Mono- and di-alkyl-[1,3,5]-dithiazinanes and their N-borane adducts revisited. Structural and theoretical study

Article abstract of DOI:10.1016/j.molstruc.2013.04.021

Structural analyses of a series of 5-alkyl-[1,3,5]-dithiazinanes [R = Me (1), iPr (2), tBu (3)], their N-BH3 adducts (1BH3-3BH3) and their 2-alkyl (R0) derivatives are reported: R = Me, R0 = Me (7); R = Me, R0 iPr (8); R = iPr, R0 =Me (10); R = tBu, R0 =Me (11); and R = Me, R0 = nBu (12). The reaction of 2-lithium-5-methyl-[1,3,5]-dithiazinane (4) with I2 affords the bis-(5-methyl-[1,3,5]-dithiazinan-2-yl) (13). Isostructural compounds: [2,5,5]-trimethyl-[1,3,5]-dithiazinan-5-ium iodide (14), 5-borane-2,5-dimethyl- [1,3,5]-dithiazinane (15) and 2,5,5-trimethyl-[1,3,5,6]-dithiazaborata (16) are compared. Structures of 7, 8 and 10-13 were determined by 11B, 13C and 1H NMR and the X-ray diffraction analyses of 2, 1BH3, 13 and 14 are reported. Optimization of two chair conformers of heterocycles 1-3, 1BH3-3BH3, 7, 9 (R = Me, R0 = tBu), 13-16 were performed by HF/6-31++G and B3LYP/6-31G(d,p) methods and their minimum energy is compared. 2-Alkyl substituents or N-BH3 anchor the [1,3,5]-dithiazinane ring conformation allowing the analyses of steric and electronic interactions as well as the lone pairs effect in these molecules.

Full text of DOI:10.1016/j.molstruc.2013.04.021

Journal of Molecular Structure 1047 (2013) 149–159
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Mono- and di-alkyl-[1,3,5]-dithiazinanes and their N–borane adducts
revisited. Structural and theoretical study
a
b
a
Angelina Flores-Parra a, , Carlos Guadarrama-Pérez , Juan Carlos Gálvez Ruiz , Sonia A. Sánchez Ruiz ,
⇑
Galdina V. Suarez-Moreno a, Rosalinda Contreras a
a Departamento de Química, Centro de Investigación y de Estudios Avanzados (Cinvestav), A.P. 14-740, México, DF 07000, Mexico
b Departamento de Ciencias Químico-Biológicas, Universidad de Sonora, Hermosillo, Sonora, Mexico
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
A series of 2,5-dialkyl-[1,3,5]-dithiazinanes derivatives is reported.
Structures were determined by 11B, 13C and 1H NMR and the X-ray diffraction analyses.
Optimization of chair conformers were performed by ab-initio methods and their minimum energy is compared.
2-R or N-BH3 anchor the ring conformation allowing the analyses of steric and electronic interactions and the lone pairs effect.
a r t i c l e i n f o
a b s t r a c t
Article history:
Structural analyses of a series of 5-alkyl-[1,3,5]-dithiazinanes [R = Me (1), iPr (2), tBu (3)], their N–BH3
adducts (1BH3–3BH3) and their 2-alkyl (R0) derivatives are reported: R = Me, R0 = Me (7); R = Me, R0 iPr
(8); R = iPr, R0 = Me (10); R = tBu, R0 = Me (11); and R = Me, R0 = nBu (12). The reaction of 2-lithium-5-
methyl-[1,3,5]-dithiazinane (4) with I2 affords the bis-(5-methyl-[1,3,5]-dithiazinan-2-yl) (13). Isostruc-
tural compounds: [2,5,5]-trimethyl-[1,3,5]-dithiazinan-5-ium iodide (14), 5-borane-2,5-dimethyl-
[1,3,5]-dithiazinane (15) and 2,5,5-trimethyl-[1,3,5,6]-dithiazaborata (16) are compared. Structures of
7, 8 and 10–13 were determined by 11B, 13C and 1H NMR and the X-ray diffraction analyses of 2, 1BH3,
13 and 14 are reported. Optimization of two chair conformers of heterocycles 1–3, 1BH3–3BH3, 7, 9
(R = Me, R0 = tBu), 13–16 were performed by HF/6-31++G and B3LYP/6-31G(d,p) methods and their min-
imum energy is compared. 2-Alkyl substituents or N–BH3 anchor the [1,3,5]-dithiazinane ring conforma-
tion allowing the analyses of steric and electronic interactions as well as the lone pairs effect in these
molecules.
Received 26 February 2013
Received in revised form 9 April 2013
Accepted 10 April 2013
Available online 18 April 2013
Keywords:
2,5-Dialkyl-[1,3,5]-dithiazinanes
N–BH3-5-alkyl-[1,3,5]-dithiazinanes
X-ray diffraction analyses
Theoretical analysis of conformers
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
31++G] in compounds 1–3 and 1BH3–3BH3 reported here afforded
useful information for better understanding the structure and reac-
We are currently working on the chemistry of [1,3,5]-heterocy-
clohexanes [1,2] such as [1,3,5]-dithiazinanes 1–3 [3,4], Scheme 1.
These compounds can act as ligands for metal atoms by coordina-
tion of the heteroatoms lone pairs [5–7]. They react with borane to
give N–BH3 adducts [8] and can also be used as building blocks in
tripodal derivatives [9] or in branched chains [10].
In continuation of our studies, here we firstly report the revised
structural analysis of compounds 1–3 [3,4] and of their N–borane
adducts (1BH3–3BH3) [8], Scheme 1. These compounds are known,
however the X-ray diffraction analyses of these N–BH3 adducts
have not been reported. Crystalline compounds 2 and 1BH3 were
analyzed by X-ray diffraction. Ab initio theoretical studies [HF/6-
tivity of 5-alkyl-[1,3,5]-dithiazinanes and their adducts. Secondly,
we report a series of [1,3,5]-dithiazinanes where one of the SCH2S
protons was substituted by an alkyl group (7, [11] 8, 10–14),
Schemes 2 and 3. A motivation for the structural analysis of these
2,5-dialkyl-[1,3,5]-dithiazinanes is their anchored rings, which al-
low to investigate the effect of sulfur lone pairs in the 1H NMR
spectra and sulfur weak interactions in conformational equilibrium
[5,6,12–17]. Our attempts to synthesize the C-2 tertbutyl deriva-
tives were unsuccessful, however the minimum energy structure
of 2-tertbutyl-5-methyl-[1,3,5]-dithiazinane (9) was calculated
and compared with that of compounds 7, 8, 10 and 11, Scheme 2.
In the third part of this paper we review the isostructural com-
pounds: [2,5,5]-trimethyl-[1,3,5]-dithiazinan-5-ium iodide (14),
5-BH3-2,5-dimethyl-[1,3,5]-dithiazinane (15) and 2,5,5-trimethyl-
[1,3,5,6]-dithiazaborata (16), and report the X-ray diffraction
⇑
Corresponding author.
E-mail address: aflores@cinvestav.mx (A. Flores-Parra).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.04.021

150
A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
R
R
N
S
S
N
R
3S
N5
BH3-SMe2
4
S
S
S
BH3
2
1
6
1BH3 R = Me
1 R = Me
iPr
2BH3 R =
3BH3 R =
iPr
2 R =
3 R =
tBu
tBu
Scheme 1. Nitrogen conformers of 5-R-[1,3,5]-dithiazinanes 1–3 and their adducts 1BH3–3BH3.
analyses of 14, Schemes 3 and 4. Calculations of compounds 14–16
are discussed and compared with those of [1,3,5]-dithiazinanes
having a free nitrogen lone pair, in order to acquire information
about the electronic density and charge distribution in the ring.
refinement for all structures (based on F2 of all data) was per-
formed by full matrix least-squares techniques with crystals
12.84 [20]. All non-hydrogen atoms were refined anisotropically.
Crystallographic data have been deposited at the Cambridge Crys-
tallographic Data Centre with numbers: 1BH3 (921690),
2
(921691), 13 (921692), 14 (921693). Copies can be obtained, free
of charge, on applications to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: +44 (0)1223 336033 or www.ccdc.cam.ac.uk/
products/csd/request/].
2. Experimental
2.1. General
Reagents were purchased from Sigma–Aldrich Chemical, Fluka
Chemika and Strem Chemical, and were used without purification.
Vacuum line techniques were employed for all manipulations with
air and moisture sensitive compounds. THF was dried by distilla-
tion from sodium–benzophenone under a nitrogen atmosphere
prior to use. Dry CDCl3, and THF-d8, were purchased from Aldrich
and used without further purification. Compounds 1–3 [3], 1BH3
[8], 4, 7 and 14–16 [11] were synthesized according to the
literature.
2.3. Calculation analysis
Calculations were performed in order to obtain the molecular
geometries using the Gaussian 98 package [21]. Geometries were
checked to be the minimal by the frequency analysis.
2.4. Synthesis of the compounds
2.4.1. 5-isoPropyl-[1,3,5]-dithiazinane 2
Melting points were obtained on a Mel-Temp II apparatus and
are uncorrected. Mass spectra in the EI mode were recorded at
20 eV on a Hewlett–Packard HP 5989A spectrometer. High resolu-
tion mass spectra were obtained by LC/MSD TOF on an Agilent
Technologies instrument with ESI as ionization source. Elemental
analyses were performed on Flash (EA) 1112 series, equipment.
NMR spectra were obtained on a Jeol GSX-270, Jeol Eclipse
To isopropylamine (7.3 mL, 84.6 mmol), water (50 mL) was
added and cooled at 0 °C, then a mixture of NaSH 95% (15.0 g,
253 mmol) and 37% aq. H2CO (31.5 mL, 423 mmol) was slowly
added. The reaction mixture was stirred for 24 h at rt. The solids
formed were filtered, washed with distilled water, and dissolved
in CH2Cl2. The CH2Cl2 solution was dried with Na2SO4, filtered,
and the solvent evaporated. Compound 2 is a crystalline colorless
solid (9.71 g, 70%). Mp 40 °C. NMR (CDCl3, 25 °C, d ppm) 1H, iPr
group: 3.78 (hept, 3J 6.5 Hz, 1H, CH), 1.14 (d, 6H, 3J 6.5 Hz, CH3).
NMR (THF-d8, ꢁ95 °C, d ppm) 13C: 34.0 (C2), 56.4 (C4/C6), iPr
group: 45.3 (CH), 20.7 (CH3). (+)TOF calcd. for (C6H14NS2)+, m/z
(uma): 164.0567; found 164.0563.
400 MHz and Bruker Avance 300 MHz. 7Li [
N
10.3976, LiCl], 1H,
13C [
N
25.145020].
2.2. X-ray crystallography
Crystallographic data were measured on a Nonius Kappa CCD
instrument with a CCD area detector using graphite-monochro-
mated Mo K
a
radiation. Intensities were measured using
u
+
x
2.4.2. 2-Lithium-5-isopropyl-[1,3,5]-dithiazinane (5)
scans. Crystal data and selected bond lengths and angles are pre-
sented in Tables 2, 3 and 5. The structures were solved using direct
methods with SHELX-97 [18], Sir 2002 and Sir 2004 [19]. The
Compound 2 (0.3 g, 1.85 mmol) was dissolved in THF (50 mL)
and cooled at ꢁ78 °C, then 1 M tBuLi solution in hexane
(1.85 mL, 1.85 mmol) was added at ꢁ78 °C. The mixture was
R
THF
Li
THF
S
S
N
nBuLi, THF
Y
Y
Li
N
R
S
1-3
S
THF
THF
4 R = Me, Y= lone pair
iPr, Y= lone pair
tBu, Y= lone pair
5 R =
6 R =
R'Cl
THF
Me
R
7
8
R' = Me, R = Me
R' =
iPr, R = Me
S
N
S
S
N
iPr
10 R' = Me, R =
11 R' = Me, R =
S
tBu
R´
tBu
9
Scheme 2. Synthesis of compounds 4–8, 10, 11. Compound 9 was calculated.

A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
151
Me
Me
Me
I2, THF
- 2 LiI
nBuLi exc
4
S
S
N
S
S
S
S
N
N
MeI
THF
nBu
MeIexc
THF
12
13
Me
-
I
+
S
N
Me
S
Me
14
Scheme 3. Synthesis of compounds 12–13 from organolithium 4.
75%). NMR (CDCl3, 25 °C, d ppm) 1H, Me group: 1.55 (d, 3J 6.9,
3H, CH3), tBu group: 1.32 (s, 9H, CH3). 13C: 45.9 (C2), 55.2 (C4/
C6), 30.2 (CH3), tBu group: 55.9 (C), 20.6 (3CH3). MS (20 eV) m/z
(%) 57.00(100), 75.00(79), 98.15(67), 135.05(36), 191.20(75).
(+)TOF calcd. for (C8H18NS2)+, m/z (amu): 192.0880; found
192.0886. Anal. calcd. for (C8H17NS2ꢃ1/2CH2Cl2): C, 43.68; H,
7.77; N, 6.00. Found: C, 44.03; H, 8.02; N, 5.92.
Me
Me
H
S
S
N
S
N
BH3
Me
Me
Me
B
H
S
16
15
Scheme 4. Compounds 15 and 16.
2.4.7. 2-nButyl-5-methyl-[1,3,5]-dithiazinane (12)
A solution of compound 1 (0.2 g, 1.5 mmol) in dried THF
(100 mL) was cooled at ꢁ78 °C, then a 1 M solution of nBuLi in hex-
ane (3.1 mL, 3.1 mmol) was slowly added. After 30 min stirring at
ꢁ78 °C, MeI (8.5 mL, 1.5 mmol) was added and left for 1 h at
0 °C. The solvent was evaporated under vacuum and the product
dissolved in CH2Cl2 (20 mL), washed with water (3 ꢂ 10 mL). The
organic phase was dried with anhydrous Na2SO4, filtered and the
solvent evaporated under vacuum to give an extremely stink vis-
cous liquid. It was distilled (0.25 mmHg, 75 °C) and compound
12 was obtained (0.23 g, 80%). NMR (CDCl3, 25 °C, d ppm) 1H, Me
group: 2.57 (s, 3H, CH3), nBu group: 1.75 (m, 2H, CH2), 1.48 (m,
2H, CH2), 1.32 (m, 2H, CH2), 0.90 (t, 3J 6.7, 3H, CH3). 13C: 36.8
(C2), 60.5 (C4/C6), 37.4 (C7), nBu group: 50.7 (CH2), 27.7 (CH2),
22.4 (2CH2), 14.0 (CH3). (+)TOF calcd. for (C8H18NS2)+, m/z (amu):
192.0875; found 192.0875. Anal. calcd. for (C8H17NS2): C, 50.21;
H, 8.95; N, 7.32. Found: C, 49.72; H, 8.51; N, 7.18.
stirred for 30 min. The lithium compound 5 was formed quantita-
tively and used in situ. NMR (THF-d8, 25 °C, d ppm) 1H, iPr group:
3.53 (hept, 3J 6.4, 1H, CH), 1.10 (d, 3J 6.4, 6H, CH3). 13C: 44.6 (C2),
57.3 (C4/C6), iPr group: 45.1 (CH), 21.1 (2CH3). d7Li: ꢁ1.71.
2.4.3. 5-tertButyl-2-lithium-[1,3,5]-dithiazinane (6)
Compound 6 was prepared following the same procedure as 5
from compound 3 (0.33 g, 1.85 mmol) and 1 M tBuLi solution in
hexane (1.86 mL, 1,86 mmol). NMR (THF-d8, 25 °C, d ppm) 1H,
tBu group: 1.29 (s, 9H, CH3). 13C: 34.9 (C2), 56.8 (C4/C6), tBu group:
55.0 (C), 29.5 (3CH3). d7Li: ꢁ1.93.
2.4.4. 2-isoPropyl-5-methyl-[1,3,5]-dithiazinane (8)
Following the procedure described in 5, to a solution of com-
pound 1 (0.5 g, 3.7 mmol)], 2-iodo-propane (0.4 mL, 3.7 mmol)
was slowly added and the mixture stirred for 15 min. Water
(10 mL) was added to the reaction mixture and the product ex-
tracted with CH2Cl2 (3 ꢂ 15 mL). The CH2Cl2 solution was dried
with Na2SO4 the solvent evaporated. Compound 8 is a viscous li-
quid (0.6 g, 90%). NMR (CDCl3, 27 °C, d ppm) 1H, Me group: 2.65
(s, 3H, CH3). 13C: 34.7 (C2), 60.6 (C4/C6), 37.4 (C7), iPr group:
59.2 (CH), 19.5 (2CH3). (+)TOF calcd. for (C7H16NS2)+, m/z (amu):
178.0724; found 178.0725. Anal. calcd. for (C7H15NS2): C, 47.44;
H, 8.54; N, 7.91. Found: C, 47.08; H, 8.69; N, 7.61.
2.4.8. Bis[5-methyl-[1,3,5]-dithiazin-2-yl] (13)
Toa solution of compound1 (0.2 g, 1.5 mmol)]inTHF (20 mL), 1 M
tBuLi solution in hexane (1.6 mmol, 1.6 mL) at ꢁ78 °C a solution of I2
(1.6 mmol) in THF (20 mL) was added. The resulting mixture was stir-
red for 15 min at rt, then water was added (2 mL) and the solvent
evaporated in vacuum. The residue was dissolved in CH2Cl2
(20 mL),washedwith water(3 ꢂ 10 mL). The organicphase wasdried
with anhydrous Na2SO4, filtered and the solvent evaporated in vac-
uum. Beige crystals of 13 were obtained (0.14 g, 72%). Mp 120 °C.
NMR (CDCl3, 25 °C, d ppm) 1H, Me group: 2.68 (s, 3H, CH3). 13C: 55.8
(C2), 61.2 (C4/C6), 37.6 (C7). (+)TOF calcd. for (C8H17N2S4)+, m/z
(uma): 269.027; found 269.0263. Anal. calcd. for (C8H16N2S4): C,
35.82; H, 6.02; N, 10.45. Found: C, 35.96; H, 5.83; N, 10.68.
2.4.5. 5-isoPropyl-2-methyl-[1,3,5]-dithiazinane (10)
Compound 10 was prepared from a solution compound 2
(0.12 g, 0.74 mmol] and MeI (4.3 mL, 1.5 mmol) following the pro-
cedure for 8. Compound 10 is a yellow viscous liquid (0.11 g, 80%).
NMR (CDCl3, 25 °C, d ppm) 1H, Me group: 1.44 (d, 3J 6.9, 3H, CH3),
iPr group: 3.53 (hept, 3J 6.4, 1H, CH), 1.10 (d, 3J 6.4, 6H, CH3). 13C:
44.6 (C2), 57.3 (C4/C6), 30.0 (CH3), iPr group: 45.1 (CH), 21.1
(2CH3). Anal. calcd. for (C7H15NS2): C, 47.41; H, 8.53; N, 7.90.
Found: C, 47.23; H, 8.48; N, 7.65.
3. Results and discussion
3.1. Heterocycles 1–3 and their N–BH3 adducts 1BH3–3BH3
2.4.6. 5-tertButyl-2-methyl-[1,3,5]-dithiazinane (11)
Compound 11 was prepared following the general procedure for
9 from a solution of compound 3 (0.13 g, 0.74 mmol)] and MeI
(5.8 mL, 1.5 mmol). Compound 11 is a viscous liquid (0.11 g,
3.1.1. Theoretical studies
The ring of compounds 1–3 is in conformational equilibrium in
solution at room temperature, whereas the N-alkyl group is

152
A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
maintained in axial position. The preferred nitrogen conformation
is attributed to the electronic repulsion between the sulfur and
nitrogen lone pairs. Our HF/6-31++G calculations indicate that con-
formers with alkyl group in axial position are more stable than
those with the alkyl in equatorial position by 37.0 (1), 34.3 (2)
and 55.0 (3) kJ/mol, Scheme 1.
BH3
N
R
N
S
S
S
S
R
BH3
Scheme 6. Chair conformers of compounds 1BH3 to 3BH3.
As we will discuss later, the C–S–C angles determined by X-ray
diffraction analyses in compounds 1–3 indicate that sulfur atoms
are partially sp3 hybridized [ꢄ40% (1) [3], ꢄ36% (2), ꢄ33% (3)
[3]]. It means that sulfur lone pairs occupy orbitals with sp3 partial
hybridization with incipient lobes in equatorial and axial positions.
The calculated electrostatic potential surface calculated for com-
pounds 1, 1BH3, 7, 14–16 by B3LYP/6-31++G(d,p) (mesh value
0.0004), is found as Supplementary material.
Reactions of compounds 1–3 with BH3–SMe2 give the corre-
sponding N-adducts (1BH3–3BH3) [4]. Compound 1BH3 is stable
in the solid state under anhydrous conditions at low temperature,
and in solution of THF for a few days. Compound 2BH3, when
standing in dry THF solution and at low temperature, is trans-
formed in a few hours into the borata heterocycle and then, into
other reduced fragments, Scheme 5. 3BH3 is so unstable that in
cool dry THF is reduced in a few minutes [4]. The different stability
of compounds 1BH3–3BH3 is attributed to the growing steric hin-
drance of the N-substituent.
Fig. 1. Superposition of the structure of compound 2 obtained by X-ray diffraction
analyses (gray), with its modeled structure (black).
According to NMR spectra, the adduct 1BH3 is in a preferred
conformation, in solution at room temperature, with the methyl
group in axial position and the BH3 in equatorial position [4]. The
ab initio calculations (HF 6-31++G) of the energy difference be-
tween the two chair conformers of compounds 1BH3–3BH3 confirm
that the preferred conformation of the molecules with the borane
rial (1.65 ppm) and the axial (1.20 ppm) protons chemical shifts.
The difference between equatorial and axial signal has been attrib-
uted to hyperconjugative interactions between antiperiplanar C–H
bonds [24,25], which also provokes the enlargement of C–Hax
bonds [26]. The 1H NMR chemical shifts of axial and equatorial pro-
tons in 5-methyl-[1,3,5]-dithiazinane (1) (270 MHz, CDCl3, ꢁ80 °C)
show an anchored conformation. The equatorial sulfur and nitro-
gen lone pairs produce a notorious gauche effect over the axial
and equatorial methylene protons, shifting them to higher fre-
quencies with respect to those of cyclohexane [27,28], Table 2. In
addition, the antiperiplanar effect of the axial sulfur lone pairs
shifts the axial methylene protons ꢄ1 ppm to higher frequencies
with respect to the equatorial ones [8].
The nitrogen lone pair effect on the chemical shifts of the meth-
ylene protons in [1,3,5]-dithiazinanes can be deduced when they
are compared with the low temperature 1H NMR spectrum of
dithiane, where the methylene protons appear overlapped [29], Ta-
ble 2. The 1H NMR spectrum of 1BH3 indicates that the N–BH3
coordination diminishes the N-lone pair effect over the axial pro-
tons which are shifted ꢄ0.5 ppm to lower frequencies with respect
to those of the free heterocycle 1.
in equatorial is the most stable [DE = 42.3 (1BH3); 49.2 (2BH3) and
26.8 (3BH3) kJ/mol], Scheme 6. The superposition of the structure
found by X-ray diffraction analysis of compound 1BH3 with the cal-
culated structure is shown in Fig. 1.
Calculated energy also means that methyl and isopropyl groups
have similar steric effects, due to the fact that the isopropyl group
in 2BH3eq is oriented with the C–H pointing towards the sulfur
atoms diminishing the steric effect between the isopropyl group
and the sulfur atoms, Fig. 2a. As expected 3BH3 with a tertbutyl
group shows a smaller energy difference between both chair con-
formers due to an increased energy of conformer 3BH3eq by the
steric repulsion between the axial tBu group and the ring, Fig. 2b.
Additional stabilization for these conformations are provide by
the hydride–proton contacts [22,23].
The calculated dipole moment of 1BH3eq is 2.9 D, whereas for
1BH3ax is 6.4 D, which is in agreement with the highest stability
of the first one. Also, calculations indicate that the dipole moments
in N–BH3 adducts are inverted with respect of the free dithiazin-
anes, Scheme 7. The calculated NBO charges [6-31++G] for dithia-
zinanes 1 and 1BH3 are in Table 1. The charges indicate that C2
is more negative than C4 and C6. Sulfur atoms have partial positive
charges, whereas the nitrogen atom is partially negative.
3.1.3. X-ray diffraction analysis of compounds 2 and 1BH3
Compound 2 crystallized from CHCl3. Its X-ray diffraction struc-
ture depicts a heterocycle in a chair conformation with the N-
isopropyl group in axial position, Fig. 3, Tables 3–5. The nitrogen
is almost planar, due to the steric repulsion between the isopropyl
group and the ring. The nitrogen sp2 character (ꢄ75%) is deduced
from angles around the nitrogen atom: C4–N–C7 (115.9°), C6–N–
C7 (115.4°) and C4–N–C6 (110.2°). The value of the C–S–C angle
(97°) corresponds to sulfur atoms with ꢄ36% of sp3 character.
The C7–H proton distances to the sulfur atoms (2.96 and 2.94 Å)
are shorter than the reported distance of the highest incidence
for C–Hꢃ ꢃ ꢃS contacts 3.21 Å [30], these contacts could contribute
to stabilize the N-alkyl preferred axial position.
3.1.2. NMR analysis
The spectroscopic behavior in [1,3,5]-dithiazinanes is different
from that of cyclohexane. The 1H NMR spectra of deuterated cyclo-
hexane-D11 recorded at low temperature let recognize the equato-
R
N
R
S
S
S
S
N
CH3
BH3
The bond lengths of [1,3,5]-dithiazinane 2 merit some discus-
sion. Equatorial C–H bonds are shorter than axial ones, C2–H
[0.902(3) vs 0.935(3) Å], C4–H [0.852(2) vs 0.952(2) Å] and C6–H
[0.881(2) vs 0.971(2) Å], due to the strong electronic effect of
BH2
Scheme 5. Isomerization of 1BH3–3BH3 adducts [1 R = Me; 2 R = iPr, 3 R = tBu] into
borata heterocycles.

A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
153
Fig. 2. Calculated structures of minimum energy for compounds 2BH3 (a) and 3BH3 (b).
pair, Table 3. The cooperative proton–hydride interactions, con-
tribute to the anchored conformation of 1BH3 in solution, as can
be deduced from the short proton–hydride contacts [22,23]. These
distances are in the range of 2.33–2.52 Å shorter than the sum of
the van der Waals radii for a proton and a hydride (RvdW = 2.65 Å),
Fig. 4.
Me
Me
S
S
N
S
N
BH3
S
1BH
3 (2.89 D)
1 (2.41 D)
Scheme 7. Calculated [HF/6-31++G] dipole moments for compounds 1 and 1BH3.
3.2. Alkylation reactions
The interest of the substitution of a C2-proton by an alkyl group
was based on the ring anchorage effect which facilitates the study
of electronic and steric interactions. Also, it was relevant to analyze
if the carbon bound to C2 could have an interaction with the vicinal
sulfur atoms. The alkylation of compounds 1–3 was performed
through the reactions of the 2-lithium compounds 4–6 with alkyl
chlorides in THF, Scheme 2.
equatorial sulfur and nitrogen lone pairs, Table 4. Interactions
n ? rꢅ between equatorial lone pairs on the sulfur atoms and
C4–N5 or C6–N5 bonds explain that S–C4 [1.826(3) Å] and S–C6
[1.828(3) Å] bonds are longer than S–C2 bonds [1.785(2) and
1.793(3) Å], and C4–N [1.434(4) Å] and C6–N [1.438(2) Å] are
shorter than axial exocyclic N–C bond [1.482(4) Å] [11], Table 3.
Compound 1BH3 crystallized in anhydrous conditions from
CDCl3. The X-ray diffraction analysis shows the BH3 group in equa-
torial position, Fig. 4, Tables 3–5. The value of C–S–C angles (97.69°
and 98.76°) indicates ꢄ35–46% of sp3 hybridization of the sulfur
atoms. N–borane coordination diminishes the nitrogen lone pair
effect whereas that of the antiperiplanar sulfur lone pairs persists.
In compound 1BH3, the intracyclic C–N bonds are elongated with
respect to those of the free dithiazinane 1 [from 1.433(4) Å in 1
to 1.494(2) Å in 1BH3], due to the suppression of the nitrogen lone
3.2.1. Lithium compounds
Compound 4 has been previously reported by us [11], whereas 5
and 6 are new. It is assumed that lithium derivative 4–6 form di-
mers by two cooperative S ? Li coordination bonds, as it was
established in the mass spectrum of compound 4 [11]. The organo-
lithium compounds 4–6 were directly characterized from the reac-
tion mixtures after solvent evaporation by lithium NMR spectra 7Li
[d = ꢁ2.4 (4), ꢁ1.9 (5 and 6) ppm, THF-d8], Table 2. 1H and 13C NMR
Table 1
Calculated NBO charges [6-31++G].
R
R
1
1
S
S
S
N
N
4
BH
R
R
2
R
R
3
2
2
S
3
6
2
16
1, 1BH3, 14, 15
1
Me
H
H
Me
Me
Me
lone pair
BH3
1BH3 Me
14
15
16
Me
Me
Me
Me
BH3
Me
C2
S1,S2
C4,C6
N
R2(H/C)
H2ax
H4ax,H6ax
H4eq,H6eq
R1
R3
1
ꢁ0.73
ꢁ0.74
ꢁ0.53
ꢁ0.53
ꢁ0.51
+0.11
+0.18
+0.27
+0.18
ꢁ0.31
ꢁ0.34
ꢁ0.38
ꢁ0.34
ꢁ0.60
ꢁ0.58
ꢁ0.44
ꢁ0.58
ꢁ0.60
+0.27
+0.28
ꢁ0.66
ꢁ0.65
ꢁ0.65
+0.25
+0.25
+0.27
+0.26
+0.25
+0.25
+0.26
+0.26
+0.26
+0.24
+0.27
+0.28
+0.27
ꢁ0.40
ꢁ0.40
ꢁ0.40
ꢁ0.40
ꢁ0.40
–
1BH3
14
15
ꢁ0.03
ꢁ0.40
ꢁ0.03
ꢁ0.40
16
+0.17S(B) ꢁ0.07
ꢁ0.36B + 0.06
+0.26H(B) ꢁ0.05
+0.25H(B) ꢁ0.05

154
A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
Table 2
1H and 13C NMR (CDCl3) data for 1BH3, 1–8, 10–16: d in ppm and (nJ(1H-1H) in Hz.
R
1
R
1
S
S
N
4
S
S
N
R
R
BH
2
2
3
R
R
6
2
3
2
1BH3, 1-8, 10-15
16
Cpd
R1
R2
R3
H2eq
H2ax
H4eq, H6eq
H4ax,H6ax
Dithianea
–
Me
–
H
–
3.81(m)
3.56
3.81(m)
4.60
2.86(m)
3.93
2.86(m)
4.95
1b,c
L pair
(dt, 2J 13.3, 4J 2.6)
3.40
(d, 2J 13.3)
4.05
(dt, 2J 12.6, 4J 2.6)
3.85
(d, 2J 12.6)
4.37
1BH3
2b,d
Me
iPr
H
H
H
BH3
(dt, 2J 13.9, 4J 2.0)
3.64
(d, 2J 13.9)
4.65
(dt, 2J 13.2, 4J 2.0)
4.29
(d, 2J 13.2)
4.80
L pair
L pair
(br d, 2J 13.9)
3.68(br s)
R0(7Li)
(d, 2J 13.9)
4.72(br s)
(d, 2J 13.0)
4.78(br s)
(d, 2J 13.0)
4.55(br s)
3b,d
tBu
4
5
6
Me
iPr
tBu
Li
Li
Li
I pair
I pair
I pair
-2.4(s)
-1.9(s)
-1.9(s)
R0(13C)
22.8
59.2
21.1
20.6
50.7
55.8
18.5
22.1
19.8
3.31(s)
3.44(s)
3.62(s)
3.22(d)
4.53(d)
4.39(d, 2J 13.2)
3.90(d, 2J 12.6)
4.55(d, 2J 13.2)
4.25(d, 2J 12.6)
7
8
Me
Me
iPr
tBu
Me
Me
Me
Me
Me
Me
iPr
Me
Me
nBu
C4H3NS2
Me
Me
Me
I pair
I pair
I pair
I pair
I pair
I pair
Me
4.11(q, 3J 6.7)
4.21(d, 3J 5.5)
4.28(q, 3J 6.9)
4.33(q, 3J 6.9)
4.41 (br s)
3.96(d, 2J 12.2)
4.07(d, 2J 13.3)c
4.39(d, 2J 13.0)
4.59(d, 2J 13.3)
4.08(d, 2J 13.5)
4.18(d, 2J 13.9)
4.79(br s)
4.69(d, 2J 12.2)
4.65(d, 2J 13.3)
4.49(d, 2J 13.0)
4.65(d, 2J 13.3)
4.67(d, 2J 13.5)
4.68(d, 2J 13.9)
4.79(br s)
10
11
12
13
14
15e
16b
4.61(s)
5.30(br s)
BH3
Me
4.10(q, 3J 6.8)
4.05(q, 3J 6.5)
3.58(d, 2J 12.4)
3.60(d, 2J 13.9)
4.47(d, 2J 12.4)
4.22(d, 2J 13.9)
a
b
c
Ref. [1].
In THF-d8.
At ꢁ80 °C.
At ꢁ95 °C.
Ref. [25].
d
e
reactions of 5 and 6 with methyl iodide afforded compounds 10
and 11, Scheme 2. Compound 9 with a tertbutyl group at C2 was
calculated, Fig. 6. Its synthesis, in our hands, was unsuccessful,
probably due to steric hindrance. When lithium compound 4 was
treated with 3 equivalents of nBuLi followed by one of methyl io-
dide in THF, a liquid compound 12, with an nBu group at C2, was
obtained in 80%, Scheme 3. The formation of 12 results from the
exchange reaction between MeI and nBuLi which affords nBuI
and MeLi [32]. Then, reaction of nBuI with lithium compound 4
gives 12. This compound has an extremely unpleasant odor. Com-
pounds 7, 8, 10–12 are liquids. The calculated structure of 7 is in
Fig. 6.
Reaction of the lithium compound 4 with one equivalent of I2 in
THF produces a coupling reaction, leading compound 13 in 72%. A
crystal of compound 13 was submitted to X-ray diffraction analy-
sis. The new compound has two coupled dithiazinanes through the
C2 atoms. Its central ethylene group bears four sulfur atoms, as de-
scribed in Scheme 3.
Fig. 3. Solid state structure of compound
analyses. Intramolecular H7ꢃ ꢃ ꢃS short distances are shown.
2
determined by X-ray diffraction
spectra showed that two THF molecules complete the coordination
sphere at the lithium [11]. 1H NMR spectra at room temperature
showed the disappearance of the C2–H proton signal. The 1H cou-
pling patterns indicate that in solution compounds 5–6 are in an
anchored conformation attributed to their dimeric structure, as it
was determined for 4 [11,31]. Calculations of axial and equatorial
carbanions of compound 1 show that the equatorial carbanion is
more stable than the axial one by 14.1 kJ/mol. The calculated min-
imum energy structure for compound 5 is in Fig. 5.
3.2.2.1. 1H NMR. The 1H NMR spectra at room temperature of
compounds 7 [11], 8, 10–13 indicate that they have a preferred
chair conformation with the 2-alkyl substituent in equatorial
and the N-alkyl group in axial position. The 2-alkyl group shifts
(ꢄ0.2 ppm) the C4, C6 methylene protons to higher frequencies
with respect of the parent dithiazinanes 1–3, Table 2. This effect
is similar to that produced by the N–BH3 coordination and
could be explained by the isolobal relationship of C–C and N–
B bonds.
3.2.2. 2-Alkyl derivatives
Reactions of compound 4 with methyl iodide and isopropyl
chloride give compounds 7 [11] and 8, respectively whereas

A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
155
Table 3
Selected bond lengths (Å) and angles (°) from X-ray diffraction analysis of 1, 1BH3, 2, 13 and 14.
R1
S
S
N
6
R2
R3
2
4
R1
R2
H
R3
1
Me
lone pair
BH3
1BH3 Me
H
2
13
14
iPr
H
lone pair
lone pair
Me
Me Dtz
Me
Me
C2–R2
S1–C2
S3–C2
S1–C6
S3–C4
C4–N5
C6–N5
1[3]
1BH3
2
13syn
13anti
14
–
–
–
1.803(4)
1.800(2)
1.793(3)
1.823(3)
1.817(3)
1.812(5)
1.794(4)
1.798(2)
1.785(2)
1.801(3)
1.812(3)
1.806(7)
1.836(4)
1.802(2)
1.828(3)
1.852(4)
1.835(4)
1.781(5)
1.836(4)
1.795(2)
1.826(3)
1.847(4)
1.845(4)
1.777(4)
1.428(4)
1.494(2)
1.434(4)
1.433(5)
1.435(4)
1.511(5)
1.433(4)
1.494(2)
1.438(2)
1.438(5)
1.435(5)
1.506(7)
1.526(4)
1.516(4)
1.521(9)
S–C2–S
S1–C2–R2
S3–C2–R2
C2–S3–C4
C2–S1–C6
1
113.7(2)
112.3(1)
113.9(1)
111.7(1)
111.6(1)
111.2(3)
–
–
–
–
–
–
97.3(2)
97.7(2)
97.0(1)
97.1(2)
98.4(1)
99.3(3)
97.8(2)
98.8(9)
97.0(1)
97.1(2)
97.5(2)
99.8(3)
1BH3
2
13syn
13anti
14
109.2(2)
109.7(2)
109.3(5)
109.2(2)
109.2(2)
108.3(5)
3.3. 2-Methyl compounds
Table 4
The treatment of the lithium compound 4 with an excess of
methyl iodide gave the ammonium derivative 14, by methylation
of C2 and quaternization of the tertiary amine, Scheme 3. The reac-
tion of compound 7 with BH3–DMS afforded the N–BH3 adduct 15.
This adduct in CDCl3 at 40 °C is transformed into its isomer, the
borata heterocycle 16 by the exchange of CH2 by BH2, Scheme 4.
The transformation was established by 1H, 13C and 11B NMR. Com-
pounds 14–16 are isostructural with a quaternary nitrogen atom.
Therefore, comparison of all these compounds is interesting. Only
the ionic molecule 14 crystallized from CHCl3, and its X-ray diffrac-
tion analysis could be performed.
C–H Selected bond lengths (Å) from X-ray diffraction analyses of 1BH3, 2 and 14.
R1
Heq
1BH3 R1= Me, R2= H,
R3= BH3
R3= lone pair
S
N
R3
Heq
R2
6
S
2
14
R1= iPr, R2= H,
R1= Me, R2= Me, R3= Me
2
4
Hax
Hax
Hax
H2ax
H2eq
H4ax
H4eq
H6ax
H6eq
1BH3
0.96(2)
1.00(2)
0.97(2)
0.93(2)
0.98(2)
0.91(2)
2
14
0.935(3)
0.992(8)
0.902(3)
–
0.952(2)
0.967(7)
0.852(2)
0.970(6)
0.971(2)
0.981(6)
0.881(2)
0.964(6)
3.3.1. NMR analysis
The 1H NMR spectrum of 2-methyldithiazinane 7 indicates a
preferred chair conformation at room temperature, contrastingly
with methyl cyclohexane and 2-methyl-1,3-dithiane which are
fluxional under the same conditions, Table 2. This result means
that the equatorial methyl group increases the already high ring
inversion energy (46 kJ/mol) of the 5-methyl-[1,3,5]-dithiazinane
(1).
Upon borane coordination of 7, methylene protons close to bor-
ane are shifted to lower frequencies due to the suppression of the
electroattracting effect of the N-lone pair. The neutral compounds
15 and 16 have similar chemical shifts, whereas the protons of the
cation 14 are considerably shifted to higher frequencies by the po-
sitive charge effect, Scheme 8.
Methylation of compound 1 (C2–H d = 4.60 ppm) to give 7 (C2–
H d = 4.11 ppm) shifts the C2-axial proton to lower frequencies.
The latter indicates that interactions between the C2–Me and the
sulfur lone pairs could occur, Table 2. This is contrasting with the
methylation of cyclohexane which shifts the C1–H axial proton
to higher frequency (d = 1.33 ppm [34]) with respect of cyclohex-
ane C–H axial protons (d = 1.20 ppm [24]).
3.2.3. X-ray diffraction analysis of compound 13
The X-ray diffraction analysis of compound 13 shows that there
are two molecules in the asymmetric unit, each one in a different
conformation. In one (13syn) the C2–C20 bond has an alternated
conformation with the four sulfur atoms and the two hydrogen
atoms in a gauche relationship, (dihedral angle H–C2–C2–H is
57.3°). The distances between the sulfur atoms of different rings
are short (3.30 and 3.36 Å) compared to the sum of the van der
Waals radii (RvdWr Sꢃ ꢃ ꢃS is 3.6 Å [33]), Fig. 7 and Tables 3–5.
In the second conformer 13anti, the C2–C20 bond has also an
alternated arrangement with the sulfur and hydrogen atoms in anti
position, Fig. 8. The distances between the sulfur atoms of the dif-
ferent rings are even shorter (3.18 Å) than 13syn, and it may be
considered as a result of stabilizing interactions [5,6,12–17]. Figs. 7
and 8 show the intramolecular sulfur–sulfur contacts in both con-
formers. The four sulfur atoms shift the C2 resonance by ꢄ20 ppm
towards higher frequencies (13C d = 55.8 ppm). This effect is calcu-
lated by comparison with 2-isopropyl-dithiazinane 10 where C2
appears at 37.4 ppm.

156
A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
Table 5
Crystal data of compounds 1BH3, 2, 13 and 14.
Me
Me
H
Me
Me
Me
Me
S
S
N
S
S
+
S
S
N
S
S
N
N
S
S
N
BH3
Me
Me
1BH3
2
14
13
Compd.
2
1BH3
13
14
Empirical formula
Molecular mass
Crystal size (mm)
C6H13NS2
163.31
C4H12BNS2
149.08
C16H32N4S8
268.47
C6H14INS2
291.22
0.3 ꢂ 0.25 ꢂ 0.1
0.5 ꢂ 0.25 ꢂ 0.15
0.35 ꢂ 0.3 ꢂ 0.17
0.2 ꢂ 0.18 ꢂ 0.1
Prism
Prism
Prism
Crystal color
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Colorless
Monoclinic
P21/n
6.2474 (1)
22.6589 (5)
6.8158 (2)
114.343 (1)
879.06 (4)
4
Colorless
Monoclinic
P21/c
7.52
9.052
11.77
94.6
798.6
4
Colorless
Orthorhombic
Pbcn
18.947
9.407
yellow
Monoclinic
Cc
12.77
7.46
13.895
13.38
b (°)
117.63
1128.72 (1)
4
1.714
3.15
V (Å3)
2476.6
8
1.44
Z
q
l
(calcd.) (mg/m3)
(mmꢁ1
1.234
0.53
1.24
0.57
)
0.73
F (000)
352
320
1136
568
Temperature (K)
293
293
293
293
2h Range for data collection
Index ranges
27.5–1.8
ꢁ7 6 h 6 7
ꢁ29 6 k 6 23
ꢁ8 6 l 6 8
3120
26.4–3.4
ꢁ9 6 h 6 9
ꢁ11 6 k 6 11
ꢁ14 6 l 6 12
6490
2.9–27.5
ꢁ20 6 h 6 24
ꢁ11 6 k 6 12
ꢁ16 6 l 6 18
27,520
2816
3.3–27.5
ꢁ10 6 h 6 9
ꢁ13 6 k 6 13
ꢁ16 6 l 6 16
5405
Reflections collected
Reflections unique
1888
1615
2103
Reflections observed (4
R (int)
r)
1347
0.03
1347
0.031
1623
0.122
2029
0.059
Number of variable
82
0.0/0.01
1.09
121
0.0313/0.1557
1.02
143
0.0275/1.9263
1.02
91
Weighting scheme [a]x/Y
GOOF
1.66
Final R (4
Final R wR2
Largest residual peak (e/Å3)
r
)
0.044
0.051
0.19
0.029
0.074
0.16
0.053
0.108
0.37
0.036
0.061
0.33
Fig. 4. X-ray diffraction structure of compound 1BH3. The [Hd+–Hdꢁ] and [Hd+–Sdꢁ
]
distances are shown.
Fig. 5. Calculated minimum energy structure for lithium compound 5. Hydrogen
atoms were deleted for clarity of the figure.
3.3.2. Ab initio calculations
Ab initio calculations [HF/6-31++G] of compounds 7 and 14–16
were performed. The dipole moments are in Scheme 9, and their
electrostatic potential in Supplementary material. Calculated NBO
charges [6-31++G] are in Table 1. The cation 14 has the more posi-
tive sulfur atoms and lesser negative nitrogen atom. In compound
16, the boron and B–H hydrogen atoms are almost neutral whereas
the nitrogen atom has almost a negative charge unity. The differ-
ence in the free enthalpy energy between the two isomers 15
and 16 is 119.1 kJ/mol, borata 16 is the most stable.
3.3.3. X-ray diffraction analyses of compound 14
The solid state structure of 14 is in Fig. 9 and data is in Tables 3–
5. Compound 14 it has the C2-methyl group in equatorial position.
The compound has two short contacts between the sulfur atoms
and the exocyclic carbon atom C9 [2.72 and 2.70 Å (RvWr C,S
3.5 Å [33])]. Examination of S–C2–C9 angles indicates that they
have the expected values (109.5(6) and 109.3(6)°) and that there
is no steric repulsion between the sulfur atoms and the methyl

A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
157
Fig. 6. Calculated structures of compounds 7 (left) and 9 (right). The distances of C–H to sulfur atoms are depicted.
Fig. 7. (a) Conformer 13syn found in the crystal of compound 13. (b) Newman projection of C2–C20 bond conformation. (c) Representations showing the van der Waals radii of
sulfur and C2 atoms.
Fig. 8. (a) Conformer 13anti found in the crystal of compound 13. (b) Newman projection of C2–C20 bond conformation. (c) Representations showing the van der Waals radii
of sulfur and C2 atoms.
Me
Me
Me
Me
-
H
I
+
S
N
S
S
N
S
N
S
N
BH3
Me
Me
Me
B
H
Me
Me
S
S
H
S
H
Me
H
H
3.96
3.60
3.58
H
4.47
4.79(br s)
H
H
H
H
H
H
H
4.22
4.56
4.11
4.05
5.30
4.10
7
15
14
16
Scheme 8. d(1H) of H-2ax and H-6 protons of compounds 7, 14–16 is shown. Compound 14 is in conformational equilibrium, data is for both equatorial and axial protons.
Me
Me
group and therefore a stabilizing interaction between the methyl
group and the two sulfur atoms may exist through Sꢃ ꢃ ꢃH and
Cꢃ ꢃ ꢃS short weak interactions. Interionic (Iꢃ ꢃ ꢃH) hydrogen bonds
stabilize the crystal [H4axꢃ ꢃ ꢃI (3.04 Å) and H6axꢃ ꢃ ꢃI (2.99 Å)] (RvWr
I,H 3.3 Å [33]). Comparison of the C2–H axial bond lengths in com-
pounds 2 [0.935(3)] and 14 [0.992(8)] shows that in 14, the C2-
methyl effect on sulfur atoms elongates the axial C–H bond.
Also the difference between free dithiazinane 2, coordinated
amine 1BH3, bis[dithiazinanyl] 13, and the dithiazinanium cation
14, is reflected in the bond length values of the ring atoms, Table 3.
S–C2 bonds are longer in compounds 14 [1.812(5) 1.806(7)], 13syn
[1.823(3) 1.801(3)], 13anti [1.817(3) and 1.812(3)] compared to 2
[1.793(3), 1.785(2)]. The S–C4 and S–C6 bonds are shorter in
1BH3 [1.802(2); 1.795(2)] and 14 [1.781(5); 1.777(4)] compared
N+
S
N
S
Me
Me
Me
Me
Me
S
S
(2.12 D)
7
(6.31 D)
14
Me
H
Me
S
S
N
S
S
N
B
Me
BH3
H
(3.38 D)
15
(5.48 D)
16
Scheme 9. Calculated [HF/6-31++G] dipole moments for compounds 7, 14, 15 and
16.
to
1 [1.836(4)], 2 [1.828(3) 1.826(3)] and 13syn [1.852(4);

158
A. Flores-Parra et al. / Journal of Molecular Structure 1047 (2013) 149–159
Fig. 9. X-ray diffraction structure of compound 14. (a) C–7H, C–9H and sulfur atoms intramolecular hydrogen bonds. (b) Representation of the van der Walls radii of C9 and
sulfur atoms.
1.847(4)], 13anti [1.835(4); 1.845(4)]. C4–N and C6–N bonds are
longer in 1BH3 [1.494(2)] and 14 [1.511(5); 1.506(7)] compare to
1 [1.428(4) 1.433(4)] and 2 [1.434(4) 1.438(2)]. The S–C2–S angles
are slightly closer in all C2 substituted compounds with respect to
non-substituted dithiazinanes.
showed that the antiperiplanar effect of axial sulfur lone pairs
elongate the axial C–H protons. The calculated atomic charges
and the electrostatic potential explain the NMR spectra trends.
Acknowledgements
We are grateful to Cinvestav for the facilities using of the super-
computer HPC-cluster Xiuhcoatl. We thank Prof. Angeles Paz-Sand-
oval for helpful discussions.
4. Conclusions
NMR data, X-ray diffraction analyses and calculations of a series
of 5-alkyl-[1,3,5]-dithiazinanes [R = Me (1), iPr (2), tBu (3)] and
their N–BH3 adducts (1BH3–3BH3) were analyzed. Agreement be-
tween the calculations and the experimental results was found.
1H NMR spectra show, that the ring CH2 protons are affected by
sulfur and nitrogen lone pairs, shifting their resonances to higher
frequencies. In addition the axial C–H protons are also shifted to
higher frequencies by the antiperiplanar effect of axial sulfur lone
pair. The latter explains why the axial protons in dithiazinanes are
found ꢄ1 ppm to higher frequencies with respect to equatorial
ones. Calculations confirmed the lone pairs effect on the electronic
density distribution and explained the 1H NMR trends.
X-ray diffraction and calculations confirmed that the more sta-
ble compounds have a chair conformation with the N-R group
(R = methyl, isopropyl or tertbutyl) in axial position and that this
conformation is also retained in the N–BH3 adducts which have
the borane in equatorial.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.
04.021.
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