Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Borane protonation

B—N bond is very covalent, owing to substantial donation of the nitrogen lone-pair in classical dative-bond fashion, and also owing to endocyclic charge transfer from the ewdo-methylene proton to the wdb-borane proton. [Pg.143]

Proton Abstraction. Although the exopolyhedral hydrogens of nido and arachno boranes are generally considered hydridic, the bridge hydrogens are acidic as first demonstrated by titration of and deuterium exchange (71). Some typical reactions are... [Pg.236]

Electrophilic Attack. A variety of boranes, heteroboranes, and metaHaboranes undergo electrophilic substitution. SusceptibiUty of boranes to electrophilic attack is often detected by deuteron—proton exchange experiments. Eor example, electrophilic hydrogen—deuterium exchange of occurs at the l-,2-,3-, and 4-positions when exposed to DCl in the presence of AlCl (81). The trend to increasing positive sites in is... [Pg.236]

The influence of boron-bonded ligands on the kinetics and mechanistic pathways of hydrolysis of amine boranes has been examined (37,38). The stoichiometry of trimetbyl amine azidoborane [61652-29-7] hydrolysis in acidic solution is given in equation 10. It is suggested that protonation occurs at the azide ligand enabling its departure as the relatively labile HN species. [Pg.262]

It follows from the preceding discussion that the unbranched H bond can be regarded as a 3-centre 4-electron bond A-H B in which the 2 pairs of electrons involved are the bond pair in A-H and the lone pair on B. The degree of charge separation on bond formation will depend on the nature of the proton-donor group AH and the Lewis base B. The relation between this 3-centre bond formalism and the 3-centre bond descriptions frequently used for boranes, polyhalides and compounds of xenon is particularly instructive and is elaborated in... [Pg.63]

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... [Pg.158]

B H +3 formed by removal of 1 bridge proton from B H +4, e.g. BsHg", BioHi3 other anions in this series such as B4H7" and BgHi2 are known though the parent boranes have proved too... [Pg.169]

The diagrams also indicate why neutral c/oio-boranes BnHn4.2 are unknown since the 2 anionic charges are effectively located in the low-lying inwardly directed orbital which has no overlap with protons outside the cluster (e.g. above the edges or faces of the Bg oct edron). Replacement of the 6 Ht by 6 further builds up the basic three-dimensional network of hexaborides MB6 (p. 150) just as replacement of the 4 H in CH4 begins to build up the diamond lattice. [Pg.177]

The so-prepared compounds are very sensitive toward air and moisture, in particular in solution. H- and C-NMR spectra prove the formation of 1 1 adducts of the type dmap—M(R2)E(Tms)2- The proton resonances of the dmap molecule are shifted to higher field, as was observed for similar borane adducts... [Pg.152]

Tetrasubstituted phosphinous amides of the type R2NPPh2 have been successfully arylated at phosphorus by the action of bromobenzene, in a process catalyzed by NiBr2, to give the aminophosphonium bromides [R2NPPh3] Br [109]. Other representative members of this class form phosphane-borane complexes [62], are aminated at phosphorus by chloramine to yield bis(amino)phos-phonium salts [110] and have been claimed to be protonated at phosphorus by ethereal tetrafluoroboric acid, as determined by NMR analysis [111]. [Pg.89]

The carbanions take up 02 and these take up protons to give hydroperoxides in good yields. But because they are explosive in nature, they are not usually isolated and on reduction with sodium sulphite on trialkyl phosphite give alcohols. Alcohols can also be prepared via hydroperoxy molybdenum complexes and alkyl boranes. These reactions are summarized as follows ... [Pg.17]

In general the deprotonation of a polyborane B H +m (to = 4,6) leads to the anions [BnHn+m-i] or [B H +m 2]2 by removal of one or two protons from a BHB 3c2e bridge with formation of a B-B single bond. Cluster expansion with a BH3 unit, usually offered as diborane in diethyl ether or tetrahydrofuran, produces borane anions [B +1H +m+2] or [B +1H +m+1]2 and these in turn on protonation give the polyboranes Bn+iHn+m+-. These may be stable species. However, in most cases they loose H2 which results in a cluster expansion by one BH unit. Several examples of this method are described in the following sections. [Pg.52]

The basicity of halogenophosphines has attracted some attention this year. A careful study of the protonation of several halogenophosphines (6) has led to the characterization by 31P n.m.r. of phosphonium species, such as (7).12 These salts show 31P shifts to relatively high fields, and large Vph values e.g., (7) has 1190 Hz.12 Attempts have been made to correlate phosphine basicity with Vpb values in borane complexes,13 and, in turn, to correlate Vpb with electrostatic interactions in the phosphorus-boron bond.13 From such correlations, the authors deduce that this bond is dominated by electrostatic contributions, and not by other factors, such as -bonding. [Pg.51]

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. [Pg.487]


See other pages where Borane protonation is mentioned: [Pg.42]    [Pg.132]    [Pg.133]    [Pg.42]    [Pg.42]    [Pg.132]    [Pg.133]    [Pg.42]    [Pg.320]    [Pg.237]    [Pg.237]    [Pg.179]    [Pg.122]    [Pg.18]    [Pg.7]    [Pg.58]    [Pg.560]    [Pg.100]    [Pg.214]    [Pg.57]    [Pg.160]    [Pg.392]    [Pg.1025]    [Pg.104]    [Pg.73]    [Pg.16]    [Pg.22]    [Pg.322]    [Pg.113]    [Pg.144]    [Pg.43]    [Pg.100]    [Pg.290]    [Pg.263]    [Pg.46]    [Pg.412]   
See also in sourсe #XX -- [ Pg.203 , Pg.206 ]




SEARCH



© 2024 chempedia.info