Catalytic dealkylation of phosphates with binuclear boron compounds

Article abstract of DOI:10.1021/ja017360m

The Salen(^tBu) ligand and its derivatives were used to prepare binuclear boron complexes. These compounds have the formula, L(BBr₂)₂ (L = Salpen(^tBu) and Salben(^tBu)). These are formed from the reaction of the corresponding L[B(OMe)₂]₂ with BBr₃. They represent a new type of binuclear boron compound. These compounds are active towards the dealkylation of many phosphates. They are also catalytically active with a stoichiometric amount of BBr₃ to trimethylphosphate. Copyright

Full text of DOI:10.1021/ja017360m

Published on Web 02/09/2002
Catalytic Dealkylation of Phosphates with Binuclear Boron Compounds
Timothy S. Keizer, Lauren J. De Pue, Sean Parkin, and David A. Atwood*
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055
Received October 23, 2001
The breaking of a phosphate ester bond is important in many
areas such as the destruction of nerve agents (Sarin and VX),1
pesticides (chloropyrifos),2 and in biological systems.3 In recent
years, methods for the catalytic cleavage of the P-O bond in
phosphate esters have been developed.4 Most of these are binuclear
systems use d-block metals such as cobalt,5 copper,6 and zinc.7
Boron compounds however, have not been examined in this regard,
despite the fact that BBr3 will, through cleavage of the O-C bond,
dealkylate alkyl and aryl ethers (eq 1)8 and silyl ethers.9 This
reagent, however, is ineffective with phosphates since phosphorus
Figure 1. Molecular structure and atom numbering scheme for 2. Selected
is electron-donating (thereby strengthening the alkyl-oxygen bond).
bond lengths (Å) and angles (deg): B(1)-O(1), 1.418(9); B(1)-N(1),
For example, BBr3 does not de-alkylate trimethyl phosphate (less
1.517(9); B(1)-Br(1), 2.023(8); B(1)-Br(2), 2.077(9), O(1)-B(1)-N(1),
than 2% in 24 h). In an effort to determine whether the presence
112.8(6); O(1)-B(1)-Br(1), 108.6(5); N(1)-B(1)-Br(1), 112.2(5);
of a chelate ligand might improve the effectiveness of boron
bromides for this reaction, new binuclear boron compounds Salpen-
(tBu)[BBr2]2 (1) and Salben(tBu)[BBr2]2 (2) have been synthesized.10
These compounds catalytically dealkylate a broad range of phos-
phate esters through cleavage of an O-C bond.
Br(1)-B-Br(2), 107.7(3).
Table 1. Percent Dealkylation of Different Phosphates with
Salpen(tBu)[BBr2]2 (1)
phosphate
conversion (%)a
(MeO)3P(O)
89
63
99
98
85
99
63
71
98
0
The binuclear boron bromides (1 and 2) are prepared in high
(EtO)3P(O)
11
yields by combining Salpen(tBu)[B(OMe)2]2 or Salben(tBu)-
(nBuO)3P(O)
[B(OMe)2]2 11 with a stoichiometric amount of BBr3 (eq 2).12 The
11B NMR shows a broad single peak for 1 and 2 at δ -0.57 and
-0.40 ppm, upfield from the related chloride analogue Salpen-
(nPentO)3P(O)
(MeO)2P(O)H
(MeO)2P(O)Me
(iPrO)2P(O)H
(PhO)2((2-Et)HexO)P(O)
(Me3SiO)3P(O)
(PhO)3P(O)
13
(tBu)[BCl2]2 (6.21 ppm).
a The percent conversion was determined by the amount of phosphate
1
remaining to the amount of alkyl bromide produced in the H NMR.
cation formation also takes place when excess BBr3 is added to
compound 1.19
In the structure14 of 2 (Figure 1), the boron atoms are in a
distorted tetrahedral geometry and trans to one another. The angle
of the ligand (N-B-O) is 112.8(6)°, which is slightly larger than
other Salen-supported binuclear systems such as Salen(tBu)-
[B(OSiPh3)2]2 15 (angle ) 104.9(3)°). The B-Br bond lengths are
2.023(8) and 2.077(9) Å for 2, which are slightly longer than those
for other four-coordinate boron dibromide compounds such as [(2-
Me2NCH2)C6H4]BBr2 (with B-Br bond distances of 2.01(1) and
2.02(1) Å).16
Salpen(tBu)[BBr2]2 cleaves an O-C bond in a multitude of
phosphate esters (Table 1).17 When combined with stoichiometric
amounts of various phosphates, 1 produces alkyl bromides and
chelated boron phosphates (eq 3). Simple and sterically encumbered
(P-O-C) linkages that possess primary and secondary sp3
R-carbons are cleaved.
Since Salpen(tBu)[BBr2]2 (1) can be generated in situ from
Salpen(tBu)[B(OMe)2]2 and BBr3, the process can be made catalytic.
The dealkylation of trimethyl phosphate occurs within 5 min
through the addition of catalytic amounts of borate to equimolar
trimethyl phosphate and BBr3 in the trimethyl phosphate to 1 ratio
of 20:1.20 Salpen(tBu)[B(OMe)2]2 dealkylates (MeO)3P(O) (75%
conversion) with BBr3 within 30 min at a substrate-to-catalyst ratio
of 200:1. Addition of BBr3 or the borate alone does not effect
dealkylation within 24 h. The boron bromide compounds show
excellent activity toward the dealkylation of different phosphates.
The activity of the boron halide compounds does not decrease with
the extension of the alkyl chain on the phosphates. However, the
activity of the boron halide compounds shows a slight decrease
with the branched phosphates such as (PhO)2((2-Et)HexO)P(O). A
The mechanism appears to be one in which a cationic intermedi-
ate [(chelate)BBr]+ coordinates the phosphate, allowing a nucleo-
philic attack by the bromide at the R-carbon. Such cations appear
readily accessible by the simple addition of a Lewis base.18 The
9
1864 VOL. 124, NO. 9, 2002 J. AM. CHEM. SOC.
10.1021/ja017360m CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
(11) Wei, P.; Atwood, D. A. Inorg. Chem. 1998, 37, 4934.
further positive attribute of this system is that these reactions can
be conducted at room temperature. Unlike the synthetic enzyme
models, the C-O cleavage appears to occur at only one metal site.
For example, preliminary results show that the compound N, -tert-
butyl (salicylideneimine) dealkylates trimethyl phosphate. Thus, this
might be a general reaction for any type of chelate.
The phosphates dealkylated in this report may be viewed as
models for the nerve agent Sarin and the pesticide chloropyrifos
since they have similar P-O-C units. Thus, chelated boron
bromides appear to be promising candidates for the decontamination
of chemical warfare agents such as VX and Sarin gas under organic
conditions. More specifically, they may be more efficient than
conventional decontamination systems that use hydroxide sources.21
(12) Experimental details for 1 and 2: Salpen(tBu)[BBr2]2 (1). To a stirring
solution of Salpen(tBu)[B(OMe)2]2 (3.0 g, 4.62 mmol) in toluene (50 mL)
was added 1M BBr3 in heptane (6.24 mL, 6.24 mmol). The reaction
mixture was stirred for 24 h. The solvent was removed and the solid
remaining behind was washed with 10 mL of hexanes. Filtration and
vacuum-drying afforded Salpen(tBu)[BBr2]2. Yield: 3.28 g (88%); mp
1
282-285 °C dec. H NMR (CDCl3): δ 1.25 [s, 18H, C(CH3)3], 1.47 [s,
18H, C(CH3)3], 2.88 [m, 2H, CH2], 4.12 [m, 4H, NCH2], 7.30 [d, 2H,
C6H2], 7.76 [d, 2H, C6H2], 8.53 [s, 2H, CHN]. 11B NMR (CDCl3): δ
-0.57 (w1/2 ) 78.2 Hz). IR(cm-1): 2963(s), 2870(w), 1626(s), 1575(s),
1472 (m), 1438(m), 1395(m), 1364(s), 1339(m), 1279(m), 1217(w). 1077-
(w), 1027(w), 904(w), 846(w), 769(w), 668(m), 641(m). MS: 766(Mx
- Br, 20), 686(Mx - 2Br, 5), 606(Mx - 3Br, 100). Anal. Calcd for
B2C33H48N2O2Br4: C 47.03(46.96), H 5.74(5.68), N 3.33(3.32). Salben-
(tBu)[BBr2]2 (2). To a stirring solution of Salben(tBu)[B(OMe)2]2 (1.0 g,
1.51 mmol) in toluene (50 mL) was added 1 M BBr3 in heptane (2.04
mL, 2.04 mmol). The reaction mixture was stirred for 18 h at room
temperature. The solution was concentrated to 10 mL, filtered, and dried.
1
Yield: 0.96 g (76%); mp: 279-280 °C dec. H NMR (CDCl3): δ 1.27
Acknowledgment. This work was supported by the National
Science Foundation NSF-CAREER award (CHE 9816155). L.J.D.
was supported by the NSF-REU program (CHE -0097668) in the
summer of 2001. NMR instruments used in this research were
obtained with funds from the CRIF program of the National Science
Foundation (CHE 997841) and from the Research Challenge Trust
Fund of the University of Kentucky.
[s, 18H, C(CH3)3], 1.42 [s, 18H, C(CH3)3], 2.21 [m, 4H, CH2], 4.08 [m,
4H, NCH2], 7.22 [d, 2H, C6H2], 7.78 [d, 2H, C6H2], 8.24 [s, 2H, CHN].
11B NMR (CDCl3): δ -0.40 (w1/2 ) 92.7 Hz). IR (cm-1): 2953(s), 2900-
(m), 2870(w), 1627(s), 1573(s), 1469(m), 1440(m), 1395(w), 1362(w),
1253(m), 1220(m), 1136(w), 1083(w), 1024(m), 863(m), 768(w), 641-
(w). MS: 780(Mx - Br, 100), 700(Mx - 2Br, 50), 620(Mx - 3Br,
40), 540(Mx - 4Br, 10). Anal. Calcd for B2C34H50N2O2Br4: C 47.66
(47.84), H 5.89 (6.22), N 3.27 (3.22).
(13) Keizer, T. S.; Parkin, S.; Atwood, D. A. J. Can. Chem. Submitted.
(14) X-ray data for 2 were collected on a Nonius CCD unit employing Mo
KR radiation. The structures were refined using the Siemens software
package SHELXTL 4.0. All of the non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were put into calculated positions.
Absorption corrections were not employed. X-ray data for 2: yellow
crystals (0.2 mm × 0.2 mm × 0.16 mm), formula C48 H66 B2 Br4 N2 O2
FW 1044.29, monoclinic, P21/c, a ) 14.0520(10) Å, b ) 30.708(2) Å, c
) 12.2780(10) Å, â ) 108.733(10)°, V ) 5017.4(6) Å3, Z ) 4, data
8848, parameters 553, R1 ) 0.0486 (for I > 2RI), wR2 ) 0.0755, GOF
(on F2) ) 1.078.
Supporting Information Available: X-ray crystallographic data
for 2 (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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(17) Dealkylation of the phosphate: In a NMR tube, phosphate was added to
a equimolar solution of the catalyst (1 or BBr3) in CDCl3 and held at
1
room temperature for 30 min. The reaction was monitored by H NMR.
(18) In a NMR tube, 5 equiv of THF was added to a solution of 1 in CDCl3
giving a 11B NMR resonance of 4.28 ppm. It is shifted downfield from 1
by 4.85 ppm, indicating the displacement of Br by THF and formation of
a cation.
(19) In a NMR tube, 3 equiv of BBr3 was added to a solution of 1 in CDCl3.
11B NMR: δ 23.10, -0.42, -21.60. Indicating the formation of BBr4
(-21.60) and Salpen(tBu)[BBr2(BBr)+] (-0.42 and 23.40).
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amounts of (MeO)3P(O) and BBr3 were added to a solution of Salpen-
(tBu)[B(OMe)2]2 in CDCl3 in the ratio of 20:1 of phosphate to borate and
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phosphate compound with no ligand present.
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JA017360M
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