Tritium and deuterium labelling studies of alkali metal borohydrides and their application to simple reductions

Abstract

Simple and facile syntheses of highly deuterated and tritiated LiBH4, NaBH4 and KBH4 were achieved by hydrogen isotope exchange with deuterium or tritium gas at elevated temperatures. The exchange products were characterized by boron, proton and deuterium or tritium NMR spectroscopy. The extent of isotope (2H or 3H) incorporation was calculated from the patterns of 11B NMR spectra. Several samples of tritiated NaBH4 were treated with BF3‐Et2O to produce tritiated borane‐THF complex, which is an electrophilic reducing agent. The utility of both the borohydride reagents and borane‐THF complex in labelling reactions was confirmed by exemplary reductions leading to specifically labelled products. The extent and orientation of labelling in the reduction products was assessed by a combination of radio‐HPLC analysis, 1H, 2H or 3H NMR and mass spectrometry.

Exchange of Hydrogen Atoms Between BH4 in LiBH4

“…We note the melting transition is obvious, with the line width decreasing markedly and J-couplings becoming evident. The J-coupling strength is 81 ± 1 Hz, in agreement with a report of LiBH 4 in alkaline methanol solution at room temperature . The spectrum of Figure displays lines with a line width fwhm (full-width at half of maximum) of 9.5 Hz.…”

Synthesis, NMR Characterization, and a Simple Application of Lithium Borotritide

“…As expected, this doublet collapsed to a singlet when 1 H was irradiated (Figure h). The observed coupling constant [ J ( 1 H− 3 H) = 13.81 Hz] matches previous observations. , The product NMR spectra shown in parts e−h of Figure were very clean and showed minimal levels of byproducts.…”

“…The pattern is centered at δ = −0.40 ppm and consists of an intense quartet [ J( 3 H− 11 B) = 84.5 Hz] due to 11 B coupling ( I = 3 / 2 , 80.42%) that is superimposed on a weaker septet [ J( 3 H− 10 B) = 28.3 Hz] due to 10 B coupling ( I = 3, 19.58%). The observed chemical shift and coupling constants [ J ( 3 H− 11 B), J ( 3 H− 10 B)] are similar to published data. ,, A weak and unresolved quartet of doublets attributable to Li 11 BH 3 H 3 was also observed at the base of the main quartet. Selective irradiation of 11 B leads to the collapse of both the Li 11 B 3 H 4 and Li 11 BH 3 H 3 quartets to give the large singlet and downfield shoulder shown in Figure b.…”

“…is centered at δ ) -0.40 ppm and consists of an intense quartet [J( 3 H-11 B) ) 84.5 Hz] due to 11 B coupling (I ) 3 / 2 , 80.42%) that is superimposed on a weaker septet [J( 3 H-10 B) ) 28.3 Hz] due to 10 B coupling (I ) 3, 19.58%). The observed chemical shift and coupling constants [J( 3 H-11 B), J( 3 H-10 B)] are similar to published data 22,27,28. A weak and unresolved quartet of doublets attributable to Li 11 BH 3 H 3 was also observed at the base of the main quartet.…”

Preparation, NMR Characterization, and Labeling Reactions of Tritiated Triacetoxy Sodium Borohydride

“…Tritiated NaBH(OAc) 3 was used to reduce a mixture of 2-naphthaldehyde and 2-acetonaphthone, and the 300 MHz 1 H and 320 MHz 3 H NMR spectra of the product are shown in Figure A−D. The observed chemical shift, coupling patterns and coupling constants were found to be similar to those previously reported. , The alcohol product was very clean and the calculated tritium incorporation (using the ratio of R-CH 2 -OH, and R-CH 3 H- OH in the 1 H spectra, R = naphthyl) was 53%, consistent with specific activity estimates from HPLC analyses.

2 300 MHz 1 H and 320 MHz 3 H NMR spectra (5.0−4.5 ppm) of the reduction product (in CD 3 OD) given by reaction with NaB 3 H(OAc) 3 : (A) 3 H coupled 1 H NMR spectrum; (B) 3 H decoupled 1 H NMR spectrum; (C) 1 H coupled 3 H NMR spectrum; and (D) 1 H decoupled 3 H NMR spectrum.

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