József Emri

Preparation and properties of novel cyano and isocyano derivatives of borane and the tetrahydroborate anion

Abstract Reaction of NaBH 3 CN with HCl in THF gave mainly NaBH 3 CNBH 2 CN, whereas in Me 2 S quantitative formation of BH 2 CN was observed. In dimethyl sulfide BH 2 CN exists in monomeric or dimeric form as Me 2 S complexes in an equilibrium with oligomers. These compounds are converted by amines into BH 2 CN complexes in nearly quantitative yield, and treatment with lithium cyanide gives LiBH 2 (CN) 2 . The reaction of the Me 2 S complexes of the corresponding bromoboranes with AgCN gives isocyanoborates AgBH n (NC) 4- n ( n = 1,2), which can be easily converted into sodium salts, which are transformed into cyanoborates by thermal isomerization. The di- and tricyanohydroborates are extremely stable towards acids, and the BH 3 CNBH 2 CN − ion is significantly more stable than the BH 3 CN − ion. On bromine oxidation in the presence of N-bases LiBH 2 (CN) 2 gives N-base · BH(CN) 2 complexes and LiBH 2 (CN)CNBH(CN) 2 . NaBH(NC) 3 is transformed into C 5 N 5 N · BH(NC) 2 upon treatment with pyridin hydrochloride.

Preparation and study of chiral boron containing amine-cyano(pyrrolyl-1)borane complexes. Preparation of (−)-(S)-α-phenylethylamine-(−)-cyano(pyrrolyl-1)borane and (+)-(R)-α-phenylethylamine-(+)-cyano(pyrrolyl-1)borane

Abstract Several amine-cyano(pyrrolyl)borane complexes [A · BH(NC4H4)CN] containing a chiral boron atom were prepared from the reactions of sodium cyanohydrodipyrrolylborate-tridioxane 1 [NaBH(NC4H4)2CN · 3C4H8O2] with amine hydrochlorides as well as by base exchange from the appropriate 4-cyanopyridine complex [4-CNC5H4N · BH(NC4H4)CN]. Amines with an sp2 N atom give stable complexes of a wide range of basicity (pKa = 0.8−9.7), in contrast to the anines with sp3 hybridized N where only the stronger bases (pKa ⪢ 7.3) are capable of giving stable complexes (mainly secondary and tertiary amines). These compounds undergo hydrolysis in neutral and alkaline media, while in strong acids they give stable boronium ions by protonation at the α-C atom of the pyrrolyl group. The compounds (−)-(S)-α-phenylethylamine-(−)-cyanopyrrolylborane and (+)-(R)-α-phenylethylamine-(+)-cyanopyrrolylborane have been prepared in pure form; in solution they undergo slow epimerization. Treatment of the appropriate complexes with KH has given the chiral boron-containing borates KBH(NC4H4)(CN)X (X = imidazolyl-, pyrazolyl-). Dicyanohydropyrrolylborane was obtained from 4-cyanopyridine-cyanopyrrolylborane and NaCN.

Preparation of N,N,N′,N′-tetramethylethylenediamine adducts of new monosubstituted boranes

Abstract TMED·2BH 2 CN (where TMED = N,N,N′,N′ -tetramethylethylenediamine) was prepared more effectively by the use of methylsulphide solution of the oligomer (BH 2 CN) n compared to previous procedures. When ethylated with Et 3 OBF 4 , TMED·2BH 2 CN was transformed into [TMED·2BH 2 CNEt][BF 4 ] 2 , which can be regarded as a cationic boron compound containing an ethylisonitrile ligand. In aqueous medium the [TMED·2BH 2 CNEt] 2+ ion (nitrilium ion) slowly hydrolyses, but it forms precipitates with large nonreactive anions (such as PF 6 − and BPh 4 − ). At elevated temperatures, the nitrilium ion was converted more rapidly and in higher yields than in the previous procedures, to TMED·2BH 2 COOH, from which TMED·BH 2 COOH can then be prepared. Anionic reactants undergo nucleophilic addition of cycloaddition reactions with the nitrilium ion, giving good yields of the new complexes TMED·2BH 2 X (where X = N -ethylthiocarbamoyl, N -ethyliminocyanomethyl and 5-(1-ethyl)tetrazolyl group).

Preparation and study of hydrolytically stable cyanohydro(pyrrolyl-1)borates and chiral boron containing amine-cyano(pyrrolyl-1)borane complexes

Abstract Lithium- and potassium cyanodihydropyrrolylborates 1 (made from monopyrrolylborane by reaction with the appropriate cyanide) as well as sodium- and potassium cyanohydrodipyrrolylborates (made from sodium hydrotripyrrolylborate with HCN and from pyridine-dipyrrolylborane with KCN, respectively) were prepared. The hydrolysis of the BH(NC4H4)2CN− ion in acidic media is approximately 250 times faster than that of the BH3CN− ion. On the other hand the BH2(NC4H4)CN− ion is easily protonated (pKa = 1.84) at the α-position of the pyrrolyl ring, and is hydrolyzed very slowly even in strongly acidic media. Reaction of the NaBH(NC4H4)2CN with pyridine- and with 4-picoline hydrochlorides (AHCl) results in the formation of compounds A·BH(NC4H4)CN containing tetracoordinated chiral boron. Considerable amounts of C5H5N·BH(NC4H4)CN are also formed in a complex reaction which took place between KBH2(NC4H4)CN and pyridine hydrochloride. These base—borane complexes also undergo α-protonation on the pyrrolyl ring in strongly acidic medium. The boronium ions ABH(NC4H5)CN+ formed in this way are stable even in concentrated mineral acids.

Bromo derivatives of amine and phosphine complexes of cyanodihydroborane. Synthesis and reactivity

Bromocyanohydroborane complexes [L·BH(Br)CN], containing a chiral boron atom and dibromocyanoborane complexes (L·BBr2CN) have been synthesized from amine- and phosphine-dihydrocyanoboranes [L·BH2CN, where L=trimethylamine (Me3N), tetramethylethylenediamine (TMEDA)/2, quinuclidine (Q), picoline (Pic), tributylphosphine (Bu3P) and triphenylphosphine (Ph3P)] and bromine. Trialkylamine-bromocyanohydroboranes show fairly high hydrolytic stability, while the analogous picoline complexes hydrolyze readily. The leaving group character of bromine in amine-bromocyanohydroboranes was tested in reactions with amines. TMEDA·2BH2CN, on reaction with tetramethylethylenediamine, picoline or on heating, is transformed into a cationic complex [(TMEDA)B(H)CN+]Br− which contains a five-membered ring. This compound is also formed under mild conditions in the reaction of Pic·BH(Br)CN and TMEDA. A chiral cationic complex [Pic(Q)B(H)CN+]Br− was formed from Q·BH(Br)CN and picoline whereas reaction of Me3N·BH(Br)CN with picoline resulted in the formation of pure [Pic2B(H)CN+]Br− through substitution and transamination. Amine-dibromocyanoboranes, in contrast to amine-bromocyanohydroboranes, did not react with amines even under harsher reaction conditions. These experiments indicate that bromine can be considered as a good leaving atom in amine-bromocyanohydroboranes and these new molecules might be useful as starting materials for the synthesis of novel boron analogs of α-amino acids.

Synthesis and characterization of some amine complexes of bromocarboxyboranes and bromo(methoxycarbonyl)boranes

Abstract Amine-carboxyboranes [A.BH2COOH; A = Me3N, Et3N, quinuclidine (Q)] are readily decarbonylated with bromine in dichloromethane to produce amine-dibromoboranes (A.BHBr2). The formation of A.BHBr2 is explained by fast loss of CO from the acid bromides A.BH(Br)COBr generated because of the action of HBr. In proton-acceptor solvents (such as H2O), however, only substitution takes place giving a chiral boron atom containing A.BH(Br)COOH and/or A.BBr2COOH. Also, bromocarboxyborane complexes can be prepared easily using bromination with N-bromosuccinimide (NBS). The products are rather stable in water under acidic conditions but bases induce fast decarbonylation followed by complete decomposition. Amine(methoxycarbonyl)boranes A.BH2COOMe (A = Me3N, Et3N and Q) are conveniently synthesized, with good yields, in methanol by treatment with a cation exchange resin (H+) as a catalyst. The bromo derivatives A.BH2-nBrn COOMe (n = 1, 2) have been prepared by treatment of amine complexes of BH2COOMe with bromine or NBS, or by esterification of the bromocarboxylic derivatives. In addition, these bromo compounds can be readily obtained in a one-pot reaction from A.BH2COOH with bromine; when esterification proceeds unexpectedly fast in parallel with the bromination. The structures of the new derivatives were substantiated by elemental analyses and IR, UV, 1H NMR and 11B NMR spectroscopic methods.

Use of a novel relationship between bond length and bond order to calculate accurate bond orders for carbon–carbon bonds

Abstract An exact relationship between bond length and bond order has been derived for the first time based on the concept of electron density. This relationship allows the calculation of sufficiently accurate bond orders and also determines the number of bond-forming electrons. According to this novel relationship between bond order and bond length, the bond order of the carbon–carbon bond in ethylene is 1.75, whereas it is 2.50 in acetylene. These bond orders are readily interpreted by the fragmentation of π-bonds and a consequent decrease in bond order, which is further supported by the chemical properties of these molecules. Assuming structure-specific fragmentation of π-bonds (i.e. one structural motif always adheres to one or two types of bond fragmentation scheme), the bond orders can be predicted for molecules containing multiple carbon–carbon bonds in excellent agreement with the experimental findings.

Complexation properties of 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7-malonate, -7,16-bis(malonate) and -7,16-bis(α-methylacetate)

The macrocycles 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-bis(malonate)(oddm), -7-malonate (odmm) and -7,16-bis(α-methylacetate)(oddp) have been synthesized. Their protonation constants and the stability constants of their complexes ML with Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Zn2+, Cd2+ and Pb2+ have been determined by pH-potentiometry at a 1 : 1 metal-to ligand ratio. The macrocycle oddm possesses high selectivity for the large metal ions. The complex [Sr(oddm)]2– is two orders of magnitude more stable than [Ca(oddm)]2– and the stability constant of [Pb(oddm)]2– is six orders of magnitude larger than that of the corresponding zinc complex. The complexes of oddm with large metal ions (Sr2+ and Ba2+) possess rigid structures, while the average lifetimes of the metal–donor bonds are relatively large, and this is indicated by the appearance of multiplet signals in the 1H NMR spectra.

Synthesis of [amine(tert-butylisonitrile) dihydroboron(III)] cations via the antimony pentachloride complexes of amine-dihydrocyanoboranes. A new route to amine-carboxyboranes

Abstract The reaction of the cyanoborane complexes L·BH 2 CN [where L = R 3 N (R = Me, Et, Pr, Bu); quinuclidine (Q) and Ph 3 P] with SbCl 5 in CCl 4 afforded 1:1 complexes. Coordination of antimony to the nitrogen atom of the cyano groupp was clearly implied by the 35–42 cm −1 shift of the ν CN cyano resonances towards the higher wave-numbers. The L·BH 2 CN·SbCl 5 complexes, carrying a bridge-head CN function, readily transformed with Bu t Cl into the [BH 2 (L)CNBu t ]SbCl 6 tert-butylnitrilium salts with high crystallization ability. In the case of L = R 3 N and Q substituents partial chlorination of the BH bond accompanied the formation of the nitrilium salt. The relatively weak coordination of the Bu t NC ligand in the above antimony complexes was indicated by the 2282–2292 cm −1 values observed for ν CN , unexpectedly high for isonitrile complexes, as well as by the exchange reactions with strong donors (i.e. Me 2 SO, Q, DMAP), giving rise to the liberation of the Bu t NC. The synthetic utility of related complexes was shown by the highly efficient transformation of [BH 2 (L)CNBu t ]SbCl 6 into Me·BH 2 C(O)NHBu t in an alkaline medium, whose acid hydrolysis furnishes Me 3 N·BH 2 COOH, a well-known and important compound in boron chemistry with high yield. Preparation of this substance with the presented new process, via the tert-butylnitrilium salt, is much more efficient than by means of the previously reported procedures employing the ethylnitrilium salt. The proposed structures of the new compounds L·BH 2 CN·SbCl 5 , [BH 2 (L)CNBu t ]SbCl 6 and Me 3 ·BH 2 C(O)NHBu t were derived from IR, 1 H and 11 B NMR spectral data.

Symmetrical and asymmetrical bis(amine)carboxyhydroboron(+1) and bis(amine)cyanohydroboron(+1) cations: synthesis and properties

Abstract Symmetrical bis(amine)(methoxycarbonyl)hydroboron(+1) cations have been prepared by treatment of trimethylaminebromo(methoxycarbonyl)borane [Me 3 N·BH(Br)COOMe] with an excess of tertiary amines (tetramethylethylenediamine, TMEDA; quinuclidine, Q), as well as pyridine bases (pyridine, Py; 4-methylpyridine, 4-Pic; 4-dimethylaminopyridine, DMAP) at higher temperature. Asymmetrical bis(amine)(methoxycarbonyl)hydroboron(+1) cations, which contain a chiral boron atom, have been synthesized, in good yields, in the reaction of 4-Pic·BH 2 COOMe and DMAP·BH 2 COOMe with amine perbromides (L·Br 2 , L = Q, Py, 4-Pic, DMAP) performed in CH 2 Cl 2 at room temperature. Besides these novel cationic carboxyboron compounds several new cationic cyanoboron complexes with an [LL′BH(CN)] + composition have been prepared by utilizing the efficient method employing amine-perbromides (L·Br 2 ; L = Py, 4-Pic, DMAP) and amine-cyanoboranes (L′·BH 2 CN, L′ = 4-Pic, DMAP). The bis(amine)boronium cations have been prepared and characterized by IR, 1 H and 11 B NMR spectra as PF 6 salts. Investigations of the water-soluble bis(amine)(methoxycarbonyl)hydroboron(+1) bromides, some of which could be prepared, showed that the hydrolytic stability of the ester group highly depends on the character of the amine: the complexes formed with tertiary amines are much more stable. This phenomenon is similar to that observed for the amine-(methoxycarbonyl)boranes (L·BH 2 COOMe, L = tertiary amine, pyridine base).