Complexation of dichlorocarbene by methylanisoles

Article abstract of DOI:10.1016/j.tetlet.2010.01.023

Dichlorocarbene generated by laser flash photolysis of dichlorodiazirine readily forms UV-vis active π- and O-ylidic complexes with methylanisole derivatives.

Full text of DOI:10.1016/j.tetlet.2010.01.023

Tetrahedron Letters 51 (2010) 1467–1470
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Tetrahedron Letters
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Complexation of dichlorocarbene by methylanisoles
Robert A. Moss , Lei Wang, Christina M. Odorisio, Karsten Krogh-Jespersen *
*
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Dichlorocarbene generated by laser flash photolysis of dichlorodiazirine readily forms UV–vis active p-
and O-ylidic complexes with methylanisole derivatives.
Received 16 December 2009
Revised 6 January 2010
Accepted 7 January 2010
Available online 14 January 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Carbenes
Kinetics
Spectroscopy
Calculations
We recently reported that dichlorocarbene (CCl
laser flash photolysis (LFP) of dichlorodiazirine, formed
2
), generated by
- and O-
and
p
-complex absorptions at 460 and 500 nm, respectively.1 In
p
the event, this proved a successful strategy, enabling us to describe
here the CCl complexes of 4-methylanisole (3), 2,4-dimethylani-
sole (4), 2,4,6-trimethylanisole (5), and 2,3,5,6-tetramethylanisole
(6).
ylidic complexes with aryl ethers such as anisole, 1,3-dimethoxy-
2
1
benzene, and 1,3,5-trimethoxybenzene. Detected by UV–vis spec-
troscopy, these complexes significantly retarded the addition of
CCl
2
to tetramethylethylene (TME). Structures, energetics, and
OMe
OMe
OMe
OMe
spectral assignments for the complexes were provided by compu-
tational studies based on density functional theory.
Mesitylene (1) and durene (2) also retarded the addition of CCl
to TME, by factors of 15 and 21, respectively, but we did not ob-
Me Me
Me
Me
Me
Me
Me
2
2
serve the UV absorptions computed for their CCl complexes in
Me
Me
Me
the 300–350 nm region.1 The sensitivity of our LFP detection sys-
tem may possibly be reduced in this spectral region due to strong
background absorption by dichlorodiazirine (kmax 324, 359 nm in
4
5
6
3
Compounds 3 and 4 were commercially available, while trime-
thylanisole 5 was obtained by methylation of 2,4,6-trimethylphenol
with dimethyl carbonate and a catalytic quantity of tetrabutylam-
monium iodide (93 °C, 19 h). Tetramethylanisole 6 was similarly
prepared from the corresponding phenol using 2,3,5,6-tetramethyl-
2
pentane).
Me
3
Me
Me
phenol obtained by ortho-methylation of 2,3,5-trimethylphenol
4
Me
with ethyl(iodomethyl)zinc. Compounds 5 and 6 were character-
1
13
Me
ized by H and C NMR spectroscopy (see Figs. S-1–S-4 in the
Supplementary data), and by mass spectrometry.
Me
Me
1
2
2
LFP of dichlorodiazirine (7) in 1:1 (v/v) pentane solutions of
It occurred to us that the substitution of a methoxy group for a
methylanisoles 3–6 affords the UV–vis spectra shown in Figure 1.
proton on arenes such as 1 or 2 would shift the electronic spectra
Each spectrum exhibits two distinct absorption peaks in the 450–
of their CCl
gion, where they might be more readily detected. For example,
the UV–vis spectrum of the CCl /anisole complex features O-ylidic
2
complexes toward the red, into the 400–500 nm re-
550 nm region, which we attribute to CCl
2
complexes.
2
N
Cl
_
CCl2
+
N
Cl
*
Corresponding authors. Tel.: +1 732 445 2606; fax: +1 732 445 5312 (R.A.M.).
E-mail addresses: moss@rutchem.rutgers.edu (R.A. Moss), krogh@rutchem.
rutgers.edu (K. Krogh-Jespersen).
N
7
8
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.01.023

1
468
R. A. Moss et al. / Tetrahedron Letters 51 (2010) 1467–1470
Table 1
Absorption maxima of CCl
Table 1 records the wavelengths of these species, and includes
-methylanisole complexesa
max (nm)
2
the previously reported O-ylidic and
p-complexes of the parent
anisole.1
Ligand
k
We note that both absorptions, in each spectrum of Figure 1,
Anisoleb
460
484
488
508
516
500
516
520
532
548
shift to longer wavelengths in response to each additional methyl
3
4
5
6
5
2
substituent imposed on the parent anisole structure. CCl usually
6
(
although not always ) behaves as an electrophile, so that this
observation accords with enhanced electron donation by the meth-
a
ylanisole derivative to an electrophilic CCl
cessive methyl group substitutions.
2
, buttressed by the suc-
In 1:1 (v/v) pentane/ligand solutions.
From Ref. 1.
b
Theoretical considerations generally support this view: calcula-
tions based on density functional theory (see the Supplementary
data for details) provide structures and energetics for the transient
(C or O) at a particular methylanisole binding site and binding en-
thalpy. (See Fig. S-9 in the Supplementary data for net atomic
charges of the methylanisoles.)
2
CCl /methylanisole species and largely rationalize their absorption
spectra. Computed complexation enthalpies are low for all identi-
fied complexes, spanning a range from À2.6 to À4.3 kcal/mol,
whereas the (standard) free energies are well into the positive
realm (ꢀ4–5 kcal/mol, PBE/6-311+G(d); see Table S-1 in the Sup-
plementary data). Thus, the binding of individual complexes is
Illustrative examples of typical O-ylidic and
p-complexes are
shown in Figure 2, using 2,4-dimethylanisole as the representative
ligand. In the O-ylidic complexes (Fig. 2, left-hand structure), the
C(carbene)-O distance is in the 2.4–2.6 Å range, and the O atom
rehybridizes toward tetrahedral coordination. The smallest C(car-
2
weak. However, there are many CCl /methylanisole configurations
with slightly variant carbene-ligand orientations and/or interac-
tion distances, all of approximately equal energies, and so the net
number of complexes formed is sufficiently large to be detected.
The binding enthalpies tend to become more favorable as the
bene)-C(phenyl) interaction distances in the p-complexes (Fig. 2,
center) are slightly longer at 2.6–2.9 Å; C(carbene) is always lo-
cated vertically above a C(phenyl) atom, and one of the Cl atoms
points toward the center of the phenyl ring.
degree of methylation increases, in particular for the
p
-type com-
The electronic transitions of lowest energy computed for each
complex (TD-B3LYP/6-311+G(d)//PBE/6-311+G(d)) are also listed
in Table S-1. As detailed previously, the O-ylidic species was com-
plexes, whereas the binding enthalpies of the O-ylidic complexes
lie within a smaller range of À2.6 to À3.5 kcal/mol. Generally,
binding increases with increasing electron richness of the aromatic
ring, but we find no simple relationship between net atomic charge
1
puted to be the thermodynamically most stable of the parent CCl
2
/
anisole complexes ( G = 3.7 kcal/mol), while the relative stabilities
D
0
0
0
0
0
0
0
.0030
.0025
.0020
.0015
.0010
.0005
.0000
0
.0018
488nm
520nm
4
84nm
0.0016
5
16nm
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
-0.0002
2
00
300
400
500
600
700
200
300
400
500
600
700
wavelength, nm
wavelength, nm
0
.0020
0
0
0
0
0
0
0
0
.0014
.0012
.0010
.0008
.0006
.0004
.0002
.0000
5
08nm
5
32nm
0.0018
516nm
0.0016
0.0014
0.0012
0.0010
0.0008
548nm
0.0006
0.0004
0.0002
0.0000
-
0.0002
-0.0002
2
00
300
400
500
600
700
200 250 300 350 400 450 500 550 600 650
wavelength, nm
wavelength, nm
Figure 1. Complexes of CCl
tetramethylanisole (6). All spectra were determined in 1:1 (v/v) pentane-ligand solutions, under nitrogen, 150 ns after the laser flash. The axes of each graph are labeled ‘A’
for absorbance) and ‘wavelength, (nm).’
2
with (upper left) 4-methylanisole (3); (upper right) 2,4-dimethylanisole (4); (lower left) 2,4,6-trimethylanisole (5); (lower right) 2,3,5,6-
(

R. A. Moss et al. / Tetrahedron Letters 51 (2010) 1467–1470
1469
of the three different
p
-complexes were about equal (
D
G ꢀ 4.4–
Table 2
Rate constants for the addition of CCl
2
to TMEa
5
.0 kcal/mol). The experimental absorption around 460 nm for
À1 À1
CCl
2
/anisole was assigned to the O-ylidic complex for which a
Solvent or additive
kadd (M
s
)
kpent/kadd
strong absorption band with significant O?p(CCl
2
) character was
predicted at 486 nm (f = oscillator strength = 0.11). The absorption
observed at 500 nm was assigned to the -complexes collectively;
individual absorbances [ (anisole)?p(CCl )] were predicted at
Pentaneb
Anisole
3
4
5
6
9
4.7 Â 10
2.6 Â 10
1.0 Â 10
7.8 Â 10
5.5 Â 10
4.9 Â 10
1.0
18
47
60
85
96
c
8
8
7
7
7
p
p
2
5
13 nm (f ꢀ 0.1) for both the C2 and C4 coordinated complexes
1
and at 534 nm (f = 0.08) for C6 coordination.
a
At 25 °C; [pyridine] = 0.12 mM for anisole, 3, and 4; 2.4 mM for 5 and 6.
Rate constant from Ref. 8.
Rate constant from Ref. 1.
2
In the present study, the absorptions of the CCl /4-methylani-
b
c
sole complexes are red-shifted by about 20 nm relative to the ani-
sole complexes. The O-ylidic CCl /3 complex is computed to be the
2
most stable and to absorb at 507 nm (f = 0.14), so that we assign
this species as responsible for the observed absorption at 484 nm
464 nm (f ꢀ 0.1). Similarly, for CCl
2
and 6, we compute p-com-
(
see Fig. 1). We have located
ring location (Table S-1), and sites C5 and C6 are particularly favor-
able for CCl binding. The resulting complexes absorb at 544 nm
f = 0.09, C5) and 557 nm (f = 0.08, C6), respectively, consequently,
p
-complexes at every possible phenyl
plexes at C1 and C4 which absorb at 506 nm and 486 nm, respec-
tively, with f ꢀ 0.1. Two absorptions of the O-ylidic complex
should center around 455 nm with overall intensities similar to
2
(
those of the
p-complexes.
we assign these complexes as the dominant contributors to the
absorption observed at 516 nm (Fig. 1).
In summary, for CCl
2
complexes with methylanisoles 5 and 6,
the calculations predict three absorption peaks in the 460–
510 nm region of similar intensities and nearly equidistant spacing
(ꢀ20 nm), whereas experiment shows only two distinct peaks in
the 510–550 nm region (cf., Fig. 1 and Table 1). We are thus unable
For CCl
-complexes at the C1 and C6 binding sites to be the most stable
cf., Fig. 2, center); both sets of complexes are predicted to absorb
2
and 2,4-dimethylanisole (4), the calculations find the
p
(
around 570 nm with substantial intensities (f ꢀ 0.1), and we assign
to make specific assignments of the CCl
spectra to individual complexes.
2
2
/5 or CCl /6 absorption
these complexes to the experimental absorption peak at 520 nm in
1
Figure 1. The O-ylidic complex (Fig. 2, left) and
formed at C5 absorb about 30–35 nm further to the blue in the
35–540 nm region (both absorptions have f = 0.12), and we assign
p
-complexes
In keeping with previous observations, complexation of CCl
2
with methylanisoles 3–6 diminishes the rate constants for the
to TME. We used the pyridine ylide method1 to
,7
5
addition of CCl
determine rate constants for the addition of CCl
and in the presence of anisole and the methylanisoles; cf. Table 2.
The apparent rate of the formation of the CCl -pyridinium ylide
2
these complexes to the observed absorption peak at 488 nm in
Figure 1.
In ligands 5 and 6, the methyl group of the methoxy unit rotates
to assume a position perpendicular to the aryl ring plane because
of steric hindrance from the adjacent methyl groups on C2 and
2
to TME in pentane
2
8, measured at 404 nm in pentane or a pentane-additive solution,
increased upon the addition of TME at a constant concentration
of pyridine. A correlation of the observed rate constants for the for-
mation of 8 versus [TME] was linear and its slope gave kadd for the
C6, thus diminishing
p-donation by the methoxy group. Although
the orbital energies of the upper occupied
p
-orbitals of the arene
7
a
become less negative (i.e., are destabilized) from anisole to the
methylanisoles 3 and 4, the rotation of the methoxy group leads
to a reversal of this trend for the HOMO’s of 5 and 6 (Table S-2
in the Supplementary data). The energy of the donor orbital enters
directly into the expression for the electronic transition energy
2 2
addition of CCl to TME. In this way, we determined that CCl
generated by LFP of diazirine 7 adds to TME in pentane with
9
À1 À1 8
k
add = 4.7 Â 10 M
s . Repetition of this experiment in 1:1 ani-
8
À1 À1
sole-pentane gave kadd = 2.6 Â 10
M
s
; see Figure 8 of Ref. 1
and Table 2, above.
and, as a result, the CCl
experience blue shifts relative to analogous complexes formed
with 3 and 4. In contrast, the observed absorptions for the CCl
complexes of 5 and 6 are red shifted by 10–30 nm, relative to com-
plexes with 4 (cf. the spectra in Fig. 1 and the data in Table 1). For
2
complexes of 5 and 6 are computed to
Similarly, we measured kadd for the CCl -TME addition in 1:1
2
solutions of pentane and methylanisoles 3–6. The results appear
in Table 2, and the corresponding correlations of kobs with [TME]
are shown in Figs. S-5–S-8 of the Supplementary data. Table 2 re-
veals that successive methylations of anisole lead to increased retar-
2
CCl
2
and 5 we located complexes of equal stability (
D
G ꢀ 3.8 kcal/
2 2
dation of CCl addition to TME. One interpretation holds that CCl
mol) at C1 and C3 (the C1 complex is shown in Fig. 2, right) which
should absorb at 490 nm and 511 nm, respectively, with f ꢀ 0.1.
The O-ylidic complex is computed to be slightly less stable
complexes with increasingly methylated anisole derivatives are
not only more stable, but also slower to transfer their complexed
carbene to TME, possibly due to a combination of steric and stabil-
ization effects. Alternatively, if complexation is reversible, the more
(D
G ꢀ 4.4 kcal/mol) and absorb slightly further to the blue at
2 2 2
Figure 2. Perspective and depth-fading illustration of CCl /methylanisole complexes: O-ylidic CCl /2,4-dimethylanisole complex (left); CCl /2,4-dimethylanisole p-complex
at C6 (center); CCl /2,4,6-trimethylanisole -complex at C1 (right).
2
p

1
470
R. A. Moss et al. / Tetrahedron Letters 51 (2010) 1467–1470
highly methylated anisoles may sequester more CCl
2
at equilibrium,
Supplementary data
thus leaving less free CCl available for addition to TME.
2
Tri- and tetramethylanisoles 5 and 6 are more potent than their
non-methoxy analogs, mesitylene (1) and durene (2), at retarding
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.01.023.
CCl
are 5.8 and 3.1, respectively. These disparities probably reflect
the additional stabilization conferred upon the mesitylene or dur-
ene CCl complexes by substitution of the electron-donating meth-
2
addition to TME: the ratios of kadd for 1 versus 5 and 2 versus
9
6
References and notes
2
1.
Moss, R. A.; Wang, L.; Odorisio, C. M.; Zhang, M.; Krogh-Jespersen, K. J. Phys.
Chem. A 2010, 114, 209.
oxy group which converts them to complexes of 5 and 6,
respectively.
2. Moss, R. A.; Tian, J.; Sauers, R. R.; Ess, D. H.; Houk, K. N.; Krogh-Jespersen, K. J.
Am. Chem. Soc. 2007, 129, 5167.
In conclusion, CCl
readily visualized - and O-ylidic complexes with methylanisoles
–6. These complexes are UV–vis active and are substantiated by
DFT computations. Complexation of CCl by the methylanisoles re-
tards the addition of the carbene to TME by factors of about 50–
00.
2
generated by LFP of dichlorodiazirine forms
3.
Taniguchi, Y.; Nobori, T.; Yamamoto, Y. Jpn. Kokai Tokkyo Koho Japanese Patent
004 123541 (22 April 2004). We thank Professor Akira Sekiguchi for
p
2
a
3
translation of the procedure.
4.
5.
6.
Lehnert, E. K.; Sawyer, J. S.; McDonald, T. L. Tetrahedron Lett. 1989, 30, 5215.
Moss, R. A. Acc. Chem. Res. 1980, 13, 58.
Moss, R. A.; Zhang, M.; Krogh-Jespersen, K. Org. Lett. 2009, 11, 1947.
2
1
7. (a) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H. J. Am. Chem. Soc.
988, 110, 5595; (b) Presolski, S. I.; Zorba, A.; Thamattoor, D. M.; Tippmann, E.
1
M.; Platz, M. S. Tetrahedron Lett. 2004, 45, 485.
Moss, R. A.; Tian, J.; Sauers, R. R.; Skalit, C.; Krogh-Jespersen, K. Org. Lett. 2007, 9,
Acknowledgments
8
.
.
4
053.
8
À1
À1) and 2 (1.5 Â 108
À1 À1
9
Values of kadd for 1 (3.3 Â 10
M
s
M
s ) appear in
We are grateful to the National Science Foundation and to the
Petroleum Research Fund for financial support.
Ref. 1.