The Reaction between Diazadienes and Element Tribromides EBr3 (E = P, B) Revisited: Metal-Free Synthesis of Halogenated N-Heterocyclic Phosphanes and Boranes

Article abstract of DOI:10.1002/ejic.201500834

Reactions of selected diazadienes with PBr₃ and BBr₃ in the presence of a tertiary amine yielding N-heterocyclic phosphanes (NHPs) and N-heterocyclic boranes (NHBs) were studied. It is demonstrated that heterocycle formation occurs e

Full text of DOI:10.1002/ejic.201500834

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DOI:10.1002/ejic.201500834
The Reaction between Diazadienes and Element
Tribromides EBr3 (E = P, B) Revisited: Metal-Free
Synthesis of Halogenated N-Heterocyclic Phosphanes and
Boranes
Dirk Herrmannsdörfer,[a] Manuel Kaaz,[a] Oliver Puntigam,[a]
Johannes Bender,[a] Martin Nieger,[b] and Dietrich Gudat*[a]
Keywords: Phosphanes / Boranes / Halogens / Reaction mechanisms / Density functional calculations
Reactions of selected diazadienes with PBr3 and BBr3 in the
presence of a tertiary amine yielding N-heterocyclic phos-
phanes (NHPs) and N-heterocyclic boranes (NHBs) were
studied. It is demonstrated that heterocycle formation occurs
even without an amine or another auxiliary reagent, and the
amine acts mainly as scavenger of Br2 formed as by-product,
but also that the additive speeds up reactions and has an
observed behaviour to the availability of two competing reac-
tion channels and provides also a rational explanation for the
different behaviour of ECl3, EBr3, and EI3 (E = B, P) in cyclo-
addition reactions with diazadienes, which is empirically
well established. In the borane series, a tribromo-NHB is
formed in a follow-up reaction between the initial products
and Br2, which seems to be the first example of an electro-
influence on the product selectivity. In extension of the re- philic backbone functionalization in an NHB. The 2-bromo
sults of earlier studies it is shown that the reactions can fol-
low two different pathways to give either 2-bromo- or 2,4-
dibromo-substituted heterocycles. Computational studies en-
abled to propose a reaction mechanism, which relates the
derivative is formed exclusively when PPh3 is used as auxil-
iary reagent. Selected products are isolated and fully charac-
terised, proving the synthetic utility of the reactions studied.
phanyl groups[6] add selectively and easily to organic
multiple-bond systems, and P-hydrogen-substituted phos-
phorus heterocycles behave as hydride transfer agents under
stoichiometric[7] and catalytic[8] conditions. That even halo-
gen-substituted heterocycles display peculiar reactivity is
exemplified by a P-chloro derivative of type IIh, which acts
as electrophilic organocatalyst in a PC cross coupling reac-
tion.[9]
Introduction
The ground-breaking discovery of stable N-heterocyclic
carbenes Ia[1] (Scheme 1) kicked off a still ongoing boom of
carbene chemistry with applications in synthesis, coordina-
tion chemistry, or catalysis.[2] In its wake, this upsurge
raised (or, in some cases, revived) interest in inorganic carb-
ene analogues, which are derived from the prototype by for-
mal isoelectronic replacement of the divalent carbon atom
by neutral or charged elements of groups 13–16 (Ib–l).[3]
Many of these species are accessed by reductive or Lewis-
acid-induced cleavage of halides from halogen-substituted
derivatives IIb–j.[4] Apart from serving as precursors of
carbene analogues, such compounds have been employed to
access heterocycles with functional substituents at the main-
group element by halide displacement. Some of these spe-
cies exhibit unique chemical properties; for instance, boron
heterocycles (IIe) with B-stannyl or -stannylenyl substitu-
ents[5] and phosphorus heterocycles (IIh) with P-phos-
Scheme 1. Structures of N-heterocyclic carbenes and their ana-
logues (Ia–k) and halogenated precursors IIb–j. (R = alkyl, aryl; X
= halogen).
Synthetically, IIb–j are usually accessed by starting from
1,4-diazadienes (diimines). The most widely used protocol
is a two-step reaction involving first treatment of the diaza-
diene with a metal-based reducing reagent to produce the
corresponding dianion, and subsequent metathetic ring
closure (either directly or after quenching with an acid)
with a suitable electrophile to give the target hetero-
[a] Institut für Anorganische Chemie, University of Stuttgart,
Pfaffenwaldring 55, 70550 Stuttgart, Germany
E-mail: gudat@iac.uni-stuttgart.de
http://www.iac.uni-stuttgart.de/akgudat
[b] Laboratory of Inorganic Chemistry, Department of Chemistry,
University of Helsinki,
P. O. Box 55, 00014 University of Helsinki, Finland
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500834.
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cycle.[4a,4b,4d,4e,7b,10] In special cases, the order of both steps sumed to proceed by a cycloaddition of a transient silylene
may be reversed.[4b,10]
to the diimine. Further parallels may be drawn to the as-
Alternatively, the conversion can also be achieved in a sembly of an N-heterocyclic iodoborane from a diazadiene
single step without a metal-based reductant. An early exam- and BI3.[10]
ple is the reaction of N,NЈ-dialkyldiazadiene 1a with PCl3 in
Altogether, these findings suggest that the “metal-free”
the presence of a tertiary amine to afford the 2,4-dichloro- syntheses of N-heterocyclic phosphanes and boranes can
substituted N-heterocyclic phosphane 2a [Scheme 2 (a)].[11] follow different pathways and may yield either mono- or
Even if no intermediates were observed, the reaction was dihalogenated products, respectively. A report on the simul-
explained through double addition of P–Cl bonds across taneous formation of both types of species[16] gives a hint
the diimine double bonds followed by dehydrohalogenation. that the reaction channels may not be mutually exclusive
This pathway was positively established for the reaction of but competitive. Considering that metal-free, one-step pro-
diazadiene 1b with BCl3. The initial addition product 3b tocols can offer a synthetically valuable approach to N-
was in this case isolated and structurally characterized, and heterocyclic element halides, further elucidation of this mat-
decayed in polar solvents under spontaneous cleavage of ter is needed in order to exploit the full potential of this
HCl to give 4b [Scheme 2 (b)].[12]
methodology. We have therefore re-examined the reactivity
of phosphorus and boron trihalides towards selected diaza-
dienes, aiming at a mechanistic analysis of this transforma-
tion, which allows us to understand the origin and implica-
tions of the two-sided reactivity. Apart from clarifying the
academically interesting question whether the cycload-
ditions may possibly involve any subvalent E–X species, we
reckoned that the results may provide valuable guidelines
for the development of selective access routes to particular
target compounds. The experimental studies focused on the
element tribromides since we anticipated that these offer the
best chance to observe two-sided reactivity.
Results and Discussion
Reactions of Phosphorus Tribromide with Diazadienes
Previous reports on reactions of PBr3 with diazadienes
mention only 2-bromo-substituted N-heterocyclic phos-
phanes (NHPs) as products.[14,17] Considering that a pos-
sible generation of 2,4-dibromo-substituted species from
these starting materials is accompanied by the formation of
HBr, we studied reactions of the diazadienes 1a–c and PBr3
in the presence of triethylamine as acid scavenger in order
to bring any two-sided reactivity to the fore. The individual
reactions were carried out by adding PBr3 to a solution
containing the diazadiene and the amine, and the phos-
phorus-containing products were analysed by 31P NMR
spectroscopy. The NHPs formed were identified by com-
parison of the observed spectroscopic data with those of
Scheme 2. One-step syntheses of N-heterocyclic halogeno-element
compounds from diazadienes and element halides. R = Cy (1a, 5a–
7a), 2,6-iPr2C6H5 (1b, 5b–7b), tBu (1c, 5c, 7c), Mes (1d, 5d–7d).
In contrast, a different outcome was observed for the re- authentic samples.
action of the heavier phosphorus halides with diazadienes.
Analysis of the NMR spectra revealed that in the reac-
Phosphorus triiodide reacted to give phosphenium triiod- tions with 1a,b complete consumption of PBr3 was achieved
ides 5 [Scheme 2 (c)].[13] Reaction of N-aryldiazadienes with at ambient temperature within 1 h, whereas the reaction
PBr3 alone was reported to be unselective, but clean conver- with 1c was slower and required approx. 18 h. All three re-
sion to N-heterocyclic phosphanes 6b,c was noted in the actions gave a mixture of two phosphorus-containing spe-
presence of cyclohexene.[14] The heterocycles 5, 6 lack a cies (see Scheme 3). The major components were identified
halogen substituent in 4-position, and product formation in all cases as the 2-bromo-NHPs 6a–c (6a,b 75%; 6c 67%
was explained by a formal cycloaddition of a monovalent by evaluation of 31P NMR signal intensities). The by-prod-
“P–X” synthon with the diazadiene. Formation of ionic 5 uct in the reactions of 1b,c – although it could not be iso-
involved further an I– abstraction from the initial cyclo- lated and fully characterized – was on the basis of an analy-
adduct by I2. In principle, these reactions resemble the syn- sis of NMR spectra of the reaction mixture tentatively as-
thesis of N-heterocyclic silanes 7 by action of a tertiary signed as phosphanyl-enamine 9.[18] Optimizing the condi-
amine on a mixture of HSiCl3/diazadiene,[15] which was as- tions for the reaction of 1b allowed to reduce the amount
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of this species and to isolate 6b in 90% yield. The by-prod-
Further experiments on the reaction of 1a with PBr3 re-
uct of the reaction of 1a with PBr3 was identified as 2,4- vealed that the product ratio 6a/8a depends crucially on the
dibromo-substituted NHP 8a. Structural assignment ob- presence or absence of triethylamine and the reaction tem-
tained from NMR spectroscopic data was in this case con- perature, whereas the choice of solvent had no prominent
firmed by a single-crystal X-ray diffraction study of a mixed influence. Thus, the amount of disubstitution product 8a
crystal containing both 6a and 8a, which was serendi- decreased substantially (to a ratio 6a/8a Ͼ 95:5) when the
pitously isolated from the reaction mixture. The crystal con- reaction was performed as described by Macdonald et al.[14]
tains molecular pairs (Figure 1) that are held together by a at room temperature in the presence of cyclohexene, but
halogen bond connecting the Br1 atom of 6a with the C- without triethylamine. This finding indicates that the for-
bound Br23 atom in 8a [Br1–Br23 3.248(7) Å]. The struc- mation of 8a is indeed promoted by the base. Interestingly,
tures of both molecular subunits are, as expected, closely the presence of cyclohexene as Br2 scavenger is in this case
similar to each other and those of 6b (see Supporting Infor- not mandatory, and even the two-component reaction be-
mation), 6c[19] and 6d[14] (R = Mes). The most prominent tween 1a and PBr3 in THF at 0 °C resulted in selective for-
differences lie in the pronounced deviation in P–Br dis- mation of 6a, which was isolated in moderate yield after
tances – the bond length in 6a [P1–Br1 2.907(1) Å] comes crystallization.[21] No spectroscopically detectable amounts
close to the extreme value observed for 6c [2.947(1) Å[19]], of 8a were observed when the reaction was conducted in
whereas the bond in 8a [2.764(1) Å] is significantly shorter the presence of triethylamine at 0 °C, but 8a became the
but still exceeds the length in 6b [2.591(1) Å] and 6d major product when the reaction was carried out at 70 °C.
[2.618(1) Å[14]], respectively. A similar trend had been pre- Under these conditions, also a small amount of 9 (ca. 5%
viously observed in chloro-substituted NHPs[20] and is of phosphorus-containing products) was produced.
mainly attributable to fine-tuning of the n(N)–σ*(P–X)
The formation of the elusive enamine 9, which was under
hyperconjugation by the inductive effects associated with certain conditions observable in the reactions of all diazadi-
the electron-releasing N-alkyl and the electron-withdrawing enes studied, can be explained as arising from a reaction of
4-halogeno substituents.
PBr3, Et3N, and Br2. The latter react, as was shown by
Belucci et al.,[22] to yield iminium salts [Et2N(+)
=
CHCHnBr3–n]Br (n = 0–3). Attempts to detect these species
in the reaction mixtures were unsuccessful, and we assume
that they are quenched by the reaction with PBr3 to give 9.
The amount of this by-product increased when PBr3 was
rapidly injected into a mixture of 1a. Even if a final proof
for the constitution of 9 and a mechanistic explanation of
its formation are still pending, its occurrence indicates that
triethylamine acts in this reaction, like cyclohexene, in the
first place as Br2 scavenger.
The main conclusions to be drawn from these experi-
ments are: (a) auxiliary reagents (triethylamine, cyclohex-
ene) are not mandatory for the formation of NHPs from
diazadienes and PBr3 but can suppress unwanted follow-up
reactions of starting materials or products with Br2 formed
as by-product; (b) whereas diazadienes react with PCl3 and
PI3 to give only 2,4-dihalo- or 2-halo-NHPs, respectively,
the reaction with PBr3 may yield both mono- and dihaloge-
nated products; (c) the product ratio varies with the molec-
ular structures of the reactants and the reaction conditions,
but the use of base as auxiliary reagent favours the forma-
tion of mixtures of mono- and dihalogenated products;[23]
(d) using triethylamine can still be advantageous for diaza-
dienes with low preference for the formation of dihaloge-
nated products (like 1b) as it allows to speed up the reac-
tions.
Scheme 3. Reactions of diazadienes 1a–c with PBr3 in the presence
of excess Et3N at ambient temperature.
Figure 1. Representation of the molecular structures of 6a and 8a
in the crystal. Thermal ellipsoids are drawn at 50% probability
level, H atoms and solvent molecule are omitted for clarity. The
Br1 atom is disordered over two positions with occupancies of 0.9
(Br1)/0.1 (Br1A). Selected bond lengths [Å]: 6a: Br1–P1 2.9069(12),
Br1A–P1 2.940(5), P1–N5 1.659(4), P1–N2 1.666(4), N2–C3
1.392(6), C3–C4 1.360(6), C4–N5 1.383(5); 8a: Br21–P21
2.7640(12), P21–N25 1.664(4), P21–N22 1.671(4), N22–C23
1.382(6), Br23–C23 1.854(5), C23–C24 1.348(6), C24–N25
1.375(6); intermolecular: Br1–Br23 3.248(7).
In order to cast further light on the mechanistic aspects
of the dual-sided reactivity to mono- and dihalogenated
NHPs, we set out to perform a computational study of the
reaction of PX3 (X = Cl, Br) with the N-methylated model
diazadiene 1e. Density functional calculations were carried
out at the b3lyp/def2-tzvp level with inclusion of solvent
effects by using a polarizable continuum model. For the
sake of simplicity, the diazadiene was assumed to adopt the
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cisoid conformation needed for the heterocycle formation.
The reaction is initiated by formation of a van der Waals
The minimum free energy reaction pathway connecting 1e complex (not shown in Figure 2a) which is a local minimum
with 6e is represented in Figure 2a; further data (including on the energy hypersurface and preorganized for a [4+1]-
calculated energies and molecular geometries) are given in cycloaddition step to give phosphorane ABr. The geometry
the Supporting Information.
of transition state TS1 suggests describing this step as con-
certed. The computations predict for ABr an unusual
square-pyramidal coordination geometry at the phosphorus
atom with short apical and long basal P–Br bonds (2.230
vs. 2.595 Å). However, a trigonal-bipyramidal conformer
representing the transition state to Berry pseudorotation is
only by 1.5 kcalmol–1 higher in energy, and the molecule is
thus best addressed as fluxional. Phosphorane ABr can
readily ionize under P–Br bond heterolysis. We found that
the resulting contact ion pair may exist in (at least) two
energetically different (ΔG = 5.0 kcalmol–1) configurations
BBr and BBrЈ, which differ in the spatial orientation of cat-
ion and anion relative to each other. Both configurations
are local minima on the energy hypersurface and can pre-
sumably easily interconvert by ion diffusion.
Formation of 6e is completed by nucleophilic attack of
Br– on a P-bound bromine atom in one of the ion pair
configurations (BBr). The search for a transition state for
this step yielded only a higher-order stationary point with
three imaginary vibrational modes. However, both ionic
constituents are at this point confined in separate solvent
cages, and a relaxed potential energy scan indicated the ab-
sence of a transition state at a later reaction stage. We as-
sume therefore that the activation barrier for this step is
intimately connected with solvent reorganization around
the reactants, which cannot be satisfactorily modelled in the
PCM approach, and that the energy and location on the
reaction coordinate of the real transition state (TS3) are
close to the stationary point found. In total, formation of
6e and Br2 from 1e and PBr3 is calculated to be slightly
exergonic (ΔG = –1.3 kcalmol–1), and the initial conversion
of 1e to ABr is the rate-determining step with ΔG϶
=
16.8 kcalmol–1.
Assuming initially that the 2,4-dibromo-NHP 8e is
formed by addition of a P–Br bond to a C–N double bond,
we also tried to locate a transition state for this process.
However, all such attempts converged to the already known
transition state TS1, and we conclude that a reaction in-
volving direct addition of PBr3 to an imine double bond of
1e is unlikely. Looking for an alternative pathway to a C-
halogenated product, we found that the contact ion pair
BBrЈ can isomerize to a ring-opened product CBr. This spe-
cies can then react under ion recombination and C–Br bond
formation to the formal imine addition product DBr. Fur-
ther conversion to a dibromo-NHP 8e may proceed by tau-
tomerization to an ene-diamine MeN(H)–CH=C(Br)–
N(Me)PBr2 (EBr) and ring closure by base-induced de-
hydrohalogenation, and was not analysed further.[24] For-
mation of DBr (ΔG = 7.6 kcalmol–1) and EBr (ΔG =
10.7 kcalmol–1) is endergonic and thermodynamically and
kinetically less favourable than that of 6e. This result is
qualitatively in accord with the experimental finding that
formation of 6a is generally preferred over that of 8a. How-
ever, it must be considered that DBr and EBr are not the
Figure 2. Calculated minimum free energy pathways for the reac-
tions of diazadiene 1e with (a) PBr3 (top), (b) PCl3 (middle), and
(c) PI3 (bottom) at the b3lyp/def2-tzvp level. Solvent effects were
modelled by using a PCM approach. BX and BXЈ (X = Br, Cl)
denote contact ion pairs with different ion arrangement.
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final products, and base trapping of the HBr formed in the modynamically unfavourable unless an external reducing
final ring closure step to yield 8e may shift the reaction agent is applied. The predicted preference of the PBr3 reac-
further to this side. Consequently, the relative free energies tion for the monobromo-NHP and its exergonic nature are
calculated for the formation of 6e and DBr/EBr do not neces- qualitatively in accord with the experimental findings. The
sarily provide a quantitative measure of the expected prod- calculations gave no evidence for the participation of sub-
uct ratio.
Qualitatively similar reaction pathways to produce either
valent P–X species.
a chloro-substituted heterocycle 9e or a formal addition
product DCl were also identified in the computational
analysis of the reaction of 1e with PCl3 (Figure 2b). Con-
trary to the reaction with PBr3, the phosphorane intermedi-
ate BCl displays the expected trigonal-bipyramidal geometry
(with a calculated activation barrier to pseudorotation of
ΔG϶ = 1.7 kcalmol–1). More importantly, formation of
monochloro-NHP 9e is even more endergonic (ΔG =
+ 19.4 kcalmol–1 vs. 1e + PCl3) than that of the formal
addition product DCl (ΔG = +11.4 kcalmol–1), which is also
reached by a lower activation barrier. The endergonic na-
ture of the transformation 1e + PCl3 Ǟ DCl + HCl implies
that trapping of hydrogen chloride by a base is again indis-
pensable as driving force for the product formation.
Reactions of Boron Tribromide with Diazadienes
Studies of syntheses of N-heterocyclic boranes (NHBs)
focused on derivatives bearing sterically demanding N-(2,6-
diisopropylphenyl) (Dipp) substituents. Reaction of BBr3
with 1b proceeded smoothly at room temperature to furnish
a mixture of three products, which were later identified as
mono- to tribromo-NHBs 11b–13b (Scheme 4).
For comparison, we also studied the reaction of 1e with
PI3. Again, the first step produces a pentacoordinate inter-
mediate AI with square-planar geometry at the phosphorus
atom. However, this species is different as it represents, ac-
cording to an NBO analysis, no phosphorane but rather a
charge-transfer complex of the diazadiene with PI3.
Furthermore, its decay does not lead to a dihalophosphon-
ium salt but results essentially in the loss of I2 to furnish
directly the N-heterocyclic phosphane 10 (Figure 2c). A for-
mal imine addition product (DI) was also identified as a
local minimum on the energy hypersurface, but was in view
of its prohibitively high energy (the calculated ΔG is
24.3 kcalmol–1 above that of GI) not further considered as
alternative reaction product. The calculations indicate that
I– abstraction from 10 by I2 to the experimentally observed
Scheme 4. Reactions of diazadiene 1b (R = Dipp) with BBr3 or
BBr3–NR3 adducts [NR3 = NEt3, N(Et)iPr2] at ambient tempera-
ture.
As with PBr3, selectivity increased upon addition of a
phosphenium triiodide [Scheme 2 (c)][13] is also slightly ex- tertiary amine. Reaction of 1b with BBr3 in the presence of
ergonic.
1 equiv. of triethylamine (as an alternative to adding the
In total, the computational studies suggest that the reac- amine to the reaction, also the preformed BBr3–amine ad-
tions of PBr3 and PCl3 with diazadienes can follow two duct could be used) gave a mixture containing only 11b and
competing reaction channels, which may both be classified 12b (16:84 ratio according to 1H NMR analysis of the crude
as oxidative addition/reductive elimination sequences. The reaction product) and amine hydrobromide. Interestingly,
oxidative addition proceeds as concerted [4+1]-cycload- 11b became the main product (ratio 11b/12b = 94:6) when
dition of the diazadiene to PX3 to give a phosphorane. The the reaction was carried out with Hünig’s base. We explain
reductive elimination occurs in two steps, the first of the different product ratio by the fact that this bulky amine
which – ionization of the phosphorane by heterolytic P–X cannot form a stable Lewis pair with BBr3 and is thus more
bond fission – is common to both reaction channels. Mono- active in quenching Br2 and suppressing the formation of
substituted NHPs are then formed by nucleophilic substitu- 12b. The major products of both reactions were isolated in
tion (SN) at a covalently bound X atom. Alternatively, the moderate yields (limited by losses during crystallisation)
ion pair can rearrange to a formal imine addition product, and fully characterized.
which may undergo further transformations to yield a di-
Monitoring the progress of a reaction of 1b with BBr3
1
halogenated NHP. The different product selectivity in reac- and triethylamine in [D8]THF by H NMR spectroscopy
tions of PBr3 and PCl3 is due to the fact that in each case revealed that the product ratio remained constant through-
a different reaction channel (leading to 6e or DCl/8e, respec- out the reaction. This finding makes it unlikely that 11b is
tively) is thermodynamically and kinetically favoured. In formed as initial product and undergoes a follow-up reac-
simple terms, the failure to observe monosubstituted NHPs tion to 12b. Rather, both species are presumably formed
in reactions of PCl3 with diazadienes is attributable to the through competing reaction channels, and the mechanistic
fact that formation of Cl2 as oxidative by-product is ther- scenario is thus similar as in reactions of PBr3 with 1a–c.
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In contrast, the tribromo-NHB 13b (which is only formed
in the absence of amines) arises possibly from subsequent
bromination of 11b or 12b. In order to verify this hypothe-
sis, we treated 11b with 1 and 2 equiv. of Br2 in the presence
of Hünig’s base. NMR spectroscopic analysis of the prod-
ucts disclosed that in the first case a mixture of 11b–13b
(with 12b as main product) had formed, and in the second
case near quantitative (Ͼ 95%) conversion to 13b had taken
place. In both cases, the tertiary ammonium bromide
formed as by-product. Tribromide 13b was isolated in satis-
factory yield after workup and was fully characterized. The
molecular structures of 12b and 13b (Figure 3) exhibit
planar five-membered rings as expected. The endocyclic
bond lengths differ not significantly from each other or
from those in 11b,[25] indicating that the Br substitution has
no visible structural impact on the heterocycle. The B–Br
bond lengths [12b: 1.911(3) Å; 13b: 1.915(3) Å] are similar
as in 11b [1.898(7) Å[25]], and the C–Br bonds are normal.
Computational studies of the reaction of diazadiene 1e
with BBr3 support the idea that mono- and dibromo-NHBs
are, in a similar manner as with PBr3, formed through com-
peting reaction channels (Figure 4).
Figure 4. Calculated minimum free energy pathways for the reac-
tions of diazadiene 1e with BBr3 at the b3lyp/def2-tzvp level. Sol-
vent effects were included by using a PCM approach. FBr and FBrЈ
denote contact ion pairs with different arrangement of ions.
Evaluation of the relative free energies of transition states
(TS2Ј and TS2) and products (11e + Br2 and 12e + HBr;
The reaction starts with the formation of a contact ion ΔΔG = 7.7 kcalmol–1) indicates that formation of 12e is ki-
pair FBr consisting of a doubly donor-stabilized heterocyclic netically and thermodynamically preferred, in line with the
dibromoborenium cation and a bromide anion. This inter- observed product distribution in the reaction of BBr3 with
mediate can further rearrange via a transition state TS2Ј 1b. Quite interestingly, the final products are less stable than
under migration of a bromine atom from the boron atom the donor-supported borenium salt FBr. This is in accord
to the carbon atom and formation of two Br–C bonds to with the fact that creation of such cations from boron tri-
give the addition product GBr, which can then loose HBr to halides and diazadienes is well known,[4b,10,16,26] and several
yield the final product 12e in a slightly exergonic step.[27] specimens have been isolated and structurally charac-
Alternatively, attack of Br– on a B-bound Br atom can pro- terized.[16,28] While the transformation of such stable ad-
duce Br2 and monobromo-NHB 11e via transition state ducts into NHBs requires an external reductant,[4b,10] reac-
TS2. The latter transformation starts from an alternative tion of 1b with BBr3 proceeds directly to the three-coordi-
configuration FBrЈ of the ion pair, which exhibits a larger nate boranes, and a donor-supported borenium salt can nei-
ion separation and is therefore higher in energy (ΔΔG = ther be spectroscopically observed nor isolated. This differ-
14.2 kcalmol–1). Although an energy difference this size ence implies that the borenium salt is in this case destabi-
seems at first glance prohibitive, the transition states TS2 lized, possibly by the influence of the bulky N-aryl groups.
and TS2Ј themselves are energetically much closer (ΔΔG =
Calculations on analogous chloroboranes were carried
5.3 kcalmol–1), making competition between both pathways out for comparison and revealed a similar picture. A differ-
a viable option. This is even more so if one considers that ence lies mainly in the fact that the free energy of 3e + Cl2
in a real reaction an intermolecular process involving the exceeds that of 4e (the chloro analogue of FBr) by
anion of an adjacent ion pair may be feasible.
28.4 kcalmol–1. As in the case of the phosphane reactions,
Figure 3. Representation of the molecular structures of 12b (left) and 13b (right; the molecule has crystallographic C2 symmetry) in the
crystal. Thermal ellipsoids are drawn at 50% probability level, H atoms and solvent molecule are omitted for clarity. Selected bond
lengths [Å]: 12b: Br1–B1 1.911(3), B1–N5 1.413(4), B1–N2 1.422(4), N2–C3 1.408(3), Br3–C3 1.872(3), C3–C4 1.332(4), C4–N5 1.410(3);
13b: Br1–B1 1.915(3), B1–N2 1.421(2), Br2–C3 1.8591(17), N2–C3 1.406(2), N2–C4 1.437(2), C3–C3Ј 1.351(3).
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the thermodynamically unfavourable formation of Cl2 as carbon atom may initiate the formation of 2,4-dihaloge-
oxidative by-product puts the monochloro-NHB 3e out of nated heterocycles. Our calculations suggest that the former
reach.
reaction channel is inaccessible for energetic reasons in re-
The marked dependence of the product ratio 11b/12b on actions of diazadienes with ECl3 and provide thus an expla-
the added amine suggests that the base serves not simply as nation for the qualitatively different reactivity of ECl3 and
auxiliary reagent, which captures the by-product (HBr or EBr3, which is empirically well known.
Br2), but may also be directly involved as nucleophile in the
The experimental studies confirm that the reactions pro-
debromination of the dihaloborenium cation. We therefore ceed in the absence of any auxiliary reagents, although the
thought it worthwhile to search for a reagent that drives the presence of a tertiary amine can improve the selectivity and
product distribution even further to the side of the mono- facilitate the workup. The product distribution in reactions
bromo heterocycle and – ideally – avoids formation of 12b with EBr3 depends strongly on the N-substituents, but can
at all. Knowing that triphenylphosphane reacts very rapidly also be influenced by the choice of the auxiliary reagent.
with Br2, we treated 1b with the BBr3–PPh3 complex. NMR Even if this diversity implies that reaction conditions need
analysis of the crude reaction product revealed a ratio of to be optimized for each single target compound and thus
11b/12b of Ͼ 98:2, indicating that indeed near quantitative impedes the development of a generally applicable synthetic
shift of the product ratio had been accomplished, and the protocol, it was shown that selected target compounds are
product could be isolated in good yield after workup accessible with comparable or even superior efficiency as
(Scheme 5).
compared to established synthetic approaches.
We have further established that tribromo-substituted N-
heterocyclic boranes are accessible by bromination of
mono- or dibrominated precursors. This reaction seems to
be the first example of electrophilic backbone functionaliza-
tion in an NHB.
Scheme 5. Reaction of diazadiene 1b (R = Dipp) with BBr3–PPh3.
Experimental Section
General Conditions: All manipulations were carried out under dry
argon in a Schlenk apparatus or a glove box. Solvents were dried
according to standard procedures. NMR spectra were recorded
with Bruker Avance AV 400 or AV 250 instruments (1H: 400.1/
250.0 MHz; 13C: 100.5/62.9 MHz; 11B: 80.2 MHz; 31P: 161.9/
101.2 MHz). Chemical shifts were referenced to ext. TMS (1H,
13C), BF3·(OEt2) (11B; Ξ = 32.083974 MHz), 85% H3PO4 (31P; Ξ =
40.480747 MHz). Coupling constants are given as absolute values.
Elemental analyses were carried out with an Elementar Micro
Cube. Melting points were determined with a Büchi B-545 melting
point apparatus in sealed capillaries. Diazabutadienes[30] and Ph3P–
BBr3[31] were synthesized as described. Et3N–BBr3 was prepared in
analogy to Et3N–BCl3.[32] Both BBr3 adducts were synthesized un-
der Schlenk conditions at 0 °C. Crude products were filtered off
and washed with dry pentane.
Initial studies on the reactions of other diazadienes than
1b with BBr3 and tertiary amines revealed that formation
of mixtures of mono- and dibromo-NHBs is observed as
well, and the reaction scheme is thus more general. How-
ever, we also noted the appearance of additional side prod-
ucts the structures of which have not yet been fully as-
signed. Obviously, further optimization is needed to turn
this approach into a generally applicable preparative
method.
Conclusions
A combined computational and experimental study al-
lowed us to establish a mechanistic picture of the reactions
of diazadienes with EX3 (E = P, B; X = Cl, Br), which can
be considered to foster the further development of selective
syntheses of N-heterocyclic phosphanes and boranes. The
computations suggest that the reactions may proceed by
two competing pathways to yield N-heterocyclic com-
pounds featuring either one or two halogeno substituents
on the ring. The initial interaction between the diazadiene
and EX3 produces an ion pair containing a cyclic dihalo-
genophosphonium or -borenium cation as key intermediate.
In reactions with phosphorus trihalides, this step represents
an oxidative addition, which is closely related to the well-
known McCormack reaction.[29] 2-Halogeno-substituted
products can then be formed by nucleophilic attack of the
halide anion on a covalently bound halogen atom in the
cation and concomitant formation of elemental X2. Alter-
Syntheses
Reactions of 1,4-Diazabutadienes with PBr3: The appropriate 1,4-
diazabutadiene (1.3 mmol), Et3N (0.74 mL, 5.3 mmol), and (if ap-
propriate) cyclohexene (0.9 mL, 9 mmol) were dissolved in CH2Cl2
(25 mL). PBr3 (0.13 mL, 1.3 mmol) was added dropwise within
10 min under the conditions stated in the text. The mixture was
stirred for 1 h and then analyzed by 31P{1H} NMR spectroscopy.
The product distribution was determined by spectral deconvol-
ution.
2-Bromo-1,3-dicyclohexyl-1,3,2-diazaphospholene
(6a):
PBr3
(0.76 mL, 8.0 mmol) was added dropwise within 10 min to a cooled
(0 °C) solution of diazabutadiene 1a (2.00 g, 9.10 mmol) in THF
(40 mL). The mixture was stirred at room temp. for 4 d. Crystalli-
zation at +7 °C produced colorless needles of 6a, which were col-
lected by filtration and dried in vacuo. Yield 1.20 g (3.6 mmol,
1
45%). H NMR (CDCl3): δ = 7.31 (s, 2 H, =CH), 4.00 (m, 2 H,
NCH), 2.46–1.07 (m, 20 H, CH2) ppm. 13C{1H} NMR (C6D6): δ
2
2
natively, migration of a halogen from E (E = B, P) to a = 120.9 (d, JPC = 8.9 Hz, =CH), 57.9 (d, JPC = 10.6 Hz, NCH),
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3
33.3 (d, JPC = 9.3 Hz, NCCH2), 25.3 (s, NCCCH2), 25.2 (s, –20 °C. The crystalline precipitate was collected by filtration and
NCCCCH2) ppm. 31P{1H} NMR (CDCl3):
193.5 ppm. recrystallized from hexane to give 115 mg (246 μmol, 18%) of a
δ
=
C14H24BrN2P (331.23): calcd. C 50.77, H 7.30, N 8.46; found C
50.40, H 7.56, N 7.89.
brownish crystalline solid. (b) With PPh3–BBr3 (Small Scale): A
mixture of diazabutadiene 1b (3.60 g, 9.56 mmol) and Ph3P–BBr3
(4.90 g, 9.56 mmol) in Et2O (50 mL) was refluxed for 12 h to give
an orange solution and a white precipitate. Solvents were removed
under reduced pressure. The residue was dispersed in pentane
(80 mL) and insoluble Ph3PBr2 removed by filtration through Ce-
lite. The filtrate was concentrated under reduced pressure to a quar-
ter of the original volume and the residual solution stored at –20 °C
to yield 3.20 g (6.84 mmol, 72%) of product in the form of pale
orange crystals. (c) With PPh3–BBr3 (Large Scale): A mixture of
diazabutadiene 1b (35.0 g, 92.9 mmol) and Ph3P–BBr3 (38.4 g,
74.9 mmol) in Et2O (600 mL) was refluxed for 48 h to give a red
solution and a white precipitate. Solvents were removed under re-
duced pressure, and the residue was dissolved in pentane (600 mL).
The insoluble Ph3PBr2 was removed by filtration and extracted
with hexane (300 mL). The combined filtrates were concentrated
under reduced pressure to a quarter of the original volume, and
the remaining solution was stored at –20 °C to yield 30.6 g
2-Bromo-1,3-dicyclohexyl-1,3,2-diazaphospholene (6a) and 2,4-Di-
bromo-1,3-dicyclohexyl-1,3,2-diazaphospholene (8a): Diazabutadi-
ene 1a (500 mg, 2.30 mmol) and Et3N (2.50 mL, 16.6 mmol) were
dissolved in toluene (10 mL). PBr3 (0.19 mL, 2.2 mmol) was added
in one batch. The mixture was stirred for 10 min. Hexane (50 mL)
was added, and precipitated solids were filtered off. Storage of the
filtrate at +7 °C produced a small amount of colourless crystals.
1
Characterization by H and 31P NMR spectroscopy and a single-
crystal X-ray diffraction study allowed to identify the product as a
1:1 mixture of 6a/8a. 8a: 1H NMR (CDCl3): δ = 6.17 (s, 1 H, =CH),
3.60 (m, 1 H, NCH), 3.26 (m, 1 H, NCH), 2.46–1.07 (m, 20 H,
1
CH2) ppm. 13C{1H} NMR (CDCl3, data from H,13C HSQC and
HMBC spectra; signals of the CH2 carbon atoms in the cyclohexyl
substituents could not be unambiguously assigned): δ = 119.5
(=CH), 105.8 (=CBr), 59.2 (NCH), 57.8 (NCH) ppm. 31P{1H}
NMR (CDCl3): δ = 171.4 ppm.
1
(65.5 mmol, 71%) of a yellow, microcrystalline powder. H NMR
2-Bromo-1,3-bis(2,2-diisopropylphenyl)-1,3,2-diazaphospholene (6b):
Et3N (3.55 mL, 25.6 mmol) was added to a solution of diazabutadi-
ene 1b (3.84 g, 10.2 mmol) in toluene (50 mL). Subsequently, PBr3
(0.95 mL, 10.2 mmol) was added dropwise. The resulting yellow
solution was stirred for 15 min until it became slightly cloudy. Ad-
dition of hexane (50 mL) induced formation of an off-white pre-
cipitate, which was removed by filtration. The filtrate was concen-
trated to dryness under reduced pressure. Recrystallization from
toluene/hexane (1:1) or THF at –20 °C afforded a light yellow, crsy-
stalline product (yield 4.46 g, 9.2 mmol, 90%) of m.p. 150 °C (dec.).
1H NMR (C6D6): δ = 7.20–6.97 (m, 6 H, C6H3), 6.17 (2 H, NCH),
3.57 [sept, 3JHH = 6.8 Hz, 4 H, CH(CH3)2], 1.31 [d, 3JHH = 6.8 Hz,
(CDCl3): δ = 7.30–7.09 (m, 6 H, ArH), 6.33 (s, 2 H, =CH), 2.98
3
3
[sept, JHH = 6.8 Hz, 4 H, CH(CH3)2], 1.23 [d, JHH = 6.8 Hz,
24 H, CH(CH3)2] ppm. 11B{1H} NMR (CDCl3): δ = 20.3 (br.)
ppm.
2,4-Dibromo-1,3,2-bis(2,6-diisopropylphenyl)-1,3-diazaborole (12b):
A mixture of diazabutadiene 1b (2.00 g, 5.31 mmol) and Et3N–
BBr3 (1.87 g, 5.31 mmol) in pentane (20 mL) was stirred at room
temp. for 12 h. Precipitated NEt3·HCl was removed by filtration
through Celite. The yellow filtrate was concentrated under reduced
pressure to a quarter of the original volume and the remaining
solution stored at –20 °C to give 0.95 g (1.74 mmol, 33%) of 12b
1
as colourless crystals of m.p. 120 °C (dec.). H NMR (C6D6): δ =
3
12 H, CH(CH3)2], 1.08 [d, JHH = 6.8 Hz, 12 H, CH(CH3)2] ppm.
3
7.27–7.07 (m, 6 H, C6H3), 6.20 (s, 1 H, =CH), 3.11 [sept, JHH
=
2
13C{1H} NMR (C6D6): 147.5 (d, JPC = 4.5 Hz, i-C6H3), 133.5 (d,
3
6.9 Hz, 2 H, CH(CH3)2], 3.09 [sept, JHH = 6.9 Hz, 2 H, CH(CH3)
5
3JPC = 9.0 Hz, o-C6H3), 129.5 (d, JPC = 2.0 Hz, p-C6H3), 124.8 (s,
2], 1.32 [d, 3JHH = 6.9 Hz, 6 H, CH(CH3)2], 1.30 [d, 3JHH = 6.9 Hz,
2
4
m-C6H3), 122.5 (d, JPC = 8.5 Hz, =CH), 29.0 [d, JPC = 2.0 Hz,
3
6 H, CH(CH3)2], 1.29 [d, JHH = 6.9 Hz, 6 H, CH(CH3)2], 1.14 [d,
5
CH(CH3)2], 25.3 [s, CH(CH3)2], 24.6 [d, JPC = 2.0 Hz, CH-
3JHH = 6.9 Hz, 6 H, CH(CH3)2] ppm. 13C{1H} NMR (C6D6): δ =
146.8 (s, i-C6H3), 145.9 (s, i-C6H3), 136.4 (s, o-C6H3), 134.6 (s, o-
C6H3), 128.9 (s, p-C6H3), 128.4 (s, p-C6H3), 123.61 (s, m-C6H3),
123.57 (s, m-C6H3), 119.4 (s, =CH), 103.5 (s, =CBr), 28.9 [s,
CH(CH3)2], 28.6 [s, CH(CH3)2], 24.0 [s, CH(CH3)2], 23.9 [s,
CH(CH3)2], 23.8 [s, CH(CH3)2], 23.4 [s, CH(CH3)2] ppm. 11B{1H}
NMR (C6D6): δ = 20.5 (br.) ppm. C26H35BBr2N2 (546.20): calcd.
C 57.17, H 6.46, N 5.13; found C 57.42, H 6.49, N 5.16.
(CH3)2] ppm. 31P{1H} NMR (C6D6):
δ = 161.4 (s) ppm.
C26H36N2PBr·C4H8O (559.56) calcd. C 64.39, H 7.93, N 5.01;
found C 63.88, H 7.89, N 4.95.
Reaction of 1,4-Bis(2,6-diisopropylphenyl)-1,4-diazabutadiene with
BBr3: A solution of BBr3 (0.25 mL, 2.65 mmol) in hexane (10 mL)
was added dropwise to a solution of diazabutadiene 1b (1.00 g,
2.65 mmol) in hexane (40 mL) at room temp. The yellow solution
instantly turned red and, after the addition was complete, back to
yellow. A colourless precipitate formed, which was filtered off. The
filtrate was concentrated to dryness. Both the precipitate and the
residue of the filtrate were dissolved in CDCl3 and characterized
by 1H and 11B NMR spectroscopy. As the spectra indicated the
presence of a product mixture, no further analyses and determi-
nation of the yield were carried out.
2,4,5-Tribromo-1,3-bis(2,6-diisopropylphenyl)-1,3,2-diazaborole
(13b): Br2 (0.50 mL, 9.8 mmol) was added to a solution of 11b
(2.00 g, 4.28 mmol) in pentane (60 mL). The solution turned red,
and a white precipitate formed. The mixture was then stirred for
24 h during which a clear red solution formed. Ethyldiisopropyl-
amine (2.00 mL, 11.8 mmol) was added and the mixture stirred for
further 30 min. The solution turned orange, and iPr2EtN·HBr pre-
cipitated as a colourless solid. Volatiles were removed under re-
2-Bromo-1,3-bis(2,2-diisopropylphenyl)-1,3,2-diazaborole
(11b).
(a) With Hünig’s Base: A solution of BBr3 (130 μL, 1.33 mmol) in duced pressure, and the residue was redissolved in pentane (60 mL).
hexane (5 mL) was added dropwise to a solution of ethyldiiso-
propylamine (230 μL, 1.33 mmol) in hexane (10 mL). The mixture
was stirred at room temp. for 30 min during which a yellowish oil
separated. Diazabutadiene 1b (500 mg, 1.33 mmol) was added, and
the mixture stirred at 50 °C for 48 h to give a dark brown solution.
Solvents and volatiles were removed under reduced pressure. The
solid was extracted with hexane (25 mL) and the resulting suspen-
sion filtered through Celite. The filtrate was concentrated under
reduced pressure to a quarter of the original volume and stored at
An insoluble fraction was removed by filtration through Celite. The
filtrate was concentrated under reduced pressure to a quarter of
the original volume and the remaining solution stored at –20 °C to
give 1.74 g (2.78 mmol, 65%) of 13b as colourless crystalline solid
of m.p. 120 °C (dec.). Analytically pure crystals suitable for a sin-
gle-crystal X-ray diffraction study were obtained by recrystalli-
zation from pentane. 1H NMR (C6D6): δ = 7.24–7.09 (m, 6 H,
3
C6H3), 3.05 [sept, JHH = 6.9 Hz, 4 H, CH(CH3)2], 1.30 [d, 3JHH
=
3
6.9 Hz, 12 H, CH(CH3)2], 1.24 [d, JHH = 6.9 Hz, 12 H, CH(CH3)
Eur. J. Inorg. Chem. 2015, 4819–4828
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2] ppm. 13C{1H} NMR (C6D6): δ = 146.6 (s, o-C6H3), 134.8 (s, ipso-
C6H3), 129.1 (s, p-C6H3), 123.7 (s, m-C6H3), 28.9 [s, CH(CH3)2],
Computational Studies: Computational studies were performed
with the Gaussian 03[34] suite of programs by using def2-tzvp basis
24.0 [s, CH(CH3)], 23.3 [s, CH(CH3)2] ppm. 11B{1H} NMR (C6D6): sets[35] (obtained from the basis set exchange home page[36,37]) on
δ = 20.5 (br.) ppm. C26H34BBr3N2 (625.09): calcd. C 49.96, H 5.48,
N 4.48; found C 49.84, H 5.51, N 4.46.
model compounds featuring N-Me instead of the larger N-alkyl or
N-Dipp substituents. Solvent effects were included by using a PCM
model as implemented in the Gaussian package (SCRF = CH2Cl2).
Molecular structures were first energy-optimized without sym-
metry constraints. Calculations of harmonic vibrational fre-
quencies were carried out in order to ensure that all stationary
points located were local minima on the energy hypersurface. Com-
puted (free) reaction energies were corrected for BSSE by using the
counterpoise method where appropriate. The NBO analysis[38] was
performed with the NBO 3.1 program as implemented in the
Gaussian package. MOLDEN[39] was used for visualization.
Crystal-Structure Determinations: Diffraction studies were carried
out with a Bruker diffractometer equipped with a Kappa APEX II
Duo CCD detector and a KRYO-FLEX cooling device with Mo-
Kα radiation (λ = 0.71073 Å) at T = 100 K (6b, 12b–13b) or 110 K
(6a/8a). The structures were solved by direct methods (SHELXS-
97[33]) and refined with a full-matrix least-squares scheme on F2
(SHELXL-2014 and SHELXL-97[33]). Semiempirical absorption
corrections were applied for all structures. Non-hydrogen atoms
were refined anisotropically, and H atoms with a riding model, on
F2. The Br1 atom in 6a/8a was disordered over two positions (rela-
tive occupations Br1/Br1a = 90:10) and was refined isotropically. Acknowledgments
A high calculated residual electron density (2.10 eA–3) at a distance
of approx. 1.72 Å from C3 is attributed to partial halogenation of
the carbon atom (either by Cl or Br, approx. 5–6%). A solvent
molecule in 6a/8a and 13b is disordered. CCDC-1059566 (13b),
-1059567 (12b), 1059568 (6a/8a), 1059569 (6b) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 6a and 8a:
Colourless crystals, C15.75H25.50Br1.50N2P, M = 393.71 gmol–1,
We thank Dr. W. Frey (Institute of Organic Chemistry, University
of Stuttgart) for the collection of X-ray data sets.
[1] A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
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2009, 109, 3612–3676; f) S. Diez-Gonzalez, N. Marion, S. P.
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Tools (Ed.: S. Diez-Gonzalez), Royal Society of Chemistry,
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¯
crystal size 0.35ϫ0.27ϫ0.13 mm, triclinic, space group P1, a =
9.2158(6) Å, b = 12.4233(8) Å, c = 15.2820(11) Å, α = 82.943(4)°,
β = 88.153(4)°, γ = 87.297(3)°, V = 1733.8(2) Å3, Z = 4, ρ(calcd.)
= 1.508 Mgm3, F(000) = 806, θmax = 28.49°, μ = 3.61 mm–1, numer-
ical absorption correction, max./min. transmission 0.8098/0.4748,
74382 reflections measured, 8655 unique reflections (Rint = 0.0499)
for structure solution and refinement with 385 parameters and 50
restraints, R1 [I Ͼ 2σ(I)] = 0.0517, wR2 = 0.1300, GooF on F2
1.060, largest diff. peak/hole 2.302/–1.766 eÅ–3. 6b: Colourless
crystals, C26H36N2PBr,
M
=
487.45 gmol–1, crystal size
[3] a) E = Si: M. Denk, R. Lennon, R. Hayashi, R. West, A. Haa-
land, H. Belyakov, P. Verne, M. Wagner, N. Metzler, J. Am.
Chem. Soc. 1994, 116, 2691–2692; b) E = Ge: W. A. Herrmann,
M. Denk, J. Behm, W. Scherer, F.-R. Klingan, H. Bock, B.
Solouki, M. Wagner, Angew. Chem. Int. Ed. Engl. 1992, 31,
1485–1488; Angew. Chem. 1992, 104, 1489; c) M = Sn: T. Gans-
Eichler, D. Gudat, M. Nieger, Angew. Chem. Int. Ed. 2002, 41,
1888–1891; Angew. Chem. 2002, 114, 1966; d) E = B: Y. Se-
gawa, M. Yamashita, K. Nozaki, Science 2006, 314, 113–115;
e) E = Ga: E. S. Schmidt, A. Jockisch, H. Schmidbaur, J. Am.
Chem. Soc. 1999, 121, 9758–9759; f) E = N: G. Boche, P. And-
rews, K. Harms, M. Marsch, K. S. Rangappa, M. Schimeczek,
C. Willeke, J. Am. Chem. Soc. 1996, 118, 4925–4930; g) E = P:
M. K. Denk, S. Gupta, A. J. Lough, Eur. J. Inorg. Chem. 1999,
41–49; h) E = As: C. J. Carmalt, V. Lomeli, B. G. McBurnett,
A. H. Cowley, Chem. Commun. 1997, 2095–2096; i) E = Sb: D.
Gudat, T. Gans-Eichler, M. Nieger, Chem. Commun. 2004,
2434–2435; j) E = S: C. D. Martin, M. C. Jennings, M. J. Fergu-
son, P. J. Ragogna, Angew. Chem. Int. Ed. 2009, 48, 2210–2213;
Angew. Chem. 2009, 121, 2244; k) E = Se: J. L. Dutton, H. M.
Tuononen, M. C. Jennings, P. J. Ragogna, J. Am. Chem. Soc.
2006, 128, 12624–12625.
0.45ϫ0.19ϫ0.16 mm, orthorhombic, space group P212121, a =
8.2051(4) Å, b = 13.6053(8) Å, c = 22.8176(15) Å, V = 2547.2(3)
A3, Z = 4, ρ(calcd.) = 1.271 Mgm–3, F(000) = 1024, θmax = 28.28°,
μ = 1.69 mm–1, 22458 reflections measured, 6256 unique reflections
(Rint = 0.0614) for structure solution and refinement with 271 pa-
rameters, numerical absorption correction, max./min. transmission
0.7889/0.5316, R1 [I Ͼ 2σ(I)] = 0.0384, wR2 = 0.0594, GooF on
F2 0.955, absolute structure parameter –0.003(6), largest diff. peak/
hole 0.497/–0.315 eÅ–3. 12b: Colourless crystals, C26H35BBr2N2, M
= 546.19 gmol–1, crystal size: 0.15ϫ0.14ϫ0.13 mm, monoclinic,
space group P21/c, a = 8.5088(5) Å, b = 17.2086(11) Å, c =
17.9580(11) Å, β = 91.221(4)°, V = 2628.9(3) A3, Z = 4, ρ(calcd.) =
1.380 Mgm–3, F(000) = 1120, θmax = 26.41°, μ = 3.10 mm–1, semi-
empirical absorption correction from equivalents, max./min. trans-
mission 0.7350/0.6718, 39706 reflections measured, 5399 unique re-
flections (Rint = 0.0957) for structure solution and refinement with
280 parameters, R1 [I Ͼ 2σ(I)] = 0.038, wR2 = 0.064, GooF on
F2 1.001, largest diff. peak/hole 0.465/–0.426 eÅ–3. 13b: Colourless
crystals, C26H34BBr3N2/pentane, M = 697.24 gmol–1, crystal size
[4] a) EX2 = SiCl2: H. tom Dieck, M. Zettlitzer, Chem. Ber. 1987,
120, 795–801; b) EX = BBr: L. Weber, E. Dobbert, H. G.
Stammler, B. Neumann, R. Boese, D. Bläser, Chem. Ber./Re-
cueil 1997, 130, 705–710; c) EX = PCl: M. K. Denk, S. Gupta,
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EX = AsCl: Ref.[3h]; e) EX = SbCl: Ref.[3i]
[5] A. V. Protchenko, M. P. Blake, A. D. Schwarz, C. Jones, P.
Mountford, S. Aldridge, Organometallics 2015, 34, 2126–2129;
L. Weber, H. B. Wartig, H.-G. Stammler, A. Stammler, B. Neu-
mann, Organometallics 2000, 19, 2891–2895.
0.47ϫ0.26ϫ0.25 mm, monoclinic, space group C2/c,
a =
22.3719(19) Å, b = 9.5588(6) Å, c = 17.6750(15) Å, β = 117.195(6)°,
V = 3361.9(5) A3, Z = 4, ρ(calcd.) = 1.378 Mgm3, F(000) = 1424,
θmax = 28.36°, μ = 3.62 mm–1, numerical absorption correction,
max./min. transmission 0.7028/0.4498, 33878 reflections measured,
4183 unique reflections (Rint = 0.0379) for structure solution and
refinement with 166 parameters and 16 restraints, R1 [I Ͼ 2σ(I)] =
0. 0251, wR2 = 0.0617, GooF on F2 1.048, largest diff. peak/hole
0.553/–0.473 eÅ–3.
Eur. J. Inorg. Chem. 2015, 4819–4828
4827
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
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C6D6. Structural assignment is based on the following data:
31P NMR: δ = 165.3 (d, JPH = 22 Hz) ppm. 1H NMR: δ = 6.83
3
(d, JPH = 22 Hz, 1 H, =CH), 2.62 (q, JHH = 7 Hz, 4 H,
3
NCH2), 0.71 (t, JHH = 7 Hz, 6 H, CH3) ppm. 13C NMR (the
signal of the Br-substituted carbon atom was not detected due
to insufficient signal/noise ratio): δ = 144.3 (d, JPC = 105 Hz,
=CH), 45.8 (d, JPC = 9 Hz, NCH2), 14.0 (CH3) ppm. In some
reactions, a mixture of BrnP{C(Br)=CHNEt2}3–n (n = 0–2) was
formed. Of these, also the signals of the disubstitution product
1
(with n = 1) could be identified: H NMR: δ = 7.20 (d, JPH
=
3
11 Hz, 2 H, =CH), 2.86 (q, JHH = 7 Hz, 8 H, NCH2), 0.79 (t,
3JHH = 7 Hz, 12 H, CH3) ppm. 13C NMR: δ = 145.1 (d, JPC
=
56 Hz, =CH), 83.7 (d, JPC = 54 Hz, =CH), 46.3 (d, JPC = 9 Hz,
NCH2), 14.2 (CH3) ppm. 31P NMR: δ = 109.1 (d, JPH = 11 Hz)
ppm.
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scopically detectable transient intermediate (31P NMR: δ =
199.1 ppm) of unknown structure, which was eventually con-
verted into 1b.
Received: August 2, 2015
Published Online: September 14, 2015
Eur. J. Inorg. Chem. 2015, 4819–4828
4828
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim