Modulation of carbenic reactivity by α-complexation to aromatics [10]

Full text of DOI:10.1021/ja973649l

1088
J. Am. Chem. Soc. 1998, 120, 1088-1089
explain the observation of a transient species.10 Here, we describe
Modulation of Carbenic Reactivity by
Re vs Ad product distribution studies for several carbenes in
isooctane, benzene, and other aromatic solvents that strongly
implicate reactiVity modulation by transient carbene-aromatic
complexes; computational studies support this contention.
The carbenes include benzylchlorocarbene (1),5 propylchloro-
carbene (2),5a and cyclopropylchlorocarbene (3).11 Each carbene
was generated photochemically (λ ) 350 nm, 25 °C) or thermally
(78 °C) from the appropriate diazirine, prepared according to
literature methods.5,11,12 The observed Re products were those
previously reported: cis- and trans-â-chloro styrenes (from 1),
cis- and trans-1-chloro-1-butene (from 2), and 1-chlorocy-
clobutene (from 3). In the presence of TME, each carbene also
afforded the anticipated cyclopropane Ad product. All products
were previously characterized,5,11 and were identified here by
capillary GC comparisons to authentic materials, GC-MS, and/
or NMR spectroscopy.
π-Complexation to Aromatics
Robert A. Moss,* Shunqi Yan, and Karsten Krogh-Jespersen*
Department of Chemistry, Rutgers
The State UniVersity of New Jersey
New Brunswick, New Jersey 08903
ReceiVed October 21, 1997
Attempts to control carbenic reactivity by intermediate car-
bene-substrate complex formation have a long history in which
the most successful example is the control of stereoselectivity in
the syn-addition of the Simmons-Smith carbenoid to cyclic allylic
or homoallylic alcohols.1 Analogous control of the reactivity of
free carbenes, which lack the carbenoid’s metal center carbene-
substrate “link”, is more problematical. Interactions of carbenes
with substrate O, S, or N n-electrons certainly afford ylides,2 but
efforts to obtain concomitant synthetic control with (e.g.) meth-
ylene3 or dichlorocarbene4 have met with limited success.
Even more difficult, because the interaction energies should
be smaller, is the modulation of carbenic reactivity by intermediate
carbene-π electron complexation. Liu and Bonneau maintain
that the transient formation of carbene-alkene π-complexes is
integral to the competition between intramolecular rearrangement
(Re) and intermolecular addition (Ad) when alkylhalocarbenes
are generated in the presence of alkenes.5,6 However, Houk’s
calculations suggest that additions of chlorocarbenes to ethylene
or tetramethylethylene (TME) proceed to cyclopropanes without
transiting bound carbene-alkene complexes.7a-c Alternatively,
broad shallow wells for complexes might occur in the reaction
enthalpy profile, but would not constitute minima on the free
energy surface.7d Additionally, Platz8 and Goodman9 offer
alternative explanations, based on the intervention of excited state
carbene precursors, for the kinetic anomalies that originally
triggered the suggestion of carbene-alkene complex participa-
tion.5,6
• •
• •
• •
C
PhCH2CCl
CH3CH2CH2CCl
Cl
2
3
1
In Table 1, we collect the Re/Ad product ratios for each
carbene, determined in isooctane, benzene, or anisole solvents,
in the presence of (e.g.) 0.25 M TME. Product ratios are averages
of three experiments with three capillary GC analyses averaged
for each experiment. Controls demonstrated that the products
were stable to the thermal conditions, and that there were no
products derived from reactions with the solvents (particularly
benzene and anisole).
With carbenes 1 and 2, the extent of 1,2-H rearrangement
clearly increases, relative to intermolecular addition to TME, as
the solvent is changed from isooctane to benzene. The enhance-
ment of Re occurs with both thermally and photochemically
generated species, and is especially pronounced with propylchlo-
rocarbene. The behavior of carbene 3 indicates that the effect
also extends to 1,2-C rearrangement. At lower [TME], the
enhancement of Re/Ad is even stronger. For example, with
photolytically generated carbenes and [TME] ) 0.11-0.13 M,
the Re/Ad distributions in isooctane vs benzene are 2.2 vs 8.3
for 1, and 15.2 vs 109 for 2.
We believe that these “solvent effects” on product distribution
are manifestations of transient carbene-aromatic π complexes
that interfere with intermolecular addition, extend the carbene
lifetimes, and afford greater opportunity for intramolecular
rearrangement (within the complex). It is of course known that
changes in solvent polarity (e.g. pentane to acetonitrile) enhance
the rate of 1,2-H shifts,8b,13 but the dielectric constants, polariz-
abilities, and viscosities of isooctane and benzene are similar,14
so that our observed effects are likely to have a more specific
genesis. Consistent with a π complex origin for the enhancement
of Re/Ad are the further increases seen in solvent anisole (Table
1), although here the polarity comes into play because the
dielectric constant increases significantly between isooctane (1.94)
Although product-determining carbene-alkene π complexes
are questionable, carbene-benzene π complexes may be more
robust, if only because their direct continuation to product will
be energetically less favorable than in the case of alkenes. Indeed,
in a recent calorimetric study of photolytic diazomethane decom-
position in benzene, Khan and Goodman invoked the formation
of weak complexes between singlet CH2 and benzene in order to
(1) Simmons, H. E.; Cairns, T. L.; Vladuchick, S. A.; Hoiness, C. M. Org.
React. 1973, 20, 1. Winstein, S.; Sonnenberg, J.; de Vries, L. J. Am. Chem.
Soc. 1959, 81, 6523.
(2) (a) Platz, M. S.; Maloney, V. M. In Kinetics and Spectroscopy of
Carbenes and Biradicals; Platz, M. S., Ed.; Plenum Press: New York, 1990;
especially pp 333f. (b) Jackson, J. E.; Platz, M. S. In AdVances in Carbene
Chemistry; Brinker, U. H., Ed.; JAI Press: Greenwich, CT, 1994; Vol. 1, pp
89f. (c) Padwa, A.; Hornbuckle, S. F. Chem. ReV. 1991, 91, 263.
(3) Young, T. A.; O’Rourke, C.; Gray, N. B.; Lewis, B. D.; Dvorak, C.
A.; Kuen, K. S.; DeLuca, J. P. J. Org. Chem. 1993, 58, 6224. Cubbage, J.
W.; Edelbach, B. L.; Kuen, K. S.; DeLuca, J. P. Tetrahedron 1997, 53, 9823.
(4) Seyferth, D.; Mai, V. A. J. Am. Chem. Soc. 1970, 92, 7412. Moss, R.
A. J. Am. Chem. Soc. 1972, 94, 6004. Ellison, R. H. J. Org. Chem. 1980, 45,
2509. Moss, R. A.; Mallon, C. B. J. Org. Chem. 1975, 40, 1368.
(5) (a) Bonneau, R.; Liu, M. T. H.; Kim, K. C.; Goodman, J. L. J. Am.
Chem. Soc. 1996, 118, 3829. (b) Liu, M. T. H. Acc. Chem. Res. 1994, 27,
287. (c) Liu, M. T. H.; Soundararajan, N.; Paike, N.; Subramanian, R. J. Org.
Chem. 1987, 52, 4223. (d) Tomioka, H.; Hayashi, N.; Izawa, Y.; Liu, M. T.
H. J. Am. Chem. Soc. 1984, 106, 454.
(10) Khan, M. I.; Goodman, J. L. J. Am. Chem. Soc. 1995, 117, 6635.
(11) Moss, R. A.; Ho, G.-J.; Shen, S.; Krogh-Jespersen, K. J. Am. Chem.
Soc. 1990, 112, 1368. Ho, G.-J.; Krogh-Jespersen, K.; Moss, R. A.; Shen, S.;
Sheridan, R. S.; Subramanian, R. J. Am. Chem. Soc. 1989, 111, 6875. Bonneau,
R.; Liu, M. T. H.; Rayez, M. T. J. Am. Chem. Soc. 1989, 111, 5973. Moss,
R. A.; Fantina, M. E. J. Am. Chem. Soc. 1978, 100, 6788.
(12) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
(13) Sugiyama, M. J.; Celebi, S.; Platz, M. S. J. Am. Chem. Soc. 1992,
114, 966.
(14) Lide, D. R., Ed. Handbook of Chemistry and Physics, 77th ed.; CRC
Press: Boca Raton, FL, 1996; pp 6-160f, 10-205f, 6-210f. The question of
“polarity” is perhaps moot for benzene because the expression of a solvent
effect would involve distortion of its π electrons and be tantamount to π
complexation.
(6) Review: Moss, R. A. In AdVances in Carbene Chemistry; Brinker, U.
H., Ed.; JAI Press: Greenwich, CT, 1994; Vol. 1, pp 59f.
(7) (a) Houk, K. N.; Rondan, N. G.; Mareda, J. J. Am. Chem. Soc. 1984,
106, 4291. (b) Houk, K. N.; Rondan, N. G.; Mareda, J. Tetrahedron 1985,
41, 1555. (c) Keating, A. E.; Garcia-Garibay, M. A.; Houk, K. N. J. Am.
Chem. Soc. 1997, 119, 10805. We thank Prof. Houk for a preprint. (d) Blake,
J. F.; Wierschke, S. G.; Jorgensen, W. L. J. Am. Chem. Soc. 1989, 111, 1919.
(8) (a) White, W. R., III; Platz, M. S. J. Org. Chem. 1992, 57, 2841. (b)
Modarelli, D. A.; Morgan, S.; Platz, M. S. J. Am. Chem. Soc. 1992, 114,
7034.
(9) LaVilla, J. A.; Goodman, J. L. Tetrahedron Lett. 1990, 31, 5109.
S0002-7863(97)03649-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/22/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 5, 1998 1089
Table 1. Rearrangement/Addition Product Distributions of
Carbenes
Re/Ada
carbene conditions T, °C isooctane
benzene
anisole
1
1
2
2
3
3
hν
∆
25 1.20 (0.05) 2.54 (0.01) 3.70 (0.08)
78 4.91 (0.28) 15.6 (0.5)
25 2.69 (0.08) 18.2 (0.4)
78 4.47 (0.12) 19.7 (0.4)
25 0.51 (0.02) 1.04 (0.02)
78 1.61 (0.07) 2.87 (0.02)
hν
∆
26.6 (0.1)
hν
∆
a
Rearrangement to addition product distribution; average deviations
Figure 1. Geometry-optimized ab initio (MP2/6-31G*) structures for
carbene-benzene π-complexes: (a) CCl2-benzene, r1 ) r3 ) 3.48 Å, r2
) 3.14 Å; (b) MeCCl-benzene, r1 ) 3.15 Å, r2 ) 3.45 Å. See text for
details.
of three experiments (with three capillary GC analyses each) in
parentheses. The product distributions are calibrated for relative GC
detector response for 1 and 2, but not for 3. [TME] ) 0.25 M.
barriers. However, with CCl2 or MeCCl (model for 2) as the
featured carbenes and benzene (Bz) or anisole as aromatic
substrates, 1:1 complexes were readily located.18 Our minimum
energy conformation located for CCl2-Bz (Figure 1a) resembles
the minimum energy structure proposed by Khan and Goodman
for CH2-Bz.10 The CCl2-Bz complex has the carbenic carbon
positioned almost directly above a Bz carbon (∼3.1 Å) but slightly
“outside” the ring; the Cl atoms are pointed inward, above the
Bz π-electron cloud. Weak interaction is indicated by the small
computed binding energy (∆E) and enthalpy (∆H) values of only
-3.23 and -2.56 kcal/mol, respectively; the free energy of
binding (∆G) is +2.62 kcal/mol, implying an equilibrium constant
for complex formation significantly less than one (Keq ∼ 10-2).
The minimum energy MeCCl-Bz complex (Figure 1b) also
has the carbenic center positioned ∼3.1 Å above a Bz carbon,
“outside” of the Bz ring, but the carbene is rotated so that the
Me group is directed toward the center of the Bz ring. The Cl
atom in this complex is well “outside” the ring. The energetics
for MeCCl-Bz complex formation are more favorable than those
for CCl2-Bz: ∆E ) -4.81 kcal/mol, ∆H ) -3.54 kcal/mol,
and ∆G ) +0.95 kcal/mol, respectively. The alignment displayed
by CCl2 and, in particular, MeCCl with respect to an individual
C-C bond maximizes the π-type interactions, as emphasized by
Hoffman.19 When the aromatic substrate is changed to the more
electron-rich anisole, the interaction energies for the π complex
increase further to ∆E ) -6.96 kcal/mol, ∆H ) -5.65 kcal/
mol, and ∆G ) +1.79 kcal/mol.20 A search for a minimum on
the MeCCl-pentane potential energy surface led to a marginally
stabilized complex showing interaction energies only of the order
of thermal energies (∆E ) -2.52 kcal/mol, ∆H ) -1.31 kcal/
mol, and ∆G ) +2.11 kcal/mol). Complete details of the
electronic structure calculations and results of additional studies
of 1:2 carbene-aromatic and 1:1:1 aromatic-carbene-alkene
“sandwich” complexes will be reported elsewhere.
or benzene (2.28) and anisole (4.30). Additionally, anisole
possesses oxygen n electrons which could also complex with the
carbenes.15
We were concerned that the observed solvent sensitivity of
Re/Ad might mainly reflect solvent-induced changes in the extent
of rearrangement products derived directly from excited-state
diazirines.6,8,9 We therefore determined correlations of Ad/Re vs
[TME] in isooctane or benzene for photolytically generated 1-3.
For carbenes 1 and 2, these correlations were curved5 in both
solvents, consistent with intervention of a second intermediate.5a
However, the reciprocal correlations, Re/Ad vs 1/[TME], were
linear, and the extents of excited-state diazirine participation could
be determined from their Y-intercepts.6,16 These were 24% from
the precursor of 1 and 51% from the precursor of 2 in isooctane,
whereas in benzene, the analogous values were 20% and 30%.
The correlations of Ad/Re vs [TME] for cyclopropylchlorocarbene
(3) were linear in both solvents, suggesting minimal contributions
of excited diazirine to 1,2-C shift product formation, in accord
with a prior analysis.8a
Although there are substantial contributions of excited diazirine
precursors to the rearrangement products when carbenes 1 and 2
are photolytically generated, this is not the origin of the benzene
vs isooctane solvent effect on Re/Ad. Thus, the excited-state
contribution to rearrangement is greater in isooctane than in
benzene for both carbenes, whereas the rearrangement-enhancing
solvent effect (Table 1) operates in benzene, not isooctane.
Moreover, the latter effect is pronounced with the thermally
generated carbenes, when excited-state involvement is minimal.
Additionally, the benzene effect is enhanced at low [TME], where
the carbenic origin of the Re products is maximal, and the effect
persists with carbene 3, where excited-state products are insig-
nificant.
Another anticipated consequence of π complex formation is
the extension of carbene lifetime in benzene vs isooctane. By
laser flash photolysis, using the pyridine probe method,2b,6,17a we
determined the rates of rearrangement of carbenes 1 and 2 in
isooctane and benzene, which afforded carbene lifetimes (τ )
1/kre). These were the following: 1, 23 ns (isooctane) and 285
ns (benzene) and 2, 33 ns (isooctane) and 121 ns (benzene).17b
The importance of π-complex formation was further examined
by ab initio computational methods. Our findings accord with
those of Houk et al.7 that stable carbene-alkene complexes, in
particular complexes between CCl2 and ethylene or TME, do not
exist; cyclopropanes form with no apparent activation energy
Our interpretation of the computational results is that ap-
preciable interaction energies may arise between carbenes and
aromatic π-donors in solution, and that carbene-aromatic com-
plexes can exist with sufficiently long lifetimes to affect the Re/
Ad behavior of (at least) alkylhalocarbenes. In the limit of strong
carbene-solvent complex formation, the complex might only be
capable of rearrangement.21
Acknowledgment. We thank the National Science Foundation for
financial support.
JA973649L
(15) Interestingly, there is a further increase in Re/Ad (to 8.00) for
photolytically generated 1 in 1,2-dimethoxybenzene (ꢀ ) 4.4514), but a decrease
(to 1.86) in trifluoromethylbenzene (under the conditions of Table 1). These
results track the electron donation/withdrawal of the solvent substituents, as
anticipated for carbene-π complex mediation.
(18) Computational procedures include geometry optimization at both the
HF/6-31G* and MP2/6-31G* levels of theory. Binding energies (∆E, 0 K)
are derived directly from the MP2/6-31G* calculations. Normal-mode analyses
at the HF/6-31G* level provide the thermodynamic corrections needed to
convert ∆E to binding enthalpy (∆H, 298 K) and free energy (∆G, 298 K).
(19) Hoffmann, R. J. Am. Chem. Soc. 1968, 90, 1475.
(16) See, for example: Moss, R. A.; Ho, G.-J. J. Phys. Org. Chem. 1993,
6, 126. Moss, R. A. Pure Appl. Chem. 1995, 67, 741.
(17) (a) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H. J.
Am. Chem. Soc. 1988, 110, 5595. (b) The rate constants of pyridine ylide
formation (kpy) are large, but a retardation in kpy is observable due to carbene
complex formation in benzene. With carbene 2, for example, kpy ) 2.7 × 109
M-1 s-1 in isooctane and 3.7 × 108 M-1 s-1 in benzene.
(20) An ylidic complex is comparable in energy.
(21) R. T. Ruck and M. Jones, Jr., find that the intramolecular reactivity
of tert-butylcarbene is sensitive to solvent, including benzene, in a manner
consistent with the effects we describe. We thank Prof. Jones for communica-
tion of these results.