Tetraacetyl Diborate1

(AcO)2BOB(OAc)2

[5187-37-1]  · C8H12B2O9  · Tetraacetyl Diborate  · (MW 273.82) (B(OAc)3)

[4887-24-5]

(selective acetylating agent for aromatic polyhydroxycarbonyl compounds;1-3 mild Lewis acid for Diels-Alder reactions of hydroxyquinones4)

Alternate Names: TADB; boron triacetate; boroacetic anhydride; pyroboroacetate; oxybis(diacetoxyborane).

Physical Data: mp 150-152 °C (sealed tube),3 146-147 °C.5

Solubility: nearly insol petroleum ether, CCl4, CS2, Et2O; especially sol dry warm benzene, EtOAc, ethylene bromide, and nitrobenzene (can be recrystallized from these solvents).3

Preparative Methods: a mixture of 31 g (0.50 mol) of Boric Acid and 165 mL (1.75 mol) of Acetic Anhydride is heated slowly with an oil bath under N2 in a flask equipped with a reflux condenser and a magnetic stirrer. At about 70 °C a vigorous exotherm (CAUTION) occurs and the reaction mixture becomes a clear solution which, upon cooling, deposits crystals of TADB. The supernatant is drawn off and the crystals either washed with dry ether or recrystallized from 200 mL CHCl3/benzene.5

Handling, Storage, and Precautions: moisture sensitive.

Structure.

There is some controversy1,6 whether the mixed anhydride of boric and acetic acid has the structure (AcO)2BOB(OAc)2 (TADB) or B(OAc)3. The material normally obtained from the reaction of boric acid (H3BO3) and acetic anhydride is (AcO)2BOB(OAc)2, at least in the crystalline state (mp ~150-152 °C).5,6 Even when prepared under conditions (BH3 + 3 HOAc) that might be expected to give B(OAc)3,5,7 the crystalline material isolated is TADB.5 There is an early report8 of the preparation of B(OAc)3, mp 121 °C, from boric anhydride (B2O3) and Ac2O. Since TADB is normally employed in excess, the constitution in solution (and the exact molecular weight) is somewhat immaterial to its use.

Selective Acetylation.

The use of TADB/acetic anhydride to acetylate selected hydroxy groups of polyhydroxyquinones was introduced by Dimroth.2,3 Thus treatment of alizarin (1) with a hot mixture of TADB and Ac2O gives, after workup with cold water, monoacetate (2).2 The reaction proceeds by way of boroacetate complex (3), which after acetylation to (4) is hydrolyzed to (2). Selective preparation of (6) from purpurin (5) was achieved analogously.2 If a carbonyl is flanked by two peri hydroxyls, only one is protected from acetylation.2 Thus treatment of (7) with TADB/Ac2O affords (8) after hydrolytic workup.2 Selective monoacetylation of xanthone (9) to (10) was accomplished similarly.9 That the product from eq 1 forms a boroacetate derivative was taken as conclusive proof that it possesses structure (11) rather than (12).10 In the absence of a flanking carbonyl, a phenol group is not protected from acetylation: reaction of (13) with TADB/Ac2O gives diacetate (14).3 Other compounds or functional group combinations that give boroacetate complexes include salicyclic acid, 8-hydroxyquinoline, peri-aminoquinones, and o-hydroxyaryl ketones.1,3

Diels-Alder Reaction Catalysis.

TADB has seen considerable recent use as a catalyst in the Diels-Alder reactions of peri-hydroxylated quinones.4 In addition to usually accelerating4,11,12 the rate of reaction, it has been employed to advantage to enhance regioselectivity. Thus in the absence of catalyst, (15) reacts with (17) to give a 45:55 ratio of (18) to (19).4 When (15) and (17) react in the presence of 2 equiv of TADB (via 20), the ratio of (18) to (19) is >19:1.4 In general, with TADB the carbonyl peri to the hydroxyl becomes the regiochemical director. Regiochemical control is best achieved with highly polarized dienes;4 however, in some11,12 (but not all)4 cases even Isoprene reacts with considerable regioselectivity. A frontier molecular orbital explanation for the regiochemical outcome induced by TADB has been advanced.13 It is to be noted that other Lewis acids can sometimes give a reversal of regiochemistry:4,11,12 in the absence of catalyst, (16) and (17) react to give a 40:60 ratio of (21) and (22), but with 0.5 equiv of Magnesium Iodide the ratio is 1:>19.4

One of the other virtues of TADB is its relatively noncorrosive nature which, for example, renders it compatible with dienes destroyed by Lewis acids such as Boron Trifluoride Etherate.4,11 Regiospecific TADB-catalyzed Diels-Alder reactions have been featured in the syntheses of (±)-bostrycin (23)14 (this paper contains experimental details), (±)-altersolanol B (24),15 (±)-ochromycinone (25),16 and (±)-epirubigenone B1 (26).17

In the presence of TADB the Diels-Alder reaction between (27) and (28) proceeds with complete (&egt;98%) facial (and regio-) selectivity (eq 3).18 Chiral analogs19,20 (e.g. 3019) of the borate complexes have been used for asymmetric Diels-Alder reactions, including one in an enantioselective synthesis of (-)-bostrycin (23).19 Quinizarin (31) does not exhibit any tendency to undergo a Diels-Alder reaction by way of its putative tautomer (32) (eq 4),21 but in the presence of TADB cycloaddition occurs.22 1,4-Diaminoanthraquinone behaves similarly.22 The boroacetate complex of (32) also acts as a dipolarophile.23

Miscellaneous.

TADB has been used for the selective cleavage of cyclic ethers, e.g. (33) -> (34) (eq 5).7 In the synthesis of (26), TADB was enlisted to aromatize the B ring in (35) when acidic and basic treatment gave (36).17


1. (a) Steinberg, H. Organoboron Chemistry; Interscience: New York, 1964; Vol. 1, pp 390-407. (b) Lappert, M. F. CRV 1956, 56, 959.
2. Dimroth, O.; Faust, T. CB 1921, 3020.
3. Dimroth, O. AC(R) 1926, 446, 97.
4. Kelly, T. R.; Montury, M. TL 1978, 4311.
5. Brown, H. C.; Stocky, T. P. JACS 1977, 99, 8218.
6. Hayter, R. G.; Laubengayer, A. W.; Thompson, P. G. JACS 1957, 79, 4243.
7. Kato, K.; Furuta, K.; Yamamoto, H. SL 1992, 565.
8. Pictet, A.; Geleznoff, A. CB 1903, 36, 2219.
9. Scannell, R. T.; Stevenson, R. JHC 1986, 23, 857.
10. Jones, E. T.; Robertson, A. JCS 1930, 1699.
11. Russell, R. A.; Collin, G. J.; Sterns, M.; Warrener, R. N. TL 1979, 4229.
12. Gupta, R. C.; Jackson, D. A.; Stoodley, R. J. CC 1982, 929.
13. Desimoni, G.; Tacconi, G.; Barco, A.; Pollini, G. P. Natural Products Synthesis Through Pericyclic Reactions; American Chemical Society: Washington, 1983; pp 191-201.
14. Kelly, T. R.; Saha, J. K.; Whittle, R. R. JOC 1985, 50, 3679.
15. Kelly, T. R.; Montury, M. TL 1978, 4309.
16. Guingant, A.; Barreto, M. M. TL 1987, 28, 3107.
17. Larsen, D. S.; O'Shea, M. D. TL 1993, 34, 1373.
18. Trost, B. M.; O'Krongly, D.; Belletire, J. L. JACS 1980, 102, 7595.
19. Kelly, T. R.; Whiting, A.; Chandrakumar, N. S. JACS 1986, 108, 3510; Kelly, T. R.; Whiting, A.; Chandrakumar, N. S. JACS 1989, 111, 5979.
20. Maruoka, K.; Sakurai, M.; Fujiwara, J.; Yamamoto, H. TL 1986, 27, 4895.
21. Kelly, T. R.; Gillard, J. W.; Goerner, Jr., R. N. TL 1976, 3873.
22. Birch, A. M.; Mercer, A. J. H.; Chippendale, A. M.; Greenhalgh, C. W. CC 1977, 745.
23. Birch, A. M.; Mercer, A. J. H.; Greenhalgh, C. W. H 1979, 12, 757.

T. Ross Kelly & Zhenkun Ma

Boston College, Chestnut Hill, MA, USA



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