Methyl-substituted stilbenes

The photochemical ring closure of certain stilbenes, eg, the highly methyl substituted compound (2) [108028-39-3], C22H2g, and their heterocycHc analogues is the basis for another class of photochromic compounds (31—33). 

The cycloaddition of substituted acrylates has been investigated with cyclic nitronate 24 (Table 2.49) (14). The cycloaddition of a 1,1-disubstituted dipolar-ophile (entry 2), proceeds in good yield, but both 1,2-disubstituted alkenes fail to react. The effect of substitution pattern on the dipolarophile was investigated with a slightly more reactive nitronate (Table 2.50) (228). Less sterically demanding alkenes such as cyclohexene, cyclopentene, and methyl substituted styrenes react, albeit at elevated temperature. The only exception is the 1,1-disubstituted alkene (entry 4), which reacts at room temperature. Both stilbene and dimethyl fumarate fail to provide the desired cycloadduct. In a rare example of the dipolar cycloaddition of tetra-substituted alkenes, tetramethylethylene reacts at 50 °C over 3 days to give a small amount of the cycloadduct (entry 7). 

A further oxidative reaction, in this case accompanied by rearrangement, has also been observed18 in the reaction of 4-benzylalkylamino-azobenzenes with Schiff s bases. Thus, reaction of the methyl-substituted azobenzene 15 with Schiff s base (16) gives at 40° to 45°C the expected stilbene 17. However, at higher temperature this product undergoes oxidative rearrangement to the A-ethylketimine (18). 

In the case of methyl-substituted 2-phenylbenzo[b]furans, the methyl groups show differing reactivity depending on the site to which they are attached. Thus, reaction of 2-(/ -tolyl)-5-methylbenzo[b]furan (34) with Schiff s base from benzaldehyde and p-chloroaniline in the presence of potassium hydroxide yields 2-(stilben-4-yl)-5-methylbenzo[bjfuran (35), whereas with potassium r-butoxide the 5-methyl group of the benzofuran ring also reacts, yielding the 5-styryl derivative 36.26,27... 

As was found in the case of the methyl-substituted 2-phenylbenzo-[6]furans (cf. 34), a similar variation in the reactivity of the methyl groups in 3,5-di(p-tolyl)-l, 2,4-oxadiazole (63) has also been observed.11 With benzalaniline in the presence of potassium hydroxide the 5-(stilben-4-yl)-3-(/ -tolyl)-l,2,4-oxadiazole (64) is formed, whereas with potassium r-butoxide both methyl groups react, leading to 3,5-di(stilben-4-yl)-l,2,4-oxadiazole (65). Some further stilbene derivatives of 1,2,4-oxadiazoles are shown in Table V. 

Apart from methyl-substituted pyridines, p-tolyl- and 4-methyl-styrylpyridines have also been found to react with benzalaniline. Thus, 2,6-diphenyl-4-(p-tolyl)pyridine (188) gives the stilbenyl derivative 189, and the stilbazole 190 gives 4-[/3-(3-pyridyl)vinyl]stilbene (191) (Table XIX).34... 

Polymers with methyl substitution on the central linkage in the stilbene mesogenic group have been prepared, Polymers 4 and 5, and show strictly nematic mesophases . The methyl group seems to act mostly to inhibit the existence of smectic order, but otherwise does not influence the mesophase formation or thermal stability very much. 

It has been deduced that the dimensionality of stilbene isomerization may not be one-dimensional, as commonly assumed, from an examination of the kinetics of isomerization of the meta-methyl substituted derivative of stilbene in decane as a function of... 

The formation of the benzyl radical by photolysis of toluene in benzene at 4.2 and 77 K has been investigated as a function of light intensity and shown to be unimolecular and biphotonic, and to occur via the lowest excited triplet state. The gas-phase photodissociation of [2,2]paracyclophane (71) into two molecules of /7-quinodimethane and analogous dissociations of two methyl-substituted derivatives have been shown to be efficient two-photon processes.A hot molecule formed by internal conversion from the initially formed singlet electronic excited state is proposed as an intermediate, a mechanism which is completely different from that of the two-photon dissociation of (71) in low-temperature matrices, which involves a triplet state. The photolyses of [2,2]para-cylcophane and [2,2]paracyclophane-l-ene have also been studied in THF and hexane matrices at 77 K, using detection by both luminescence and absorption spectroscopy. The products from both these compounds in THF matrices included alkyl derivatives of rra/15-stilbene, and in hexane matrices alkylphenan-threnes. Cycloreversion of so-called cyclodimers of naphthalene derivatives with benzene or furan, e.g. (72), occurs efficiently upon UV irradiation. ... 

The electronic spectroscopy of trans-isomers of 3-(N-phenylamino)stilbene (mlc), 3-(N-methyl-N-phenylamino)stilbene (mid), 3-(N,N-diphenylamino)stilbene (mle), and 3-(N-(2,6-dimethylphenyl)amino)stilbene (mlf) and their double-bond constrained analogues, m2a-m2c and m2e, were studied [71]. When compared with trans-3-aminostilbene (mla), mlc-mle displayed a redshift of the So —> Si absorption and fluorescence spectra, lower oscillator strength and fluorescence rate constants, and more effldent Si —> Ti intersystem crossing. The N-Ph derivatives mlc-mle had lower fluorescence quantum yields and higher photoisomerization quantum yields. The role of Si —> Ti transition in the amino-substituted stilbenes as the predominant nonradiative decay pathway was discussed. The excited triplet (Ti) state formation of stilbene dendrimers (tetramethoxystilbene (generation G) GO, Gl,... 

A number of methyl-substituted 1,2-diphenylethanols have been prepared in this way for use as convenient precursors of the relevant stilbenes [86] (Scheme 25). 

Photoaddhion of electron donor olefins such as vinyl ethers and stilbene to variously methyl and halogeno-substituted 1,4-benzoquinones resulted in the formation of dihydrobenzofurans via a dienone-phenol rearrangement of the primary product spirooxetanes <96H(43)619>. High-temperature water seems to be an alternative to use of acid catalysts or organic solvents by the cyclization of allyl phenyl ethers to dihydrobenzofurans <96JOC7355>. 

The excited-state behavior of 1,1,2,2-tetraphenylethene (TPE) has been studied by means of picosecond fluorescence, absorption, and Raman spectroscopies and picosecond optical calorimetry. It has been shown that, like stilbene, TPE derivatives substituted with minimally perturbing stereochemical labels such as methyl groups undergo efficient photoisomerization. However, unlike stilbene, strong spectroscopic evidence exists for the direct detection of the twisted excited singlet state, 5ip herein but traditionally designated as of TPE. 

Of particular interest are cases in which substituents are present in ortho positions to the reacting methyl group. Apart from o-chloro-substituted derivatives, those with methoxy, carbonamido, sulfonamido, and phenylsulfonyl groups have been reported to undergo reaction.68 The 2-(3-chloro-4 methyIphenyl)-5-methoxybenzotriazole (163) has been found to be particularly reactive, yielding, with Schiff s base 98, the stilbene 164.54... 

On the other hand, the addition of a quaternary ammonium salt to the reaction medium accelerates the isomerization of the radical intermediate [36]. Thus, the epoxidation of c/j-stilbene in the presence of A -benzylquinine salt gives rranr-stilbene oxide with 90% ee as major product (Table 6B.1, entry 24). This protocol provides an effective method for the synthesis of trans-epoxides. In contrast to the epoxidation of c/s-di- and tri-substituted olefins for which complexes 11-13 are the catalysts of choice, the best catalyst for the epoxidation of tetra-substituted conjugated olefins varies with substrates (Table 6B.1, entries 27 and 28) [37]. The asymmetric epoxidation of 6-bromo-2,2,3,4-tetramethylchromene is well-promoted by complex 14 and that of 2-methyl-3-phenylindene, by complex 12a. 

---