Dolores Castillo

NMR and X-ray diffraction studies of two bicyclic borates containing chiral boron and nitrogen atoms

Abstract Ring closure between the nitrogen and boron atoms during the syntheses of bicyclic organoboron compounds occurs under asymmetric induction, since reactions of optically active N-methyl-N-(1-methyl-2-phenyl-2-hydroxyethyl)glycines with 4-bromophenylboronic acid or with 1,4-phenylenediboronic acid provides the corresponding bicyclic boron derivatives (N → B)-4-bromophenyl[N-methyl-N- (1-(S)-methyl-2-(R)-phenyl-2-oxyethyl)aminoacetate-O,O′,N]boron (3a–3b), (N → B)-4-bromophenyl[N-methyl-N-(1-(S)-methyl-2-(S)-phenyl-2-oxyethyl)aminoacetate-O,O′,N]boron (4a–4b), (N → B)(N′ → B′)-1,4-phenylenebis[N-methyl- N-(1-(S)-methyl-2-(R)-phenyl-2-oxyethyl)aminoacetate-O,O′,N]bis-boron (5) and (N → B)(N′ → B′)-1,4-phenylenebis[N-methyl-N-(1-(S)-methyl-2-(S)-phenyl-2-oxyethyl)aminoacetate-O,O′,N]bisboron (6) in high stereochemical yields. The configuration of the nitrogen and boron atoms in 6 and in (N → B) phenyl[N-methyl-N-(1-(S)-phenyl-2-oxyethyl)aminoacetate-O,O′,N]boron (2) is established by single crystal X-ray diffraction studies.

Synthesis and molecular structures of dimeric boron compounds

A series of eleven new dimeric boron chelates: [B(SAE)(m-NH2C6H4)]2 (3a), [B(SAE)(p-CF3C6H4)]2 (3b), [B(SAE)(pCHOC6H4)]2 (3c), [B(SAE)(p-BrC6H4)]2 (3d), [B(SAE)(p-EtOC6H4)]2 (3e), [B(SAE)(m-CH3COC6H4)]2 (3f), [B(SAE)(pCH3COC6H4)]2 (3g), [B(SAE)(o-F, p-FC6H3)]2 (3h), [B(SAE)(m-F, p-FC6H3)]2 (3i), [B(SAE)(m-F, m-FC6H3)]2 (3j) and [B(SAE)(o-F, o-FC6H3)]2 (3k), prepared by condensation of 2-salicylideneamino hydroxyethane (H2SAE) with arylboronic acids such as amino-, trifluoromethyl-, bromo-, formyl-, acetyl-, ethoxy- and difluoro-boronic acid allowed a study of the influence of different substituents in the B-phenyl moiety on coordination of the boron atom. In all cases, dimeric derivatives were formed and the molecular structures for [B(SAE)(p-CH3OC6H4)]2 (2e), 3b, 3d and 3h were established by X-ray diffraction analysis where the values of the intramolecular (NB) bond lengths confirmed a donor‐acceptor character.

X-ray crystallographic study of boroxazolidones obtained from l-ornithine, l-methionine, kainic acid and 2,6-pyridinedicarboxylic acid

Abstract In the present contribution, the boroxazolidones prepared from l -ornithine, l -methionine, kainic acid and 2,6-pyridinedicarboxylic acid have been studied by X-ray crystallography. A comparison of the structural data with corresponding boroxazolidines, bicyclic boronates and tricyclic borates has shown that in boron complexes with a boroxazolidone ring the B–O bond is longer in comparison to boron complexes with a boroxazolidine ring. At the same time the N→B bond length is shorter indicating that the hydrolytic stability of the complexes with boroxazolidone rings is enhanced.

Through-bond modulation of N→B ring formation shown by NMR and X-ray diffraction studies of borate derivatives of pyridyl alcohols

The preparation of diphenyl(2-pyridylmethoxy-O,N)borane (2a), diphenyl[1-(2-pyridyl)ethoxy-O,N]borane (2b), diphenyl[α-(2-pyridylbenzyloxy-O,N)]borane (2c), diphenyl[2-(2-pyridyl)ethoxy-O,N]borane (4), benzene-1,2-diyldioxy(2-pyridylmethoxy-O,N)borane (5) and benzene-1,2-diyldioxy[2-(2-pyridyl)ethoxy-O,N]borane (6) is reported. The cyclic structures 2a and 4 were established by X-ray diffraction studies, the N→B bond distance being 1.642 A for 2a and 1.685 A for 4. Complete assignment of the 1H and 13C NMR spectra of 2a–c and 4 was achieved from two dimensional HETCOR data. Additional evidence for the strength of the five- and six-membered rings in boron esters was obtained from variable-temperature measurements which show partial ring opening for the six-membered ring compounds at 180 °C. The observation that five-membered rings are more stable than six-membered ones is attributed to sigma-assistance by through-bond interactions.

Study of cyclic borinates obtained from piperidine- and piperazine alcohols by spectroscopic methods and X-ray crystallography

Abstract A series of eleven new 2-aminoethyl- and 3-aminopropyl borinate derivatives with a coordinative N→B bond has been synthesized by condensation reactions between piperidine- as well as piperazine alcohols and diphenylborinic acid. The products obtained are analogous to N-spiro compounds and bicyclic systems and have been characterized by spectroscopic methods and X-ray crystallography. Thereby the N→B bond and the geometry of this new heterocyclic systems have been studied in more detail.

Dynamic NMR and X-ray diffraction study of (N-B) -diphenyl(2-aminoethoxy) borane derivatives of ephedrines', and pseudoephedrines

Abstract The preparation and characterization of various (N-B)-diphenyl-(2-aminoethoxy)boranes derived from ephedrine and pseudoephedrine derivatives ( 1b–6b ) are reported: (N-B)-diphenyl(1-( R )-phenyl-2-( S )-methyl-2-aminoethoxy)borane ( 1b ), (N-B)-diphenyl(1-( R )-phenyl-2-( R )-methyl-2-aminoethoxy)borane ( 2b ), (N-B)-diphenyl[ N -( R )-methyl-(1-( R )-phenyl-2-( S )-methyl-2-aminoethoxy)]borane ( 3b - trans ), (N-B)-diphenyl[ N -( S )-methyl-(1-( R )-phenyl-2-( S )-methyl-2-aminoethoxy)]borane ( 3b - cis ), (N-B)-diphenyl[ N -( S )-methyl-(1-( R )-phenyl-2-( R )-methyl-2-aminoethoxy)]borane ( 4b - trans ). (N-B)-diphenyl[ N,N -dimethyl-(1-( R )-phenyl-2-( S )-methyl-2-aminoethoxy)]borane ( 5b ) and (N-B)-diphenyl[ N,N -dimethyl-(1-( R )-phenyl-2-( R )-methyl-2-aminoethoxy)]borane ( 6b ). The five membered N → B cyclic structures 1b–6b were assigned based on 1 H-. 13 C-, 11 B- and 15 N-NMR data and all compounds except for 3b - trans were subjected to X-ray diffraction analysis showing N → B bond lengths of 1.66(2) and 1.64(2)A for 1b , 1.657(9) and 1.664(9))A for 2b , 1.68(2) A for 3b - cis , 1.66(1)A for 4b - trans , 1.744(8)A for 5b and 1.74(1)A for 6b . The study of the intramolecular N → B coordination by means of dynamic NMR spectroscopy afforded Δ G ‡ values of 67.9, 70.9, 64.8, 68.2, 49.7 and 52.7 kJ mol −1 for the dissociation of the N → B bond in compounds 1b–6b respectively. The results show that steric interactions between the substituents at the (2-aminoethoxy)borane ring determine the stability of the N → B bond as well as the nitrogen configuration. Theoretical calculations of the electrostatic charges for the boron and nitrogen atoms in 1b , 2b , 3b - cis , 3b - trans , 4b - cis , 5b and 6b show that the increase of positive charge on the nitrogen atom causes a shift to lower frequencies in the 15 N NMR spectra.

Facile synthesis of 1,3,6-oxadiazepines from 2,2′-(1,2-ethanediyldiimino)bisphenols

Abstract A useful sequence of reactions for the syntheses of a variety of heterocyclic systems including an oxadiazepine ring is described. The key step involves the condensation of substituted 2,2′-(1,2-ethanediyldiimino)bisphenols with ethanedial to provide stereoselectively the 6a,7a-trans-6,6a,7a,8,15,16-hexahydro[1,4]benzoxazine[4′,3′:6,7][1,3,6]oxadiazepino[2,3-c][1,4]-benzoxazine-6,8-diol framework, as established by X-ray diffraction analyses.

Comparative Structural Characterization of the Biquaternized N-CH3 and N-BH3 Derivatives of the cis-Cyclen and cis-Cyclam Condensation Products with Glyoxal

Abstract The biquaternized N-CH3 and N-BH3 derivatives of the cis-cyclen (1,4,7,10-tetraazacyclododecane) and cis-cyclam (1,4,8,11-tetraazacyclotetradecane) condensation products with glyoxal have been prepared and structurally characterized by NMR spectroscopy and in the case of the BH3 adducts additionally by X-ray crystallography. A comparison of the structural data between the four compounds and the starting materials permitted: (i) to establish their overall conformation, (ii) to discuss the structural modifications caused by the N-CH3 and N-BH3 functional groups and (iii) to understand, why the bialkylated tetraazapolycycles can be reduced to the cross-bridged tetraazamacrocycles, in contrast to the borane adducts.

X-ray crystallographic study of three (N→B)-borinates prepared from 8-hydroxyquinoline and 2-hydroxypyridine

Abstract8-Hydroxyquinoline and 2-hydroxypyridine have been reacted with diphenylborinic acid or 9-BBN; the molecular structure of the resulting heterocycles has been studied by X-ray crystallography. A structural comparison of the so formed five- and six-membered heterocycles with similar complexes obtained from aliphatic amino alcohol and α-amino acid derivatives shows significant differences for the N→B, B−O and B−C bond lengths and some of the inner cycle bond angles. Other structural parameters discussed in this respect are the sum of bond lengths at the boron atom, the sum of bond angles in the heterocycle and the tetrahedral character of the boron atom. On the basis of these parameters a qualitative comparison of heterocycle stability is possible.

13C NMR and Crystallographic Study of Thymolsulfonephthalein

Up to now the correct structure of the undissociated thymolsulfone- phthalein has not been determined unequivocally and three possible resonance structures have been proposed so far: a zwitterionic one, a quinoid one, and a lactonoid one. The present 13 C NMR and X-ray crystallographic study resolves this problem and it can be explicitly shown that the correct structure is zwitterionic with some delocalization of positive charge into the phenol rings. The correct assignment of the completely dissociated blue basic form is also included and permits the conclusion that a fourfold-charged resonance structure is as probable as a quinoid one.