Michael A. Beckett

A convenient n.m.r. method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity

Abstract The reaction rates from viscosity measurements of Lewis acid catalysed epoxide (oxirane) polymerizations have been correlated with Gutmanns Acceptor Number (AN) scale derived from measurement of 31 P n.m.r. chemical shifts of Et 3 PO (TPO) dissolved in the boron-containing Lewis acid.

Amine adducts of triarylboroxines: Synthesis and characterization of adducts of tri(2-tolyl) boroxine and crystal structures of (4-MeC6H4)3B3O3 and (4-MeC6H4) 3B3O3 · 4-picoline

Abstract Seven 1:1 adducts of the triarylboroxine (2-MeC6H4)3B3O3 with N-donor ligands (4-picoline, cyclohexylamine, pyridine, 3-picoline, piperidine, isoquinoline, benzylamine) have been prepared by reaction of equimolar quantities of amine and (2-MeC6H4)3B3O3 in Et2O at room temperature. These adducts have been characterized by elemental analysis, melting point, IR spectroscopy and by 1H and 13C NMR spectroscopy. Room temperature solution 1H NMR spectra of these complexes suggest that rapid ligand dissociation—recombination exchange processes are occurring. The related compounds (4-MeC6H4)3B3O3 and (4-MeC6H4)3B3O3 · 4-picoline have been characterized in the solid state by single crystal X-ray diffraction studies. Solid state 11B MAS NMR results are reported for triarylboroxines (4-MeC6H4)3B3O3 and (3,5-Me2C6H3)3B3O3 and for the amine adducts (4-MeC6H4)3B3O3 · cyclohexylamine, (4-MeC6H4)3B3O3· isoquinoline, (4-MeC6H4)3B3O3 · benzylamine, (3,5-Me2C6H3)3B3O3 · 4-picoline and (3,5-Me2C6H3)3B3O3 · morpholine.

Synthesis and Structural Characterization of an Unprecedented Nonmetal Cation Polyborate Salt Containing Two Different “Isolated” Polyborate Anions: [H2en]2[B4O5(OH)4][B7O9(OH)5]·3H2O (en = H2NCH2CH2NH2)

The nonmetal cation polyborate salt of stoichiometry [H(2)en](2)[B(11)O(18)(OH)]·7H(2)O is obtained from the reaction of 1,2-diaminoethane and boric acid (1:5 ratio) in H(2)O/MeOH. An X-ray crystallographic study of the product reveals that the polyborate moiety is composed of two isolated hydrated polyborate anions: [B(4)O(5)(OH)(4)](2-) and [B(7)O(9)(OH)(5)](2-). The structure is templated by the cations with the anions forming a supramolecular H-bonded network, augmented by additional H-bonds involving the waters of crystallization and the cations.

Synthesis and Characterisation of Amine Adducts of Tri(4-bromophenyl) boroxine, Tri(3-nitrophenyl)boroxine, and Tri(3-aminophenyl)boroxine; Molecular Structure of 3-Picoline·tri(4-bromophenyl)boroxine

Twenty five 1:1 adducts of the triarylboroxines (4-BrC6H4)3B3O3 (PBB) (3-NO2C6H4)3 B3O3 (MNB), and (3-NH2C6H4)3B3O3 (MAB) with N donor ligands (cyclohexylamine, 4-picoline, 3-picoline, piperidine, morpholine, isobutylamine, triethylamine, pyridine, of ligand and triarylboroxine in Et2O at room temperature. All 25 adducts have been characterised by elemental analysis, Mpt, IR and by 1H and 13C NMR. MNB and 4 of its adducts have been characterised by 11B MAS NMR. In solution variable temperature 1H NMR of selected PBB derivatives and of MNB-cyclohexylamine indicate that a ligand dissociation-recombination process is occuring with ΔG+ of ca. 39–54 kj·mol 1. The crystal and molecular structure of 3-picoline PBB has been determined by a single-crystal X-ray diffraction study. The colourless compound crystallises in the orthorhombic space group (Pbca) with a = 14.1710(10), b = 15.446(5), and c = 22.418(7) A, with Z = 8, calculated density = 1.737 Mg/M3, and final R values [I > 2 σ(I)] of R1 = 0.0376, and wR 2 =...

Synthesis and characterization of amine adducts of tri(4-tolyl)boroxine and tri(3,5-xylyl)boroxine: molecular structure of (4-MeC6H4)3B3O3·cyclohexylamine

Abstract 19 1 : 1 adducts of the triarylboroxines (4-MeC 6 H 4 ) 3 B 3 O 3 (PTB) and (3,5-Me 2 C 6 H 3 ) 3 B 3 O 3 (MXB) with N -donor ligands (cyclohexylamine, 4-picoline, 3-picoline, piperidine, morpholine, isobutylamine, methylamine, dimethylamine, isoquinoline and benzylamine) have been prepared by reaction of stoichiometric quantities of ligand and tri-arylboroxine in Et 2 O at room temperature. All 19 adducts have been characterized by elemental analysis, M pt, IR and 1 H and 13 C NMR. C 6 H 11 NH 2 · PTB has been characterized in the solid state by a single-crystal X-ray diffraction study. Solid-state 11 B MAS NMR results for PTB and its adducts with cyclohexylamine and isoquinoline are reported. In solution-variable temperature 1 H NMR of the morpholine, cyclohexylamine, isoquinoline and benzylamine adducts of PTB and MXB indicate that a ligand dissociation-recombination process is occurring with ΔG ‡ of ca 43–49 kJ mol −1 .

cyclo-Boratrisiloxane and cyclo-diboratetrasiloxane derivatives and their reactions with amines: crystal and molecular structure of (p-BrC6H4BO)2(Ph2SiO)2

The new cyclo -diboratetrasiloxanes (RBO) 2 (R′ 2 SiO) 2 (R= p -BrC 6 H 4 , o -MeC 6 H 4 , m -NH 2 C 6 H 4 , m -NO 2 C 6 H 4 , Bu n ; R′ 2 =Ph 2 , R=Ph, p -BrC 6 H 4 , m -NO 2 C 6 H 4 ; R′ 2 =oct n /Me) and cyclo -boratrisiloxanes (RBO)(R′ 2 SiO) 2 (R= p -BrC 6 H 4 , m -NH 2 C 6 H 4 ; R′ 2 =Me 2 , Ph/Me) were prepared in high yield by condensation reactions. cyclo -Diboratetrasiloxane·amine adducts (1:1) were obtained for (PhBO) 2 (Ph 2 SiO) 2 with cyclohexylamine, triethylamine, piperidine, and isobutylamine and for ( o -MeC 6 H 4 BO) 2 (Ph 2 SiO) 2 with morpholine; in contrast the cyclo -boratrisiloxanes were unreactive towards amines. The Lewis acidity of these borasiloxane rings was determined by the Gutmann’s acceptor number (AN) method with well-defined ranges observed for the cyclo -diboratetrasiloxanes (46–62) and the cyclo -boratrisiloxanes (22–28). The crystal and molecular structure of ( p -BrC 6 H 4 BO) 2 (Ph 2 SiO) 2 is reported. New compounds were characterised by elemental analysis, m.p., 1 H- and 11 B-NMR, and IR spectroscopy; solids were characterised by solid-state 11 B MAS-NMR spectroscopy.

π-Bonding in B–O ring species: Lewis acidity of Me3B3O3, synthesis of amine Me3B3O3 adducts, and the crystal and molecular structure of Me3B3O3.NH2iBu·MeB(OH)2

The synthesis and characterisation (m.p., elemental analysis, IR, NMR) of some previously unreported Me3B3O3. L adducts (L=piperidine, isobutylamine, morpholine, 3-picoline, benzylamine) are described. The crystal and molecular structure of (Me3B3O3NH2Bu)-Bu-i . MeB(OH)(2) has been determined by a single-crystal X-ray diffraction study. The structure reveals an unusual H-bond interaction between MeB(OH)(2) and a ring O atom of the six-membered B3O3 ring of the trimethylboroxine adduct, Me3B3O3. (NH2Bu)-Bu-i. The direction of the H-bond strongly suggests sp(2) hybridisation for this O atom. The Lewis acidity of Me3B3O3, Ph3B3O3, 1,3,5-Ph3B3N3Me3 and 1,3,5-Ph3C6H3 have been estimated by Gutmanns method and data interpreted in terms of relative K-bonding strength within these ring systems.

A New Decaoxidooctaborate(2−) Anion, [B8O10(OH)6]2–: Synthesis and Characterization of [Co(en)3][B5O6(OH)4][B8O10(OH)6)]·5H2O (en = 1,2-Diaminoethane)

The synthesis and X-ray diffraction structure of [Co(en)3][B5O6(OH)4][B8O10(OH)6]·5H2O (1) are reported. Compound 1 arises through a selective-templating process from a Dynamic Combinatorial Library of polyborate anions. Compound 1 contains two different polyborate species, with [B8O10(OH)6](2-) being particularly novel. It is comprised of fused tetraborate and pentaborate anions with a 4-coordinate B atom and a 3-coordinate O atom in common.

Synthesis and Lewis acidity of triorganosilyl and triorganostannyl esters of orthoboric, metaboric, and arylboronic acids

The synthesis of 30 new triorganosilyl and triorganostannyl esters of orthoboric, metaboric, and arylboronic acids are described: (R3SiO)3B {R3=nBuMe2, PhMe2, [2-(3-cyclohexenyl)ethyl]Me2, (nOctadecyl)Me2, nHex3}; (R3MO)3B3O3 {M=Si: R3=nPr3, nBu3, nHex3, EtMe2, nBuMe2, [2-(3-cyclohexenyl)ethyl]Me2, [(Me2HC)(Me)2C]Me2, (nOctadecyl)Me2, PhMe2, Ph2Me, Ph3. M=Sn, R=Ph, Me}; (R3MO)2BAr {M=Si: Ar=Ph, R3=nPr3, nBu3, Ph2Me; Ar=C6H4Br-4, R=Ph, nPr; Ar=C6H4Me-2, R=Ph, nPr. M=Sn: R=Ph, Ar=Ph, C6H4Br-4, C6H4Me-2; R=nBu, Ar=Ph, C6H4Br-4}. The esters were characterised by elemental analysis, m.p./b.p., 1H-, 13C-, and 11B-NMR and IR spectroscopy; solids were characterised by solid-state 11B MAS NMR spectroscopy. The Lewis acidity of liquid compounds was determined by Gutmann’s acceptor number (AN) method and well-defined ranges were observed for the compound classes {(R3MO)3B, 23–36; (R3MO)2BAr, 23–31; (R3MO)3B3O3, 44–81}. The effect of the R3Si or R3Sn group was to enhance the Lewis acidity at boron relative to organic esters. Increasing the steric bulk of the R3M moiety significantly reduced AN values and there was little differentiation in AN values between R3Si and R3Sn derivatives.

Protodeboronation of arylboronic acids and triarylboroxines in Bu20/THF

Abstract The arylboron compounds Ph 3 B 3 N 3 Me 3 , Ar 3 B 3 O 3 , or ArB(OH) 2 (Ar  Ph, p -BrC 6 H 4 , p -MeC 6 H 4 , m -NH 2 C 6 H 4 react with [Cr(CO) 6 ] in refluxing Bu 2 O/THF (9:1) to afford the (η 6 -arene) tricarbonyl-chromium(O) complexes of the protodeboronated substrates in yields of up to 55%. The proposed mechanism involves protodeboronation of the arylboron compound followed by complexation of the free arene by the chromium carbonyl moiety.