Modelling of transition state in grignard reaction of rigid N-(Aryl)imino-Acenapthenone (Ar-BIAO): A Combined experimental and computational study

Article abstract of DOI:10.1071/CH14399

We present a combined synthetic and computational study on the addition of Grignard reagents RMgBr/RMgI (R≤Me, Et) to various sterically rigid N-(aryl)imino-acenapthenone (Ar-BIAO) (Ar≤2,6-iPr2C6H3 (1), 2,6-Me2C6H3 (2), and 2,4,6-Me3C6H2 (3) ligands). In

Full text of DOI:10.1071/CH14399

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH14399
Modelling of Transition State in Grignard Reaction of Rigid
N-(Aryl)imino-Acenapthenone (Ar-BIAO): A Combined
Experimental and Computational Study
A
A
A
A B
,
Srinivas Anga, Sayak Das Gupta, Supriya Rej, Bhabani S. Mallik,
A B
,
and Tarun K. Panda
A
Department of Chemistry, Indian Institute of Technology Hyderabad, Ordnance
Factory Estate, Yeddumailaram 502205, Telengana, India.
B
Corresponding authors. Email: tpanda@iith.ac.in; bhabani@iith.ac.in
We present a combined synthetic and computational study on the addition of Grignard reagents RMgBr/RMgI (R ¼ Me,
Et) to various sterically rigid N-(aryl)imino-acenapthenone (Ar-BIAO) (Ar ¼ 2,6-iPr2C6H3 (1), 2,6-Me2C6H3 (2), and
2,4,6-Me3C6H2 (3) ligands). In the experimental method, when compounds 1–3 were treated with RMgBr (R ¼ Me, Et) at
room temperature, the corresponding racemic N-(aryl)imino-acenapthylene-1-ol (Ar-BIAOH) compounds (Ar ¼ 2,6-
iPr2C6H3, R ¼ Me (1a), Et (1b); Ar ¼ 2,6-Me2C6H3, R ¼ Me (2a), Et (2b); and Ar ¼ 2,4,6-Me3C6H2, R ¼ Me (3a), Et (3b))
were obtained in yields up to 82 %. The Ar-BIAOH compounds were characterized by spectroscopic and combustion
analyses. The solid state structures of compounds 1a–3a were established by single-crystal X-ray diffraction analysis. To
model the transition state of the Grignard reaction with asymmetrical and sterically rigid Ar-BIAO ligands having three
fused rings containing exo-cyclic carbonyl and imine functionalities, we carried out computational analysis. During our
study, we have considered the gas phase addition of CH3MgBr to 2 and the model system of 2-(methylimino)pentanone
(29). We have carried out ab initio (HF/3–21G*) and density functional theory calculations with the hybrid density
functional B3LYP/6–311þG(2d,p) to probe two major aspects: (1) the stability of an intra-molecular chelation involving
magnesium, carbonyl oxygen, and imine nitrogen and (2) to suggest a probable transition state and a mechanistic pathway.
The computational investigation suggests the formation of a tetra-coordinated magnesium complex as the transition state
for the Grignard reaction.
Manuscript received: 19 June 2014.
Manuscript accepted: 5 September 2014.
Published online: 1 December 2014.
R2Mg ꢀ MgX2
2 RMgX
Introduction
With over a 100-year-old history, the Grignard reaction is one of
the most potent methods for C–C bond formation.[1] The
importance and widespread application of the Grignard reagent
was realized soon after its discovery in 1900 and resulted in
Victor Grignard receiving the Nobel Prize in 1912. Most of the
Grignard reagents (RMgX) are easily prepared by the reaction of
organic halides RX (X ¼ Cl, Br, I) with activated magnesium
metal in ether or THF solvent. The basic reaction involves
addition of RMgX across carbonyl–oxygen bonds in aldehydes
and ketones to form secondary and tertiary alcohols, respec-
tively. The synthetic use of the Grignard reagent has evolved
with time. The use of Grignard reagents continues to be an
important step in several natural product syntheses.[2] Addi-
tionally, metal-catalyzed Grignard reactions[3] and application
of Grignard reagents in homo and cross coupling reactions[4] are
being extensively explored by modern day synthetic chemists.
Due to their immense utility, the mechanism and structure of
Grignard reagents have been constantly under investigation
using synthetic, X-ray analyses, and spectroscopic and compu-
tational methods.[5,6] The complexity of the mechanism arises
from the wide number of possibilities depending on the solvents
used, alkyl group of the Grignard reagent, and several other
aggregated
species
Scheme 1. Schlenk equilibrium species.
factors. The Grignard reagent commonly represented as RMgX,
exists in solution in several complex forms; the most common
one is represented by the Schlenk equilibrium (Scheme 1),[7]
whereas in ether solvents, RMgXꢀ2Et2O has been established as
one of the most stable forms of the Grignard reagent.[8]
However the exact form of the Grignard reagent in solution
continues to intrigue chemists as several ab initio and density
functional theory (DFT) calculations have been reported to
ascertain the stability of the different suggested forms of the
Grignard reagent.[9,10]
In our ongoing project, we have undertaken the synthesis of
several recemic N-(aryl)imino-acenapthylene-1-ol (Ar-BIAOH)
compounds as potential ligands to stabilize alkaline earth metal
ions. During this study, we were highly interested in exploring
the coordination behaviour and bonding environments of the
Journal compilation Ó CSIRO 2014
www.publish.csiro.au/journals/ajc

B
S. Anga et al.
TBDMS
CH3
RBr, Mg, THF
H3Oꢀ
O
O
R
Ar
N
Ar
N
OH
O
O
O
N
O
O
N
Ar ꢁ 2,6-iPr2C6H3 (1)
2,6-Me2C6H3 (2)
Ar ꢁ 2,6-iPr2C6H3, R ꢁ Me (1a), Et (1b)
2,6-Me2C6H3, R ꢁ Me (2a), Et (2b)
2,4,6-Me3C6H2, R ꢁ Me (3a), Et (3b)
CH3
CH3
2,4,6-Me3C6H2 (3)
Chart 1. a-Alkoxy carbonyls differing in steric constraints. TBDMS
refers to tert-butyldimethylsilyl.
Scheme 2. Syntheses of Ar-BIAOH from respective Ar-BIAO
compounds.
Here, we present the synthesis of various racemic N-(aryl)
imino-acenapthylene-1-ol (Ar-BIAOH) compounds (Ar ¼ 2,6-
iPr2C6H3, R ¼ Me (1a), Et (1b); Ar ¼ 2,6-Me2C6H3, R ¼ Me
(2a), Et (2b); and Ar ¼ 2,4,6-Me3C6H2, R ¼ Me (3a), Et (3b)) by
reaction of corresponding (Ar-BIAO) (Ar ¼ 2,6-iPr2C6H3 (1),
2,6-Me2C6H3 (2), and 2,4,6-Me3C6H2 (3) ligands with either
MeMgBr or EtMgBr in diethylether solution. We also report a
computational study of Grignard reaction to model the transition
state of the magnesiumꢀꢀꢀBAIO complex in the course of the
reaction to understand the mechanism.
N
O
O
Me
N
Ar-BIAO (2)
2-(Methylimino)cyclopentanone (2؅)
Chart 2. Real (2) and model (29) systems selected for computational
investigation.
Results and Discussion
Ar-BIAO ligand towards the magnesium ion in the transition
state of the Grignard reaction. Several attempts to isolate the
magnesium intermediate in this particular Grignard reaction
were unsuccessful. The rigid three ring-fused system of the
imino-acenapthenone moiety and the bulky aryl group of the Ar-
BIAO ligand make the system highly sterically constrained. To
the best of our knowledge, there have not been many mechanis-
tic studies of Grignard reaction on such geometrically con-
strained systems, thereby inspiring us to probe the probable
transition states and mechanisms of these kinds of reactions.
Thus, efforts were made to model the illusive transition state Ar-
BIAOꢀꢀꢀMg complex by computational approach to understand
the mechanism of the reaction using DFT analysis.
The racemic N-(aryl)imino-acenapthylene-1-ol (Ar-BIAOH)
i
mixtures (Ar ¼ 2,6- Pr2C6H3, R ¼ Me (1a), Et (1b); Ar ¼ 2,6-
Me2C6H3, R ¼ Me (2a), Et (2b); and Ar ¼ 2,4,6-Me3C6H2,
R ¼ Me (3a), Et (3b)) were obtained by the treatment of
respective N-(aryl)imino-acenapthenone (Ar-BIAO) and alkyl
magnesium halides (MeMgI (for 1a, 2a, 3a) and EtMgBr (for
1b, 2b, 3b)) at ambient temperature, generating yields of up to
82 % (Scheme 2). The compounds 1a, 1b, 2a, 2b, 3a, 3b were
characterized by Fourier transform infrared spectroscopy
(FT-IR), 1H and 13C{1H} NMR spectroscopy, and high-
resolution mass spectrometry (HRMS). The solid state struc-
tures of 1a, 1b, 2a, and 3a were established by single-crystal
X-ray diffraction analysis.
There have been several reports on theoretical studies carried
out on the addition of Grignard reagent to a- and b-alkoxy
carbonyls considering the Cram chelation model[11] and the
Felkin–Anh[12] model as the two possible pathways. There has
been constant investigation on Cram’s rules with respect to the
addition of organometallic reagents across carbonyl double
bonds.[13] Ye et al.[14] reported Grignard reaction in a-alkoxy
carbonyls going through a chelated as well as a non-chelated
intermediate in two substrates differing in their steric constraints
(Chart 1). Very recently Hevia et al. reported the synthesis and
characterization of mixed metal homoleptic lithium magnesiate
reagents incorporating the silyl-substituted alkyl ligand
CH2SiMe3 in the presence of a variety of Lewis base donors
such as tetrahydrofuran, 1,4-dioxane, N,N,N0,N0-tetramethy-
lethylenediamine (TMEDA), and N,N,N0,N00,N00-pentamethyl-
diethylenetriamine (PMDETA).[15] They also reported that
ZnCl2-mediated addition reactions of Grignard reagents to
ketones could enhance the chemoselectivity of ketones.[16]
Thus, based on these studies, we were interested to investi-
gate the possibility of intra-molecular chelation of magnesium
with carbonyl oxygen and the exocyclic imine nitrogen a- to the
carbonyl carbon. Furthermore, theoretical studies have been
carried out on the model system 2-(methylimino)pentanone that
mimics the fundamental features of the Ar-BIAO ligand under
consideration. The consideration of a system with less steric
constraints helps us to explore the influence of steric bulk on the
transition state (Chart 2).
The FT-IR spectrum of compound 1a shows a strong absorp-
tion peak at 1660 cmꢁ1 corresponding to the C¼N bond stretch-
ing. The broad absorption band at 3306 cmꢁ1 can be attributed to
hydroxyl (O–H) bond stretching. Similar broad peaks were
observed for the remaining five compounds 1b, 2a, 2b, 3a,
and 3b in the region of 3250–3350 cmꢁ1 indicating the presence
of hydroxyl group in the respective molecules. In the 1H NMR
spectrum of compound 1a, four doublet signals at d ¼ 1.18, 1.08,
0.95, and 0.75 ppm in 3 : 3 : 3 : 3 ratio are obtained that can be
assigned to the methyl protons attached to isopropyl groups on
the 2,6-diisopropylphenyl moiety. A sharp singlet was obtained
at d ¼ 1.86 ppm for the methyl proton attached to the quaternary
carbon atom. The resonance of the CH proton present in the
isopropyl groups was observed as two quartets of quartets that
appeared as pseudo-septets at d ¼ 2.91 and 2.74 ppm indicating
the presence of two different kinds of isopropyl groups. The
resonances of the protons attached to the acenapthene moiety
and the phenyl ring are in the expected region and match the
parent Ar-BIAO ligand. In the 13C{1H} NMR spectra of
compounds 2a and 2b, the imine carbon is mostly de-shielded
and appears at d ¼ 174.6 ppm, and the phenyl carbon attached to
nitrogen atom generates a signal at d ¼ 146.3 ppm. The reso-
nance at d ¼ 78.8 ppm was obtained for the carbon atom
attached to the alcohol group. Similarly for the remaining five
Ar-BIAO-ol compounds, the presence of alcohol group, imine
group, and phenyl moiety is confirmed by FT-IR and 1H and 13
{1H} NMR analyses.
C

Modelling of Transition State in Grignard Reaction
C
Table 1. Crystallographic details of compounds 1a, 1b, 2a, and 3a
Compound
1a
1b
2a
3a
CCDC Number
Empirical formula
Formula weight
Crystal system
Space group
998574
998575
C27H31Cl2NO (1b.DCM)A
998576
998577
C25H27NO
357.48
C21H19NO
301.37
C22H21NO
315.40
456.43
Triclinic
P-1
Triclinic
Monoclinic
P21/c
Triclinic
P-1
P-1
˚
a [A]
˚
b [A]
˚
c [A]
8.6868(10)
8.9781(12)
14.0518(19)
92.717(11)
105.543(11)
107.993(11)
994.0(2)
2
10.7614(7)
11.2065(10)
12.0353(13)
110.492(9)
103.284(7)
101.698(7)
1257.74(19)
2
21.1517(16)
11.0520(7)
15.6075(12)
90
8.1845(6)
13.2124(11)
17.9902(18)
102.434(8)
102.180(7)
93.473(6)
1845.5(3)
4
a [8]
b [8]
g [8]
111.338(9)
90
3
˚
Volume [A ]
3398.4(4)
8
Z
ꢁ3
˚
Calculated density [mg A
]
1.194
1.205
1.178
1.135
Temperature [K]
˚
Wavelength [A]
293(2)
293(2)
293(2)
293(2)
1.54184
1.54184
1.54184
0.560
1.54184
0.535
Absorption coefficient
0.552
2.450
y range for data collection [8]
Absorption correction
Reflections collected
Unique reflections
3.30 to 70.81
Multi-Scan
6855
4.15 to 70.88
Multi-Scan
8670
4.49 to 70.78
Multi-Scan
13435
3.79 to 70.91
Multi-Scan
11705
3729 (Rint ¼ 0.0234)
97.3
4717 (Rint ¼ 0.0222)
97.2
6383 (Rint ¼ 0.0277)
97.5
6639 (Rint ¼ 0.0292)
93.4
Completeness to y [%]
Data/restraints/parameters
R indices (R1; wR2)
GOOF
3729/0/250
0.0473; 0.1200
1.044
4717/0/286
0.0662; 0.1791
1.043
6383/0/424
0.0723; 0.1609
1.017
6639/0/444
0.0791; 0.1991
1.107
A1b.DCM indicates that compound 1b possesses one molecule of methylene dichloride (DCM ¼ CH2Cl2) in the unit crystal cell as detected from X-ray
structure.
Single-crystal X-ray quality crystals of racemic compounds
1a, 1b, 2a, and 3a were obtained from dichloromethane at room
C9
temperature. The solid state structures of these compounds were
established by single-crystal X-ray diffraction. It was observed
that even compounds 1a, 1b, and 3a crystallized as single
isomers (R for 1a, S for 1b, and R for 3a); compound 2a
crystallized as both S and R isomers in the respective asymmet-
ric unit. Compounds 1a and 1b crystallized in triclinic space
group P-1 having two molecules in the respective unit cell. The
mesityl derivative 3a crystallizes also in triclinic space group
P-1, but with four molecules in the unit cell. Compound 2a
crystallized in monoclinic space group P21/c having eight
molecules in the unit cell. The structural details of compounds
1a, 1b, 2a, and 3a are given in Table 1. Fig. 1 represents the
molecular structure of 1b (S-isomer) and the molecular struc-
ture of compound 1a is given in the Supplementary Material
(Fig. S1).The molecular structures of 3a (R-isomer) and the
enantiomers of compound 2a are given in Figs S2 and S3,
respectively in the Supplementary Material.
In compounds 1a, 1b, 2a, and 3a, the C–O bond length of
˚
1.414(2)–1.422(2) A is within the range of a typical C–O single
˚
bond; the C–N bond length ranges from 1.268(2) to 1.275(2) A,
and is in agreement with the imine bond length, and these
distances are comparable with those in parent Ar-BIAO
ligands.[17–19] The most interesting observation is that the sp3
carbon atom attached to the hydroxyl group is almost coplanar
C10
C11
C6
C7
C21
C5
C23
C20
C27
C1
C24
C4
C14
C3
C25
C2
C15
N1
O1
C13
C18
C16
C12
Fig. 1. Solid state structure of compound 1b (S-isomer); ellipsoids are
˚
drawn to encompass 30 % probability. Selected bond lengths [A] and bond
angles [8]: O1–C2 1.414(2), N1–C1 1.268(2), N1–C14 1.432(2), C1–C27
1.489(2), C1–C2 1.559(2), C2–C3 1.511(3), C2–C12 1.534(2), C1–N1–C14
119.36(15), N1–C1–C27 131.70(16), N1–C1–C2 119.97(15), C27–C1–C2
108.21(14), O1–C2–C3 110.19(15), O1–C2–C12 110.40(14), C3–C2–C12
112.02(14), O1–C2–C1 111.24(13), C3–C2–C1 101.67(14), C12–C2–C1
111.07(14), C4–C3–C8 119.08(19).
˚
˚
˚
(0.043 A for 1a and 0.159 A for 1b, 0.176 and 0.041 A for 2a,
˚
and 0.084 A for 3a above the plane) with the respective ace-
Computational Study
napthene plane. The dihedral angles between the aryl ring
and the acenapthene ring of the respective compounds (82.838
for 1a, 76.198 for 1b, 76.068 and 76.898 for 2a, and 87.858 for
3a) are quite similar and slightly deviated from orthogonal
arrangement.
We have reacted CH3MgI, prepared in situ from magnesium and
methyl iodide in ether, with Ar-BIAO (1) at room temperature.
The reaction solution turned violet and was stirred for 6 h. After
evaporation of solvent, a violet solid was obtained. After

D
S. Anga et al.
˚
Table 2. Selected bond lengths [A] and angles[8] for ligand 2a by
different methodologies
Bond
Experiment
HF/3–21G(d)
B3LYP/6–311þG(2d,p)
C13
C1
O1–C2
1.426
1.274
1.427
1.253
1.425
1.268
C4
C12
N1–C1
N1–C14
1.434
1.427
1.415
C1–C12
1.484
1.486
1.480
C3
C1–C2
1.561
1.548
1.557
C2
C2–C4
1.516
1.516
1.517
C14
C2–C3
1.520
1.536
1.533
N1
O1
C1–N1–C14
N1–C1–C12
N1–C1–C2
C12–C1–C2
O1–C2–C4
O1–C2–C3
C4–C2–C3
O1–C2–C1
C4–C2–C1
C3–C2–C1
C13–C4–C2
118.08
131.17
120.17
108.63
108.97
112.85
111.96
110.87
102.15
109.53
109.51
124.96
133.21
119.16
107.55
113.11
110.71
110.16
111.49
102.63
108.38
106.41
123.48
133.04
118.75
108.12
110.43
110.31
112.27
110.51
102.53
110.57
108.78
Fig. 2. Optimized structure (HF/3–21G (d)) of ligand 2a. (Grey, blue, and
red spheres represent carbon, nitrogen, and oxygen atoms, respectively.
Hydrogen atoms have been omitted for clarity.
washing with pentane, a light violet powder was obtained, and
we attempted to crystallize it. However, all efforts to crystallize
the powder were unsuccessful. We tried to characterize the
compound by multi-nuclear NMR and it was observed that the
compound was not soluble in common deutoriated solvents.
Thus, to learn insights of magnesium species generated by
reaction between Ar-BIAO and MeMgI, we carried out a series
of computational calculations. We planned to carry out a
computational study on the addition of Grignard reagents across
Ar-BIAO ligands to get an insight into the coordination envi-
ronment of magnesium atom, most probable transition state, and
details of the mechanistic pathway. Our investigation is
focussed on the gas phase reaction of CH3MgBr with Ar-BIAO
ligand 2 containing a moderately steric moiety of 2,6-
dimethylphenyl group at the imine nitrogen. For a better
understanding of the role of the steric factors on the transition
state and reaction pathway, we computationally investigated
the Grignard reaction on the model system 2-(methylimino)
pentanone (29) that mimicked the fundamental a-iminocarbonyl
functionality of Ar-BIAO ligands. However, a simple methyl
substituent at the imine nitrogen and the five-membered ring
of cyclopentanone instead of a rigid fused ring system of
acenaphthenone moiety ensured that 29 was much less geo-
metrically constrained as compared with 2.
C2
C2
O1
C3
C4
C1
C3
N1
N1
C4
Mg1
Br1
Mg1
C1
Br1
R؅
P؅
C3
C4
C1
C2
O1
N1
C3
C4
C2
O1
N1
Mg1
C1
Mg1
Br1
Br1
R
P
Computational Methodology
Fig. 3. Pre- (R9 and R) and post-complexes (P9 and P) for the model and
real systems, respectively.
Computational studies have been performed using both ab
initio and DFT methods to optimize and calculate energetics of
different chemical species under consideration. All calculations
have been carried out with Gaussian 09[20] suite of programs.
We have used Gauss View[21] package for visualization of
optimized geometries and analysis of computational results.
To reduce the cost of computation, the procedure for calculating
single point energies using B3LYP/6–311þG(2d,p)[22] on struc-
tures optimized at a lower level (HF/3–21G(d)) has been
followed; B3LYP functional is usually insensitive to the geom-
etry optimization level.[23] Unless otherwise specified, the
reported energies include B3LYP single point energy with zero
point energy corrections from the Hartree–Fock (HF) method.
This method compared with the experimental results of the so-
called G2-molecule set has a reasonable maximum absolute
deviation value. Furthermore, quadratic synchronous transit
(QST)[24] has been used to locate a transition state, and QST2
calculations (without an initial guess transition state (TS)) were
applied to predict the most probable TS.
Analysis of Results
The experimental data were in excellent agreement with the
optimized structure of 2a (Fig. 2) established by computational
methods. Furthermore, we observed that the structural para-
meters obtained by optimizing at HF/3–21G(d) and B3LYP/6–
311þG(2d,p) showed minor deviations (Table 2). So, we have
decided to follow the combined method for real systems without
compromising the accuracy of the result.
The pre-complexes (R9 and R) and post-complexes (P9 and
P) were optimized without any geometrical constraints (Fig. 3).
Mg1–N1 bond distance for R in the optimized structures is
˚
2.27 A; intact Mg1–O1 bond indicates that both nitrogen and
oxygen are simultaneously coordinated to magnesium. From the
experimental investigation, the Grignard reagent preferentially
reacts with the electronegative carbonyl oxygen than the imine

Modelling of Transition State in Grignard Reaction
E
Cl
2.36 Å
C2
Br1
C3
C2
C2
C3
O1
N1
C3
C4
N1
C3
O1
O1
2.17 Å
N1
2.21 Å
Mg1
C1
C2
2.42 Å O1
4.63 Å
Mg1
Cl
2.22 Å
N1
Mg1
Br1
C4
Br1
Mg1
C1
Br1
Fig. 5. Transition states (TS9) and (TS) for the model and real systems,
respectively. (Grey, blue, and red spheres represent carbon, nitrogen, and
oxygen atoms, respectively; bromine and magnesium atoms are marked as
Br1 and Mg1, respectively; hydrogen atoms have been omitted for clarity).
N1
C3
C4
intra-molecular chelation between Mg and N was stabilized by
ꢁ13.81 kcal molꢁ1 compared with the non-chelated structure.
By carrying out similar studies with the real system, the
interaction between magnesium and nitrogen was preferred
despite the sterically bulky 2,6-dimethylphenyl group at the
imine nitrogen. The primary difference between the two struc-
tures was the re-orientation of the phenyl group to facilitate the
C3
C4
N1
C2
O1
C2
O1
3.99 Å
2.27 Å
Mg1
Mg1
Br1
C1
Br1
C1
˚
Mg–N coordination. The chelated structure (Mg1–N1 ¼ 2.27 A)
Fig. 4. Chelated and non-chelated structures of considered systems. (Grey,
blue, and red spheres represent carbon, nitrogen, and oxygen atoms,
respectively; bromine and magnesium atoms are marked as Br1 and Mg1,
respectively; hydrogen atoms have been omitted for clarity).
was stabilized by ꢁ11.26 kcal molꢁ1 compared with the non-
˚
chelated model (Mg1–N ¼ 3.99 A).
For the model system, the post-complex (P9) was stabilized
by ꢁ23.9 kcal molꢁ1 when compared with the pre-complex
˚
(R9). The C1–C2 bond lengths were of 4.36 and 1.54 A in the
nitrogen. Due to the asymmetrical nature of the ligand, and the
presence of two coordinating sites (oxygen and nitrogen atoms),
we were motivated to explore the structure and energetics of the
transition state and probe the possibility of the intra-molecular
chelation involving the magnesium of the Grignard reagent.
It was very difficult to guess and optimize a transition state
with a single imaginary frequency; hence, we decided to predict
the most probable TS through a systematic approach. As it is
evident from the post-complex, a covalent bond between the
methyl carbon (C1) of the Grignard reagent and carbonyl carbon
(C2) was formed in the course of the reaction. So, we scanned
the C1–C2 bond length without any symmetry restrictions to
perform a series of energy minimizations where the correspond-
ing distance was varied at a regular interval and the boundary
limits were the C1–C2 bond lengths in the pre- and post-
complex-optimized structures (Fig. 3). The probable high-
energy transition state was evident from the plots obtained,
and the energies of the TS were calculated by the combined
method mentioned above. To check the accuracy of the above
procedure, QST2 calculations (without a guess transition state)
were performed to interpolate the TS between the pre- and
post-complexes. The same methodology was followed for the
model and real systems. The transition states were characterized
by one single imaginary frequency, which, on visualization, was
the vibration of the bond formed between C1 and C2 atoms.
The predicted transition states from the Hartree–Fock and
density functional methods for the model system are very close
to each other.
pre-complex (R9) and post-complex (P9), respectively. Thus, for
locating the probable TS structure, the C1–C2 bond was scanned
within this range that led to a high energy transition structure
˚
with C1–C2 distances between 2.76 and 1.96 A (Fig. S2,
Supplementary Material). For further investigation, this interval
˚
was scanned at intervals of 0.1 A, and an energetically higher
˚
structure was obtained with C1–C2 ¼ 2.36 A from the scan plot,
˚
with the corresponding Mg1–N1 bond distance of 2.17 A.
Finally, we conclude that the C1–C2 and Mg1–N1 distances
correspond to the most probable transition state TS9 (Fig. 5) that
was found at 19.51 kcal molꢁ1 above that of the pre-complex R9
(Fig. 3). The transition state obtained from QST2 calculations
(18.75 kcal molꢁ1 above R9) was in close agreement with TS9
˚
both energetically and structurally. The C1–C2 (2.41 A) and
˚
Mg1–N1 (2.18 A) bond lengths in the predicted transition state
using QST2 calculations (Table S2, Supplementary Material)
˚
showed only minor differences of 0.05 and 0.03 A when com-
pared with TS9.
By carrying out systematic scan and QST2 calculations
using B3LYP/6–311þG(2d,p), we observed that both these
methods predict almost the similar transition state located at
17.62 kcal molꢁ1 above the pre-complex. Even the structural
˚
parameters exhibit negligible deviations of 0.03 and 0.01 A for
the C1–C2 and Mg1–N1 bond lengths, respectively.
For the real system, post-complex P is stabilized by ꢁ23.72
kcal molꢁ1. From the variation of the C1–C2 bond length
˚
between 4.32 and 1.62 A, a high-energy transition state was
˚
evident between 2.22 and 2.82 A (Fig. 6).
For the pre-complex R9, two possibilities were considered
and optimized to explore the chelation and non-chelation
coordinations. In one of the optimized geometries, the imine
nitrogen atom was coordinated to the magnesium ion (Mg1–
On investigating this interval by varying the C1–C2 bond
˚
length by 0.1 A, we arrived at a transition state located at
19.79 kcal molꢁ1 (Fig. 6) above the pre-complex (R) with
˚
C1–C2 and Mg1–N1 distances of 2.42 and 2.21 A, respectively.
˚
N1 ¼ 2.22 A) and in the other case, the interaction between
The transition state obtained by QST2 calculations was at
16.73 kcal molꢁ1 compared with R. The C1–C2 bond lengths
˚ ˚
of 2.42 and 2.41 A and Mg1–N1 bond lengths of 2.21 and 2.20 A
˚
these two atoms (Mg–N1 ¼ 4.63 A) (Fig. 4) was made negligible
by increasing the inter-atomic distance. The structure with

F
S. Anga et al.
C3
C4
C3
C1
C2
C3
C2
O1
C4
N1
N1
C4
N1
C2
O1
TS
C1
TS
TSꢃ
O1
Mg1
C1
Mg1
Br1
Mg1
19.51
19.79
Br1
Br1
R
P
ꢂ2.31466 ꢄ 106
ꢂ2.31467 ꢄ 106
ꢂ2.31468 ꢄ 106
ꢂ2.31469 ꢄ 106
ꢂ2.3147 ꢄ 106
ꢂ2.31471 ꢄ 106
ꢂ2.31472 ꢄ 106
ꢂ2.31473 ꢄ 106
Scan
TS
Rꢃ
0.0
R
R
P
Pꢃ
P
ꢂ23.99
ꢂ23.72
4.5
4.0
3.5
3.0
2.5
2.0
1.5
C1–C2 Bond length [Å]
Fig. 7. Energy profiles for the model and real systems.
Fig. 6. Plot of energy [kcal molꢁ1] versus carbonyl C and methyl C
˚
(C1–C2) distance [A] for the real system.
extremely spontaneous in nature which is evident from the high
stabilization energy of post-complex (P) of approximately
ꢁ24 kcal molꢁ1. From the theoretical analysis, we also conclude
that the reaction is initiated by both nitrogen and oxygen
simultaneously coordinating with magnesium to form a stable
five-membered metallacycle ring through intra-molecular che-
lation. The coordination behaviour of magnesium was similar to
that of cobalt complexes previously reported by our group.
˚
Table 3. Selected bond lengths [A] and angles [8] for the real system
transition states obtained by different methodologies
Bond
Manual (HF/B3LYP)
(TS ¼ 19.79 kcal molꢁ1
QST2 (HF/B3LYP)
(TS ¼ 16.73 kcal molꢁ1
)
)
C1–C2
2.42
2.21
2.41
2.20
Experimental
Mg1–N1
C2–O1
1.24
1.25
General Methods
C1–Mg1
2.22
2.26
All manipulations involving air- and moisture-sensitive com-
pounds were carried out in an argon atmosphere using the
standard Schlenk technique. Dichloromethane, ether, and
petroleum ether were dried with P2O5 followed by distillation,
and kept under molecular sieves before use. Ethyl acetate was
dried under K2CO3 followed by distillation under inert atmo-
sphere. 1H NMR (400 MHz) and 13C{H} NMR (100 MHz)
spectra were recorded on a BRUKER AVANCE III-400 spec-
trometer. BRUKER ALPHA FT-IR was used for FT-IR mea-
surements. HRMS was conducted on an Agilent Technology
Q-TOF instrument. Elemental analyses were performed with
a BRUKER EURO EA at the Indian Institute of Technology
Hyderabad. Compounds 1–3 were prepared according to liter-
ature procedure.[17]
Mg1–Br1
C2–C3
2.35
2.35
1.55
1.55
C3–N1
1.26
1.26
N1–C4
1.44
1.44
C2–C1–Mg1
C2–O1–Mg1
N1–Mg1–O1
N1–Mg1–Br1
C2–C3–N1
62.67
90.22
80.94
122.67
116.82
61.38
89.48
81.88
123.82
116.89
(Table 3) in the transition states obtained by scanning the C1–C2
bond and QST2 calculations, respectively, revealed structural
similarities between the states obtained by the above two
methods. The C1–Mg1 bond length shows a deviation of
ꢁ1
˚
0.04 A, which could be the driving force for the 3 kcal mol
Preparation of Compounds 1a, 2a, and 3a
difference; in the course of the reaction, this is the bond that is
broken. From Fig. 7, we observe that both the model and the real
systems follow similar reaction pathways and energetic profiles
despite their sterically different behaviours.
An ether solution of methyl magnesium iodide was prepared
from Mg turnings (128 mg, 5.27 mmol), catalytic amounts of I2
and methyl iodide (0.58 mL, 9.37 mmol) in dry ether (10 mL) as
per standard procedures. To the in situ generated Grignard
reagent, an ether solution of respective Ar-BIAO ligand
(0.58 mmol) was added dropwise via a syringe at ambient
temperature. The violet reaction mixture was stirred for 3 h at
ambient temperature. The reaction mixture was quenched by
saturated NH4Cl solution and extracted with ether (3 ꢂ 10 mL).
The combined organic layers were dried over Na2SO4 and the
solvent was removed using a rotary evaporator. The white crude
product was crystallized from dichloromethane.
Conclusion
We have demonstrated the synthesis and characterization of
various racemic N-(aryl)imino-acenapthylene-1-ol from their
corresponding N-(aryl)imino-acenapthenone compounds by
Grignard reagent. To understand the coordination environment
of magnesium atom, most probable transition state, and details
of the mechanistic pathway, we have carried out DFT calcula-
tions for the model system 2-(methylimino)pentanone (29) and
real system 2. Thus, the modelling the transition state of
Grignard reaction with Ar-BIAO ligand provides us an elaborate
insight into the mechanistic pathway. The reaction was
1a: Yield 165 mg, 79 %; mp 1268C. nmax (neat)/cmꢁ1 3306br
(O–H), 2961, 2925, 2867, 1660 (C¼N), 1597, 1433, 1193, 1012,
781, 729. dH (CDCl3, 400 MHz) 7.81 (1H, d, ArH), 7.75 (1H, d,
ArH), 7.66 (1H, d, ArH), 7.60 (1H, t, ArH), 7.25 (1H, t, ArH),

Modelling of Transition State in Grignard Reaction
G
7.18 (3H, m, ArH), 6.45 (1H, d, ArH), 3.25 (1H, br s, OH), 2.91
(1H, qq, CH(CH3)2), 2.74 (1H, qq, CH(CH3)2), 1.86 (3H, s,
CH3), 1.18 (3H, d, CH(CH3)2), 1.08 (3H, d, CH(CH3)2), 0.95
(3H, d, CH(CH3)2), 0.75 (3H, d, CH(CH3)2). dC (100 MHz,
CDCl3) 174.6 (C¼N), 146.3, 142.9, 138.7, 136.5, 135.9, 131.1,
129.6, 129.1, 128.6, 128.1, 125.1, 124.3, 123.78, 123.72, 123.4,
119.7, 78.8 (C–OH), 28.4, 28.0, 27.6, 23.48, 23.45, 23.1. Found:
C 83.56, H 7.48, N 3.72 %; (MþH)þ 358.2170. Anal. Calc. for
C25H27NO: C 83.99, H 7.61, N 3.92 %; (MþH)þ 358.2165.
2a: Yield 131 mg, 71 %; mp 1288C. nmax (neat)/cmꢁ1 3344br
(O–H), 2966, 2919, 2852, 1661 (C¼N), 1592, 1434, 1196, 1010,
779, 735. dH (CDCl3, 400 MHz) 7.89 (1H, d, ArH), 7.82 (1H, d,
ArH), 7.69 (2H, m, ArH), 7.34 (1H, t, ArH), 7.11 (3H, m, ArH),
6.58 (1H, d, ArH), 3.45 (1H, br s, OH), 2.12 (3H, s, CH3), 2.00
(3H, s, CH3), 1.93 (3H, s, CH3). dC (CDCl3, 100 MHz) 175.5
(C¼N), 147.3, 142.8, 138.5, 131.0, 129.5, 129.3, 128.6, 128.3,
126.2, 125.8, 125.0, 124.0, 123.0, 119.7, 78.8 (C–OH), 27.59
(CH3), 17.6 (CH3), 17.5 (CH3). Found: C 83.29, H 5.98,
N 4.53 %; (MþH)þ 302.1544. Anal. Calc. for for C21H19NO:
C 83.69, H 6.35, N 4.65 %; (MþH)þ 302.1539.
6.52 (1H, d, ArH), 3.36 (1H, br s, OH), 2.27 (2H, q, CH2CH3),
2.09 (3H, s, o-CH3), 1.88 (3H, s, o-CH3), 0.76 (3H, t, CH2CH3).
dC (CDCl3, 100 MHz) 141.1, 139.5, 131.0, 130.2, 129.7, 128.8,
128.7, 128.4, 126.5, 125.9, 125.2, 124.2, 122.8, 120.4, 82.2
(C–OH), 34.1 (CH2CH3), 18.1 (CH3), 17.7 (CH3), 8.6
(CH2CH3). Found: C 83.47, H 6.39, N 4.21 %; (MþH)þ
316.1701. Anal. Calc. for C22H21NO: C 83.78, H 6.71,
N 4.44 %; (MþH)þ 316.1696.
3b: Yield 150 mg, 71 %; mp 1228C. nmax (neat)/cmꢁ1 3308br
(O–H), 2966, 2916, 2855, 1660 (C¼N), 1601, 1434, 1187, 1010,
782, 735. dH (CDCl3, 400 MHz) 7.85 (2H, dd, ArH), 7.68 (2H, s,
ArH), 7.35 (1H, t, ArH), 6.95 (2H, d, ArH), 6.65 (1H, d, ArH),
3.35 (1H, br s, OH), 2.35 (5H, s, p-CH3 and CH2CH3), 2.14 (3H,
s, o-CH3), 1.91 (3H, s, o-CH3), 0.82 (3H, t, CH3). dC (CDCl3,
100 MHz) 181.0 (C¼N), 141.1, 139.5, 131.1, 129.3, 129.0,
128.5, 128.2, 125.0, 122.7, 120.2, 82.0 (C–OH), 34.0 (CH2CH3),
20.9 (Ar-CH3), 17.9 (Ar-CH3), 17.5 (Ar-CH3), 8.5 (CH2CH3).
Found: C 83.40, H 6.78, N 4.01 %; (MþH)þ 330.1860. Anal.
Calc. for C23H23NO: C 83.85, H 7.04, N 4.25 %; (MþH)þ
330.1852.
3a: Yield 159 mg, 82 %; mp 1298C. nmax (neat)/cmꢁ1 3246br
(O–H), 2961, 2922, 2853, 1660 (C¼N), 1598, 1431, 1194, 1013,
781, 728. dH (CDCl3, 400 MHz) 7.89 (1H, d, ArH), 7.82 (1H, d,
ArH), 7.68 (2H, m, ArH), 7.35 (1H, t, ArH), 6.95 (2H, d, ArH),
6.65 (1H, d, ArH), 3.51 (1H, br s, OH), 2.37 (3H, s, p-CH3), 2.08
(3H, s, CH3), 1.93 (6H, d, o-CH3 and p-CH3). dC (CDCl3,
100 MHz) 174.9 (C¼N), 145.3, 143.0, 138.4, 133.0, 131.0,
129.6, 129.2, 129.1, 129.0, 128.5, 128.3, 125.7, 125.2, 124.9,
122.9, 119.6, 78.7 (C–OH), 27.6, 22.3, 20.9, 17.5. Found: C
83.43, H 7.51, N 4.26 %; (MþH)þ 316.1700. Anal. Calc. for
C22H21NO: C 83.78, H 7.71, N 4.40 %; (MþH)þ 316.1696.
X-Ray Crystallographic Studies of 2, 4–7
Single crystals of compounds 1a, 1b, 2a, and 3a were grown
from dichloromethane at room temperature. In each case, a
crystal of suitable dimensions was mounted on CryoLoop
(Hampton Research Corp) with a layer of light mineral oil, and
placed in a nitrogen stream at 150(2) K. All measurements were
made on Oxford Supernova X-calibur equipped with Eos CCD
˚
detector with graphite monochromated CuKa (1.54184 A) radi-
ation. The crystal data and structure refinement parameters are
summarized in the Table 1. The structures were solved by direct
methods (SIR2004)[25] and refined on F2 by full-matrix least-
squares methods using SHELXL-97.[26] Non-hydrogen atoms
were anisotropically refined. H atoms were included in the
refinement on calculated positions riding on their carrier atoms.
The function minimized was [Sw(F2o ꢁ Fc2)2] (w ¼ 1/[s2Fo2 þ
(aP)2 þ bP]), where P ¼ (Max(Fo2,0) þ 2Fc2)/3 with s2(Fo2) from
counting statistics. The functions R1 and wR2 were (S||Fo| ꢁ
|Fc||)/S|Fo| and [Sw(F2o ꢁ Fc2)2/S(wFo4)]1/2, respectively. The
ORTEP-3 program[23] was used to draw the molecule. Crystal-
lographic data (excluding structure factors) for the structures
reported in this paper have been deposited at the Cambridge
Crystallographic Data Centre as supplementary publication
nos CCDC 998574–998577. Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: þ (44)1223–336–033; email:
deposit@ccdc.cam.ac.uk or via http://www.ccdc.cam.ac.uk/
data_request/cif.
Preparation of Compounds 1b, 2b, and 3b
An ether solution of ethyl magnesium bromide was prepared
from Mg turnings (128 mg, 5.27 mmol), catalytic amounts of I2
and ethyl bromide (0.7 mL, 9.37 mmol) in dry ether (10 mL) as
per standard procedures. After formation of Grignard reagent,
an ether solution of respective Ar-BIAO ligand (0.58 mmol) was
added dropwise via a syringe at ambient temperature. The violet
reaction mixture was stirred for 3 h at ambient temperature. The
reaction mixture was quenched by saturated NH4Cl solution and
extracted with ether (3 ꢂ 10 mL). The combined organic layers
were dried over Na2SO4 and the solvent was removed using a
rotary evaporator. The white crude product was crystallized
from dichloromethane.
1b: Yield 162 mg, 74 %; mp 1228C. nmax (neat)/cmꢁ1 3309br
(O–H), 2962, 2927, 2868, 1658 (C¼N), 1560, 1434, 1187, 1011,
783, 729. dH (CDCl3, 400 MHz) 7.75 (2H, m, ArH), 7.61 (2H, m,
ArH), 7.18 (4H, m, ArH), 6.44 (1H, d, ArH), 3.12 (1H, br s, OH),
2.97 (1H, sept, CH(CH3)2), 2.69 (1H, sept, CH(CH3)2), 2.24
(2H, m, CH2CH3), 1.19 (3H, d, CH(CH3)2), 1.05 (3H, d, CH
(CH3)2), 0.99 (3H, d, CH(CH3)2), 0.80 (3H, t, CH2CH3), 0.78
(3H, d, CH(CH3)2). dC (CDCl3, 100 MHz) 141.0, 139.5, 131.0,
128.4, 127.8, 125.0, 123.7, 123.3, 123.2, 120.2, 82.0 (C–OH),
33.93 (CH(CH3)2), 28.3 (CH(CH3)2), 27.8 (CH2CH3), 23.7 (CH
(CH3)2), 23.3 (CH(CH3)2), 23.1 (CH(CH3)2), 22.85 (CH
(CH3)2), 8.3 (CH2CH3). Found: C 70.72, H 6.38, N 2.87 %;
(MþH)þ 372.2328. Anal. Calc. for C26H29NO: C 71.05, H 6.85,
N 3.07 %; (MþH)þ 372.2322.
Supplementary Material
The geometries and absolute energies of all calculated structures
are available on the Journal’s website.
Acknowledgements
This work was supported by the Council of Scientific and Industrial
Research (CSIR) under the project No. 01(2530)/11/EMR-11 and start-up
grant from IIT Hyderabad. SA thanks University Grants Commission, India,
for the PhD fellowship.
2b: Yield 159 mg, 79 %; mp 1218C. nmax (neat)/cmꢁ1 3307br
(O–H), 2966, 2917, 2878, 1659 (C¼N), 1592, 1434, 1187, 1011,
782, 765. dH (CDCl3, 400 MHz) 7.81 (1H, d, ArH), 7.75 (1H, d,
ArH), 7.61 (2H, t, ArH), 7.26 (1H, t, ArH), 7.04 (3H, m, ArH),
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