New Reactivity Patterns in 3H-Phosphaallene Chemistry [Aryl-P=C=C(H)-tBu]: Hydroboration of the C=C Bond, Deprotonation and Trimerisation

Article abstract of DOI:10.1002/chem.202002506

3H-Phosphaallenes, R?P=C=C(H)C?R’ (3), are accessible in a multigram scale on a new and facile route and show a fascinating chemical reactivity. BH₃(SMe₂) and 3 a (R=Mes*, R’=tBu) afforded by hydroboration of the C=C bonds of two phosphaallene molecules an unprecedented borane (7) with the B atom bound to two P=C double bonds. This compound represents a new FLP based on a B and two P atoms. The increased Lewis acidity of the B atom led to a different reaction course upon treatment of 3 a with H₂B-C₆F₅(SMe₂). Hydroboration of a C=C bond of a first phosphaallene is followed in a typical FLP reaction by the coordination of a second phosphaallene molecule via B?C and P?B bond formation to yield a BP₂C₂ heterocycle (8). Its B?P bond is short and the B-bound P atom has a planar surrounding. Treatment of 3 a with tBuLi resulted in deprotonation of the β-C atom of the phosphaallene (9). The Li atom is bound to the P atom as demonstrated by crystal structure determination, quantum chemical calculations and reactions with HCl, Cl-SiMe₃ or Cl-PtBu₂. The thermally unstable phosphaallene Ph?P=C=C(H)-tBu gave a unique trimeric secondary product by P?P, P?C and C?C bond formation. It contains a P₂C₄ heterocycle and was isolated as a W(CO)₄ complex with two P atoms coordinated to W (15).

Full text of DOI:10.1002/chem.202002506

Accepted Article
Accepted Article
Title: New reactivity patterns in 3H-phosphaallene chemistry [Aryl-
P=C=C(H)-tBu] – hydroboration of the C=C bond, deprotonation
and trimerisation
Authors: Werner Uhl, Jonas C. Tendyck, Alexander Hepp, and Ernst
Ulrich Würthwein
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10.1002/chem.202002506
Chemistry - A European Journal
FULL PAPER
DOI: 10.1002/chem.200((will be filled in by the editorial staff)
New reactivity patterns in 3H-phosphaallene chemistry [Aryl-P=C=C(H)-tBu] – hy-
droboration of the C=C bond, deprotonation and trimerisation
Jonas C. Tendyck,[a] Alexander Hepp,[a] Ernst-Ulrich Würthwein,*[b] Werner Uhl*[a]
Abstract:
3H-Phosphaallenes,
R- atom led to a different reaction course tonation of the β-C atom of the phos-
P=C=C(H)-R’ (3), are accessible in a upon treatment of 3a with H2B- phaallene (9). The Li atom is bound to
multigram scale on a new and facile C6F5(SMe2). Hydroboration of a C=C the P atom as demonstrated by crystal
route and show a fascinating chemical bound of a first phosphaallene is fol- structure determination, quantum chem-
reactivity. BH3(SMe2) and 3a (R = Mes*, lowed in a typical FLP reaction by the ical calculations and reactions with HCl,
R’ = tBu) afforded by hydroboration of coordination of a second phosphaallene Cl-SiMe3 or Cl-PtBu2. The thermally
the C=C bonds of two phosphaallene molecule via B-C and P-B bond for- unstable phosphaallene Ph-P=C=C(H)-
molecules an unprecedented borane (7) mation to yield a BP2C2 heterocycle (8). tBu gave a unique trimeric secondary
with the B atom bound to two P=C dou- Its B-P bond is short and the B bound P product by P-P, P-C and C-C bond for-
ble bonds. This compound represents a atom has a planar surrounding. Treat- mation. It contains a P2C4 heterocycle
new FLP based on a B and two P atoms. ment of 3a with tBuLi resulted in depro- and was isolated as a W(CO)4 complex
The increased Lewis acidity of the B
with two P atoms coordinated to W (15).
processes. Reports on the reactivity of phosphaallenes are limited,
and only few secondary products are reported such as transition metal
complexes (η1- and η2-coordination)[1,2,9] and cyclic dimers.[7e,10,11]
Few reactions with Main Group elements or their compounds, such
as n-butyllithium, sulfur, hydrogen peroxide, protic reagents or dich-
lorocarbene, are known.[7f,12] The central C atom of the P=C=C
moiety bears a partial negative charge and is attacked by pro-
tons.[1,13,14] Recently, we found facile access to various 3H-phos-
phaallenes (3) in a multigram scale and excellent yields by treatment
of dialkynylphosphines (1) with commercially available dialkylalu-
minium hydrides (e.g. iBu2AlH; Scheme 1).[2] The products are
formed via hydroaluminiation of an alkynyl group, generation of
mixed alkenyl-alkynyl aluminium intermediates (2)[15] and elimina-
tion of aluminium alkynides. This method is suitable for the synthesis
of persistent and transient species, the latter were trapped by coordi-
nation to transition metal atoms (η1- and η2-coordination).[2]
Introduction
Phosphaallenes, R-P=C=CR’2, form a fascinating class of com-
pounds.[1,2] They have cumulated P=C and C=C double bonds, are
highly reactive and only isolable with bulky substituents R such as
Mes* (2,4,6-tBu3C6H2) to prevent decomposition. They were gener-
ated for the first time about 40 years ago by the seminal work of Yo-
shifuji[3,4] and further studied by the groups of Appel[5,6] and
Märkl.[7,8] The latter group reported in 1988 on the first 3H-phos-
phaallenes, R-P=C=C(H)-R’ (e.g. R = Mes*, R’ = tBu), which had a
H atom bound to their β-C atom and were obtained by rearrangement
upon treatment of R-P(H)-C≡C-R’ with a strong base.[7] With the
Lewis-basic P atom, the P=C and C=C bonds and the C-H bond in β-
position to phosphorus these molecules have four different functiona-
lities and are applicable as promising starting materials in secondary
[a]
MSc. J. C. Tendyck, Dr. A. Hepp, Prof. Dr. W. Uhl
Institut für Anorganische und Analytische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstrasse 30, 48149 Münster, Germany
Fax: (+)49 251 8336660
e-mail: uhlw@uni-muenster.de
[b]
Prof. Dr. E.-U. Würthwein
Organisch-chemisches Institut and Center for Multiscale Theory and
Computation (CMTC)
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster, Germany
E-Mail: wurthwe@uni-muenster.de
Scheme 1. Synthesis of 3H-phosphaallenes: (i) +H-AlR’’2; (ii) –R’’2Al-C≡C-R’; R = Mes,
Trip, Mes*, CH(SiMe3)2; R’ = tBu, Ph, Adamantyl, C6H11; R’’ = Et, iBu, CH(SiMe3)2.
The facile accessibility of 3H-phosphaallenes enabled extensive
studies on their chemical properties. A new dimer (4; Scheme 2) was
isolated, which had a P-P single bond and an alkynyl and an alkenyl
group bound to the P atoms.[2] Irradiation of a transient species af-
forded a tricyclic compound (5) with a strained three-membered
phosphirane ring.[14] It represents an unprecedented isomer of the
Supporting information (NMR spectra, DFT calculations) for this arti-
cle is available on the WWW under http://www.chemeurj.org/ or from
the author.
1
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10.1002/chem.202002506
Chemistry - A European Journal
starting phosphaallene. Thermolysis of two room temperature stable
Mes* compounds afforded 1-benzo-dihydrophosphetes (6) with an-
nulated C6 and PC3 rings.[14] Their formation proceeds by an unusual
intramolecular nucleophilic aromatic substitution with the replace-
ment of an alkyl group at the aromatic ring. The phosphorus bound H
atom of 6 allowed catalyst-free hydrophosphination reactions and the
generation of bulky phosphine ligands.[14] In this paper we focus on
reactions at the vinylidene part of the phosphaallenes and report on
the isolation of a novel trimeric product of an unstable derivative.
Figure 1. Molecular structure and numbering scheme of 7. Displacement ellipsoids are
drawn at the 40% level. H atoms with exception of H1, H2A/H2B and H5A/H5B and
methyl groups of the Mes* substituents are omitted. Important bond lengths (Å) and an-
gles (°): P1-C1 1.686(2), P2-C4 1.690(2), B1-C1 1.544(2), B2-C4 1.549(2), C1-C2
1.514(2), C4-C5 1.514(2), C1-B1-C4 126.7(1), B1-C1-P1 109.2(1), B1-C1-C2 120.5(1),
B1-C4-P2 110.0(1), B1-C4-C5 128.0(1).
7 showed a singlet at a low field (δ = 326.8) of the 31P{1H} NMR
spectrum. The chemical shift corresponds to values found for phos-
phaalkenes that have a relatively electropositive atom (Si of SiMe3,
Sn of SnR3 or B atoms in various groups) attached to their central C
atom.[23] The 13C NMR signal of the P=C moiety (δ = 200.8) is in the
normal range of phosphaalkenes, and the 11B NMR spectrum shows
a broad resonance in the range expected for three-coordinate B atoms
(δ = 60.4).[24] A sharp absorption of the B-H stretching vibration was
observed for the terminal B-H bond in the characteristic range of the
IR spectrum at 2413 cm–1. 7 has an interesting molecular structure
with a coordinatively unsaturated Lewis acidic B atom and two Lewis
basic P atoms in a single molecule. It represents a rare example of a
frustrated Lewis pair,[25] in which the presence of two basic atoms
may allow the chelating activation of substrates.[26] However, the B
atom of 7 is not activated by an electron withdrawing group, hence,
its Lewis acidity may not be high enough to act as an efficient FLP[27]
and its reactivity may be somewhat limited. We, therefore, applied
the borane H2B-C6F5 with the electron withdrawing C6F5 group atta-
ched to boron.[28]
Scheme 2. Secondary products of 3H-phosphaallenes (R’ = tBu, adamantyl).
Results and Discussion
Hydroboration
Hydroboration is a powerful method for the reduction of unsatura-
ted compounds and the generation of functional molecules[16,17] and
has been applied also for the reduction P-C double or triple bonds.[18-
20] Phosphaalkenes, R-P=CR’2, afforded preferably products with P-
H and C-B bonds.[18,19] In contrast, treatment of phosphaalkynes,
P≡C-R, with secondary boranes yielded both possible products with
the formation of P-H or P-B bonds.[20] Hydroboration of phosphaal-
lenes has not been reported previously. We treated Mes*-P=C=C(H)-
tBu (3a) with equimolar quantities of H3B(SMe2) in n-hexane at room
temperature, but based on NMR data and crystal structure determina-
tion the unexpected insertion of two phosphaallene moieties into dif-
ferent B-H bonds of a BH3 molecule gave the di(phosphaalkenyl)bo-
rane (7). The reaction in the correct molar ratio of 2 : 1 afforded 7 in
a high yield of 87% (Scheme 3). Interestingly, hydroboration of the
C=C bonds is preferred over the hydroboration of the heteronuclear
P=C bonds (Figure 1). The P=C distances of 1.688 Å on average cor-
respond to standard values of phosphaalkenes.[21] Caused by the dif-
ferent hybridization of the central C atoms (sp2 versus sp) they are
slightly longer than in phosphaallene 3a [1.651(3) Å].[2] The B atoms
have a trigonal planar surrounding (sum of angles = 360°), and the B-
C distances are with 1.547(av) Å in the normal range of B-C single
bonds to sp2-C atoms.[22]
Treatment of 3a with H2B-C6F5(SMe2) in a molar ratio of 2 : 1 in
n-hexane at -78 °C afforded after recrystallization of the crude pro-
duct from CH2Cl2 yellow crystals of compound 8 in 70% yield.
Crystal structure determination revealed an unexpected molecular
structure which is different from that one of 7. Once again two for-
mula units of the phosphorus compound reacted with one equivalent
of the borane. But now the formation of three new bonds (B-P, P-C,
B-C) afforded a five-membered P2C2B heterocycle, which adopted a
slightly distorted envelope conformation with the atom P2 0.57 Å
above the average plane of the four remaining ring atoms. Both P=C
bonds of the phosphaallenes and a C=C bond were reduced to single
bonds, one of the C=C bonds [1.345(9) Å] is retained and is an
exocyclic position of the P2C2B ring. The atom P2 has the expected
trigonal pyramidal surrounding with sum of the bond angles = 325.9°
and shows normal endocyclic P-C bond lengths of 1.853(2) and
1.846(2) Å. In contrast, P1 has surprisingly a trigonal planar coordi-
nation sphere (sum of the angles = 359.0°) with short P1-C4 [1.787(2)
Å] and P1-B1 distances [1.797(2) Å]. These observations support the
assumption of a π-interaction between the lone pair at phosphorus and
the empty p-orbital at boron and the occurrence of a P=B double bond,
which may be favored by the strong electron-withdrawing character
of the C6F5 group at boron. Few structurally authenticated compounds
containing a P=B bond with planar surrounded P atoms and short P-
B bond lengths are reported in the literature, in two cases the B atoms
are also bound to C6F5 groups.[29] Quantum chemical calculations on
the latter compounds suggested a highly polar bonding character of
Scheme 3. Hydroboration of phosphaallene 3a.
2
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10.1002/chem.202002506
Chemistry - A European Journal
the B-P bond.[29b,c] These compounds have been used for the hetero-
lytic cleavage of H2 molecules and for the dehydrogenation of am-
mineboranes.
Deprotonation
While the ligand properties and the reactivity of the P=C and C=C
bonds of phosphaallenes have been reported in the literature, the reac-
tivity of the C-H bond is almost unexplored and to the best of our
knowledge lithium compounds derived from 3H-phosphaallenes by
deprotonation are not known. Yoshifuji treated a 3H-phosphaallene
with 0.5 equivalents of n-butyllithium and isolated a 3,4-diphosphi-
nidenecyclobutene.[10,11a] In some cases a transient phosphaallene was
postulated to be deprotonated by a strong base.[11c,d] But no
spectroscopic or theoretical evidences were provided to support such
a mechanism. A persistent phosphaallene, R-P=C=CR’2, was re-
ported to react with n-butyllithium by addition of the Li-C moiety to
the P=C bond.[12a] The product was not isolated and characterized, but
quenched by addition of methanol. We treated 3a with equimolar
quantities of tert-butyllithium and isolated a colorless highly viscous
liquid of 9a (Scheme 4) which could not be purified and isolated in a
crystalline form. It showed very broad resonances in the NMR spectra,
which could not be assigned completely. Deprotonation of the C-H
group is confirmed by the missing signal of the vinylic H atom in the
characteristic range of 1H NMR spectrum at about δ = 5.70.[2] The P
atom shows a broad resonance in the 31P{1H} NMR spectrum at δ = -
136, which is considerably shifted to a higher field compared to 3b
and in the typical range of lithium phosphides.[31] Two 13C NMR sig-
nals at δ = 97.7 and 117.1 confirm the presence of a C≡C triple bond
and the missing phosphaallene unit. A suggestion for the molecular
constitution of 9a is given in Scheme 4, it is based on the structures
of the TMEDA adduct 9b and of some secondary products (see be-
low).
Figure 2. Molecular structure and numbering scheme of 8. Displacement ellipsoids are
drawn at the 40% level. H atoms with exception of H2A/H2B and H5 and the tBu groups
of the Mes* substituents are omitted. Important bond lengths (Å) and angles (°): P1-C4
1.787(2), P1-B1 1.797(2), P2-C1 1.853(2), P2-C4 1.846(2), B1-C1 1.596(2), C4-C5
1.345(2), C4-P1-B1 104.76(8), P1-B1-C1 113.6(1), B1-C1-P2 105.7(1), C1-P2-C4
103.64(7), P1-C4-P2 103.14(8).
Two resonances were observed in a narrow range of the 31P{1H}
NMR spectrum at δ = 30.3 and 25.3. The first one is broad and did
not allow the identification of a coupling pattern. It results from the
atom P1, which is bound to the boron atom, and its broadness is
caused by the quadrupole moment of the boron nuclei. The second P
2
atom (P2) shows a doublet with a coupling constant JPP of 89.2 Hz.
A broad resonance was observed in the 11B NMR spectrum at δ = 53.2.
B and P NMR shifts are similar to those reported for B=P com-
pounds,[29] but are not really indicative for this specific bonding situ-
ation. The 13C NMR signals of the exocyclic C=C bond were detected
in the expected range at δ = 142.2 and 136.2 as doublets of doublets
due to the coupling to two chemically different P atoms.
The different reaction pathways observed for the hydroboration
of phosphaallene 3a (Scheme 3) are clearly influenced by the electron
withdrawing C6F5 group at boron. The first step of both reactions may
be the same and comprise the addition of a B-H bond to the C=C
double bond of the P=C=C group. The postulated intermediate has an
intact P=C bond, and the B atom is bound to the inner C atom, similar
to the structure of compound 7. The resulting constitution resembles
the situation in geminal Frustrated Lewis pairs.[25] In contrast to the
synthesis of 7 with the insertion of a C=C double bond into the second
B-H bond, the Lewis acidity of the B atom is considerably enhanced
in the C6F5 borane. It is a strongly polarizing acceptor and favors the
approach of the nucleophilic P atom of a second phosphaallene. The
P=C bond becomes polarized by the P-B interaction, and the α-C
atom reacts as an electrophile with the P atom of the FLP-type inter-
mediate to form the five-membered heterocycle of 8. A similar reac-
tion has been postulated based on quantum chemical calculations and
comprises the trapping of a highly unstable phosphaallene by a tran-
sient Al/P-based FLP analogous to 2 (Scheme 1) to yield an AlP2C2
heterocycle (see below for further discussion).[15c] In other cases,
hydroalumination of sterically less shielded alkynylphosphines with
dialkylaluminum hydrides resulted in the formation of geminal Al/P
FLPs, which could not be isolated, but reacted spontaneously with the
triple bond of a second equivalent of the starting alkynylphosphine by
generation of five-membered heterocycles.[30] In the final step of the
formation of 8 1,2-hydrogen shift from boron to the C atom of the P-
C-B group is required.
Scheme 4. Deprotonation of the 3H-phosphaallene 3a.
A similar lithium phosphide, Li[P(Mes*)-C≡C-R’] (R’ = SiMe3),
was claimed to be formed by deprotonation of the corresponding P-H
alkynylphosphine with methyllithium in the presence of TMEDA.[8]
It was not isolated or characterized, but immediately trapped by addi-
tion of Cl-SiMe3.[8] A phosphaallene was isolated, and from the con-
stitution of the product an equilibrium was postulated between li-
thium phosphide and a deprotonated phosphaallene. A similar
equilibrium has been postulated for a phenyl substituted species (R’
= Ph), because quenching with Cl-SiMe3 resulted in the formation of
a mixture of the phosphaallene and the corresponding alkynylphos-
phine.[10a] NMR spectroscopy or the generation of secondary products
did not confirm such an equilibrium in our case. In addition, quantum
chemical calculations with 9a (TPSSTPSS/def2tzvp+GD3BJ)[32a-g]
showed that the Li phosphinide structure depicted in Scheme 4 is
energetically highly preferred over the isomer with Li bound to the β-
C atom. Similar to the results of crystal structure determination of 9b
3
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10.1002/chem.202002506
Chemistry - A European Journal
(see below) the optimized structure has the Li atom additionally coor-
dinated to the α- and β-C atoms of the ethynyl substituent. The shift
of the Li atom to the β-C atom does not result in an energetic mini-
mum, it is in contrast energetically highly unfavorable and can defi-
nitely be excluded. Interestingly, the H compounds behave different.
The phosphallene structure is slightly favoured over the P-H phos-
phine tautomer by 4.32 kcal/mol. These results confirm that an
equilibrium between Li alkynylphosphinide (9a, experimentally ob-
served) and a deprotonated compound with an intact phosphaallene
moiety is at least in our case not to be expected.
relatively large coupling constants of all SiMe3 atoms to phosphorus,
29Si satellites in the 31P{1H} NMR spectrum and the typical reso-
nances of an ethynyl group in the 13C NMR spectrum at δ = 118.0 (P-
C≡C) and 76.8 (P-C≡C). All observations are in accordance with the
phosphine structure given in Scheme 5. 11 was isolated as a highly
viscous liquid and could not be crystallized. P-P bond formation was
observed upon treatment of 9a with Cl-PtBu2. The crystalline com-
pound 12 was isolated in 88% yield. It showed two doublets in the
1
31P{1H} NMR spectrum at δ = 42.5 (PtBu2) and -65.0 with a JPP
coupling constant of 190.5 Hz. The high field resonance is in the ty-
pical range of aryl-ethynylphosphines,[2,15c] while the signal at a lower
field resembles values of dialkyldiphosphines. e.g. P2tBu4.[36] The
stretching vibration of the C≡C bond was detected in the normal range
of the IR spectrum at 2149 cm–1. The P-P bond length of 2.245(av) Å
corresponds to standard values (Figure 4).[37] The P-C distances cor-
relate to the respective hybridization of the C atoms and are 1.755(av)
(P1-C1; alkynyl), 1.865(av) (P1-C41; aryl) and 1.902(av) Å (P-tBu).
Both P atoms have a trigonal pyramidal surrounding with sum of the
angles of 317.6 (P1) and 308.3° (P2; average values of two indepen-
dent molecules). The difference may reflect the different bulk of the
substituents.
In order to unambiguously characterize the deprotonated compound
9a, we treated the reaction mixture with an equimolar quantity of
N,N,N’,N’-tetramethylethylenediamine (TMEDA, Scheme 4) and
isolated yellow crystals of the TMEDA adduct 9b in 82% yield. The
31P{1H} NMR signal was slightly shifted to a lower field (δ = -104.9)
compared to 9a; a coupling to the Li atom was not observed.[33] The
C atoms of the ethynyl unit resonated in a relatively narrow range at
δ = 111.1 and 114.1 and confirm the presence of a C≡C triple bond.
The stretching vibration of the C≡C group was observed in the IR
spectrum at 1994 cm–1, which is slightly outside of the characteristic
range and may reflect the Li-ethynyl interaction (see below). The Li
atom is coordinated by the P atom with a P-Li distance of 2.618(3) Å,
which is in the upper range of values reported for lithium phosphides
in the literature (Figure 3).[33,34] It is additionally coordinated in a
chelating manner by the N atoms of the TMEDA ligand and shows
short contacts to the C atoms of the ethynyl group with Li1-C1 =
2.120(3) and Li1-C2 = 2.344(3) Å, which are in the characteristic
range of Li-C interactions in organo lithium derivatives.[35] The shor-
ter one may reflect the importance of electrostatic interactions and
charge separation with a relatively high negative partial charge at the
α-C atom of the ethynyl group. These Li-C interactions may cause the
deviation of the C-C≡C-P group from linearity [P-C≡C 163.4(1) and
C≡C-C 161.5(5)°; main component of the disordered group].
Scheme 5. Secondary reactions of compound 9a.
Figure 3. Molecular structure and numbering scheme of 9b. Displacement ellipsoids are
drawn at the 40% level. H atoms are omitted. Important bond lengths (Å) and angles (°):
P1-C1 1.753(2), C1-C2 1.217(2), P1-Li1 2.618(3), C1-Li1 2.120(3), C2-Li1 2.344(3), P1-
C1-C2 163.4(1), C1-C2-C3 161.5.
Crystal structure determination confirms the phosphide character of
9 and excludes the originally anticipated phosphaallene nature. We
conducted some secondary reactions with 9a and were particularly
interested in clearly excluding the equilibrium between both possible
isomers in solution. Treatment of 9a with HCl, Cl-SiMe3 and Cl-
PtBu2 (Scheme 5) did not result in traces of products with a phos-
phaallene structural motif. HCl yielded the P-H phosphine 10, which
has been obtained previously on another route[2,7a] and is easily iden-
tified by it characteristically large 1JPH coupling constant of 245.0 Hz.
Cl-SiMe3 afforded in 70% yield the silylphosphine 11 which showed
Figure 4. Molecular structure and numbering scheme of 12. Displacement ellipsoids are
drawn at the 40% level. H atoms are omitted. Important bond lengths (Å) and angles (°),
average values of two independent molecules: P1-P2 2.245, P1-C1 1.755, C1-C2 1.202,
P1-C41 1.865(3), P2-tBu 1.902, C1-P1-C41 107.0, C5-P2-C6 109.5.
A novel trimeric secondary product of an unstable phosphaallene
Unstable phosphaallenes or persistent derivatives form various se-
condary products upon irradiation or in the heat. Dimers are known
since many years which resulted from [2+2]- or [2+3]-cycloaddition
reactions and have four-membered P2C2 or C4 rings (head-to-tail[6,7e]
and head-to-head dimerization[10a,b,11a,b,d]) or adopt diphosphafulvene
4
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10.1002/chem.202002506
Chemistry - A European Journal
structures with five-membered P2C3 heterocycles.[11b-d] Only recently
we observed the formation of two unprecedented secondary products
which are shown in Scheme 2 (4 and 5). The tricyclic phosphirane 5
is an isomer of a phosphaallene and obtained by rearrangement, C-H
bond activation and P-C bond formation.[14] Compound 4 represents
an unusual, asymmetric dimer with a P-P bond.[2] One P atom bears
an alkynyl group, while the second one is bound to an alkenyl group.
The generation of 3H-phosphaallenes 3 by hydroalumination of dial-
kynylphosphines and elimination of aluminium alkynides[2] allows
the generation of persistent (R = Mes*) and transient species [R =
Trip, Mes, CH(SiMe3)2], the latter could be trapped by coordination
to transition metal atoms or showed a selective rearrangement reac-
tion to yield compound 4 (Scheme 1). Ph-P=C=C(H)-tBu (3b) with
the relatively small Ph group attached to P is accessible on a similar
route (Scheme 6) and was identified by its characteristic 31P NMR
shift of δ = 64.6.[15a] It is very reactive and starts to decompose before
the intermediately formed hydroalumination product similar to 2 was
completely consumed, as was shown by thorough NMR investigati-
ons into the kinetics of these reactions.[15a] Decomposition results in
the formation of several products (see Experimental Part). One of
these crystallized from the reaction mixture and was identified as
compound 13 (Scheme 6) which contains a five-membered AlP2C2
heterocycle and results from the reaction of an intermediately formed
FLP (2, Scheme 1) with the transient phosphaallene 3b.[15] Another
secondary product (14) showed resonances of three different P atoms
with three coupling constants of 12.6, 7.8 and 168.1 Hz. The latter is
and P3 almost in a plane (maximum deviation 0.13 Å), the vinylic C
atom C11 is 0.89 Å above the plane. The trimer coordinates to the W
atom in a chelating manner via bonds to P1 and P3 [2.503(av) Å].
These P atoms have a distorted tetrahedral coordination sphere, while
the atom P2 has a trigonal pyramidal surrounding. A benzene solution
of the crystallized compound 15 showed three very similar sets of re-
sonances in the 31P{1H} NMR spectrum with an intensity ratio of 7 :
2 : 1 (see Experimental Part). The three P atoms have a chiral
surrounding, and we assume that diastereomeric molecules are for-
med. Few data are discussed exclusively for the main component. The
chemical shift of the P atom P2, which is not coordinated to W, is in
the range of resonances observed for 14 (δ = -35.1). In accordance
with their increased coordination numbers, the signals of the other P
atoms are shifted to a lower field (δ = 52.6 and 12.1). The 1JPP coup-
ling constant remains almost unchanged, but the 2JPP and 3JPP values
increase to 20.9 and 26.9 Hz.
1
characteristic of a JPP coupling and indicates the presence of a P-P
bond. The concentration of 14 increased continously, while the reso-
nances of the phosphaallene 3b disappeared after two days.[15a] The
mixture of products with overlapping NMR signals did not allow the
further characterization or unambiguous identification of 14.
Figure 5. Molecular structure and numbering scheme of the W complex 15. Displacement
ellipsoids are drawn at the 40% level. H atoms with exception of the vinylic ones and CO
ligands are omitted. Important bond lengths (Å) and angles (°): P1-W1 2.496(1), P3-W1
2.506(1), P1-P2 2.252(1), P2-C11 1.818(4), P3-C11 1.838(4), C51-C52 1.408(6), P1-
C11-P3 113.0(2), C11-P2-C51 97.8(2), C11-P3-C6 104.1(2).
The mechanism for the unexpected formation of trimer 14 was
studied in detail by quantum chemical DFT model calculations.[32] On
the basis of B3LYP/6-31G(d)[32b,c] + GD3BJ[32d,e] geometry optimiza-
tions TPSSTPSS/def2tzvp[32f,g]+GD3BJ[32d,e] calculations including
the experimentally used solvent n-hexane (PCM[32h]) were performed.
Trimerisation proceeds also in the dark, and the reaction outcome
seems not to be influenced by light. Thus, in accord with the reaction
conditions (thermal reactions) only closed shell calculations were per-
formed. Discussion of relative Gibbs free energies (ΔG298K) is based
on the PCM-n-hexane solvent sphere model (kcal/mol) with respect
to the sum of two molecules of 1-phosphallene A and one molecule
of B (activated form of A; Scheme 7, see also Supporting Information
for details). In all model calculations the tBu group was replaced by
a Me group. We used a retro-synthetic approach, starting from the
final product K (the methyl analogue of 14) employing reaction path
calculations (stepwise varying the interatomic distances of the respec-
tive atoms) to localize transition states and minima. Their intercon-
nections were checked by IRC-calculations.
Scheme 6. Trimeric secondary product of phosphaallene 3b and its W(CO)4 complex [(i)
+ HAlR2, - R2Al-C≡C-tBu; (ii) + W(CO)5(THF), - THF, - CO; R = CH(SiMe3)2].
We, therefore, treated the reaction mixture with in-situ generated
W(CO)5(THF)[38] in order to favor crystallization and to isolate 14 by
complexation (Scheme 6). Chromatographic work-up and crystalliza-
tion of the oily residue from n-hexane afforded yellow crystals of 15
in an expectedly low yield of 17% based on diethynylphosphine.
Crystal structure determination revealed a unique molecular structure
with a six-membered P2C4 heterocycle and an exocyclic P-P bond to
one of the P atoms of the heterocycle. A trimer of the starting phos-
phaallene is formed which represents a new type of secondary pro-
ducts. The P-P bond length is with 2.252(2) Å in the normal range
(Figure 5).[37] The central P2C4 ring adopts an envelope conformation
with the sp2-hybridized C atoms C51, C52 and C6 and the P atoms P2
Initial calculations showed that reactions of neutral species led to
high kinetic barriers. Thus, in accordance with the reaction conditions,
electrophilic catalysis was considered using Lewis acidic AlMe3 as a
catalyst, which represents any electrophilic Al species present in the
reaction mixture. Our suggested reaction mechanism starts with the
highly reactive Lewis acid-base complex B, which reacted overall via
several steps with two neutral phosphaallenes A to yield the trimeric
compound K.
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202002506
Chemistry - A European Journal
In the first step, we suggest an approach of the activated form B
to the 1-phosphaallene A leading via transition state C and P-C bond
formation to the P-C-P-species D. D contains a vinylidenephospho-
nium cation with cumulated P=C and C=C bonds and a highly reac-
tive, electron rich central allene C atom (NBO charge: -0.3603). This
C atom approaches the ortho-C atom of a P-phenyl group to form a
six-membered P2C4 heterocycle F via transition state E (NBO-charge
at the ortho-arene C atom: -0.2067) in an unusual electrocyclization
reaction. After intra- or intermolecular proton shift the intermediate
G is formed, which already has the central structural motif of the tri-
mer with two annulated six-membered rings, the aromatic C6 ring and
the P2C4 heterocyle. The activating Lewis acid is coordinated by a P
atom. Intermediate G reacts with a second 1-phosphaallene molecule
B, which is activated by migration of AlMe3. Interestingly, we were
able to localize a stationary point for the van der Waals complex H,
which is close in energy to the transition state I. Relaxation and P-P-
bond formation results in the intermediate diphosphine J in a highly
exothermic reaction. Compound K, as the analogue of the experimen-
tally obtained trimer 14, is finally formed after proton shift and re-
lease of AlMe3. As impressively shown, the formation of the trimeric
phosphaallene requires a complicated multistep process and proceeds
via relatively low activation barriers (maximum 12.4 kcal/mol) which
are in accordance with the mild reaction conditions. Overall the tri-
meric formula unit (14) is thermodynamically highly favoured over
the starting material (3b), but its selective formation requires activa-
tion by a Lewis acid.
Conclusion
3H-Phosphaallenes are highly functional molecules with Lewis basic
P atoms, P=C and C=C double bonds and a C-H bond in β-position to
phosphorus. They are known since the eighties of the last century, but
their reativity was scarcely elucidated. Recently we found facile ac-
cess to persistent and reactive species in a multigram scale, which
allows systematic investigations into their fascinating chemical prop-
erties. In this contribution we report on two hydroboration reactions
with the persistent compound Mes*-P=C=C(H)-tBu. The insertion of
C=C bonds into two B-H bonds was observed for BH3, while the elec-
tron withdrawing C6F5 groups of H2B-C6F5 gave hydroboration fol-
lowed by coordination of a second phosphaallene molecule and gen-
eration of a BP2C2 heterocycle. The first compound represents a frus-
trated Lewis pair with the chelating arrangement of two P atoms and
has intact P=C bonds, while in the second case an intermediate FLP
with an activated Lewis acidic B atom favoured adduct formation.
The B-P bond of the latter product is short, and its P atom has a planar
environment. Its polarity should result in a nice reactivity as shown
for a related molecule in the literature. Deprotonation of the C-H bond
resulted in formation of a lithium alkynylphosphide with the Li atom
coordinated to phosphorus and the P=C double bond replaced by a P-
C single bond. The reactivity of the P=C bonds was demonstrated
with the isolation of a trimeric secondary product of a sterically low
shielded phosphaallene. It represents a nice addition to known decom-
position products of phosphaallenes and results from P-P, P-C and C-
C bond formation on a multistep pathway with low activation barriers,
as shown by DFT calculations. Once more, these results confirm the
exceptional and highly promising reactivity of phosphaallenes and
their secondary products.
Experimental Section
General methods: All manipulations were carried out under purified argon,
using standard Schlenk techniques. Solvents were distilled from drying agents
and degassed (THF and toluene over sodium/benzophenone; n-pentane and n-
hexane over LiAlH4; pentafluorobenzene and dichloromethane over molecular
sieves). NMR spectra were recorded in benzene-d6 and C6D5-CD3 at ambient
probe temperature using the following Bruker instruments: Avance I (1H,
7
400.13; 13C, 100.62; 31P, 161.98; 29Si, 79.49 MHz; 11B, 128.38 MHz; Li,
155.51 MHz; 15N, 40.55 MHz), Avance III (1H, 400.03; 13C, 100.60; 31P,
161.93 MHz; 29Si, 79.47 MHz) and referenced internally to residual solvent
resonances (chemical shift data in δ). 13C, 31P, 7Li, 11B and 15N NMR spectra
were all proton-decoupled. The assignment of NMR spectra is based on HSQC,
HMBC, DEPT135, DEPT19.5 and H,H-ROESY data. Elemental analyses
were determined by the microanalytic laboratory of the Westfälische Wil-
helms Universität Münster. IR spectra were recorded as KBr pellets on a
Shimadzu Prestige 21 spectrometer, electron impact mass spectra on a Finni-
gan MAT 95 mass spectrometer, HRMS (ESI-TOF) on Orbitrap LTQXL. The
phosphaallene 3a[2] and the borane H2B-C6F5(SMe2)[28] were synthesized ac-
cording to literature procedures. Improved procedures for the syntheses of Ph-
P(C≡C-tBu)2 and H-Al[CH(SiMe3)2]2 are given below. BH3(SMe2), Ph-PCl2,
tBu2PCl, solutions of tert-butyl- and n-butyl lithium in n-pentane, TMEDA,
Me3SiCl and a solution of HCl in diethyl ether are commercially available and
were used as purchased.
Ph-P(C≡C-tBu)2:[39] A cooled (0 °C) solution of 3,3-dimethyl-1-butyne (6.42
g, 78.3 mmol) in 100 mL of diethyl ether was treated with a solution of n-
butyllithium in n-hexane (1.6 M, 48.9 mL, 78.3 mmol). The solution was war-
med to room temperature and stirred for 1 h. The resulting solution was added
to a cooled (0 °C) solution of Ph-PCl2 (7.00 g. 39.1 mmol) in 20 ml of diethyl
ether. The mixture was warmed to room temperature overnight. All volatiles
were removed in vacuum. The residue was extracted with 100 mL of n-pen-
tane. After filtration the solvent was removed in vacuum, and the remaining
highly viscous liquid was distilled in vacuum (70 °C, 10–3 Torr) (7.18 g, 68%).
31P{1H} NMR (162 MHz, C6D6, 300 K): δ = -60.7.
[41]
H-Al[CH(SiMe3)2]2:[40] Al[CH(SiMe3)2]3 (23.9 g, 47.4 mmol) was cooled
with liquid N2 and treated with AlH3⸱1.49NMe2Et (4.12 mL, 3.29 g, 23.7
mmol) without a solvent. The mixture was warmed to room temperature and
heated to 120 °C for 2 h. The amine was removed in vacuum (10–3 Torr) at
100 °C. The residue was treated with hot n-hexane. Filtration and cooling af-
forded colorless needles of the hydride (23.5 g, 96%).
Scheme 7. Mechanism for the formation of K (methyl analogue of 14);
[TPSSTPSS/def2tzvp+GD3BJ+PCM (n-hexane); G298K in kcal/mol with respect to the
sum of 2A + B].
6
This article is protected by copyright. All rights reserved.

10.1002/chem.202002506
Chemistry - A European Journal
Hydroboration of 3H-3-tert-butylphosphaallene 3a with BH3·SMe2; syn-
thesis of 7. A solution of 3H-3-tert-butylphosphaallene 3a (0.35 g, 0.98 mmol)
in 10 mL of n-hexane was cooled to -78 °C and treated with BH3·SMe2 (46
µL, 0.037 g, 0.49 mmol). The mixture was warmed to room temperature over-
night. All volatiles were removed in vacuum. The residue was recrystallized
from 1,2-difluorobenzene at 3 °C to obtain 7 as yellow crystals (0.36 g, 87%).
The crystals enclosed one molecule of difluorobenzene per formula unit of 7.
M.p. 210 °C; 1H NMR (400 MHz, [D8]toluene, 300 K): δ = 7.48 (s , 4H; m-H),
6.30 (s br., 1H; B-H), 2.08 (overlap, m, 4H; CH2) 1.63 (s br., 36H; o-CMe3),
1.36 (s, 18H; p-CMe3), 0.96 ppm (s, 18H; CH2-tBu); 13C{1H} NMR (100 MHz,
[D8]toluene, 300 K): δ = 200.8 (m br.; P=C), 154.6 (o-C Mes*), 149.8 (p-C
Mes*), 141.2 (m br.; i-C Mes*), 121.4 (m-C Mes*), 53.3 (pseudo-t, 2JC,P = 4JC,P
= 6.8 Hz; CH2), 38.4 (br.; o-CMe3), 35.0 (p-CMe3), 34.0 (br.; o-CMe3), 33.8
(CH2-CMe3), 31.6 (p-CMe3), 31.1 ppm (CH2-CMe3); 11B{1H} NMR (160 MHz,
[D8]toluene, 300 K): δ = 60.4 ppm; 31P{1H} NMR (162 MHz, [D8]toluene, 300
K): δ = 326.8 ppm; IR (KBr): 휈̃ = 2957 (vs), 2901 (vs), 2866 (vs) ν(CH)); 2509
(vw), 2413 (m) ν(BH); 1778 (vw), 1751 (vw), 1616 (w), 1589 (m), 1508 (s)
ν(C=C), aryl; 1476 (s), 1464 (m), 1393 (s), 1362 (vs), 1258 (vs), 1240 (s)
δ(CH3); 1202 (m), 1128 (w), 1090 (m), 1045 (m), 1024 (vw), 1001 (vw), 984
(vw), 930 (vw), 903 (w), 874 (m), 845 (m), 756 (s) ν(CC), ν(BC); 646 (vw),
596 (w) aryl; 569 (vw), 488 (vw), 471 (vw), 451 (vw) cm–1 ν(PC), δ(CC); MS
(EI+, 20 eV, 323 K): m/z (%): 731 (15) [M]+, 674 (21) [M–tBu]+, 617 (9) [M–
2tBu]+, 485 (22) [M–Mes–H]+; elememtal analysis calcd (%) for C48H81P2B +
C6H4F2: C 76.7, H 10.1; found: C 76.5, H 10.0.
temperature overnight. Alternatively n-butyllithium may be used, but requires
warming to 75 °C. The intermediate lithium compound 9a was isolated as a
yellow highly viscous liquid, which could not be crystallized from different
solvents. It showed broad resonances in the NMR spectra, which did not allow
the complete assignment. Selected NMR data: 13C{1H} NMR (100 MHz, C6D6,
300 K): δ = 117.1 (P-C≡C), 97.7 ppm (P-C≡C); 31P{1H} NMR (162 MHz,
C6D6, 300 K): δ = -136.0 ppm. N,N,N′,N′-Tetramethylethylenediamine
(TMEDA) (42 µL, 0.28 mmol) was, therefore, added to the reaction mixture.
The solution was concentrated and cooled to 3°C to obtain yellow crystals of
1
9b (0.11 g, 82%). M.p. 137 °C; H NMR (400 MHz, C6D6, 300 K): δ = 7.34
(d, 4JH,P = 1.7 Hz, 2H; m-H), 2.01 (s, 18H; o-CMe3), 1.71 (s, 12H; NMe2), 1.67
(s, 4H; N-C2H4-N), 1.41 (s, 9H; p-CMe3), 1.26 ppm (s, 9H; C≡C-tBu); 13C{1H}
NMR (100 MHz, C6D6, 300 K): δ = 152.3 (d, 2JC,P = 6.3 Hz; o-C Mes*), 146.2
(d, 1JC,P = 79.1 Hz; i-C Mes*), 142.0 (s; p-C Mes*), 121.1 (d, 3JC,P = 1.8 Hz;
m-C Mes*), 114.1 (d, 2JC,P = 8.8 Hz; C≡C-tBu), 111.1 (d, 1JC,P = 89.1 Hz; C≡C-
tBu), 56.6 (N-C2H4-N), 45.8 (NMe2), 39.2 (d, 3JC,P = 1.1 Hz; o-CMe3), 34.4 (p-
CMe3), 33.8 (d, 4JC,P = 11.3 Hz; o-CMe3), 32.7 (d, 4JC,P = 2.6 Hz; C≡C-CMe3),
32.0 (p-CMe3), 31.3 ppm (d, 3JC,P = 2.2 Hz; C≡C-CMe3); 7Li{1H} NMR (105
MHz, C6D6, 300 K): δ = 0.98 ppm; 15N{1H} HMBC-NMR (41 MHz, C6D6,
300 K): δ = 19 ppm; 31P{1H} NMR (162 MHz, C6D6, MHz, 300 K): δ = -104.9
ppm; IR (KBr): 휈̃ = 2970 (vs), 2947 (vs), 2901 (vs), 2864 (vs), 2793 (s) ν(CH);
1994 (s) ν(C≡C); 1722 (vw), 1653 (vw), 1589 (m), 1528 (w) ν(C=C), aryl;
1464 (vs), 1391 (s), 1360 (vs), 1285 (s), 1240 (vs) δ(CH3); 1198 (s), 1179 (m),
1157 (m), 1128 (w), 1098 (vw), 1078 (vw), 1063 (w), 1034 (s), 1020 (m), 947
(m), 930 (w), 901 (vw), 878 (m), 808 (vw), 791 (w), 770 (w), 743 (m) ν(CC),
ν(CN); 648 (vw), 623 (w), 596 (w) aryl; 544 (w), 482 (m), 438 (w) cm–1 ν(PC),
δ(CC); MS (EI+, 20 eV, 298 K): m/z (%) = 357 (14) [M–Li(TMEDA)]+; ele-
mental analysis calcd for C30H54PLiN2: C 74.9, H 11.3, N 5.8; found: C 74.5,
H 11.2, N 5.4.
Hydroboration of 3H-3-tert-butylphosphaallene 3a with H2BC6F5·SMe2;
synthesis of 8. A cooled (-78 °C) solution of 3H-3-tert-butylphosphaallene 3a
(0.39 g, 1.09 mmol) in 10 mL of n-hexane was treated with BH2C6F5·SMe2
(0.13 g, 0.54 mmol). The mixture was warmed to room temperature overnight.
All volatiles were removed in vacuum. Compound 8 crystallized from a satu-
rated solution in dichloromethane at 3 °C as yellow crystals (0.37 g, 70%).
The crystals enclosed one molecule of CH2Cl2 per formula of 8. M.p. 137 °C;
1H NMR (400 MHz, C6D6, 300 K): δ = 7.70 (overlap, dd, 4JH,P = 2.1 Hz, 4JH,H
= 2.1 Hz, 1H; m-H Mes* PB), 7.70 (overlap, dd, 4JH,P = 3.3 Hz, 4JH,H = 2.1 Hz,
Reaction of 9a with hydrogen chloride; synthesis of 10.[2,7a] A solution of
3H-3-tert-butylphosphaallen 3a (0.10 g, 0.28 mmol) in 5 mL of n-hexane was
treated at -78 °C with a solution of tert-butyllithium (147 µL, 1.9 M, 0.28
mmol) in n-pentane. The mixture was warmed to room temperature overnight.
After cooling to -78 °C, a solution of HCl in diethyl ether (280 µL, 1 M, 0.28
mmol) was added. The mixture was warmed to room temperature. All volatiles
were removed in vacuum. The solid residue was treated with 10 mL of n-pen-
tane. After filtration and removal of all volatiles from the filtrate compound
102 was obtained as a colorless oil (0.092 g, 92%). 1H NMR (400 MHz, C6D6,
300 K): δ = 7.60 (d, 3JH,P = 2.5 Hz, 2H; m-H), 5.94 (d, 1JH,P = 245.0 Hz, 1H;
P-H), 1.75 (s, 18H; o-CMe3), 1.23 (s, 9H; p-CMe3), 1.02 ppm (s, 9H; C≡C-
CMe3); 31P{1H} NMR (162 MHz, C6D6, 300 K): δ = -102.1 ppm.
4
4
1H; m-H Mes* PC2), 7.64 (dd, JH,P = 4.6 Hz, JH,H = 2.1 Hz, 1H; m-H PC2),
7.49 (dd, 4JH,P = 4.6 Hz, 4JH,H = 2.1 Hz, 1H; m-H Mes* PB), 6.08 (dd, 3JH,P
=
50.4 Hz, 3JH,P = 12.6 Hz, 1H; C=CH), 2.85 (dd, 2JH,P = 25.4 Hz, 3JH,H = 5.3 Hz,
1H; P-CH-B), 2.08 (ddd, 3JH,P = 24.1 Hz, 2JH,H = 14.8 Hz, 3JH,H = 5.3 Hz, 1H;
CH2), 1.92 (s, 9H; o-CMe3 Mes* PC2), 1.91 (s, 9H; o-CMe3 Mes* PB), 1.89
(overlap, 1H; CH2), 1.80 (s, 9H; o-CMe3 Mes* PC2), 1.53 (s, 9H; o-CMe3
Mes* PB), 1.31 (s, 9H; p-CMe3 Mes* PC2), 1.18 (s, 9H; p-CMe3 Mes* PB),
0.93 (s, 9H; C=C-CMe3), 0.45 ppm (s, 9H; CH2-CMe3); 13C{1H} NMR (100
MHz, C6D6, 300 K): δ = 161.8 (dd, 2JC,P = 35.8 Hz, 4JC,P = 2.3 Hz; o-C Mes*
Reaction of 9a with chlortrimethylsilane, synthesis of 11. A solution of 3H-
3-tert-butylphosphaallene 3a (0.39 g, 1.09 mmol) in 10 mL of n-hexane was
cooled to -78 °C and treated with a solution of tert-butyllithium in n-pentane
(570 µL, 1.9 M, 1.09 mmol). The mixture was warmed to room temperature
overnight. Chloro-trimethylsilane (0.12 g, 137 µL, 1.09 mmol) was added to
the cooled (-78 °C) solution. Warming to room temperature, filtration and eva-
2
2
PC2), 158.0 (d, JC,P = 2.8 Hz; o-C Mes* PB), 157.6 (d, JC,P = 6.2 Hz; o-C
Mes* PC2), 156.2 (dd, 2JC,P = 10.2 Hz, 4JC,P = 2.5 Hz; o-C Mes* PB), 153.3 (d,
4JC,P = 3.4 Hz; p-C Mes* PB), 151.3 (d, 4JC,P = 2.0 Hz; p-C Mes* PC2), 142.2
2
1
(dd, JC,P = 15.1 and 9.4 Hz; C=CH), 136.2 (dd, JC,P = 53.8 and 27.8 Hz;
C=CH), 131.2 (dd, 1JC,P = 60.0 Hz, 3JC,P = 5.8 Hz; i-C Mes* PC2), 127.3 (m-C
3
3
Mes* PC2), 126.0 (d, JC,P = 10.8 Hz; m-C Mes* PB), 124.6 (d, JC,P = 13.0
Hz; m-C Mes* PB), 123.1 (d, 3JC,P = 13.4 Hz; m-C Mes* PC2), 120.8 (d, 1JC,P
= 39.9 Hz; i-C Mes* PB), 116 (s. br.; i-C C6F5), 50.2 (dd, 2JC,P = 9.9 Hz, 3JC,P
1
poration afforded a yellowish oil of 11 (0.33 g, 70%). H NMR (400 MHz,
4
C6D6, 300 K): δ = 7.49 (d, JH,P = 2.7 Hz, 2H; m-H), 1.76 (s, 18H; o-CMe3),
1.30 (s, 9H; p-CMe3), 1.22 (s, 9H; C≡C-tBu), 0.06 ppm (d, 2JH,P = 5.5 Hz, 9H;
TMS); 13C{1H} NMR (100 MHz, C6D6, 300 K): δ = 157.1 (d, 2JC,P = 12.8 Hz;
o-C Mes*), 149.2 (d, 4JC,P = 2.7 Hz; p-C Mes*), 129.7 (d, 1JC,P = 39.2 Hz; i-C
3
= 39.7 Hz, CH2), 41.1 (o-CMe3 Mes* PC2), 40.6 (d, JC,P = 2.0 Hz; o-CMe3
Mes* PB), 40.1 (d, 3JC,P = 9.6 Hz; o-CMe3 Mes* PC2), 39.4 (d, 3JC,P = 3.1 Hz,
3
o-CMe3 Mes* PB), 37.6 (dd, JC,P = 6.0 and 4.2 Hz, C=CH-CMe3), 35.4
3
2
Mes*), 122.1 (d, JC,P = 8.5 Hz; m-C Mes*), 118.0 (d, JC,P = 4.4 Hz; C≡C-
tBu), 76.8 (d, 1JC,P = 23.5 Hz; C≡C-tBu), 39.0 (d, 3JC,P = 4.5 Hz; o-CMe3), 34.9
(d, 5JC,P = 0.9 Hz; p-CMe3), 34.4 (d, 4JC,P = 7.4 Hz; o-CMe3), 31.4 (p-CMe3),
31.0 (d, 4JC,P = 1.7 Hz; C≡C-CMe3), 29.4 (d, 3JC,P = 1.6 Hz; C≡C-CMe3), -0.1
(o-CMe3 Mes* PC2), 35.1 (d, 5JC,P = 1.0 Hz; p-CMe3 Mes* PB), 34.9 (overlap;
p-CMe3 Mes* PC2), 34.9 (overlap, d, 4JC,P = 13.2 Hz; o-CMe3 Mes* PC2), 34.6
(overlap; o-CMe3 Mes* PB), 34.5 (overlap; o-CMe3 Mes* PB), 32.0 (dd, 3JC,P
= 3.4 Hz, 3JC,P = 1.3 Hz; CH2-CMe3), 31.2 (p-CMe3 Mes* PC2), 31.0 (p-CMe3
Mes* PB), 30.8 (br., overlap; P-CH-B), 30.3 (C=CH-CMe3), 29.0 ppm
(CH2-CMe3); with the exception of i-C the resonances of the C6F5 group are
poorly resolved and could not be identified and assigned unambiguously;
11B{1H} NMR (128 MHz, C6D6, 300 K): δ = 53.2 ppm; 19F{1H} NMR (376
MHz, C6D6, 300 K): δ = -127.2 (dd, JF,F = 23 and 10 Hz, 1F; o-F), -128.9 (dd,
JF,F = 24 and 8 Hz, 1F; o-F), -154.9 (td, JF,F = 20 and 4 Hz, 1F; p-F), -162.1
(ddd, JF,F = 25, 20 and 9 Hz, 1F; m-F), -163.7 ppm (ddd, JF,F = 24, 20 and 9
Hz, 1F; m-F); 31P{1H} NMR (162 MHz, C6D6, 300 K): δ = 30.3 (br.; B-P),
25.3 ppm (d, 2JP,P = 89.2 Hz; PC2); IR (KBr): 휈̃ = 2957 (vs), 2905 (vs), 2868
(vs) ν(CH); 1643 (m), 1597 (s), 1514 (vs) ν(C=C), aryl; 1474 (vs), 1393 (vs),
1362 (vs), 1298 (m), 1279 (vw), 1234 (s) δ(CH3); 1211 (s), 1194 (m), 1136 (s),
1123 (s), 1061 (w), 1013 (m), 974 (vs), 901 (vw), 880 (m), 820 (w), 770 (m),
741 (s), 704 (vw) ν(CC), ν(CF), ν(CB); 665 (w), 600 (w) aryl; 581 (vw), 525
(vw), 503 (vw), 484 (vw), 447 (vw) cm–1 ν(PC), ν(BP), δ(CC); MS (EI+, 20
eV, 353 K): m/z (%) = 897 (25) [M]+, 651 (100) [M–Mes*–H]+; elemental
analysis calcd (%) for C54H80P2BF5 + CH2Cl2: C 67.3, H 8.4; found: C 67.6, H
8.5.
2
ppm (d, JC,P = 15.4 Hz; TMS); 29Si{1H} NMR (79 MHz, C6D6, 300 K): δ =
1
8.5 ppm (d, JP,Si = 15.2 Hz); 31P{1H} NMR (162 MHz, C6D6, 300 K): δ = -
109.8 ppm; MS (EI+, 20 eV, 303 K): m/z (%) = 430 (77) [M]+, 357 (32) [M–
TMS]+.
Reaction of 9a with di-tert-butylchlorophosphine; synthesis of 12. A solu-
tion of 3H-3-tert-butylphosphaallen 3a (0.39 g, 1.09 mmol) in 10 mL of n-
hexane was cooled to -78 °C and treated with a solution of tert-butyllithium
in n-pentane (570 µL, 1.9 M, 1.09 mmol. The mixture was warmed to room
temperature overnight. After cooling to -78 °C, di-tert-butylchlorophosphine
(0.20 g, 205 µL, 1.11 mmol) was added. Warming to room temperature, filt-
ration, removal of the solvents in vacuum and recrystallization of the residue
from a saturated solution in prentafluorobenzene afforded colorless crystals of
12 at -30 °C 0.48 g, 88%). M.p. 137 °C; 1H NMR (400 MHz, C6D6, 300 K): δ
3
= 7.50 (s br., 2H; m-H), 1.90 (s br., 18H; o-CMe3), 1.53 (d, JH,P = 10.5 Hz,
9H; P-CMe3), 1.28 (s, 9H; p-CMe3), 1.23 (s, 9H; C≡C-tBu), 0.83 ppm (d, 3JH,P
= 10.3 Hz, 9H; P-CMe3); 13C{1H} NMR (100 MHz, C6D6, 300 K): δ = 159.6
4
1
(br.; o-C Mes*), 150.5 (d, JC,P = 2.5 Hz; p-C Mes*), 128.8 (dd, JC,P = 48.5
Hz, 2JC,P = 13.3 Hz; i-C Mes*), 125.9 (br.; m-C Mes*), 122.9 (br.; m-C Mes*),
116.5 (d, JC,P = 2.7 Hz; C≡C-tBu), 81.4 (dd, JC,P = 29.1 Hz, JC,P = 6.5 Hz;
Deprotonation of 3H-3-tert-butylphosphaallene 3b, synthesis of 9. A solu-
tion of 3H-3-tert-butylphosphaallene 3a (0.10 g, 0.28 mmol) in 5 mL of n-
hexane was cooled to -78 °C and treated with a solution of tert-butyllithium
(147 µL, 1.9 M, 0.28 mmol) in n-pentane. The mixture was warmed to room
2
1
2
1
2
C≡C-tBu), 41.9 and 40.1 (br.; o-CMe3), 35.3 (dd, JC,P = 35.1 Hz, JC,P = 4.7
Hz; P-CMe3), 35.1 (br.; o-CMe3), 34.7 (d, 5JC,P = 0.9 Hz; p-CMe3), 34.3 (br.;
7
This article is protected by copyright. All rights reserved.

10.1002/chem.202002506
Chemistry - A European Journal
3
1
3
o-CMe3), 33.6 (dd, 1JC,P = 33.8 Hz, 2JC,P = 22.8 Hz; P-CMe3), 31.5 (dd, 2JC,P
=
27.0 Hz, JP,P = 14.8 Hz; P3), 17.7 (dd, JP,P = 138.2 Hz, JP,P = 14.8 Hz;
1
2
4.1 Hz, 3JC,P = 13.0 Hz; P-CMe3),31.3 (p-CMe3), 30.5 (d, 4JC,P = 1.8 Hz; C≡C-
P1), -44.7 ppm (dd, JP,P = 138.2 Hz, JP,P = 27.0 Hz; P2). Less abundant
CMe3), 30.0 (dd, 2JC,P = 5.6 Hz, 3JC,P = 13.8 Hz; P-CMe3), 29.2 ppm (d, 3JC,P
=
diastereomer: 31P{1H} NMR (162 MHz, C6D6, 300 K): δ = 65.3 (dd, 2JP,P
=
3
1
3
1.6 Hz; C≡C-CMe3); 31P{1H} NMR (162 MHz, C6D6, 300 K): δ = 42.5 (d, 1JP,P
26.8 Hz, JP,P = 26.6 Hz; P3), 16.2 (dd, JP,P = 163.6 Hz, JP,P = 26.6 Hz;
P1), -25.3 ppm (dd, 1JP,P = 163.6 Hz, 2JP,P = 26.8 Hz; P2). IR (KBr): 2961 (w),
2930 (vw), 2903 (vw), 2866 (vw) ν(CH); 2068 (s), 2016 (w), 1977 (m), 1925
(vs), 1869 (s) ν(CO); 1634 (vw), 1589 (vw), 1557 (vw) ν(C=C), aryl; 1474
(vw), 1460 (vw), 1435 (vw), 1362 (vw), 1250 (vw) δ(CH); 1196 (vw), 1090
(vw), 941 (vw), 891 (vw), 872 (vw), 781 (vw), 758 (vw), 745 (w), 725 (vw)
ν(CC); 694 (vw), 644 (vw), 627 (vw) aryl; 596 (m), 577 (m), 538 (w), 482
(vw) cm–1 ν(PC), δ(CC); MS (EI+, 20 eV, 453 K): m/z (%) = 866 (71) [M]+,
838 (100) [M–CO]+, 810 (45) [M–2CO]+, 782 (72) [M–3CO]+, 754 (88) [M–
4CO]+; HRMS (ESI-ORBITRAP): m/z: calcd for C40H46O4P3W+H: 867.2120;
found 867.2008; elemental analysis gave the correct hydrogen content, the
carbon content was always found too low.
1
= 190.5 Hz; PtBu2), -65.0 ppm [d, JP,P = 190.5 Hz, P(Mes*)(ethynyl)]; IR
(KBr): 2970 (vs), 2949 (vs), 2899 (vs), 2860 (vs), 2706 (vw) ν(CH); 2149 (w)
ν(C≡C); 1707 (w), 1690 (vw), 1653 (vw), 1589 (s), 1530 (m), 1514 (w)
ν(C=C), aryl; 1476 (vs), 1460 (s), 1391 (s), 1362 (vs), 1310 (w), 1254 (vs),
1238 (sh) δ(CH3); 1198 (m), 1175 (s), 1123 (m), 1070 (vw), 1018 (w), 984
(m), 939 (w), 926 (w), 895 (w), 872 (m), 806 (w), 770 (m), 752 (w), 716 (vw)
ν(CC); 648 (w), 596 (m) aryl; 561 (w), 519 (w), 471 (vw), 453 (vw) cm–1 ν(PC),
δ(CC); MS (EI+, 20 eV, 313 K): m/z (%) = 502 (18) [M]+, 445 (16) [M–tBu]+,
389 (61) [M–tBu–butene]+, 357 (69) [M–PtBu2]+; elemental analysis calcd for
C32H56P2: C 76.4, H 11.2; found: C 76.0; H 10.8.
Trimer of phenyl-3H-3-tert-butylphosphaallene (14). Phenyldi(tert-
butylethinyl)phosphine 1a (0.64 g, 2.37 mmol) was dissolved in 15 mL of n-
hexane and treated with a solution of H-Al[CH(SiMe3)2]2 (0.82 g, 2.37 mmol)
in 45 mL of n-hexane. The mixture was stirred for 4 d at room temperature.
Several compounds were detected in its 31P NMR spectrum: the phosphaallene
trapping product 13[15] (δ = -10.3 and -95.0 ppm), the intermediate FLP 2 [R =
Ph, R’ = tBu, R’’ = CH(SiMe3)2] shown in Scheme 1 (δ = -43.3 ppm)[15a] and
the starting dialkynylphosphine 1a (δ = -60.8 ppm).[39] A signal at δ = -59.6
ppm could not be assigned. The trimer 14 of the phosphaallene 3b was identi-
fied by the characteristic coupling pattern in the 31P NMR spectrum: 31P{1H}
NMR (162 MHz, C6D6, 300 K): δ = -17.6 (JP,P = 12.6 and 7.8 Hz; P3), -25.1
(1JP,P = 168.1 Hz, JP,P = 7.8 Hz), -43.3 ppm (1JP,P = 168.1 Hz, JP,P = 12.6 Hz).
X-Ray crystallography. Crystals suitable for X-ray crystallography were ob-
tained by crystallisation from 1,2-difluorobenzene (3 °C; 7), dichloromethane
(3 °C; 8), the reaction mixture (3 °C; 9b), pentafluorobenzene (-30 °C; 12) or
n-hexane (-45 °C; 15). Intensity data was collected on Bruker D8 Venture dif-
fractometer with monochromated Mo K radiation. The collection method in-
volved -scans. Data reduction was carried out using the program SAINT+.[42]
The crystal structures were solved by direct methods using SHELXTL.[43] Non-
hydrogen atoms were first refined isotropically followed by anisotropic refine-
ment by full matrix least-squares calculation based on F2 using SHELXTL.[43]
H atoms were positioned geometrically and allowed to ride on their respective
parent atoms. A tBu group of 7, 9b and 15 and two tBu groups of 8 were
disordered; the respective atoms were refined on split positions. The crystals
of 8 enclosed a molecule of CH2Cl2 per formula unit, which was disordered
over three positions (0.22:0.49:0.29). 12 crystallized with two independent
moleculecules in the unit cell. CCDC–2001342 (7), -2001344 (8), -2001343
(9b), -2001341 (12) and -2001340 (15) contain the supplementary crystallo-
graphic data for this paper. These data are provided free of charge by the Cam-
bridge Crystallographic Data Centre.
W(CO)4 complex (15) of the phosphaallene trimer. A solution of impure 14
in n-hexane was synthesized as described in the previous section. It was trea-
ted with a solution W(CO)5(THF) which was in-situ generated by irradiation
of W(CO)6 (1.11 g, 3.15 mmol) in 40 mL of THF. The mixture was stirred for
2 h at room temperature. All volatiles were removed in vacuum, and the crude
product was purified by column chromatography (SiO2, eluent: n-hexane/E-
tOAc). The water content of the usually employed silica gel is sufficient to
completely destroy the organoaluminum by-products by hydrolysis. A yellow
oil of compound 15 remained after removal of the solvents. Pure 15 was ob-
tained as yellow crystals by crystallization from a saturated solution in n-he-
xane at -45 °C (0.35 g, 17%). M.p. 172 °C. Three very similar sets of reso-
nances were observed in the NMR spectra, which probably result from dias-
tereomers of 15. The ratio of the isomers is 7:2:1. Only the NMR spectra of
the main isomer could be assigned completely, the remaining two isomers
were identified by their characteristic 31P{1H} NMR data. Assignment is based
on the numbering scheme of the atoms in the molecular structure (Figure 5):
Phenyl group at P1: C41 to C46; phenyl group at P3: C91 to C96, hindered
rotation results in six 1H and 13C{1H} NMR signals; bridging aryl group: C51
to C56 with C51 and C52 in the P2C4 ring; vinyl group at P1: C1-C2-C3-
C31/C32/C33; vinyl group bridging P2 and P3: C11 to C133; vinyl group
Acknowledgements
We are grateful to Prof. Herbert Mayr (LMU München) for very helpful dis-
cussions.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Phosphaallenes • hydroboration • frustrated Lewis pairs • lithium
phosphides • trimerisation
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1
bridging P3 and C52: C6 to C83. Main diastereomer: H NMR (400 MHz,
C6D6, 300 K): δ = 8.65 (dd, 3JH,P = 12.3 Hz, 3JH,H = 7.4 Hz, 1H; C92-H), 8.33
(dd, 3JH,P = 11.4 Hz, 3JH,H = 8.2 Hz, 2H; C42/C46-H), 7.71 (ddd, 3JH,P = 16.0
3
4
3
Hz, JH,H = 7.3 Hz, JH,H = 1.6 Hz, 1H; C56-H), 7.56 (d, JH,H = 7.3 Hz, 1H;
3
3
C53-H), 7.48 (dd, JH,P = 8.0 Hz, JH,H = 6.8 Hz, 1H; C96-H), 7.21 (overlap,
1H; C93-H), 7.20 (overlap, 3H; C43/C45-H and C94-H), 7.19 (overlap, 1H;
C54-H), 7.11 (ddd, 4JH,P = 1.4 Hz, 5JH,P = 1.4 Hz, 3JH,H = 7.3 Hz, 1H; C55-H),
3
7.06 (overlap, 2H; C44-H and C95-H), 6.51 (d, JH,P = 32.1 Hz, 1H; C7-H),
5.55 (overlap, dd, 3JH,P = 30.9 Hz, 4JH,P = 4.3 Hz, 1H; C2-H), 5.53 (overlap, dd,
2JH,P = 20 Hz, 3JH,P = 15 Hz, 1H; C1-H), 5.48 (dd, 3JH,P = 30.0 Hz, 3JH,P = 18.0
Hz, 1H; C12-H), 1.17 (s, 9H; C8-tBu), 0.88 (s, 9H; C3-tBu), 0.72 ppm (s, 9H;
C13-tBu); 13C{1H} NMR (100 MHz, C6D6, 300 K): δ = 209.8 (dd, 2JC,P = 26.3
Hz, 2JC,P = 5.6 Hz; CO), 207.0 (dd, 2JC,P = 27.8 Hz, 2JC,P = 6.6 Hz; CO), 203.6
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(dd, 2JC,P = 6.8 Hz, JC,P = 3.5 Hz; CO), 202.4 (dd, JC,P = 8.9 Hz, JC,P = 7.2
2
2
2
2
2
2
Hz; CO), 158.7 (d, JC,P = 11.8 Hz; C7), 158.2 (ddd, JC,P = 22.3 Hz, JC,P
=
3
2
3
10.3 Hz, JC,P = 3.5 Hz; C12), 157.1 (dd, JC,P = 2.5 Hz, JC,P = 1.8 Hz; C2),
148.2 (ddd, 2JC,P = 4.2 Hz, 2JC,P = 1.5 Hz, 3JC,P = 1.5 Hz; C52), 138.9 (dd, JC,P
= 59.0 and 4.5 Hz; C56), 138.8 (ddd, 1JC,P = 30.7 Hz, 2JC,P = 23.7 Hz, JC,P
=
4
3.9 Hz; C41), 138.3 (overlap, dd, 1JC,P = 14 Hz, 1JC,P = 12 Hz; C11), 137.0 (d,
2JC,P = 22.3 Hz; C92), 136.1 (ddd, JC,P = 29.5 Hz, JC,P = 8.0 Hz, JC,P = 1.5
1
3
4
Hz; C91), 135.7 (dd, 2JC,P = 7.3 Hz, 3JC,P = 12.8 Hz; C42), 133.3 (ddd, 1JC,P
=
27.7 Hz, 2JC,P = 4.0 Hz, 2JC,P = 4.0 Hz; C51), 132.5 (C96), 131.8 (d, 1JC,P = 24.6
Hz; C6), 131.4 (overlap; C54 and C94), 131.2 (d, 3JC,P = 2.3 Hz; C95), 130.5
(d, 4JC,P = 2.4 Hz; C44), 129.5 (d, 3JC,P = 13.1 Hz; C93), 129.2 (C53), 128.8 (d,
3JC,P = 10.4 Hz; C43), 127.1 (d, 3JC,P = 18.0 Hz; C55), 116.9 (dd, 1JC,P = 28.0
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2
3
3
Hz, JC,P = 6.5 Hz; C1), 37.4 (d, JC,P = 1.7 Hz; C8), 35.7 (d, JC,P = 2.5 Hz;
C3), 35.3 (dd, 3JC,P = 9.6 Hz, 3JC,P = 1.6 Hz, 3JC,P = 1.3 Hz; C13), 31.6 (dd, 4JC,P
= 9.5 Hz, 4JC,P = 0.9 Hz; C131-C133), 31.1 (C81-C83), 29.9 ppm (C31-C33);
31P{1H} NMR (162 MHz, C6D6, 300 K): δ = 52.6 (dd, 2JP,P = 26.9 Hz, 3JP,P
=
20.9 Hz, 1JP,W = 241.3 Hz; P3), 12.1 (dd, 1JP,P = 161.8 Hz, 3JP,P = 20.9 Hz, 1JP,W
= 223.1 Hz; P1), -35.1 ppm (dd, 1JP,P = 161.8 Hz, 2JP,P = 26.9 Hz; P2). Second
diastereomer: 31P{1H} NMR (162 MHz, C6D6, 300 K): δ = 51.4 (dd, JP,P
=
2
8
This article is protected by copyright. All rights reserved.

10.1002/chem.202002506
Chemistry - A European Journal
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Chemistry - A European Journal
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10.1002/chem.202002506
Chemistry - A European Journal
Entry for the Table of Contents
Phosphaallene Reactivity
J. C. Tendyck, A. Hepp, E.-U.
Würthwein*, W. Uhl*
Page – Page
Renewed interest: 3H-Phos-
a BP2C2 heterocycle and deprotono-
phaallenes are highly functional
compounds with a fascinating per-
spective in various chemical trans-
formations. Hydroboration of the
C=C bond afforded a BP2 FLP or
ation yielded a reactive Li phos-
phide. A new secondary product was
obtained by trimerisation of a steri-
cally low shielded phosphaallene via
P-P, P-C and C-C bond formation.
New reactivity patterns in 3H-
phosphaallene chemistry [Aryl-
P=C=C(H)-tBu] – hydroboration
of the C=C bond, deprotonation
and trimerisation
Keywords: Phosphaallenes • hydroboration • frustrated Lewis pairs •lithium phosphides • trimerisation
11
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