Wavelength-dependent photochemistry of diazo Meldrum's acid and its spirocyclic isomer, diazirino Meldrum's acid: Wolff rearrangement versus isomerization

Article abstract of DOI:10.1021/ja029528p

Photoreaction of diazo Meldrum's acid (1) shows a unique wavelength selectivity. At 254 nm it results in efficient (φ₂₅₄ = 0.34) Wolff rearrangement, while irradiation with 355 nm light leads to a completely different process, isomerization into corresponding cyclic α,α″-dicarbonyl diazirine 2 (φ₃₅₀ = 0.024). UV photolysis of diazirine 2 is accompanied by two competing processes: loss of nitrogen followed by the Wolff rearrangement and isomerization into diazo compound 1. Thermal decomposition of 1 leads to clean Wolff rearrangement, while heating of 2 causes quantitative conversion into diazo isomer 1. Copyright

Full text of DOI:10.1021/ja029528p

Published on Web 01/16/2003
Wavelength-Dependent Photochemistry of Diazo Meldrum’s Acid and Its
Spirocyclic Isomer, Diazirino Meldrum’s Acid: Wolff Rearrangement versus
Isomerization
Aneta Bogdanova and Vladimir V. Popik*
Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403
Received November 29, 2002 ; E-mail: vpopik@bgnet.bgsu.edu
Wavelength dependence is a rarely observed phenomenon in
organic photochemistry, as most photoreactions occur from the
lowest singlet or triplet excited state of the molecule.1,2 The
composition of product mixtures in photolysis of diazo compounds,
however, sometimes depends on the wavelength of irradiation.3 In
most cases this dependence is due to the secondary photoreactions
of primary products or due to a separate excitation of weakly
interacting chromophores. There are only a handful of examples
of “real” wavelength dependence, the phenomenon in which
excitation to the lowest and higher excited states results in the
formation of different products.3b,c In this work we report a
Figure 1. UV spectra of ca. 0.0001 M solutions of diazo Meldrum’s acid
(1, solid line) and diazirine 2 (dotted line) in methanol.
remarkable wavelength selectivity of photoreactions of diazo
Meldrum’s acid (1) and its diazirino isomer, 6,6-dimethyl-5,7-dioxa-
1,2-diaza-spiro[2,5]oct-1-ene-4,8-dione (2).4 We also demonstrate
the first example of the direct photo-Wolff reaction of R,R′-
dicarbonyl diazirine.
Scheme 1
The UV spectrum of the diazo Meldrum’s acid (1) has a strong
absorbance at 248 nm and much weaker band at 329 nm (Figure
1). Position of these absorbance bands is virtually independent from
the polarity of the solvent. No fluorescence or phosphorescence
could be detected at room temperature in solutions of 1 using 254-
and 330-nm excitation light. This fact indicates a fast nonradiative
depopulation of excited states.
Table 1. Product Ratios Observed in Low-Conversion Photolyses
of Diazo Meldrum’s Acid (1) in Methanol at Different Wavelengths
The UV spectrum of isomeric diazirine 2 is very different: it
shows only two weak shoulders on the tail of the short-wavelength
band at ca. 244 nm and at ca. 284 nm (Figure 1).
UV irradiation of the methanolic solution of diazo Meldrum’s
acid (1) results in the formation of two major products: ketoester
3, the product of the Wolff rearrangement, and diazirine 2, the
product of isomerization of starting diazo compound (Scheme 1).
Small amounts of Meldrum’s acid (4) were also detected in the
reaction mixtures.
wavelength of irradiation λ (nm)
product
254
254/sens.
266a
300
350
355a
Fraction in Product Mixture
2
3
4
0.065
0.91
0.025
0
0
1
0.17
0.83
-
0.17
0.81
0.02
0.55
0.42
0.03
0.94
< 0.06
-
quantum yield
0.34
0.025
a Monochromatic light using frequency up converted output of Nd:YAG
laser; 20-30% conversion.
The thermal decomposition of 1 in methanol at 80 °C, on the
other hand, results only in Wolff rearrangement and in the
quantitative formation of ketoester 3.5 This observation agrees well
with DFT calculations at the B3PW91/6-311+G(d,p) level that
predict an energy barrier for the loss of nitrogen from 1 is 6-9
kcal mol-1 lower than the barrier for isomerization reaction.6
The ketoester 3-to-diazirine 2 ratio was found to be strongly
dependent on the wavelength of irradiation in direct photolyses.
Product ratios presented in the Table 1 were measured at low
conversion (ca. 10%) to ensure that these values were not affected
by secondary photochemistry.
The Wolff rearrangement is the main process when diazo
Meldrum’s acid (1) is irradiated with the wavelength close to λmax
of the major absorbance band.7 At a longer wavelength, isomer-
ization of diazo Meldrum’s acid 1 into diazirine 2 becomes more
pronounced,8 and at 350 nm diazirine becomes the major product.
This dramatic change of reactivity becomes even more evident when
1 is irradiated with a 355-nm monochromatic light. In this case
diazirine 2 is produced in a high yield, and only traces of ketoester
3 are observed. The 254- and 350-nm photolyses of diazo
Meldrum’s acid differs not only in the major photoproduct but also
in the quantum yields (Table 1). Irradiation into the major
absorbance band of 1 results in a chemical transformation 14 times
more efficiently than irradiation into the weaker band.
According to TD-B3PW91/6-311++G(3df,2p) calculations the
longer-wavelength absorbance band corresponds to the excitation
of 1 to the lowest singlet excited state (S1) by HOMO f LUMO
transition.6 The HOMO of diazo Meldrum’s acid (1) is a p-type
orbital, which is mainly localized on the carbon of the diazogroup
and is orthogonal to the plane of the COCN2CO fragment. The
LUMO is a π* orbital of NtN bond, which lies in the plane. Thus,
HOMO-to-LUMO transition is a forbidden process due to poor
orbital overlap, which agrees with the low extinction coefficient
of the 329-nm band. The thermal deactivation of S1 to the ground
9
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10.1021/ja029528p CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Table 2. Results of Low-Conversion Photolyses of Diazirine (2) in
Methanol at Different Wavelengths
wavelength of irradiation λ (nm)
product
254
300
350
Fraction in Products Mixture
1
3
4
0.04
0.93
0.02
0.44
0.56
0.002
0.36
0.63
0.01
state is very rapid, evidenced by the fact that no fluorescence or
phosphorescence is observed. A small fraction of S1 population
(ca. 3%) undergoes isomerization into diazirine 2. We believe that
this reaction proceeds from the excited-state S1 rather than from
“hot” ground state as thermal reaction of 1 does not produce
diazirine 2 and DFT calculations predict isomerization to be much
slower than the Wolff rearrangement.6
of R,R′-dicarbonyl diazirine 2. The results of photolysis at 300 nm
are especially indicative. At 300 nm diazirine 2 and diazo
Meldrum’s acid (1) have very similar extinction coefficients (19
and 23 M-1 cm-1, respectively), and at low conversion of starting
diazirine 2 absorbs practically all light. The photodecomposition
of 1 becomes noticeable only when the conversion reaches ca. 30%.
The Wolff rearrangement of diazirine 2 apparently proceeds via
a dicarbonyl carbene. The absence of O-H insertion products, on
the other hand, indicates an extremely short lifetime of this
intermediate. The rate of reaction of structurally similar dicar-
The short-wavelength irradiation of diazo Meldrum’s acid results
in the formation of a higher excited state, most probably S2. The
major component of this excitation is HOMO-1 to LUMO transition.
The HOMO-1 is a nonbonding orbital localized on carbonyl
oxygens, which lies in the plane of diazocarbonyl fragment and
has a good and in-phase overlap with LUMO. This excited state
undergoes an extremely rapid loss of nitrogen. For the latter reaction
to compete efficiently with an internal conversion (φ254 ) 0.34), it
should proceed at 1012 s-1 or a faster rate.1b The rest of the excited
molecules undergo internal conversion to S1, participation of which
is evident from the formation of small amounts of diazirine 2, before
falling onto the ground-state surface. The much higher quantum
yield of 254 nm photolysis, on the other hand, provides an argument
against the formation of a Wolff rearrangement product from a “hot”
S1. According to MP2 calculations corresponding dicarbonyl
carbene lies in a very shallow energy well (<1 kcal M-1), indicating
that Wolff rearrangement of 1 is essentially a concerted process.5
The laser flash photolysis of the diazo Meldrum’s acid (1)
conducted at 248 nm using KrF laser demonstrated that the
formation of a corresponding ketene was complete during the laser
pulse (ca 20 ns).9 This allows us to estimate the lower rate limit
bomethoxycarbene with methanol is 1.5 × 109 M-1 s-1 11
. This value
allows us to estimate the lower rate limit for the Wolff rearrange-
ment of the cyclic dicarbonyl carbene at 1011 s-1
.
The thermal reaction of diazirine 2 results in quantitative reverse
isomerization to diazo Meldrum’s acid. Theoretical analysis of the
ground-state reactivity of the diazirine 2 shows that isomerization
to 1 is the only feasible process.
Acknowledgment. We are grateful to the National Institutes
of Health for partial support of this project (CA91856-01A1). A.B.
thanks the McMaster Endowment for research fellowship.
Supporting Information Available: Preparation, photolysis, quan-
tum yield measurements, and spectral data for compounds 1-3;
Gaussian 98 output files for DFT calculations. (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
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for the photo-Wolff rearrangement of 1 at 108 s-1
.
The triplet sensititized photolysis of 1 results in a quantitative
formation of Meldrum’s acid (4). This observation is in line with
the accepted mechanism of photochemical reduction of R-diazo-
carbonyl compounds through triplet carbonylcarbene intermediate.10
The absence of the ketoester 3 in reaction mixtures of the triplet
sensitized photolyses indicate that the spin equilibration of this
carbene is much slower than the reaction of the triplet state with
methanol. The small amounts of Meldrum’s acid found in direct
photolyses of 1 is apparently due to the low efficiency intersystem
crossing yielding triplet excited state of 1.
UV irradiation of diazirino Meldrum’s acid (2) results in the
processes similar to that observed in photolysis of its diazo
isomer: Wolff rearrangement to produce ketoester 3, isomerization
to 1, and formal reduction to 4 (Scheme 2).
The composition of product mixtures in the photolysis of
diazirine 2 also depends on the wavelength of irradiation. This
dependence, however, is less pronounced: the ketoester 3 is always
the major product, while the yield of diazo compound 1 increases
at longer wavelength (Table 2). The product ratios shown in the
Table 2 were obtained at ca. 10% conversion of the starting material.
Analysis of the data summarized in Table 2 allows us to conclude
that ketoester 3 can be formed directly in the Wolff rearrangement
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(4) Preparation of compounds 1, 2, and 3 is presented in the Supporting
Information.
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L.; Shelvin, P. B.; Jones, M., Jr. J. Org. Chem. 1977, 42, 2931.
(8) Formation of small amounts of diazirine 2 in photolysis of 1 was reported
previously: Nikolaev, V. A.; Khimich, N. N.; Korobitsyna, I. K. Khim.
Geterotsikl. Soedin. 1985, 321; Livinghouse, T.; Stevens, R. V. J. Am.
Chem. Soc. 1978, 100, 6479.
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