K. Sukumar Varma

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.

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.

Synthesis and characterisation of cyclo-boratetrasiloxane, (RBO)(Me2SiO)3 (R = nBu, Ar), derivatives

Abstract Low temperature reactions of dilute solutions of 1,5-dichloro-1,1,3,3,5,5-hexamethyltrisiloxane with boronic acids {RB(OH) 2 ; R= n Bu, C 6 H 4 Me-2, C 6 H 4 Me-3, C 6 H 4 Me-4, C 6 H 4 OMe-3, C 6 H 4 OMe-4, C 6 H 4 Br-2, C 6 H 4 Br-3, C 6 H 4 Br-4} in the presence of a twofold excess of Et 3 N afforded the 8-membered ring products, cyclo -boratetrasiloxanes, (RBO)(Me 2 SiO) 3 , in moderate yields. New compounds were colourless oils and were characterised by elemental analysis, NMR ( 1 H, 11 B, 13 C, 29 Si), IR and MS. The cyclo -boratetrasiloxanes are weakly Lewis acidic, with acceptor number (AN) values of ∼30, but do not form adducts with amines.

Synthetic studies on extended S-donor ligands as potential molecular conductors

The syntheses of vinylenedithiolethione (VDT-DTT) and its dimerised derivative bis(vinylenedithio)tetrathiafulvalene (BVDT-TTF) is reported as part of our studies on new synthetic pathways to planar extended S-donor ligands. These and other derived ligands have been discussed as materials for the synthesis of new organic conductors and one-dimensional metals.

Trimethoxyboroxine as an ‘oxygen-transfer’ reagent: a non-aqueous ‘sol–gel’ route to alkali-free borosilicate glass

A synthesis of alkali-free borosilicate gel from Si(OEt)4 and (MeO)3B3O3 in non-aqueous solvents is reported; the formation of the gel proceeds via rapid transesterification/oxygen transfer with elimination of B(OR)3; removal of volatiles from the gel, followed by drying (60 oC, 12 h) and furnacing in air (600 oC, 20 min) resulted in alkali-free borosilicate glass.

Lewis Acidity in Haloalkyl Orthoborate and Metaborate Esters

The Lewis acidities of a series of haloalkyl orthoborate and metaborate esters have been determined by Gutmanns 31P NMR method. The introduction of halogens into the alkyl group of the borate generally increases the Lewis acidity at boron. Detailed analysis of data indicate that systematic variations correlate with Tafts electronic σ* substituent parameters.

THE INTERACTION OF BENZYL ALCOHOL WITH TRIMETHOXYBOROXINE : AMINE ADDUCTS OF TRIARYLBOROXINES AS MODEL COMPOUNDS

The interaction of benzyl alcohol with trimethoxyboroxine has been investigated by hydrogen-1 and carbon-13 NMR and IR spectroscopy. These techniques indicate formation of a weak Lewis acid/base adduct, additionally stabilized by a hydrogen-bonding interaction, which undergoes rapid ligand exchange at room temperature. The system has been modelled by the synthesis and characterization of a series of amine-triarylboroxine adducts. These adducts rapidly undergo ligand dissociation-recombination in solution with ΔG+ of ca. 40-50 kJ/mol.

Sulphur—Containing Donors and Ligands for Molecular Conductors

Abstract The structures and properties of molecular conductors based on metal complexes of the sulphur-containing donor ligands mnt and dmit are described.

Improved synthesis and crystal structure of tetrakis(benzoylthio)tetrathiafulvalene. A useful precursor to functionalised bis(ethylenedithio)tetrathiafulvalene derivatives

A simple one-step procedure is reported for the preparation of tetrakis(benzoylthio)tetrathiafulvalene (2) using octacarbonyldicobalt. The crystal structure of 2 is discussed and the preparation of some charge-transfer salts of 2 and their electrical conductivity has been determined. The use of 2 as a synthon for functionalised bis(ethylenedithio)tetrathiafulvalene derivatives is described.