Boronium salts

Yamamoto Y, Koizumi T, Katagiri K, Furuya Y, Danjo H, Imamoto T, Yamaguchi K (2006) Facile synthesis of highly congested 1,2-diphosphinobenzenes from bis(phosphine)boronium salts. OrgLett 8(26) 6103-6106. doi 10.1021/ol062717i... 

Although boronium salts are most usually formed as iodides, other anions have been incorporated through metathesis reactions and ion exchange procedures in appropriate solvents. Examples of anions used are listed in Table 4. 

Pyrolysis of certain boronium salts leads to the anion serving as a ligand to boron 97>. 

Direct reaction of BX3 (X = halogen) with secondary amines produces amino-dihalogenoboranes, ammonium salts, and bis(amine) boronium salts as well as the simple adducts. This complication may be avoided by the halogenation of amino-boranes, and the syntheses of R2NH,BX3 were described for X = Cl or Br, R = Me or Et. ... 

Dibromomethyl [10B]borane is a colorless liquid, which fumes strongly in contact with moisture. Larger quantities when exposed to air may ignite bp 59°/736 torr, 5 11B(10B) 65.2 ppm, 5 H 1.42 ppm (5% in CC14). The compound dissolves readily in various hydrocarbons. It forms stable addition products with tertiary nitrogen bases and is readily solvolyzed by alcohols as well as primary and secondary amines. Boronium salts may form with the amines. Reaction with ethers ruptures the C-0 bond. 

A closely related series of adducts, viz. L,BH2l, where L = NMe3, py 3-Mepy, 4-Mepy, or PPh3, has also been reported. The addition of suitable amines or phosphines to solutions of these adducts in benzene leads to the formation of boronium salts containing two different donors co-ordinated to B [LL BHa] I, where L = PMe3, PPhg, NMe3, etc. H and "B n.m.r. data were listed for these wherever possible. ... 

Dihydroboronium derivatives of t-Bu-BisP with different cotmter anions were prepared, as shown in Scheme 56. The reaction of BisP with BH2Br afforded the boronium salt 175 which possessed a bromide ion. The dihydroboronium derivative of (S,S)-l,2-bis(tert-butylmethylphosphino)ethane 175 (t-Bu-BisP ) was prepared by the reaction of t-Bu-BisP 174 with catecholborane and used as chiral diphosphine ligand precursor in Rh-catalyzed asymmetric hydrogenated of methyl (Z)-acetamidocinnamate to afford the hydrogenation product in up to 94% ee. Complexes of iron(III) and P-chiral phosphine oxides 176 are catalysts for the asymmetric Diels-Alder reaction of iV-acrylamide dienophiles [106]. 

The same group persisted in the preparation of 1,2-diphosphinobenzenes and finally met success with a clever use of hzj(phosphine)boronium salts. In these compounds, two secondary phosphines are connected through a BH2 bridge. They are prepared by the reactions depicted in Scheme 5.26. 

Scheme 1 S.24 Formation of a stable borylene, boronium salt and radical cation [295].

Borane adducts R3N,BR3 and boronium salts [(R3N)2BR2] X , formal analogues of alkanes, are described together with other co-ordination compounds on pp 99-104. 

Borane (BH3) adducts of some nitrogen heterocyclic bases e.g. quinoline, aminopyridines) have been prepared. Halogenoboranes form boronium salts with such heterocycles, and some containing two different bases attached to a single boron atom, of the type (R -pyridine)(R -pyridine)aBH + 2I, have been identified. The syntheses and molecular structure determinations of complexes of phosphoranes with borane have been reported. The molecular structure of MejP.BCls has been determined by gas electron diffraction. ... 

The bidentate ligands (RR C(pz)2) react with R"2BX compounds, where X is a leaving group, to form boronium cations [(RR C(pz)2)BR"2]+ isosteric with pyrazaboles and isolable as PF6 salts.202... 

Spectroscopic (i.r., u.v.—visible, and n.m.r.) data were given for a number of boronium chelate salts of the type [BLgl+X", where Lg is ethyl acetoacetate, 2-acetylphenol, etc., and X = SbCl, CIO, or FeCl. Factors influencing electron delocalization in the chelate ring were considered in detail. 

An amino-pendant NHC-stabilized boronium cation was prepared from the reaction of a LiBr adduct of an NHC with borane in THF, giving a bromide salt. Subsequent reaction with silver triflate and lithium borohydride gave the amino-pendant NHC adduct with two equivalents of borane, one at the carbeneic carbon and one at the pendant nitrogen (Scheme 15.9). A four-membered metallacycle intermediate with a B-H-B motif has been postulated [108]. 

The boronium ionic liquid, [BH2(L )(L")][NTf2] (L = 3-methylimidazole, L" = trimethylamine), melting at 60°C, has been structurally characterised [193], revealing only weak cation-anion interactions and the anion in the low-energy trans conformation. The structure of the [BH2(CN)2] salt of [bis(l-methyl-lJT-imidazole-3-yl)dihydroboronium] ... 

Boron anions in the form of tetravalent boron compounds can serve as anions in ILs, combined with the conventionally used cations such as iV, Af -dialkylimidazolium, A(-alkylpyridinium, tetraalkylam-monium, and tetraalkylphosphonium. The properties of the salts are very similar to those of salts with other anions. Boronium-cation-based RTILs are also promising candidates as electrolytes in... 

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