Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives

Article abstract of DOI:10.1039/c1dt11235j

Reactions of the β-diketiminato n-butyl magnesium complex, [HC{(Me)CN(2,6-ⁱPr₂C₆H₃)} ₂MgⁿBu], with a range of substituted pyridines and fused-ring quinolines in the presence of PhSiH₃ has been found to result in dearomatisation of the N-heterocyclic compounds. This reaction is proposed to occur through the formation of an unobserved N-heterocycle- coordinated magnesium hydride and subsequent hydride transfer via the C2-position of the heterocycle prior to hydride transfer to the C4-position and formation of thermodynamically-favoured magnesium 1,4-dihydropyridides. This reaction is kinetically suppressed for 2,6-dimethylpyridine while the kinetic product, the 1,2-dihydropyridide derivative, was isolated through reaction with 4-methylpyridine (4-methylpyridine), in which case the formation of the 1,4-dihyropyridide is prevented by the presence of the 4-methyl substituent. X-ray structures of the products of these reactions with 4-methylpyridine, 3,5-dimethylpyridine and iso-quinoline comprise a pseudo-tetrahedral magnesium centre while the regiochemistry of the particular dearomatisation reaction is determined by the substitution pattern of the N-heterocycle under observation. The compounds are all air-sensitive and exposure of the magnesium derivatives of dearomatised pyridine and 4-dimethylaminopyridine (DMAP) to air resulted in ligand rearomatisation and the formation of dimeric μ²- η²-η²-peroxomagnesium compounds which have also been subject to analysis by single crystal X-ray diffraction analysis. An unsuccessful extension of this chemistry to N-heterocycle hydrosilylation is suggested to be a consequence of the low basicity of the silane reagent in comparison to the pyridine substrates which effectively impedes any further interaction with the magnesium centres. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt11235j

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PAPER
Magnesium hydrides and the dearomatisation of pyridine and quinoline
derivatives†
Michael S. Hill,* Gabriele Kociok-Ko¨hn, Dugald J. MacDougall, Mary F. Mahon and Catherine Weetman
Received 28th June 2011, Accepted 31st August 2011
DOI: 10.1039/c1dt11235j
Reactions of the b-diketiminato n-butyl magnesium complex, [HC{(Me)CN(2,6-iPr2C6H3)}2MgnBu],
with a range of substituted pyridines and fused-ring quinolines in the presence of PhSiH3 has been
found to result in dearomatisation of the N-heterocyclic compounds. This reaction is proposed to occur
through the formation of an unobserved N-heterocycle-coordinated magnesium hydride and
subsequent hydride transfer via the C2-position of the heterocycle prior to hydride transfer to the
C4-position and formation of thermodynamically-favoured magnesium 1,4-dihydropyridides. This
reaction is kinetically suppressed for 2,6-dimethylpyridine while the kinetic product, the
1,2-dihydropyridide derivative, was isolated through reaction with 4-methylpyridine (4-methylpyridine),
in which case the formation of the 1,4-dihyropyridide is prevented by the presence of the 4-methyl
substituent. X-ray structures of the products of these reactions with 4-methylpyridine,
3,5-dimethylpyridine and iso-quinoline comprise a pseudo-tetrahedral magnesium centre while the
regiochemistry of the particular dearomatisation reaction is determined by the substitution pattern of
the N-heterocycle under observation. The compounds are all air-sensitive and exposure of the
magnesium derivatives of dearomatised pyridine and 4-dimethylaminopyridine (DMAP) to air resulted
2
2
2
in ligand rearomatisation and the formation of dimeric m -h -h -peroxomagnesium compounds which
have also been subject to analysis by single crystal X-ray diffraction analysis. An unsuccessful extension
of this chemistry to N-heterocycle hydrosilylation is suggested to be a consequence of the low basicity
of the silane reagent in comparison to the pyridine substrates which effectively impedes any further
interaction with the magnesium centres.
precedented by several heterogeneous processes,4 the initial report
Introduction
of homogeneous pyridine hydrosilylation was only described in
1998. In this case a [Cp2TiMe2]/PhMeSiH2 system was reported
to effect the transformation of a variety of 3- and 4-substituted
and 3,5-disubstituted pyridines to the N-silylated 1,2- and 1,4-
dihydropyridines or, dependent upon substitution patterns, the
corresponding tetrahydropyridine products.5 This process was
reasoned to proceed via a Ti–H pyridine coordination/insertion
and silane metathesis mechanism (Scheme 1) and an inability
to dearomatise pyridines with 2- or 6-substitution patterns was
attributed to the substrate steric demands. Although similar
limitations have been observed in very recent work described
by Nikonov and co-workers, in this case the catalytic ability
of the cationic ruthenium complex [Cp(iPr3P)Ru(NCCH3)]PF6
to effect hydrosilylation of 3- and 5-substituted pyridines to
the dihydropyridines was not compromised by the concomitant
reduction of these products to tetrahydropyridines.6 In further
related work, Crabtree and co-workers have developed a reduction
of quinolines by silanes in the presence of rhodium or iridium
catalysts7 and a very recent report has described an extension of
this chemistry to effect a palladium-catalysed silaboration of a
variety of pyridines.8
While recent interest in the dearomatisation and hydrogenation of
aromatic nitrogen heterocycles (pyridines, quinolines) has been
driven by the prevalence of these functions in many natural
products and pharmaceuticals,1 a number of reductive and group
1 and group 13-based dearomatisation strategies have been known
for some time.2 Lansbury’s reagent, [Li{Al(1,4-dihydropyrid-1-
yl)4], for example, is generated by reaction of LiAlH4 and pyridine
and has been known to effect the selective reduction of ketones in
the presence of carboxylic and ester groups for some fifty years.3
Applications of these types of reagents in further syntheses where
the reduced N-heterocyclic core is retained are, however, relatively
limited. More attractive, therefore, is the development of milder,
more conveniently employed and, ideally, catalytic reagents suit-
able for the conversion of a wide range of N-heterocyclic substrates
to isolable dearomatised derivatives which are then available for
use as synthons in subsequent multi-step syntheses. Although
Department of Chemistry, University of Bath, Claverton Down, Bath, BA2
7AY, UK. E-mail: msh27@bath.ac.uk
† CCDC reference numbers 831736–831741. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt11235j
12500 | Dalton Trans., 2011, 40, 12500–12509
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well-defined b-diketiminato-supported n-butylmagnesium com-
plex, [HC{(Me)CN(2,6-iPr2C6H3)}2MgnBu], I, with pyridine in
the presence of PhSiH3, results in heterocycle dearomatisation by
a hydride transfer which occurs with the formation of isolable
magnesium compounds containing 1,2- and 1,4-dihydropyridide
anions as the respective kinetic and thermodynamic products.13
In this contribution we describe extensions of this behaviour to a
range of substituted pyridine and quinoline substrates and outline
attempts to establish a catalytic variant of this reactivity which is
reminiscent of the titanocene-based system illustrated in Scheme 1.
Scheme 1 Proposed mechanism for the titanocene-catalysed hydrosilyla-
tion of pyridine.5
Our own interest in this area derives from our attempts to
develop a catalytic and stoichiometric reaction chemistry for
the abundant and environmentally benign group 2 elements
heavier than beryllium that takes them beyond their traditional
applications as, for example, Grignard reagents or Hauser bases.9
Much of this initial work has explored the reactivity of the
redox-inactive divalent cations of these elements by analogy with
chemical behaviour previously observed for similar d0 complexes
of early transition elements or lanthanides in their highest and
trivalent oxidation states respectively. In this regard, Diaconescu
and co-workers have recently shown that both N-heterocycle
dearomatisation and C–H activation may be combined into an
elegant group 3-centred scheme which results in C–C coupling
reactivity between the two relevant heterocycles (Scheme 2).10
In earlier work Kiplinger and co-workers had also shown that
dearomatisation of terpyridines may be readily induced through
reactions with lanthanide alkyls.11
Results and discussion
We have previously reported that addition of pyridine to a toluene
solution of compound I results in the rapid and clean formation of
the n-butylmagnesium pyridine adduct, compound 1 (Scheme 3).13
A series of NMR scale reactions demonstrated that this process
occurs for every monocyclic and fused ring pyridine investigated
in this study and no evidence for magnesium n-butyl-induced
substrate dearomatisation was observed even under forcing con-
ditions. This observation contrasts markedly with those of Okuda
of the reactivity of pyridine with more reactive calcium bis-allyl
species and our own observations of the reactivity of heavier group
2 bis(trimethylsilyl)methanides, [M{CH(SiMe3)2}2(THF)2] (M =
Ca, Sr, Ba) with bis(imino)pyrines in which case rapid formation
of the respective allylated and alkylated pyridine was observed
upon combination of the reagents.14,15 As the reactivity with
compound I was assessed to be straightforward and unambiguous,
only the reaction utilising one equivalent of 2-methylpyridine
was repeated on a preparative scale resulting in the isolation of
pale yellow crystals of compound 2 which were suitable for X-
ray diffraction analysis. The single crystal X-ray structure of the
Mg(2)-containing molecule (there are two unique molecules in
the unit cell) of compound 2 is illustrated in Fig. 1, while selected
bond length and angle data and details of the crystallographic
analysis are provided in Tables 1, 2 and 3, respectively. Like
compound 1, compound 2 is a four-coordinate pseudo-tetrahedral
magnesium complex in which the coordination sphere of the Mg
atoms is provided by the bidentate b-diketiminate ligand and
the n-butyl and 2-methylpyridine ligands. The bond lengths and
angles are perturbed only very slightly from those observed in
compound 1 and the structure is otherwise unremarkable and will
not, therefore, be discussed further here.13
Scheme
2 Group 3-catalysed pyridine ortho-CH activation and
dearomatisation/coupling.10
Following the procedure of Jones and Stasch for magnesium
hydride generation from compound I,16 we have previously
reported that addition of PhSiH3 to a solution of compound 1 or to
I in the presence of pyridine results in the formation of magnesium
compounds containing 1,2- and 1,4-dihydropyridide (identified as
compound 3 in this submission) anions as the respective kinetic
and thermodynamic products. Accordingly, a series of preparative
scale reactions were undertaken through addition of two molar
equivalents of the relevant pyridine or quinoline derivative to
a toluene solution of compound I. For monocyclic pyridine
derivatives this gave a yellowing of the solution, whereas for
bicyclic quinoline derivatives a red solution formed. A further
molar equivalent of phenylsilane was then added and the reactions
heated at 60 ◦C for 48 h. Crystalline samples of compounds 4–10
were then isolated by slow cooling and crystallisation from the
saturated reaction solutions (Schemes 3 and 4).
Although a somewhat ‘lanthanide mimetic’ reactivity has
emerged from our studies of the group 2-catalysed hydroele-
mentation of C E multiple bonds (E = CR2, NR, O), and
examples of N-heterocycle dearomatisation are well precedented
in the chemistry of groups 1 and 13,2,3 examples of well-defined
behaviour comparable to the second reaction step depicted in
Scheme 2 are relatively unexplored for group 2-based systems.
On the basis of a series of detailed NMR spectroscopic in-
vestigations, van der Kerk and Budzelaar deduced that the
outcome of the room temperature reaction between MgH2 and
pyridine was exclusively the magnesium bis(1,4-dihydropyridide)
isomer, which formed via the intermediacy of short-lived but
observable 1,2-dihydropyridide species.12 In support of these
observations we have recently reported that the reaction of the
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˚
Table 1 Selected bond lengths (A) for compounds 2, 6, 7, 10, 11 and 12
2
6
7
10
11
12
Mg(1)–N(1)
Mg(1)–N(2)
Mg(1)–N(3)
Mg(1)–N(4)
N(3)–C(30)
N(3)–C(34)
C(30)–C(31)
C(31)–C(32)
C(32)–C(33)
C(33)–C(34)
2.074(2)a
2.04793)
2.044(2)
1.993(3)
2.044(2)
1.418(5)
1.401(4)
1.341(5)
1.439(6)
1.426(6)
1.308(5)
2.0521(12)
2.0555(12)
1.9827(13)
2.1282(13)
1.394(2)
1.386(2)
1.334(2)
1.503(2)
1.504(2)
1.340(2)
2.0289(15)
2.0347(15)
1.9926(16)
2.1301(15)
1.468(3)
1.356(3)
1.502(3)
1.422(4)
1.431(4)
1.366(3)
2.1090(17)
2.1073(18)
2.1966(19)
1.9716f
1.9583(12)g
—
—
—
—
—
2.141(3)
2.112(3)
2.160(3)
1.969(3)f
1.972(3)g
—
—
—
—
—
2.075(2)b
2.216(4)c
2.144(5)d,e
—
—
—
—
—
—
a Mg(2)–N(4) 2.073(2); b Mg(2)–N(5) 2.081(2); c Mg(2)–N(6) 2.2058; d Mg(1)–C(30); e Mg(2)–C(69) 2.118(3); f Mg(1)–O(1); g Mg(1)–O(1)¢.
Table 2 Selected bond angles (◦) for compounds 2, 6, 7, 10, 11 and 12
2
6
7
10
11
12
N(1)–Mg(2)–N(2)
N(1)–Mg(1)–N(3)
N(1)–Mg(1)–N(4)
N(2)–Mg(1)–N(3)
N(2)–Mg(1)–N(4)
N(3)– Mg(1)–N(4)
Mg(1)–N(3)–C(30)
Mg(1)–N(3)–C(34)
C(30)–N(3)–C(34)
91.32(9)
109.94(17)
114.40(17)a
107.35(17)
128.91(19)b
—
—
—
—
92.36(10)
125.65(11)
107.60(10)
118.74(12)
106.94(10)
103.30(12)
122.6(2)
93.55(5)
95.89(6)
89.00(7)
100.40(7)
107.21(5)c
148.55(5)d
151.54(7)e
106.19(6)f
48.8g
88.70(11)
122.64(5)
107.33(5)
124.93(6)
101.06(5)
104.75(6)
126.46(10)
119.85(10)
112.89(12)
124.10(7)
103.52(6)
120.59(7)
106.00(6)
104.6896)
121.98(13)
121.19(13)
112.17(17)
97.94(12)
107.00(11)c
149.27(13)d
149.44(13)e
105.44(11)f
49.14(13)g
126.9(2)
109.6(3)
—
—
a N(1)–Mg(1)–C(30); b N(2)–Mg(1)–C(30); c N(1)–Mg(1)–O(1); d N(2)–Mg(1)–O(1); e N(1)–Mg(1)–O(1)¢; f N(2)–Mg(1)–O(1)¢; g O(1)–Mg(1)–O(1)
Table 3 Crystallographic data for compounds 2, 6, 7, 10, 11 and 12
2
6
7
10
11
12
Molecular formula
Formula weight (g mol-1)
Crystal system
C39H57LiMgN3
592.19
Monoclinic
P21/c
15.8710(2)
24.1760(4)
19.3490(3)
90
98.312(1)
90
C41H56MgN4
629.21
Monoclinic
C2/c
35.9897(12)
15.3126(3)
14.6944(5)
90
105.416(1)
90
C43H60MgN4
657.26
Orthorhombic
P212121
10.5988(1)
18.1105(2)
21.0057(2)
90
C47H56MgN4
701.27
Monoclinic
P2/a
18.3245(2)
10.8579(1)
22.0748(3)
90
108.966(1)
90
C71.5H96Mg2N6O2
1120.16
Monoclinic
P21/n
13.0076(2)
14.3714(2)
18.6803(3)
90
94.299(1)
90
C72H102Mg2N8O2
1160.24
Monoclinic
P21/n
14.1320(7)
13.7311(5)
17.9409(11)
90
95.414(3)
90
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
90
90
3
˚
V (A )
7346.17(19)
8
0.077
7806.7(4)
8
0.077
4032.04(7)
4
0.077
4153.68(8)
4
0.079
3482.22(9)
2
0.080
3465.9(3)
2
0.083
Z
m (mm-1)
r (g cm-3)
1.071
1.071
1.083
1.121
1.068
1.112
q range (◦)
4.27 to 27.48
0.0663, 0.1442
0.1506, 0.1919
133599/16753/
0.1155
4.04 to 25.02
0.0663, 0.1561
0.1199, 0.1913
55881/6839/
0.0882
3.54 to 27.52
0.0384, 0.0980
0.0426, 0.1014
55112/9226/
0.0481
3.98 to 27.48
0.0523, 0.1217
0.0812, 0.1403
61721/9439/
0.0599
3.53 to 27.48
0.0723, 0.2008
0.0909, 0.2128
55826/7938/
0.0477
4.68 to 25.15
0.0845, 0.2027
0.1291, 0.2394
24351/6008/
0.1851
R1, wR2 [I > 2s(I)]
R1, wR2 (all data)
Measured/independent
reflections/Rint
The products of 2-methylpyridine (4), 3-methylpyridine (5)
and 3,5-lutidine, (7) dearomatisation under these conditions were
identified as the respective 1,4-dihydropyridide derivatives by a
6.3 ppm and meta-protons shifted to between d 4.2 to d 5.0 ppm.
In contrast to this behaviour, no reaction was observed with
2,6-lutidine, while 4-methylpyridine was converted selectively to
the 1,2-dihydropyridide derivative, compound 6. Consistent with
our previous proposal, we interpret these latter observations to
indicate that hydride transfer generally occurs via an initially
formed 1,2-dihydropyridide derivative. In the case of 2,6-lutidine,
this process is completely suppressed by the presence of the two
methyl groups positioned adjacent to the lutidine nitrogen centre.
Similarly, for compound 6 the initial 1,2-dihydropyridide is formed
1
combination of H (COSY) and 13C NMR spectroscopy. In line
with the previously reported data of Budzelaar and co-workers,12a,b
in all cases the new sp3 CH2 protons were found to resonate
between d 3.30 and d 4.00 ppm, while the alkenic components
of the newly formed dihydropyridide substituent were observed
as a series of multiplets at chemical shifts upfield from the
aromatic region; ortho-protons were found within d 5.4 to d
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Scheme 3 Dearomatisation of pyridine derivatives by 1 and PhSiH3.
converted completely to the product of hydride transfer to the
4-position to form compound 8 by heating at 60 ◦C in toluene
for a further 96 h. This latter transformation contrasts markedly
with the previous observation of Jones and Stasch of the similarly
four-coordinate magnesium hydride species, [HC{(tBu)CN(2,6-
iPr2C6H3)}2Mg(H)(DMAP)] in which the hydride maintains its
integrity as a terminally-bonded ligand.16 The observation of
both potential isomers during the early stages of the reaction
implies that the ultimate conversion to the symmetrical 1,4-
dihydropyridide-containing isomer, compound, 8, again occurs
under overall thermodynamic control. This observation was borne
out by calculations performed on model complexes of compound
8, its 1,2-dihydropyridide isomer and a DMAP-coordinated
magnesium hydride species in which the complete b-diketiminate
ligand was replaced by [HC{(H)CN(Me)}]- in the interests of
computational expense. Despite this simplification, geometry
optimisations performed at the B3LYP density functional theory
with LAN2DZ pseudopotentials (and basis set) implemented in
the Gaussian03 suite of programmes provided a good approx-
imation of the local bond lengths and angles about each Mg
atom in comparison to the crystallographically deduced structures
(vide infra).17
Fig. 1 ORTEP representation (25% probability ellipsoids) of the X-ray
structure of compound 2. Hydrogen atoms and iso-propyl methyl groups
are deleted for clarity.
Consistent with the experimentally observed transformation
into compound 8, the isomer containing the 1,4-dihydropyridide
anion was calculated to be marginally lower (1.02 kcal mol-1) in
total energy than its 1,2-dihydropyridide isomer, while both species
were more significantly lower (1,2-isomer, 9.01 kcal mol-1; 1,4-
isomer 10.02 kcal mol-1) than the alternative pyridine-adducted
1
magnesium hydride formulation, [HC{(H)CN(Me)}Mg(H)-k -
NC5H4-4-NMe2].C5H4N-4-NMe2. These data reflect our previous
DFT examination of the dearomatisation of pyridine and earlier
MNDO calculations which assessed the heats of formation, DHf,
of the various potential isomers of pyridine reduction, albeit
with less thermodynamic discrimination between the alternative
isomeric forms.13,18 While the reaction to produce compound 8 is
effectively identical to that employed by Jones and Stasch to syn-
thesise [HC{(tBu)CN(2,6-iPr2C6H3)}2Mg(H)(DMAP)],16 the out-
come of the two reactions is quite different. The dearomatisation-
based syntheses reported herein generally required elevated
Scheme 4 Dearomatisation of quinoline derivatives by 1 and PhSiH3.
but can not then convert to the 1,4-dihydropyridide observed for
compounds 3, 4, 5, 7 or 8.
In contrast to the reactivity observed when a methyl group
occupies the 4-position of the pyridine substrate, treatment of
4-dimethylaminopyridine (DMAP) under analogous conditions
was found to result in the production of a mixture of the 1,2-
and 1,4-hydride transfer products in a 3 : 1 ratio, which could be
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temperatures to occur and, thus, this latter compound may again
be assigned as a kinetic reaction product, which may display
some enhanced kinetic stability resulting from the increased
steric demands of the tert-butyl-substituted b-diketiminate ligand
employed in the earlier study. Analogous calculations performed
upon model complexes containing dearomatised 4-methylpyridine
revealed a similar thermodynamic preference of 1.55 kcal mol-1
for the formation of the 1,4-dihydropyridide isomer over the
corresponding 1,2-dihydropyridide isomer. We thus tentatively
ascribe the formation of compound 6 to occur under overall kinetic
rather than thermodynamic control.
The fused ring substrates quinoline and iso-quinoline un-
derwent clean dearomatisation to form compounds 9 and 10.
In the case of compound 9, complete conversion to the 1,4-
dihydroquinolide product was observed while for the isomeric
derivative, compound 10, selective hydride transfer occurred to
the 2-carbon centre adjacent to the iso-quinoline nitrogen. In
this latter case the reaction is necessarily unambiguous as the 4-
position of the heterocyclic ring is blocked by the fused aromatic
system (Scheme 4).
Representative crystal structures of the dearomatised pyridine
derivatives were acquired after slow crystallisation from toluene
at room temperature for the 1,4-dihydropyridide derivative of
3,5-lutidine, compound 7, the 1,2-dihydropyridide derivative of
4-methylpyridine, compound 6, and the iso-quinoline derivative,
compound 10. The results of these analyses are presented in Fig. 2–
4, while selected bond length and angle data and details of the X-
ray experiments are provided in Tables 1, 2 and 3, respectively. In
a manner similar to the previously described 1,2-dihydropyridide
complex of pyridine itself,13 the methylene hydrogen atoms were
disordered between the 2-carbon and/or 6-carbon centres (C30
and C34) of the reduced methylpyridine moiety of compound
6. These were, thus, omitted from the refinement because of the
inability to determine precisely which of these best represents a
CH2 moiety.
Fig. 3 ORTEP representation (25% probability ellipsoids) of the X-ray
structure of compound 6. Hydrogen atoms and iso-propyl methyl groups
are deleted for clarity.
Fig. 4 ORTEP representation (25% probability ellipsoids) of the X-ray
structure of compound 10. Hydrogen atoms except those attached to C(30)
and iso-propyl methyl groups are deleted for clarity.
Each of the three structures feature a pseudo-tetrahedral mag-
nesium atom in which the coordination sphere is provided by
the bidentate b-diketiminate ligand and single equivalents of the
relevant dearomatised and non-dearomatised pyridine derivative.
While the magnesium-bound dihydropyridide N(3) centres are
not significantly pyramidalised [Rbond angles = 6: 359.1◦; 7: 359.2◦;
10: 355.3◦], the pyridine and dihydropyridide donors are readily
discriminated by the alternation of double and single bonds
about the deraromatised ligand as well as the non-planarity of
the heterocycle introduced upon incorporation of the saturated
methylene carbon centre. The differing bonding modes (cova-
lent/electrostatic versus dative covalent) are also reflected in the
Fig. 2 ORTEP representation (25% probability ellipsoids) of the X-ray
structure of compound 7. Hydrogen atoms except those attached to C(32)
and iso-propyl methyl groups are deleted for clarity.
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respective Mg(1)–N(3) [6: 1.993(3); 7: 1.9827(13); 10: 1.9926(16)
˚
˚
A] and Mg(1)–N(4) [6: 2.140(3); 7: 2.1282(13); 10: 2.1301(15) A]
distances linking the Mg centre to the two heterocycles with the
latter distances effectively identical to the Mg–NDMAP distance
˚
[2.135(3) A] within the similarly four-coordinate magnesium
hydride species, [HC{(tBu)CN(2,6-iPr2C6H3)}2Mg(H)(DMAP)],
in which the hydride maintains its integrity as a terminally-bonded
ligand.16
The aluminate derivative, Lansbury’s reagent,3 and previously
reported dihydropyridide derivatives of magnesium and zinc have
been reported as air-sensitive and to behave as selective reducing
agents for a variety of organic substrates.12,19 Although none of
this latter reactivity will be reported in this submission, all of
the compounds 3–10 were found to display a marked sensitivity
toward molecular and/or atmospheric oxygen. Exposure of reac-
tion solutions or solutions of the isolated compounds to the open
atmosphere was observed to result in rapid decolourisation of the
distinctive (red or orange) colours attributed to the formation of
the dearomatised pyridine species. In two cases, the previously
reported 1,4-dihydropyridide derivative, 3, and the product of
DMAP dearomatisation, compound 8, serendipitous reactions
with atmospheric O2 were found to result in the formation of
isolable quantities of the well-defined dimeric magnesium peroxide
species, compounds 11 and 12. The compounds were characterised
Fig. 6 ORTEP representation (25% probability ellipsoids) of the X-ray
structure of compound 12. Hydrogen atoms and iso-propyl methyl
groups are deleted for clarity. Symmetry transformations used to generate
equivalent atoms: #1 -x+2,-y,-z.
1
by H and 13C NMR spectroscopy and, in the case of 11, by
CHN microanalysis. X-ray diffraction analyses were performed
on both compounds after crystallisation from saturated solutions
in toluene at room temperature. The molecular structures of
compounds 11 and 12 are shown in Fig. 5 and 6, while selected
bond length and angle data and details of the X-ray analyses are
provided in Tables 1, 2 and 3, respectively. The structures of both
compounds were found to be centrosymmetric and to contain
five-coordinate Mg centres in which the coordination sphere of
each group 2 atom is provided by a single bidentate b-diketiminate
ligand, a molecule of the relevant pyridine donor (11, pyridine; 12,
DMAP) and a side-on m-h -h -O2 peroxide dianion which bridges
symmetrically between the two magnesium atoms.
Although two peroxo-centred mixed metal magnesium/alkali
metal ‘inverse crown’ structures, [{[(Me3Si)2N]4M2Mg2(O2)}•]
(M = K or Li), have been reported previously,20 compounds 11
and 12 appear to be the first examples of monometallic magnesium
peroxo species. The Mg–O bond lengths within the structures of
2
2
˚
˚
compounds 11 (1.9583(12), 1.9716 A) and 12 (1.969(3), 1.972(3) A)
are shorter than those observed in the potassium ‘inverse crown’
20b
˚
species (2.010(2), 2.015(1) A), while the O–O separation of
the peroxide units of the pyridine coordinated compounds (11,
˚
1.625(5); 12, 1.638(5) A) is somewhat longer than in both of the
previously reported [{[(Me3Si)2N]4M2Mg2(O2)}•] systems (M =
˚
K, 1.583; M = Li, 1.551 A). While these latter measurements
for compounds 11 and 12 are undoubtedly long, they are not
unprecedented and similar distances have been observed in several
high oxidation state early metal derivatives such as tetrafluoro(2-
21
˚
methylpyridine-N-oxide)peroxotantalate(V) (1.67(5) A) and in
22
˚
a dimeric tetraaryl porphyrin hafnium(IV) complex (1.618 A).
We are satisfied to discount the possibility that compounds
11 and 12 are m-hydroxo species through comparison with the
previously reported THF-solvated complex [HC{(Me)CN(2,6-
iPr2C6H3)}2Mg(OH)(THF)]2, which bears the same b-diketiminate
ligand.23 In this case the O–O separation is elongated to more
˚
than 2.46 A and the relevant O–Mg–O¢ bond angles are conse-
quently considerably more obtuse (76.78◦ versus 48.8◦ for 11 and
49.14(13)◦ for 12).
The possibility of
a
magnesium-catalysed hydrosilyla-
tion/dearomatisation procedure reminiscent of the titanium-
catalysed process illustrated in Scheme 1 is dependent upon
consecutive magnesium hydride-induced dearomatisation and
subsequent Si–H/Mg–N metathesis steps (Scheme 5). The vi-
ability of the first of these processes for a variety of pyridine
Fig. 5 ORTEP representation (25% probability ellipsoids) of the X-ray
structure of compound 11. Hydrogen atoms and iso-propyl methyl
groups are deleted for clarity. Symmetry transformations used to generate
equivalent atoms: #1 -x +1,-y,-z+2.
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Goss Scientific Instruments Ltd. and dried over molten potassium
before distilling under nitrogen and storing over molecular sieves.
Compounds I and 3 were synthesised by literature procedures and
all silanes and pyridine substrates were purchased from Sigma-
Aldrich.25 CHN microanalysis was performed by Mr Stephen
Boyer of London Metropolitan University. In NMR assignments
Dipp = 2,6-di-iso-propylphenyl.
Synthesis of compound 2
Compound I (0.75 g, 1.51 mmol) was dissolved in toulene (10 mL)
and a solution of 2-methylpyridine (0.14 g, 1.72 mmol) in toluene
(2 mL) was added at room temperature. The resulting pale yellow
solution was stirred for one hour. The solution was concentrated
and 2 was isolated as pale crystals after storage at -30 ◦C.
(0.67 g, 75%). 1H NMR (300 MHz, C6D6, 300 K) dH (ppm):
1.40 (24H, d, JHH = 6.9 Hz, CH(CH3)2), 1.95 (6H, s, NC(CH3),
3.45 (2H, m, CH(CH3)2), 5.21 (1H, s, NC(CH3)CH), 6.68 (2H, s,
NCHCH), 6.70 (2H, s, NC(CH3)CH), 7.07 (2H, t, JHH = 7.8 Hz,
NCHCHCH), 7.35 (6H, s, Ar–H), 9.02 (2H, s, NCH). 13C NMR
(75.5 MHz, C6D6, 300 K) dC (ppm): 168.44 (s, NCH), 159.71 (s,
NC(CH3)), 150.71 (s, NC (Ar)), 146.64 (s, NCHCHCH), 142.92
(s, p-C6H3), 138.27 (s, NC(CH3)), 125.51 (s, NC(CH3)CH), 124.15
(s, m-C6H3), 121.59 (s, NCHCH), 95.20 (s, NC(CH3)CH), 32.75
(s, CH(CH3)2), 32.31 (s, CH(CH3)2), 28.74 (s, NC(CH3)), 24.82 (s,
NC(CH3)), 24.63 (s, CH(CH3)2), 24.45 (s, CH(CH3)2). Elemental
analysis (%) for C39H57MgN3 (found); C 79.09 (78.91); H 9.72
(9.60); N 7.10 (7.00).
Scheme 5 Proposed mechanism for the magnesium-catalysed hydrosily-
lation of pyridine derivatives.
derivatives is clearly demonstrated by the isolation of compounds
3–10, while we have previously employed the latter metathesis
reactivity between [Mg{N(SiMe3)2}2] and PhSiH3 to synthesise a
high nuclearity magnesium hydride species.24 Reactions between
the pyridine derivative, 3, and the quinoline and iso-quinoline
complexes, compounds 9 and 10, with an excess of PhSiH3 were,
thus, studied on an NMR scale. Monitoring of reactions in d8-
◦
toluene performed at 60 C for a period of one week evidenced
no conversion of the pyridine starting materials, while heating
to 90 ◦C caused the b-diketiminato magnesium complexes to
begin to decompose with the liberation of the free di-imine
ligand. Similarly, attempted catalytic reactions of 10 mol% of the
magnesium n-butyl complex, I, with pyridine and PhSiH3 at 60 ◦C
provided ca. 10% consumption of the pyridine and silane reagents
and stoichiometric formation of compound 3 based upon the
starting quantity of compound I employed. It appears, therefore,
that the silane reagent employed as a potential source of hydride is
insufficiently reactive to engage in any further metathesis reactivity
with the magnesium dihydropyridide species. We believe that this
lack of reactivity is due to the low Lewis basicity of the silane in
comparison to the pyridine substrates and a consequent inability
to engage the Mg–N bond in an appropriate metathesis transition
state (inset, Scheme 5).
In summary, we have observed that reactions of in situ generated
magnesium hydrides and aromatic pyridine derivatives result
in N-heterocycle dearomatistion which generally results in the
formation of well-defined 1,4-dihydropyridide derivatives as the
thermodynamic reaction products. We are continuing to study the
reductive reactivity of these well defined species and to elaborate
their reactions with more potent hydride sources in attempts to
achieve some level of catalytic pyridine heterofunctionalisation
with this abundant and benign element and its heavier group 2
congeners. This work will be reported in subsequent publications.
General procedure for dearomatisation reactions
Compound I (1 mmol, 500 mg) was dissolved in toluene (ca.
5 mL) followed by the addition of 1 equivalent of the relevant
pyridine or quinoline derivative. The reactions were then stirred
at room temperature overnight before further equivalents of the
N-heterocycle and◦phenyl silane were added. The reactions were
then heated to 60 C for 48 h, before cooling and concentration
of the solution provided crystalline products suitable for isolation
by filtration in ca. 50–60% yield.
Compound 4 (dearomatisation of 2-methylpyridine)
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): 0.72 (6H, d, JHH
=
9 Hz, CH(CH3)2), 0.87 (6H, d, JHH = 6 Hz, CH(CH3)2), 1.50 (6H, d,
JHH = 6 Hz, CH(CH3)2), 1.76 (6H, d, JHH = 6 Hz, CH(CH3)2), 2.10
(3H, s, NC(CH3)CHCH2), 2.13 (9H, br.s, NC(CH3)), 2.98 (2H, m,
CH(CH3)2), 3.29 (2H, m, CH(CH3)2), 4.05 (2H, s, NCHCHCH2),
4.59 (1H, m, NC(CH3)CHCH2), 4.74 (1H, m, NCHCHCH2), 5.44
(1H, s, (NC(CH3))2CH), 5.82 (1H, d, JHH = 6 Hz, NCHCHCH2),
6.79 (2H, m, m-Pyr), 7.16 (1H, m, p-Pyr), 7.45–7.66 (6H, m, Ar),
9.37 (1H, m, o-Pyr). 13C NMR (75.5 MHz, C6D6, 300 K) dC (ppm):
169.59 (o-Pyr), 139.39 (m-Pyr), 139.08 (m-Pyr), 126.11, 125.20,
123.99, 95.76 ((NC(CH3))2CH), 93.33 (NC(CH3)CHCH2), 92.49
(NCHCHCH2), 29.33 (NCHCHCH2), 28.30 (CH(CH3)2), 25.26
(NC(CH3)-Pyr), 25.15 (NC(CH3)CHCH2), 25.02 (CH(CH3)2),
24.72 (CH(CH3)2). Elemental analysis (%) for C41H56MgN4
(found); C 78.31 (78.09); H 8.95 (8.84); N 8.67 (8.80).
Experimental section
All manipulations were carried out using standard Schlenk line
and glovebox techniques under an inert atmosphere of either ni-
trogen or argon. NMR experiments were conducted in Youngs tap
NMR tubes made up and sealed in a Glovebox. NMR data were
collected on a Bruker AV300 spectrometer operating at 75.5 MHz
(13C). Solvents (Toluene, THF, hexane) were dried by passage
through a commercially available (Innovative Technologies) sol-
vent purification system, under nitrogen and stored in ampoules
over molecular sieves. C6D6 and d8-toluene were purchased from
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Compound 5 (dearomatisation of 3-methylpyridine)
mental analysis (%) for C43H60MgN4 (found); C 78.58 (78.10); H
9.20 (8.77); N 8.52 (8.14).
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): 0.35 (6H,
d, JHH = 6 Hz, (CH3)2CH), 1.08 (6H, d, JHH = 6 Hz,
(CH3)2CH), 1.30 (6H, d, JHH = 6 Hz, (CH3)2CH), 1.51 (3H, s,
NCHC(CH3)CH2, 1.54 (3H, s, NCHC(CH3)(CH)3), 1.68 (6H,
d, JHH = 9 Hz, (CH3)2CH), 1.75 (6H, s, NC(CH3)CH), 2.82
(2H, m, (CH3)2CH), 3.53 (2H, m, NCHC(CH3)CH2(CH)2), 3.64
(2H, m, (CH3)2CH), 4.30 (1H, m, NCHCHCH2), 4.92 (1H, s,
Compound 8 (dearomatisation of 4-dimethylaminopyridine)
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): 0.80 (6H, d,
JHH = 9 Hz, CH(CH3)), 1.33 (6H, d, JHH = 6 Hz, CH(CH3)),
1.49 (6H, d, JHH = 6 Hz, CH(CH3)), 1.77 (6H, d, JHH = 9 Hz,
CH(CH3)), 1.97 (NC(CH3)), 2.16 (6H, s, HN(CH3)2), 2.80 (6H, s,
N(CH3)2), 3.20 (2H, m, CH(CH3)), 3.77 (1H, m, CH(CH3)), 3.88
(2H, m, CH(CH3)), 3.91 (2H, m, NCHCHCH(N(CH3)2)), 5.15
(1H, s, NC(CH3)CH), 5.86 (NCH), 6.43 (1H, t, JHH = 6 Hz,
NCHCHCH(N(CH3)2), 7.23–7.49 (8H, m, Ar), 8.32 (2H, m, o-
Pyr). 13C NMR (75.5 MHz, C6D6, 300 K) dC (ppm): 169.87
(o-Pyr), 146.63, 143.62, 142.19, 138.57, 135.89, 130.21, 126.43,
126.12, 125.16, 124.01 (NCHCHC(H)(NMe2)), 122.13, 118.71,
117.67, 94.95 ((NC(CH3))2CH), 92.37 (NCHCHCH(NMe2)),
29.38 (NCHCHC(N(CH3)2), 27.87 (NCHCHCH(N(CH3)2),
25.21 (CH(CH3)2), 25.12 (CH(CH3)2) 24.35 (CH(CH3)2), 23.55
(CH(CH3)2). Elemental analysis (%) for C43H32MgN4 (found): C
75.14 (75.26); H 9.09 (8.97); N 12.23 (12.18).
NC(CH3)CH), 5.29 (1H, s, NCHC(CH3)), 5.65 (1H, d, JHH
6 Hz, NCHCHCH2), 6.36 (1H, m, m-Pyr), 6.67 (1H, d, JHH
=
=
6 Hz, p-Pyr), 7.03 (2H, m, m-Ar(Diip)), 7.17 (2H, m, p-Ar(Dipp)),
7.28 (2H, m, m-Ar(Dipp)), 8.25 (2H, m, o-Pyr). 13C NMR
(75.5 MHz, C6D6, 300 K) dC (ppm): 169.48 (o-Pyr), 145.42,
143.76, 142.23, 138.16 (NCHCH2), 133.54 (NCHC(CH3)), 125.88,
124.87, 123.90, 100.41 (NC(CH3)CH), 94.40 (NCHC(CH3)),
92.51 (NCHCH), 31.80 (NCHCHCH2), 29.52 (CH(CH3)2),
28.43 (CH(CH3)2), 25.35 (CH(CH3)2), 24.77(CH(CH3)2), 24.56
(CH(CH3)2), 24.47 (CH(CH3)2), 24.35 (NC(CH3)2), 22.96 (CH3-
3Pic), 18.00 (NCHC(CH3)CH2). Elemental analysis (%) for
C41H56MgN4 (found); C 78.26 (78.09); H 8.97 (8.84); N 8.90 (8.80).
Compound 6 (dearomatisation of 4-methylpyridine)
Compound 9 (dearomatisation of quinoline)
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): 0.38 (6H, d, JHH
=
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): -0.06 (6H, d,
JHH = 6 Hz, (CH3)2CH), 0.79 (6H, d, JHH = 6 Hz, (CH3)2CH),
1.25 (6H, d, JHH = 6 Hz, (CH3)2CH), 1.34 (6H, d, JHH = 6 Hz,
(CH3)2CH), 1.77 (6H, s, NC(CH3), 2.73 (2H, m, (CH3)2CH), 3.61
(2H, m, (CH3)2CH), 3.94 (2H, m, NCHCHCH2), 4.24 (1H, m,
NCHCH), 5.26 (1H, NC(CH3)CH), 5.41 (1H, m, NCH), 5.44 (1H,
m, NCCH), 6.20 (1H, m, 3-H-Quin), 6.34 (1H, m, NC(CH)2CH),
6.60 (1H, m, NCCHCH), 6.89 (1H, m, N(CH)2CH2CCH),
6.91–7.28 (9H, m, Ar), 7.57 (1H, m, 7-H-Quin), 8.67 (1H, d,
JHH = 6 Hz, 8-H-Quin), 9.08 (1H, s, 2-H-Quin). 13C NMR
(75.5 MHz, C6D6, 300 K) dC (ppm): 169.87 (NC), 169.71 (NCH),
155.60, 150.70, 146.67, 146.38, 143.74, 143.62, 142.19, 138.57,
135.89, 130.21, 126.43, 125.14, 124.01 (NCHCHCH2), 122.13,
118.71, 117.67, 94.95 (NC(CH3)CH), 92.37 (NCHCHCH2),
29.58 (CH(CH3)2), 29.38 (NCHCHCH2), 27.87 (CH(CH3)2),
25.21(CH(CH3)2), 25.12 (CH(CH3)2), 25.00 (CH(CH3)2), 24.75
(CH(CH3)2), 24.35 (NC(CH3), 23.55 (CH(CH3)2). Elemental
analysis (%) for C47H56MgN4 (found); C 80.50 (80.38), H 8.05
(8.08); N 7.99 (7.92).
6 Hz, (CH3)2CH), 1.10 (6H, d, JHH = 6 Hz, (CH3)2CH), 1.29
(6H, d, JHH = 6 Hz, (CH3)2CH), 1.55 (3H, s, Pyr-CH3), 1.57
(6H, d, JHH = 6 Hz, (CH3)2CH), 1.76 (6H, s, NC(CH3)CH),
1.95 (3H, s, NCH2CHC(CH3)), 2.82 (2H, m, (CH3)2CH), 3.58
(2H, d, JHH = 3 Hz, NCH2), 3.64 (2H, m, (CH3)2CH), 4.32
(1H, m, NCH2CH), 4.94 (1H, s, NC(CH3)CH), 4.99 (1H, m,
NCHCH), 6.17 (1H, d, JHH = 6 Hz, NCH), 6.32 (2H, d,
JHH = 6 Hz, m-Pyr), 7.04 (2H, m, m-Ar(Dipp)), 7.16 (2H, m,
p-Ar(Dipp)), 7.25 (2H, m, m-Ar(Diip)), 8.42 (2H, d, JHH = 6 Hz,
o-Pyr). 13C NMR (75.5 MHz, C6D6, 300 K) dC (ppm): 169.59
(o-Pyr), 148.77, 145.37, 143.49, 142.28, 135.81 (NCH), 126.02,
124.82, 124.00, 98.03 (NC(CH3)CH), 94.48 (NCHCH2), 94.42
(NCH2CH), 48.71 (NCH2), 29.49 (CH(CH3)2), 28.43 (CH(CH3)2),
25.22 (CH(CH3)2), 24.82 (CH(CH3)2), 24.37 (NC(CH3)2), 24.33
(CH(CH3)2), 21.59 (CH3- 4-Pic), 20.96 (NCH2CHC(CH3)). No
meaningful microanalytical data could be obtained for this
compound.
Compound 7 (dearomatisation of 3,5-lutidine)
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): 0.41 (6H, d,
JHH = 6 Hz, (CH3)2CH), 1.09 (6H, d, JHH = 6 Hz, (CH3)2CH),
Compound 10 (dearomatisation of iso-quinoline)
1.29 (6H, d, JHH = 6 Hz, (CH3)2CH), 1.58 (6H, d, JHH
=
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): 0.08 (6H, d, JHH
=
6 Hz, (CH3)2CH), 1.61 (6H, s, N(CHC(CH3))2CH2), 1.62
(6H, s, N(CHC(CH3)2CH), 1.77 (6H, s, NC(CH3)), 2.87 (2H,
m, (CH3)2CH), 3.33 (2H, s, N(CHC(CH3)2CH2), 3.64 (2H, m,
(CH3)2CH), 4.93 (1H, s, NC(CH3)CH), 5.42 (2H, s, NCH), 6.56
(1H, s, p-Pyr), 7.04 (2H, m, m-Ar(Dipp)), 7.19 (2H, p-Ar(Dipp)),
7.28 (2H, m, m-Ar(Dipp)), 8.03 (2H, s, o-Pyr). 13C NMR
(75.5 MHz, C6D6, 300 K) dC (ppm): 169.41 (o-Pyr), 147.17, 145.69,
143.98, 142.28, 140.95, 135.08, 133.17 (NCHC(CH3)CH2), 125.78,
124.91, 123.82, 98.88 (NC(CH3)CH), 94.33 (NCHC(CH3)CH2),
37.79 (NCHC(CH3)CH2), 29.46 (CH(CH3)2), 28.49 (CH(CH3)2),
25.36 (CH(CH3)2), 24.62 (CH(CH3)2), 24.52 (CH(CH3)2), 24.38
(NC(CH3)), 22.72 (CH3- 3,5Lut), 17.93 (NCHC(CH3)CH2). Ele-
9 Hz, (CH3)2CH), 0.98 (6H, d, JHH = 6 Hz, (CH3)2CH), 1.27 (6H,
d, JHH = 6 Hz, (CH3)2CH), 1.48 (6H, d, JHH = 9 Hz, (CH3)2CH),
1.78 (6H, s, NC(CH3), 2.82 (2H, m, (CH3)2CH), 3.63 (2H, m,
(CH3)2CH), 3.94 (2H, s, NCH2), 5.00 (1H, s, NC(CH3)CH),
5.54 (1H, d, JHH = 6 Hz, NCHCH), 6.31 (1H, d, JHH = 6 Hz,
NCH), 6.64 (1H, d, JHH = 6 Hz, NCH2CCH), 6.92–7.25 (14H,
m, Ar–H), 8.33 (1H, s, 2-H-iQuin), 8.69 (1H, s, 8-H-iQuin). 13C
NMR (75.5 MHz, C6D6, 300 K) dC (ppm): 169.79 (NCH), 145.20,
142.49, 136.80, 133.26, 129.67, 128.80, 126.08, 124.86, 124.17,
121.72, 119.64 (NCHCH), 94.55 (NC(CH3)CH), 52.82 (NCH2),
29.47 (CH(CH3)2), 28.32 (CH(CH3)2), 25.25 (CH(CH3)2), 24.87
(CH(CH3)2), 24.50 (CH(CH3)2), 24.38 (NC(CH3). Elemental
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analysis (%) for C47H56MgN4 (found); C 80.50 (80.38); H 8.05
(7.91); N 7.99 (7.83).
the symmetry imposed disorder and its diffuse nature, it could not
be modelled in any logical way. Hence PLATON SQUEEZE was
employed, and allowance has been made herein for the presence
of one molecule of toluene per unit cell based on the results from
using this algorithm.29
Compound 11
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): 0.65 (12H, d,
JHH = 6 Hz, CH(CH3)2), 1.15 (12H, d, JHH = 9 Hz, CH(CH3)2),
1.66 (12H, s, NC(CH3), 3.05 (8H, m, CH(CH3)2), 4.97 (2H, s,
NC(CH3)CH), 6.79 (4H, m, NCH2CH2), 6.99–7.15 (14H, m, Ar),
8.61 (4H, s, NCH2). 13C NMR (75.5 MHz, C6D6, 300 K) dC (ppm):
168.59 (s, NCH2), 148.68 (s, N(CH2)2CH), 142.65 (s, NCH2CH2),
129.66 (s, CCH2), 126.03, 124.65, 123.96, 64.64 (s, NC(CH3)CH),
28.61, 25.88, 25.31, 24.59, 21.76. Elemental analysis (%) for
C68H92N6Mg2O2 (found): C 76.04 (75.93); H 8.63 (8.52); N 7.82
(7.84).
References
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Angew. Chem., Int. Ed., 2011, 50, 4069; (c) A. R. Karitzky, S. Rachwal
and B. Rachwal, Tetrahedron, 1996, 52, 15031; (d) D. M. Stout and
A. I. Meyers, Chem. Rev., 1982, 82, 223; (e) A. Lavilla, J. Chem. Soc.,
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2 (a) J. G. Keay, Adv. Heterocycl. Chem., 1986, 39, 1; (b) R. A. Sulzbach,
J. Organomet. Chem., 1970, 24, 307; (c) K. Ziegler and H. Zeiser, Chem.
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L. Dunbar, L. Horsburgh and R. E. Mulvey, Angew. Chem., Int. Ed.
Engl., 1996, 35, 753.
Compound 12
1H NMR (300 MHz, C6D6, 300 K) dH (ppm): 0.80 (12H, d,
JHH = 9 Hz, CH(CH3)), 1.33 (12H, d, JHH = 6 Hz, CH(CH3)),
1.97 (12H, s, NC(CH3)), 2.80 (6H, s, N(CH3)2), 3.20 (4H,
m, CH(CH3)), 5.15 (2H, s, NC(CH3)CH), 7.23–7.49 (16H, m,
Ar), 8.32 (4H, m, o-Pyr). 13C NMR (75.5 MHz, C6D6, 300
K) dC (ppm): 169.87 (o-Pyr), 146.63, 143.62, 142.19, 138.57,
135.89, 130.21, 126.43, 126.12, 125.16, 122.13, 118.71, 117.67,
94.95 ((NC(CH3))2CH), 29.38 (NCHCHC(N(CH3)2), 27.87
(NCHCHCH(N(CH3)2), 25.21 (CH(CH3)2), 25.12 (CH(CH3)2)
24.35 (CH(CH3)2), 23.55 (CH(CH3)2). No meaningful microan-
alytical data could be obtained for this compound.
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X-ray crystallography
Data for 2, 6, 7, 10, 11 and 12 were collected at 150 K on a Nonius
Kappa CCD diffractometer equipped with an Oxford Cryosystem
low temperature device, using graphite monochromated Mo-
˚
Ka radiation (l = 0.71073 A). Data were processed using the
Nonius Software.26 Structure solution, followed by full-matrix
least squares refinement was performed using the programme suite
X-SEED for compounds for 2, 6, 7, 10 and 11 while WINGX-
1.7027,28 was used for compound 12. Noteworthy points related to
the structure determinations follow.
The asymmetric unit of 2 consisted of 2 molecules. Disorder
was modelled as follows: the ligand based on N(3) was modelled
over 2 positions in a 50 : 50 ratio, while that based on N(6) in
addition to the butyl group based on C(30) were modelled over 2
sites in a 70 : 30 split. Disordered aromatic groups were treated as
rigid hexagons in the refinement and only partial atoms with 50%
occupancy or above were refined anisotropically. For 6 data were
truncated to a Bragg angle of 25◦ due to a decline of diffraction
intensity beyond this point. This is most likely to be related to
the small crystal dimension. The electron density in the reduced
methylpyridine moiety based on N(3) was quite smeared, and
suggested that this functionality may be disordered. However,
ardent efforts to model the same over two sites did not improve
convergence. The hydrogens attached to C(30) and C(34) have been
omitted from the refinement because of an inability to determine
precisely which of these best represents a CH2 moiety. For 10 the
asymmetric unit comprised half of a dimer molecule plus a small
region of disordered solvent. Both fragments straddled inversion
centres. The solvent bore a resemblance to toluene, but because of
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