Dihydroazulenes

As appropriate model compounds for these reactions240 the bridgehead substituted di hydro-4-methyleneazulenes 474 were employed. Allyl-, crotyl- and propargyl-substituted dihydroazulenes 474 and 476 can be easily rearranged to the 4-substituted azulenes 475 and 477 (equations 179 and 180) whereas all attempts to obtain 4-benzylazulene 479 by rearrangement of precursor 478 gave only polymeric products (equation 181). Undoubtedly, this failure can be explained by the fact that the Cope rearrangement becomes very... 

A method for highly efficient asymmetric cyclopropanation with control of both relative and absolute stereochemistry uses vinyldiazomethanes and inexpensive a-hydroxy esters as chiral auxiliaries263. This method was also applied for stereoselective preparation of dihydroazulenes. A further improvement of this approach involves an enantioselective construction of seven-membered carbocycles (540) by incorporating an initial asymmetric cyclopropanation step into the tandem cyclopropanation-Cope rearrangement process using rhodium(II)-(5 )-N-[p-(tert-butyl)phenylsulfonyl]prolinate [RhjtS — TBSP)4] 539 as a chiral catalyst (equation 212)264. 

We demonstrate by using ultrafast time resolved spectroscopy that the photoconversion from dihydroazulene (DHA) to vinylheptafulvene (VHF) is governed by two mechanisms The ring opening proceeds on the excited energy surface on the picosecond time scale. It is followed by an internal conversion to the VHF ground state that is accelerated by the presence of a conical intersection in the case of cyclopenta-DHA. This conical intersection hinders the photoinduced back reaction from the final VHF products. However, we successfully photo-converted the cyanophenyl-VHF-cis back to the DHA in an experiment with two delayed pulses. This opens the route to the development of bistable dihydroazulene switches. 

Electro-photoswitching devices possess a dual mode operation resulting from different electrochemical properties in the two photo-interconvertible states. They may display both photochromism and electrochromism. They allow the photomodulation of electrochemical properties as was shown to occur in photochromic dihydroazulene derivatives [8.247]. Separate photochemical and electrochemical interconversion has been realized in systems containing a photoactive azo [8.248, 8.249] or dihydroazulene [8.250] unit appended with an anthraquinone moiety. 

Optoelectronic Molecular Switches Based on Dihydroazulene-Vinylheptafulvene (DHA-VHF)... 

Scheme lb Synthesis of dihydroazulenes via corresponding vinylheptafulvenes AT = reflux. 

When covalently attached to electron transfer active subunits, the DHA-VHF couple can facilitate chemical and physical switching of electronic properties, as a result of photochemically induced rearrangement accompanied by a change in the redox potential. An interesting example of such a switching system is the compound containing a dihydroazulene component and a covalently attached anthraquinone moiety.1311 This system is able to act as a multimode switch, assisted by various processes such as photochromism, reversible electron transfer, and protonation-deprotonation reactions (Scheme 8). 

Scheme 8 Light-driven multimode molecular switching of the electron transfer active dihydroazulene 31a.

Fig. 25 Information storage in dihydroazulene/vinylhepta-fulvene systems attached to heteroaromatic groups.

Scheme 11 Conception for a four-step cyclic process with biphotochromic compounds. The notation open/closed for isomer 46 refers to the dithienyl moiety in its open constitution and the dihydroazulene moiety in its closed one. This notation applies equally to 47, 48, and 49.

Molecular switches based on the dihydroazulene-vinylheptafulvene system are discussed in Chapter 3. Chiroptical switching was achieved with a carbohydrate-modified chiral polyazuleneJ721... 

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