Darstellung und Struktur von 1‐Oxa‐3,5‐diaza‐1,3,5‐hexatrienen. Die 1‐Oxa‐3,5‐diaza‐1,3,5‐hexatrien/2H‐1,3,5‐Oxadiazin‐Tautomerie

Abstract

Key Words: l-Oxa-3,5-diaza-1,3,5-hexatrienes / 2H-1,3,5-0xadiazines / l-Oxa-3,5-diaza-l,3,5-hexatriene -2H-1,3,5-0xa-diazine tautomerism / Ring-chain tautomerism / Calculations, ab initio Synthesis and Structures of l-Oxa-3.5-diaza-l.3,5-hexatrienes. The l-Oxa-3,5-diaza-1.3,5-hexatriene/2H-1.3.5-Oxadiazine TautomerismReaction of N-lithio-or N-trimethylsilyl-substituted imines 1 with N-acylimidic acid derivatives 2 provides l-oxa-3,5-diaza-1,3,5-hexatrienes 3 in fair to good yields. Depending on the substitution pattern, the acyclic compounds 3 form an equilibrium with their cyclic tautomers, e.g. 2H-1,3,5-oxadiazines 4.Aromatic substituents at C-6 favor the acyclic form 3, whereas aliphatic groups (e.g. tert-butyl) lead to the exclusive formation of the ring tautomer 4. For a typical example (3d/4d) the free enthalpy of activation for the ring/chain tautomerism was determined to be ca. 12.5 kcal/mol. Detailed spectroscopic data ('H-, 13C-NMR, IR, UV) for both tautomers are given. The XRay analysis of the acyclic compound 3a shows a central C = N bond with Z configuration and with s-gauche conformations of the adjacent C = O and C = N systems. According to quantum mechanical ab initio calculations (MP2/6-31G'//6-31G') for the C3H4N20 model compounds, the ring tautomer 6 is by ca. 6 kcal/mol lower in energy than the acyclic forms ?. Within the series of open-chain tautomers 7, gauche forms are lower in energy than s-trans conformers, indicating that amide resonance is more important for these systems than polyene delocalization. s-cis Forms were found to be transition states. The acyclic conformers 7 are rather flexible; barriers of rotation for both C -N bonds are calculated to 4 -5 kcal/mol only (RHF/ 3-21G//3-21G).In den letzten Jahren ist die Bedeutung von Heterobutadienen in organischen Synthesen, besonders in Cycloadditionen"], stark gewachsen. Dies regte uns dazu an, die Reaktivitat dieser Verbindungen auf der Grundlage ihrer elektronischen und strukturellen Eigenschaften zu interpretieren. Die von uns in diesem Zusammenhang an 1-0xa-3-azabutadienen[*] und 1,3-Dia~abutadienen[~] durchgefuhrten Untersuchungen zeigten die interessante Stereochemie dieser Verbindungen auf, wobei niedrige Barrieren fur Rotationen um die C -N-Einfachbindungen dieser Amidin-bzw. Carbonsaureamid-verwandten Verbindungen hervorzuheben sind. Ein wesentlicher strukturbestimmender Faktor ist dabei die in Konkurenz zur n/n-Konjugation auftretende n/ 7c-Wechselwirkung, die zur Begunstigung von verdrillten Strukturen fuhrt. Diese Ergebnisse motivierten uns zu einer eingehenden Untersuchung der Strukturchemie verlangerter azavinyloger Heteroen-Systeme. In diesem Beitrag werden die strukturellen Eigenschaften der kaum bekannten 1-Oxa-3,5-diaza-l,3,5-hexatrieneL4I 3 untersucht. Neben den elektronischen Wechselwirkungen werden besonders die Auswirkungen der additiven Verknupfung der molekularen Bindungsdipole im C = N -C = N -C = 0-System auf die Molekulstruktur berucksichtigt. In dieser Atomanordnung sind Molekule rnit ungewohnlich groBen Dip...

Improved Reaction and Activation Energies of [4+2] Cycloadditions, [3,3] Sigmatropic Rearrangements and Electrocyclizations with the Spin‐Component‐Scaled MP2 Method

“…The set of neutral reactions consists of two parent systems, the heavily studied 4 e electrocyclic ring opening of cyclobutene [32,33] and the 6 e ring closure of 1,3,5-hexatriene, [34,35,36] and two reactions that contain heteroatoms, namely the ring closure of 1-oxa-3,5-diazahexatriene, [37] which also illustrates ring-chain tautomerism, and the important oxepine-benzeneoxide interconversion. [38] All these reactions have been well documented in the literature.…”

1,3,5‐Triazapenta‐1,3‐dienes: Useful Building Blocks for the Synthesis of1,2‐Dihydro‐1,3,5‐triazines and Oligonitriles

“…As a formal nitrile trimer they present the next longer homologue in the oligonitrile family after the “dimers” 1,3,5‐triazapenta‐1,3‐dienes 1 . From the X‐ray diffraction of 9a (Figure 6), a three‐dimensional structure was obtained, showing three ( Z )‐configured C=N bonds and three C–N bonds with gauche conformation resulting in a helix‐type arrangement of the oligonitrile main chain (torsional angles along the N=C–N=C–N=C–N=C–NH 2 chain: –1.23°, 77.56°, –7.15° 76.06°, 6.23° and 26.59°), as it is often found in oligonitrile chemistry 4,10,11,13. We interpret these structural properties by substantial electronic interactions of the lone pair of the imino nitrogen atoms with the adjacent C=N double bonds (n–π* interaction), which seem to be more important compared to polyene‐type π–π* conjugation, which would induce a planar structure.…”

“…The structural ambiguity of 3 and 6 is best explained by ring‐chain tautomerism, which is typical for unsaturated hetero chains of the oligonitrile type (Scheme ) 10,11. Obviously, the equilibrium is very sensitive with respect to small changes of the substitution pattern of the compounds involved,12 as can be seen by comparing 3d with 6a , which differ only in the substituent at N1 (phenyl vs. cyclohexyl).…”

“…The X‐ray diffraction structure of 11b (Figure 7) presents a 3 1 ‐helix (torsional angles along the chain: 40.73°, 8.93°, 65.58°, 0.67°, 63.81°, 15.00°), whereas 11a shows a more irregular structure (torsional angles along the chain:–87.73°, 7.92°, –119.96°, 1.58°, 128.94°, –14.61°). Obviously, small differences in the steric requirement of the peripheral groups lead to different structures in the solid state,where easy conformational changes about the C–N bonds may be caused by the crystal packing forces during the crystallization 10…”

(5‐Imino‐4,5‐dihydro‐3H‐pyrrol‐2‐yl)amines as Sterically Restrained 1,3,5‐Triazapenta‐1,3‐dienes: Useful Building Blocks for the Synthesis of Oligonitriles

“…Use of benzoyl chloride as the trapping electrophile gave the 1-oxa-3,5,7-triazahepta-1,3,5-triene derivative 3b. According to the X-ray determination, 3b combines the structural features typical of an N-acylimine [12] or a 1-oxa-3,5-diaza-1,3,5-hexatriene, [13] a gauche conformation (62.4°) of the C=O group with respect to the adjacent C=N bond as a consequence of intense nitrogen lone pair/C=O π*-orbital interactions (Scheme 2, Figure 4), with the planar π system of the dihydropyrrole subunit.…”