A Novel Reactivity of Phosphanylalumane (>P?Ala Saturated Interelement Bond to Olefins

Article abstract of DOI:10.1002/chem.202101649

The reversible addition of olefins to a phosphanylalumane, P?Al single-bond species, was investigated. The P?Al bond added to ethylene and relatively small terminal alkenes (propylene and hex-1-ene) at room temperature to give the corresponding alkene adducts. Heating the terminal alkene adducts released the corresponding alkenes and regenerated the P?Al bond, but no release of ethylene was observed even under vacuum conditions. The reactivity of ethylene adduct as a new saturated C₂ vicinal P/Al-based FLP was also investigated. The ethylene adduct was found to undergo complexation with nitriles to give the corresponding nitrile adducts to the Al center, which retained the ethylene tether as in the case of the corresponding P/B-based FLP. However, the reactivity of ethylene toward CO₂ and benzaldehyde differed from that of the P/B system giving the corresponding adducts.

Full text of DOI:10.1002/chem.202101649

Accepted Article
Accepted Article
Title: A Novel Reactivity of Phosphanylalumane (>P–Al<): Reversible
Addition of a Saturated Interelement Bond to Olefins
Authors: Tatsuya Yanagisawa, Yoshiyuki Mizuhata, and Norihiro
Tokitoh
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To be cited as: Chem. Eur. J. 10.1002/chem.202101649
Link to VoR: https://doi.org/10.1002/chem.202101649
01/2020

10.1002/chem.202101649
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A Novel Reactivity of Phosphanylalumane (>P–Al<): Reversible
Addition of a Saturated Interelement Bond to Olefins
Tatsuya Yanagisawa,[a] Yoshiyuki Mizuhata*[a, b] and Norihiro Tokitoh*[a, b]
[a]
Dr. T. Yanagisawa, Prof. Dr. Y. Mizuhata, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
E-mail: mizu@boc.kuicr.kyoto-u.ac.jp
[b]
Prof. Dr. Y. Mizuhata, Prof. Dr. N. Tokitoh
Integrated Research Consortium on Chemical Sciences
Uji, Kyoto, (Japan)
Supporting information for this article is given via a link at the end of the document.
Abstract: The reversible addition reactions of olefins with a
phosphanylalumane, P–Al single bond species, were investigated.
The P–Al bond added to ethylene and relatively small terminal
alkenes (propylene and hex-1-ene) at room temperature to give the
corresponding alkene-adducts. Heating of the terminal alkene-
adducts released the corresponding alkenes and regenerated the P–
Al bond, but no release of ethylene was observed even under
vacuum conditions. The reactivity of ethylene-adduct as a new
saturated C2-vicinal P/Al-based FLP was also investigated. The
ethylene-adduct was found to undergo complexation with nitriles to
give the corresponding nitrile adducts to the Al center, which retained
the ethylene-tether as in the case of the corresponding P/B-based
FLP. However, the reactivity of ethylene-adduct toward CO2 and
benzaldehyde differed from that of the P/B-system giving the
corresponding adducts.
irreversible 1,2-addition reaction of >P–B< bond to a terminal
allene by rhodium catalyst is known.[13]
On the other hand, phosphanylboranes, >P–B< bond
compounds, are known to react with various small molecules,
including H2, without catalyst due to their weak P→B π
interactions.[13,14] Based on this background, we made a working
hypothesis that >P–Al< single-bond compounds (phosphanyl-
alumanes) having a long σ-bond between the third-row elements
would show decrease of the π-interaction, acquire effective
interaction with olefins, and are suitable for the investigation of
the reversible behavior with olefins (Scheme 1b). Thus, further
exploration of the reactivity of phosphanylalumanes can develop
not only transition metal-mimicking reactivity of main group
compounds but also unprecedented small molecule activation by
saturated interelement species.
Reversible behavior (oxidative addition and reductive
elimination) of main group element compounds toward small
molecules has been emphasized from the viewpoint of
alternatives to transition metal complexes.[1] Reversible
reactions of various main group element compounds with small
molecules have been reported so far.[2] In the case of olefins, a
variety of main group element compounds, such as a distannyne
(–SnSn–),[3] digermynes (–GeGe–),[4] digallenes (–
Ga=Ga–),[5] silylenes (>Si:),[6] germylenes (>Ge:),[7] Mg(I)
reagents,[8] and a diazadiborinine,[9] have been known to
undergo reversible reactions. Most recently, it has also been
found in Al(I) species (Scheme 1a).[10] In addition, reversible
additions of B/P-FLP to alkene or alkyne are known.[11] Thus, the
reversible addition/elimination behavior with olefins has currently
been limited to low-coordinated species and FLPs. Their
electron-deficient moiety (the highly reactive π* or vacant p
orbitals) plays a major role on the interaction with olefins.
Especially, the orbital interaction between dimetallynes and
ethylene is like the transition metal-ethylene interaction
interpreted with the Dewar-Chatt-Duncanson model.[12] Taking
into account of the synergistic orbital interactions with olefins
mentioned above, a direct bond between Lewis acid (LA) and
Lewis base (LB) can work similarly (Figure 1b). However, it is
easy to imagine that electronic perturbations between LA and LB
can inactivate each function. Actually, regarding the addition
reaction to olefin, there is no report on >N–B< bond, and only
Scheme 1. (a) Examples of reversible addition reactions of ethylene with
distannyne and Al(I) compounds. (b) Postulated interaction of LA–LB bond with
ethylene.
Furthermore, such alkene conversions are important in
forming C–E (E = main group element) bond(s), which will
introduce LA (Al) and LB (P) onto the resulting alkanes in the
case of phosphanylalumanes. The LA/LB-substituted alkanes
will be of worth as new reactive compounds like frustrated Lewis
pairs (FLPs). For example, Erker and Stephan et al. obtained
ethylene-bridged P/B-based FLP A by the hydroboration reaction
of dimesitylvinylphosphane (Mes2PCH=CH2) with HB(C6F5)2
(Scheme 2).[15]
1
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Scheme 4. Reaction of 1 with ethylene.
Scheme 2. Synthesis of P/B-based FLP A by Erker and Stephan et al.
Unfortunately, as with the P/B system A, no crystals suitable
for X-ray crystallography have been obtained for 2H. Therefore,
the structural optimization for 2H was performed at the B3LYP-
D3/6-31G(d) level of theory. The 31P NMR chemical shift of the
optimized structure was in good agreement with the observed
one (Figure S34, Table S3), suggesting that the DFT-optimized
structure (gas phase) is well matched to that in solution. The
optimized P···Al distance (2.663 Å) was much longer compared
to that of 1 (2.348 Å) and the sum of the covalent bond radii (rAl
+ rP = 2.37 Å),[18] and the natural bond orbital corresponding to
the P–Al σ-bond was not found (Table S4). Therefore, the P–Al
σ-bond of 1 seems to be weakened by addition to ethylene.
Recently, we have succeeded in the synthesis and isolation
of novel phosphanylalumane 1 with carbon-based protecting
groups on the P and Al atoms.[16] With this new class of stale E–
E’ bond compounds, phosphanylalumanes, new research for the
addition reactions to olefins and the synthesis of reactive species,
possibly acting as the saturated P/Al-based intramolecular FLPs,
should be opened for discussion. Here, we found the reversible
addition reaction to alkenes by the P–Al σ-bond species for the
first time (Scheme 3). In other words, the P–Al σ-bond of 1, an
P(III)–Al(III) compound, was weakened by the addition to
terminal alkenes (RCH=CH2) in the case of the relatively small
substituents (R = H, Me, and n-Bu) to afford alkene-adducts 2R,
and the P–Al bond of 1 is regenerated by the elimination of the
alkenes on heating in the case of R = Me (2Me) and n-Bu (2Bu).
To ensure the generation of ethylene-adducts 2H, a variety of
reactions were also examined.
Scheme 3. Reversible addition reactions of 1 with alkenes described in this
work.
Figure 1. Scope of alkenes. [a] Estimated by 1H and 31P{1H} NMR
measurement of the reaction mixture (in %). [b] Isolated yield.
A degassed solution of 1 in C6D6 was instantly exposed to an
excess amount of ethylene gas (ca. 1 atm) at –196 °C. After
warming to room temperature, the pale-yellow solution of 1
Next, the scope of alkenes was examined, and the results
are summarized in Figure 1. The reaction of 1 with propylene
was not completed within 5 min even with the exposure to an
excess amount of propylene gas (ca. 1 atm), unlike ethylene
case. NMR spectra after 24 h at room temperature showed the
formation of propylene-adduct 2Me (65%) and the partial
decomposition (probably hydrolysis) of 1, giving Mes2PH (30%).
Therefore, 2Me could not be isolated by any means. In the case
of hex-1-ene, the reaction was slow even in the presence of an
excess amount of hex-1-ene. The ratio of 1 to hex-1-ene-adduct
2Bu reached a static state (about 4:1) after leaving it for several
days. Further investigation revealed that the static state was
attained 21 hours after the treatment of 1 with two equivalents of
hex-1-ene at room temperature. This result suggests the
reversibility of the reactions at room temperature (Table S1,
Figure S1). When more amount of the alkene was used (e.g.,
neat condition of hex-1-ene), only the decomposition giving
Mes2PH was promoted. Terminal alkenes having a relatively
large substituent and internal alkenes were found to be
unreactive.
1
turned into colorless within 5 minutes (Scheme 4). The H and
13C{1H} NMR and their two-dimensional NMR spectroscopic
studies suggested the formation of the corresponding cis-1,2-
adduct 2H. This compound is the first example of a saturated C2-
bridged P/Al analogue of P/B system A.[15]
In the 1H NMR spectrum, the quartet signal corresponding to
the P-bound CH2 moiety of 2H appeared at 2.78 ppm (q, 2JPH
=
3
8 Hz and JHH = 8 Hz), which is comparable to that of the P/B
system A [2.87 ppm (q, 2JPH = 8 Hz and 3JHH = 8 Hz)].[15] On the
other hand, the double-triplet signal corresponding to the Al-
bound CH2 moiety of 2H appeared at 0.99 ppm (dt, 3JPH = 36 Hz
and 2JHH = 16 Hz), which is considerably up-field shifted than that
of A [2.29 ppm (dt, 3JPH = 42 Hz and 2JHH = 8 Hz)], reflecting the
lower electronegativity of Al than B (Al: 1.47 < B: 2.01).[17] In
addition, the 31P{1H} NMR signal of 2H (0.4 ppm) appeared in
higher magnetic field than that of A (20.6 ppm). This result
indicated that the interaction between the P and Al atoms in 2H
was weaker than that between the P and B atoms in A due to the
elongation of bonds containing the Al atom in 2H.
2
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The position of methyl substituent of 2Me was determined by
the measurement of 1H, 1H{31P}, and 1H-1H COSY NMR spectra
(Figures S10-S12). The relatively large coupling (16.8 Hz) was
observed between the methyl protons and the P atom, strongly
suggesting that the methyl group should be placed on the C atom
intermolecular FLPs.[20] Ethylene-adduct 2H was found to be
extremely more stable than other thermodynamically
unfavourable alkene-adducts (2Me and 2Bu) (2H: –72.1 kcal
mol–1 < 2Me: 2.2 kcal mol–1 and 2Bu: 2.9 kcal mol–1).
Correspondingly, the barrier for alkene elimination (2R→TS4)
was the largest for the case of 2H (2H: 23.2 kcal mol–1 > 2Me:
20.1 kcal mol–1 and 2Bu: 19.9 kcal mol–1). These results well
explained the experimental results that the alkene was
eliminated on heating in the cases of 2Me and 2Bu.
2
adjacent to the P atom (3JHP). In this model, the JHP of the
3
methine proton (<8 Hz) was smaller than the JHP of the
methylene proton (61 Hz), showing the tendency same as 2H
and A. Although the low occurrence of 2Bu relative to the 1 and
hex-1-ene prevented complete assignment of the protons, the
31P NMR chemical shift (15.9 ppm) and the 3JPH value (63 Hz) of
2Bu were quite similar to those of 2Me (16.4 ppm and 61 Hz,
respectively), suggesting that hex-1ene added to 1 in the
direction same as 2Me. Therefore, the alkyl-substituted C atom
of terminal alkenes selectivity connected to the P atom of 1, due
to its small electron density.
Since crystals suitable for X-ray crystallographic analysis
have not been obtained for ethylene-adduct 2H at this stage,
capturing of 2H by further reactions was attempted. In the P/B
system A, pivalonitrile is known to act as a LB to give nitrile-
adduct.[21] Similarly, the reaction of P/Al system 2H with
pivalonitrile gave the corresponding nitrile-adduct 5Bu as judged
by NMR spectroscopy (Scheme 6a). However, 5Bu was an oily
product, and hence it was difficult to isolate and obtain single
crystals suitable for X-ray crystallographic analysis. The use of
benzonitrile instead of pivalonitrile afforded 5Ph as a crystalline
compound, and the structural analysis revealed the retainment
of the ethylene bridge (Scheme 6b).[22] This result clearly
supports the structure of 2H.
No elimination of ethylene from 2H was observed even on
heating up to 105 °C in the solid state under high vacuum
condition (<10–1 Pa) or in a toluene solution. When a C6D6
solution of 2Me was heated at 60 °C for 1.5 h, the 1H NMR
signals corresponding to 2Me disappeared and the signals for 1
and propylene were observed. Similarly, heating of the reaction
mixture containing 2Bu at 50 °C for 1.5 h resulted in the
disappearance of 2Bu and the increasing of 1 and hex-1-ene.
(a)
(b)
Mes2P
Al(t-Bu)2
2H
P
R
C N
C6D6, r.t.
N
(R = t-Bu, Ph)
R
C
Al
(R = t-Bu)
5Ph (R = Ph)
16%
65%
5Bu
Mes2P
N
Al(t-Bu)2
Scheme 6. (a) Nitrile-coordination to 2H to give the corresponding adduct 5.
(b) Thermal ellipsoid plot (50% probability) of 5Ph. Hydrogen atoms and
solvent molecules are omitted for clarity.
P/B system A is known to bind CO2 in the presence of an
excess amount of CO2 to give CO2-adduct under –20 °C, and it
liberates CO2 above –20 °C in CH2Cl2, regenerating A.[23] In
addition, the groups of Uhl and Lammertsma have successfully
captured CO2 using C1-geminal P/Al-FLP at room
temperature.[24] Binding of CO2 is useful in terms of CO2
reduction, and intermolecular P/Al-FLP based CO2 capture and
stoichiometric reduction have been reported by Stephan et al.[25]
However, we found that the reactivity of 2H with CO2 was
completely different from those of A and C1-geminal P/Al-FLP.
The exposure of 2H to a CO2 atmosphere afforded compound 62.
In this case, contrary to our expectation, CO2-trapping product 7
was not obtained. Compound 62 was the dimeric form of the CO2-
inserted product into the P–Al bond of 1. (Scheme 7). Compound
62 can be obtained directly by exposing 1 to a CO2 atmosphere.
The CO2 insertion into the P–Al bond of 1 was much faster than
the previously reported insertion in the case of (Me3Si)2P–
Al(tmp)2 (tmp = 2,2,6,6-tetramethylpiperidino).[26]
Scheme 5. Calculated reaction pathways for the reactions of 1 with alkenes.
Relative Gibbs energies (ΔG in kcal mol–1) were calculated at the TPSSTPSS-
D3/6-311+G(2d,p)<Al,P>,6-31G(d,p)<C,H>//B3LYP-D3/6-31G(d) level of
theory, and are shown in parentheses.
We estimated the energy profile for the reactions of 1 with
alkenes by theoretical calculations to investigate the reversible
behavior. Scheme 5 shows the calculated reaction pathways for
the addition of alkenes (ethylene, propylene, and hex-1-ene) to
1 with the values of relative energies. The reactions should
proceed via the interaction with the 3p(Al) orbital of 1 and the π
orbital of the alkene to form the π-complex intermediate
(INT3),[19] and then pass through the transition state TS4. This
syn-type addition differs from anti-addition proposed in the B/P
3
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Scheme 8. Calculated reaction mechanisms for the reaction of 2H with CO2 and ethylene-release from 2H (a) and the reaction of 1 with CO2 (b). Relative Gibbs
free energies (ΔG in kcal mol–1) were calculated at the B3LYP-D3/6-31G(d) level of theory and are shown in parentheses.
from that for the reaction of 1 with CO2 (ΔΔG‡ = 7.1 kcal mol–1:
1→TS12).
Scheme 7. Reactions of CO2 with 1 and 2H. [a] One step from 1. [b] Two steps
via 2H.
The structure of 62 was established by X-ray crystallography
to confirm the dimeric, 8-membered ring form in the solid state,
in which the two Al atoms were bridged with a phosphanyl-
carbonate ligand (Mes2PCO2)– (Figure S32).[22] The
(Mes2PCO2)– acts as the bidentate chelating ligand sandwiching
the (t-Bu)2Al cation, [(t-Bu)2Al]+.
Scheme 9. (a) Reactions of phenylacetylene with 1 and 2H. (b) Reactions of
diphenylacetylene with 1 and 2H. [a] Judged by NMR spectra. [b] Ref. 28.
The factors of ethylene elimination were clarified by
theoretical calculations (Scheme 8, Figure S35). The barrier to
release ethylene from 2H (ΔΔG‡ = 25.5 kcal mol–1: 2H→TS4H)
was larger than that of the reaction of 2H with CO2 giving the
corresponding CO2 adduct 9 via INT8 and TS9 (ΔΔG‡ = 7.4 kcal
mol–1: 2H→TS9). The release of ethylene from 7 was found to
proceed with the barrier (ΔΔG‡ = 17.8 kcal mol–1: 7→TS10) lower
than that in the case of 2H. Therefore, ethylene is eliminated not
from 2H directly, but from 7. The barrier for the reaction of 2H
with CO2 (ΔΔG‡ = 7.4 kcal mol–1: 2H→TS9) did not differ greatly
Similar elimination of ethylene from 2H is also observed in
the case of the reaction with phenylacetylene.[27] When 2H was
treated with phenylacetylene, 2H was quantitatively converted
into Mes2PH and [PhC≡CAl(t-Bu)2]2 in accompany with the
elimination of ethylene (Scheme 9). This is the same
combination of products for the reaction of
1
with
phenylacetylene.[28] Considering the results of theoretical
calculations for the reaction of 2H with CO2, the reaction of 2H
4
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with phenylacetylene passed through the intermediate INT13,
leading to the elimination of ethylene to give Mes2PH and
[PhC≡CAl(t-Bu)2]2. On the other hand, 1 and 2H showed different
reactivity toward diphenylacetylene (Scheme 9b). Phosphanyl-
alumane 1 underwent cis-addition to diphenylacetylene under
heating conditions to give 14,[28] whereas 2H did not react with
diphenylacetylene under heating.
16. As the result, two molecules of benzaldehyde were converted
into two different products. In our system, the Al moiety of 2H can
be greatly involved in the capture of the oxygen atoms of
benzaldehyde as a LA.
To conclude, we found the reversible addition reactions of
olefins with phosphanylalumane 1 as the first case for single-
bond compounds of main group elements. Although the
substrate scopes of alkenes are limited to terminal alkenes (R-
CH=CH2) having relatively small substituents (R = H, Me, and n-
Bu), the corresponding alkene-adducts 2 were obtained. In the
case of R = Me and n-Bu, the P–Al bond of the alkene-adducts
(2Me and 2Bu) was recombined in accompany with the liberation
of corresponding alkene on heating. On the other hand, ethylene
was not liberated from 2H even under heating conditions. We
demonstrated the reactivity of 2H as a new C2-vicinal P/Al-based
FLP. In the reaction of 2H with nitriles, nitriles were coordinated
to Al as linear LBs, bringing the retention of ethylene-bridge like
the case of P/B-system. However, reactions of 2H with CO2,
phenylacetylene, and benzaldehyde gave characteristic
products that involved elimination of ethylene from the initial
intermediates.
These experimental and theoretical results suggested that
the addition reactions of 1 with CO2 and phenylacetylene should
pass through characteristic ethylene-bridged intermediates, 7
and INT13, respectively. Therefore, we expected that the
transformation of small molecules, which could not be achieved
by 1 due to the decomposition or no reactivity, could be achieved
by using 2H.
Acknowledgements
This work was supported by KAKENHI (JP24109013,
JP26620028, JP19H05528, JP18H01963, and JP19H05635)
and Integrated Research Consortium on Chemical Science
(IRCCS). T.Y. gratefully acknowledges a Research Fellowship
for Young Scientists (JP19J14359) from the Japan Society for
the Promotion of Science (JSPS). This study was supported by
the Joint Usage/Research Center [JURC, Institute for Chemical
Research (ICR), Kyoto University] by providing access to a
Bruker Avance III 600 NMR spectrometer. We are furthermore
grateful for computation time, which was provided by the Super
Computer Laboratory (ICR, Kyoto University). Elemental
analyses were carried out at the Microanalytical Laboratory of
the ICR (Kyoto University).
Scheme 10. (a) Reactions of 1 and 2H with benzaldehyde. (b) Plausible
mechanism for the generation of 15 and 16.
Conflict of interest
While benzaldehyde is also known to form the cyclic adducts
with the P/B-system A[21] and C1-geminal P/Al-system,[29] the
reaction of 1 with benzaldehyde afforded a complicated mixture
probably via the generation of a radical species arisen from the
carbonyl moiety (Scheme 10a, top). On the other hand, a C6D6
solution of 2H was treated with an excess amount of
benzaldehyde to give one set of signals corresponding to t-Bu
moieties (Scheme 10a, bottom). X-Ray crystallographic analysis
revealed the formation of benzyloxy(di-t-Bu)aluminum dimer 15
(Figure S33).[22] The corresponding P-containing moieties turned
into acylphosphine 16[30] as a major product. The reaction
mechanism can be explained as an intramolecular Tishchenko-
type reaction (Scheme 10b).[31]
The authors declare no conflict of interest.
Keywords: addition reaction • aluminum • olefin •
phosphanylalumane • reversible behavior
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10.1002/chem.202101649
Chemistry - A European Journal
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Entry for the Table of Contents
Phosphanylalumane (P–Al single-bond compound) was reacted
with relatively small terminal olefins (ethylene, propylene, and
hex-1-ene) at room temperature. Heating of propylene- and
hex-1-ene-adducts gave the regeneration of the P–Al bond with
liberation the corresponding olefins. An unprecedented
reactivity of the ethylene-adduct with the release of ethylene
was demonstrated, in sharp contrast to the conventional P/B-
system.
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