Using the Secondary PH/BH Functional Groups of an Active Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction and Reactions with Nitriles and Isonitriles

Article abstract of DOI:10.1002/anie.202000612

Reaction of the secondary alkynyl(Mes*)PH phosphane 2 with (Fmes)BH₂?SMe₂ gives the geminal PH/BH frustrated Lewis pair (FLP) 3. The PH and the BH functions are jointly used in the reduction of carbon monoxide under mild reaction conditions to give the [P]-CH₂-O-[B] product. A subsequent cycloaddition sequence results in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile to give a P-benzamidine, and it couples two isonitriles at the FLP framework.

Full text of DOI:10.1002/anie.202000612

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Accepted Article
Title: Using the Secondary PH/BH Functional Groups of an Active
Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction
and Reactions with Nitriles and Isonitriles
Authors: Gerhard Erker, Jun Li, Constantin G. Daniliuc, Christian
Mück-Lichtenfeld, and Gerald Kehr
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202000612
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10.1002/anie.202000612
Angewandte Chemie International Edition
RESEARCH ARTICLE
Using the Secondary PH/BH Functional Groups of an Active
Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction
and Reactions with Nitriles and Isonitriles
Jun Li,[a] Christian Mück-Lichtenfeld,[a] Constantin G. Daniliuc,[a] Gerald Kehr,[a] and Gerhard Erker*[a]
Dedication ((optional))
We used our recently described (Fmes)BH2 reagent 1a[8] for
Abstract: Reaction of the secondary alkynyl(Mes*)PH phosphane 2
with the (Fmes)BH2•SMe2 reagent gives the geminal PH/BH frustrated
preparing the new geminal PH/BH FLP system 3. The borane was
Lewis pair 3. The P-H and the B-H functions are jointly used in the
reacted with the secondary alkynylphosphane 2 (pentane, r.t. 4h).
reduction of carbon monoxide under mild conditions to give the [P]-
Workup involving crystallization from pentane at -35 °C gave the
CH2-O-[B] product 11. A subsequent cycloaddition sequence results
PH/BH containing product 3 as a yellow crystalline material in
in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile
39% yield. Compound 3 was characterized by an X-ray crystal
to give a P-benzamidine, and it couples two isonitriles at the FLP
structure analysis (see Fig. 1). It confirmed that a regioselective
framework.
cis-1,2-BH addition reaction had taken place to the acetylene
carbon-carbon triple bond of the reagent 2. The product 3 features
the Z-configurated C1-C2 carbon-carbon double bond and it has
Introduction
both the geminal Mes*(H)P- and (Fmes)(H)B- substituents
attached at carbon atom C1. The phosphane geometry is trigonal-
Intramolecular hydrocarbyl-bridged frustrated Lewis pairs (FLPs)
pyramidal, whereas the coordination geometry at boron is trigonal
have served as reactive dihydrogen splitting reagents and
planar. The borane Lewis acid has its substituents oriented in the
subsequently as components of active metal-free hydrogenation
catalysts.[1] In addition, some intramolecular P/B FLPs have
shown unique stoichiometric reactions with carbon monoxide or
nitrogen monoxide.[2] They served as the templates for various
unusual reactions, e.g. the HB(C6F5)2 reduction of CO to the
formyl stage and beyond.[3] In such cases, bulky tertiary
phosphane and borane components made the FLP systems.[4]
The respective hydrogen functionalities then needed to be
provided externally, be it e.g. by means of the heterolytic cleavage
of the dihydrogen molecule or the involvement of external H[B]
borane reagents. We have now prepared a rare example of a
geminal[5,6,7] intramolecular PH/BH pair, that contains a secondary
R1R2PH functionality combined with a R2R3BH borane building
block. We found a variety of unique reactions where both the
neutral PH and BH functionalities were actively involved in small
molecule reactions, most notably the stoichiometric carbon
monoxide to -CH2-O- reduction. The formation and
characterization of the new PH/BH FLP and some representative
reactions will be presented and discussed in this account.
plane of the C1-C2 double bond. The B and the P atoms each
bear a -H substituent. The B-H vector is oriented toward the inside
of the P1-C1-B1 wedge; the P-H vector is oriented outside. There
is no direct interaction between the P-based Lewis base and
boron Lewis acid.
Results and Discussion
Formation of the PH/BH FLP 3 and its reactions with carbon
monoxide
[a]
Dr. J. Li, Dr. C. Mück-Lichtenfeld, Dr. C. G. Daniliuc, Dr. G. Kehr,
Prof. Dr. G. Erker
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstraβe 40, 48149 Münster (Germany)
E-mail: erker@uni-muenster.de.
Scheme 1. Borane assisted CO reduction at the FLP 3.
Supporting information for this article is given via a link at the end of
the document.
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Compound 3 shows a 11B NMR feature at δ 64.4 in CD2Cl2
solution at 243 K, and a 31P NMR signal at δ -69.9 (1JPH ~ 233 Hz).
It shows the olefinic 1H NMR signal at δ 6.29 [3JPH = 22.8 Hz; 13C:
yield. The X-ray crystal structure analysis revealed the presence
of a six-membered heterocyclic core structure which has
incorporated the B(C6F5)2 moiety from the added borane reagent
and the B/P pair from 3 with its stabilizing Fmes and Mes*
substituents (see Fig. 2). The carbon monoxide molecule has
been split. Its oxygen atom is found bridging between the pair of
boron atoms and its carbon atom has become reduced to -CH2-,
which is found in a bridging position between B2 and P1. In
solution compound 4 shows the core heteroatom resonances at δ
-16.4 (31P, 1JPH ~ 464 Hz) and δ 39.3, -1.5 (11B). The newly formed
[P]-CH2-[B] group shows up at δ 3.12/2.51 (1H) and δ 20.9 (13C),
respectively (see the Supporting Information for further details).
We assume a pathway of the formation of compound 4 that
involves the addition of the in situ generated borane carbonyl
[(C6F5)2B(H)CO] to the P/B pair followed by formation of the η2-
formyl borane intermediate 6 at the P/B FLP framework, similar
as we had previously described it for the formylborane formation
at the Mes2P(CH2)2B(C6F5)2 FLP.[3b,c] Internal hydride attack
would then directly give the observed product 4.
2
δ 176.8 (C2, JPC = 15.6 Hz), δ 142.9 (C1, broad, 1JPC ~ 32 Hz)]
and the tBu resonance at δ 1.15 [9H; 13C: δ 37.3 (C), δ 28.8 (CH3)].
The FLP 3 reacts analogously with (Fmes)BH2/CO. In this case,
we assume the intermediate formation of the heterocycle 7 after
H2-elimination (H2 was detected by NMR). Chromatographic
workup in this case gave the H2O addition product 8 (see the
Supporting Information for its characterization including the X-ray
crystal structure analysis).
Compound 3 reacts itself slowly with carbon monoxide. This
reaction takes a markedly different course. Treatment of the in
situ generated PH/BH FLP with CO under somewhat forcing
conditions (toluene, 50 bar CO, 7d, 100 °C) gave the FLP CO-
reduction product 11, which we isolated as a crystalline solid in
25% yield after crystallization from pentane at -35 °C.
Figure 1. A view of the molecular structure of the geminal PH/BH frustrated
Lewis pair 3 (thermal ellipsoids are set at 15% probability).[20] Selected bond
lengths (Å) and angles (°) of molecule A (mol. B shows similar values): P1A-
C1A: 1.859(5), B1A-C1A: 1.540(8), C1A-C2A: 1.360(7), B1A-C1A-P1A:
120.5(4), C11A-P1A-C1A: 100.8(2), C11A-P1A-H1A: 106.(2), C1A-P1A-H1A:
104.(3), C1A-B1A-C31A: 121.5(5), C1A-B1A-H2A: 120.(3), C31A-B1A-H2A:
118.(3), C31A-B1A-C1A-C2A: -4.7(1), B1A-C1A-C2A-C3A: -170.2(5), P1A-
C1A-C2A-C3A: 2.1(8).
Figure 2. Molecular structure of compound 4 (thermal ellipsoids are set at 30%
probability).[19] Selected bond lengths (Å) and angles (°): O1-B1: 1.320(2), O1-
B2: 1.490(2), P1-C7: 1.792(2), B2-C7: 1.655(2), B1-O1-B2: 130.1(1), C7-P1-
C1: 104.1(1), O1-B1-C1: 124.7(2), B1-C1-P1: 110.4(1), B2-C7-P1: 109.1(1),
C1-C2-C3: 139.4(2).
Scheme 2. Carbon monoxide reduction to the formaldehyde stage by the
PH/BH pair 3.
We used the FLP-framework of compound 3 to achieve the
borane assisted reduction of carbon monoxide. For that reaction,
3 was treated with CO in the presence of Piers’ borane (1b)
[(C6F5)2BH]. The reaction of 3 with (C6F5)2BH/CO was carried out
in d6-benzene at 80 °C for the duration of 1d. Workup involving
chromatography gave the product 4 as a crystalline solid in 25%
In solution (CD2Cl2), compound 11 shows the 1H NMR signals of
the pair of diastereotopic methylene hydrogen atoms of the newly
2
formed -CH2O- group adjacent to phosphorus at δ 5.58 (dd, JHH
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10.1002/anie.202000612
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= 11.8 Hz, 2JPH = 3.5 Hz) and δ 5.18 [dd, with 2JPH = 37.3 Hz; 13C:
δ 77.2 (1JPC = 30.1 Hz)]. The 31P NMR resonance (δ -32.4, q, JPF
= 5.9 Hz) shows coupling to one Fmes CF3 group. There is
hindered rotation around the B-C(Fmes) vector. The 11B NMR
signal of compound 11 occurs at δ 48.0 and there are the usual
NMR signals of the framework [e.g. =CH-tBu at δ 6.06 (1H)/ 167.8
(13C)] and the Mes* and Fmes substituents at phosphorus and
boron, respectively.
Compound 11 was characterized by X-ray diffraction. The
structure (see Fig. 3) shows the newly formed heterocyclic five-
membered core including the [P]-CH2-O-[B] unit that was formed
by reduction of the CO molecule. Carbon atom C1 is part of the
Z-substituted exocyclic olefinic unit. The phosphorus coordination
geometry is trigonal-pyramidal (ΣP1CCC = 318.1°), whereas the
boron center is trigonal-planar (ΣB1CCO = 359.8°). We note that
the attachment of the bulky Mes* substituent at P1 is markedly
bent (angle P1-C11···C14: 147.3°).
82 :18 ratio in a combined yield of 27%. Formation of such E-/Z-
isomers has often been observed in this and related chemistry.[15]
It probably occurs acid catalyzed (see below). The major E-15
isomer shows a 31P NMR resonance at δ 89.1 (Z-15: 66.0), which
shows coupling to one CF3 group of the Fmes ligand at boron.
Both compounds feature a broad 11B NMR signal at δ 32.9. The
major E-15 isomer shows a characteristic smaller cis 3JPH =13.9
Hz coupling constant to the =CHtBu proton (δ 4.83) as compared
to Z-15 (δ 6.32, trans 3JPH = 18.4 Hz).
It has long been known that B-H boranes do not reduce carbon
monoxide unless catalyzed.[9-11] We assume that the reaction
sequence leading to the formation of 11 is initiated by a typical
FLP/CO reaction, namely the 1,1-P/B-addition to the carbonyl
carbon atom[12] to generate the intermediate 9. This can then
undergo hydride addition. We assume that this then leads to the
η2-formylborane stage of 10, similar to a related example that we
recently isolated from the reaction of Mes2P-CH2CH2-B(H)Fmes
with carbon monoxide.[8] Internal proton transfer then may convert
the intermediate 10 to the observed reaction product 11 (see
Scheme 2).
Figure 4. A projection of the molecular geometry of compound E-15 (thermal
ellipsoids are set at 30% probability).[20] Selected bond lengths (Å) and angles
(°): P(1)-C(1): 1.859(2), C(1)-C(2): 1.342(3), C(1)-B(1): 1.565(3), P(1)-C(1)-B(1):
104.6(1), C(8)-N(3)-C(41)-C(42): 127.8(2), ΣN3CCC: 359.9, ΣN1CNP: 336.5,
ΣN2CNP: 357.2, ΣPCNC: 307.4.
The major E-15 isomer was characterized by an X-ray crystal
structure analysis (see Fig. 4). It shows the bicyclic framework.
The boron atom B1 and two nitrogen atoms (N3, N1) are close to
trigonal-planar. The nitrogen atom N2 and the phosphorus atom
P1 feature trigonal-pyramidal coordination geometries.
Other reactions of the FLP 3 making use the PH/BH
combination of functionalities: reactions with organic nitriles.
Figure 3. A projection of the molecular structure of the CO-reduction product 11
(thermal ellipsoids are set at 15% probability).[20] Selected bond lengths (Å) and
angles (°): P(1)-C(7): 1.885(3), B(1)-O(1): 1.349(4), O(1)-C(7): 1.449(4), B(1)-
O(1)-C(7): 115.0(3), O(1)-C(7)-P(1): 109.2(2), C(1)-P(1)-C(11): 112.8(1), C(1)-
P(1)-C(7): 91.8(1), C(11)-P(1)-C(7): 113.4(2), O(1)-B(1)-C(1): 117.0(3), O(1)-
B(1)-C(31): 118.2(3), C(1)-B(1)-C(31): 124.6(3).
We reacted the PH/BH system 3 with p-tosylcyanide. Workup
after a reaction time of 7d at r.t. gave product E-17a as a yellow
solid in 54% yield. The 1H NMR spectrum shows the NH signal at
δ 5.17 and a singlet of the adjacent ring CH moiety at δ 6.24 (13C:
1
1
82.2, JPC = 49.3 Hz). In addition, we have monitored the H/13C
NMR signals of the exocyclic trisubstituted olefinic unit [=CH-: δ
4.32 (1H), 157.6, 2JPC = 16.8 Hz (13C)] and the resonances of the
substituents (tBu, Mes*, Fmes).
Compound 11 can formally be regarded as a cyclic frustrated
Lewis pair and it reacts as such with N-phenyltriazolindione 12
(r.t., CDCl3) to form the heterobicyclic product 15 with liberation of
formaldehyde (14) [detected by NMR spectroscopy: δ 9.72 (1H),
δ 194.6 (13C)[13]]. It can safely be assumed that the reactive
cycloaddition reagent 12 adds to the cyclo-P/B FLP 11 to
generate the heteronorbornane derivative 13 (see Scheme 2).
This may then undergo “retro-cycloaddition”[14] to liberate
formaldehyde (14) with formation of 15 which we isolated on a
preparative scale as a mixture of the E-15/Z-15 isomers in a ca.
The X-ray crystal structure analysis of compound E-17a (see the
Supporting Information for details and the depicted structure)
confirmed that the H+/H- pair of the PH/BH reagent had reduced
the incipient C=N unit. We note that the exocyclic trisubstituted
C=C double bond of product E-17a features an inverted geometry
as compared to the starting material 3.[15]
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We carried out a few additional experiments that were in situ
analyzed by NMR spectroscopy and supported by a DFT
calculation to arrive at a justified rationalization of the underlying
mechanistic scheme. We found that the reaction of 3 with
tosylcyanide initially gave a mixture of the compounds Z-16a and
Z-17a (ca. 5 min, r.t.). Only with time this was converted to the
thermodynamically favored compound E-17a as the major
product (the spectra are depicted in the Supporting Information).
We assume that this occurs acid catalyzed. A DFT analysis
confirmed that the E-17a isomer is favored at the equilibrium state
(see Scheme 3). [16]
22b. These observations indicate that compound 20b may
possibly serve as the intermediate of the formation of the product
22b by means of a ring-opening fragmentation reaction pathway
(see Scheme 4).
Scheme 4. Reaction of the FLP 3 with benzonitrile. DFT calculated Gibbs
energies (in kcal mol-1; PW6B95-D3//TPSS-D3/def2-TZVP+COSMO-RS
(pentane)) of the Ph containing series (b) are given in [ ]. From the reaction of
3 with p-bromobenzonitrile we isolated the analogous final products 22c.
Scheme 3. Reaction of the FLP 3 with p-tosylcyanide with DFT calculated
relative Gibbs energies [in kcal mol-1; PW6B95-D3//TPSS-D3/def2-TZVP+
COSMO-RS(pentane)].
The reaction of the FLP 3 with benzonitrile (pentane, r.t., 3 d)
furnished a slightly different outcome. Workup gave the product
22b in 57% yield. Crystallization from pentane at -35 °C produced
single crystals of the phospha-amidine type compound[17,18] all E-
22b (see Scheme 4). It was characterized by X-ray diffraction (see
the Supporting Information for details). In solution compound 22b
showed some complicated behavior. We observed several
compounds by NMR spectroscopy, probably phospha-amidine
isomers.[19] However, their detailed structures could not be
assigned so far with certainty. The reaction of 3 with p-
bromobenzonitrile gave a similar result. Compound all E-22c was
also characterized by an X-ray crystal structure analysis (see the
Supporting Information for the structural and spectroscopic
characterization of the 22b and 22c isomers and further details).
A closer investigation showed that the reaction of 3 with
benzonitrile (CD2Cl2, r.t., overnight) gave mainly the initial product
Z-16b [NMR: δ -2.1 (11B), δ -173.2 (15N, 1JNH ~ 80 Hz), δ 8.52 (1H
of =NH)]. Heating compound Z-16b converted it to a mixture of
the zwitterionic borata-alkene/iminium compound 20b [NMR: δ
28.6 (11B), δ -231.3 (15N, 1JNH ~ 84 Hz), δ 7.83 (d, 3JPH = 17.3 Hz,
1H of =NH)] and the isomer E-16b [NMR: δ -5.8 (11B), δ -187.4
(15N, 1JNH ~ 80 Hz), δ 7.98 (1H of =NH)], the latter probably again
formed by acid catalyzed isomerization. Further thermolysis
resulted in an increasing conversion to the final isomer mixture of
Scheme 5. Reaction of compound 3 with benzoyl chloride.
The reaction of the PH/BH FLP with benzoyl chloride follows a
related reaction pattern to yield 24. In this case the ring opening
reaction probably takes place by HCl elimination (Scheme 5, see
the Supporting Information for details).
Reaction of the PH/BH FLP 3 with isonitriles
One could have envisaged that the PH/BH FLP 3 reacts with an
isonitrile analogously as with carbon monoxide, but that is not the
case. We added (2,6-dimethylphenyl)isocyanide (CN-Xyl, two
molar equiv.) to compound 3, generated in situ in pentane. The
resulting dark red solution was stirred at r.t. for 3 d. Workup gave
a mixture of red and yellow solids. It contained the products 30
and 31 in a ratio of ca. 1 : 3 plus some minor (ca. 11 %) as yet
unidentified components. Compounds 30 and 31 were manually
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separated and each component of the mixture was characterized
by X-ray diffraction and by NMR spectroscopy.
moiety. Both these hydrogen atoms eventually ended up at the
pair of nitrogen atoms. The structure of 30 has a peculiarity: the
C7-N1 and C8-N2 bond lengths are quite different (N1-C7:
1.351(3)Å, ΣN1CCH = 353°, C7-C8: 1.405(3) Å), and the C8-B1
bond is short (1.495(4) Å, cf. C1-B1: 1.577(3) Å), indicating some
borata-alkene character (see Fig. 5).
In CD2Cl2 solution compound 30 shows a 11B NMR resonance at
δ 46.3. The “endiamine” 13C NMR resonances are quite different
from each other [δ 174.2 and 134.8 (broad)]. We note hindered
rotation around the B-Fmes as well as the P-Mes* vectors. The
1
pair of H NMR NH resonances are also quite different (δ 6.27
and δ 3.96). The 31P NMR signal was located at δ -12.5.
We assume that the formation of compound 30 starts with
isonitrile insertion into the B-H bond of 3. The resulting iminoacyl
intermediate 26 is prone to nucleophilic attack by a second
isonitrile equivalent followed by P-C bond formation to form the
five-membered ring. Proton transfer followed by tautomerization
would then rationalize the formation of the observed product 30
[pathway (a) in Scheme 6].
The X-ray crystal structure analysis of compound 31 shows that it
was also formed by the reaction of 3 with two xylyl-isocyanide
equivalents. Compound 31 is an isomer of 30, but it shows a
markedly different framework (see Fig. 6). Both isonitrile derived
carbon atoms are found bonded to boron, one as a neutral imine
the other as a positively charged iminium ion. The boron atom still
bears a hydride and the Fmes group. The rest of the perimeter
bears the tBu substituent and a hydrogen at carbon atom C2, both
directly originating from the starting material 3, and carbon atom
C1 is part of the exocyclic phospha-imine functional group.
The hand-picked red crystals of compound 31 were dissolved in
CD2Cl2 to obtain the NMR data of the compound. It shows the
typical 1H NMR signals of the Fmes and Mes* groups, both
featuring hindered rotation. The 31P NMR resonance is at δ 381.9
and the 11B NMR signal at δ -12.5 (d, 1JBH ~ 75 Hz).
Scheme 6. Reaction of compound 3 with the CN-Xyl isonitrile.
Figure 5. Molecular structure of compound 30 (thermal ellipsoids are set at 30%
probability).[20] Selected bond lengths (Å) and angles (°): N1-C7: 1.351(3), N2-
C8: 1.454(3), P1-C7: 1.808(2), B1-C8: 1.495(4), C7-C8: 1.405(3), C1-B1:
1.577(3), C7-N1-C41: 125.3(2), C7-P1-C1: 93.3(1), C2-C1-B1: 139.0(2), C8-
B1-C1: 111.1(2), C51-N2-C8: 124.7(2).
Figure 6. A view of the molecular structure of compound 31 (thermal ellipsoids
are set at 15% probability).[20] Selected bond lengths (Å) and angles (°): P1-C1:
1.706(4), C1-C2: 1.538(6), C11-N1: 1.320(5), C51-N2: 1.278(5), C1-C11:
1.475(6), C11-B1: 1.604(6), C1-P1-C21: 108.5(2), C11-C1-C2: 109.9(3): C51-
N2-C52: 120.6(4), C2-C51-B1: 107.0(4), C11-B1-C51: 99.8(3), C11-N1-C12:
129.2(4).
The X-ray crystal structure analysis of compound 30 shows the
presence of a heterocyclic five-membered core. It is constructed
of the core atoms of 3 and an endiamine unit that was formed by
carbon-carbon coupling between two isocyanide units. Both the
PH proton and the BH hydride were used in the formation of this
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For a rationalization of the formation of compound 31, we assume
a pathway that also starts with the isonitrile adduct 25 (see
Scheme 6). This is apparently set for C-C coupling with the
adjacent “enphosphane” to generate 27. The methylene-
phosphonium (i.e. phospha-iminium) unit is prone for attack by
the second isonitrile nucleophile. Ring opening rearrangement
followed by H+ equilibration would then provide a possible
pathway for the formation of the observed isomer 31.
Keywords: boron • phosphorus • carbon monoxide • reduction •
frustrated Lewis pair •
[1]
Reviews: a) D. W. Stephan, G. Erker, Angew. Chem. 2010, 122, 50-81;
Angew. Chem. Int. Ed. 2010, 49, 46-76; b) D. W. Stephan, G. Erker, Top.
Curr. Chem. 2013, 332, 85-110; c) D. W. Stephan, G. Erker, Angew.
Chem. 2015, 127, 6498-6541; Angew. Chem. Int. Ed. 2015, 54, 6400-
6441; d) D. J. Scott, M. J. Fuchter, A. J. Ashley, Chem. Soc. Review 2017,
46, 5689-5700; e) W. Meng, X. Feng, H. Du, Acc. Chem. Res. 2018, 51,
191-201; f) J. Lam, K. M. Szkop, E. Mosaferi, D. W. Stephan, Chem. Soc.
Rev. 2019, 48, 3592-3612.
Conclusions
[2]
a) A. J. P. Cardenas, B. J. Culotta, T. H. Warren, S. Grimme, A. Stute, R.
Fröhlich, G. Kehr, G. Erker, Angew. Chem. 2011,123, 7709-7713; Angew.
Chem. Int. Ed. 2011, 50, 7567-7571; b) M. Sajid, A. Stute, A. J. P.
Cardenas, B. J. Culotta, J. A. M. Hepperle, T. H. Warren, B. Schirmer, S.
Grimme, A. Studer, C. G. Daniliuc, R. Fröhlich, J. L. Petersen, G. Kehr,
G. Erker, J. Am. Chem. Soc. 2012, 134, 10156-10168; c) R. Liedtke, F.
Scheidt, J. Ren, B. Schirmer, A. J. P. Cardenas, C. G. Daniliuc, H. Eckert,
T. H. Warren, S. Grimme, G. Kehr, G. Erker, J. Am. Chem. Soc. 2014,
136, 9014-9027; d) J. C. M. Pereira, M. Sajid, G. Kehr, A. Wright, B.
Schirmer, Z. W. Qu, S. Grimme, G. Erker, P. Ford, J. Am. Chem. Soc.
2014,136,513-519.
We had recently presented FLP examples that contained a single
secondary PH[5e] or BH functionality[8] (compounds 32 and 33, see
Scheme 7). Compound 32 showed typically FLP reactivities, but
no exceptional behavior originating from the PH functional group.
Compound 33 reduced carbon monoxide to the formyl stage (34),
but then the reaction stopped.[8]
[3]
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Scheme 7 Examples of PH or BH containing FLPs.
[4]
[5]
The here newly presented PH/BH system 3 has a different quality.
This system has the ability to utilize both the incipient protolytic
feature at phosphorus and the hydridic one at boron jointly in a
sequential series of reaction steps for the reductive conversion of
added small molecules. The reaction of the PH/BH FLP with CO
is a typical example. The CO reduction at 3 takes a course that
brings both the PH and the BH hydrogen to the carbonyl carbon
atom, bringing it to the formaldehyde state. We could even
liberate formaldehyde as the free carbonyl compound in a
subsequent reaction. The reactions of 3 with nitriles and even with
an isonitrile take somewhat different routes. In the case of the
reaction of 3 with TsCN the H+/H- pair undergoes 1,2-addition at
the C=N unit formed by cyanide addition to the P/B pair. In the
case of the arylcyanide reactions, a similar pathway is observed
that only differs in the course of the stabilization of the
intermediate. In the case of the isonitrile coupling reaction we
observe H+/H- transfer to give an isonitrile coupling product, where
formally an “Umpolung” of the reactivity of one of the hydrogen
reagents has taken place during the course of the overall reaction,
complementary to the PH/BH reaction of carbon monoxide to 11.
It seems that the presence of a pair of neutral secondary PH/BH
functionalities opens interesting pathways in small molecule
activation and conversion.
[6]
[7]
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. J. L. thanks the Alexander von
Humboldt foundation for a stipend.
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[20] CCDC 1952808, 1952809, 1952810, 1952811,1952812, 1952813,
1952814, 1952815, 1962693, 1965201 and 1968694 contain the
supplementary crystallographic data for this paper. These data are
provided free of charge by The Cambridge Crystallographic Data Centre.
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Entry for the Table of Contents (Please choose one layout)
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RESEARCH ARTICLE
Jun Li, Christian Mück-Lichtenfeld,
Constantin G. Daniliuc, Gerald Kehr,
Gerhard Erker*
Page No. – Page No.
Using the Secondary PH/BH
Functional Groups of an Active
Geminal Frustrated Lewis Pair for
Carbon Monoxide Reduction and
Reactions with Nitriles and Isonitriles
The geminal secondary phosphane/borane PH/BH pair reduces carbon monoxide
to a formaldehyde unit at the P/B template. Formaldehyde is liberated in a
subsequent reaction.
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