Synthesis and the thermal and catalytic dehydrogenation reactions of amine-thioboranes

Article abstract of DOI:10.1021/ic3008188

A series of trimethylamine-thioborane adducts, Me₃N· BH₂SR (R = tBu [2a], nBu [2b], iPr [2c], Ph [2d], C₆F ₅ [2e]) have been prepared and characterized. Attempts to access secondary and primary amine adducts of thioboranes via amine-exchange reactions involving these species proved unsuccessful, with the thiolate moiety shown to be vulnerable to displacement by free amine. However, treatment of the arylthioboranes, [BH₂-SPh]₃ (9) and C₆F ₅SBH₂·SMe₂ (10) with Me₂NH and iPr₂NH successfully yielded the adducts Me₂NH· BH₂SR (R = Ph [11a], C₆F₅ [12a]) and iPr ₂NH·BH₂SR (R = Ph [11b], C₆F₅ [12b]) in high yield. These adducts were also shown to be accessible via thermally induced hydrothiolation of the aminoboranes Me₂N=BH ₂, derived from the cyclic dimer [Me₂N-BH ₂]₂ (13), and iPr₂N=BH₂ (14), respectively. Attempts to prepare the aliphatic thiolate substituted adducts R₂NH·BH₂SR′ (R = Me, iPr; R′ = tBu, nBu, iPr) via this method, however, proved unsuccessful, with the temperatures required to facilitate hydrothiolation also inducing thermal dehydrogenation of the amine-thioborane products to form aminothioboranes, R₂N= BH(SR′). Thermal and catalytic dehydrogenation of the targeted amine-thioboranes, 11a/11b and 12a/12b were also investigated. Adducts 11b and 12b were cleanly dehydrogenated to yield iPr₂N=BH(SPh) (22) and iPr₂N=BH(SC₆F₅) (23), respectively, at 100 °C (18 h, toluene), with dehydrogenation also possible at 20 °C (42 h, toluene) with a 2 mol % loading of [Rh(μ-Cl)cod]₂ in the case of the former species. Similar studies with adduct 11a evidenced a competitive elimination of H₂ and HSPh upon thermolysis, and other complex reactivity under catalytic conditions, whereas the fluorinated analogue 12a was found to be resistant to dehydrogenation.

Full text of DOI:10.1021/ic3008188

Article
pubs.acs.org/IC
Synthesis and the Thermal and Catalytic Dehydrogenation Reactions
of Amine-Thioboranes
Alasdair P. M. Robertson, Mairi F. Haddow, and Ian Manners*
Department of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS U. K.
S
* Supporting Information
ABSTRACT: A series of trimethylamine-thioborane adducts, Me3N·BH2SR
(R = tBu [2a], nBu [2b], iPr [2c], Ph [2d], C6F5 [2e]) have been prepared
and characterized. Attempts to access secondary and primary amine adducts
of thioboranes via amine-exchange reactions involving these species proved
unsuccessful, with the thiolate moiety shown to be vulnerable to
displacement by free amine. However, treatment of the arylthioboranes,
[BH2−SPh]3 (9) and C6F5SBH2·SMe2 (10) with Me2NH and iPr2NH
successfully yielded the adducts Me2NH·BH2SR (R = Ph [11a], C6F5 [12a])
and iPr2NH·BH2SR (R = Ph [11b], C6F5 [12b]) in high yield. These
adducts were also shown to be accessible via thermally induced hydrothiolation of the aminoboranes Me2NBH2, derived from
the cyclic dimer [Me2N-BH2]2 (13), and iPr2NBH2 (14), respectively. Attempts to prepare the aliphatic thiolate substituted
adducts R2NH·BH2SR′ (R = Me, iPr; R′ = tBu, nBu, iPr) via this method, however, proved unsuccessful, with the temperatures
required to facilitate hydrothiolation also inducing thermal dehydrogenation of the amine-thioborane products to form
aminothioboranes, R2NBH(SR′). Thermal and catalytic dehydrogenation of the targeted amine-thioboranes, 11a/11b and
12a/12b were also investigated. Adducts 11b and 12b were cleanly dehydrogenated to yield iPr2NBH(SPh) (22) and iPr2N
BH(SC6F5) (23), respectively, at 100 °C (18 h, toluene), with dehydrogenation also possible at 20 °C (42 h, toluene) with a 2
mol % loading of [Rh(μ-Cl)cod]2 in the case of the former species. Similar studies with adduct 11a evidenced a competitive
elimination of H2 and HSPh upon thermolysis, and other complex reactivity under catalytic conditions, whereas the fluorinated
analogue 12a was found to be resistant to dehydrogenation.
unexplored. Research published in the mid 20th century
documents the synthesis of amine adducts of thioboranes by
various methods, focusing primarily on tertiary amine
adducts.36−38 Mikhailov and co-workers, however, also reported
the synthesis of secondary amine-thioborane adducts,39 and
briefly discussed the competitive elimination of hydrogen and
the respective thiol from these species when heated to beyond
60 °C.39 The characterization of the initial adducts and
products, however, was limited compared to modern standards
with key experimental evidence provided solely by elemental
microanalysis and cryoscopic measurements.39 Of particular
relevance, given the current interest in metal-catalyzed
dehydrogenation of amine-boranes and their derivatives, is
the lack of any reports on the transition metal-catalyzed
eliminations of hydrogen from amine-thioboranes. Further-
more, a route to the regeneration of spent ammonia-borane fuel
following hydrogen release has recently been reported,
employing a B-thiolation process.40,41 The regeneration initially
involves reaction of benzenethiol or benzene-1,2-dithiol with
borazine and polyborazylene to produce dithiolated borane
adducts of ammonia, making further investigation of B-
thiolated amine-borane adducts particularly pertinent.
INTRODUCTION
■
The catalytic dehydrogenation of amine-borane adducts has
gained substantial attention in recent years as a result of the
large reductions in reaction temperature and increased reaction
rates that are possible relative to thermal hydrogen release.
Over the past decade the field has grown dramatically, and a
wide range of metals and main group species have now been
shown to be catalytically active for such processes.1−20 Research
in the area has also been accelerated by the interest in
ammonia-borane, NH3·BH3, and related species as potential
hydrogen storage and transfer media.21−28 Furthermore,
recently it has also been reported that catalytic dehydrocou-
pling allows access to poly(alkylaminoboranes), [RNH-BH2]n,
inorganic analogues of polyolefins.11,29 The dehydrogenation of
NH3·BH3 and its derivative borazine, [HN-BH]3, on metallic
surfaces have also recently been shown to provide a route to
meshes and, significantly, thin films of boron nitride, which may
find applications in graphene-based electronic devices.30
To date, however, reports of the catalytic dehydrogenation of
amine-borane adducts have almost exclusively discussed the
reactivity of adducts substituted at nitrogen.31 These studies
have demonstrated that both the thermodynamics of hydrogen
release, and the nature of the dehydrogenated products are
defined by the choice of substituent(s) at nitrogen.32−35 In
contrast, the dehydrocoupling reactivity of amine-boranes
containing heteroatom substitution at boron is virtually
Received: April 20, 2012
Published: July 16, 2012
© 2012 American Chemical Society
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11B NMR spectroscopy in CDCl3 solution indicated shifts of
−4.2, −3.7, and −1.9 ppm for 2a, 2d, and 2e, respectively,
which appeared as triplets, JBH = 100−115 Hz, upon proton
coupling. Both the shift and the coupling pattern are consistent
with the formation of a new four-coordinate boron environ-
ment in each case, with two hydrogen substituents at boron, as
In this paper we present the results of our attempts to
prepare monothiolated borane adducts of various amines, and
investigations of the dehydrogenation chemistry of the resulting
species under thermal and catalytic conditions.
RESULTS AND DISCUSSION
■
1
expected for monothiolated borane adducts. H and 13C NMR
(a). Trimethylamine-Thioborane Adducts,
Me3N·BH2SR. (i). Synthesis of Me3N·BH2SR (R = tBu [2a],
nBu [2b], iPr [2c], Ph [2d], and C6F5 [2e]). Literature reports of
the reaction of Me3N·BH3 (1) with aliphatic thiols at high
temperature to produce trialkylthioborates provided a potential
route to B-thiolated amine-borane adducts.42 It was postulated
that the initial preparation of trimethylamine-thioboranes,
Me3N·BH2SR, could then be followed by an amine-exchange
process to yield potential dehydrocoupling substrates (Scheme
1).
spectroscopy (and 19F NMR spectroscopy in the case of 2e)
also confirmed the successful incorporation of the thiolate
moiety into the amine-borane, with chemical ionization mass
spectrometry (CI-MS) and elemental microanalysis also
consistent with the assigned compositions. Atom connectivity
in each case was assigned unequivocally by single crystal X-ray
diffraction studies, carried out on crystals grown by sublimation
under high vacuum (Figure 1). The respective compounds all
crystallize with a single molecule in the asymmetric unit, and
are monomeric in nature. The length of the central B−N bond
varies slightly with the substituent at boron, with a slight
contraction apparent ([2a, 1.629(2) Å], [2d 1.623(2) Å], and
[2e, 1.617(2) Å]) as the nominal electron-withdrawing ability
of the thiol group increases.44 The electronic effects of the
thiolation appear, however, to be minor in comparison with the
steric effect of adding a bulky group at boron, with no overall
contraction of the B−N bond apparent relative to the starting
material, amine-borane 1 (1.617(6) Å).45
Scheme 1. Proposed Synthetic Route to Primary Amine-
Thioborane Adducts: (a) Initial Synthesis of
Trimethylamine-Thioboranes, (b) Subsequent Amine-
Exchange to Remove Tertiary Amine Moiety
Syntheses of Me3N·BH2SnBu (2b) and Me3N·BH2SiPr (2c)
were carried out in analogous fashion, with the products
isolated as highly moisture sensitive liquids (Table 1). Adduct
2b could not be purified beyond 95% based on integration of
the 11B NMR spectrum, with the impurities postulated to be
the multisubstitution products Me3N·BH(SnBu)2 (δB 5.6 [d,
JBH = 127 Hz]) and B(SnBu)3 (δB 58.7 (s) ppm). Adduct 2c
could be obtained more cleanly (∼99%), but despite attempted
purification of both products by distillation and sublimation
under reduced pressure, neither compound provided acceptable
elemental microanalysis data.
Utilizing a tertiary amine-borane precursor in this manner
also enables temperatures in excess of 100 °C to be employed
during the synthesis, conditions under which adducts
containing N−H and B−H bonds have been shown to
thermally dehydrocouple.2,22,43 Through a modification of the
method of Hawthorne,42 we were able to prepare a series of
monothiolated derivatives of 1 of the form Me3N·BH2SR in
high yield from commercially available starting materials
(Scheme 2).
The multisubstitution observed on reaction with HSnBu and
indeed HSiPr at higher temperatures can presumably be
explained on the basis of the relative steric encumbrance of the
various thiols. In the case of the synthesis of 2a, for example,
the increased steric bulk of HStBu enabled a clean synthesis of
the monosubstituted product, 2a, at higher temperatures in the
presence of a 5-fold excess of thiol.
Scheme 2. Generalized Thermal Synthesis of Me3N·BH2SR
(ii). Amine-Exchange Reactions of Me3N·BH2SR (R = tBu
[2a], nBu [2b], and iPr [2c]). Amine-exchange reactions are
well-documented within the amine-borane literature, and serve
as a means of transferring the Lewis acidic borane moiety
between two amines.31,46,47 These reactions are understood to
occur via nucleophilic substitution at boron, through SN1 or
SN2 type mechanisms depending on the relative steric
encumbrance of the borane moeity.48−50 Using this method,
often over multiple reaction cycles, the substitution of one
amine moiety for another can, in many cases, be readily
achieved.
The adducts were synthesized via thermolysis of a mixture of
amine-borane 1 and the relevant thiol at 100 to 150 °C (Table
1). The adducts Me3N·BH2StBu (2a), Me3N·BH2SPh (2d),
and Me3N·BH2SC6F5 (2e) were isolated as stable, crystalline
solids following removal of the solvent under high vacuum,
under which conditions residual 1, a highly volatile solid, and
unreacted thiol were also removed. Analysis of these adducts by
Table 1. Selected Properties of the Adducts Me3N·BH2SR (R
= tBu, nBu, iPr, Ph, C6F5)
It was therefore postulated that treatment of the trimethyl-
amine-thioborane adducts with excess MeNH2 could act as a
route to methylamine-thioboranes, of interest as potential
dehydrocoupling substrates. However, treatment of the adducts
2a−c, that is, aliphatic thiolate substituted adducts, with
MeNH2 led to unexpected reactivity, with no amine-exchange
reaction prevailing. In fact on treatment with excess MeNH2,
these adducts proved to be susceptible to nucleophilic
displacement of the thiolate moiety by the free amine to
δB
melting point
B−N bond length
a
adduct
(ppm)
(°C)
(Å)
Me3N·BH2StBu (2a)
Me3N·BH2SnBu (2b)
Me3N·BH2SiPr (2c)
Me3N·BH2SPh (2d)
Me3N·BH2SC6F5 (2e)
−4.2
−1.5
−2.8
−3.7
−1.9
74−75
1.629(2)
80−81
112−113
1.623(2)
1.617(2)
a
All recorded in CDCl3 solution.
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Figure 1. Molecular structures of (a) 2a, (b) 2d, and (c) 2e, with thermal ellipsoids at the 50% probability level. All hydrogen atoms omited for
clarity.
produce nonthiolated amine-borane products. For example,
upon treatment of a solution of 2a with a 10-fold excess of
MeNH2, 11B NMR spectroscopy indicated the primary
products of the reaction with excess amine to be the amine-
borane MeNH2·BH3 (3) (δB −19.3 [q, JBH = 96 Hz]),2 and the
bisaminoborane, HB(NHMe)2 (4) (δB 27.5 [d, JBH = 127
Hz]),51 in an approximately 1:1 ratio (Scheme 3). The relative
with free amines, analogous experiments were carried out with
the thiophenol and pentafluorothiophenol derivatives. It was
postulated that the increased electron withdrawing effect of
these thiolate groups, particularly in the polyfluorinated case,
may lead to a stronger B−N bond, and consequently a reduced
tendency to be nucleophilically displaced by free amine,
assuming an SN1 type mechanism. This suggestion was
confirmed on treatment of these species with excess MeNH2
solution.
Scheme 3. Attempted Amine-Exchange of Adducts 2a−c with
MeNH2
Upon treatment of a tetrahydrofuran (THF) solution of 2d
with a 10-fold excess of MeNH2 at 20 °C, no reaction was
observed over 18 h by 11B NMR spectroscopy, with only
unreacted starting material present in solution. However, the
use of the secondary amine Me2NH did result in somewhat
more successful amine-exchange reactions to produce a species
consistent with the formation of Me2NH·BH2SPh (11a), as
evidenced by the observation of a triplet in the 11B NMR
spectrum at −7.9 ppm of coupling constant 113 Hz.
Unfortunately, this reaction also yielded ∼44% conversion to
an unknown species by 11B NMR spectroscopy with a chemical
shift of −2.5 ppm, appearing as a broad singlet, which proved to
be inseparable from the desired product (Scheme 4). A
subsequent clean synthesis of the adduct via alternative means
(vide infra), however, confirmed the partially successful amine-
exchange process in this case. Again this difference in reactivity
between Me2NH and MeNH2 can be attributed to the
increased donor ability of Me2NH, which is likely to produce
a more thermodynamically favorable amine-exchange process.
Treatment of 2e with excess MeNH2 or Me2NH solution at
20 °C over 18 h produced no reaction with either amine, as
evidenced by 11B NMR spectroscopy. It is likely that the
increased electron withdrawing effect of the C6F5 group is
responsible for the reduced reactivity as anticipated. The lack of
reactivity toward both amine-exchange or thiolate displacement
may indicate that in this case the electron withdrawing effect of
the SC6F5 group increases the B−N bond strength to the extent
that dissociation of the initial adduct, 2e, to its respective amine
integrals of the two products were consistent with a
disproportionation mechanism leading to their formation. It
is postulated that the reaction proceeds via initial attack of the
amine at the boron center, liberating the thiolate moiety into
solution. Identical reactivity was observed upon treatment of 2b
and 2c with MeNH2.
Repeating the amine-exchange process on adduct 2a with the
secondary amine Me2NH produced similar results, with the
major products Me2NH·BH3 (5) (δB −14.2 [q, JBH = 96 Hz])
and (Me2N)2BH (6) (δB 28.1 [d, JBH = 129 Hz]), respectively.
However, in the cases of 2b and 2c as substrates, over 18 h
minor quantities of the desired species, Me2NH·BH2SR,
appeared to be formed (δB −6.4 [t, JBH = 111 Hz, BH2SnBu]
and −5.4 [t, JBH = 111 Hz, BH2SiPr] ppm respectively)
although these proved inseparable from the primary products.
This difference in reactivity toward MeNH2 and Me2NH can be
attributed to the increased donor ability of the secondary
amine, which may result in more favorable thermodynamics for
adduct formation than in the case of MeNH2, where
nucleophilic displacement of the thiolate moiety is preferred.
(iii). Amine-Exchange Reactions of Me3N·BH2SPh (2d) and
Me3N·BH2SC6F5 (2e). Following the unexpected reactivity of
the trimethylamine-thioboranes containing aliphatic thiolates
Scheme 4. Amine-Exchange of 2d with 10 equiv of Me2NH, 20 °C
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and borane components becomes negligible. This would
prevent any amine-exchange via the likely SN1 mechanism,
and may also hinder thiolate displacement, which is also likely
to occur via a three-coordinate boron intermediate because of
the ready availability of the vacant p-orbital at boron for
electron donation.
60 h to furnish Ph3CSBH2·THF (8b) which was isolated as a
colorless solid. Recrystallization of this material from a hexane/
THF solution at −40 °C produced large block like crystals
suitable for study by X-ray diffraction, which confirmed the
expected coordination of a molecule of THF to the thioborane
moiety through the oxygen lone-pair (Figure 2).
(b). Thioborane Trimers, [BH2−SR]3, and Adducts
RSBH2·SMe2. Following the unexpected reactivity of tertiary
amine-thioboranes with respect to amine-exchange chemistry,
alternative synthetic routes to secondary amine-thioborane
adducts were investigated. Two potential routes were pursued,
the first via reaction of amines with preformed thioborane
trimers or adducts as discussed by Mikhailov, and the second
via hydrothiolation reactions of aminoboranes.
(i). Synthesis of B−S Trimers [BH2−SR]3 (R = tBu [7a], nBu
[7b], iPr [7c], Ph [9]) and the Related Adduct C6F5SBH2·SMe2
(10). A series of reports in the mid-20th century document the
reaction of liquid diborane and thiols to produce various
products including polymeric B−S species, but primarily cyclic
trimers of the form [BH2−SR]3.37,52,53 Significantly, such
trimers have been reported to cleanly yield the amine adduct of
the thioborane moiety upon treatment with neat tertiary or
secondary amine.39
The synthesis of a series of B−S trimers was therefore carried
out, using BH3·THF as a more convenient source of the BH3
moiety. Addition of neat aliphatic thiols HSR (R = tBu, nBu
and iPr) to a solution of BH3·THF at −78 °C before warming
slowly to 20 °C led to the quantitative formation of the trimeric
species [BH2−StBu]3 (7a), [BH2−SnBu]3 (7b) and [BH2−
SiPr]3 (7c), which appeared as broad triplets in the 11B NMR
spectrum, with chemical shifts between −15 and 18 ppm, and
coupling constants of ∼100 Hz (Scheme 5). Upon isolation,
Figure 2. Molecular structure of 8b, with thermal ellipsoids at the 50%
probability level. All hydrogen atoms omitted for clarity.
Because of the apparent decomposition of the THF adducts
of PhSBH2 and C6F5SBH2 following reaction of HSPh and
HSC6F5 with BH3·THF, the synthesis of [BH2−SR]3 (R = Ph
(9), C6F5) was then attempted from BH3·SMe2 in dichloro-
methane (DCM). The synthesis of 9 was previously reported
via a similar method by Paetzold and co-workers,57 in hexane
solution, and proved equally as facile in DCM at 20 °C.
Thioborane 9 was isolated in high yield as a colorless solid,
which appeared as a broad triplet in the proton coupled 11B
NMR spectrum at −14.2 ppm in CD2Cl2. Interestingly,
dissolution of this solid in THF at −78 °C appeared to
produce the expected THF adduct, 8a, (δB 3.6 [t, JBH = 120
Hz] ppm), which in the absence of free thiophenol or
BH3·THF was stable for at least 1 h at 20 °C.
Scheme 5. Synthesis of [BH2−SR]3
The synthesis of [BH2−SC6F5]3 was not documented in the
literature, and was attempted via an analogous method to that
for the thiophenol system. A notably slower reaction occurred
to yield complete conversion to a new product, which appeared
at −8.2 ppm in the 11B NMR spectrum, and gave a triplet (JBH
= 124 Hz) on proton coupling. This product was initially
assigned as the expected cyclic trimer [BH2−SC6F5]3. However,
upon isolation of the colorless solid product, subsequent
multinuclear NMR studies (1H NMR (CD2Cl2) δH 2.29 ppm,
13C NMR (CD2Cl2) δC 22.6 ppm), and CI-MS evidenced the
continued presence of an SMe2 moiety, suggesting in fact the
formation of a monomeric dimethylsulfide adduct of the form
C6F5SBH2·SMe2 (10). It is probable that the increased steric
bulk and electron-withdrawing nature of the pentafluorophenyl
moiety in this case disfavors the formation of a trimeric
product, with coordination of the less sterically demanding,
electron-rich SMe2 moiety quenching the electron deficiency at
boron.
7a54−56 was found to be the only solid product, with 7b and 7c
both involatile oils. CI-MS in all cases confirmed the formation
of the desired trimeric structures, with strong signals for both
the molecular ion, and the corresponding monomers, BH2−SR,
observed in each case. Purification of the liquid products by
distillation to levels appropriate for elemental microanalysis was
not possible because of their involatility under high vacuum and
temperature sensitivity, although satisfactory analysis was
obtained on the tBu substituted species, which was purified
by vacuum sublimation at 50 °C, and also characterized by
single-crystal X-ray diffraction (see Supporting Information,
section 3b).
Attempted syntheses of the aryl substituted trimers via this
method proved to be less successful, with a mixture of products
obtained on reaction of BH3·THF with HSPh or HSC6F5. On
the basis of existing literature reports37,57 and our own
observations, it was probable that the initial trimeric products
of such reactions were cleaved in THF solution to produce
simple adducts of the form RSBH2·THF, which subsequently
reacted further as evidenced by 11B NMR spectroscopy.
Attempts to cleanly isolate the adduct PhSBH2·THF (8a) to
confirm this assertion were unsuccessful. Repeating this
chemistry with HSCPh3, however, gave a clean reaction over
(ii). Reactivity of [BH2−SR]3 (R = tBu [7a], nBu [7b], iPr
[7c], Ph [9]), and C6F5SBH2·SMe2 (10) with Tertiary and
Secondary Amines. Following the successful synthesis of
thioboranes 7a−c and 9 and dimethylsulfide adduct 10, the
synthesis of amine adducts from these species was investigated.
Reaction of trimers 7a−c and 9, and adduct 10 with Me3N in
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toluene or THF solution at −78 °C cleanly yielded the
expected adducts, Me3N·BH2SR, as previously characterized
from the thermal syntheses, confirming the applicability of each
of these reagents as precursors to amine-thioborane adducts
(Scheme 6).58
Scheme 6. Synthesis of Me3N·BH2SR from (a) [BH2−SR]3
and (b) Adduct 10
Figure 3. Molecular structures of amine-thioboranes (a) 11a and (b)
11b, with thermal ellipsoids at the 50% probability level. All hydrogen
atoms omited for clarity.
However, upon treatment of the thioboranes with secondary
amines, different reactivity was also observed. In the cases of
thioboranes 7a−c, treatment with a stoichiometric quantity of
Me2NH in THF solution at 0 or −78 °C produced primarily
the products associated with disproportionation, as previously
discussed, namely, amine-borane 5 and bisaminoborane 6
(Scheme 7). Following the previous experiments regarding
range expected for a formal B−N single bond in a secondary
amine-borane adduct.45,59−61 Both the nitrogen and boron
centers are pseudotetrahedral as would be expected for sp3
hybridization.
An analogous methodology was used in the preparation of
the related adduct iPr2NH·BH2SPh (11b), which was isolated
as a white solid in 88% yield from reaction of neat iPr2NH with
a THF solution of 9 at −78 °C (Scheme 8). The 11B NMR
spectrum of this compound in CDCl3 contained a single peak, a
Scheme 7. Reaction of Me2NH with 7a−c in THF
1
triplet at −12.3 ppm, with a coupling constant of 100 Hz. H
and 13C NMR spectra confirmed the presence of both the
iPr2NH and the BH2SPh moieties, with further evidence for the
composition provided by CI-MS. Subsequent recrystallization
of the compound from a toluene/hexane mixture at −40 °C
produced large colorless crystals suitable for single crystal X-ray
analysis which confirmed the expected connectivity (Figure
3b). The compound was found to crystallize in the monoclinic
space group P21/n with one molecule per asymmetric unit. The
central B−N bond measures 1.625(15) Å, consistent with a
single B−N bond in a iPr2NH adduct,59 and is slightly
increased relative to the analogous Me2NH adduct. This
elongation can be explained by the increased steric bulk of the
isopropyl substituents, which are likely to interact more
strongly with the substituents at boron.
An attempted synthesis of Me2NH·BH2SC6F5 (12a) from
dimethylsulfide adduct 10 was less successful than with the
nonfluorinated analogue. Stoichiometric reaction of Me2NH
and 10 yielded three distinct products, apparent in the 11B
NMR spectrum as a triplet at −5.8 ppm (JBH = 115 Hz) [50%],
a broad singlet at −2.5 ppm [40%], and a quartet at −14.2 (JBH
= 97 Hz) [10%] postulated to be amine-borane 5. Although the
mixture of products could not be separated, the peak at −5.8
ppm was consistent with the formation of the desired adduct
12a based on the change in chemical shift relative to the related
nonthiolated adduct, 5, and a coupling constant comparable to
that for 2e (JBH = 115 Hz) [The compound was later
successfully synthesized via an alternative method, providing
spectral data in support of its presence on this case, vide infra].
In contrast, the reaction of 10 with iPr2NH produced a
significantly cleaner reaction, with quantitative conversion to
iPr2NH·BH2SC6F5 (12b) over 1 h at 20 °C based on 11B NMR
spectroscopy. The product was isolated as a colorless solid, and
possessed a single peak in the 11B NMR spectrum in CDCl3 at
−10.5 ppm, which split into a broad triplet on proton coupling.
1H, 13C, and 19F NMR spectra were also consistent with the
assigned structure, suggesting the presence of both iPr2NH and
amine-exchange reactions of Me3N·BH2SR, such disproportio-
nation is not surprising, but is in contrast to published results
with regard to the clean synthesis of secondary amine adducts
of thioboranes via this method.39 Altering the conditions to
increase dilution produced no change in the reaction products,
with those attributed to disproportionation continuing to
dominate. Treatment of the trimers 7a−c with neat Me2NH or
iPr2NH produced no reaction at ambient temperature.
Attempts to perform the analogous chemistry using the
phenyl substituted trimer 9 proved to be significantly more
successful as a route to amine-thioborane adducts. Reaction of
9 with Me2NH in THF at −78 °C produced the desired adduct,
Me2NH·BH2SPh (11a), in 91% yield (Scheme 8). The 11B
Scheme 8. Synthesis of Me2NH·BH2SPh (11a) and
iPr2NH·BH2SPh (11b)
NMR spectrum of the crystalline white solid product in CDCl3
contained a single peak at −7.6 ppm consistent with the
expected four-coordinate boron environment, splitting to a
1
triplet on proton coupling (1JBH = 108 Hz). H and 13C NMR
spectra were both consistent with the formation of the desired
product, and elemental microanalysis and CI-MS both
confirmed the expected composition. Recrystallization of the
product at −40 °C from a toluene/hexane mixture produced
crystals suitable for X-ray diffraction study, which confirmed the
expected atom connectivity (Figure 3a). The compound
crystallized as thick colorless plates, in the orthorhombic
space group Pna21, with 4 molecules in the asymmetric unit.
The average B−N bond length of 1.607(3) Å is within the
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C6F5SBH2 moieties. Recrystallization from hexanes at −60 °C
produced crystals suitable for X-ray diffraction study,
confirming the expected structure based on spectroscopic
data (Figure 4). The compound crystallized in the monoclinic
Figure 5. Molecular structure of 12a, with thermal ellipsoids at the
50% probability level. All hydrogen atoms omited for clarity.
12b, via reaction of the monomeric aminoborane iPr2NBH2
(14) with the respective thiol, in high yield demonstrating its
general applicability to this chemistry.
Attempts to utilize this method in the preparation of adducts
containing aliphatic thiolates, however, which had proved
inaccessible by other methods (vide supra), were unsuccessful.
Reaction of 13 with HStBu, for example, at 60 °C produced a
mixture of products, with a peak assigned to the desired adduct
Me2NH·BH2StBu (15) (δB −7.6 [t, JBH = 111 Hz]) appearing
as a minor component of the 11B NMR spectrum of the crude
reaction mixture (ca. 8% of products). A significant proportion
of the product mixture was assigned as the aminothioborane
Me2NBH(StBu) (16) (38%), which appeared as a doublet at
38.6 ppm, JBH = 144 Hz. In this case it appears that the rate of
formation of 15 was lower than that of its consumption via
thermal dehydrogenation to form 16 (Scheme 10). Reducing
Figure 4. Molecular structure of 12b, with thermal ellipsoids at the
50% probability level. All hydrogen atoms omited for clarity.
space group P21/c, with two essentially equivalent molecules
per asymmetric unit. The structure was broadly analogous to
that of the nonfluorinated analogue 11b, with effectively no
contraction observed in the B−N bond length (1.621(2) Å).
The geometries around both boron and nitrogen are both close
to tetrahedral, as expected for 4-coordinate, that is, sp3
hybridized, centers of both elements.
(iii). Hydrothiolation of Secondary Aminoboranes: an
Alternative Synthesis of R2NH·BH2SR. A recent publication of
Dixon and co-workers that demonstrated the addition of thiols
across B−N multiple bonds40 provided impetus for us to
investigate the hydrothiolation of aminoboranes as an
alternative route to amine-thioboranes. The synthesis of 12a,
which could not be achieved cleanly by other means, was
therefore attempted via this methodology. Heating a toluene
solution of the thermally-labile dimer [Me2N-BH2]2 (13) in the
presence of 2 equiv of HSC6F5 to 70 °C led to complete
consumption of the aminoborane within 18 h, as evidenced by
11B NMR spectroscopy, with the appearance of a new peak at
−5.8 ppm, apparent as a triplet, JBH = 115 Hz, in the proton
coupled spectrum. Removal of the solvent furnished the new
product as a colorless solid, identified by multinuclear NMR
spectroscopy, CI-MS and elemental microanalysis as the
desired amine-thioborane 12a in 82% isolated yield (Scheme
9).
Scheme 10. Attempted Synthesis of Dialkylamine-
Thioborane Adducts via Thermolytic Hydrothiolation
the reaction temperature to 50 °C produced no significant
change in product composition, with the rate of both processes
dropping at lower temperatures. Similar reactivity was observed
on reaction of HStBu with aminoborane 14 and indeed on
reaction of 13 with other aliphatic thiols (e.g., HSnBu, HSiPr).
In all cases, therefore, the isolation of the desired amine-
thioborane R2NH·BH2SR was not feasible because of the
competitive thermal elimination of hydrogen from the
thioborane adduct.
(c). Attempted Synthesis of Primary Amine Adducts
of Thioboranes: RNH2·BH2SR. (i). Reaction of MeNH2 with
[BH2−SR]3 (R = tBu [7a], nBu [7b], iPr [7c], and Ph [9]) and
C6F5SBH2·SMe2 (10). Reaction of MeNH2 with B−S trimers
7a−c derived from aliphatic thiols produced identical reactivity
to that observed on reaction with Me2NH in THF at −78 °C.
The sole reaction products in this case appear again to be those
of disproportionation, namely, MeNH2·BH3 (3) and HB-
(NHMe)2 (4), with no evidence of the formation of the desired
amine-thioborane adducts (Scheme 11).
To probe the effect of THF on this reaction, an analogous
experiment was performed using the liquid amine, nBuNH2, in
the absence of solvent. This again led to disproportionation, in
this case to yield products assigned by 11B NMR spectroscopy
as nBuNH2·BH3 (17) (δB −20.4 [q, JBH = 94 Hz] ppm)62 and
Scheme 9. Synthesis of 12a via Hydrothiolation
Recrystallization of the solid product from toluene/hexanes
at −40 °C produced crystals suitable for an X-ray diffraction
study, which confirmed the expected atom connectivity (Figure
5). The compound crystallized in the triclinic space group P1 as
̅
colorless plates, with a single molecule in the asymmetric unit,
with a structure again closely related to that of the
nonfluorinated analogue 11a.
The hydrothiolation methodology was readily extended to
produce the previously characterized adduct 11a, and also 11b/
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(δB −14.0)64 respectively, and the coupling constant of the
former compound.
Scheme 11. Reaction of 7a−c with Excess MeNH2 in THF
Solution
It is postulated that the less well-defined reaction with
MeNH2 is due to the reduced donor ability of this amine
relative to Me2NH and iPr2NH, respectively, which would
result in a less favorable adduct forming reaction. This could
result therefore in significantly increased levels of competing
side reactions, alongside the formation of the desired 1:1
adduct.
Reaction of the related dimethylsulfide adduct, 10, with
excess MeNH2 in THF solution at −78 °C did not yield either
the expected amine-thioborane adduct or the previously
observed disproportionation products. The product in this
case, was found to be the salt [(MeNH2)2BH2][SC6F5] (20),
which was isolated as a colorless solid (Scheme 12). This
(nBuNH)2BH (18) (δB 27.3 [d, JBH = 115 Hz] ppm),51 with no
evidence for the formation of a stable thioborane adduct.
Reaction of MeNH2 with the phenyl substituted trimer 9,
however, produced different reactivity. In this case, addition of
a solution of MeNH2 to a THF solution of 9 produced rapid
conversion to a complex mixture of products by 11B NMR
spectroscopy. The major species within the mixture, however,
accounted for ∼70% of the total soluble boron content, and
appeared as a triplet at −11.5 ppm (JBH = 111 Hz). Both the
chemical shift and the coupling constant of this species were
consistent with the expected amine-thioborane adduct,
MeNH2·BH2SPh (19), based upon the analogous secondary
amine-thioborane 11a. Removal of the solvent from the
reaction mixture yielded an oily solid, which was immediately
triturated with hexanes. Although the resulting solid products
could not be further purified in the bulk by sublimation or
recrystallization under various conditions, recrystallization from
a DCM/hexane mixture at −40 °C fortuitously produced a
colorless plate suitable for study by single crystal X-ray
diffraction, which confirmed the presence of the desired
product, 19 (Figure 6). This compound was found to crystallize
Scheme 12. Reaction of MeNH2 and 10 in THF at 20 °C
compound was insoluble in most common NMR solvents, but
was completely soluble in d8-THF producing a poorly resolved
triplet in the 11B NMR spectrum at −8.7 ppm, of coupling
constant ∼90 Hz. The 1H{11B} NMR spectrum was consistent
with the assigned structure, with peaks assigned to the Me (δH
2.43 ppm) and BH2 (δH 2.03 ppm) environments integrating in
a 6:2 ratio. In addition, the 13C and 19F NMR spectra, although
broadly uninformative, confirmed the inclusion of a MeNH2
and C6F5 moieties, respectively.
Recrystallization of the isolated solid from THF/hexanes at
−40 °C produced colorless crystals suitable for X-ray diffraction
analysis, which confirmed the product to be salt 20 (See
Supporting Information, Section 3a). It is likely that the
formation of this compound results from the displacement of
the thiolate anion from the borane by nucleophilic attack by the
primary amine, although it is unclear why in this case the
reaction does not continue to furnish bisaminoborane 4 as
observed in initial amine-exchange chemistry (vide supra). It is
conceivable that the formation of this compound may be due to
the increased stability of the [SC6F5]− anion relative to those
produced from aliphatic thiols. Interestingly, analogous
products, [(RNH2)2BH2][SR′], were proposed by Mikhailov
and co-workers to result from the treatment of the aliphatically
substituted trimers [BH2−SR′]3 (R′ = Me, Et) with primary
amines, RNH2 (R = Me, Et),65 which were not observed in our
studies of related species.
It should also be noted that the stoichiometry of the
formation of this salt clearly dictates the necessity of an excess
of MeNH2, and it was conceivable that the desired product
could be formed upon stoichiometric reaction of the amine
with adduct 10. Reaction in this manner, however, yielded only
mixtures of the starting material and the salt product based on
11B NMR spectroscopy.
Figure 6. Molecular structure of 19, with thermal ellipsoids at the 50%
probability level. All hydrogen atoms omited for clarity.
in the orthorhombic space group Pbca, and contained a single
molecule per asymmetric unit. The substituents at nitrogen and
boron were found to be pseudotetrahedral in nature as
expected for sp3 hybridization at both centers. The boron−
nitrogen bond length of 1.586(2) Å was of the magnitude
expected for a B−N single bond within a primary amine-borane
adduct,45,63 and shows the expected contraction from that of
the analogous Me2NH adduct (B−N: 1.607(3) Å) based on
steric arguments.
Although the complete purification of this species could not
be achieved, it is likely that the molecular structure
demonstrated by X-ray crystallographic study is representative
of ∼70% of the overall composition, and it is possible to
tentatively assign the 11B NMR spectral data by close
comparison with that of other similar species. The chemical
shift and coupling constant of the major product (δB −11.5, JBH
= 111 Hz) are within the expected range for the target
compound, based on the relative shift between amine-
thioborane 11a (δB −7.6, JBH = 108 Hz) and amine-borane 5
(ii). Attempted Synthesis of MeNH2·BH2SR (R = Ph [19],
C6F5) via Hydrothiolation of [MeNH-BH2]3 (21a) and [MeNH-
BH2]n (21b). The lack of a clean synthesis of primary amine-
thioborane adducts by other methods led to the investigation of
hydrothiolation, as demonstrated for secondary adducts, as an
alternative route to these species. Unfortunately however, the
reaction of various thiols with the cyclic oligomeric and linear
polymeric aminoboranes, [MeNH-BH2]3 (21a) and [MeNH-
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BH2]n (21b), at elevated temperatures did not lead to thiolated
products. The only product of such reactions was N,N′,N″-
trimethylborazine, [MeN-BH]3, the reported thermolysis
product of both aminoboranes, the formation of which may
be enhanced by mild acid catalysis resulting from the presence
of the thiol (see Supporting Information, Section 4).
(d). Reactivity of B-Thiolated Adducts with Respect to
H2 Release. (i). Thermal and Catalytic Dehydrogenation of
iPr2NH·BH2SPh (11b) and iPr2NH·BH2SC6F5 (12b). Thermal
dehydrogenation of iPr2NH·BH2SPh (11b) was investigated
initially as a prelude to catalytic studies. A solution of the
adduct was, therefore, heated to 100 °C over 18 h in toluene
solution. Over this period, 11B NMR spectroscopy demon-
strated the growth of a new peak at 37.9 ppm, a doublet of
coupling constant 141 Hz, postulated to be due to iPr2N
BH(SPh) (22), along with the concomitant consumption of the
starting material (Scheme 13a). Upon removal of the volatile
crystallize in the monoclinic space group P21/n, with a single
molecule in the asymmetric unit. The core of the molecule is
almost perfectly planar, with the angle between the S(1)−
B(1)−H(1) and (C7)−N(1)−C(10) planes measuring 0.87°,
as generally observed in monomeric aminoboranes.59,68 Both
nitrogen and boron centers are in effectively trigonal-planar
environments as expected following sp3 to sp2 hybridization at
both centers following hydrogen loss. The B−N bond length of
1.385(16) Å shows the expected contraction from that of 11b,
1.625(15) Å, consistent with the increased bond order
associated with the formation of a B−N double bond in this
system. To our knowledge, this is the first crystallographic
characterization of such an aminothioborane.
As discussed previously, the employment of transition metal
catalysts has been shown to facilitate hydrogen release from
simple amine-borane adducts, and it was of interest to
investigate similar catalysis in this context. Therefore, we
attempted the Rh-catalyzed dehydrogenation of 11b, using the
precursor complex [Rh(μ-Cl)cod]2 (cod = 1,5-cyclooctadiene).
Using a Rh loading of 2 mol %, the adduct was found to be
completely dehydrogenated to produce solely aminothioborane
22 over 42 h at 20 °C (Scheme 13b). In this case, the product
could be isolated via sublimation from the catalyst following
removal of the volatiles under high vacuum, with multinuclear
NMR spectroscopy confirming the identity of the product as
that previously characterized.
Scheme 13. Thermal and Catalytic Dehydrogenation of 11b
components of the reaction mixture, the product was furnished
as a colorless oil, which was subsequently recrystallized from
hexane solution at −40 °C to yield a colorless solid, which
melted at ambient temperature. The 11B NMR spectrum in
CDCl3 showed a single peak at 38.7 ppm, a doublet with
coupling constant 139 Hz, consistent with a three-coordinate
boron environment, with a single hydrogen substituent. The
chemical shift in this case was strongly indicative of a
monomeric aminothioborane,66 and was consistent with that
observed for the unsubstituted aminoborane analogue iPr2N
BH2 (14) (δB 35.1 ppm).2,66 1H and 13C NMR spectroscopies
indicated the presence of two inequivalent isopropyl environ-
ments consistent with the limited rotation around the central
BN bond present in monomeric aminoboranes, and also
confirmed the inclusion of a phenyl moiety within the product.
Accurate mass CI-MS was also consistent with the assignment
of the product as aminothioborane 22. Adduct 11b was,
therefore, shown to be cleanly dehydrogenated, to produce the
monomeric aminothioborane 22 in 59% isolated yield.67
Crystallization of the liquid product at −40 °C from hexane
solution yielded large colorless plates suitable for single crystal
X-ray analysis, confirming the expected atom connectivity in
the aminothioborane (Figure 7). The compound was found to
Analogous thermal dehydrogenation reactions with the
perfluorinated analogue, iPr2NH·BH2SC6F5 (12b), were also
observed. Heating a solution of this adduct to 100 °C over 18 h,
also led to a relatively clean dehydrogenation to form the
monomeric aminothioborane iPr2NBH(SC6F5) (23), along
with small quantities of aminoborane (14) (δB 35.1 ppm),
suggesting the release of HSC6F5 also occurs as a minor
pathway. Aminothioborane 23 was isolated as a colorless solid,
which showed a single resonance in the 11B NMR spectrum in
CDCl3 at 37.0 ppm (d, JBH = 127 Hz), consistent with a three
coordinate boron center bound to a single hydrogen
1
substituent. The H and 13C NMR spectra were unremarkable,
but were in support of the assigned monomeric structure with
resonances consistent with two inequivalent isopropyl groups
apparent in both spectra. The 1H{11B} spectrum also showed a
resonance at 4.84 ppm which integrated to 1 proton, consistent
with the single hydrogen environment at boron, and in line
with the analogous signal observed for 22 at 5.59 ppm.
Catalytic dehydrogenation of 12b was also attempted under
analogous conditions to those successfully employed in the
dehydrogenation of 11b. Upon treatment of a solution of 12b
with 2 mol % [Rh(μ-Cl)cod]2 over 18 h at 20 °C, however,
analysis by 11B NMR spectroscopy indicated this adduct was
not cleanly dehydrogenated. Over this period, 20% of the initial
adduct was consumed to form a mixture of 5 products
including iPr2NH·BH3 (δB −21.1 ppm) and 14 (δB 34.6 ppm),
along with a small amount of the desired aminothioborane 23
(δB 37.1 ppm) suggesting the presence of several competing
side reactions.
(ii). Thermal and Catalytic Dehydrogenation of
Me2NH·BH2SPh (11a) and Me2NH·BH2SC6F5 (12a). In contrast
to the clean dehydrogenative chemistry observed for 11b,
heating a toluene solution of Me2NH·BH2SPh (11a) to 100 °C
over 42 h, appeared to produce a competitive elimination of
hydrogen and thiophenol. Analysis of the crude reaction
mixture by 11B NMR spectroscopy indicated the major
products of the reaction to be Me2NBHSPh (24), (δB 39.4
Figure 7. Molecular structure of 22, with thermal ellipsoids at the 50%
probability level. Hydrogens bonded to carbon omited for clarity.
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[t, JBH = 153 Hz] ppm, {35%})69 and cyclodiborazane 13 (δB
4.8 [t, JBH = 110 Hz],2 {12%}), with significant quantities of
11a remaining unreacted (39%). Continued heating over 68 h
led to a mixture containing 41% of 24, 3% of 13, and 22%
unreacted 11a remaining (Scheme 14). Separation of the
complex mixture of products was, however, unsuccessful,
preventing further characterization of novel aminothioborane
24.
[Rh(μ-Cl)cod]2, Rh(PHCy2)3Cl, and the FLP Me3SiOTf/
TMPH respectively, but was not successful.73
DISCUSSION
■
The differing reactivity of the four secondary amine-
thioboranes, 11a/11b and 12a/12b, with respect to hydrogen
release is likely to be dictated by a combination of steric and
electronic effects within the adducts. In a previous computa-
tional study, we investigated the thermodynamics of dehydro-
genation for a range of amine-borane adducts,32 including
iPr2NH·BH3 (26) and Me2NH·BH3 (5), unsubstituted
analogues to the thiolated adducts studied in this case. The
dehydrogenation of 26 to form iPr2NBH2 (14) was
calculated to have ΔG = −16.5 kcal/mol, with the analogous
reaction for 5 calculated to have ΔG = −11.5 kcal/mol. It is
reasonable to suggest, therefore, that upon B-thiolation, a
similar relationship may hold, with H2 release from 11b/12b
more favorable than in the related methyl substituted species,
11a/12a. Such an assertion is directly in line with experimental
results. Furthermore, the observation of the elimination of thiol
in the case of thermolysis of 11a and to a lesser degree 12b
suggests that elimination of HSR is also favorable, and this side
reaction would become increasingly significant where the
elimination of H2 was not strongly favored. The prevalence of
this mode of reactivity in the case of 11a is again in line with
this suggestion.
Scheme 14. Thermolysis of 11a at 100 °C, Toluene Solution
This competitive elimination correlates closely with that
proposed under similar conditions by Mikhailov,39 and implies
little thermodynamic advantage from elimination of hydrogen
rather than thiophenol, despite the release of a gaseous product
in the former case. It was therefore of interest to investigate
whether the dehydrogenation reaction could be selectively
facilitated under catalytic conditions. However, reactions with
[Rh(μ-Cl)cod]2, as used for in the successful dehydrogenation
of 11b, and also two further efficient amine-borane
dehydrogenation catalysts: IrH2POCOP (POCOP = κ3-1,3-
(OPtBu2)2C6H3])11 and “Cp2Ti”,6 produced none of the
aminothioborane observed thermally, nor indeed aminoborane
13, the product of thiophenol elimination. However, reaction
with the Wilkinson’s catalyst analogue, Rh(PHCy2)3Cl70 (2 mol
%, 16 h, toluene) cleanly yielded ∼16% of 24, with no apparent
formation of 13. Allowing this reaction to stir for a further 120
h, however, did not result in a complete conversion to the
desired product, which is in fact consumed to produce a new
unidentified product at 19 ppm in the 11B NMR spectrum,
which remained a singlet on proton coupling. An alternative
means of dehydrogenation employing stoichiometric quantities
of the frustrated Lewis pair (FLP) Me3SiOTf/2,2,6,6-
tetramethylpiperidine71 (TMPH) was also attempted. Treat-
ment of a toluene solution of 11a, with a stoichiometric
quantity of this FLP at 20 °C, however, resulted in the clean
elimination of thiol to produce cyclic diborazane 13 as the
major boron containing product (98%), although a trace
amount of the aminothioborane 24 was also detected (Scheme
15).
In the case of 12a the lack of reactivity at 100 °C or under
catalytic conditions evidence an unfavorable elimination of both
H2 and HSC6F5. The former may be rationalized by the
combination of the presence of Me groups at nitrogen, as
discussed above, and the highly electron withdrawing SC6F5
group at boron, both of which are likely to disfavor H2
release.32 The unfavorable elimination of HSC6F5 is, however,
less easily explained. Nevertheless, it is noteworthy that the
analogous elimination of HSPh is also sluggish from 11a (only
12% of cyclodiborazane 13 is formed after 42 h at 100 °C),
suggesting subtle effects related to thiol loss may be
operational. With regard to the limited catalytic reactivity of
the fluorinated adducts, it is likely that the high steric bulk of
the thiolate group in this case impairs coordination to the active
catalytic centers, thus hampering their action.
SUMMARY
■
The synthesis of a broad range of amine-thioborane adducts has
been developed and detailed investigations of a variety of routes
to such species carried out. Thermal dehydrogenation of
secondary amine-thioborane adducts has been demonstrated,
along with the first example of a metal catalyzed dehydrogen-
ation of such an adduct.
Scheme 15. Reaction of 11a with Me3SiOTf/TMPH
A series of trimethylamine-thioboranes, Me3N·BH2SR (R =
tBu [2a], nBu [2b], iPr [2c], Ph [2d], C6F5 [2e]), have been
prepared and characterized. The use of these adducts as
precursors to secondary and primary amine-thioboranes via
amine-exchange reactions was also investigated, but was found
to be unfeasible due to nucleophilic displacement of the thiolate
moieties. However, the thioborane trimer [BH2−SPh]3 (9) and
the dimethylsufide adduct C6F5SBH2·SMe2 (10) were found to
act as effective precursors to a range of amine-thioboranes.
These precursors were found to cleanly form the expected
thioborane adducts with Me3N, as characterized via thermal
synthesis, but more significantly also formed Me2NH·BH2R (R
= Ph [11a], C6F5 [11b]) and iPr2NH·BH2SR (R = Ph [12a],
Clearly under these conditions, and indeed on repeating the
reaction using 11b,72 the elimination of HSPh is more favorable
than the elimination of H2 as observed in nonthiolated systems.
The fluorinated adduct, Me2NH·BH2SC6F5 (12a), proved to
be significantly less reactive toward dehydrogenation. Remark-
ably, thermolysis of this adduct at 100 °C in toluene solution
over 18 h, produced no evidence of small molecule elimination
by 11B NMR spectroscopy, with only unreacted 12a remaining
in solution. Repeating the thermolysis at 150 °C in tetraglyme
over the same period resulted in complete decomposition of
the initial adduct to form a series of unknown products.
Catalytic dehydrogenation of 12a was also attempted using
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respectively. All four secondary amine adducts were charac-
terized crystallographically, and as such represent the first
structural characterization of this family of compounds.
Studies of the thermal and catalytic dehydrogenation of 11a/
11b and 12a/12b evidenced facile hydrogen loss under mild
conditions from both diisopropylamine adducts 11b and 12b.
These adducts were cleanly dehydrogenated at 100 °C to form
the monomeric aminothioboranes iPr2NBHSR (R = Ph
[22], C6F5 [23]), with the former adduct also shown to be
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Cl)cod]2. The closely related adduct 11a was found to
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respectively, with Rh-catalyzed dehydrogenation favoring 24
only in the early stages of reaction. The fluorinated analogue,
12a, could not be cleanly dehydrogenated under thermal or
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ASSOCIATED CONTENT
■
S
* Supporting Information
Full experimental details, crystallographic information and
details of additional experiments. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Ian.Manners@Bristol.ac.uk.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.P.M.R and I.M. thank the EPSRC for funding.
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Inorganic Chemistry
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(67) At 20 °C in toluene solution, a very gradual elimination of H2
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(14) [85%], iPr2NBH(SPh) (22) [15%].
(73) Treatment of toluene solutions of the adducts with 2 mol % of
each catalyst over 18 h, however, produced <5% conversion to
unknown products, none of which were consistent with the expected
aminothioborane, Me2NBH(SC6F5). Treatment of a toluene
solution of this adduct with the FLP Me3SiOTf/TMPH at 20 °C
over 18 h, yielded solely cyclic diborazane 13, again indicating a
preferred elimination of the thiolate moiety.
8264
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