Interaction of 1,3,2,4-benzodithiadiazines with aromatic phosphines and phosphites

Article abstract of DOI:10.1002/hc.21209

Although an interaction between hydrocarbon and fluorocarbon 1,3,2,4-benzodithiadiazines (1) and P(C₆H₅)₃ continuously produces chiral 1,2,3-benzodithiadiazol-2-yl iminophosporanes (2; in this work, 5,7-difluoro derivative 2a ) via 1:1 condensation, an interaction between 1 and other PR₃ reagents gives different products. With R = OC₆H₅ and both hydrocarbon and fluorocarbon 1, only X=P(OC₆H₅)₃ (X = S, O) were identified in the complex reaction mixtures by ¹³C and ³¹P NMR and GC-MS. With R = C₆F₅, no interaction with the archetypal 1 was observed but catalytic addition of atmospheric water to the heterocycle afforded 2-amino-N-sulfinylbenzenesulfenamide (4). With electrophilic B(C₆F₅)₃ instead of nucleophilic P(C₆F₅)₃, only adduct H₃N→B(C₆F₅)₃ and a new polymorph of C₆F₅B(OH)₂ were isolated and identified by X-ray diffraction (XRD). A molecular structure of 2a was confirmed by XRD, and the π-stacked orientation of one of phenyl groups and heterocyclic moiety was observed. This structure is in general agreement with that calculated at the RI-MP2 level of theory, as well as at three different levels of DFT theory with the PBE and B3LYP functionals. Mild thermolysis of 2a in a dilute decane solution gave persistent 5,7-difluoro-1,2,3-benzodithiazolyl (3a) identified by EPR in combination with DFT calculations.

Full text of DOI:10.1002/hc.21209

Heteroatom Chemistry
Volume 26, Number 1, 2015
Interaction of 1,3,2,4-Benzodithiadiazines with
Aromatic Phosphines and Phosphites
Tatiana D. Grayfer,1,2 Alexander Yu. Makarov,1,3
Irina Yu. Bagryanskaya,1,2 Irina G. Irtegova,1 Yuri V. Gatilov,1,2
and Andrey V. Zibarev1,3,4
1Institute of Organic Chemistry, Russian Academy of Sciences, 630090 Novosibirsk, Russia
2Department of Natural Sciences, National Research University–Novosibirsk State University,
630090 Novosibirsk, Russia
3Department of Chemistry, National Research University–Tomsk State University, 634050 Tomsk,
Russia
4Department of Physics, National Research University–Novosibirsk State University, 630090
Novosibirsk, Russia
Received 19 December 2013; revised 11 June 2014
ture of 2a was confirmed by XRD, and the π-stacked
ABSTRACT: Although an interaction between hydro-
orientation of one of phenyl groups and heterocyclic
carbon and fluorocarbon 1,3,2,4-benzodithiadiazines
moiety was observed. This structure is in general agree-
(1 ) and P(C6H5)3 continuously produces chiral 1,2,3-
ment with that calculated at the RI-MP2 level of the-
benzodithiadiazol-2-yl iminophosporanes (2; in this
ory, as well as at three different levels of DFT theory
work, 5,7-difluoro derivative 2a ) via 1:1 condensa-
with the PBE and B3LYP functionals. Mild thermoly-
tion, an interaction between 1 and other PR3 reagents
sis of 2a in a dilute decane solution gave persistent 5,7-
gives different products. With R
OC6H5 and both
difluoro-1,2,3-benzodithiazolyl (3a ) identified by EPR
in combination with DFT calculations. 2014 Wiley
Periodicals, Inc. Heteroatom Chem. 26:42–50, 2015;
View this article online at wileyonlinelibrary.com.
DOI 10.1002/hc.21209
hydrocarbon and fluorocarbon 1, only X=P(OC6H5)3
(X = S, O) were identified in the complex reaction mix-
tures by 13С and 31Р NMR and GC-MS. With R = C6F5,
no interaction with the archetypal 1 was observed but
catalytic addition of atmospheric water to the hetero-
cycle afforded 2-amino-N-sulfinylbenzenesulfenamide
(4 ). With electrophilic B(C6F5)3 instead of nucle-
ophilic P(C6F5)3, only adduct H3N→B(C6F5)3 and a
new polymorph of C6F5B(OH)2 were isolated and iden-
tified by X-ray diffraction (XRD). A molecular struc-
ꢀC
INTRODUCTION
1,3,2,4-Benzodithiadiazines (1, Chart 1) represent
a relatively rare class of compounds possess-
ing formal features of antiaromaticity in com-
bination with moderate thermal stability. Their
heteroatom reactivity is of obvious fundamental
interest [1]. Particularly, it was found that 1,3,2,4-
benzodithiadiazine and its derivatives, including
low- and high-fluorinated ones (as well as 1-Se
congener of trifluoro derivative) react with PPh3
Correspondence to: Alexander Yu. Makarov; e-mail:
makarov@nioch.nsc.ru.
Contract grant sponsor: Ministry of Education and Science of
the Russian Federation (project of Joint Laboratories of Siberian
Branch of the Russian Academy of Sciences and National Re-
search Universities).
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
ꢀC
2014 Wiley Periodicals, Inc.
42

Interaction of 1,3,2,4-Benzodithiadiazines with Aromatic Phosphines and Phosphites 43
CHART 1 Chemical structures and atom numbering of 1,3,2,4-benzodithiadiazines (1), 1,2,3-benzodithiazol-2-yl iminophos-
phoranes (2), and 1,2,3-benzodithiazolyls (3).
(Ph = C6H5) to give chiral 1,2,3-benzodithiazol-
2-yl iminophosphoranes (2, Chart 1) inaccessible
by other approaches including classical Kirsanov
and Staudinger reactions [2]. The exact mecha-
nism of this seemingly soft-El–soft-Nu orbitally
controlled reaction is unclear. Formally, 1,3,2,4-
benzodithiadiazines act in this reaction as they are
isomeric singlet nitrenes (observed in a cryogenic
matrix as a product of their photochemical rear-
rangement [1, 3]), which iminate oxidatively the P
atom of the phosphine. In the crystalline state, com-
pounds 2 reveal interesting structural feature as one
of Ph groups and the hetero ring are always face-
to-face oriented (π-stacked) with interplanar sepa-
ration being shortened as compared with the sum of
corresponding van der Waals radii [1,2]. The nature
of this is not entirely clear. In solution, compounds
2 easily form corresponding 1,2,3-benzodithiazolyls
(Herz radicals; 3; Chart 1) [4], candidate-building
blocks for magnetically active and/or conductive ma-
terials [5], covering derivatives inaccessible in other
ways (e.g., fluorinated derivatives) [2, 5]. This ap-
proach to Herz radicals can be considered as the
mildest one.
In the present work, we report on new data
on the interaction between compounds 1 and PPh3
as well as on its attempted extension to other PR3
reagents (R = C6F5 (PhF), OPh) with different nu-
cleophilicities. The latter can be comparatively es-
timated through (nP)−1 ionization energy, which is
7.80 and 9.18 eV for PPh3 and P(PhF)3, respectively,
in HeI PES spectra, and 7.83 and 8.60 eV for PPh3
and P(OPh)3, respectively, measured by electron im-
pact ionization [6].
SCHEME 1
1c with electrophilic B(PhF)3 instead of nucle-
ophilic P(PhF)3 resulted in a complex mixture, whose
workup gave an adduct H3N→B(PhF)3 and a new
polymorph of C6F5B(OH)2 identified by XRD. The
RI-MP2 and DFT calculations on 1b qualitatively re-
produced its conformation observed in the crystal
[2a].
RESULTS AND DISCUSSION
It was found that at –70°C and in toluene solu-
tion 6,8-difluoro-1,3,2,4-benzodithiadiazine (1a) in-
teracts with PPh3 to give a new iminophosphorane
2a (Scheme 1) in the isolated yield of 37%. This is
almost two times higher than the yield of its isomer
2d (Chart 1) synthesized under identical conditions
[2].
The structure of 2a was confirmed by XRD, and
face-to-face orientation of a Ph group and the het-
erocycle was observed (Fig. 1; discussion is given
below).
With the archetypal compound 1b, it was also
found that the interaction between PPh3 and com-
pounds 1 is solvent dependent and concentration in-
dependent. With toluene as the solvent, high dilution
of the reaction mixture (obtained at –70°C by simul-
taneous addition of solutions of 1 mmol of 1b and
PPh3 in 5 mL of toluene to 30 mL of the same sol-
vent during 1 h) did not improve the yield of a target
product 2b reported earlier (16% [2b]). Performed in
diethyl ether or dichloromethane instead of toluene,
the interaction under discussion did not give product
2b; instead, unidentified tar was obtained.
With PPh3, a new iminophosphorane 2a was
obtained, structurally characterized by XRD and
converted into a new 1,2,3-benzodithiazolyl radi-
cal 3a identified by EPR. From a slow reaction
with P(OPh)3, only S P(OPh)3 and O P(OPh)3
were identified, whereas P(PhF)3 was found to
catalyze hydrolysis of 1b affording 2-amino-N-
sulfinylbenzenesulfenamide 4. The interaction of
Heteroatom Chemistry DOI 10.1002/hc

44 Grayfer et al.
SCHEME 3
tial [2a,7] whose values can be used for comparative
estimation of their Lewis acidity. Reduced Lewis ba-
sicity of P(OPh)3 leads to zero yield of the imination
product.
In the toluene solution of 1b, P(PhF)3 remained
unchanged even under refluxing (19F NMR). It is
known that compounds 1 are stable under these
conditions [8] and their decomposition with for-
mation of radicals 3 requires higher temperatures
[2a,b,9]. After evaporation of the reaction mixture
and dissolving the residue in CDCl3 without pro-
FIGURE 1 XRD structure of 2a (displacement ellipsoids at
30%).
1
tection from atmospheric moisture, H NMR spec-
trum of the reaction solution revealed, however, the
presence of 2-amino-N-sulfinylbenzenesulfenamide
(4, Scheme 3) together with starting 1b. Previously,
this product was obtained by 1:1 addition of atmo-
spheric H2O to 1b catalyzed by SbPh3 [2a] (cf. [10]).
Taking into account that compound 1b itself does
not add H2O at the same conditions, one can con-
clude that P(PhF)3 also very effectively catalyses ad-
dition of atmospheric water to 1b, and that Lewis
bases catalytic contribution to heteroatom reactiv-
ity of compounds 1 is worth of further investigation.
Seemingly, this result gives a new example of cat-
alytic action of frustrated Lewis pair [11].
Overall, the data obtained lead to conclusion
that the scope of interaction between compounds
1 and PR3 derivatives affording iminophosphoranes
2 is limited because only PPh3 gives the target com-
pounds whereas other PR3 tried (R = OPh, PhF) do
not.
Other PR3 compounds tried (R = OPh, PhF)
revealed different heteroatom reactivity. Reactions
required higher temperatures but even at 20°C
P(OPh)3 interacted with both hydrocarbon 1b and
fluorocarbon 1c derivatives very slowly to give af-
ter several days complex reaction mixtures in which
only X P(OPh)3 (X = S, O, in proportion 1:1 and
1:2.3, respectively) were identified by 13С and 31Р
NMR and GC-MS techniques (Scheme 2). In the case
of X = S, initial compounds 1 were obviously the
source of sulfur (desulfurization of sulfur–nitrogen
rings, cages, and chains by PR3 compounds is well
known [1,2c]), whereas with X = O a source of oxy-
gen is not entirely clear. The experiments were per-
formed under argon, but their duration was 12–15
days; and atmospheric oxygen might enter the reac-
tion systems to oxidize P(OPh)3 into O P(OPh)3.
The interaction of compounds 1 with the phos-
phorus reagents is thought to begin with neutral-
ization of Lewis acid (compound 1) by Lewis base
(phosphine or phosphite). The yields of compounds
2 are higher for starting 1 with higher calculated
electron affinity and experimental reduction poten-
In experiments with compound 1c and elec-
trophilic B(PhF)3 instead of nucleophilic P(PhF)3,
a complex mixture of products was observed by
19F NMR. The products were extremely moisture
sensitive and decomposed fast in contact with
SCHEME 2
Heteroatom Chemistry DOI 10.1002/hc

Interaction of 1,3,2,4-Benzodithiadiazines with Aromatic Phosphines and Phosphites 45
FIGURE 2 Molecular conformation of 2b optimized at the RI-MP2/L1 (top left), PBE/3z (top right), B3LYP/6-311G (bottom left)
and PBE/L22 (bottom right) levels of theory.
air during workup of the reaction mixture. Only
adduct H3N→B(PhF)3 and a new polymorph of
C6F5B(OH)2 were isolated and identified by XRD
(in the Supporting Information). One can think that
these compounds came from air-moisture hydrol-
ysis of the primary reaction products. It should
be noted that little is known about reactivity of
chalcogen-nitrogen heterocycles related to com-
pounds 1 toward BR3 derivatives. In this context,
only molecular complexes between BR3 and 2,1,3-
benzochalcogenadiazoles can be mentioned [12].
The XRD structure of compound 2a (Fig. 1) gives
another example of face-to-face orientation of one
of the Ph groups and the heterocyclic moiety typi-
cal of compounds 2. For previously studied deriva-
tives, it was observed that the corresponding inter-
planar separation decreases with an increase in the
number of fluorine atoms as 3.52 (2b), 3.49 (2d),
H···N interaction, the C F···π interaction of the flu-
orine atom of the C F bond of one molecule with a
fluorinated benzene ring of another is observed; the
˚
atom-to-plane distance is 3.44 A.
It remains unclear whether the conformation
observed is a result of intramolecular interactions
(including π-stacking interactions of the arene-
(poly)fluoroarene type, [2a and references therein]),
or of intermolecular ones (packing forces of the crys-
tal lattice), or of their interplay. To investigate the
geometry of free molecules of 2, quantum chemical
calculations of the archetypal compound 2b were
performed. Previously DFT methods were success-
fully used for the calculation of structure and proper-
ties of 1, 3, 1,2,3-benzodichalcogenazolium cations
and some other related compounds [1, 3, 4a,c,8d–
f,9, 13, 14]. The MP2 method also gave satisfactory
results for 1,2,3-benzodithiazolium cation but not
good ones for the compounds 1 [14].
˚
3.43 (2e), and 3.39 (2c) A [2] (for chemical struc-
tures, see Chart 1). Compound 2a, however, does
In this work, the RI-MP2 and DFT optimization
of molecular geometry of 2b at four different lev-
els of theory gives molecular conformations (Fig. 2)
whose shape is similar to that observed by XRD
(Fig. 1). At the same time, experimental and theo-
retical molecular geometries differ in two important
aspects: (1) the S S bond in the XRD structures (2b,
not fit into this sequence since the observed inter-
˚
planar separation of 3.53 A not only exceeds that
for isomeric 2d but also is the biggest separation
found so far. In the crystal, weak hydrogen bonds
˚
C
H···N (H···N 2.55 A, C H···N 156°) lead to for-
mation of one-dimensional chains. In addition to the
Heteroatom Chemistry DOI 10.1002/hc

46 Grayfer et al.
SCHEME 4
Upon moderate heating in dilute hydrocarbon
CHART 2 Structure of 2b corresponding to the B3LYP/6-
solutions or, in some cases, just dissolving in chloro-
form at ambient temperature, compounds 2 trans-
form into radicals 3 [1, 5, 9]. Both approaches were
applied to 2a. Under heating of 2a in decane so-
lution at 120°C, an EPR spectrum corresponding
to 5,7-difluoro-1,2,3-benzodithiazolyl (3a) was ac-
quired (Scheme 4 and Fig. 3). At the same time,
the chloroform solution of 2a was EPR-silent over
a 2-week period of observation. Previously, the for-
mation of radicals 3 under the latter conditions was
observed by EPR in the case of 2b–d but not of 2e
[2a,b,9]. Seemingly, the different ratios of rate con-
stants of radicals’ generation and decay may be a
reason of this situation.
The electrochemical reduction of 2c was also
tried in this work. It was suggested that its rad-
ical anion will decompose into persistent 4,5,6,7-
tetrafluoro-1,2,3-benzodithiazolyl [9] and anion
Ph3P N−. The reduction peak at –0.96 V was ex-
pectedly irreversible, but the reaction system was
EPR-silent.
311G calculations.
˚
2.17 [2b]; 2a, 2.18 A) is markedly shorter than in the
calculated structures (RI-MP2, 2.57; PBE/3z, 2.46;
˚
PBE/L22 2.32; B3LYP, 2.90 A) where it is conforma-
tion dependent (in the Supporting Information) and
(2) the intercentroid separation in the XRD struc-
˚
tures of 3.56 A [2a] is significantly smaller than in the
DFT-calculated ones (PBE/3z, 4.18; PBE/L22, 4.02;
˚
B3LYP, 5.52 A); at the same time, RI-MP2 results
˚
(3.48 A) agree well with the experiment (for detailed
results of the calculations, see the Supporting In-
formation). Some other conformations of 2b were
also optimized by the same methods. In many cases,
they were less stable than the discussed conforma-
tions only by a few kcal mol−1 and revealed shorter
˚
S
S bonds (down to 2.18 A; see the Supporting In-
formation for details).
˚
The B3LYP/6-311G distance of 2.90 A is too
long for the S S bond whose typical length is 2.02–
˚
2.08 A [15]. In this respect, the calculated structure
is closer to the structure shown in Chart 2 than to
the structure represented in Chart 1.
EXPERIMENTAL
General
1H, 13C, 19F, and 31P NMR spectra were measured
with a Bruker AV-300 machine at frequencies of
300.13, 75.47, 282.40, and 121.49 MHz, respectively,
for CDCl3 solutions, except for C6D6 solutions
of products from experiments with P(OPh)3
and toluene-C6D6 solutions of products from
Overall, one may think that the experimental
molecular conformation under discussion is an in-
terplay of inter- and intramolecular interactions. The
calculations also reveal high flexibility of the com-
pound 2b, which is worth of further investigation
(for instance, by methods of variable-temperature
NMR).
3460
3470
3480
3490
3460
3470
3480
3490
FIGURE 3 Experimental (left) and simulated (right) EPR spectrum of 3a. The experimental (B3LYP/6-31G calculated) hfc
3
4
6
5
7
constants (G): aN 7.8 (8.5), aH 2.6 (3.6), aH 3.4 (4.1), aF 2.6 (3.7), aF 3.8 (4.6).
Heteroatom Chemistry DOI 10.1002/hc

Interaction of 1,3,2,4-Benzodithiadiazines with Aromatic Phosphines and Phosphites 47
experiments with B(PhF)3. The standards were TMS
tions 1167, wR2 = 0.1117, S = 1.10 for all reflections
(R = 0.0396 for 1075 F > 4σ). The obtained crystal
structures were analyzed for short contacts between
nonbonded atoms using the PLATON program [22].
(1Н, 13С), C6F6 (19F; δ = –162.9 ppm with respect to
CFCl3), and 85% H3PO4 (31P).
High-resolution MS spectra (IE, 70 eV) were
obtained using a Thermo DFS mass-spectrometer.
GC-MS determinations were performed with a
Hewlett-Packard G1800A GDC instruments, and
with an Agilent Technologies apparatus compris-
ing a gas chromatograph Agilent 6890N and a
chromatography–mass-spectrometry system Agilent
5973N.
Compounds 1 [6], PR3 (R = OC6H5 [16], C6F5
[17]), and B(PhF)3 [18] were prepared as described
before; other chemicals were commercially avail-
able. Solvents were dried with common drying
agents.
Quantum Chemical Calculations
Theoretical hfc constants of radical 3a were ob-
tained at the B3LYP/6-31G level of theory with the
GAMESS program [23a]. Molecular conformations
of compounds 2 were calculated with the GAMESS
(B3LYP) and PRIRODA (RI-MP2, PBE) [23b,c]. The
L1 (an analog of cc-pVDZ) [23d], L22 (cc-pCVTZ),
and 3z basis sets were used as implemented in the
PRIRODA program.
Cyclic Voltammetry of Iminophosphorane 2c
EPR Spectroscopy
The CV measurements on compound 2c (10−3 M so-
lution in dry MeCN) were performed at 298 K in
an argon atmosphere with a PG 310 USB potentio-
stat (HEKA Elektronik). A stationary platinum elec-
trode (S = 0.08 cm2) was used. A supporting elec-
trolyte was 0.1 M Et4NClO4. The peak potential was
quoted with reference to a saturated calomel elec-
trode (SCE).
EPR spectra were acquired using a Bruker ESP-300
spectrometer (modulation frequency, 100 kHz; mod-
ulation amplitude, 0.05 G; MW power, 265 mW).
Simulations of the experimental EPR spectra were
performed with the Winsim 2002 program [19]. The
accuracy of the hyperfine coupling (hfc) constants
was 1 × 10−2 G.
Syntheses
X-Ray Diffraction
Compound 2a. At –70°C and under argon, a so-
lution of 0.284 g (1.1 mmol) of PPh3 in 5 mL of
toluene was added dropwise to a stirred solution of
0.221 g (1.1 mmol) of 1a in 5 mL of toluene over a pe-
riod of 1 h. The reaction mixture was slowly warmed
up to 20°C and stirred for additional 1.5 h. Then 15
mL of hexane was layered over the reaction mixture
to produce a two-layered system. The system was
kept at ambient temperature until mutual diffusion
of solvents ceased. The solution and tarry residue
were separated, and the solution was layered again
with 15 ml of hexane. Compound 2a was obtained in
the form of dark-red crystals (0.187 g, 37%) suitable
to XRD, m p 126–127оС. NMR, δ: 1Н: 7.62–7.53 (9H,
broad unresolved signal), 7.45–7.39 (6H, broad unre-
solved signal), 6.35 (1H, d), 6.05 (1H, d); 13С: 161.8,
160.4, 156.0, 132.8 (two overlapping signals), 128.6,
126.4, 107.9, 97.8, 92.4; 19F: 58.1, 44.3; 31P: 19.7. MS
M+, m/z, measured (calculated for C24H17F2N2PS2):
466.0530 (466.0533). Found (calculated), С 61.87
(61.79), H 3.60 (3.67), N 5.92 (6.00), F 8.17 (8.14),
P 6.58 (6.64), S 13.90 (13.75).
The XRD data were collected with a Bruker Kappa
Apex II CCD diffractometer using φ, ω scans
of narrow (0.5°) frames with Mo Kα radiation
(λ = 0.71073 A) with a graphite monochromator
˚
at T = 240 (2) K. The structures were solved by
direct methods and refined by full-matrix least-
squares method against all F2 in anisotropic ap-
proximation (isotropic for H atoms in compound
C6F5B(OH)2) using the SHELX-97 programs set
[20]. The H atoms positions were calculated with
the riding model for 2a, and the H atom po-
sitions for C6F5B(OH)2 were located from the
difference Fourier map. Absorption corrections
were applied empirically using SADABS programs
[21]. Compound 2a: monoclinic, space group Cc,
˚
a = 9.788(2), b = 15.898(3), c = 14.307(3) A, β =
3
˚
92.432(7)°, V = 2224.3 (8) A , Z = 4, C24H17F2N2PS2,
Dc = 1.393 g cm−3, μ = 0.342 mm−1, F(0 0 0) =
960, crystal size 0.04 × 0.04 × 0.04 mm3, indepen-
dent reflections 2872, wR2 = 0.1424, S = 1.05 for
all reflections (R = 0.0545 for 2051 F > 4σ). Com-
pound C6F5B(OH)2: monoclinic, space group P21,
˚
a = 7.455(1), b = 5.0070(7), c = 10.055 (2) A, β =
Reaction of 1b and 1c with P(OPh)3
3
˚
96.784(6)°, V = 372.7(1) A , Z = 2, C6H2F5BO2, Dc =
1.888 g cm−3, μ = 0.214 mm−1, F(0 0 0) = 208, crys-
At –70°C and under argon, a solution of 0.310 g (1.0
mmol) of P(OPh)3 in 5 mL of toluene was added
tal size 0.04 × 0.04 × 0.9 mm3, independent reflec-
Heteroatom Chemistry DOI 10.1002/hc

48 Grayfer et al.
dropwise to a stirred solution of 1.0 mmol of 1b or
1c in 5 mL of toluene over a period of 1 h. The re-
action mixture was slowly warmed up to 20°C and
kept further at this temperature. The 31P NMR spec-
tra were measured periodically, indicating the dis-
appearance of the signal of P(OPh)3 (δ = 128 ppm)
after 12 days (1b) or 15 days (1c). The main signals in
the 31P NMR spectra of the reaction mixtures belong
to O P(OPh)3 (–17 ppm) and S P(OPh)3 (54 ppm)
in proportion 1:2.3 (1b) or 1:1 (1c). The presence
of O P(OPh)3 and S P(OPh)3 in the reaction mix-
tures was also confirmed by 13C NMR. According to
1H or 19F NMR spectra, the mixtures also contained
starting 1b or 1c. The solvent evaporation under re-
duced pressure gave tarry products from which un-
consumed compounds 1 were recovered in the yield
of 10–15% by sublimation at 50oC/1 Torr. GC-MS
analysis of the residue revealed the same ratio of
O P(OPh)3 and S P(OPh)3 as indicated above.
H3N→B(PhF)3 [24] was obtained in the form of dark-
purple crystals suitable to XRD and identified by the
unit cell parameters. Another part of the reaction
solution was evaporated and left at ambient temper-
ature for 1 week. White needles of C6F5B(OH)2 ap-
peared on the walls of the vessel that were suitable
for XRD.
CONCLUSIONS
The general character of reaction between 1,3,2,4-
benzodithiadiazines and PPh3 affording chiral
1,2,3-benzodithiazol-2-yl iminophosphoranes inac-
cessible by other ways was confirmed with another
example, as well as ability of the iminophospho-
ranes to produce 1,2,3-benzodithiazolyls (Herz rad-
icals) under mild conditions. However, it was found
that this reaction cannot be transferred from PPh3
to P(OPh)3 and P(PhF)3. The first one produces
only unidentified tar together with S P(OPh)3 and
O P(OPh)3 as a result of a slow reaction. The sec-
ond one does not react at all under studied con-
ditions but, similar to SbPh3, effectively catalyzes
water addition to 1,3,2,4-benzodithiadiazines. The
lower nucleophilicity of P(OPh)3 and P(PhF)3 as
compared with PPh3 seems to be the cause of
these findings. Thus, more reactive P-nucleophiles
like donor-substituted triarylphosphines, alkyl- and
aminophosphines are thought to be promising for
further development of this method for the synthe-
sis of iminophosphoranes.
The XRD structure of 2a gave another example
of face-to-face orientation of one of the Ph groups
and the heterocyclic moiety typical of the 1,2,3-
benzodithiazol-2-yl iminophosphoranes. This spe-
cial structure is in a general agreement with the re-
sults of both post-HF and DFT calculations. Overall,
one may think that the XRD molecular conformation
of the 1,2,3-benzodithiazol-2-yl iminophosphoranes
is an interplay of inter- and intramolecular interac-
tions. The former are packing forces of the crystal
lattice, whereas the latter in the case of fluorocar-
bon derivatives may involve π-stacking interactions
of the arene-(poly)fluoroarene type.
Transformation of 1b in the Presence of
P(C6F5)3
Under a CaCl2 tube, a solution of 0.445 g (0.8 mmol)
of P(C6F5)3 in 5 mL of toluene was added to a stirred
solution of 0.140 g (0.8 mmol) of 1b in 5 mL of
toluene, and the reaction mixture was refluxed for
2 h. Upon cooling to 20°C, 19F NMR spectrum was
measured to reveal unchanged P(C6F5)3. The solvent
was distilled off, and the residue was dissolved in
CDCl3. Two last operations were performed without
protection from atmospheric moisture. According to
the 1H NMR, CDCl3 solution contained a mixture of
starting 1b and a water-addition product 4 [2a] in a
42:58 ratio.
Generation of Radical 3a
Under argon, a solution of 7 mg (1.5 × 10−5 mol) of
2a in 1 mL of degassed decane was kept at 120°C for
40 min, cooled to 20°C, and its EPR spectrum was
measured to reveal radical 3a.
Interaction of 1c with B(C6F5)3 and Isolation of
a New Polymorph of C6F5B(OH)2
In experiments with 1c and electrophilic B(PhF)3
instead of nucleophilic PR3 reagents, a complex mix-
ture of products was observed, with only adduct
H3N→B(PhF)3 and a new polymorph of C6F5B(OH)2
being isolated and identified by XRD.
At –70°C and under argon, a solution of 0.399 g
(0.8 mmol) of B(C6F5)3 in 6 mL of toluene was added
dropwise to a stirred solution of 0.187 g (0.8 mmol)
of 1c in 5 mL of toluene over a period of 80 min
during which the solution was turning dark-brown.
Upon warming to 20°C, 19F NMR spectrum was mea-
sured to reveal a complex mixture of unassigned sig-
nals. A part of the solution was evaporated, and the
residue was crystallized twice from hexane without
protection from atmospheric moisture. An adduct
ACKNOWLEDGMENTS
The authors are grateful to Prof. Dr. Vadim V. Bardin
for generous gift of B(C6F5)3 and helpful discussions,
to Dr. Nadezhda V. Vasilieva for the electrochemical
Heteroatom Chemistry DOI 10.1002/hc

Interaction of 1,3,2,4-Benzodithiadiazines with Aromatic Phosphines and Phosphites 49
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for help in some preparations. They also thank
the Collective Chemical Service Center of Siberian
Branch of the Russian Academy of Sciences for in-
strumental facilities.
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