The Photochemical Reaction of 1 , 1-dicyano-3-phenylbut-1-ene . Simultaneous Occurrence of π-methane and di-π-methane Rearrangements

A fotólise direta de 1,1-diciano-3-fenilbutene-1 (3-MDCN) foi pesquisada a temperatura ambiente em solventes de diferentes polaridades (hexano, diclorometano e acetonitrilo). Foram obtidos fotoprodutos originários dos processos di-π-metano e π-metano (migração de hidrogênio 1,2). As estruturas dos produtos foram determinadas por H-NMR, GC/MS, IV e cromatografia. Os resultados das determinações dos rendimentos quânticos relativos e as análises cromatográficas de irradiações sequênciais evidenciaram que i) não ocorrem reações secundárias, até a altas conversões; ii) o rearranjo di-π-metano é mais afetado pelas variações de solvente que o rearranjo π-metano. Não foram observados produtos por fotosensitização com acetofenona ou acetona. A presença de mecanismo simultâneos e os efeitos de solvente foram considerados como evidência de excitações localizadas e deslocalizadas sobre a superfície de energia potencial.

In the presence of a second homoconjugated π-bond the di-π-methane rearrangement usually predominates, as proved by Hixson 14 and Zimmerman and Little 15 (Scheme 3).
The formation of different cyclopropanes from the same starting compound by both mechanisms should provide a good opportunity for comparative studies, since the reactivity ratios could be obtained directly from the quantum yields or from the chemical yields at low conversions.

General methods
Irradiations at 254 nm were performed in a Rayonet photoreactor (The Southern New England Ultraviolet Co.) with low-pressure mercury arc lamps (RPR2537).In general, 3 or 4 mL of the solution of 3-MDCN with concentrations between 4.25 and 11.8 mM were placed in quartz tubes and deoxygenated by flushing thoroughly with nitrogen.After irradiation for the required time, 1 mL of the solution of the chromatographic standard was added and the reaction mixture was analyzed by gas chromatography.
All chemicals and solvents (spectrophotometric or HPLC grade) were from Aldrich or Merck and used without further purification.
2-Methyl-2-phenylpropanal used as a precursor in the synthesis of 3-DMDCN was obtained by a Grignard reaction between methyl mandelate and CH 3 I with pinacol formation (78%), followed by H 2 SO 4 /AcOH/I 2 treatment (yield ~60%), as described by Wolfe et al. 19 The aldehyde was converted to 3-DMDCN with 61% yield by a 2-MDCN, was synthesized using similar conditions 20 .This compound was used as actinometer and for chromatographic and spectroscopic comparisons, as its photolysis rendered a single product.The quantum yield, previously determined 16 , using both trans-cis isomerization of trans-1-phenylbut-2-ene and potassium ferrioxalate, is 0.018.

Photolysis of 3-MDCN
In order to identify the reaction products a preparative photolysis was carried out.Thus, 1.6 mmoles of the dicyanoolefin 3-MDCN in hexane (100 mL) were degassed and irradiated for 5 h.A yellow oil was obtained after concentration of the reaction mixture in vacuum.The fraction recovered by bulb-to-bulb distillation showed three products by GC analysis (areas corresponding to 66, 10 and 19%, approximately), as well as the substrate (3%).Chro-matographic and spectroscopic comparisons with CP.1 obtained by catalytic thermal synthesis 21 and CP.2, obtained by the of photolysis 2-MDCN 16  The absorption spectrum of 3-MDCN and its time evolution during photolysis are shown in Fig. 1.A blueshift of the band around 230-220 nm during the photolysis, as well as the general decrease of the absorbance at wavelengths below 280 nm, suggests a bichromophoric interaction between the phenyl group and the double bond, which will be destroyed during the reaction.
It can be seen from Fig. 2, that the sum of the amounts of both cyclopropanes obtained during the photolysis is always equal to the consumption of 3-MDCN.This indicates a competition between the mechanisms leading to  both photoproducts.Similar behavior can also be observed when the reaction is carried out in acetonitrile or dichloromethane.In addition, the ratio between both cyclopropanes also remains constant throughout the reaction, up to high conversions.This proves that there is no photochemical interconversion between the products CP.1 and CP.2, as expected from the low extinction coefficients of the cyclopropanes at the irradiation wavelength (i.e., at 254 nm, ε3-MDCN ≅ 5000, εCP.1 ≅ εCP.2 ≅ 200 M -1 cm -1 ).

Solvent effect
The photolysis of 3-MDCN was performed in three solvents with different polarity.The quantum yields for both cyclopropanes (i.e. both types of rearrangements) are shown in Table 1.The overall conversion is influenced by the solvent polarity due to the effect on the di-π-methane rearrangement.The formation of CP.1 always prevails over that of CP.2 (π-methane rearrangement), the latter practically not being affected by the change of solvent.

Photolysis of 3-DMDCN
To compare with the results obtained for 3-MDCN, a hexane solution of 3-DMDCN (2.57mmol/100mL) was photolyzed for 6 h.Only one product was observed, corresponding to the di-π-methane rearrangement.It crystallized in the reaction solvent (white crystals, m.p. = 62-65 °C) with a yield of 41% and was identified as CP.

Discussion
The absorption spectrum of 3-MDCN suggests a bichromophoric interaction as the extinction coefficients in the 230 and 270 nm regions (ε 270 = 3610 and ε230 = 10000 M -1 cm -1 ) are larger than those corresponding to the separate chromophores (ε 261 max = 295 M -1 cm -1 for toluene).The same behaviour has been observed also for 2-MDCN 16,20 .The increase of the extinction coefficient for similar compounds was assumed to be due to the excitation to a delocalized state (aromatic π + double bond π*) [22][23][24] .In this interaction the phenyl group acts as the donor and the double bond as the acceptor, giving rise to a charge transfer complex in the vertical excited state.Assuming that the 1,2-migration of H results from electronic excitation on the double bond, the observation of products resulting from both mechanisms from the same compound, suggests the excitation to a delocalized orbital.
The absorption peaks in the short wavelength region of the spectrum of 3-MDCN can be ascribed to the transitions to localized and delocalized states, both having singlet character.Those states correspond to potential energy surfaces that lead to different photoproducts.On the lower energy surface, corresponding to the delocalized excited state, excitation leads to a charge-transfer complex (CTC), which will take to the di-π-methane rearrangement.The increase of solvent polarity will stabilize this complex, decreasing the quantum yield of CP. b ratio between the areas of the peaks of chromatographic analysis.
Figure 2. Time evolution of the concentrations of the dicyanoolefin 3-MDCN (1), and its photoproducts cyclopropane CP.1 (2) and cyclo- propane CP.2 (3), during photolysis in hexane at 254 nm.Line (4) corresponds to the sum of the concentrations of substrate and photoproducts at any time.
energy scheme is shown in Fig. 3.The small effect of the solvent polarity on ΦCP.2 seems to indicate that the excited state surface leading to the 1,2-photomigration of H has a more accentuated radical character.The pathway on this surface should not be affected by the motions leading to the surface corresponding to the di-π-methane rearrangement ("touching" motion between both π moieties 25 ).
On the surface leading to the CP.1 formation, the charge transfer complex in the excited state is stabilized by polar solvents, decreasing the formation rate of this cyclopropane.This decrease in the efficiency of di-π-methane rearrangement (Φ CP.1 ) with increasing solvent polarity has already been observed before 16,20 .It was considered an evidence for the competition between the deactivation of the excited singlet state by reversible electron transfer from the phenyl group to the dicyanovinyl moiety, and the formation of a 1,4-diradical leading to homolytic ring-opening and reclosure on the di-π-methane pathway.Both processes originate from the same intramolecular motion from the vertical excited state ("touching").This deactivation process by electron transfer between two chromophores was also proposed for di-π-methane rearrangements in the triplet state 26 .
Previous studies on similar systems proved that both mechanisms (π-methane 4,14 and di-π-methane 22,24 ) originate from singlet excited states.For the structurally related compound in which one of the CN groups is replaced by an ester, the sensitization with acetophenone leads to isomerization around the double bond, with no formation of cyclopropanes.The latter were only observed upon direct photolysis with 254 nm light.From these results it can be assumed that triplet states are not involved in the photoreactions of 3-MDCN and 3-DMDCN, which should proceed via a singlet excited state.
In general, from data found in the literature (Scheme 3) it can be seen that the diπ-methane rearrangement is much more efficient than the π-methane reaction.As expected, the photomigration is easier for hydrogen than for methyl group, as found when comparing the photochemistry of 3-MDCN with that of 3-DMDCN, where only the di-πmethane process occurs, with the same efficiency as for 3-MDCN (φ CP.3 = 0.054).The photochemistry of 2-MDCN should show the same products as 3-MDCN,wouldn't it be by the inhibition of the 1,2 photomigration by substitution on the end carbon of the migration, due probably to steric and electronic effects 4 .Thus, 3-MDCN seems to be the only compound for which both mechanisms occur simultaneously with comparable efficiency.

Figure 3 .
Figure 3. Potential energy surface diagram for the photoreactions leading to di-π-methane (B) and π-methane reactions (C).GS correponds to the ground state.

Table 1 .
Solvent effect on the quantum yields for the photoreaction of 3-MDCN a .