Angewandte
Communications
Chemie
the ring-closed N-centered radical 9. Hydrogen atom abstrac-
tion by NO would then eventually form the observed CO/NO
coupling product 4. The resulting in situ formed nitroxyl
HNO would then probably decompose to give N2O and water,
which would subsequently serve to hydrolyze one equivalent
of the starting material 3 to yield 5 if it was not removed by
other means, for example, by the addition of molecular sieves.
This mechanistic scheme is supported by the results of
DFT calculations. All of the structures were optimized at the
dispersion-corrected TPSS-D3 level of theory,[24–26] with
a Gaussian AO basis set of def2-TZVP quality,[27,28] followed
by single-point energy calculations at the PW6B95-D3
level[25,26,29] with the same basis set. The conductor-like
screening model for real solvents (COSMO-RS) solvation
model[30,31] was used to compute the solvation Gibbs free
energies for toluene as the solvent (for details, see the
Supporting Information). It revealed that the initial B–O
dissociation step (i.e., opening of the six-membered ring)
leading to 7 is endergonic and the subsequent NO addition is
almost thermoneutral (Scheme 3), but ring closure to give the
heterocyclic radical intermediate 9 is strongly exergonic
(À72.6 kcalmolÀ1 in the case of two mesitylene groups on
the phosphorus atom). For the validity of this reaction
mechanism, it is significant that the next step, the H-atom
abstraction by NO to eventually give the diamagnetic CO/NO
coupling product 4 and nitroxyl (HNO), is again strongly
exergonic, with a calculated reaction energy of À37 kcalmolÀ1
with -PMes2. In short, the formation of product 4, which
contains coupled CO and NO molecules, is strongly favored
Figure 3. X-ray crystal structure analysis of the h2-formylborane P/B
FLP adduct 3b. Thermal ellipsoids are shown at 50%. Selected bond
À
À
À
lengths (ꢂ) and angles (8): C3 P1 1.835(2), C3 O1 1.462(2), O1 B1
1.571(2), C3 B2 1.603(2), O1 B2 1.528(2), SC3POB =292.2,
SO1BCB =359.8.[35]
À
À
scopic characterization of 4b). In the presence of the 1,4-
cyclohexadiene H-atom donor, the reaction of 3b with NO
took a different course: in this case the formaldehyde P/B
FLP adduct 5b was formed at the expense of the CO/NO
coupling product 4b. We isolated compound 5b from the
reaction mixture in 60% yield after crystallization (see the
Supporting Information for characterization).
=
We assume that the CO/NO coupling sequence is initiated
by the ring closure, in which a central C N double bond and
À
À
by reversible dissociation of the B O bond in the six-
a B O bond are formed, and the H-atom abstraction, which is
accomplished by a NO molecule.
membered ring of the starting material 3 (Scheme 3). This is
supported by trapping of the resulting opened intermediate 7
by various donors.[23] We assume that NO can add to the
intermediate 7 to form 8. It is likely that the P-formylborane
moiety in 8 has a reactivity that can be illustrated by an ylidic
resonance form (or even an equilibrating ylidic isomer).[23a]
Since the [B]-NO intermediate probably has electrophilic
Formally, our overall reaction could be regarded as
involving disproportionation of two NO equivalents to NO+
and (singlet or triplet) NOÀ,[32] with the former being used to
form the CO/NO coupling product 4 by reaction with the 3/7
system followed by loss of a proton, and the later generating
HNO by protonolysis, although in reality our reaction very
likely follows a completely different mechanistic pathway
involving typical radical chemistry. However, our study shows
that NO/HNO chemistry, which is so important physiologi-
cally,[33,34] can be performed in a completely artificial environ-
ment and can serve to couple the essential two-atom-
containing main-group-element oxides carbon monoxide
and nitrogen monoxide .
À
character at nitrogen (similar to HNO), subsequent C N
bond formation can occur by nucleophilic attack to generate
Experimental Section
Preparation of compound 4a: A mixture of compound 3a (320.0 mg,
0.315 mmol), molecular sieves (3 ꢁ, 630 mg), and toluene (20 mL)
was degassed by freeze-pump-thaw cycles (ꢂ 2). Then the cooled
(08C) reaction mixture was carefully evacuated and exposed to NO
gas (1.5 bar). After the reaction mixture was stirred at room
temperature for 2 hours, it was filtered and the separated molecular
sieves were washed with CH2Cl2 (2 ꢂ 5 mL). The solvent of the filtrate
was then removed under reduced pressure, the resulting residue was
washed with n-pentane (2 ꢂ 10 mL) and dried under reduced pressure.
The obtained solid material was crystallized from CH2Cl2 (1 mL) and
n-pentane (5 mL) to give compound 4a as white powder (180.1 mg,
0.172 mmol, 55%).
Scheme 3. Possible reaction scheme for the CO/NO coupling reaction
with computed Gibbs energies {kcalmolÀ1} at the PW6B95-D3/def2-
TZVP(COSMO-RS, toluene)//TPSS-D3/def2-TZVP level of theory. The
values refer to reaction free energies for the step denoted by the arrow
(see the Supporting Information for details).
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
These are not the final page numbers!