Formylborane formation with frustrated Lewis pair templates

Article abstract of DOI:10.1002/anie.201307551

Boranes R₂BH react with carbon monoxide by forming the respective borane carbonyl compounds R₂BH(CO). The formation of (C₆F₅)₂BH(CO) derived from the Piers borane, HB(C₆F₅)₂, is a typical example. Subsequent CO-hydroboration does not take place, since the formation of the formylborane is usually endothermic. However, an η²-formylborane was formed by CO-hydroboration with the Piers borane at vicinal phosphane/borane frustrated Lewis pair (FLP) templates. Subsequent treatment with pyridine liberated the intact formylborane from the FLP framework, and (pyridine)(C ₆F₅)₂B-CHO was then isolated as a stable compound. This product underwent typical reactions of carbonyl compounds, such as Wittig olefination. Channeling frustration into productivity: An elusive borane carbaldehyde was liberated from the product of carbon monoxide hydroboration at a frustrated Lewis pair template by treatment with pyridine and isolated as the donor-stabilized adduct (see scheme; Mes=mesityl). In this way, the thermodynamic restrictions of CO insertion into boron-hydrogen bonds could be circumvented. Copyright

Full text of DOI:10.1002/anie.201307551

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DOI: 10.1002/anie.201307551
Borane Carbonyl Derivatives
Formylborane Formation with Frustrated Lewis Pair Templates**
Muhammad Sajid, Gerald Kehr, Constantin G. Daniliuc, and Gerhard Erker*
Abstract: Boranes R2BH react with carbon monoxide by
forming the respective borane carbonyl compounds
R2BH(CO). The formation of (C6F5)2BH(CO) derived from
the Piers borane, HB(C6F5)2, is a typical example. Subsequent
CO-hydroboration does not take place, since the formation of
the formylborane is usually endothermic. However, an “h2-
formylborane” was formed by CO-hydroboration with the
Piers borane at vicinal phosphane/borane frustrated Lewis pair
(FLP) templates. Subsequent treatment with pyridine liberated
the intact formylborane from the FLP framework, and
We first treated compound 3a with dihydrogen (60 bar). It
reacted at room temperature to give the product 4, which was
isolated as a crystalline solid in 77% yield. X-ray crystal-
structure analysis showed that the formyl group was reduced
[13]
À
and its C O linkage cleaved.
The structure contains
a saturated seven-membered heterocycle that is annulated
with the norbornane framework. The CO-derived methylene
group was found to bridge a phosphonium and a borate unit
À
À
[P1 C8 1.805(2) ꢀ, C8 B2 1.637(4) ꢀ, P1-C8-B2 119.4(2)8].
The former carbonyl oxygen atom had become protonated
À
À
(pyridine)(C6F5)2B CHO was then isolated as a stable com-
pound. This product underwent typical reactions of carbonyl
compounds, such as Wittig olefination.
and was found to bridge the two borate units [B2 O1
À
1.554(4) ꢀ, O1 B1 1.601(4) ꢀ, B1-O1-B2 129.6(2)8]. The
newly formed seven-membered ring adopted a typical cyclo-
heptane-like boat conformation with the newly formed [B]-
CH2-[P] group at the tip [C8-B2-O1-B1 35.9(3)8, C8-P1-C2-
C3 À40.5(2)8; Figure 1]. In solution, compound 4 showed 1H/
B
urg and Schlesinger had shown in 1937 that diborane
(B2H6) reacted with carbon monoxide to give borane carbonyl
(H3BCO),[1] a low-boiling liquid that reversibly dissociated at
a low CO partial pressure. Even under forcing conditions,
borane carbonyl did not react further to give formylborane as
a result of its unfavorable thermodynamics (Scheme 1).[2–7] In
Scheme 1. Reactions of BH boranes with carbon monoxide. Mes=me-
sityl (2,4,6-trimethylphenyl).
Figure 1. A view of the molecular structure of compound 4 (thermal
ellipsoids are shown with 30% probability).[26]
À
general, the carbonylation of free [B] H-containing boranes
seems not to lead to the formation of the respective borane
carbaldehydes because of the endothermicity of this reac-
tion.[2–7] We recently prepared a small series of annulated
formylborane-like compounds[8] by the treatment of CO with
HB(C6F5)2 at frustrated Lewis pair (FLP) templates.[9–12] We
have now carried out reactions of these FLP-stabilized
formylborane derivatives with some remarkable outcomes.
13
À
À
C NMR signals for the [B] CH2 [P] methylene group at d =
3.25, 2.75/19.2 ppm (31P: d = 33.2 ppm, 11B: d = 3.2 and
0.5 ppm). It showed 19F NMR signals for the four diastereo-
topic C6F5 groups and an OH 1H NMR resonance at d =
5.78 ppm (for details, see the Supporting Information).
[8]
À
The B1 O1 bond in the starting material 3a is very long.
Therefore, we assume (endothermic) equilibration of 3a with
its open form 5a, which then serves as a reactive boron/
oxygen FLP[14] to activate dihydrogen with formation of the
intermediate 6a. Intramolecular hydride attack would then
readily open the adjacent three-membered ring to eventually
yield the product 4 (Scheme 2).
This description of the reaction is supported by the
reaction of the formylborane FLP adduct 3b with pyridine
derivatives. Compound 3b was obtained analogously to 3a by
[*] Dr. M. Sajid, Dr. G. Kehr, Dr. C. G. Daniliuc,[+] Prof. Dr. G. Erker
Organisch-Chemisches Institut der Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: erker@uni-muenster.de
[+] X-ray crystal-structure analysis.
[**] Financial support from the European Research Council is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307551.
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Scheme 2. Reaction of compound 3a with dihydrogen.
Scheme 4. Liberation of a formylborane from compound 3a.
Supporting Information). Compound 9 was isolated by
crystallization and characterized by X-ray diffraction, C,H,N
elemental analysis, and spectroscopy.
X-ray crystal-structure analysis showed that the unique
À
formylborane (C6F5)2B CHO had been liberated in this
reaction and that we had isolated it as its pyridine adduct
(Figure 3 and Scheme 4). The boron atom in compound 9 is
tetracoordinated. It has bonded to it a pair of C6F5 groups
Scheme 3. Reaction of compound 3b with pyridines.
facile hydroboration of carbon monoxide with the Piers
borane, HB(C6F5)2,[15] at the FLP template 2b (Scheme 3).[8]
The treatment of 3b with, for example, 4-methylpyridine in
CH2Cl2 at room temperature instantly resulted in selective
À
cleavage of the connecting B1 O1 bond to give the product of
pyridine addition 7, the open “h2-formylborane” phosphane
adduct. X-ray crystal-structure analysis of compound 7b
showed that the three-membered substructure was still intact
À
À
À
(O1 C3 1.439(3) ꢀ, O1 B2 1.465(4) ꢀ, B2 C3 1.598(4) ꢀ,
À
P1 C3 1.789(3) ꢀ; Figure 2 and Scheme 3) and that the
Figure 3. A projection of the molecular structure of the formylborane
product 9 (thermal ellipsoids are shown with 30% probability).[26]
À
À
[B1 C11 1.646(3) ꢀ, B1 C21 1.649(3) ꢀ, C11-B1-C21
À
113.9(2)8], the pyridine donor ligand [B1 N31 1.610(3) ꢀ],
and the formyl group.[7,16] Compound 9 features a B1 C1
À
À
bond length of 1.649(3) ꢀ, which is in the typical range for B
2
À
C(sp ) single bonds. The C1 O1 bond length [1.210(2) ꢀ] is
short, and the B1-C1-O1 angle is 126.3(2)8. In solution
(CD2Cl2), compound 9 showed typical 1H/13C NMR aldehyde
signals at d = 11.24/233.1 ppm. The 11B NMR resonance
occurred at d = À5.1 ppm, and we observed a single set of
19F NMR signals for the pair of symmetry-equivalent C6F5
substituents with the expected chemical shift difference of
Dd(19Fm,p) = 7.4 ppm.
Figure 2. Molecular structure of compound 7b (thermal ellipsoids are
shown with 30% probability).[26]
The proposed pathway of the favored FLP-assisted CO-
reduction/hydroboration[8] was strongly supported by the
outcome of two additional experiments. We exposed the
Piers borane, HB(C6F5)2, to carbon monoxide under carefully
selected reaction conditions (for details, see the Supporting
Information) and were indeed able to isolate the borane
methylpyridine donor was attached to the pendent borane
À
functionality (B1 N51 1.638(4) ꢀ).
This result prompted us to treat the FLP–CO/HB(C6F5)2
reduction product 3a[8] with excess pyridine. Compound 3a
reacted rapidly with pyridine (2.4 equiv) in CH2Cl2 to give the
products 8 and 9. Product 8 was characterized as the pyridine
adduct of the free FLP 2a (Scheme 4; for details, see the
carbonyl (C6F5)2B(H)CO (10; Scheme 5). Compound 10
13
À ꢀ
showed a C NMR [B] C O resonance at d = 169.2 ppm
(223 K) and a 11B NMR signal at d = À30.6 ppm (d, JBH
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Scheme 5. Formation and reactions of the borane carbonyl 10.
Scheme 6. Some reactions of the formylborane 9.
ꢁ 95 Hz; 253 K).[17] Single crystals of compound 10 were
obtained from a solution in CH2Cl2 under a CO atmosphere
(2.5 bar) at À408C. X-ray crystal-structure analysis of com-
pound 10 showed a tetracoordinated boron atom with
a pseudotetrahedral geometry [sum of the C-B-C angles:
Schwarz reagent [(Cp2Zr(H)Cl] to give 13. Finally, compound
9 was employed in a typical carbon–carbon bond-forming
reaction of carbonyl compounds: Treatment of 9 with the
=
phosphorous ylide Ph3P CH2 gave the Wittig olefination
À
À
331.78, B C11 1.616(2) ꢀ, B C21 1.609(2) ꢀ]. The linear
product 14, which was isolated in 66% yield after chromato-
1
À ꢀ
À
=
[B] C O unit [B1-C1-O1 174.7(2)8] showed a B1 C1 bond
graphic workup [ H NMR: d = 6.84, 5.66, 4.97 (-CH CH2);
length of 1.601(2) ꢀ. The carbonyl C1 O1 bond is short at
C NMR: d = 145.8, 124.6 ppm (-CH CH2); 11B NMR: d =
13
À
=
1.107(2) ꢀ (Figure 4).[18]
À2.1 ppm].
In summary, we were able to show that the unique borane
À
carbaldehyde (C6F5)2B CHO (isolated as its pyridine-stabi-
lized form 9) can readily be obtained by reduction of carbon
monoxide with the borane HB(C6F5)2 at the intramolecular
frustrated Lewis pair 2a, followed by liberation from the
template by treatment with pyridine. In this way, the
thermodynamic restrictions of CO insertion into the boron–
hydrogen bond[19–21] can be circumvented. This reaction
sequence impressively demonstrates the potential of frus-
trated Lewis pairs in small-molecule binding and activa-
tion.[11,12,22–25] We are looking forward to investigating and
developing the chemistry of borane carbaldehydes, now that
such systems can be made in a convenient straightforward
way.
Figure 4. Molecular structure of the borane carbonyl 10 (thermal
ellipsoids are shown with 30% probability).[26]
Received: August 27, 2013
Revised: November 11, 2013
Published online: December 11, 2013
We also exposed the HB(C6F5)2/CO mixture to 4-methyl-
pyridine to check whether a direct pathway could be opened
to the formylborane pyridine adduct 9 via an alleged
Keywords: aldehydes · boron · carbon monoxide ·
formylboranes · frustrated Lewis pairs
.
À
(C6F5)2B CHO formylborane intermediate. However, only
the corresponding (C6F5)2BH(pyridine) adduct 11 was
obtained. Compound 11b (R = CH3) was unequivocally
identified by X-ray diffraction (for details, see the Supporting
Information).
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It turned out that it behaved similarly to the way one would
expect for a normal aldehyde. The treatment of 9 with
HB(C6F5)2 resulted in reduction of the formyl group. After
chromatographic workup we obtained the corresponding
1
boryl methanol product 12 [60% yield; H NMR: d = 3.97
(CH2), 1.18 ppm (OH); 11B NMR: d = À1.7 ppm; Scheme 6;
for details, see the Supporting Information]. The formyl
group of compound 9 was also reduced by treatment with the
1120
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