single conformational form.
It should be noted that the secondary amino-methylene Mel-
drum’sacidanalogues(e.g., 6and 7, Scheme1)differstructurally
from the present secondary amino-substituted isoxazolones
(e.g., 14, 16, and 18, Scheme 2) in an important way. Intramolec-
ular hydrogen bonding from the NH groups to the carbonyl
groups C5=O5 of 14, 16, and 18 is not possible because of the
more acute bond angles within the five-membered isoxazolone
rings (105–110◦), which draw O5 away from the NH group
and out of H-bonding range. In contrast, strong intramolecu-
lar hydrogen bonding has been seen in related (six-membered
ring) Meldrum’s acid analogues such as 6 and 7 (Scheme 1),
where the 120◦ angles at the carbonyl and C6 atoms orient the
carbonyl O-atoms towards the adjacent NH groups.
Intheabsenceofdioxan, recrystallizationof 18fromaqueous
MeOH gave a solvent-free form, which exhibited a markedly dif-
ferent structure (Fig. 2, bottom right).The molecule 18 resides on
a crystallographic mirror plane, which includes the phenylisoxa-
zolonemoietysothetworingsarestrictlycoplanar. Furthermore,
the isoxazolone ring and the plane defined by N3/C6/N3ꢀ (sym-
metry operation x, −y + 1/2, z) are orthogonal (twist angle
90.0◦), and the C4–C6 bond length (1.466(3) Å) approaches that
of a typical C–C single bond. This is the largest twisting angle
observed in an alkene of this type and is equalled only in the
anionic compound 3. In this conformation, each N3–H3 group
donates a hydrogen bond to N2ꢀ of an adjacent molecule (sym-
metry operation x, y, z − 1), which results in a polymeric chain
(Fig. 1). It is pertinent that these two amino-H atoms are syn and
capable of simultaneously forming hydrogen bonds to the same
acceptor. This is in contrast to the anti conformation of N3–
H3/N4–H4 in the structure of 18·1/2(C4H8O2), where the two
N–H groups point in different directions and donate hydrogen
bonds to different molecules.
The 90◦ C4–C6 torsional angle in 18 is similar to that of the
Meldrum’s acid derivative 2·H2O (83◦), where hydrogen bond
donation to the water molecule helps to ‘tie back’ the two syn
N–H groups. Essentially the same feature is seen in 18, where
hydrogen bond donation by the pair of symmetry related N3–H3
groups to N2ꢀ (of an adjacent molecule) is found. In each case
the tert-butyl groups are forced to occupy space above and below
the adjacent heterocyclic ring.
The last compound structurally characterized was the bis-t-
butylimidazolidinederivative19.Thisanalogue, likecompounds
12, 13, 15, and 17, bears no hydrogen bond donors so the struc-
ture is unaffected by inter- or intramolecular hydrogen bonding.
As is evident from Fig. 4, the isoxazolone and imidazolidine
rings are orthogonal (interplanar twist angle 89.0(6)◦). The con-
necting C4–C6 bond is 1.458(3) Å and consistent with a single
Calculations
Structural calculations were carried out at the B3LYP/6-31G(d)
level of theory, which has proved to be reliable for related
molecules.[3,4] The calculations were performed for molecules in
the gas phase and with an imposed solvent field using the polar-
izable continuum model (PCM) self-consistent reaction field

RESEARCH FRONT
1072
D. Kvaskoff et al.
Table 1. Comparison of relevant experimental and calculated structural data for compounds 12–19. C4–C6 dihedral and twisting angles [◦],
C4–C6 bond distances [Å], and barriers to rotation [kcal mol−1] (1 kcal mol−1 = 4.186 kJ mol−1
)
Angle/bond
Exp
B3LYP/6-31G(d)
SCRF-B3LYP/6-31G(d)
12
13
14
15
16
17
18
C3(Ph)–C4–C6–S1
C5(O)–C4–C6–S1
C3(Ph)–C4–C6–S2
C5(O)–C4–C6–S2
C4–C6
−22.1
−14.1
161.1
167.2
−20.5
156.3
160.2
150.0
160.2
−27.6
−17.6
−23.0
1.369
26.2
359.7/360.0
1.380
1.389
C4–C6 twisting angleA
Sum of bond anglesB
Rotation barrier
16.8
359.6/359.8
22.1
22.1
359.7/359.7
C3(Ph)–C4–C6–S1
C5(O)–C4–C6–S1
C3(Ph)–C4–C6–S2
C5(O)–C4–C6–S2
C4–C6
C4–C6 twisting angleA
Sum of bond anglesB
Rotation barrier
−26.2
146.7
155.2
−32.0
1.375
30.0
−21.8
143.7
169.0
−24.0
141.9
165.3
−25.5
−28.8
1.384
1.389
30.8
359.8/360.0
22.7
32.2
359.9/360.0
359.8/360.0
C
C3(Ph)–C4–C6–S
C5(O)–C4–C6–S
C3(Ph)–C4–C6–N3
C5(O)–C4–C6–N3
C4–C6
C4–C6 twisting angleA
Sum of bond anglesB
Rotation barrier
−38.7
129.9
145.7
−27.2
138.6
156.6
−31.5
136.1
152.0
−45.6
−37.5
−40.5
1.422
1.393
1.408
45.1
359.2/359.9
35.7
358.9/359.9
18.9
38.5
359.1/359.9
C3(Ph)–C4–C6–N4
C5(O)–C4–C6–N4
C3(Ph)–C4–C6–N3
C5(O)–C4–C6–N3
C4–C6
C4–C6 twisting angleA
Sum of bond anglesB
Rotation barrier
−43.2
127.8
137.9
−31.3
138.7
149.7
−35.8
136.8
145.0
−51.0
−40.2
−42.5
1.427
1.404
1.422
48.1
359.5/360.0
37.2
359.4/360.0
18.0
39.9
359.6/360.0
C3(Ph)–C4–C6–N3
C5(O)–C4–C6–N3
C3(Ph)–C4–C6–N4
C5(O)–C4–C6–N4
C4–C6
C4–C6 twisting angleA
Sum of bond anglesB
Rotation barrier
−139.4
42.9
−152.7
35.7
−148.2
38.4
42.5
29.7
33.3
−135.2
−141.9
−140.1
1.420
1.399
1.417
42.9
360.0/360.0
34.1
358.6/359.9
10.4
36.6
359.2/359.9
C3(Ph)–C4–C6–N3
C5(O)–C4–C6–N3
C3(Ph)–C4–C6–N4
C5(O)–C4–C6–N4
C4–C6
C4–C6 twisting angleA
Sum of bond anglesB
Rotation barrier
46.9
−127.9
−136.0
49.3
1.433
48.6
359.8/360.0
33.4
−147.9
−136.5
42.2
1.408
39.3
359.5/360.0
10.7
41.5
−131.8
−139.8
47.0
1.428
44.9
359.8/360.0
C3(Ph)–C4–C6–N3
C5(O)–C4–C6–N3
C3(Ph)–C4–C6–N4
C5(O)–C4–C6–N4
C4–C6
−89.8
90.2
89.8
28.9
−138.6
−150.3
42.2
65.6
−112.0
−116.0
66.4
−90.2
1.466
1.404
1.449
(Continued)

RESEARCH FRONT
Highly Twisted C=C Double Bonds
1073
Table 1. (Continued)
B3LYP/6-31G(d)
Angle/bond
Exp
90.0
SCRF-B3LYP/6-31G(d)
C4–C6 twisting angleA
Sum of bond anglesB
Rotation barrier
36.8
359.2/360.0
8.0
66.1D
359.8/360.0
360.0/360.0
19 (minimum 1)E
C3(Ph)–C4–C6–N3
C5(O)–C4–C6–N3
C3(Ph)–C4–C6–N4
C5(O)–C4–C6–N4
C4–C6
C4–C6 twisting angleA
Sum of bond anglesB
129.4
−59.6
−47.1
123.9
1.420
53.5
113.6
−64.0
−69.1
113.2
1.448
66.2D
359.5/359.9
359.9/360.0
19 (minimum 2)E
C3(Ph)–C4–C6–N3
C5(O)–C4–C6–N3
C3(Ph)–C4–C6–N4
C5(O)–C4–C6–N4
C4–C6
C4–C6 twisting angleA
Sum of bond anglesB
−89.7
86.7
96.2
91.2
−88.8
−91.2
88.8
1.446
89.9
90.2
−89.9
−90.9
89.0
1.458
89.5
−87.5
1.458
89.0(6)
359.9/359.7
360.0/360.0
360.0/360.0
10
20
21
Rotation barrier
Rotation barrier
Rotation barrier
8.0
19.6
21.2
C
C
AThe twisting angle is the dihedral angle between the planes defined by the atoms C3, C4, and C5 on the isoxazolone ring and the C6(X)(Y) exocyclic
substituents.
BThe sum of bond angles subtended at C4/C6, respectively, establishing trigonal planar geometry.
CSee Table 3.
DThis angle increases to 70◦ when using acetonitrile as a solvent (ε 36.6).
EMinima 1 and 2 for compound 19 are calculated energy minima; minimum 1 has not been identified experimentally.
100
Table 2. Comparison of exocyclic C4–C6 bond lengths [Å] and
π*/π orbital occupation quotients for these bonds, calculated at
80
B3LYP/6-31G(d) and SCRF-B3LYP/6-31G(d) levels
R2 ϭ 0.6789
60
Compound
Bond length
π*/π quotient
Calc.
Exp.
40
20
12
12 SCRF
13
13 SCRF
14
14 SCRF
15
15 SCRF
16
16 SCRF
17
17 SCRF
18
18 SCRF
19 (minimum 1)
19 (minimum 1) SCRF
19 (minimum 2)
19 (minimum 2) SCRF
1.380
1.389
1.384
1.389
1.393
1.408
1.404
1.422
1.399
1.417
1.408
1.428
1.404
1.449
1.420
1.448
1.446
1.458
0.207
0.237
0.220
0.245
0.258
1.369
1.375
1.422
1.427
1.420
1.433
1.466
0
Ϫ0.3
Ϫ0.4
Ϫ0.5
Ϫ0.6
Ϫ0.7
Ϫ0.8
Ϫ0.9
A
–
Charge on isoxazolone fragment
0.287
0.339
0.275
0.323
0.311
Fig. 5. Correlation between the sums of NPA charges on isoxazolone
fragment and the twisting angles in 12–19 (including both minima of 19).
A
–
0.285
(SCRF) method (ε = 20.7, simulating acetone). The purpose of
the solvent field is to provide a dielectric environment, which
mimics the compound in solution or in the crystal by stabilizing
charge-separated, zwitterionic structures. The calculated struc-
tures are in reasonably good agreement with the experimental
data, especially at the SCRF level (Table 1). The solvent field
typically causes small increases in the C4–C6 bond lengths and
the twisting angles.
A
–
0.349
A
–
–
–
A
A
1.458
AThe NBO population analysis does not reveal a C4=C6 double bond,
thereby indicating strong zwitterionic character of this compound.

RESEARCH FRONT
1074
D. Kvaskoff et al.
0.4
0.35
0.3
(72–81 ppm) are also in keeping with sizeable negative charges
on these carbon atoms. The corresponding chemical shifts for
compounds 12/13 and 20/21 are 107–113 ppm, and for ‘nor-
mal’isoxazolones bearing only alkyl substituents at C6, they are
113–117 ppm.[15]
R 2 ϭ 0.9831
R2 ϭ 0.7427
R2 ϭ 0.999
The push–pull effect can be quantified in terms of a correla-
tion between C=C bond lengths and the quotient of occupation
numbers of the occupied and unoccupied π orbitals (π/π*).[10]
The π* occupancy is a measure of the electron donation into that
orbital, which destabilizes the planar conformation of the alkene.
The data for the isoxazolones (Table 2) show excellent correla-
tion between the calculated C4=C6 bond lengths and the π/π*
quotients regardless of the computational method (gas phase or
SCRF) as shown in Fig. 6 (R2 = 0.98). The correlation is poorer
(R2 = 0.74) but still shows the correct trend when the experi-
mental bond lengths are compared with the π*/π quotients for
the gas phase. The poor correlation is attributable largely to the
long C4–C6 bond in 18 (1.466 Å), which is much longer than
predicted and not a double bond at all in the solvent-free crys-
tal structure. It would be desirable to compare the experimental
bond lengths with the π/π* occupancies for the solvent field
calculations, which provide the more accurate structures, but a
natural bond orbital (NBO) population analysis does not indi-
cate a double bond at all for compounds 14 and 17–19 at the
SCRF level (Table 2); accordingly, π/π* occupancies cannot be
calculated. The correlation is excellent for the remaining four
compounds for which solvent field π/π* occupancies can be
calculated (12, 13, 15, and 16; R2 = 0.99; Fig. 6).
0.25
0.2
Exp versus gas phase
1.440 1.460
Calc
Exp versus SCRF
0.15
1.360
1.380
1.400
1.420
1.480
Bond lengths
Fig. 6. Graph of π*/π orbital occupation quotients of the central C4=C6
double bonds (ordinate) versus bond lengths (Å; abscissa). The graph shows
correlation of the experimental bond lengths with the π*/π orbital occupa-
tion quotients calculated at the SCRF level (compounds 12, 13, 15, and 16
only; R2 = 0.99), correlation of the calculated bond lengths with the calcu-
lated π*/π quotients for compounds 12–19 in solution (SCRF) and gas phase
(R2 = 0.98), and correlation of the experimental bond lengths for compounds
12–18 with the π*/π quotients calculated for the gas phase (R2 = 0.74).
NC
i-Pr
i-Pr
S
S
S
S
i-Pr
i-Pr
N
N
O
O
O
O
20
21
Scheme 3.
Rotational Barriers
All of the isoxazolones described here exhibit only one set of
1H NMR signals each, thereby suggesting rapid rotation about
the formal C4=C6 double bonds at room temperature. Barri-
ers to rotation, ꢀG‡ = 15–16 kcal mol−1, were measured for
While the correlation between experimental and calculated
twisting angles is good, theory tends to predict smaller angles.
This is particularly noticeable in the case of 18, where the cal-
culated angles for the gas phase (37◦) and the simulated solvent
(66◦ in acetone, and 70◦ in acetonitrile) are evidently too small.
In the case of 19, too, we first found a gas phase minimum
(minimum 1) with a twisting angle of 53.5◦, which increased to
66.2◦ in acetone and 70◦ in acetonitrile. However, there exists
a second minimum (minimum 2) for 19, which possesses an
orthogonal arrangement (89◦) and is only 0.2 kcal mol−1 less
stable than minimum 1 in the gas phase, whereas it is the pre-
ferred structure in the simulated acetone solvent (0.9 kcal mol−1
below minimum 1). When the free energies are calculated, min-
imum 2 becomes the absolute minimum, 1.4 kcal mol−1 below
minimum 1, even in the gas phase.
Detailed natural bond orbital (NBO) analysis of the elec-
tronic nature of the observed phenomenon reveals that there
is a good correlation between charge separation and twisting
angle (Fig. 5). The sum of the natural population analysis (NPA)
charges on the isoxazolone fragment (isoxazolone + phenyl
ring + C4) increases with increasing twisting of the molecule. A
similar relationship is observed by comparing the C4 charges
with the central dihedral angles. This charge separation is
because of the push–pull effect of the substituents, as the
hybridization of the central carbon atoms C4 and C6 is almost
unchanged in the series investigated herein, hence ruling out
π/π* electron separation. Beyond that, within the same substi-
tution pattern (bis-amino or bis-thio), there is no influence of
the groups attached to C6 on the natural electron configuration.
Details can be found in the Accessory Publication, Table S1.
The high-field 13C chemical shifts of C4 in compounds 15–19
1
compounds 13, 20, and 21 (Scheme 3) by dynamic H NMR
spectroscopy in DMSO solution. The results are presented in
Table 3.
Comparison with DFT-derived barriers shows good agree-
ment, with the calculated energies for 13, 20, and 21 being
∼5 kcal mol−1 higher, probably because of the smaller twist
angles in the calculated structures (Table 1).
Details of coalescence temperature measurements, line shape
analysis, and calculations are given in the Accessory Publica-
tion. Compound 17 showed only one sharp methyl group signal
down to −80◦C (CD3OD solution), thereby indicating that this
temperature is still far from coalescence and suggesting that
ꢀG‡ for rotation may be below 10 kcal mol−1 (calculated barrier
10.7 kcal mol−1; see Table 1). The rotational barriers for com-
pounds 10 and 18 were calculated to be ∼8 kcal mol−1 (Table 1).
These results are in keeping with a pronounced contribution by
the zwitterionic structure 11 in these molecules.
Conclusions
Push–pull-substituted ethylenes 12–19 exhibit variable twisting
about the exocyclic C4=C6 double bonds (26–90◦) and elon-
gation of these bonds (1.37–1.47 Å) as determined by X-ray
crystallography. In the crystal structures of compounds 18 and
19, the exocyclic C4–C6 bonds are essentially single bonds, and
the twisting angle is 90◦. The twisting angles about the exo-
cyclic double bonds in the isoxazolones are similar to those
found in Meldrum’s acid derivatives[3,4] and are ascribed in both

RESEARCH FRONT
Highly Twisted C=C Double Bonds
1075
Table 3. Activation parameters for internal rotation in 13, 20, and 21 at coalescenceA
(1 kcal mol−1 = 4.186 kJ mol−1
)
Compd
Tc
[K]
k
[s−1
ꢀH‡
[kcal mol−1
ꢀS‡
[kcal mol−1
ꢀG‡
[kcal mol−1
]
]
]
]
13
20
21
325 0.5
315 0.5
330 0.5
400
360
108
2
2
2
10.00 0.2
9.28 0.4
9.40 0.4
−16.02 0.8
−17.43 1.0
−20.99 1.3
15.20 0.5
14.77 0.7
16.34 0.9
ATc coalescence temperature; k rate constant for exchange of magnetically inequivalent
proton sites at Tc.
cases to the strong push–pull character of these double bonds.
Syntheses
Compounds 12,[14] 13,[14] 14,[13] and 16[13] were prepared as
described previously.
Microwave reactions were performed in Pyrex tubes using
a CEM Discover BenchMate reactor (300W) operating at a
frequency of 2.45 GHz.
Safety Note: cyanide solutions are potentially lethal and
should only be used in a well ventilated fume hood with
appropriate personal protective equipment.
Calculations at the SCRF-B3LYP/6-31G(d) level are in general
agreement with the experimental structures. The largest discrep-
ancies are in the case of 18, where a twisting angle of at most 70◦
is predicted. The hydrogen-bonded structure of 18 (Fig. 3) may
help to stabilize the orthogonal arrangement in this case, but this
cannot be the reason for the extreme twisting of compound 19.
The computational results for 19 demonstrate that two distinct,
almost isoergic, energy minima are possible, one of them having
the orthogonal structure.A possibility, to be investigated further,
is that two distinct energy minima may be achievable for many
of these molecules, i.e., one where the rings are perfectly orthog-
onal (as in compounds 18 (solvent free) and 19) and one where
the rings attempt to be as co-planar as possible whilst minimiz-
ing clashes with the ring substituents (as seen in all of the other
structures including the dioxan solvate of compound 18).
NBO population analysis indicates that compounds 14 and
17–19 do not possess a C4=C6 double bond, i.e., they are highly
zwitterionic. There is a good correlation between the twisting
angle and the calculated charge separation, measured by the
negative charge on the isoxazolone moiety.
None of the twisted alkenes show significant pyramidaliza-
tion of the olefinic carbon atoms C4 and C6; these atoms are
trigonal, with the sums of bond angles close to 360◦. This is also
true for the calculated structures at the level of theory used here.
The thermodynamic parameters of activation for rota-
tion about the C4=C6 bonds were determined by 1H
NMR spectroscopy for compounds 13, 20, and 21 (ꢀG‡ =
15–16 kcal mol−1). All the data suggest a pronounced contribu-
tion by the zwitterionic canonical structure 11, which possesses
a formal C4–C6 single bond. The calculated barriers to rotation
about these C=C bonds are lower in the isoxazolones than in
the analogous Meldrum’s acid derivatives (Table 1). Rotational
barriers as low as 8, 8, and 11 kcal mol−1 have been calculated
here for compounds 18, 10, and 17, respectively.
4-[Bis(dimethylamino)methylene]-3-phenylisoxazol-
5(4H)-one 15
Anhydrous dimethylamine (99%) was bubbled through a solu-
tion of 4-[bis(methylthio)methylene]-3-phenylisoxazol-5(4H)-
one 12 (0.212 g, 0.8 mmol) in 1,4-dioxan (20 mL) at 0◦C for
10 min. An exothermic reaction occurred, and the solution
was stirred for another 20 min at room temperature, during
which time a precipitate formed. The solid was filtered off,
washed with diethyl ether (10 mL), and recrystallized from
methanol. Yield 0.20 g (97%), white crystals, mp 259–260◦C.
δH (400 MHz, DMSO) 2.71 (s, 6H, NMe2), 2.89 (s, 6H, NMe2),
7.28–7.31 (m, 2H), 7.38–7.40 (m, 3H). δC (100 MHz, DMSO)
41.2 (NMe2), 41.4 (NMe2), 77.3 (C4), 126.5, 128.6, 129.5,
131.3, 162.5 (C3), 164.5 (C5), 173.1 (C6) (Found: C 64.65,
H 6.68, N 16.47. Calc. for C14H17N3O2: C 64.85, H 6.61,
N 16.20%.)
4-(1,3-Dimethylimidazolidin-2-ylidene)-3-phenylisoxazol-
5(4H)-one 17
A solution of 12 (0.212 g, 0.8 mmol) and N,Nꢀ-dimethyl-
ethylenediamine (0.07 g, 0.8 mmol) in 1,4-dioxan (1 mL) was
irradiated in the microwave reactor at 95◦C for 30 min. The
solution was then cooled in ice, and the precipitate thus formed
filtered off, washed with diethyl ether (10 mL), and recrystal-
lized from methanol. Yield 0.178 g (87%), colourless crystals,
mp 245–247◦C. δH (400 MHz, CD3OD) 2.79 (s, 6H, NCH3),
5.48 (s, 4H, CH2), 7.48 (s, 5H, Ph). δC (100 MHz, DMSO)
35.3 (NCH3), 49.3 (CH2), 68.3 (C4), 126.6, 128.8, 129.3, 131.5,
161.2(C3), 162.4(C5), 172.7(C6). m/z(EI+)257(100%)[M+•],
212 (19), 199 (30), 143 (17), 103 (14), 81 (12) (Found: C 65.09,
H 5.81, N 16.50. Calc. for C14H15N3O2: C 65.35, H 5.88,
N 16.33%.)
Experimental
Computational Methods
All calculations were performed using the Gaussian 03 program
package.[16] DFT with the B3LYP functional[17] together with
Pople’s 6-31G(d)[18] basis set was used as the default level of the-
ory. This has been shown to be a reliable approach in the study
of related systems.[3,4] The nature of all stationary points as true
minimawasconfirmedbycalculatingharmonicfrequencies.The
influence of a polar solvent environment was taken into account
by employing the PCM SCRF[19] approach with ε = 20.7, sim-
ulating acetone as solvent. Population analysis has been carried
out using the natural bond order (NBO) version 5.G,[20] which
is implemented in our local version of Gaussian 03.
4-[Bis(tert-butylamino)methylene]-3-phenylisoxazol-
5(4H)-one 18
Compound 18 was prepared from 12 (0.212 g, 0.8 mmol) and
tert-butylamine (0.117 g, 1.6 mmol) under microwave irradia-
tion by the method described for 17 except that the reaction
time was 60 min. The product was recrystallized from methanol
to yield 0.256 g (81%) of white crystals, mp 174◦C (dec.). δH

RESEARCH FRONT
1076
D. Kvaskoff et al.
(400 MHz, CD3OD) 1.34 (s, 18H, t-Bu), 7.44–7.48 (m, 5H,
Ph). δC (100 MHz, CD3OD) 29.7 (CH3), 54.8, 81.1 (C4), 128.9,
129.8, 130.7, 131.9, 158.1 (C3), 164.4 (C5), 176.7 (C6) (Found:
C 68.17, H 8.28, N 13.20. Calc. for C18H25N3O2: C 68.54,
H 7.99, N 13.32%.)
170 (26), 129 (18), 102 (21), 75 (21), 43 (100). δH (200 MHz,
CDCl3) 1.14 (br s, 6H), 1.39 (br s, 6H), 3.56 (m, 1H), 4.29 (m,
3
3
1H), 7.60 (d, J 8.5, 2H), 7.74 (d, J 8.5, 2H). δC (50 MHz,
CDCl3) 22.3 (CH3), 43.4 (SCH), 108.2 (C4), 113.6 (C4ꢀ),
118.0 (CN), 129.3 (C2ꢀ), 132.0 (C3ꢀ), 133.7 (C1ꢀ), 159.8 (C3),
166.8 (C5), 180.8 (CS2). δC (DEPT 135) 22.3, 129.3, 132.0.
νmax (KBr)/cm−1 2927w, 2228m, 1716s, 1474s, 1367m, 864m.
(Found: C 59.22, H 5.17, N 7.93. Calc. for C17H18N2O2S2: C
58.93, H 5.24, N 8.08%.)
When compound 18 was recrystallized from 1,4-dioxan,
crystals of
a
hemi-dioxan adduct were obtained
(18·1/2(C4H8O2)), mp 171–174◦C.
4-(1,3-Di-tert-butylimidazolidin-2-ylidene)-
3-phenylisoxazol-5(4H)-one 19
4-[Bis-((1-methylethyl)thio)methylene]-3-(2,2-dimethyl-
propyl)isoxazol-5(4H)-one 21
The title compound 19 was prepared from 12 (0.530 g, 2.0 mmol)
and N,Nꢀ-di-tert-butyl-ethylenediamine (98%, Aldrich, 0.344 g,
2.0 mmol) by refluxing in 1,4-dioxan (15 mL) for 24 h.The prod-
uct was left to precipitate at room temperature, collected by
vacuum filtration, and recrystallized from ethanol/water (1:1)
to yield 0.150 g (22%), yellow crystals, mp 291◦C (dec.). δH
(400 MHz, CDCl3) 1.33 (s, 18H, CH3), 3.99 (d, 4H, CH2),
7.33–7.37 (m, 3H), 7.60–7.63 (d, 2H). δC (100 MHz, CDCl3)
29.1 (CH3), 44.9 (CH2, DEPT-135), 60.3 (Me3-C), 71.9 (C4),
126.1, 128.7, 129.4, 130.9, 159.4 (C3), 165.1 (C5), 173.8 (C6).
(HRMS (ESI, CH3OH): Found: 364.1995 [M + Na+]. Calc. for
C20H27N3NaO2: 364.1995.)
Ethyl 5,5-dimethyl-3-oxo-hexanoate was prepared by a modifi-
cation of the procedure described by Rathke.[22] Ethyl acetate
(7.5 mL, 0.075 mol, dried over 4 Å molecular sieves) was
added dropwise to a freshly prepared solution of lithium
bis(trimethylsilyl)amide (105 mL, 0.15 mol, 1.46 M in dryTHF)
at −78◦C under nitrogen.After 10 min, tert-butylacetyl chloride
(10.5 mL, 0.075 mol, prepared from the acid and SOCl2) was
added dropwise, with stirring. After 60 min, 20% aq. HCl was
cautiously added and the mixture was allowed to warm to room
temperature before diluting with ether (250 mL). The organic
phase was washed with saturated sodium carbonate (100 mL)
and water (100 mL), and dried over MgSO4. The solvent was
evaporated and the residue distilled, collecting the fraction boil-
ing at 55–56◦C (5 Pa). Yield: 11.5 g (83%). m/z 186 [M+•]. δH
(200 MHz, CDCl3) 0.96 (s, 9H, t-Bu), 1.25 (t, 3H, Et), 2.36 (s,
2H, CH2), 3.35 (s, 2H, CH2), 4.12 (q, 2H, Et). δC (50 MHz,
CDCl3) 14.1 (CH3), 29.7 (C6), 30.9 (C5), 51.1 (C2), 54.8 (C),
61.1 (CH2), 167.1 (C1), 201.0 (C3). νmax (neat)/cm−1 2957s,
1746s, 1716s, 1644m, 1366m, 1235s, 1034m (Found: C 64.89,
H 10.02. Calc. for C10H18O3: C 64.49, H 9.74%.)
3-(2,2-Dimethyl-propyl)isoxazol-5(4H)-one was prepared
from ethyl 5,5-dimethyl-3-oxo-hexanoate analogously to the
preparation of 3-phenylisoxazol-5(4H)-one;[13,21] bp 69–70◦C
(5 Pa), colourless oil. Yield 77%, mp 46–48◦C. m/z 155 [M+•].
δH (200 MHz, CDCl3) 0.99 (s, 9H, t-Bu), 2.32 (s, 2H, CH2), 3.40
(s, 2H, CH2). δC (50 MHz) 29.6 (CH3), 31.1 (C), 37.7 (CH2),
42.5 (C4), 165.5 (C3), 175.4 (C5). νmax (KBr)/cm−1 1740m
(Found: C 61.64, H 8.84, N 8.79. Calc. for C8H13 NO2: C 61.91,
H 8.44, N 9.03%.)
Compound 21 was prepared from 3-(2,2-dimethyl-propyl)
isoxazol-5(4H)-one in analogy with 12.[13] Chromatography on
silica gel (eluent CH2Cl2) afforded 3.91 g of a yellow oil (68%),
mp 45◦C. δH (200 MHz, CDCl3) 0.99 (s, 9H, t-Bu), 1.35 (d, 12H,
CH3), 2.89 (s, 2H, CH2), 4.00 (m, 2H, CH). δC (50 MHz, CDCl3)
22.8 (CH3), 29.7 (t-Bu), 32.4 (C), 39.7 (CH2), 43.3 (CH), 113.1
(C4), 159.6 (C3), 167.4 (C5), 177.4 (CS2). δC (DEPT-135) 22.8
(CH3), 29.7, 39.7, 43.3. νmax (KBr)/cm−1 2960s, 1728s, 1488s,
1367m, 1129m, 1040m, 921m, 867m. m/z 315 (66%) [M+•], 272
(91), 240 (33), 230 (43), 216 (24), 198 (100), 174 (39), 157 (32),
142 (14), 128 (16), 99 (13), 71 (50), 57 (51), 43 (91) (Found: C
57.26, H 8.17, N 4.24, S 20.27. Calc. for C15H25NO2S2: C 57.11,
H 7.99, N 4.44, S 20.33%.)
4-[Bis((1-methylethyl)thio)methylene]-3-(4-cyanophenyl)-
isoxazol-5(4H)-one 20
4ꢀ-Aminoacetophenone (27 g, 0.2 mol) was suspended in 6 M
HCl (80 mL) and diazotized with 2.5 M sodium nitrite (80 mL,
0.2 mol) at 0◦C. The yellow resulting diazonium solution was
added in small portions to copper cyanide (0.2 mol) dissolved
in 4.5 M sodium cyanide (45 mL; caution) in a large beaker
(foaming), maintaining the temperature below 10◦C. After com-
plete addition, the mixture was heated to 60◦C for 10 min and
cooled to room temperature again. It was then extracted with
ether (5 × 200 mL), washed with water (500 mL), dried over
MgSO4, and evaporated to dryness. The excess cyanide in
the aqueous phase was destroyed with cold bleach (caution).
Recrystallization from ether/CH2Cl2 gave 18.6 g (72%) of
4ꢀ-cyanoacetophenone, mp 58◦C.
To a suspension of sodium hydride (2.3 g, 0.1 mol) in dry
DME (20 mL) and diethylcarbonate (10 mL, 0.08 mol) was
added a solution of 4ꢀ-cyanoacetophenone (9.6 g, 0.066 mol) in
DME (40 mL) at room temperature while stirring under nitrogen.
After refluxing for 30 h, the mixture was poured onto a mixture
of ice (500 g) and conc. HCl (10 mL). The resulting solid was
collected and recrystallized from diethyl ether/ethyl acetate to
give 11.80 g (82%) of 4-cyanobenzoylacetate, mp 67–68◦C, bp
162◦C (70 Pa). m/z (EI+) 217 [M+•]. νmax (neat)/cm−1 2210m.
δH (200 MHz, CDCl3) 1.32 (t, 3J 7, 3H), 3.98 (s, 2H, keto form
30%), 4.26 (q, 3J 7, 2H), 5.70 (s, 1H, enol form 70%), 7.68 (d,
3J 8.8, 2H), 7.84 (d, 3J 8.8, 2H) (Found: C 66.46, H 5.15, N 6.36.
Calc. for C12H11NO3: C 66.35, H 5.10, N 6.45%.)
3-(4-Cyanophenyl)isoxazol-5(4H)-one was prepared from
4-cyanobenzoylacetate by adaptation of the method for 3-
phenylisoxazol-5(4H)-one.[13,21] Yield 3.9 g (69%), white crys-
tals, mp 155–157◦C. Subl. 100◦C (10−2 Pa). m/z (EI+) 186
Crystallography
[M+•]. δH (200 MHz, CDCl3) 3.79 (s, 2H), 7.47 (d, J 8, 2H),
Intensity data for compounds 12, 14–18, 18·1/2(C4H8O2), and
19 were measured on an Oxford Diffraction Gemini S Ultra CCD
diffractometer. Data were corrected for Lorentz and polariza-
tion effects and a MULTAN absorption correction was applied
(when necessary) using multiple symmetry equivalent reflec-
tions using the CrysAlis program (Oxford Diffraction). Data for
3
7.65 (d, 3J 8, 2H). νmax (KBr)/cm−1 2929w, 2232m, 1797s,
1167m, 877m, 845m.
Compound 20waspreparedfrom3-(4-cyanophenyl)isoxazol-
5(4H)-one by the method described for 12.[14] Yield 4.3 g (80%),
mp 115◦C. m/z 346 (34%) [M+•], 303 (11), 229 (76), 185 (15),

RESEARCH FRONT
Highly Twisted C=C Double Bonds
1077
compound 13 were measured on an Enraf-Nonius CAD4 four
circle diffractometer and data reduction and empirical absorp-
tion correction (ψ-scans) were carried out within the WinGX
package.[23] Monochromated MoKα radiation (λ 0.71073 Å) was
used in each case. All structures were solved by direct meth-
ods and refined on F2 using SHELX-97.[24] All non-hydrogen
atoms were refined anisotropically.Alkyl and aryl H-atoms were
included at calculated positions using a riding model whilst
hydrogens bound to nitrogen atoms were first identified from
difference Fourier maps then constrained using a riding model.
Figs 1 and 2 were produced with ORTEP3[25] while Fig. 3 was
drawn with PLUTON.[26]
c 7.1127(2) Å, V 1782.8(1) Å3, Dc (Z 4) 1.175 g cm−3, F(000)
680, µ(MoKα) 0.078 mm−1, 4956 reflections, 1644 unique (Rint
0.0364, 2θmax 50◦), 944 with I > 2σ(I); R 0.0386 (obs. data),
wR2 0.0905 (all data), goodness of fit 0.853.
Compound 19: C20H27N3O2, M 341.45,T 293 K, orthorhom-
bic, space group Pn21a (No. 33), a 13.1416(8) Å, b 13.511(1) Å,
c 10.6856(6) Å, V 1897.3(2) Å3, Dc (Z 4) 1.195 g cm−3, F(000)
736, µ(MoKα) 0.078 mm−1, 4845 reflections, 2415 unique (Rint
0.0373, 2θmax 50◦), 1194 with I > 2σ(I); R 0.0393 (obs. data),
wR2 0.0876 (all data), goodness of fit 0.775.
Crystallographic data in CIF format have been deposited with
the Cambridge Crystallographic Data Centre with CCDC ref-
erence numbers 734489–734493 (compounds 12–17), 734494
(18·1/2(C4H8O2)), 736771 (18), and 742119 (19). Copies of
the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)
1223-336-033, Email: deposit@ccdc.cam.ac.uk).
Crystal Data
Compound 12: C12H11NO2S2, M 265.34, T 293 K, monoclinic,
space group P21/n (No. 14, variant of P21/c), a 8.9736(5) Å,
b 8.1893(4) Å, c 16.8007(8) Å, β 98.026(5)◦,V 1222.6(1) Å3, Dc
(Z 4) 1.442 g cm−3, F(000) 552, µ(MoKα) 0.423 mm−1, 5850
reflections, 2140 unique (Rint 0.0281, 2θmax 50◦), 1576 with
I > 2σ(I); R 0.0325 (obs. data), wR2 0.0751 (all data), goodness
of fit 0.926.
Accessory Publication
1
Details of H NMR coalescence measurements, determination
of activation parameters, Cartesian coordinates and energies
for calculated species, and details of NBO population analysis
(Table S1) are available on the Journal’s website.
Compound 13: C16H19NO2S2, M 321.44, T 293 K, mono-
clinic, space group P21/c (No. 14), a 8.451(1) Å, b 19.192(3) Å,
c 11.081(1) Å, β 110.48(1)◦, V 1683.6(4) Å3, Dc (Z 4)
1.268 g cm−3, F(000) 680, µ(MoKα) 0.319 mm−1, 3165 reflec-
tions, 2955 unique (Rint 0.0285, 2θmax 50◦), 2243 with I > 2σ(I);
R 0.0399 (obs. data), wR2 0.1154 (all data), goodness of fit 1.051.
Compound 14: C12H12N2O2S, M 248.3, T 293 K, mono-
clinic, space group P21/n (No. 14, variant of P21/c), a
8.658(1) Å, b 9.661(1) Å, c 14.264(2) Å, β 97.32(1)◦, V
Acknowledgement
This research was supported by the Australian Research Council. R.K.
gratefully acknowledges generous allocation of computer time at the CSC
Oldenburg.
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