Organic compounds -- part of the class 532-570 series – Organic compounds – Chalcogen in the nitrogen containing substituent
Reexamination Certificate
1998-04-07
2001-06-19
Barts, Samuel (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Chalcogen in the nitrogen containing substituent
C564S463000
Reexamination Certificate
active
06248885
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides a novel class of borane adducts with selected trialkylamines, and more particularly relates to borane-trialkylamines wherein at least one alkyl is a branched chain C
4
-C
8
alkyl, preferably tert-butyl or tert-octyl, and their use in the hydroboration and reduction of organic compounds. The novel borane-amine adducts of this invention have a number of advantages over the presently available agents.
PRIOR ART
Borane adducts with amines are versatile reagents exhibiting many different properties as compared to the metal borohydrides. For example, they are soluble in a variety of solvents, including hydrocarbons or even water, and in some cases can be used in an acidic medium. Many adducts have been synthesized. See for example, Long, L. H. in W. J. Mellor
A Comprehensive Treatise on Inorganic and Theoretical Chemistry;
Longman: London, 1981, Supplement Vol. 5, Part B1, p 1.; and Meller, A. In Gmelin
Handbook of Inorganic and Organometallic Chemistry;
Springer: Berlin, 1992, 4th Supplement, Vol. 3, p 1. Several are commercially available. They find various uses, e.g., as fuel additives, polymerization catalysts, polymer stabilizers and stain removers, in metal plating and in the dye and pharmaceutical industries. See Lane, C. F.
Aldrichimica Acta
1973, 6, 51. Most of these applications are based on their reducing properties.
In contrast, the use of borane-amine adducts for hydroboration is rather limited due to strong complexation, which renders their reactivity low as compared to the weaker borane adducts with ethers and sulfides. For example, borane-triethylamine does not hydroborate 1-octene at room temperature and only very slowly in refluxing tetrahydrofuran (THF). See Brown, H. C. et al.
Inorganic Chem.
1984, 23, 2746.
Amines as borane carriers offer significant advantages often giving adducts of low sensitivity to moisture and air and readily soluble in representative solvents. Environmentally important is an easy recovery of the amine from the hydroboration products, making possible its ready recycling. The significance of these factors becomes apparent with the growing importance of diborane for the synthesis of pharmaceuticals and other valuable compounds. However, the well established reagents, borane-tetrahydrofuran and borane-dimethylsulfide (BMS) suffer a number of disadvantages for large-scale commercial applications as discussed below.
Borane-tetrahydrofuran is a valuable reagent for the hydroboration of olefins and for the reduction of organic compounds. It suffers from the disadvantage in that the solutions are unstable over a period of time. U.S. Pat. No. 3, 882, 037 discloses stabilized borane-tetrahydrofuran solutions which permit storage of such solutions for relatively longer periods of time. However, the inherent availability only as a relatively dilute solution in tetrahydrofuran poses a drawback to commercial use of this reagent.
Borane-methyl sulfide (BMS) is much more stable than borane-tetrahydrofuran and is widely used for both hydroboration and reduction [See Burg et al.,
J. Am. Chem. Soc.
76, 3307 (1954) and Coyle et al.,
J. Am. Chem. Soc.
81, 2989 (1959)]. However, it suffers from the serious disadvantage in that it yields a product which contains free dimethyl sulfide. The free dimethyl sulfide is highly volatile, b.p. 38° C., flammable and has a very noxious odor. Moreover, it is not soluble in water, so it cannot be disposed of by washing it away with water.
Borane-1, 4-thioxane (U.S. Pat. No. 4, 298, 750) is another valuable hydroboration agent. It has both lower volatility and milder odor than dimethyl sulfide. It has a limited solubility in water and can be easily oxidized to the corresponding sulfoxide, which is miscible in water. This agent is a liquid, 8 M in BH
3
, stable over prolonged periods. Unfortunately, this commercially available reagent is relatively costly compared to borane-tetrahydrofuran and borane-dimethyl sulfide.
The growing importance of borane reagents for the synthesis of pharmaceuticals and other compounds and the problems associated with other well established borane adduct hydroboration agents, e.g., low concentration and stability, high volatility, flammability, unpleasant odor, as discussed above, create a need for easy to handle, stable and environmentally benign hydroborating agents as discussed specifically below.
Thus, the search continues for effective, versatile borane derivatives which are as effective as the commercially available reagents but which overcome the disadvantages of noxious odor, expense, volatility, and lack of water solubility. The amines of the present invention are well suited for that purpose. They have an agreeable odor, form neat adducts, are highly concentrated in borane, are soluble in various solvents, and the amine can be readily removed and recovered from hydroboration products.
It is wholly surprising that the borane-trialkylamines of the present invention are valuable hydroborating agents since the ethyl derivative is a hindered amine widely used as a proton scavenger [Raber, D. J. et al.,
Tetrahedron Lett.,
4741 (1971)] but which forms a borane adduct that hydroborates 1-octene in tetrahydrofuran in two hours at room temperature. The compounds of this invention, on the other hand, hydroborate 1-octene in tetrahydrofuran in less than an hour, and in most cases in 15-30 minutes.
SUMMARY OF THE INVENTION
The present invention provides novel borane adducts of trialkylamines represented by the formula:
H
3
B.NRR
1
R
2
wherein R is a branched chain alkyl having 4 to 8 carbon atoms, R
1
and R
2
are the same or different straight or branched chain alkyl or alkoxy having from 1 to 5 carbon atoms, and B is boron.
Presently preferred compounds are those wherein R is tert-butyl or tert-octyl.
For ease of discussion, illustrative compounds prepared and tested are referred to with a number and letter designation as shown in the following table.
Compound
R
R
1
R
2
1a
t-Bu
—CH
2
CH
2
OCH
2
CH
2
—CH
2
CH
2
OCH
2
CH
2
1b
t-Bu
Et
Et
1c
t-Bu
Pr
n
Pr
n
1d
t-Bu
—CH
2
CH
2
OMe
—CH
2
CH
2
OMe
1e
t-Bu
Bu
i
Bu
i
2a
t-Bu
Me
Bu
i
2b
t-Bu
Me
Pr
i
2c
t-Bu
Et
Bu
i
2d
t-Bu
Pr
n
Bu
i
2e
t-Bu
Et
Pr
i
3a
t-Oct
Me
Me
3b
t-Oct
Et
Me
3c
t-Oct
—CH
2
CH
2
OCH
2
CH
2
—CH
2
CH
2
OCH
2
CH
2
3d
t-Oct
Et
Et
3e
t-Oct
Bu
i
Me
3f
t-Oct
Pr
n
Pr
n
Generally speaking, the novel compounds of this invention are conveniently prepared in a straight-forward procedure by passing diborane into a neat amine at 0° C. in a bubbler provided with a stirrer. Excess diborane not absorbed by the amine is absorbed in a down-stream bubbler containing tetrahydrofuran over mercury and cooled in ice water. A mercury bubbler is connected to the exit. Diborane is passed into the amine until the concentration of excess borane in THF reaches approximately 1 M. The borane-amine adduct is stirred overnight at room temperature prior to disconnecting the bubblers and analyzed for active hydride following the procedure described by Brown, H. C.,
Organic Syntheses via Boranes,
J. Wiley: New York, 1975, p. 191, using a 2 M hydrochloric acid-glycerol-water (2:1:1) hydrolysis solution.
In accordance with the invention, borane-amine adducts are prepared by passing diborane into a neat amine at temperatures in the range of 0° C. to 25° C. until no more diborane is absorbed. A solution of an amine in a suitable solvent, preferably diethyl ether, can also be used.
The complexing ability of the amines toward borane was tested by the exchange of BMS (borane-methylsulfide) and BH
3
.THF (borane- tetrahydrofuran) mixed in 1:1 molar ratio. The amount of borane taken by an amine in the equilibrium was determined by
11
B NMR and is shown in Tables 1, 2 and 3. Values for the exchange with borane-tetrahydrofuran, a 1 M solution, should be considered less quantitative since THF is in large excess.
The borane adducts of this invention are highly reactive, hydroborating 1-octene is tetrahydrofuran at room temperature in less than 1 hour. A number of the adducts are liquids above 0° C
Barts Samuel
Sigma-Aldrich Co.
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