Nicholas C. Norman

Transition metal catalysed diboration

Homogeneous transition metal catalysed hydroboration reactions are discussed briefly, providing the background which led to the recent discovery of metal catalysed diborations [additions of the B–B bond in B2(OR)4 compounds] of alkynes, alkenes, 1,3-diynes, 1,3-dienes, and ά,βunsaturated ketones. Catalysed additions of B–Si and B–Sn bonds are also presented. Metal catalysed synthesis of organoboron compounds is a useful new synthetic method.

OXIDATIVE ADDITION OF BORON-BORON, BORON-CHLORINE AND BORON-BROMINE BONDS TO PLATINUM(0)

Abstract The synthesis and spectroscopic characterisation of the new diborane(4) compounds B2(1,2-O2C6Cl4)2 and B2(1,2-O2C6Br4)2 are reported together with the diborane(4) bis-amine adduct [B2(calix)(NHMe2)2] (calix=Butcalix[4]arene). B–B bond oxidative addition reactions between the platinum(0) compound [Pt(PPh3)2(η-C2H4)] and the diborane(4) compounds B2(1,2-S2C6H4)2, B2(1,2-O2C6Cl4)2 and B2(1,2-O2C6Br4)2 are also described which result in the platinum(II) bis-boryl complexes cis-[Pt(PPh3)2{B(1,2-S2C6H4)}2], cis-[Pt(PPh3)2{B(1,2-O2C6Cl4)}2] and cis-[Pt(PPh3)2{B(1,2-O2C6Br4)}2] respectively, the former two having been characterised by X-ray crystallography. In addition, the platinum complex [Pt(PPh3)2(η-C2H4)] reacts with XB(1,2-O2C6H4) (X=Cl, Br) affording the mono-boryl complexes trans-[PtX(PPh3)2{B(1,2-O2C6H4)}] as a result of oxidative addition of the B–X bonds to the Pt(0) centre; the chloro derivative has been characterised by X-ray crystallography.

Platinum Catalysed Diboration of Terminal Alkenes with Chiral Diborane(4) Compounds

Abstract Chiral diborane(4) compounds, synthesised from B 2 (NMe 2 ) 4 and the relevant chiral diols, react with terminal alkenes in the presence of Pt(dba) 2 (dba = dibenzylidene acetone) affording 1,2-diboryl compounds which have diastereoselectivities up to 60% d.e.

Synthesis and reactivity of dichloroboryl complexes of platinum(II)

The reaction between [Pt(nbe)3] (nbe=norbornene), two equivalents of the phosphines PPh3, PMePh2 or PMe2Ph and 1 equivalent of BCl3 affords the platinum dichloroboryl species [PtCl(BCl2)(PPh3)2], [PtCl(BCl2)(PMePh2)2] and [PtCl(BCl2)(PMe2Ph)2]. All three complexes were characterised by X-ray crystallography and reveal that the boryl group lies trans to the chloride. With PMe3 as the phosphine, the complex [PtCl(BCl2)(PMe3)2] is isolated in high yield as a white crystalline powder although crystals suitable for X-ray crystallography were not obtained. Crystals were obtained of a product shown by X-ray crystallography to be the unusual dinuclear species [Pt2(BCl2)2(PMe3)4(micro-Cl)][BCl4] which reveals an arrangement in which two square planar platinum(II) centres are linked by a single bridging chloride which is trans to a BCl2 group on each platinum centre. The reaction of [PtCl(BCl2)(PMe3)2] with NEt3 or pyridine (py) affords the adducts [PtCl{BCl2(NEt3)}(PMe3)2] and [PtCl{BCl2(py)}(PMe3)2], respectively, both characterised spectroscopically. The reaction between [PtCl(BCl2)(PMe3)2] and either 4 equivalents of NHEt2 or piperidine (pipH) results in the mono-substituted boryl species [PtCl{BCl(NEt2)}(PMe3)2] and [PtCl{BCl(pip)}(PMe3)2], respectively, the former characterised by X-ray crystallography. Treatment of either [PtCl(BCl2)(PMe3)2] (in the presence of excess NEt3) or [PtCl{BCl(NEt2)}(PMe3)2] with catechol affords the B(cat) (cat=catecholate) derivative [PtCl{B(cat)}(PMe3)2] which is also formed in the reaction between [Pt(PMe3)4] and ClB(cat) and also from the slow decomposition of [Pt{B(cat)}2(PMe3)2] in dichloromethane over a period of months. The compound [Pt{B(cat)}2(PMe3)2] was prepared from the reaction between [Pt(PMe3)4] and B2(cat)2.

Synthesis of a phospha-arsene and a phosphastibene; the first compounds with phosphorus–arsenic and phosphorus–antimony double bonds

The reaction of (Me3Si)2CHMCl2(M = As or Sb) with (2,4,6-But3C6H2)PH2 in the presence of 1,5-diazabicyclo[5.4.0]undec-5-ene affords the double-bonded compounds, (2,4,6-But3C6H2)PMCH(SiMe3)2(M = As or Sb)

An examination of the structures of iodosylbenzene (PhIO) and the related imido compound, PhINSO2-4-Me-C6H4, by X-ray powder diffraction and EXAFS (extended X-ray absorption fine structure) spectroscopy

Structural data derived from X-ray powder diffraction and EXAFS spectroscopy are presented for iodosylbenzene (PhIO) and the imido analogue, PhINSO2-4-Me-C6H4, which indicate that these compounds are polymeric in the solid state.

Reactions of co-ordinated ligands. Part 33. Mononuclear η2-vinyl complexes: synthesis, structure, and reactivity

Treatment of the four-electron alkyne cation [Mo(η2-PhC2Ph){P(OMe)3}2(η-C5H5)][BF4] with K[BHBus3] affords the η2-vinyl or metallacyclopropene complex [[graphic omitted]HPh}{P(OMe)3}2(η-C5H5)](2). The related complexes [[graphic omitted]HPh}{P(OMe)3}2(η-C5H5)](3),[[graphic omitted]H(C6H4Me-4)}{P(OMe)3}2(η-C5H5)](4), [[graphic omitted]H(C6H4Me-4)}{P(OMe)3}2(η-C5H5)](5), and [[graphic omitted]Ph2}{P(OMe)3}2(η-C5H5)](6) are obtained on reaction of the corresponding lithium diarylcuprate with the respective alkyne cation [Mo(η2-R1C2R2){P(OMe)3}2(η2-C5H5)][BF4](R1= But, R2= H; R1= Pri, R2= H; R1= Me, R2= Ph). The structures of (2), (3), and (6) have been determined by single-crystal X-ray diffraction studies. The molecules show close similarities; each has a molybdenum atom to which a vinyl moiety is co-ordinated via one short and one long Mo–C bond. These molecules may be described either as η2(3e)-vinyl or metallacyclopropene complexes. The orientation of the C2 vinyl group relative to the Mo{P(OMe)3}2(η-C5H5) fragment is discussed in terms of the torsion angles. In (2) and (3), Cα of the vinyl group lies closer to the plane of the η-C5H5, ligand than does Cβ, whereas in (6) the reverse orientation is observed. The solution n.m.r. spectra of (2) and (3) have distinct 31P environments, whereas in (6) the phosphorus environments are equivalent. This is discussed in terms of a rotational movement, an extended Huckel molecular orbital analysis suggesting that the orientation and fluxional behaviour of the η2-vinyl ligands parallel those of the related alkyne complexes. Reaction of [Mo(η2-PhC2CH2Ph){P(OMe)3}2(η-C5H5)][BF4] with K[BHBus3] affords a separable mixture of isomeric complexes [[graphic omitted]HPh}{P(OMe)3}2(η-C5H5)], which differ only in the orientation of the η2-vinyl moiety. The complex (6) slowly rearranges in solution to an η3-allylic complex this being explained in terms of an η2 to σ change in the bonding mode of the vinyl ligand. A similar transformation is suggested to explain the formation of η3- allylic complexes on reaction of [Mo(η2-RC2H){P(OMe)3};2(η-C5H5)][BF4](R = But or Pri) with LiCuMe2. Extension of the dimethyl- or diphenyl-cuprate reactions to the cations [Mo(η2- R1C2R2){P(OMe)3}2(η-5H5)][BF4] provides evidence for competing reaction pathways involving either direct attack on the metal centre or on a co-ordinated alkyne carbon. The regioselectivity of the latter reaction is discussed in terms of steric and electronic effects.

Diborane(4) compounds incorporating thio- and seleno-carboranyl groups

The reaction between B2(NMe2)4 and the carborane dithiol C2B10H10(SH)2, followed by addition of HCl affords the [NH2Me2]+ salt of the dianion [B2Cl2(S2C2B10H10)2]2−, which has been characterised by X-ray crystallography. A similar reaction utilising the carborane diselenol C2B10H10(SeH)2 (generated in situ) afforded a compound containing the dianion [B2(Se2C2B10H10)3]2−.

ISOLATION AND STRUCTURAL CHARACTERIZATION OF NOVEL COMPOUNDS CONTAINING B4O2 RINGS

Two compounds containing planar six-membered B4O2 rings have been synthesized and characterized by X-ray crystallography, namely [B4O2(OH)(4)].[NH2Me2][Cl] and [B4O2(dab)(2)] (dab = 1,4-Bu-t-1,4-diazabutadiene), the former compound providing a model for one possible structure of boron monoxide. {BO}(x)

Notes. Oxidation and reduction of phosphorus–phosphorus and arsenic–arsenic double bonds. An electrochemical study of two diphosphenes and a diarsene

The diphosphenes [(Me3Si)3C]2P2(1) and (2,4,6-But3C6H2)2P2(2) and the diarsene [(Me3Si)3C]2As2(3) undergo electrochemical reduction in tetrahydrofuran (thf) solution to the corresponding anion radicals [{(Me3Si)2C}2P2]˙–(4), [(2,4,6-But3C6H2)2P2]˙–(5), and [{(Me3Si)3C}2As2]˙–(6), respectively. Anion radicals (4) and (5) were sufficiently stable to permit the acquisition of e.s.r. data. The products of oxidation of (1), (2), and (3) were much more difficult to characterise. The oxidation of (1) in CH2Cl2, is irreversible at 25 °C; however, at –75 °C a one-electron oxidation occurs to the unstable cation radical [{(Me3Si)3C}2P2]˙+. The oxidation of both (2) and (3) is irreversible even at –75 °C.