bluuebunny/crossref_metadata_2025_split huggingface dataset:
Struktur von α‐(Dimethylamino)benzyllithium in Lösung: Dynamisches Gleichgewicht zwischen einer η1‐ und einer η3‐Spezies
Authors: Ahlbrecht Hubertus , Harbach Jürgen , Hauck Tim , Kalinowski Hans‐Otto 
Published: July 2007

Structure of α‐(Dimethylamino)benzyllithium in Solution: Dynamic Equilibrium between an η1 and an η3 SpeciesTemperature‐dependent 1H‐ and 13C‐NMR‐spektroscopic shifts observed in tetrahydrofuran are indicative of an equilibrium between a high‐temperature η3 and a low‐temperature η1 species. MNDO calculations are in accord with this view, and HOESY experiments proof it, too. The equilibrium is controled by solvation or complexation, low Lewis basicity of the solvent or bidentate ligands favoring the η3, high Lewis basicity or tridentate ligands the η1 species. Equilibrium as well as rate constants have been estimated, and from their temperature dependence the thermodynamics of the equilibrium and the activation barriers were calculated.
Conformation in the Solid State and in Solution of 3,3′(4H,4′H)‐Spirobi[2H‐1,5‐benzodioxepin]
Authors: Grochowski Jacek , Rutkowska Maria , Rys Barbara , Serda Pavel , Snatzke Günther 
Published: July 2007

AbstractThe “spiro‐coplanar” conformation of 6,6′‐methoxycarbonyl‐3,3′(4H,4′H)‐spirobi[2H‐1,5‐benzodioxepin] (1b) and the 3P configuration of the corresponding menthol ester 1c have been determined from chiroptical data by application of the exciton theory. The solid‐state conformation of the central part of the menthol ester molecule is “spiro‐coplanar”, too, with nearly D2 symmetry. The menthol moieties have chair conformations, and the absolute configurations at their chiral centers have been assigned using enantiomer‐sensitive Bijvoet pairs; they are in agreement with the known absolute configuration of menthol.
Von vierfach funktionalisierten Cyclophanen zu rohrförmigen Molekülen
Authors: Schröder Axel , Karbach Detlef , Güther Ralf , Vögtle Fritz 
Published: July 2007

From Fourfold Functionalized Cyclophanes to Tube‐Shaped MoleculesA series of fourfold functionalized 2,11‐dithia‐ and 2,11‐diaza[3.3]cyclophanes 15–17 and 18–22, resp., is prepared by using a new multistep‐strategy starting from 7 or m‐xylene, respectively. The cyclophanes are used to construct tube‐shaped molecules of type 24 by macrocyclization reactions of e. g. 21 and 22. Macrocycle 24 turns out to be conformationally flexible.
Stereochemie der Denitrogenierung von optisch aktivem 9,10‐Dihydro‐5,6‐diazalumibullvalen zu optisch aktivem 3,4‐Dihydrosemibullvalen
Authors: Askani Rainer , Andermann Thomas , Müller Karl Michael 
Published: July 2007

Stereochemical Course of the Denitrogenation of Optically Active 9,10‐Dihydro‐5,6‐diazalumibullvalene to Optically Active 3,4‐DihydrosemibullvaleneThermal decomposition of optical active diazalumibullvalene derivative (+)‐6 gives optical active dihydrosemibullvalene (+)‐5. The absolute configurations of optical active 6 and 5 were determined by correlation to tricyclooctanone 9 and 2‐cyclopentene‐1‐acetic acid (21), respectively. From these results it is concluded that thermal denitrogenation of 6 occurs by concerted six‐electron cycloreversion competing most likely to a multi‐step N2 elimination via the symmetrical diradical intermediates 8 and 7.
Ein neuartiger Einstieg in Wittig‐Umlagerungen – Eine stereoselektive [1,2]‐Wittig‐Umlagerung mit Konfigurationsumkehr am Carbanion‐Zentrum
Authors: Hoffmann Rolf , Brückner Reinhard 
Published: July 2007

A Novel Entry into Wittig Rearrangements – A Stereoselective [1,2]‐Wittig Rearrangement with Inversion of Configuration at the Carbanion CenterThe O,Se‐ketal 10, through lithium naphthalenide induced reductive cleavage of its C–Se bond, was converted into the α‐lithio ether 11a with an equatorially oriented CO bond. At ‐78°C, 11a undergoes a rapid [1,2]‐Wittig rearrangement to provide stereoselectively the homoallylic alcohol 12a with an axial OH group. Thus, the rearrangement proceeds with inversion of configuration at the carbanion center.
Metallkomplexe von Farbstoffen, III. Übergangsmetall‐Komplexe mit Pyrrolindigo
Authors: Schmidt Christoph , Wagner Hans‐Ulrich , Beck Wolfgang 
Published: July 2007

AbstractMetal Complexes of Dyes, III[1,2]. – Transition Metal Complexes of Pyrrole Indigo The dianions of octahydroindigo and 4,4′‐dibutyl‐5,5′‐dimethylpyrrole indigo (L) react with metal salts and chloro‐bridged complexes [(μ‐Cl)M(Cl)PR3]2 (M = Pd, Pt; R = nBu, Et), [(μ‐Cl)M(Cl)(η5‐C5Me5)]2 (M = Rh, Ir) to give the bis‐N,O chelate complexes [M(L – 2H+)]n (M = Cu, Zn), (L – 2H+)[M(Cl)(PR3)]2 (M = Pd, Pt), and (L – 2H+)[M(Cl)(η5‐C5Me5)]2 (M = Rh, Ir). The electronic spectra of the soluble complexes with phosphane or C5Me5 ligands show a large bathochromic shift of the absorptions in the visible region compared to the free neutral ligands which is in accordance with PPP calculations of model compounds.
Derivate des Borols, XIX. (Borol)dicarbonylcobaltat‐Ionen: Synthese, Struktur und Reaktivität
Authors: Herberich Gerhard E. , Carstensen Tobias , Englert Ulli 
Published: July 2007

Derivatives of Borole, XIX[1]. – (Borole)dicarbonylcobaltate Ions: Synthesis, Structure, and Reactivity[3]Reduction of the dinuclear (borole)carbonylcobalt complexes [(3,4‐R2C4H2BPh)Co(CO)2]2 (3a: R = H, b: R = Me) with Na/Hg in THF at –30°C affords the anions [(3,4‐R2C4H2BPh)‐Co(CO)2]− (2a,b−) which can be isolated as [Na([15]crown‐5)]+ salts. The structural data of PPh4 · 2a indicate a strong Co – C(CO) interaction and relatively weak cobalt‐to‐boron bonding. Methylation of the anions 2a,b− gives labile metal alkyl complexes (3,4‐R2C4H2BPh)Co(CO)2Me while protonation and subsequent addition of phosphanes afford boracyclopentenyl complexes as e.g. (C4H5BPh)Co(CO)2(PMe3) (5a). The latter reacts with Ph3CBF4 to give the salt [(C4H4BPh)Co(CO)2‐(PMe3)]BF4 (6 · BF4). At –80CC cation 6+ undergoes hydride addition at the carbonyl carbon to give the formyl species (C4H4BPh)Co(CO)(CHO)(PMe3) (7) which transforms to the hydride (C4H4BPh)CoH(CO)(PMe3) (8). At room temperature the same reaction produces 5a. Deuteration studies show that the oxidative formation of 6+ from 5a and the formation of 5a by addition of hydride to 6+ are nonstereospecific processes.
Thiiran als Schwefelquelle für das 3z4e‐Bindungssystem im Komplex {(η5‐C5H5)(CO)2Mn}2S
Authors: Beuter Georg , Drobnik Stefan , Lorenz Ingo‐Peter , Lubik Alfred 
Published: July 2007

AbstractThiirane as Sulfur Source for the 3c4e Bonding System in the Complex {(η5‐C5H5)(CO)2Mn}2SThiirane (2) is used as source for the directed formation of the symmetrically bridged complex { (3) by its reaction with (η5‐C5H5)(CO)2MnTHF (1). Compound 3 can be oxidized, protonated, or methylated to give the {(η5‐C5H5)(CO)2Mn}2SSO complex {SO (4) or the SR+ cations [{(η5‐C5H5)(CO)2Mn}2SR]+ (R = H, CH3) (5, 6). Compounds 5 and 6 show an equilibrium between the open and closed form in solution at room temperature. According to an X‐ray structural analysis 3 crystallizes in the triclinic space group Pη1 and shows the open form IIa with a 3c4e bonding system which is isolobal to SO2, whereas the analogous rhenium complex has the isomeric closed form IIb with a metal – metal bond which is isolobal to thiirane.
Phosphoniothiolat–Komplexe: Darstellung und Umlagerung in Phosphan–Komplexe
Authors: Fischer Helmut , Fluck Klaus H. , Troll Carsten 
Published: July 2007

Phosphoniothiolate Complexes: Synthesis and Rearrangement to Phosphane ComplexesPentacarbonyl(thiobenzaldehyde)tungsten (1) reacts with phosphanes PR3 to give the phosphoniothiolate / complexes (CO)5W[SC(H)Ph(PR3)] (2) (RMe, Et, Ph, 4–MeC6H4). Similarly, the thiobenzophenone complex (CO)5W[SCPh2] (3) affords (CO)5W[SC(Ph)2PMe3] (4). The phosphoniothiolate complexes obtained from 1 and PPh2H or P(2,4,6–Me3C6H2)H2 rapidly rearrange by formal S,P migration of (CO)5W and P,C migration of H to form the phosphate complexes (CO)5W{P‐(Ph)2[C(Ph)H(SH)]} (5) and (CO)5W{P(2,4,6–Me3C6H2)H‐[C(Ph)H(SH)]} (6/6′).
Die hypothetischen Phosphor–Cluster P6 und P8 – welche Isomeren wären die stabilsten?
Authors: Janoschek Rudolf 
Published: July 2007

The Hypothetic Phosphorus Clusters P6 and P8 – Which Isomers Would Be the Most Stable Ones?The hexaphosphabenzvalene 3 turned out to be the most stable P6 isomer applying ab initio MP4(SDQ)/CEP–DZ* calculations, and therefore plays a favorite role for synthesis. The cubic P8 is replaced by a polycyclic structure 8 which is more stable by 49 kcal mol‐1, but still thermodynamically unstable with respect to 2 P4.
On the Reactions of Fe(CH2)+ with 1,7–Octadiene in the Gas Phase
Authors: Stöckigt Detlef , Schwarz Helmut 
Published: July 2007

AbstractIn contrast to the reactions of “bare” Fe+ with 1,7–octadiene (1), the chemistry of Fe(CH2)+ with 1 represents a “textbook” example for a gas–phase metathesis process to generate in a clean fashion Fe(C2H4)+. Similarly, for the formation of Fe(C6H6)+ a mechanism is suggested which is in keeping with the experimental findings. In contrast, the reactions of Fe(CH2)+ with 1 leading to Fe(C3H4)+ and Fe(C4H6)+ are so complex that a coherent description is not yet possible.
Mehrfachbindungen zwischen Hauptgruppenelementen und Übergangsmetallen, CXIV. Organorhenium(VII)‐oxide
Authors: Herrmann Wolfgang A. , Kühn Fritz E. , Romão Carlos C. , Huy Hoa Tran , Wang Mei , Fischer Richard W. , Kiprof Paul , Scherer Wolfgang 
Published: July 2007

Multiple Bonds between Main Group Elements and Transition Metals, CXIV. — Organorhenium(VII) OxidesAlkylrhenium(VII) oxides of formulae RReO3 and RReO3 · L (L = quinuclidine) result from dirhenium heptaoxide and dialkylzinc compounds at low temperatures in THF solution. Unbranched, noncyclic organorhenium oxides prove to be less thermolabile than branched‐chain derivatives, with the thermal stability decreasing with increasing chain length. Complexes with branched carbon chains normally can be isolated as 1:1 adducts of a nitrogen base, e.g. quinuclidine. A first chiral alkylmetal oxide and base‐stabilized cyclopropyltrioxorhenium(VII) are also described.
Pyridinium N‐Phenoxide Betaines and Their Application to the Characterization of Solvent Polarities, XIX. The Halochromism of Pyridinium N‐Phenoxide Betaine Dyes in Acetonitrile Solution
Authors: Reichardt Christian , Asharin‐;Fard Siamak , Schäfer Gerhard 
Published: July 2007

AbstractThe halochromism of the negatively solvatochromic pyridinium N‐phenoxide betaine dye 1 has been measured in acetonitrile solution and linearly correlated with the effective charge of the cations of the added alkaline and alkaline earth salts. The observed negative halochromism of betaine dye 1 constitutes a new type of true halochromism, in contrast to the trivial halochromism found for the first time by Baeyer and Villiger in 1902.
Trimethylsilylcyanid als Umpolungsreagens, XX. Zur Regioselektivität der Addition von Carbonyl‐;Verbindungen an umgepolte α,β‐ungesättigte Aldehyde
Authors: Hünig Siegfried , Schäfer Matthias 
Published: July 2007

Trimethylsilyl Cyanide — A Reagent for Umpolung, XX. — On the Regioselectivity of the Addition of Carbonyl Compounds to α,β‐Unsaturated Aldehydes after UmpolungBy addition totBuMe2SiCN to α,β‐unsaturated aldehydes5the silylated cyanohydrins6are produced. Deprotonation of6provides allylic anions which can be attached to aldehydes and ketones in α‐position (kinetic control) (→8) or γ‐position (thermodynamic control) (→9). In contrast to the Me3Si group which has been applied so far thetBuMe2Si group is stable under the reaction conditions. Therefore, the ratio of α/γ adducts can be tuned from >95:5 to <5:95 depending on the substitution pattern of6, the aldehydes and ketones, the reaction time and temperature, and even by some additives. The γ adducts9are easily transformed into γ‐butyrolactones27by acidic hydrolysis.
Darstellung und Charakterisierung von Anthrylenmagnesium‐Innerkomplexen
Authors: Benn Reinhard , Bogdanović Borislav , Brüning Martin , Grondey Hiltrud , Herrmann Wolfram , Kinzelmann Hans‐;Georg , Seevogel Klaus 
Published: July 2007

Preparation and Characterization of Inner Complexes of AnthrylenemagnesiumAnthracene derivatives containing ether or amino groups in 9‐ or 9,10‐position (1 – 10) react with magnesium to form new magnesium anthracene inner complexes (11 – 20). These complexes as well as their organic starting materials have been extensively characterized by spectroscopy and chemical means. It was found that solvent ligands which are present in all magnesium anthracene complexes known to date can be partially or totally replaced by inner complexation. For some of the new magnesium anthracene inner complexes the complex formation rates have been determined.
Crystal Structure of the Explosive Parent Benzyne Precursor: 2‐Diazoniobenzenecarboxylate Hydrate
Authors: Horan Christopher J. , Barnes Charles L. , Glaser Rainer 
Published: July 2007

AbstractThe structure of the highly unstable benzyne precursor 2‐diazoniobenzenecarboxylate (3) has been determined by single‐crystal X‐ray diffraction. The structure is discussed in comparison to ab initio results for several conformers of 3, related aromatic diazonium ions, and phenyl cation and also to crystal structures of simple diazonium ion salts and of benzoates. Structural features and characteristic distortions are related to the electron density distributions and to intra‐ and intermolecular interactions between the neighboring functional groups.
Allenyl Enolates — A New Class of Chiral Ambident Nucleophiles, 1. Reaction with C and Si Electrophiles
Authors: Arndt Silke , Handke Gabriele , Krause Norbert 
Published: July 2007

AbstractThe regio‐ and stereoselectivity of the reaction of allenyl enolates obtained by 1,6‐addition of lithium dimethylcuprate to acceptor‐substituted enynes 1 with C and Si electrophiles is examined. It is found that allyl bromide reacts at C‐4 to give 4‐substituted 2,4‐dienoates 2; in contrast, methyl triflate and carbonyl compounds react at C‐2 to yield 2‐substituted β‐allenic esters 3–12. The hard electrophile chlorotrimethylsilane reacts at the enolate oxygen atom to provide allenyl enol ether 14. The regioselectivity of these trapping reactions depends only on the nature of the electrophile; the stereoselectivities, on the other hand, are a function of both the steric properties of the allenyl enolate and the type of electrophile. The regio‐and stereoselectivities are rationalized in terms of charge control and frontier‐orbital control, of kinetic and thermodynamic control, and of steric interactions in intermediates and transition states.
Preparation and Some Reactions of (Acylamino)diorganoboranes
Authors: Yalpani Mohamed , Köster Roland , Boese Roland 
Published: July 2007

AbstractThe carboxamides RCONH2 [RPh (1), tBu (2)] react with BEt3 and with (9H‐9‐BBN)2 to give the (acylamino)diethylboranes 3, 4 and the 9‐acylamino‐9‐BBN 5, 6, respectively. Compounds 3 and 4 are largely monomeric (NMR) in solution and dimeric as crystalline solids [X‐ray structure of (4)2]. The 9‐BBN derivatives 5 and 6 are associated and probably polymeric in solution (NMR). The 1:1 adducts 7 and 8 containing the ring (X‐ray structure of 7) are formed on the reaction of 3 and 4 with (9H‐9‐BBN)2 by borane exchange. The former is also obtained directly by treating compound 1 or 2 with (9H‐9‐BBN)2. Compound 7 reacts at 140–150°C to give the air‐stable heterocycle 11 (X‐ray structure).
Synthese und Struktur des ersten Tellur‐haltigen Borazin‐Derivats und einer Tellur‐haltigen Bor – Stickstoff‐Spiro‐Verbindung
Authors: Koch Hans‐Joachim , Roesky Herbert W. , Besser Susanne , Herbst‐Irmer Regine 
Published: July 2007

Synthesis and Structure of the First Tellurium‐Containing Borazine Derivative and a Tellurium‐Containing Boron – Nitrogen Spirocyclic CompoundWe describe the synthesis of the first tellurium‐containing borazine derivative MeN[PhBN(Me)]2TeCl2 (2). Compound 2 has been prepared by the reaction of MeN[PhBN(Me)SiMe3]2 and TeCl4. The spirocyclic compound [PhB(tBuN)2]2Te (4) has been synthesized by the reaction of PhB(tBuNLi)2 (3) with TeCl4 in a molar ratio of 2:1. The single‐crystal X‐ray structures of 2 and 4 are reported.
Monofunktionelle tetraedrische Zink‐Komplexe L3ZnX [L3Tris(pyrazolyl)borat]
Authors: Alsfasser Ralf , Powell Anne K. , Vahrenkamp Heinrich , Trofimenko Swiatoslaw 
Published: July 2007

Monofunctional Tetrahedral Zinc Complexes L3ZnX [L3Tris(pyrazolyl)borate]The pyrazolyl borates K[HBpz3*] (abbreviated KL3, pz*3‐phenyl‐, 3‐tolyl‐, 3‐anisyl‐3,5‐diphenyl pyrazolyl) react with zinc salts ZnX2 (XCl, Br, I, NO3) to form the mononuclear neutral tetrahedral zinc complexes L3ZnX (1 – 4). The corresponding alkylzinc complexes L3ZnR (5 – 8; RMe, Et, tBu, Ph) result either from L3ZnCl and LiR or from ZnEt2 and KL3. These alkylzinc compounds are remarkably stable towards oxidation and hydrolysis. For specific cases the reaction of L3ZnR with carboxylic acids is found to yield the carboxylates L3ZnOCOR (9, 10), and likewise with thioacetic acid to yield the thioacetates L3ZnSCOMe (11). Facile cleavage of L3ZnR with thiols and selenophenol produces the thiolates L3ZnSR (12, 13) and the selenolates L3ZnSePh (14). The complexes L3ZntBu which are the most reactive of the alkylzinc compounds are cleaved by H2O, H2S, NH3 and various OH and NH compounds with formation of the bis(ligand) complexes Zn(L3)2 (15). Crystal structure determinations of one L3ZnX complex each for XNO3 (4a), CH3 (5a), SCOMe (11a), and SEt (12a) confirm the nature of the compounds and the relation of 4a and 12a to the active centers of zinc‐containing enzymes.
Cycloadditionen von Benzothiet und aromatischen Nitroso‐Verbindungen
Authors: Saul Klaus , Eckes Heinz‐Ludwig , Meier Herbert , Jacob Dominic 
Published: July 2007

Cycloaddition Reactions of Benzothiete and Aromatic Nitroso CompoundsThe new heterocyclic ring systems 4H‐3,1,2‐benzoxathiazines 4 and 2,3‐dihydro‐1,2‐benzothiazol 1‐oxides 6 can be synthesized by cycloaddition of benzothiete (1) and aromatic nitroso compounds 3. The regioselectivity of the primary step and the secondary thermal or acid‐catalyzed rearrangements are discussed.
Reactivity of Dithiazinanes towards BH3, BD3 and BF3. New Heterocycles: 5,5‐Dimethyl‐1,3‐dithia‐5‐azonia‐4‐boratacyclohexane and 6,6‐Dideuterio‐5‐methyl‐5[D1]methyl‐1,3‐dithia‐5azonia‐4‐boratacyclohexane. A Method for the Dimethylation and Monodeuteriomethylation of Primary Amines
Authors: Flores‐Parra Angelina , Cadenas‐Pliego Gregorio , Martínez‐Aguilera Luz Maria R. , Lourdes García‐Nares M. , Contreas Rosalinda 
Published: July 2007

Abstract5‐Methyl‐and 5‐tert1‐butyl‐1,3,5‐dithiazinane (1 and 7) react with BH3·THF, and Et2O·BF3 to yield the N1‐coordinated adducts 2 – 4 (with BH3, BD3, and BF3, respectively). The conformations and spectroscopic properties of the adducts are discussed. The reactions of 1 and 7 with BY3·THF lead to the six‐membered boron heterocycles 5,5‐dimethyl‐1,3‐dithia‐5‐azonia‐4‐boratacyclohexane (5), 4,4‐dideuterio‐5‐methyl‐5[D1]methyl‐1,3‐dithia‐5‐azonia‐4‐boratacyclohexane (6), and 5‐tert1‐butyl‐5‐methyl‐1,3‐dithia‐5‐azonia‐4‐boratacy‐clohexane (8). Compounds 2 and 5 are isolobal isomers. The reaction of BH3 or BD3 with 7 affords, after heating, tert1‐butyldimethylamine – borane (9) and tert1‐butyldi([D1]methyl)1‐amine – trideuterioborane (11), respectively. The dithiazinane derivatives may be used in organic synthesis for the dimethylation of primary amines and the preparation of alkyldimethylamines with monodeuterated methyl groups.
(Ether‐phosphan)eisen(II)1‐Komplexe mit den Koordinationszahlen Vier, Fünf und Sechs
Authors: Lindner Ekkehard , Geprägs Michael , Fawzi Riad , Steimann Manfred 
Published: July 2007

(Ether‐phosphane)iron(II) Complexes with the Coordination Numbers Four, Five, and SixPhotolysis of cis,cis,trans1‐Br2(OC)2 Fe(P∑O)2 (1a, b) [O,PPh2PCH2CH2OCH3 (a), Ph2PCH2C4H7O (b)] results in the formation of the iron(II) complexes Br2Fe(P∼O)2 (2a), [BrFe‐(P∩O)(μ1‐Br)]2 (4b), and [trans1‐Br2Fe(P∩O)(μ1‐CO)]2 (5a) with the coordination numbers four, five, and six, respectively. 2a crystallizes in the triclinic space group P\documentclass{article}\pagestyle{empty}\begin{document}$\bar{1}$\end{document} with Z2 and has two monodentated O,P functions (P∼O: η1‐P1‐coordinated; P∩O:η2‐O,P1‐coordinated). The dimeric complex 4b with two O∩P‐chelated ligands crystallizes in the orthorhombic space group Pbca with Z4. 2a and 4b, which react under CO pressure to give the starting complexes 1a, b, are also accessible from the ether‐phosphanes 3a, b and anhydrous FeBr2.
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
Authors: Buhmann Martin , Würthwein Ernst‐Ulrich , Möller Manfred H. 
Published: July 2007

Synthesis and Structures of 1‐Oxa‐3,5‐diaza‐1,3,5‐hexatrienes. The 1‐Oxa‐3,5‐diaza‐1,3,5‐hexatriene/2H‐1,3,5‐Oxadiazine TautomerismReaction of N1‐lithio‐or N1‐trimethylsilyl‐substituted imines 1 with N1‐acylimidic acid derivatives 2 provides 1‐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. tert1‐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 (1H‐, 13C‐NMR, IR, UV) for both tautomers are given. The X‐Ray 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 C3H4N2O model compounds, the ring tautomer 6 is by ca. 6 kcal/mol lower in energy than the acyclic forms 7. 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).
Substituenteneffekte auf die CC‐Bindungsstärke, 13. Kinetische und thermodynamische Stabilität von α1‐alkylierten α1‐Amino‐carbonsäureestern – Stabilisierungsenergie von α1‐Amino‐α1‐(ethoxycarbonyl)alkyl‐Radikalen
Authors: Schulze Rainer , Beckhaus Hans‐Dieter , Rüchardt Christoph 
Published: July 2007

Substituent Effects on the Strength of CC Bonds, 13[1]. – Kinetic and Thermodynamic Stability of α1‐alkylated α1‐Amino Acid Esters – Energy of Stabilization of α1‐Amino‐α1‐(ethoxycarbonyl)alkyl RadicalsThermolytic cleavage of 1a–c and 2a–c into radicals in solution are clean first‐order processes. The activation parameters for CC bond homolysis were determined. Compared with the corresponding data for hydrocarbons of equal strain a decrease of bond strength of 14.4 kcal/mol results by the combined interaction of the amino and the ethoxycarbonyl substituents. The heat of formation of 5 was determined by combustion calorimetry [δHco (1)– 1207.75 ± 0.20 kcal/mol] combined with the heat of vaporization (δHvo13.31 ± 0.10 kcal/mol). Compared with the additive behaviour of group increments the resulting heat of formation (δHfo (g)– 112.02 ± 0.24 kcal/mol] suggests a synergetic, geminal destabilization of Hsyn1.9 ± 1.4 kcal/mol. Combination of the kinetic data of bond homolysis of 1a–c and 2a–c with the synergetic ground‐state destabilization leads to a synergetic, capto‐dative radical stabilization of Hsyn (rad)6.7 kcal/mol.