Synthesis and characterization of new (N → B) phenyl substituted[ N -benzyliminodiacetate- O , O ′ , N ]boranes

The synthesis of nine (N → B) phenyl substituted[N-benzyliminodiacetate-O,O’,N]boranes 3a - 3i is reported herein. These compounds were characterized by 1 H, 13 C, 11 B, HETCOR, NOESY, infrared spectroscopy, mass spectrometry and in the case of compounds 3d and 3g also by 19 F NMR. All compounds exhibit a bicyclic structure due to the presence of an intramolecular N → B coordination bond. The structure of 4-chlorophenyl[N-benzyliminodiacetate-O,O’,N]borane 3e was further established by a single crystal x-ray diffraction study. The correlation between δ ( 11 B) of compounds 3a , 3d - 3i and σ Hammett values shows that the strength of the N → B bond depends on the electronic factors of the substituent on the B-phenyl group.

0][11][12][13][14][15][16][17][18][19][20][21] Comparison of the δ ( 11 B) values for compounds 3a-3i with the unsubstituted (N→B) phenyl[Nbenzyliminodiacetate-O,O',N]borane [12.5 ppm], 11 shows that: a) electron donating substituent at para position (3a) decrease the N→B coordination bond, while electron-withdrawing groups (3b, 3d-3f) strengthens this bond; b) electron-withdrawing groups at the meta position (3g-3i) increase the N→B coordination bond.This shows that δ ( 11 B) is sensitive to inductive and resonance factors and there should exist a correlation with σ Hammett values. 22ble 1. 1     The 13 C NMR data for compounds 3a-3i are summarized in Table 2.For all compounds the assignment of C-2 and C-3 are based on HETCOR experiments. Tus C-2 correlates with the signals showing an AB coupling, which appear in the range between 3.86 and 4.49 ppm and C-3 correlates with the single signal between 3.75 and 3.87 ppm. Th C-8 signal is not observed in any of the compounds; C9 to C-13 in 3d exhibit a doublet, while C9, C10, C11 and C14 in 3g appear quartets due to coupling with F atoms.

Mass spectrometry
The 70 eV EI mass spectra of compounds 3b and 3d-3i do not exhibit the molecular ion, while the spectra of 3a and 3c show the molecular ion.The following important fragment ions are observed, in the spectra of 3a, 3b, as well as 3d-3i; the base peak is at m/z = 91 and corresponds to tropylium ion; 3c exhibits the corresponding base peak at m/z = 198 [C 6 H 5 CH 2 -C 6 H 4 OCH 3 ] + .All compounds exhibit the fragment ion [C 6 H 5 CH 2 -C 6 H 4 R] + and the fragment ion at m/z = 42 [CH 2 NCH 2 ] + .Compounds 3e, 3i and 3f exhibit fragment ions containing 35 Cl, 37 Cl and 79 Br, 81 Br, respectively.Scheme 2 shows some fragment ions and a possible fragmentation pattern.

Infrared spectroscopy
The IR spectra exhibit the ν (C=O) carbonyl oxygen band in the range between 1758 and 1772 cm -1 , and a band due to B-O between 1292 and 1304 cm -1 .Also the band due to N→B is in the range between 1026 and 1034 cm -1 .Table 2. 13

X-Ray diffracction
Suitable crystals of 3e for X-ray analysis were obtained from methylene chloride; the molecular structure and crystallographic numbering is showing in figure 2. In general the bond distances are within the values characteristic of analogous compounds. 13,18 H 19 , which are significantly shorter than the sum of the van der Walls radii for nitrogen and hydrogen atoms (2.75Å), as well as oxygen and hydrogen (2.70Å), 23 In addition, the following intermolecular contact is observed between O 1 ….H 11A 2.3919(0.0249)Å.

Experimental Section
General Procedures.N-benzyliminodiacetic acid 1 was prepared according to our methodology. 24Reagents 2a-2i were purchased from Aldrich Co. Atomic scattering factors were taken from the International Tables for X-ray Crystallography. 25ata reduction were performed by Denzo. 26All calculations were carried out using the SHELXL-97 (Sheldrick 1997) 27 and the molecular graphics by Diamond 2.1. 28 The procedure outline is general for the preparation of compounds 3a to 3i Scheme 1 H and 11 B NMR data of 3a-3i: δ H and δ B [ppm] and coupling constants J [Hz] = p-CH 3 3b: R = p-CHO 3c: R = p-OCH 3 3d: R = p-F 3e: R = p-Cl 3f: R = p-Br 3g: R = m-CF 3 3h: R = m-NO 2 3i: R = m-Cl

a 2 J
. b 3 J. c J H-F .Thus, a plot of δ ( 11 B) for 3a, 3d-3i compounds versus σ Hammett values (Fig.2) gives the equation σ Hammett = -0.666[δ( 11 B)] + 7.6224, with a correlation coefficient R 2 = 0.9036.These data confirm that the strength of the N→B coordination bond is governed by electronic factors.The 19 F NMR spectra of 3d and 3g compounds exhibit a triplet of triplets at -113.10 ppm ( J = 9.0, 6.4 Hz) and a single signal at -61.23 ppm, respectively.

3i. Issue in Honor of Prof. Eusebio Juaristi ARKIVOC 2005 (vi) 366-376 ISSN 1424-6376 Page 372
13, planar as indicate by the torsion angles (Table3).The aryl-B and N-Bn groups are bent away from N-B, as indicated by the angle values C 7 -N 1 -B 1 112.48 o (13) and C 14 -B 1 -N 1 114.78 o(14).The molecular structure establishes the bicyclic structure showing a N→B bond length of 1.683(2) Å, the value being comparable to the N→B bond length in analogous compounds.13,18Thismolecule shows a bicyclooctane structure with torsion angle of the junction, 17.25 o (20).