Reaction of triethyl phosphite with boron tribromide

Article abstract of DOI:10.1007/s007060070070

Reaction of boron tribromide with triethyl phosphite led to the formation of triethyl tribromoborophosphate 1, a complex bearing a P→B bond.

Full text of DOI:10.1007/s007060070070

Monatshefte fuÈr Chemie 131, 707±710 (2000)
Reaction of Triethyl Phosphite with Boron
Tribromide
Jarosøaw Lewkowski1;Ã, Jacques Mortier2, and Michel Vaultier2
1
   Â
Synthese et Electrosynthese Organique, Universite Rennes 1, F-35042 Rennes Cedex, France
Department of Organic Chemistry, University of èodz, PL-90136 èodz, Poland
Â
2
Á
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Summary. Reaction of boron tribromide with triethyl phosphite led to the formation of triethyl
tribromoborophosphate 1, a complex bearing a P!B bond.
Keywords. Boron tribromide; Complexes; NMR Spectroscopy; Triethyl phosphite; Lewis acids.
Introduction
In 1959, Gerrard et al. published their results on the reaction between triphenyl
phosphite and boron tribromide [1]. Contrary to their expectation, the reaction did
not lead to the P!B adduct, but yielded phosphorus tribromide and triphenyl
borate. Later, they reported on the reaction of tributyl phosphite with boron
trichloride resulting in dibutyl phosphorchloridite and butoxyboron dichloride,
stating that boron tribromide reacted with trialkyl phosphite in the same way [2].
Binder and Palmtag presented the reaction of triethyl phosphite with boron
tri¯uoride, which did not give any compound bearing a B-P bond [3]. The reaction
resulted in products of so-called `mutual replacement'. Very recently, Stasi and Just
[4] tested the reaction of triethyl phosphite with boron trichloride and boron
tri¯uoride. They did not observe the formation of a B-P complex, but detected the
corresponding diethyl phosphorhalidites instead.
We present here the results of our study on the reaction of triethyl phosphite
with boron tribromide, unexpectedly leading to a complex bearing a boron-
phosphorus bond.
Results and Discussion
The reaction of triethyl phosphite with boron tribromide was carried out in
chloroform for 18 hours at room temperature leading to triethyl tribromoboropho-
sphate (1), ethoxybromophosphono bromoboronite (2), phosphorus tribromide (3),
and triethyl borate (4) in a 10:5:1:1 ratio (Scheme 1).
Ã
Corresponding author

708
J. Lewkowski et al.
Scheme 1
Attempts to separate the products failed due to their instability. NMR and mass
spectroscopic studies of the mixture, however, allowed a characterization of the
components.
The 11B NMR signal of 1 occurred at 18.8 ppm as a doublet (1JPB ˆ 268 Hz),
its 31P NMR signal at 38.5 ppm as a quartet (1JPB ˆ 270 Hz), thus clearly indicating
the formation of a phosphorus-boron bond. EI-MS showed the molecular ion peak
at m/z ˆ 417. The 11B and 31P NMR signals of 2 were observed at 18.3 and
198.4 ppm as singlets. Its 1H NMR spectrum showed the presence of ethoxy groups
linked to phosphorus and to boron atoms, the character of the 11B and 31P NMR
signals suggesting tricoordination of boron and phosphorus. The molecular ion
peak (EI-MS) was found at m/z ˆ 228. Phosphorus tribromide (3) gave a 31P NMR
signal at 228.3 ppm; the 11B NMR resonance of triethyl borate (4) occurred at
18.0 ppm.
In order to disclose the mechanism of this reaction, it was performed under
NMR monitoring. Several minutes after mixing the reagents, formation of 1
(ꢀP ˆ 38.5 and 18.8 ppm), 6 (ꢀP ˆ 69.0 ppm, ꢀB ˆ
15.6 25.7 ppm), and 7
(ꢀP ˆ 141.3 ppm, ꢀB ˆ 27.6 ppm) in a 10:1:1 ratio was observed. 1H NMR
measurements showed the signals of the evolving bromoethane. One hour later,
the signals of compound 7 had disappeared.
Within the next six hours, the intensities of the signals of 6 decreased,
and formation of compounds 2 (ꢀP ˆ 198.3 ppm, ꢀB ˆ 18.3 ppm) and 3

Triethyl Tribromoborophosphate
709
(ꢀP ˆ 228.2 ppm) was observed. Three new signals appeared in the 31P NMR
spectrum of the mixture, and we suggest the formation of the following
intermediates: 8 (ꢀP ˆ 184.2 ppm), 10 (ꢀP ˆ 209.7 ppm), and 12 (ꢀP ˆ 217.1 ppm).
They were identi®ed by their 31P NMR signals which were in accordance with
literature data [5, 6]. However, attempts of their isolation failed. After 12 more
hours, only the signals of the end products were observed.
The mechanism of the reaction remains unclear. Nevertheless, we suggest that
®rst triethyl phosphite reacts with boron tribromide, formating 1 and intermediate
7. 7 in turn immediately reacts with one molecule of boron tribromide to yield
complex 6. Reaction of 6 triethyl phosphite leads to two molecules of ethyl
ethoxybromophosphono bromoboronite (2).
The mechanism of the formation of 3 and 4 is even less clear. Most probably,
fragmentation of 6 leads to the formation of diethyl phosphorbromidite (8) and
ethoxyboron dibromide (9). These two compounds undergo the mutual replace-
ment to give ethyl phosphordibromidite (10) and diethoxyboron bromide (11)
which, in the course an intermolecular rearrangement, afford phosphorus
tribromide (3) and triethyl borate (4). We also suppose that compounds 10 and
11 react with each other to give compound 12 which rearranges subsequently to the
end product 2 (Scheme 1).
Repetition of the reaction employing boron trichloride under the same
experimental conditions did not give rise to any compound bearing a B-P bond.
Diethyl phosphorochloridite (ꢀP ˆ 167.4 ppm) and ethyl dichloroboronite
(ꢀB ˆ 31.6 ppm) were obtained as products which is in accordance with literature
data [1±4].
It is dif®cult to explain why the reaction of boron tribromide with phosphite
leads to the formation of a P-B bond whereas the reaction of boron trichloride with
phosphites does not. It is possible that the complex ꢀRO†3P ! BCl3 occurs as an
intermediate but is so unstable that its detection is beyond our technological
possibilities. The problem is still under investigation.
Experimental
All NMR spectra were recorded on a Bruker AC 300 spectrometer (1H: internal TMS; 11B: external
Et2O Á BF3 31P: external H3PO4); electron impact mass spectra were measured on a Varian MAT 311
instrument.
Reaction of triethyl phosphite with boron tribromide
Boron tribromide (0.68 g, 2.72 mmol) was dissolved in CHCl3 under argon, cooled to 20ꢁC, and
triethyl phosphite (0.45 g, 2.72 mmol) was added. The mixture was allowed to warm up to room
temperature and was stirred for 12 h. The precipitate was collected by ®ltration to give 0.08 g of a
white solid. The ®ltrate was evaporated to give 0.73 g of a mixture of products.
Triethyl tribromoborophosphate (1; C6H15O3PBBr3)
3
3
1H NMR (300 MHz, CDCl3): ꢀ ˆ 4.47 (dq, JPH ˆ 7.0 Hz, JHH ˆ 7.0 Hz, CH2), 1.46 (dt,
3JPH ˆ 0.8 Hz, 3JHH ˆ 7.0 Hz, CH3) ppm; 31P NMR (121MHz, CDCl3): ꢀ ˆ 38.39 (q, 1JPB ˆ 270.0Hz)
ppm; 11B NMR (96 MHz, CDCl3): ꢀ ˆ 18:8 (1JPB ˆ 270.0Hz) ppm; EI-MS: m/z ˆ 417 (M ).
‡

710
J. Lewkowski et al.: Triethyl Tribromoborophosphate
Ethoxybromophosphono bromoboronate (2; C4H10O3PBBr2)
3
3
1H NMR (300 MHz, CDCl3): ꢀ ˆ 4.31 (dq, JPH ˆ 8.8 Hz, JHH ˆ 7.1 Hz, P-OCH2CH3), 4.01 (q,
3
3
J ˆ 7.1 Hz, B-OCH2CH3), 1.40 (dt, JPH ˆ 0.5 Hz, JHH ˆ 7.1 Hz, P-OCH2CH3), 1.25 (t, J ˆ 7.1 Hz,
B-OCH2CH3) ppm; 31P NMR (121 Mhz, CDCl3): ꢀ ˆ 198.4 (s) ppm; 11B NMR (96 MHz, CDCL3):
‡
ꢀ ˆ 18.9 (broad s) ppm; EI-MS m/z ˆ 308 (M ).
Phosphorus tribromide (3; PBr3)
31
‡
P NMR (121 MHz, CDCl3): ꢀ ˆ 228.3 (s) ppm; EI-MS: m/z ˆ 271 (M ).
Triethyl borate (4; C6H15O3B)
1H NMR (300 MHz, CDCl3): ꢀ ˆ 4.25 (q, J ˆ 7.1 Hz, CH2, 1.23 (t, J ˆ 7.1 Hz, CH3) ppm; H), 11B
‡
NMR (96 MHz, CDCl3): ꢀ ˆ 18.0 (s) ppm; EI-MS: m/z ˆ 146 (M )
Complex 6 (C4H10O3PB2Br5)
3
3
1H NMR (300 MHz, CDCl3): ꢀ ˆ 4.40 (dq, JPH ˆ 7.1 Hz, JHH ˆ 7.1 Hz, CH2, 1.46 (dt,
3JPH ˆ 0.8 Hz, JHH ˆ 7.1 Hz, CH3) ppm; 31P NMR (121 MHz, CDCl3): ꢀ ˆ 68.70 (q,
3
1JPB ˆ 218.7 Hz) ppm; 11B NMR (96 MHz, CDCl3): ꢀ ˆ 25.7 (s), 15.6 (d, JPB ˆ 218.7 Hz) ppm.
1
Intermediate 7 (C4H10O3PBBr2)
3
3
1H NMR (300 MHz, CDCl3): ꢀ ˆ 4.08 (dq, JPH ˆ 7.1 Hz, JHH ˆ 7.1 Hz, CH2), 1.08 (dt,
3
3JPH ˆ 0.5 Hz, JHH ˆ 7.1 Hz, CH3) ppm, 31P NMR (121 MHz, CDCl3): ꢀ ˆ 141.3 (s) ppm; 11B
NMR (96 MHz, CDCl3): ꢀ ˆ 27.6 (s) ppm.
Acknowledgements
J. L. thanks Dr. Ilya Gridnev (Russian Academy of Sciences, Moscow) for helpful dis-
cussions.
References
[1] Frazer MJ, Gerrard W, Patel JK (1959) Chem Ind 728
[2] Gerrard W, Lindsay M (1960) Chem Ind 152
[3] Binder H, Palmtag J (1972) Z Naturforsch B: Anorg Chem, Org Chem 27B: 753
[4] Stasi L, Just G (1999) Tetrahedron Lett 40: 2283
[5] Gorenstein DG, Shah DO (1984) In: Gorenstein DG (ed) Phosphorus-31 NMR. Principles and
Applications. Academic Press, New York
[6] Emsley JW, Feeney J, Sutcliffe LH (1967) High Resolution Nuclear Magnetic Resonance
Spectroscopy, vol 2. Pergamon Press, Oxford
Received November 19, 1999. Accepted (revised) January 21, 2000