Organic compounds -- part of the class 532-570 series – Organic compounds – Borate esters
Reexamination Certificate
1999-12-22
2001-03-20
Lambkin, Deborah C. (Department: 1613)
Organic compounds -- part of the class 532-570 series
Organic compounds
Borate esters
Reexamination Certificate
active
06204405
ABSTRACT:
This invention relates to new, economical and convenient procedures for the preparation of catecholborane. These processes provide catecholborane in high yields and chemically pure form.
PRIOR ART
Catecholborane is one of the most versatile boron hydride reagents available for synthetic chemists. It has found a multitude of applications as a selective reducing and hydroborating agent (Brown, H. C.; Gupta, S. K.
J. Am Chem. Soc.,
1971, 93, 1816. Brown, H. C.; Gupta. S. K.
J. Am Chem. Soc.,
1971, 93, 4062. Lane, C. F.; Kabalka, G. W.
Tetrahedron
1976, 32, 981. Kabalka, G. W. Org.
Prep. Proc. Intl.,
1977, 9, 133. Kabalka, G. W.; Baker, Jr., J. D.; Neal, G. W.
J. Org. Chem.,
1977, 42, 512. VanNieuwenhze, M. S.
Encyclopedia of Reagents for Organic Synthesis;
Wiley: New York, vol. 2, Ed. Paquette, L. A., 1995, p1017. Brown, H. C.
Organic Synthesis via Boranes;
Aldrich Chemical Co., Inc.; Milwaukee, Wis., 1997; Vol. 1.). It has been effectively used in the preparation of alkyl- and alkenylboronic acids (Brown, H. C.; Gupta, S. K.
J. Am Chem. Soc.,
1972, 94, 4370. Brown, H. C.; Gupta, S. K.
J. Am Chem. Soc.,
1975, 73, 5249. Brown, H. C.; Chandrasekhran, J.
J. Org. Chem.,
1983, 48, 5080). Particularly its usage in conjunction with chiral oxazaborolidine and chiral transition-metal complex catalysts provides a unique tool for the synthesis of chiral alcohols in very high enantioselectivities (Mannig, D; Noth, H.
Angew. Chem., Int. Ed. Engl.,
1985, 24, 878. Burgess, K.; Ohlmeyer, M. J.
Chem. Rev.,
1991, 91, 1179. Corcy, E. J.; Helal, C. J.
Angew. Chem., Int. Ed. Engl.,
1998, 37, 1986).
Though catecholborane can be prepared by other procedures (Newson, H. C.; Woods, W. G.
Inorg. Chem.,
1968, 7, 177. Suseela, Y.; Periasamy, M.
J. Organomet. Chem.,
1993, 450, 47), the most preferred way of preparation is by the reaction of catechol with borane-tetrahydrofuran or borane-methyl sulfide (eq. 1) (Brown, H. C.; Gupta, S. K.
J. Am Chem. Soc.,
1971, 93, 1816. Brown, H. C.; Mandal, A. K.; Kulkarni, S. U.
J. Org. Chem.,
1977, 42, 1392).
The product catecholborane was isolated by distillation (bp 50° C./50 mmHg). The catecholborane thus obtained is moisture sensitive, but is stable in dry air and can be stored at 0° C. for long periods. However, this procedure does suffer from some disadvantages, such as the waste of two equivalents of active hydrides and the liberation of large amounts of hydrogen, which may be of concern in large-scale applications. Also, the commercially available catecholborane contains considerable amounts of borate impurities. On the other hand, the increasing use of catecholborane in materials and medicinal chemistries warrants more economic and convenient options for the synthesis of this important borane reagent, with increased chemical purity. The present invention provides new, economical and convenient procedures for the preparation of catecholborane.
SUMMARY OF THE DISCLOSURE
New, economical and convenient procedures for the preparation of catecholborane in high chemical pure form using tri-O-phenylene bis borate, readily prepared from the reaction of catechol with boric acid, and diborane or borane-Lewis base complexes. This procedure utilizes only one molar equivalent of a boron-hydrogen bond per mole of catecholborane, instead of the three molar equivalents of boron-hydrogen bonds required by the present procedure. As used herein, the following terms have the following meanings:
“Borate 2” stands for tri-O-phenylene bis borate.
“Cyclic ethers” refers to ethers, such as tetrahydrofuran, tetrahydrofuran, dioxane and the like.
“Organic sulfides” refers to compounds such as dimethyl sulfide, diisoamyl sulfide and thioxane.
“Tertiary amines” refers to amines such as triethylamine, tert-butyldiethylamine, N,N-dimethylaniline and N-ethyl-N-isopropylaniline.
“Halogenated hydrocarbons” include both aliphatic and aromatic such as dichloromethane, 1,2-dichloroethane and chlorobenzene.
“Hydrocarbon solvents” include both aliphatic and aromatic such as n-heptane, toluene and tetralin, with boiling points in the range of 50-300° C.
“RB” refers to a round-bottom flask.
As mentioned in the prior art, the current popular preparation of catecholborane using the reaction of catechol with borane-methyl sulfide involves the wastage of two equivalents of active hydride. Also, the resulting excess evolution of hydrogen makes this process somewhat disadvantageous for large-scale preparations. In order to reduce the cost of catecholborane preparation and to avoid the formation of hydrogen gas side product, a modified synthesis of catecholborane was envisioned through the exchange of >B—H of diborane to >B—O— of tri-O-phenylene bis borate (hereafter called “borate 2”). This was examined according to the following equation.
The borate 2 can be conveniently prepared from inexpensive catechol and boric acid according to the reported procedures (Thomas, L. H.
J. Chem. Soc.,
1946, 820. Gerrard, W.; Lappert, M. F.; Mountfield, B. A.
J. Chem Soc.,
1959, 1529. Mehrotra, R. C.; Srivastava, G.
J. Chem Soc.,
1961, 4046).
Initially, the synthesis of catecholborane was attempted by the reaction of borate 2 (white solid, mp 102° C.) with diborane at room temperature. However, the reaction proceeds very slowly at room temperature, but the exchange was greatly accelerated at higher temperatures. The reaction proceeds very rapidly at 102° C., at which temperature the borate 2 melts and catecholborane was obtained in 80% yield after distillation. However, diborane gas is sensitive to high temperatures in the absence of Lewis bases and may result in unwanted higher boranes. This problem was addressed by using an inert solvent that dissolves borate 2 and does not interfere with the isolation procedure for catecholborane.
Accordingly, the borate 2 was taken in toluene and diborane gas was bubbled into this solution. The progress of the reaction was followed by
11
B NMR examination of the reaction mixture. Here also, the reaction of borate 2 with diborane is very slow at room temperature. It was greatly accelerated at higher temperatures (90-100° C.) and catecholborane was obtained in 85% yield
11
B NMR, +28.2 ppm, d). The reaction temperatures could be brought down considerably by performing the reaction in glymes, such as di-, tri-, or tetraglymes, in which borate 2 is somewhat soluble. Thus the borate 2 was taken in tetraglyme and diborane gas was passed into this mixture. The diborane gas readily reacts with the tetraglyme solution of borate 2 at 70° C., and catecholborane was formed in 85% yield and 15% of unreacted borate 2 was also observed (by
11
B NMR, +22.3 ppm). The catecholborane thus obtained was distilled out from tetraglyme conveniently in 83% yield with 98% chemical purity (by
11
B NMR, +28.2 ppm, d). The
11
B NMR examination of the residual tetraglyme showed only the presence of unreacted borate 2 (15%). To this tetraglyme solution, additional borate 2 (85%) can be added and treated with diborane (Scheme 1). This provides an ideal procedure for the synthesis of catecholborane.
The other glymes, such as diglyme, and triglyme, also give comparable results and preparation of catecholborane was carried out without problems with the isolation of catecholborane. However, monoglyme, a low boiling glyme has a boiling point close to that of catecholborane which complicates the isolation of catecholborane.
Further, this exchange of >B—H of borane to >B—O— of tri-O-phenylene bis borate can be also affected by using borane-Lewis bases as >B—H source, in solvents that are inert to borane and catecholborane.
Fhus, heating the borate 2 with borane-methyl sulfide in toluene to 100° C. (bath temperature) for 3 h provides clean catecholborane. The
11
B NMR showed the disappearance of the peak due to BMS at −20.2 ppm and the appearance of a new peak due to catecholborane at +28.2 ppm (doublet). It also showed some (10%) starting borate 2. This may be due to some loss of borane due to the higher reaction temperature. Using a sma
Lambkin Deborah C.
Sigma-Aldrich Co.
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