Deformative transition of the menschutkin reaction and helical atropisomers in a congested polyheterocyclic system

Article abstract of DOI:10.1021/jo501815y

A 4,7-phenanthroline polycyclic 1A designed for probing the limits of the Menschutkin reaction was synthesized in a six-step sequence. The rotational barrier of the phenyl ring nearby the N-methyl group in rac-2A was estimated to be 18.1 kcal/mol from VT-NMR experiments, making them a new type of helical atropisomer. The methylation rate constants of 9 and 1A with MeI was found to be 2.22 × 10^-4 and 9.62 × 10^-6 s^-1 mol^-1 L, respectively; thus, the formation rate of (P/M)-2A is one of the slowest rates ever reported for a Menschutkin reaction. The N-methyl protons in (P/M)-2A exhibit a significant upfield shift (Δδ 1.0 ppm) in its ¹H NMR, compared to those without a nearby phenyl, indicating a strong CH-π interaction is involved. Conformational flexibility in dipyridylethene 9 is clearly shown by its complexation with BH₃ to form helical atropisomers (P,P/M,M)-10. The pK_a values of the conjugate acids of 1A and 9 in acetonitrile were determined to be 4.65 and 5.07, respectively, which are much smaller compared to that of pyridine 14a (pK_a = 12.33), implying that the basicity, nucleophilicity, and amine alkylation rates of 1A and 9 are markedly decreased by the severe steric hindrance of the flanking phenyl rings in the polyheterocycles.

Full text of DOI:10.1021/jo501815y

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Article
Deformative Transition of Menschutkin Reaction and Helical
Atropisomers in a Congested Polyheterocyclic System
Yung-Yu Chang, Tse-Lok Ho, and Wen-Sheng Chung
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo501815y • Publication Date (Web): 03 Oct 2014
Downloaded from http://pubs.acs.org on October 16, 2014
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Deformative Transition of Menschutkin Reaction and Helical Atropisomers in a
Congested Polyheterocyclic System
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Yung-Yu Chang, Tse-Lok Ho,* and Wen-Sheng Chung*
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Department of Applied Chemistry, National Chiao-Tung University, Hsinchu, 30050,
Taiwan-ROC
tselokho@yahoo.com; wschung@nctu.edu.tw
Graphic Abstract
Abstract
A 4,7-phenanthroline polycyclic 1A designed for probing the limits of the Menschutkin
reaction was synthesized in a six-step sequence. The rotational barrier of the phenyl ring
nearby the N-methyl group in rac-2A was estimated to be >>18.1 kcal/mol from VT-NMR
experiments making them a new type of helical atropisomers. The methylation rate of 9 and
1A with MeI was found to be 2.22 × 10-4 and 9.62 × 10-6 s-1mol-1L, respectively, thus, the
formation rate of (P/M)-2A is one of the slowest rates ever reported for a Menschutkin
reaction. The N-methyl protons in (P/M)-2A exhibit a significant upfield shift ( 1.0 ppm) in
its 1H-NMR, compared to those without a nearby phenyl, indicating a strong CH- interaction
is involved. Conformational flexibility in dipyridylethene 9 is clearly shown by its
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complexation with BH3 to form helical atropisomers (P,P/M,M)-10. The pKa values of the
conjugate acids of 1A and 9 in acetonitrile were determined to be 4.65 and 5.07, respectively,
which are much smaller compared to that of pyridine 14a (pKa = 12.33), implying that the
basicity, nucleophilicity, and amine alkylation rates of 1A and 9 are markedly decreased by the
severe steric hindrance of the flanking phenyl rings in the polyheterocycles.
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Introduction
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Bimolecular nucleophilic substitution (SN2) reaction plays an important role in organic
syntheses1 and biochemical processes2. Since the Menschutkin reaction3 which converts a
tertiary amine to a quaternary ammonium salt by reaction with an alkyl halide, is a classical
model for SN2 reaction. The alkylation of various alkyl and heteroatom-substituted pyridines
to quaternary ammonium ions can provide information regarding electronic, steric, and solvent
effects in chemical reactions, it has intrigued chemists for decades to study the kinetics and
transition state (TS) structures of this reaction.4 Linear free energy relationship, statistic
algorithm, and theoretical calculations have long been used in studying the steric and
electronic effects on Menschutkin reactions. For example, Brown and co-workers4m-w have
tried to quantify the steric effects in amine alkylations by measuring the reaction rates of
mono- and multiple alkyl substituted pyridines with iodoalkanes and various other alkyl
halides. Their studies indicated that on the one hand, the introduction of alkyl group(s) to the
ortho-position(s) of pyridines increased the steric environment which in turn increased the
activation energy and decreased the reaction rate. On the other hand, similar steric effects on
the reaction rates were observed when bulkier alkyl halides reacted with unsubstituted
pyridines.
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Clarke and Rothwell4l also demonstrated that the effects of ortho-substitution on the SN2
reaction of pyridine could primarily be attributed to steric hindrance. However, Arnett and
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Reich reported that the SN2 reactions of pyridines show considerably higher activation free
energies when positive charges are developed on the pyridines. The differences are caused by
the higher intrinsic barriers of the reactions with methyl iodide, in which the breaking of the
CI bond requires additional reorganization energy.4i Consequently, the effect of ortho-alkyl
substituents on the quaternization of pyridines was considered to be predominantly steric in
nature, and the effect was important in determining the TS structure, activation energy, and
reactivity of Menschutkin reaction.
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The CH-interaction has been difficult to measure because (1) it is relatively weak
(0.52.5 kcal/mol)5 and (2) most molecules without special design are usually quite flexible in
geometries. Most of the reported CH- interactions have been focused on the studies of single
crystals in which the distances between the tips of the C-H bond to the center of phenyl rings
are smaller than 2.90 Å,6 the sum of the van der Waals radii of the interacting H and C atoms.
Note that weak molecular forces play important roles in the molecular assembly in
supramolecular chemistry,7c,d,7g,7j biochemistry,7h,i,7n,7p,7k crystallography,7m,7o,7q asymmetric
catalysis,7e,f and reaction mechanisms.7a,b Recently, the interactions between CH- and -
of an aromatic ring with intra- and/or intermolecular functional groups were studied using
quantum mechanical modeling and these results suggest that CH-π and - interactions can
direct a reaction stereoselectively.7a
We report here the synthesis of a rigid polycyclic 4,7-phenanthroline compound 1A to
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study the steric effect of a flanking phenyl ring on its methylation reactions (with methyl
iodide, CH3SO3CH3, and Meerwein’s reagent) and BX3 (BH3THF and BF3OEt2)
complexation. These reactions, if successful, would provide us non-classical atropisomers and
ring current shielding between flanking phenyl ring and the N-alkyl substituent. The synthetic
route of the fluorinated compounds 1B and 1C can be regarded as one of the ideal model for
preparation of 2A; however, we were surprised by the overwhelming importance of steric
effects over the opposing ring current shielding of fluorine, as revealed by the 19F-NMR
spectra. Even with the counter-effect the largest steric deshielding on record still showed up.8a
The interest of such a phenomenon to one of us dates back to 1969 in a report on the
determination of the C(15) stereochemistry of the Lycopodium alkaloid annotinine.8b
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Results and Discussion
The synthesis of 1A from 813b involves a two-step sequence, which includes the
application of tandem Diels-Alder reaction9 followed by an iodine induced photocyclization10
(see Scheme 1). The compound 8 is accessible from a commercial compound 3 by a short
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reaction sequence involving condensation of 3 with catalytic amount of sodium cyanide,11
oxidation of 5 with iodine,12 aldol condensation of 6 with dibenzyl ketone,13b and dehydration
of 1,3-diphenyl-2-propanone 7 with POCl3.13c Compound 9 is obtained through the tandem
Diels-Alder reactions between 8 and 1,5-cyclooctadiene based on similar reaction conditions
of other polycarbocyclic structures.14a,b Intermolecular Diels-Alder reaction between the
cyclopenta-2,4-diene-1-one 8 with 1,5-cyclo-octadiene led to the formation of a carbonyl
bridged intermediate I, which underwent a decarbonylation to afford the bicyclic
cyclohexa-1,3-diene II. The bicyclic cyclohexa-1,3-diene II then underwent an intramolecular
Diels-Alder (IMDA) reaction due to conformational flexibility of the fused eight-membered
ring and resulted in the formation of the polycyclic 9 in only 49% yield. It is disappointing to
obtain product 9 in such a low yield; however, similar yields (425%) were reported14c in the
synthesis of related tetracyclic compounds. The pyridinyl substituents in the
cyclohexa-1,3-diene intermediate II may be electronically unfavorable for an IMDA reaction
causing its low yield. Apparently, the synthetic pathways of 2A are slightly different from
those of 1A due to the retardation by intramolecular hydrogen bonding interactions in 5 and 7
and unfavorable electronic effects in IMDA reaction of pyridinyl diene.
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The photochemical behavior of the stilbazoles (styrylpyridines) and 1,2-bis-
pyridylethylenes have been extensively explored for several decades, which in general leads to
CC cyclization.15a-d To our surprise, Berdnikova recently reported the photochemical
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reactions of 2-styrylquinolines by Hg lamp irradiation, leading to unexpected CN cyclization
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OH
O
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(b)
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OH
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(a)
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1A
Scheme 1. (a) Cat. NaCN, EtOH, reflux, 3 h, 67% ; (b) I2, DCM, rt, 15 h, 37% ; (c) KOH,
dibenzyl ketone, EtOH, reflux, 1 h, 84% (d) POCl3, pyridine, 85 oC, 66%, (e)
1,5-cyclooctadiene, reflux, 24 h, 49%; (f) cat. I2, Rayonet, 254 nm, THF, 8 h, 75%.
instead of the traditional CC cyclization.15e The photocyclization of 9 (with a stilbazole unit)
to 1A (4,7-phenanthroline) was performed under the irradiation of a Rayonet photoreactor
(max = 254 nm) at room temperature for 8 h, which gave the desired CC cyclization of
phenanthroline structure 1A in 75% yield (Scheme 1, f). The structure of 1A was determined
1
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by H and C NMR, DEPT-135, HRMS and eventually confirmed by a single crystal X-ray
structural analysis (Figures 1, S11, and 12, SI).
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Figure 1. X-ray crystal structures of 1A.
In the current series the access of 1A permitted us to examine the amine alkylations under
a highly congested environment. The N atoms of the phenanthroline part of 1A are in close
proximity with the centroid of the phenyl groups with a distance of ca. 2.70 Å for both N1 to
C14-C19 and N2 to C29-C34 (Figure 1). The distance is significantly smaller than the sum of
van der Waals radii of C and N atoms (3.2 Å),16 which implies that the molecular spaces
between the phenanthroline and the phenyl group in 1A are very congested. Importantly, for
the classical SN2 reaction the steric environments surrounding the nucleophilic center exert a
crucial influence as the electrophile must be collinearly accommodated.3d We speculated that
the phenanthroline would be difficult to undergo an SN2 reaction with methyl iodide. Indeed,
heating 1A with a large excess of MeI in MeCN under reflux for 2 d did not lead to noticeable
product. A noticeable reaction required 7 d of reflux (Scheme 2).
The product was confirmed to be the mono-N-methyl phenanthrolinium iodide 2A by 1H-
and 13C-NMR, DEPT-135, HRMS and X-ray single crystal structural analysis (Figure 3a), but
the dimethylated product 2B was not obtained. If the methylation reagents were replaced by
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methyl methanesulfonate and Meerwein’s reagent (CH3OBF4), no methylation products 2C
and 2D were obtained (Scheme 2b and 2c), even though Meerwein’s reagent is considered to
be a strong methylation reagent.17
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Scheme 2. conditions and reagents: (a) 100 eq. MeI/CH3CN, reflux 7 d, 51%. (b) 100 eq.
MeOSO2Me /CH3CN, reflux 7 d, no reaction. (c) 6.6 eq. Me3OBF4, dry CH2Cl2, 0oC, 10 hr, no
reaction.
In principle, 2-substituted pyridines and 4,7-phenanthroline derivatives could be regarded
as good control compounds of the reactions discussed above; furthermore, there have been
many reports discussing their methylation reactions.4 For example, Gallo and coworkers
explored the methylation rates of the 2-substituted pyridines with methyl iodide at 298 and 303
K, respectively.4i,w They found that the relative methylation rate of 14e (R = t-Bu) decreased
by a factor of 5000 compared to that of 14a (R = H) implying that steric effect of the
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2-substituents in compounds 14ae plays a very important role (Figure 2). On the other hand,
the reflux of 4,7-phenanthroline with methyl halides could lead to high yields of dimethylated
salts in a short time.18 The methylation of 1A with excess methyl iodide under reflux was quite
sluggish which did not run to completeness even after 28 d. We estimated the amine alkylation
rates (k’) of 9 and 1A with excess methyl iodide to be roughly 2.22 × 10-4 and 9.62 × 10-6
s-1mol-1L under seal tube heating condition, respectively (Table S1 and S2, SI); in contrast, the
methylation rate of 2-t-butylpyridine (14e) was found to be 6.36 s-1mol-1L.4i,w Thus, the
relative methylation rate of 1A is ~6 orders of magnitude smaller than that of a hindered
2-t-butylpyridine 14e (see Figure 2). Apparently, the intramolecular steric effect on the amine
alkylations of polycyclic 9 and 1A is even more severe when compared to that of 14e with a
2-t-butyl group on the pyridine.
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Figure 2. The relative methylation rates of 2-substituted pyridines 14a-e at 30oC,4i 9, and 1A
with methyl iodide in acetonitrile. The methylation rate of 14a at 25oC was from reference 4w.
1H NMR signals of the bridgehead protons of 2A at C24 and C27 appeared as two
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multiplets at  3.57 and 2.91 ppm (Figure 3b) because they experienced different magnetic
environments due to the restricted rotation of the flanking phenyl ring. X-ray single crystal
structures of 2A revealed that there are two stable enantiomers, which could be identified as
atropisomers P- and M-2A (vide infra). The assignment of P and M descriptors was done by
viewing the cross between the two lines containing N1-N2 and C14-C29 in their X-ray
structures (Figure 4). On the one hand, if the turn is clockwise the absolute configuration is P;
on the other hand, if the turn is counterclockwise, then the absolute configuration is M. If the
flanking phenyl rings in 2A could undergo free rotation at high temperature, the stereoisomers
P-2A and M-2A would interconvert to each other. Variable-temperature NMR (VT-NMR)
experiment was then implemented to measure the rotational energy barrier of the flaking
phenyl ring nearby the N-Me group of 2A (Figure 5). However, even at 393 K there was very
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(a)
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Figure 3. (a) X-ray single crystal structure of 2A and (b) its partial NOESY spectrum.
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Figure 4. Stereochemical descriptors of P- and M-2A.
little merging movement on the bridgehead proton signals of H24 and H27, implying a very
high energy barrier for the single bond (C29-C28) rotation of the phenyl ring. The energy
barrier for the restricted rotation of the phenyl ring of 2A was estimated to be >> 18.1 kcal/mol
by VT-NMR. Hence, the highly rigid stereoisomers P- and M-2A were unable to interconvert
Figure 5. VT-1H-NMR (500 MHz) spectra of the protons H23, H24, and H27 and N-Me of 2A
in DMSO-d6.
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to each other through the rotation of the flanking phenyl ring at high temperature due to the
severe steric hindrance between the phenyl ring and N-methyl group.
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We did not obtain the di-methylated product of 1A possibly because the N-methyl
phenanthrolinium ring of 2A becomes more electron deficient, inductively hampering a second
equivalent of N-methylation reaction to occur. Since the distance of N1 to the centroid of
phenyl (C14-C19) was measured to be 2.64 Å in 2A, which is even shorter by 0.06 Å
compared to that in 1A, implying that there may have a more severe steric hindrance between
the unmethylated nitrogen (N1) and the flanking phenyl ring of 2A compared to those in 1A,
thus, increasing the barrier for compound 2A for further methylation.
In the crystal structure of 2A, one of the N-methyl protons of 2A is located 2.20 Å above
the centroid of the phenyl ring (C29-C34, Figure 3a). Such a distance is shorter than the
commonly used cutoff distance, 2.90 Å, for CH interactions.6 As expected, shielding of the
N-methyl signal by the phenyl ring was manifested [H 4.0 (13C signal C 51.6 ppm)], a H 1.0
ppm upfield shift in comparison with that of 4-methyl-4,7-phenanthrolinium iodide (H 5.0).19
It should be noted that a series of exquisitely designed skeletons with short contact distances of
alkoxy protons with nearby phenyl groups were designed and synthesized by two research
groups to measure the intramolecular CH interactions in solution, however, only small
1
upfield shifts (ca. 0.10.3 ppm) were observed in their H-NMR signals compared to those
without nearby phenyl groups.6a,b Similar upfield shift of ca. 1.0 ppm was reported by
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Shimizu on a molecular balance which also exploring the shielding of aryl CH by a nearby
phenyl ring.5a To the best of our knowledge, there has been few report on the NCH3
interactions in pyridinium and/or phenanthrolinium derivatives. Recently, Natsugari reported
a shielding of 0.93 ppm on NCH3by a phenyl group on a series of 1,5-benzodiazepine
derivatives.6e
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More significant is the structures of 1A and 2A delineated by single crystal X-ray
crystallographic analysis (Figures 1 and 3a). The torsional angle of N2-C8-C7-C28 in the
crystal structure of 2A was found to be 42.7o, which is significantly larger than those in
compound 1A (N2-C8-C7-C28, 18.0o) and 2A (N1-C5-C6-C13, 1.3o). The torsional angle of
the carbon skeleton C28- C7-C6-C13 of 2A was found to be 22.8o, which is also noticeably
larger than that (13.2o) in 1A. Interestingly, the torsional angle of H12-C12-N2-C35 in
compound 2A was shown to be 14.3o instead of the expected coplanarity. These results imply
that the presence of an N-methyl group in 2A brings in an even more severe overall ring strain
than that in 1A. Significantly, while in 1A the phenanthroline is essentially flat and it deviates
from the plane containing C13-C28 and C6-C7 by 8o, the methyl salt 2A shows a larger angle
of 17o, with mutual tilting of the N-methylpyridinium ring and its proximal phenyl group. This
altered geometry suggests that prior to reaching the SN2 reaction transition state the 1A
molecule must undergo cooperative clockwise and counterclockwise rotations, respectively, on
the part of the phenanthroline portion and the phenyl group. Both such molecular movements
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are necessary to clear space for trapping the alkylating agent. The observation is reminiscent of
induced fit theory20,21 for certain enzymatic reactions, in which the substrate leads to
conformational changes in the enzyme such that the active site achieves the exact
configuration required for the reaction to occur.
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Next, we considered complexation of 1A with BX3. The flat boron species should be
more easily inserted into the gap between the nitrogen lone pair and the phenyl group, and the
initial complex is structurally akin to the transition state of the SN2(C) reaction, just before
configurational inversion of the electrophilic center. Complexation of 9, The complexation of
1A, with borane did give a 1:2-adduct 10 quantitatively (Scheme 3)22 which displays distinct
NMR characteristics.
The bridgehead hydrogen pairs at C(20)/C(23) and C(27)/C(24) become magnetically
nonequivalent. Thus, the singlet at H 3.02 is split into two multiplets (H 3.11 and 3.51). The
Scheme 3. conditions and reagents: (a) 3.0 eq. 50% BH3 in THF-d8 at 0oC, 2.5 h,
quantitatively and (b) 4.0 eq. 50% BH3 in THF-d8 at 0oC, 7 d, no reaction.
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downfield shift is attributable to a hindered rotation of the phenyl ring such that its plane is
closer to one of the bridgehead hydrogens (and similarly on the other half of the molecule).
Even more remarkable is the appearance of nine separate groups of absorption for the
aromatic protons in 10, which we identified by COSY, NOESY, 11B, and other NMR
techniques (Figures S1520 and S28, SI). The five protons of the phenyl group (Figure 6b)
experience different magnetic environments as a result of spatial interactions with the
borane-bearing pyridine unit. X-ray diffraction of 10 (Figure 6a) indicates the two heterocycles
are forced out of conjugation with the central double bond.
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(a)
(b)
Figure 6. (a) X-ray crystal structure of 10 and (b) its partial 2D-H,H-COSY spectrum.
The dipyridine -system and its potential conformational mobility in compound 9 allow it
to react with two equivalents of BH3 forming the bis-N-borane complex 10 quantitatively.
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9
Surprisingly, because there are two rotationally hindered heterocycles, the bishelical P,P- and
M,M-10 could be regarded as novel atropisomers. The helical descriptors of 10 can be
assigned by X-ray analysis, which is done by facing the two axes of C5-C6 and C8-C7 on
compound 10. On the one hand, if the turn is clockwise the helical descriptor is P; on the other
hand, if the turn is anticlockwise the helical descriptor is M. If the two axes are both
counterclockwise or clockwise they are described as M,M- or P,P-10, respectively (Figure 7).
VT-NMR study was used to assess the rotational energy barrier of these molecules;
unfortunately, we did not see any merging of the proton signals of 10 at the temperature limit
(323 K) of chloroform, implying a very high energy barrier (at least 15.9 kcal/mol) for the
rotation of the flanking phenyl and the heterocyclic groups (Figures S36 and S37, SI).
However, complexation of 1A with borane in THF-d8 for 7 d showed no reaction when
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monitored by H-NMR (Figure S27, SI) instead of the expected bis-N-borane product 11. It
was undoubtedly attributed to the severe intramolecular steric hindrance of 1A and its inability
to assume a conformation amenable to the methylation transition state. Notably, we have not
been able to obtain the BH3 complexes of 1A. Unfortunately, in our hands complexation of 9
and 1A with BF3 yielded only the protonated salts 12 and 13, apparently due to adventitious
moisture of the reagent. The product of the complexation of 1A with BF3 was identified as the
1
mono-protonated 13 by X-ray single crystal structure analysis (Figure 8). However, the H-,
13C, and 11B-NMR and COSY spectra of adduct 13 in MeOH-d4 showed
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Figure 7. Helical descriptors of P,P- and M,M-10.
Figure 8. X-ray single crystal structure of 13.
unambiguously symmetrical signals instead of the expected asymmetric one (Figures S2326
and S30, SI). The discrepancy between those obtained from NMR spectroscopy and that of
X-ray single crystal analysis could be resolved if one attributes the NMR results to the fast
exchange between the MeOH-d4 and N-H proton of adduct 13. The fast exchange of N-H
proton with d4-methanol proton is justified due to strong hydrogen-bonding interactions.
Interestingly, neither the di-N-protonated nor the di-N-methylated product of 1A was obtained,
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which might be due to similar stereoelectronic factors in 13 and 2A. Furthermore, Uv-vis
spectroscopy was applied to explore the pKa values of the conjugate acids of 9, 1A, and 2A.
The pKa value of the conjugate acids of 1A and 9 in acetonitrile was determined to be 4.65
and 5.07, respectively, by titration with trifluoromethanesulfonic acid (Figure S38, SI).23a-e The
pKa values of the conjugate acids of 1A and 9 are much smaller compared to that of pyridine
14a (pKa = 12.33),23e implying that the basicity, nucleophilicity, and amine alkylation rates of
1A and 9 are markedly decreased by the severe steric hindrance of the flanking phenyl rings in
the polyheterocycles.23a,f Even though the pKa2 of the conjugate acid of mono-protonated 1A
and the pKa of the conjugate acid of 2A were unable to be determined, they proved that steric
effects of the flanking phenyl group and electro-deficient effects are prohibiting them for
further protonation.
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Conclusion
A 4,7-phenanthroline polycyclic system 1A designed for probing the limit of the
Menschutkin reaction was synthesized in a six-step sequence including tandem Diels-Alder
reaction followed by an iodine induced photocyclization. The formation of mono-methylated
product 2A from the amine alkylation of 1A with MeI was found to be very sluggish due to
intramolecular steric effect. The amine alkylation rates of 9 and 1A with excess methyl iodide
1
were determined to be roughly 2.22 × 10-4 and 9.62 × 10-6 s-1mol-1L by H NMR. Thus, the
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relative methylation rate of 1A is ~6 orders of magnitude smaller than that of a hindered
2-t-butylpyridine 14e!! The rotational barrier of the phenyl ring nearby the N-methyl group in
rac-2A was estimated to be >>18.1 kcal/mol from VT-NMR experiments making them a new
type of helical atropisomers. Furthermore, the N-methyl group in 2A exhibits a significant
upfield shift (= 1.0 ppm) in its 1H-NMR comparing to those without a nearby phenyl which
reveals the strong intramolecular CH interactions in our system. The flexibility in
conformational change of the dipyridylethene is clearly shown by the complexation of 9 with
BH3 to form bishelical atropisomers (P,P- and M,M)-10 in high yield. To date, there has been
no report on the determination of the absolute configurations of these helical atropisomers
(P/M)-2A and (P,P/M,M)-10. The pKa values of the conjugate acids of 1A and 9 are much
smaller compared to that of pyridine 14a, implying that the basicity, nucleophilicity, and amine
alkylation rate of 1A are markedly decreased by the severe steric hindrance of the flanking
phenyl rings in the polyheterocycles.
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Experimental Section
Column chromatography was performed on silica gel 70-230 or 230-400 mesh; thin-layer
Chromatography (TLC) was performed on aluminum plates coated with silica gel 60 F254.
1
Melting points were determined with a melting-point apparatus and are uncorrected. H NMR
spectra were measured with 500, 400, and 300 MHz spectrometer with the residual solvent
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peaks (usually CHCl3, DMSO and MeOH) as the internal standard. Natural abundance 13C
NMR spectra were recorded using pulse Fourier Transform techniques with 500, 400, and 300
MHz spectrometer operating at 125, 100, and 75.4 MHz, respectively. 11B NMR spectra were
measured on a 500 MHz NMR spectrometer operating at 160.5 MHz with the solvent peak
(BF3 OEt2/CDCl3 = 15%) as an external standard (B 0 ppm). High-resolution mass
spectrometry (HRMS) was obtained with a magnetic sector type analyzer using ESI, EI and
FAB method. UV/Vis spectra were recorded with a spectrophotometer and solvents were of
HPLC grade. Compounds 5,11f 6,12 7,13b and 8,13b were prepared according to literature reports.
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Synthesis of 5.11f A suspension of pyridine 2-carbaldehyde 3 (11.30 g, 0.11 mol) and catalytic
amount of sodium cyanide (1.10 g, 0.02 mol) in EtOH (200 mL) and H2O (50 mL) was heated
at reflux for 2 h. After cooling to room temperature, the solvents were removed under reduced
pressure. The residue was partitioned between H2O (100 mL) and CH2Cl2 (200  3 mL). The
combined organic layer was dried over anhydrous MgSO4 and evaporated. The resulting
residue was recrystallized from CH2Cl2 and EtOH to afford the product 2,2'-pyridoin 5 as an
orange solid (7.9 g, 67%). Mp: 155156 oC (lit.11e 156157 oC); 1H NMR (300 MHz, CDCl3):
13
H13.22 (s, 2H), 8.478.44 (m, 2H), 7.917.79 (m, 4H), 7.207.15 (m, 2H); C NMR
(75 MHz, CDCl3): δC 156.5 (C), 145.6 (CH), 137.5 (CH), 135.8 (C), 121.1 (CH), 119.4 (CH).
FAB-MS: m/z 214 (M+).
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Synthesis of 6.12 A suspension of 5 (5.0 g, 23.3 mmol) and iodine (0.08 g, 23.3 mmol) in
CH2Cl2 (100 mL) was stirred at room temperature for 12 h. The solvent was removed under
reduced pressure. The residue was dissolved in EtOAc (100 mL), washed with 10% aqueous
solution of Na2S2O3 (50 mL), saturated aqueous solution of Na2CO3 (50 mL). The organic
layer was separated and dried over anhydrous MgSO4, then the solvent was evaporated under
reduced pressure. The resulting residue was recrystallized from CH2Cl2 and MeOH to afford
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o
o
the product 6 as a brown solid (1.8 g, 37%). Mp: 152153 C. (lit.11e 156157 C); 1H NMR
(300 MHz, CDCl3): H8.57 (dd, J1 = 4.1Hz and J2 = 0.8 Hz, 2H), 8.20 (d, J = 7.8 Hz,
2H), 7.96-7.90 (m, 2H), 7.517.47 (m, 2H); 13C NMR (75 MHz, CDCl3): C 196.9 (C),
151.5 (C), 149.4 (CH), 137.2 (CH), 127.9 (CH), 122.3 (CH). FAB-MS: m/z 213 (M+).
Synthesis of 7.13b Potassium hydroxide (0.38 g, 6.7 mmol) was added slowly to a vigorously
stirred solution of dibenzylketone (6.26 g, 29.8 mmol) and 6 (4.76g, 22.4 mmol) in EtOH (25
mL). After 1 h, the voluminous white precipitate was filtered off with suction, washed with
EtOH and dried in vacuum to afford the product 7 as a white solid (2.4 g, 84%). Mp: 242243
oC; 1H NMR (300 MHz, CDCl3): H 8.76 (s, 2H), 8.398.38 (m, 2H), 7.567.51 (m, 2H),
7.38 (d, J = 8.0 Hz, 2H), 7.23 (t, J = 3.0 Hz, 6H), 7.15-7.10 (m, 2H), 6.99-6.95 (m, 4H),
13
4.80 (s, 2H); C NMR (75 MHz, CDCl3): C 213.3 (C), 162.5 (C), 146.5 (CH), 137.3 (CH),
133.3 (CH), 131.0 (CH), 127.7 (CH), 127.1 (CH), 123.2 (CH), 122.4 (CH), 82.2 (C), 66.0
(CH). EI-MS: m/z 422 (M+).
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Synthesis of 8.13b The POCl3 (6.48g, 15.3mmol) was added slowly to a solution of 7 (8.93g,
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21.1 mmol) in pyridine (38 mL). The reaction mixture was stirred at 85 C for 14 h. After
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cooling to room temperature, the solvent were removed under reduced pressure. The solid
o
residue was dissolved in CH2Cl2 (500 mL) and the solution was cooled to 0 C, then washed
with a saturated aqueous solution of Na2CO3 (250 mL). The organic phase was separated and
dried over anhydrous MgSO4. The solvent was removed under reduced pressure and the
brownish red crude product was recrystallized from CH2Cl2 and EtOH to afford the product 8
o
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as dark red solid (3.9 g, 66%). Mp: 198199 C (lit.13c 200201 C); H NMR (300 MHz,
CDCl3): H 8.358.33 (m, 2H), 7.567.50 (m, 2H), 7.287.21 (m, 12H), 7.117.06 (m,
13
2H); C NMR (75 MHz, CDCl3):C 200.7 (C), 153.2 (C), 153.1 (C), 149.0 (CH), 135.9 (CH),
130.1 (C), 130.0 (CH), 128.1 (CH), 128.0 (CH), 126.7 (C), 125.0 (CH), 122.4 (CH). EI-MS:
m/z 386 (M+).
Synthesis of 9. A solution of 8 (2.0 g, 5.18 mmol) in 1,5-cyclooctadiene (50 mL) was heated
at reflux for 24 h. After cooling to room temperature, the solvent were removed under reduced
pressure. The resulting residue was purified by flash column chromatography
(Hex/EtOAc/NEt3 = 6:3:1) to afford the product 9 (Rf = 0.3) as a white solid (0.1 g, 4%). Mp:
290292 oC; 1H NMR (300 MHz, CDCl3):H 7.96 (d, J = 4.3 Hz, 2H), 7.297.27 (m, 4H),
7.026.97 (m, 6H), 6.89 (t, J = 7.2 Hz, 2H), 6.64 (d, J = 7.8 Hz, 2H), 6.546.50 (m,
2H), 3.02 (s, 4H), 1.96 (d, J = 9.2 Hz, 4H), 1.57 (d, J = 9.0 Hz, 4H); 13C NMR (75 MHz,
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CDCl3):C 159.0 (C), 147.3 (CH), 143.9 (C), 143.0 (C), 134.2 (CH), 128.3 (CH), 127.0 (CH),
125.8 (CH), 125.0 (CH), 119.5 (CH), 56.1 (C), 45.8 (CH), 24.8 (CH2). EI-MS: m/z 466 (M+);
HR-MS m/z calcd for C34H30N2 (M+) 466.2409; found 466.2407.
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Synthesis of 1A. A mixture of 9 (0.28 g, 0.60 mmol) and catalytic amount of iodine (0.02 g,
0.006 mmol) in THF was stirred at room temperature and irradiated at 300 nm in a Rayonet
photoreactor for 8 h. The solvent was removed under reduced pressure and the residue was
dissolved in CH2Cl2 (100 mL), washed with 10% aq. Na2S2O3 (50 mL), and saturated aq.
Na2CO3 (50 mL). The organic layer was separated and dried over anhydrous MgSO4, then the
solvent was evaporated under reduced pressure. Flash column chromatography [eluent:
o
1
hexane/EtOAc v/v = 3:7] afforded 1A as a white solid (0.21 g, 75%). Mp: 293295 C; H
NMR (500 MHz, CDCl3): H 8.62 (d, J = 8.1 Hz, 2H), 8.18 (d, J1 = 2.7 Hz, 2H),
7.387.35 (m, 4H), 7.237.15 (m, 8H), 2.87 (s, 4H), 2.02 (d, J = 9.1 Hz, 4H), 1.75 (d,
13
J = 9.4 Hz, 4H); C NMR (125 MHz, CDCl3): δc 147.8 (C), 146.7 (CH), 145.5 (C), 142.7
(C), 129.4 (CH), 128.4 (CH), 126.8 (CH), 124.5 (CH), 123.6 (C), 119.2 (CH), 55.5 (C), 47.4
(CH), 25.4 (CH2). EI-MS: m/z 464 (M+); HR-MS m/z calcd for C34H28N2(M+) 464.2247; found
464.2246. The single crystal of 1A was recrystallized from a mixed solvent of
dichloromethane and ethanol (2:8 v/v).
X-ray single crystal data for 1A: C34H28N2, M = 464.58, monoclinic, a = 12.9576(8) Å, b =
14.6324(9) Å, c = 14.6459(16) Å, = 99.246(4)°, = 102.984(4)°, = 113.991(3)°, V =
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2371.0(3) Å3, space group P-1, Z = 4, calculated density 1.302 Mg/m-3, crystal dimensions
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(mm ): 0.72  0.27  0.07 mm T = 200(2) K, (MoKa) = 1.54178 Å, = 0.0776 mm-1, 18271
,
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reflections collected, 8215 independent (Rint = 0.0374), 649 parameter refined on F2, R1 =
0.0731, R2[F2] = 0.1393 (all data), Goodness-of-fit (GOF) on F2 1.028, max = 1.452 eÅ-3.
CCDC 987532 contains the supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data-request/cif.
Synthesis of 2A. A solution of MeI (2.84 g, 22.0 mmol) and 1A (0.1 g, 0.22 mmol) in
acetonitrile was heated at reflux for 7 d, solvent was removed under reduced pressure, and the
residue was purified by flash column chromatography [eluent: CH2Cl2/MeOH 9:1] to afford an
orange product 2A (0.06 g, 51%), which was recrystallized from EtOH / CH2Cl2. Mp:
o
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296297 C; H NMR (500 MHz, DMSO-d6): H 9.86 (d, J = 8.2 Hz, 1H), 9.23 (dd, J1 =
8.5 Hz and J2 = 1.4 Hz, 1H), 8.75 (d, J = 7.9 Hz, 1H), 8.37 (dd, J1 = 4.2 Hz and J2 = 1.5
Hz, 1H), 8.088.05 (m, 2H), 7.76 (d, J= 7.8 Hz, 1H), 7.58 (dd, J1 = 8.3 Hz and J2 = 4.2
Hz, 1H), 7.47(t, J = 7.2 Hz, 1H),(t, J = 7.2 Hz, 1H), 7.267.18 (m, 3H),
7.137.10 (m , 1H), 6.876.84 (m, 1H), 6.20 (d, J = 8.0 Hz, 1H), 4.0 (s, 3H), 3.57 (d, J
= 6.4 Hz, 1H), 2.972.95 (m, H), 2.92m, 1H, 2.322.28 (m, 3H), 2.202.14 (m,
1H), 1.961.95 (m, 1H), 1.891.85 (m, 1H), 1.691.65 (m, 1H), 1.221.14 (m, 1H),
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0.700.68 (m, 1H); C NMR (125 MHz, d6-DMSO): δC 153.2 (C), 149.9 (CH), 146.0 (CH),
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145.5 (C), 144.6 (C), 141.9 (C), 139.8 (CH), 139.7 (CH), 135.8 (C), 132.1 (CH), 129.6 (CH),
129.2 (CH), 128.7 (CH), 128.0 (CH), 127.6 (CH), 127.5 (CH), 127.3 (CH), 126.8 (CH), 126.9
(CH), 126.4 (CH), 125.2 (CH), 122.3 (C), 122.0 (CH), 58.7 (C), 57.1 (CH), 55.7 (C), 53.8
(CH), 51.6 (CH3), 42.1 (CH), 40.6 (CH), 25.6 (CH2), 25.4 (CH2), 24.8 (CH2), 24.2 (CH2).
ESI-MS: m/z 479.25 (M+); HR-MS m/z calcd for C35H31N2 (M+) 479.2482; found 479.2487.
The single crystal of 2A was obtained by recrystallization from dichloromethane/hexane (3:7
v/v).
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X-ray single crystal data for 2A: C35H31IN2, M = 606.52, monoclinic, a = 7.52510(10) Å, b =
12.2650(2) Å, c = 14.7230(3) Å, = 81.8260(10)°, = 83.4550(10)°, = 83.8270(10)°, V =
1330.52(4) Å3, space group P-1, Z = 2, calculated density 1.514 Mg/m-3, crystal dimensions
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(mm ): 0.38  0.28  0.1 mm T = 200(2) K, (MoKa) = 0.71073 Å, = 1.231 mm-1, 343
,
reflections collected, 4625 independent (Rint = 0.0443), 343 parameter refined on F2, R1 =
0.0375, R2[F2] = 0.0884 (all data), Goodness-of-fit (GOF) on F2 1.085, max = 0.449 eÅ -3.
CCDC 987534 contains the supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data-request/cif.
Synthesis of 10. To a suspension of 9 (30.0 mg, 0.064 mmol) in dry THF (2.0 mL) was added
to 50% BH3·THF (0.384 mL, 1M in THF) at 0°C under nitrogen. After vigorous stirring for 2.5
h, the solvent was removed under reduced pressure to yield 10 (31.6 mg, quantitative yield) as
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a colourless solid. Mp: 260261 C (dec.); H NMR (500 MHz, CDCl3): H 1H NMR (500
MHz, CDCl3) δH = 8.06 (d, J = 5.7 Hz, 2H), 7.81 (d, J = 7.9 Hz, 2H), 7.67 (d, J = 7.9 Hz, 2H),
7.35 (t, J = 7.3 Hz, 2H), 7.23 (d, J = 7.9 Hz, 2H), 7.08 (m, 2H), 6.97 (m, 2H), 6.89
(t, J = 7.3 Hz, 2H), 6.786.75 (m, 2H), 3.523.49 (m, 2H), 3.123.09 (m, 2H), 2.5 (br., BH3),
2.01.89 (m, 4H), 1.651.59 (m, 2H), 1.461.39 (m, 2H); 13C NMR (125 MHz, CDCl3): δC
156.9 (C), 147.2 (CH), 142.2 (C), 139.0 (C), 137.0 (CH), 129.0 (CH), 128.3 (CH), 128.1 (CH),
127.5 (CH), 126.8 (CH), 125.7 (CH), 122.1 (CH), 56.6 (C), 46.8 (CH), 44.5 (CH), 24.6 (CH2),
24.5(CH2); 11B NMR (160.5 MHz, CDCl3): δB -13.86 (br). FAB-MS: m/z 492 (M-2H)+;
HR-MS m/z calcd for C34H34N2B2 (M-2H)+ 492.2908; found: 492.2904. The single crystal of
10 was obtained by recrystallization from CH2Cl2/hexane (2:8 v/v).
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X-ray single crystal data for [10]6·CH2Cl2: C205H218B12Cl2N12, M = 3050.53, monoclinic, a =
13.1252(3) Å, b = 32.3680(8) Å, c = 41.4449(10) Å, = 90°, = 95.4200(10)°, = 90°, V =
17528.6(7) Å3, space group P-21/n, Z = 4, calculated density 1.156 Mg/m-3, crystal dimensions
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-1
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(mm ): 0.25  0.17  0.04 mm T = 200(2) K, (MoKa) = 0.71073 Å, = 0.095 mm , 122380
reflections collected, 30740 independent (Rint = 0.0685), 343 parameter refined on F2, R1 =
0.1540, R2[F2] = 0.3106 (all data), Goodness-of-fit (GOF) on F2 0.987, max = 1.027 eÅ-3.
CCDC 987535 contains the supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data-request/cif.
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Synthesis of 12. To a suspension of 9 (19.3 mg, 0.041 mmol) in dry THF (2.0 mL) was added
BF3·OEt2 (0.021 mL, 0.166 mmol) at 0°C under nitrogen. After vigorous stirring for 2.5 h, the
solvent was removed under reduced pressure to yield 12 (19.4 mg, quantitative yield) as a
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colorless solid. Mp: 242243 C; H NMR (500 MHz, CDCl3): H 8.15 (d, J = 5.6 Hz, 2H),
8.00 (t, J = 7.6 Hz, 2H), 7.83 (d, J = 8.0 Hz, 2H), 7.427.38 (m, 6H), 7.12 (t, J = 7.6 Hz, 4H),
7.00 (t, J = 7.2 Hz, 2H), 3.28 (s, 4H), 2.04 (d, J = 9.2 Hz, 4H), 1.61 (d, J = 9.6 Hz, 4H);
13C NMR (125 MHz, CDCl3): δC 150.7 (C × 2), 146.0 (CH × 2), 141.6 (C × 2), 141.1 (C × 2),
140.4 (CH × 2), 130.3 (CH × 2), 128.8 (CH × 2), 127.7 (CH × 2), 127.0 (CH × 2), 124.8 (CH
× 2), 57.5 (C × 2), 46.3 (CH × 4), 24.8 (CH2 × 4); 11B NMR (160.5 MHz, MeOH-d4): δB 0.64.
ESI-MS: m/z 234.3 (M2+); HR-MS m/z calcd for C34H32N2 (M2+) 234.1277; found 234.1275.
Synthesis of 13. To a suspension of 1A (20.0 mg, 0.043 mmol) in dry THF (3.0 mL) was
added to BF3·OEt2 (0.022 mL, 0.172 mmol) at 0°C under nitrogen. After vigorous stirring for
2.5 h, the solvent was removed under reduced pressure to yield 13 (21.2 mg, quantitative yield)
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as a colorless solid. Mp: 291292 C; H NMR (500 MHz, CDCl3): H 9.63 (d, J = 8.5 Hz,
2H), 8.44 (dd, J1 = 5.0 and J2 = 1.5 Hz, 2H), 7.85 (dd, J1 = 8.5 and J2 = 5.0 Hz, 2H),
7.647.62 (m, 4H), 7.517.48 (m, 6H), 3.09 (s, 4H), 2.16 (d, J = 9.5 Hz, 4H), 1.80 (d, J =
9.5 Hz, 4H); 13C NMR (125 MHz, CDCl3): δC 145.6 (C), 145.2 (C), 139.4 (C), 130.5 (CH),
129.7 (CH × 3), 128.8 (CH), 126.5 (C), 122.9 (C), 57.6 (C), 49.2 (CH), 26.1 (CH2); 11B NMR
(160.5 MHz, MeOH-d4): δB 1.05. ESI-MS: m/z 465.2 (M+); HR-MS m/z calcd for C34H29N2
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(M+) 465.2325; found 465.2332.
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X-ray single crystal data for 13: C34H29BF4N2, M = 552.40, monoclinic, a = 8.2710(5) Å, b =
12.9503(8) Å, c = 24.7661(14) Å, = 90°, = 99.143(4)°, = 90°, V = 2619.0(3) Å3, space
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group P-21/c, Z = 4, calculated density 1.401 Mg/m-3, crystal dimensions (mm3): 0.42  0.25 
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0.13 mm T = 200(2) K, (MoKa) = 0.71073 Å, = 0.101 mm-1, 12509 reflections collected,
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4485 independent (Rint = 0.0616), 370 parameter refined on F2, R1 = 0.1042, R2[F2] = 0.1649
(all data), Goodness-of-fit (GOF) on F2 1.039, max = 0.377 eÅ-3. CCDC 987533 contains the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data-request/cif.
Acknowledgements
We thank Mr. T.-S. Kuo of the Advanced Instrumentation Center of the National Taiwan
Normal University for X-ray crystallographic analysis. We also thank the Ministry of Science
and Technology (MOST) and the MOE ATU program of the Ministry of Education, Taiwan,
the Republic of China, for financial support.
Supporting Information
1
13
Crystallographic data of compounds 1A, 2A, 10, and 13; H- and C-NMR spectral data for
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compounds 1A, 2A, 510, and 13; B NMR spectra for compounds 10, 12, and 13; kinetic
data for methylation rate constants, and UV/vis titrations for pKa determination. This material
is available free of charge via the Internet at http://pubs.acs.org.
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