Angewandte
Communications
Chemie
system (3.021(1) ꢀ vs. 2.8457(8) ꢀ), which we propose leads
ation, and one of the magnesium aryl complexes 4–7
to the more facile generation of transient magnesium(I)
radical intermediates, [( Nacnac)MgC], and subsequent arene
(Scheme 2). All of these reactions were completely selective
for the depicted isomers, as determined by H NMR spectro-
Ar
1
reduction. It is noteworthy that irradiation of benzene
solutions of less bulky, and less kinetically protected,
scopic analyses of the total reaction mixtures, and each of the
products could be isolated from the reaction mixtures as
Ar
magnesium(I) compounds, [{( Nacnac)Mg} ] (Ar= mesi-
colorless crystalline solids (see Figure 2 for molecular struc-
2
[21]
[22]
[24]
tyl or 2,6-xylyl ), did not lead to reduction of benzene,
but instead to disproportionation to [Mg( Nacnac) ] and
tures).
It is noteworthy that when the reactions were
Ar
1
monitored over time by H NMR spectroscopy, no inter-
mediates were observed in the formation of the magnesium
xylyl complexes, though in the first few hours of the reaction
that gave the magnesium tolyl, 4, resonances corresponding to
that compound and a reaction intermediate were present. The
signals for the intermediate remained until all of the
magnesium(I) starting material was consumed (after ca.
14 h), then decreased in intensity at the expense of signals
for the final product 4. It is believed the intermediate is
a “Birch-like” reduction product similar to 1–3, which is
further evidenced by the results of calculations (see below).
Quenching of 4–7 with I2 gave mono(iodo)-arenes with
complete selectivity for the formation of the regio-isomer
that corresponds to that of the magnesium aryl starting
material.
2
[
23]
magnesium metal. Moreover, heating solutions of 1 at 608C
for 12 h led to clean conversion of the compound to
magnesium phenyl and hydride products in a similar fashion
to the higher temperature thermolysis of bulkier 2.
The NMR spectroscopic data for orange micro-crystalline
[15]
[
15]
1
are identical to those reported by Harder and co-workers,
who prepared the compound in low yield, as part of a product
mixture, as described above. Similar to that compound, and 2,
1
the solution state H NMR spectrum of 3 displays a singlet
resonance at d = 3.82 ppm (cf. d = 3.75 ppm for 1; d =
[
15]
2À
6
3
.78 ppm for 2 ) for the [C H ] dianion. This indicates
6
a fluxional process involving rapid MgÀC bond breaking and
re-forming on the NMR timescale. The X-ray crystal structure
of orange 3 was determined, and its molecular structure is
depicted in Figure 1. This shows it to be closely related to the
cyclohexadienediyl bridged structure of 2.
The very high yielding nature and regioselectivity of the
reactions that gave 4–7 is remarkable. These outcomes can be
I
I
Given the facility by which benzene could be reduced by
compared to the Al and Mg induced arene CÀH activations
photo-activated magnesium(I) compounds, attention turned
mentioned in the introduction. Those reactions typically
require forcing conditions or transition metal catalysts to
proceed, they often give rise to two or more isomers, and
these isomers can include benzylic products arising from
competing CÀH activations of toluene or xylene methyl
Dip
to irradiation of solutions of [{( Nacnac)Mg} ] in toluene, or
2
one of the three isomers of xylene. Blue light was chosen for
these irradiations, so as to allow the reactions to be carried out
on preparative scales in glass walled Schlenk flasks. With that
Dip
said, UV irradiation of arene solutions of [{( Nacnac)Mg}2]
groups. In the current study, the metallations of toluene and
meta-xylene were found to occur selectively at a C-position
meta to a methyl group (yielding 4 and 6), as is often the case
in thin walled 5 mm NMR tubes gave similar reaction
outcomes. That is, the blue light irradiations (24–96 h at
I
2
08C) led to the near quantitative formal CÀH activations of
with the major isomer of product mixtures resulting from Al
II [5f]
[7–9]
the arenes, to give ca. 50:50 mixtures of the magnesium(II)
(and Mg
) induced arene metallations.
Although the
Dip
hydride, [{( Nacnac)Mg(m-H)} ], which is stable to irradi-
formation of 5 also involves metallation at a site meta to one
methyl group, there are no instances of a main group reagent
selectively metallating ortho-xylene to give a 1-metallo-2,3-
dimethylphenyl product (as is 5). That being the case,
2
0
Crimmin and co-workers have reported that a Pd catalyzed
alumination of ortho-xylene yields the ortho-aluminated
[
7a]
product as the minor isomer.
Some initial investigations of the chemoselectivity and
functional group tolerance of photoactivated arene magne-
siations have been carried out using fluorobenzene, anisole
and N,N’-dimethylaminoaniline as
magnesium(I) compound [{( Nacnac)Mg} ] does not react
substrates. The
Dip
2
[25]
with any of these in solution at room temperature.
However, blue light irradiation of solution of
{( Nacnac)Mg} ] in fluorobenzene led to the chemo-selec-
a
Dip
[
2
tive activation of its strong, and normally inert, CÀF bond,
leading to the essentially quantitative formation of biphenyl
Dip
and the known compound, [{( Nacnac)Mg(m-F)} ], after
2
6
hours. Computational studies suggest this reaction involves
Figure 1. Molecular structure of 3 (25% thermal ellipsoids; hydrogen
a radical defluorination mechanism (see SI for further
details). Reactions of photoactivated [{( Nacnac)Mg} ] in
the presence of anisole or N,N’-dimethylaminoaniline were
atoms omitted; TCHP substituents shown as wire-frame for sake of
Dip
[
28]
2
clarity). Selected bond lengths [ꢁ] and angles [8]: Mg1–C6 2.360(3),
Mg1–C3 2.373(3), Mg2–C1 2.391(3), Mg2–C2 2.553(3), Mg2–C4 2.391-
not as clean as those involving fluorobenezne, and likewise,
(
7
3), Mg2–C5 2.622(3), C1–C2 1.386(4), C4–C5 1.380(4); C6-Mg1-C3
0.31(9), C1-Mg2-C4 69.78(9), C2-Mg2-C5 65.24(9).
did not yield any CÀH activated products. Instead C ÀO or
Ph
Angew. Chem. Int. Ed. 2021, 60, 7087 –7092
ꢀ 2021 Wiley-VCH GmbH
Jones, Dafydd D. L.
