Hydroboration of Phosphaalkynes by HB(C6F5)2

Article abstract of DOI:10.1002/chem.201602860

The hydroboration of phosphaalkynes with Piers’ borane (HB(C₆F₅)₂) generated unusual phosphaalkenylboranes [RCH=PB(C₆F₅)₂]₂that persisted as dimers in both solution and the solid state. These P₂B₂heterocycles underwent ring opening when subjected to nucleophiles, such as pyridine and tert-butylisocyanide, to yield monomeric phosphaalkenylborane adducts RCH=PB(C₆F₅)₂(L). DFT calculations were performed to probe the nature of the interaction of phosphaalkynes with boranes.

Full text of DOI:10.1002/chem.201602860

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Accepted Article
Title: Hydroboration of Phosphaalkynes by HB(C6F5)2
Authors: Lauren Longobardi; Tim Johnstone; Rosalyn Falconer; Chris Russell; Dou-
glas Wade Stephan
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Hydroboration of Phosphaalkynes by HB(C6F5)2
Lauren E. Longobardi,[a] Timothy C. Johnstone,[a] Rosalyn L. Falconer,[b] Christopher A. Russell,[b] and
Douglas W. Stephan*[a]
Abstract: The hydroboration of phosphaalkynes with Piers’ borane
(HB(C6F5)2) generates unusual phosphaalkenylboranes
bond to a phosphaalkyne has been reported only once, to the
best of our knowledge. In that case, the double hydroboration of
[RCH=PB(C6F5)2]2 that persist as dimers in solution and the solid
state. These P2B2 heterocycles undergo ring opening when
subjected to nucleophiles, such as pyridine and tert-butylisocyanide,
tert-butylphosphaalkyne (tBuCP 1a) with HBCat (Cat
=
catechol) generated gem-diboryl substituted primary
a
phosphine (Figure 1, C).[17] The regiochemistry of this 1,2-
addition reaction is governed by the electronegativity difference
between carbon and phosphorus and the inherent polarity of the
δ+PCδ- bond This regiochemistry of 1,2-addition has also been
observed in related hydrogermylation[18] and hydrostannation
chemistry.[19] In contrast, the aforementioned reaction of HBEt2
to
yield
monomeric
phosphaalkenylborane
adducts
RCH=PB(C6F5)2(L). DFT calculations were performed to probe the
nature of the interaction of phosphaalkynes with boranes.
Hydroboration is a powerful chemical transformation in which
B–H bonds are added across unsaturated units in alkynes,
olefins, imines, and carbonyl compounds.[1] These additions are
typically facilitated by transition metal catalysts.[2] The
widespread utility of this chemical reaction in organic
synthesis has been realized since the initial work by H.C. Brown
in the 1960s.[3] Although related chemistry has been achieved to
exploit the utility of carboborations[4] and haloborations,[5] more
recent innovations have included the development of metal-free
catalysts for hydroboration and the specific design of borenium
catalysts for trans-hydroborations of alkynes.[6] Despite the
broad utility of hydroboration in organic chemistry, exploitation of
this reaction in main group inorganic chemistry has drawn little
attention.
with
a Ti-phosphaalkyne complex resulted in B–P bond
formation.[16] Although this reaction does not involve B–H
addition to the PC bond, it suggests that steric demands might
also play
a
role in determining the regiochemistry of
phosphaalkyne addition reactions. Herein we report the reaction
of phosphaalkynes with the highly electrophilic borane
HB(C6F5)2,[20] which affords the first examples of simple B–P
bonded phosphaalkenylboranes and a novel class of P/B
heterocycles.
The isolation of stable, singlet P2B2 diradicaloids by
Bertrand and co-workers[7] and the advent of frustrated Lewis
pairs (FLPs),[8] are two examples of chemistry that has focused
attention on compounds containing phosphorus and boron. In
seeking to exploit hydroboration for the preparation of novel P/B
compounds, we noted that low-coordinate phosphorus species
such as phosphaalkenes[9] and phosphaalkynes,[10] have a rich
and diverse reactivity but few examples of boron-containing
derivatives exist.[11] Phosphaalkynes have been reported to react
with BBr3,to yield 1,2-addition products (Figure 1 A),[12] and also
polyhedral boranes, resulting in either linkage of B10 units by
phosphaalkene fragments[13] or incorporation of the PC
fragment into the polyhedral cluster.[14] Phosphaalkynes have
Figure 1 Reported products arising from reactions between phosphaalkynes
and boranes.
To study the reactivity of phosphaalkynes with highly
electrophilic boranes, 1a was exposed to one equivalent of the
strong Lewis acid B(C6F5)3. Multinuclear NMR spectroscopy
showed no evidence of coordination or carboboration[4c] even
after prolonged heating, in contrast to the reactivity of B(C6F5)3
with alkynes.[4c] We attribute the lack of interaction to the low
basicity and nucleophilicity of the P-centre.[21] Similarly,
treatment of 1a with B(C6F5)3/PtBu3 or B(C6F5)3/PPh3 at ambient
or elevated temperatures showed no addition across the PC
bond, in contrast to the reported reactivity of frustrated Lewis
also been shown to insert into boroles[15] and
a
Ti-
phosphaalkyne complex was observed to undergo addition of
HBEt2 to afford a unique P/B-Ti complex (Figure 1, B).[16]
Aside from Ti species B, the direct addition of an R2B–H
pairs (FLPs) with CC bonds.[22] Treatment of 1a with one
equivalent of Piers’ borane (HB(C6F5)2)[20] in CH2Cl2, however,
afforded a colourless crystalline solid 2a in 67% yield after
workup (Scheme 1). Single crystals for X-ray diffraction were
obtained via diffusion of pentane into a CH2Cl2 solution of 2a
at -35 °C (Figure 2a). The solid state structure revealed that
hydroboration of the phosphaalkyne had occurred and produced
a phosphaalkenylborane, which dimerized to form a P2B2 four-
membered ring.[23] The phosphaalkene P=C bond lengths were
1.651(2) and 1.6470(19) Å. The P2B2 ring is distorted from
planarity in a butterfly conformation featuring a dihedral angle of
23.32(8)°. P–B–P bond angles of 81.14(8)° and 81.01(8)°, and
B–P–B bond angles of 96.17(9)° and 96.24(9)° were found in the
central ring. The P–B bond lengths in 2a ranged from 2.009(2) –
2.013(2) Å; these distances are at the short end of those of
phosphinoborane dimers, [R2B–PR′2]2, which range from
[a]
[b]
L. E. Longobardi, Dr T. C. Johnstone, Professor Dr D. W. Stephan
Department of Chemistry
University of Toronto
80 St. George St, Toronto, ON, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
R. L. Falconer, Dr C. A. Russell
School of Chemistry
University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
Supporting information for this article is given via a link at the end of
the document.

Chemistry - A European Journal
10.1002/chem.201602860
COMMUNICATION
2.004(4) to 2.096(5) Å,[24] but are longer than those of P2B2
diradicaloids, which range from 1.8904(15) to 1.900(2) Å.[7] The
short P–B distances in 2a are perhaps due to the reduced steric
demands of the bridging phosphaalkene units or the
exhibit any coupling to each other. A similar phenomenon has
been observed for trans-diphosphine metal complexes,[25] and
the structurally analogous dimeric four-membered heterocycle
[LiPPh2]2.[26] Further confirmation of this interpretation is derived
from ssNMR data. Fortuitously, dimer 2a sits on a general
position in the solid state and thus the two P-centres are
crystallographically inequivalent. Consequently, the CP-MAS
ssNMR 31P{1H} spectrum of 2a (Figure S33) exhibits two 31P
hybridization
of
the
P-centre.
2
resonances that couple to each other with Jpp ~ 1520 Hz.
Simulation of the solution state 1H NMR resonance of the olefinic
proton of 2a as an ABX multiplet using 1520 Hz as JAB suggests
that the coupling between the two phosphorus centres is
sufficiently large to collapse the second order ABX multiplet into
an apparent triplet (Figure S34).
Given the difference in the electronegativities of carbon and
phosphorus, and the polar nature of the PC bond, it is
interesting to note that the hydroboration occurs with formation
of P–B and C–H bonds. This pattern of reactivity stands in
contrast to literature reports of B–H,[17] Ge–H,[18] and Sn–H[19]
phosphaalkyne addition reactions, in which hydride adds to the
more electropositive phosphorus affording
a P–H bond.
Interestingly, the formation of 2a is more akin to reactions of
phosphaalkynes with Ru–H species described by Hill, Jones[27]
and Crossley,[28] reinforcing the analogy between electrophilic
boranes and transition metals. We also noted that, unlike the
previously
reported
double
addition
of
HBCat
to
phosphaalkyne,[17] Piers’ borane undergoes a single 1,2-addition
to 1a, and any excess borane remains unreacted in solution, as
evidenced by multinuclear NMR spectroscopy.
To probe the mechanism of reaction and the observed regio-
chemistry in the formation of 2a, DFT computations were
performed. Inspection of the f–(r) Fukui function clearly reveals
that the site of electrophilic attack is the PC triple bond, and
not the lone pair on the phosphorus atom (Figure S36). This
view is in accord with the partial positive charge on the
phosphorus atom resulting from the difference in
electronegativity between phosphorus and carbon. Relaxed
potential energy surface (PES) scans of the distance between
the centre of the PC triple bond and the boron atom of either
Piers’ borane or B(C6F5)3 were performed. These surface scans
reveal that the energy of the system experiences a minimum as
Piers’ borane approaches the triple bond. In contrast, the energy
monotonically rises as B(C6F5)3 and phosphaalkyne approach
one another (Figure S35). The latter result suggests that steric
repulsion between the C6F5 rings and the tert-butyl group
precludes approach of B(C6F5)3 and the nucleophilic π cloud of
the phosphaalkyne. The minimum from the PES scan with Piers’
Scheme 1. Reactions of phosphaalkynes 1a/b with HB(C6F5)2 to produce 2a/b.
Figure 2 POV-ray depictions of (a) 2a and (b) 2b. H atoms omitted for clarity.
The 11B NMR spectrum of 2a revealed a sharp singlet at -5.3
ppm and the 19F NMR spectrum showed three resonances
at -127, -153, and -162 ppm. These data support the assignment
of 2a as a four-coordinate boron species, consistent with a dimer
in solution. 31P NMR data revealed a broad singlet at 183 ppm, a
drastic downfield change in chemical shift from that of
phosphaalkyne 1a (-69 ppm)[11a], consistent with the formation of
a
phosphaalkene. The 1H NMR spectrum showed two
resonances at 8.03 and 1.04 ppm, with relative integrations of
1:9, which were assigned as the olefinic and tBu resonances,
respectively. Surprisingly, the resonance at 8.03 ppm appears
as a triplet with an apparent coupling constant of J = 8.2 Hz. The
triplet resonance persists even when recorded at differing
magnetic field strengths and temperatures (25 to -35 °C). The
1H{31P} spectrum, however, reveals this resonance as a singlet,
indicating that the fine structure of the signal arose from coupling
between the H and P nuclei. 13C{1H} NMR data showed
resonances at 178, 41, and 30 ppm, all of which were apparent
borane was used as the starting point for
a geometry
optimization. In the optimized configuration, the BH is directed
towards the carbon atom of the phosphaalkyne (Figure 3). This
configuration is favourable as it minimizes steric interactions
between the C6F5 rings and the tert-butyl group. These results
are congruent with chemical intuition and account for the lack of
reactivity with B(C6F5)3 and the observed regiochemistry of the
reaction with Piers’ borane.
The analogous reaction of 1-adamantylphosphaalkyne
(1-AdCP 1b) with Piers’ borane resulted in formation of the
dimeric species 2b in 68% yield (Scheme 1). Compound 2b also
1
triplets. The multiplicity of the H and 13C NMR signals appears
to arise from virtual coupling to the pair of strongly coupled 31P
nuclei in 2a. The two putatively coupled phosphorus centres in
2a have identical chemical shifts in solution and thus do not

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COMMUNICATION
shows virtual coupling of the P-atoms to 1H and 13C nuclei in
solution state NMR spectra. However, unlike 2a, the P2B2 unit of
2b is centrosymmetric in the solid state (Figure 2b), with B–P
163 ppm, while the 11B NMR resonance appears at -22 ppm.
The 31P{1H} NMR spectrum features a pentet at 266 ppm, with J
= 36 Hz, while in the 31P{19F} spectrum, the resonance collapsed
to a doublet with J = 24 Hz. This evidence suggests that the
phosphorus centre couples to both the olefinic proton and the
four ortho-fluorines of the two C6F5 rings. This reactivity
contrasts with that of phosphinoborane dimers [(C6F5)2BPR2]2
29]
which proved unreactive toward donor molecules.[24,
Collectively these data are consistent with the formulation of 3a
as tBuCH=PB(C6F5)2(CNtBu).
The adduct 3a was consistently isolated as a pale oil, so
analogous chemistry with 2b was undertaken with the goal of
obtaining crystallographic data. Compound 2b also reacted
cleanly
Ad)CH=PB(C6F5)2(tBuNC)
with
tert-butylisocyanide,
3b, which
affording
was confirmed
(1-
crystallographically. Additionally, when 2a was treated with
pyridine, the resulting adduct tBuCH=PB(C6F5)2(pyr) 4a could be
isolated as a crystalline material (Scheme 2). Species 3b
exhibits solution NMR spectra with similar properties to those of
3a, while 4a has a 11B NMR resonance at -0.8 ppm. X-ray data
for both 3b and 4a confirmed the formulations (Figure 4). For
both molecules, the B-centre adopts a pseudo-tetrahedral
geometry. The average B–Cisocyanide bond distance of 3b is
1.61(1) Å, and the B–N bond distance of 4a is 1.612(3) Å. The
P–B bond lengths average 2.040(7) Å in 3b and was found to be
2.029(2) Å in 4a. These P–B distances are significantly longer
than those of (C6F5)2BPR2 (R = tBu, Cy),[24] which were found to
be 1.786(4) Å and 1.762(4) Å. This difference presumably
reflects the poorer sigma-donor ability of sp2 vs sp3 phosphorus.
The P=C bond lengths average 1.657(6) Å in 3b and 1.669(2) Å
in 4a with B–P–C average angles of 103.7(3)° and 105.1(1)°,
respectively. These P=C bond lengths fall within the range
typically observed for phosphaalkenes (1.661(6)-1690(2) Å).[18,
30] These represent the first examples of phosphaalkenylboranes
to be synthesized, and crystallographically characterized.
distances of 2.021(3) and 2.023(4) Å. The B–P–B and P–B–P
angles were found to be 98.5(1)° and 81.5(1)°, respectively, and
the P=C double bond length is 1.649(3) Å, similar to that of 2a.
Figure 3. Ball-and-stick (a and b) and space-filling (c and d) depictions of the
optimized geometry of Piers’ borane approaching 1a viewed from top (left) and
side (right)..
Scheme 2. Reactions of dimers 2a/b with tert-butylisocyanide and pyridine to
produce 3a/b and 4a, respectively.
Investigations of the reactivity of dimers 2a/b revealed
their thermal instability. Both decompose in C6D5Br with modest
heat (60 °C) or upon standing at room temperature in
halogenated solvents over several hours. Given the short P–B
bond lengths and the strong NMR spectroscopic coupling
observed in 2a/b, we anticipated the dimers to be unreactive
towards small molecules. Indeed, 2a shows no reactivity with H2,
CO2, or CO. Upon treatment with tert-butylisocyanide, however,
a rapid and clean reaction occurred, affording a new species 3a,
which we propose to be the monomeric phosphaalkenylborane
adduct (Scheme 2). This assignment was supported by loss of
the 1H NMR virtual triplet of 2a, and the appearance of a doublet
at 8.75 ppm with J = 24 Hz. Compound 3a gives rise to a 19F
NMR spectrum comprising three resonances at -130, -157, and -
Figure 4. POV-ray depiction of (a) 4a and (b) 3b. H atoms omitted for clarity.
In summary, hydroboration of phosphaalkynes with Piers’
borane proceeds with an unexpected regiochemical outcome to
give phosphaalkenylboranes, which are dimeric in the solid and
solution state. Despite their apparently short P–B bonds, the
P2B2 rings of these compounds were readily cleaved in the
presence of donors, affording monomeric phosphaalkenylborane
adducts. The products 2, 3, and 4 are unique examples of

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COMMUNICATION
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D.W.S. is grateful for the award of a Canada Research Chair.
L.E.L., T.C.J., and D.W.S. acknowledge NSERC for supporting
this research. R.L.F. thanks the EPSRC Centre for Doctoral
Training in Chemical Synthesis (EP/G036764/1) and the
University of Bristol, for a PhD studentship. L.E.L. thanks Digital
Specialty Chemicals for funding, and the NMR staff at the
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acknowledge the Canadian Foundation for Innovation, project
number 19119, and the Ontario Research Fund for funding of
the Centre for Spectroscopic Investigation of Complex Organic
Molecules and Polymers.
Keywords: hydroboration • phosphaalkyne • phosphaalkene •
heterocycles • phosphaalkenylborane
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P. C. Junk, A. F. Richards, M. Waugh, J. Organomet. Chem.
2003, 665, 127-134.

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Lauren E. Longobardi, Timothy C.
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Hydroboration of Phosphaalkynes
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