A phenyliodonium ylide as a precursor for dicarboethoxycarbene: Demonstration of a strategy for carbene generation [1]

Full text of DOI:10.1021/ja000334o

5210
J. Am. Chem. Soc. 2000, 122, 5210-5211
Communications to the Editor
A Phenyliodonium Ylide as a Precursor for
Dicarboethoxycarbene: Demonstration of a Strategy
for Carbene Generation
Margaret B. Camacho, Aurora E. Clark,
Tabitha A. Liebrecht, and JoAnn P. DeLuca*
Department of Chemistry
Central Washington UniVersity
Ellensburg, Washington 98926-7539
ReceiVed January 31, 2000
ReVised Manuscript ReceiVed April 4, 2000
Thermal or photochemical decomposition of carbene precur-
sors, especially diazocompounds and diazirenes, leads in many
cases to both a free carbene intermediate and products formed
directly from the precursor.1 This ability of precursors to mimic
carbenes often complicates mechanistic studies. For example,
,2
direct photolysis of dimethyl diazomalonate (1a) leads to products
1
stemming from reactions of substrate with :C(CO
2
Me)
2 2
, :C(CO -
3
1
3
2
Me) , and photoexcited 1a ( 1a*) as outlined in Scheme 1. In
situations such as this, alternative modes of carbene generation
have proved valuable in characterizing the reactive species. Here
4
Figure 1. B3LYP/LANL2DZ optimized structures.
we describe a strategy for identifying new carbene precursors and
Scheme 1
1
report the facile generation of :C(CO
2
Et)
2
by thermal decom-
position of iodonium ylide 1b, as well as a supporting compu-
tational study.
Reaction with compounds containing atoms with unshared
electron pairs to produce ylides is one of the typical reactions of
singlet carbenes (eq 1). If reversible, this reaction might be useful
6
with carbene, or at least carbenoid, intermediates, yet they have
not previously been exploited as sources of true carbenes.
Although they are known to be less stable than the analogous
diazonium ylides, no information is available concerning the
strength of the carbene-nucleophile bond in iodonium ylides.
Paying particular attention to these ylides vs diazo compounds
as sources of free carbenes, we have started to carefully
characterize the photochemical and thermal decompositions of
selected iodonium ylides through both computational and labora-
tory studies.
5
as a source of carbenes. Inspection of eq 1 leads to the prediction
that the weaker the carbene-nucleophile bond, the more likely
that thermal, and possibly photochemical, decomposition of the
ylide will lead to free carbenes without intervention of masquer-
ading precursors.
As the first step in the computational study, structures were
calculated for N -CH , HI-CH (2) and PhI-CH (3), using
the B3LYP density functional method and LANL2DZ basis set,
and the Gaussian 94 software package. Two stable conformations
were found for both HI-CH and PhI-CH and were confirmed
Iodonium ylides have been shown to decompose under thermal,
photochemical, and catalytic conditions to give products consistent
2
2
2
2
7
8
9
(1) (a) Frey, H. M.; Stevens, I. D. R. J. Chem. Soc. 1965, 3101. (b) Mansoor,
A. M.; Stevens, I. D. R. Tetrahedron Lett. 1966, 1733. (c) Chang, K.-T.;
Shechter, H. J. Am. Chem. Soc. 1979, 101, 5082. (d) Chambers, G. R.; Jones,
M., Jr. J. Am. Chem. Soc. 1980, 102, 4516. (e) Modarelli, D. A.; Morgan, S.;
Platz, M. S. J. Am. Chem. Soc. 1992, 114, 7034.
2
2
as minima through calculation of vibrational frequencies. The
calculated structures and selected bond distances are shown in
Figure 1. The C-I bond distance in the more stable conformation
(
2) Kirmse, W. Carbene Chemistry, 2nd ed.; Academic Press: New York,
971.
3) (a) Jones, M., Jr.; Ando, W.; Hendrick; M. E.; Kulczycki, A.; Howley,
P. M.; Hummel, K. F.; Malament, D. S. J. Am. Chem. Soc. 1972, 94, 7469.
b) Wulfman, D. S.; Poling, B.; McDaniel, R. S., Jr. Tetrahedron Lett. 1975,
519. (c) Wang, J.-L.; Toscano, J. P.; Platz, M. S.; Nicolaev, V.; Popik, V. J.
Am. Chem. Soc. 1995, 117, 5477.
4) See, for example: (a) Chen, N.; Jones, M., Jr.; White, W. R.; Platz, M.
1
of HI-CH is shorter than the C-I bond distance calculated for
2
(
(6) Stang, P. J.; Zhdankin, V. V. Chem. ReV. 1996, 96, 1123.
(7) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(8) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.
(
4
(9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson,
B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.;
Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara,
A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley,
J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez,
C.; Pople, J. A. Gaussian 94 (Revision E.2); Gaussian, Inc.: Pittsburgh, PA,
1995.
(
S. J. Am. Chem. Soc. 1991, 113, 4981. (b) Fox, M. J.; Scacheri, J. E. G.;
Jones, K. G.; Jones, M., Jr.; Shevlin, P. B.; Armstrong, B.; Sztyrbicka, R.
Tetrahedron Lett. 1992, 33, 5021. (c) Armstrong, B. M.; McKee, M. L.;
Shevlin, P. B. J. Am. Chem. Soc. 1995, 117, 3685. (d) Nigam, M.; Platz, M.
S.; Showalter, B. M.; Toscano, J. P.; Johnson, R.; Abbot, S. C.; Kirchhoff,
M. M. J. Am. Chem. Soc. 1998, 120, 8055.
(5) Padwa, A.; Hornbuckle, S. F. Chem. ReV. 1991, 91, 263.
1
0.1021/ja000334o CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/10/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 21, 2000 5211
Table 1. Relative Energies (kcal/mol) Calculated at the B3LYP/
LANL2DZ Level of Theory
intermediate on 1b. Iodocyclohexane is also formed, possibly by
radical reactions stemming from a triplet carbene or cleavage of
C-I bonds.
a
a
E
rel
H°rel
E
rel
H°rel
HI-CH
HI-CH
2
(2, conf a)
(2, conf b)
0.0 0.0
6.0 6.2
HI + CH
2, conf a + N
PhI + CH
3, conf a + N
2
-N
2
0.0
0.0
The reaction with 3-heptene was used to examine the spin state
of the intermediates. Heating a mixture of 1b and cis-3-heptene
led to formation of iodobenzene and cyclopropane 5 as well as
several minor products (eq 3). Heating a mixture of 1b and trans-
2
2
44.1 44.2
0.0 0.0
37.6 38.5
PhI-CH
PhI-CH
2
(3, conf a) 0.0 0.0
(3, conf b) 0.3 0.3
2
-N
2
2
2
a
Total electronic energies were corrected for zero-point energies
scaled by a factor of 0.9806. See: Scott, A. P.; Radom, L. J. Phys.
Chem. 1996, 100, 16502.
CH
between the carbon atom of the CH
atom in PhI-CH
3
I (2.19 Å) at the B3LYP/LANL2DZ level. The distance
fragment and the iodine
is also shorter than the C-I distance in CH I.
2
2
3
These shorter C-I bond distances are consistent with some double
bond character in the ylide C-I bonds. The phenyl C-I bond
3-heptene led to formation of iodobenzene, cyclopropane 6, and
distance in PhI-CH is longer than the C-I distance calculated
2
for iodobenzene, possibly indicating less interaction between the
phenyl π-system and the iodine substituent.
several minor products (eq 4). For both reaction mixtures GC/
To compare the strength of the carbene-nucleophile bonds,
2
the energy changes associated with transfer of the CH fragment
from HI or PhI to N
were calculated to be exothermic with over 35 kcal/mol released.
The bond dissociation energy for the C-N bond in CH
calculated using experimental heats of formation, is 50 kcal/mol.1
were calculated (Table 1).10 Both reactions
2
2
2
N ,
0b
The thermal decomposition of the iodonium ylides to give
carbenes should be facile, requiring an enthalpy change of no
more than 15 kcal/mol. Properties of the excited states have not
been calculated, but it is plausible that decomposition of photo-
excited iodonium ylides might be very rapid. Our prediction and
the computational results point to iodonium ylides as potential
carbene precursors.
MS analysis of the minor products was consistent with the
structures of the anticipated products of C-H insertion. GC
analysis showed that compound 5 accounted for less than 1% of
the cyclopropanated products when 1b was heated with trans-3-
heptene, and 6 accounted for only 2% of the cyclopropanes when
In the laboratory, ylide 1b was prepared by the reaction of
iodobenzene diacetate, diethyl malonate, and base, followed by
recrystallization from benzene/hexane, and stored in the freezer.11
Ylide 1b, rather than the dimethyl ester, was chosen because of
its greater solubility in hydrocarbon solvents. Ylide 1b was still
only sparingly soluble in alkane solvents at room temperature,
yet dissolved readily at 100 °C to give a pale yellow solution
that became colorless as 1b decomposed. Heating a stirred mixture
of 1b and cyclohexane in a sealed tube at 100 °C for 45 min
1
b was heated with cis-3-heptene. The very high degree of
stereospecificity indicates cyclopropanation of the double bond
by a singlet carbene, with cyclopropanation faster than intersystem
crossing to the more stable carbene triplet state.2
Photolysis of a mixture of 1b and cis-3-heptene produced low
yields of cyclopropanes 5 and 6 in the ratio 34:66. Photolysis of
1b and trans-3-heptene led to cyclopropanes 5 and 6 in the ratio
22:78. Since results of thermal decomposition indicate that
produced iodobenzene and C-H insertion product 4 as the major
reaction of singlet carbene with the alkene is faster than
intersystem crossing under these conditions, photoexcited 1b is
the intermediate that likely undergoes intersystem crossing and
leads to triplet carbene (see Scheme 1). In comparison, Jones et
1
products (eq 2), consistent with the formation of either :C(CO
2
-
al. found that about 10% of the “wrong” isomer was formed when
a was irradiated in alkene solution.3 The greater extent of
a
1
nonstereospecific reaction seen in direct photolysis of 1b is
probably a result of more efficient intersystem crossing due to
1
3
the presence of the iodine atom (heavy atom effect).
3
2
Et)
produced in 80% of the theoretical yield and 4 was produced in
4
2 2 2
or :C(CO Et) as reaction intermediate(s). Iodobenzene was
As predicted on the basis of our hypothesis and calculation of
bond dissociation energies for model ylides, thermal decomposi-
tion of 1b under mild conditions appears to be an efficient source
1
6% of the theoretical yield, as determined from a H NMR
spectrum of a crude product mixture to which diethyl fumarate
had been added as a standard. Photolysis (450W Hanovia Hg
lamp, Pyrex, 45 min) of a stirred suspension of 1b in cyclohexane
led to formation of iodobenzene in 77% yield.12 Product 4 was
produced in very low yield (<2% of theory). The low yield of 4
may result from the low solubility of 1b in cyclohexane at room
temperature. Analysis by GC/MS indicates substantial amounts
of carbene dimer, possibly produced by attack of a carbene
1
1
3
of :C(CO
tions of the reactivity of :C(CO
2 2
Et) , uncontaminated by 1b* or 1b*. Further explora-
1
2 2
Et) , produced from 1b, vs the
intermediates produced from 1a are in progress. We are also
applying this strategy to the identification of other carbene
precursors.
Acknowledgment. We thank the donors of the Petroleum Research
Fund, administered by the American Chemical Society, the Camille and
Henry Dreyfus Foundation, the M. J. Murdock Trust, and Central
Washington University for financial support.
(
10) (a) Heats of reaction (kcal/mol) calculated at the B3LYP/LANL2DZ
level compare reasonably well with those calculated using experimental heats
of formation. CH + CH I f CH CH I + N , -51 (calcd) vs -55 (exptl).
CH + CH f CH CH + N
2
N
2
3
3
2
2
2
N
2
4
3
3
2
, -48 (calcd) vs -52 (exptl). (b) Experimental
heats of formation from Mallard, W. G., Ed. NIST Chemistry WebBook; http://
JA000334O
www.nist.gov/chemistry/.
(
11) (a) Neiland, O.; Karele, B. J. Org. Chem. USSR (Engl. Transl.) 1965,
1
, 1854. (b) Schank, K.; Lick, C. Synthesis 1983, 392.
12) Ylide 1b, λmax) 280 nm in benzene solution.
(13) Barltrop, J. A.; Coyle, J. D. Excited States in Organic Chemistry; John
Wiley & Sons: New York, 1975.
(