High-level Computational Study of the Site-, Facial- and Stereoselectivities for the Diels-Alder Reaction Between o-Benzoquinone and Norbornadiene

Ab initio and DFT quantum chemical calculations have been applied to a study of the Diels-Alder reaction of o-benzoquinone as diene and norbornadiene as dienophile. Transition states for the different reactions are located and activation energies estimated. The prefered exo-À-facial selectivity and exo,endo-stereoselectivity exhibited in this cycloaddition are readily predicted using RHF/3-21G or higher levels of calculations. Differences between experimentally observed results and calculations may be explained by the postulation of a second, nonconcerted biradical mechanism leading to formation of hetero Diels-Alder products

Pyramidalized olefins : a DFT study of the homosesquinorbornene and sesquibicyclo[2.2.2]octene nuclei

Density functional theory (B3LYPl6-31G*) was used to study a series of homosesquinorbornenes and sesquibicyclo[2.2.2loctenes. The compounds in which the two faces of the double bond are different are predicted to have a pyramidal double bond with butterfly bendings (v) ranging from 1.8 to 17.9". The degree of pyramidalization of these central double bonds is greater in the homosesquinorbornenes than in the sesquibicyclo[2.2.2loctenes

Sulfur-bridged molecular racks : O,S-sesquinorbornadienes, CNS-[3] and CNOS [4]polynorbornanes

Thiophene has been reacted under high-pressure (10 kbar) and temperature (1000C) with Nmethylmaleimide and N-phenylmaleimide to produce Diels-Alder exo- and endo-adducts in modest yields (25-36%). Reaction of 7-oxanorbornadiene-2,3-dicarboxylic anhydride 6, a highly reactive dienophile, with thiophene occurred under high-pressure (10 kbar) at 100oC to yield stereoisomeric 1:1-adducts by site selective attack at the maleic anhydride type of π-bond. This approach afforded the first examples of syn- and anti- heterobridged sesquinorbornadiene anhydrides 8 and 9 containing a sulfur bridge. Similar reaction of isobenzothiophene with 6 was even more facile as reaction occurred at room temperature and atmospheric pressure to yield benzo-analogues 12 and 13. Thermal fragmentation involving loss of furan and sulfur occurred from both classes of adducts under FVP (370 0C, 0.005 mbar) to produce phthalic anhydride or naphthalene-2,3-dicarboxylic anhydride respectively. The putative 7-thianorbornadiene intermediates 20 and 22, generated by loss of furan, were not detected. Reaction of exo-N-methyl 7-thianorbornene-5,6-dicarboxylic imide 4a with the ester-activated cyclobutenoaziridine 16 provided access to CNS-[3]polynorbornane 18, while similar addition of the exo,endo-isomer of O,S-benzosesquinorbornadiene 13 to 16 afforded the CNOS-[4] olynorbornane 19. These are the first S-bridged [n]polynorbornanes to be reported. Molecular modelling (AM1) has shown that Sn-[n]polynorbornadienes have a curved topology greater than On- n]polynorbornadienes but less than Nn-[n]polynorbornadienes

The synthesis of rigid chromophore-spacer-chromophore dyads and three-armed triads by the 1,3-dipolar reaction of cyclobutene epoxides with aromatic dipolarophiles

A series of polyaromatic hydrocarbons (PAHs) or their aza derivatives were reacted as dipolarophiles with ring-fused cyclobutene epoxides (CEs) in the 1,3-dipolar cycloaddition protocol to form 1:1 cycloadducts. The dihydroforms of the PAH chromophores contained in the rigid alicyclic scaffold were dehydrogenated to regenerate the original PAH chromophore and constituted a new route to rigid chromophore-nσ-PAH dyads separated by an alicyclic scaffold. The anthracene 1:1 adduct derived from the reaction with the 5,8-dimethoxynaphthalene-containing CE was reacted a second time to produce stereoisomeric three-armed triads in which naphthalene chromophores were attached rigidly onto scaffold. Two approaches were used to generate the incipient cyclic 1,3-dipolar intermediates involved in cycloaddition process. The first method employed ring-opening of functionalised CEs, the other method produced the dipolar reagent in situ by loss of dinitrogen from the initially formed adduct resulting from treatment of fused norbornenes with 2,5-bis(trifluoromethyl)-1,3,4-oxadiazole

Preparation of the first isobenzofuran containing two ring nitrogens : a new diels-alder diene for the synthesis of molecular scaffolds containing one or more end-fused 3,6-di(2-pyridyl)pyridazine ligands

Preparation of a stable, crystalline isobenzofuran containing two ring-nitrogen atoms, 4,7-di(2-pyridyl)-5,6-diazaisobenzofuran (diaza-IBF) (12), is reported here for the first time. Diaza-IBF was prepared using the s-tetrazine-induced fragmentation of 1,4-di(2pyridyl)-5,8-epoxy-5,8-dihydrophthalazine (9), for which a synthesis is provided. Diaza-IBF was also prepared by flash vacuum pyrolysis (FVP) of the isomeric N-methyl-1,4-di(2-pyridyl)-5,8-epoxy-5,6,7,8-tetrahydrophthalazine-6,7-dicarboximides (18) and (19), formed in three steps from furan, N-methyl maleimide, and 3,6-di(2-pyridyl)-s-tetrazine. Diaza-IBF was employed in cycloaddition protocols with alkenes to form scaffold mono- or bis-3,6-di-(2pryidyl)-pyridazine ligands having novel molecular architectures

The preparation of stereoisomeric tricyclo[4.2.1.02,5]nona-7-ene-3,4-dicar­boximides and anhydrides literature corrections and new products /

The bis-homo Diels-Alder reactions of maleimide and N-methylmaleimide with quadricyclane are shown to produce mixtures containing both the exo, anti- and exo, syn-tricyclo-[4.2.1.02,5] nona-7-ene-3, 4-dicarboximides, whereas the isomeric endo, anti-adducts are available from the reaction of cyclopentadiene with the appropriate cyclobutene-3, 4-dicarboximides. The related reaction of quadricyclane with maleic anhydride had been erroneously reported to form single adducts; our work shows that two stereoisomers are actually formed and these have been chemically related to the maleimide and N-methylmaleimide adducts. The proton chemical shifts of the adduct formed by reaction of cyclobutene-3, 4-dicarboxylic anhydride with cyclopentadiene have also been miss-assigned. Preparation of [3] and [5] polynorbornane scaffolds with terminal succinimides have been effected by coupling the stereoisomeric adducts at the norbornene π-bond by using 2, 5-bis(trifluoromethyl)-1, 3, 4- oxadiazole and bisepoxide. The spacer shapes and dimensions have been determined by molecular modelling