Electrocyclic Electron deficiency

The same authors extended the [2 + 2 + 2]-cycloaddition methodology to the use of highly electron-deficient tricarbonyl compounds such as ketomalonates (Equation (33)).360 In that particular case, the reaction does not stop at the initial stage of 277-pyrans 164. Instead, a thermally induced electrocyclic ring opening occurred to form the corresponding cyclopentenes 165 as final product. 

The ring expansion of arenes by electron-deficient singlet nitrenes is by far the most versatile synthetic route to 1H -azepines. The first l//-azepines were prepared independently in 1963 by Hafner, and by Lwowski, and their coworkers. They found that ethoxycarbonyl-nitrene (Scheme 26, path a R=C02Et), generated by photolysis of ethyl azidoformate, adds to benzene to give initially the unstable azanorcaradiene (227), electrocyclic ring... 

The relative rates for electrocyclic ring openings of cyclopropyl ions [200] are shown in Eqs (173,174). For the cations the faster rates are exhibited by compounds in which an acceptor is directly bonded to the electron deficient center (a-a arrangement), whereas precursors with a donor substituent at the center open most slowly. 

The benzannulation reaction tolerates a range of alkyl and aryl allcynes, which may bear additional functionalities. The simultaneous presence of two bulky substituents directly attached to the C=C bond, as for example in bis(trimethylsilyl)ethyne, however, blocks the final electrocyclization and causes the reaction to stop at the vinyl ketene stage [44]. Neither very electron-rich nor very electron-poor allcynes can undergo benzannulation. Strongly electron-deficient allcynes such as hexafluorobut-2-yne cannot adequately compete with car-... 

The ruthenium-catalyzed [2+2+2] cycloaddition of 1,6-diynes was performed with an electron-deficient carbonyl double bond, activated with two electron-withdrawing groups, to produce conjugated dienones via electrocyclic ring opening of the expected cycloadduct [101] (Eq. 77). 

A highly electron-deficient carbon-oxygen double bond can also participate in the co-cyclotrimerization with alkynes under the ruthenium catalysis. The cycloaddition of commercially available diethyl ketomalonate with the diynes 21 proceeded at 90 °C in the presence of 5-10 mol % Cp RuCl(cod). The expected fused 2ff-pyrans 27, however, underwent thermal electrocyclic ringopening to produce cyclopentene derivatives 28 (Eq. 14) [23]. 

Owing to the less aromatic character of thiophene compared to benzene it can be involved in many electrocyclic reactions for example, thiophene can react as a dienophilic 2rt-component or as a diene (47t-component) in Diels-Alder reactions. Furthermore, [2-1-2] cycloadditions are possible using electron-deficient counterparts. 

Whereas inter- and intramolecular Diels-Alder reactions normally require electron-deficient dienophiles, the 67r-electrocyclization proceeds with a large vaiiety of substituents on a hexatriene. In one such approach, the intramolecular Heck-type reaction of a 2-bromo-1 -en-(ft> — l)-yne 66 is used as a trigger to initiate an intermolecular Heck coupling with an alkene to form the conjugated 1,3,5-hexatriene 67 which eventually cyclizes in a 67T-electrocyclic process (Scheme 3-21) [173]. In many cases, aromatization of the cyclohexadiene 68 formed primarily occurs to yield carbo- and heterobicyclic compounds of type 70 [173a,b]. But with alkyl ethenyl ethers the cyclohexadienes 69 can be obtained in moderate yields [173b]. 

Electrocyclic ring closure of l-oxa-3,5-diaza-l,3,5-hexatrienes forms 2//-l,3,5-oxadiazines (see Section 6.18.10.1.1), while 4//-l,3,5-oxadiazines arise by (4 - - 2) t cycloaddition of electron-deficient l-oxa-3-aza-1,3-dienes with carbonitriles (see Section 6.18.10.2.2.i) and by cycloaddition of hexa-halogenopropanones (1 equiv.) with dialkylcyanamides (2 equivs.) (see Section 6.18.10.3.l.i). 

Acetylenes, particularly ones with electron-withdrawing substituents such as Me02CC=CC02Me, are known to have a rich cycloaddition chemistry. Since alkynyl(phenyl)iodonium species are highly electron-deficient alkynes, they are expected to be excellent cycloaddition partners in a variety of electrocyclic processes. 

---