1,2-Dimethyldiborane Explained

1,2-Dimethyldiborane is an organoboron compound with the formula [(CH<sub>3</sub>)BH<sub>2</sub>]2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alkylborane.[1] 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement.[2] 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.

An isomer of 1,2-dimethyldiborane is 1,1-dimethyldiborane, known as unsymmetrical dimethyldiborane, which has two methyl groups on one boron atom. Other methylated versions of diborane including methyldiborane, trimethyldiborane, tetramethyldiborane. Trimethylborane exists as a monomer.

Preparation

Methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.[3] [4]

In a more modern synthesis, 1,2-dimethyldiborane is produced by treating lithium methylborohydride with hydrogen chloride:[1]

2 LiCH3BH3 + 2 HCl → (CH3BH2)2 + 2 H2 + 2 LiClInstead of hydrogen chloride, methyl iodide or trimethylsilyl chloride can be used.

Lithium methylborohydride can be made by treating methylboronic esters with lithium aluminium hydride.[5]

Miscellaneous routes

Methylboranes arise the reaction of diborane and trimethylborane. This reaction produces 1-methyldiborane, 1,1-dimethyldiborane, 1,1,2-trimethyldiborane, and 1,1,2,2-tetramethyldiborane. By treating monomethyldiborane with ether, dimethyl ether borane (CH3)2O.BH3 leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane.[6] The reaction is complex.

Tetramethyl lead reacts with diborane to give a range of methyl-substituted diboranes, ending up at trimethylborane, but including 1,1-dimethyldiborane, and trimethyldiborane. Other products are hydrogen gas and lead metal.[7]

Other methods to form methyldiboranes include treating hydrogen with trimethylborane between 80 and 200 °C under pressure, or treating a metal borohydride with trimethylborane in the presence of hydrogen chloride, aluminium chloride or boron trichloride. If the borohydride is sodium borohydride, then methane is a side product. If the metal is lithium, then no methane is produced.[3] dimethylchloroborane and methyldichloroborane are also produced as gaseous products.[3]

When Cp2Zr(CH3)2 reacts with diborane, a borohydro group inserts into the zirconium-carbon bond, and methyl diboranes are produced.[8]

In ether dimethylcalcium reacts with diborane to produce dimethyldiborane and calcium borohydride:[9]

Ca(CH3)2 + 2 B2H6 → Ca(BH4)2 + B2H4(CH3)2

1,2-Dimethyldiborane is produced by the room temperature disproportionation of trimethyldiborane.[10]

Physical and spectroscopic properties

cis-1,2-Dimethyldiborane melts at −132.5 °C; trans-1,2-dimethyldiborane melts at −102 °C.[11] The cis-1,2-dimethyldiborane molecule has point group Cs. A trans-1,2-dimethyldiborane molecule has point group C2. Unsymmetrical dimethyldiborane melts at −150.2 °C.[12] Vapour pressure is approximated by Log P = 7.363−(1212/T).[12] The vapour pressure for the symmetrical isomer is given by Log P = 7.523−(1290/T).[12]

Gas chromatography can be used to determine the amounts of the methyl boranes in a mixture. The order of elution are: diborane, monomethyldiborane, trimethylborane, 1,1-dimethyldiborane, 1,2-dimethyldiborane, trimethyldiborane, and last tetramethyldiborane.[13]

The nuclear resonance shift for the bridge hydrogen is 9.55 ppm for the unsymmetrical isomer and 9.73 ppm for the symmetrical isomers, compared to 10.49 for diborane.[14]

Reactions

Methylborane shows little tendency to disproportionate (redistribute) at room temperature. It reacts stepwise with alkenes to produce mono and dialkylmethylboranes. More methylated boranes are less stable.[5]

1,2-Dimethyldiborane slowly converts to 1,1-dimethyldiborane.[15]

Methylborane hydrolyzes to methylboronic acid:[6]

(MeBH2)2 + 4 H2O → CH3B(OH)2 + 4 H2Symmetrical dimethyldiborane reacts with trimethylamine to yield a solid adduct trimethylamine-methylborane (CH3)3N·BH2CH3.[6]

When dimethyldiborane is combined with ammonia and heated, B-methyl borazoles are produced. These borazoles can have one, two or three methyl groups substituted on the boron atoms.[16] [17]

Under normal conditions dimethyldiborane does not react with hydrogen.[18]

Related species

Extra reading

Notes and References

  1. Srebnik. Morris. Cole. Thomas E.. Brown. Herbert C.. Methylborane - a remarkable unhindered monoalkylborane which achieves the controlled sequential hydroboration of representative alkenes. Tetrahedron Letters. January 1987. 28. 33. 3771–3774. 10.1016/s0040-4039(00)96380-9.
  2. Low. M. J. D.. Characteristic Infrared Frequencies of Methyldiboranes. The Journal of Chemical Physics. 1968. 48. 5. 2386–2387. 10.1063/1.1669454. 1968JChPh..48.2386L.
  3. Long. L. H.. Wallbridge. M. G. H.. 646. The Chemistry of Boron. Part VI. New Preparative Methods and Decomposition Studies Relating to Methyldiboranes. Journal of the Chemical Society (Resumed). 1965. 3513–3520. 10.1039/JR9650003513.
  4. Schlesinger. H. I.. Walker. A. O.. Hydrides of Boron. IV. The Methyl Derivatives of Diborane. Journal of the American Chemical Society. April 1935. 57. 4. 621–625. 10.1021/ja01307a009.
  5. Brown. Herbert C.. Cole. Thomas E.. Srebnik. Morris. Kim. Kee Won. Hydroboration. 79. Preparation and Properties of Methylborane and Dimethylborane and Their Characteristics as Hydroborating Sgents. Synthesis of Tertiary Alcohols Containing Methyl Groups via Hydroboration. The Journal of Organic Chemistry. December 1986. 51. 25. 4925–4930. 10.1021/jo00375a031.
  6. Bell. R. P.. Emeléus. H. J.. The Boron Hydrides and Related Compounds. Quarterly Reviews, Chemical Society. 1948. 2. 2. 132. 10.1039/QR9480200132.
  7. Holliday. A.K.. N. Jessop. G.. The Reaction of Tetramethyllead with Diborane. Journal of Organometallic Chemistry. November 1967. 10. 2. 291–293. 10.1016/s0022-328x(00)93089-4.
  8. Marsella. John A.. Caulton. Kenneth G.. Dealkylation of Zirconium(IV) by Borane: the Intimate Mechanism of an Alkyl Transfer Reaction. Journal of the American Chemical Society. May 1982. 104. 9. 2361–2365. 10.1021/ja00373a005.
  9. Book: James. B. D.. Wallbridge. M. G. H.. Lippard. Stephen J.. Progress in Inorganic Chemistry, Volume 11. 1970. Wiley. 0471-54081-1. 185. https://books.google.com/books?id=8N6nFvNzfv0C&pg=PA185. Metal Tetrahydroborates.
  10. Lehmann. Walter J.. Wilson. Charles O.. Shapiro. I.. Infrared Spectra of Alkyldiboranes. V. Tri- and Tetramethyl- and Ethyldiboranes. The Journal of Chemical Physics. 1961. 34. 3. 783. 10.1063/1.1731675. 1961JChPh..34..783L.
  11. Hedberg. Lise. Hedberg. Kenneth. Kohler. David A.. Ritter. David M.. Schomaker. Verner. Electron-diffraction investigations of the molecular structures of cis- and trans-1,2-dimethyldiborane. Journal of the American Chemical Society. May 1980. 102. 10. 3430–3434. 10.1021/ja00530a021.
  12. Book: Onak. Thomas. Stone. F. G. A.. West. Robert. Advances in Organometallic Chemistry. 1 January 1966. Academic Press. New York, London. 284. 14 August 2015. 9780080580043.
  13. Seely. G. R.. Oliver. J. P.. Ritter. D. M.. Gas-Liquid Chromatographic Analysis of Mixtures Containing Methyldiboranes. Analytical Chemistry. December 1959. 31. 12. 1993–1995. 10.1021/ac60156a032.
  14. Leach. John B.. Ungermann. Charles B.. Onak. Thomas P.. Proton magnetic resonance studies on methyl and chloro substituted diboranes. Journal of Magnetic Resonance. January 1972. 6. 1. 74–83. 10.1016/0022-2364(72)90088-1. 1972JMagR...6...74L.
  15. Lehmann. Walter J.. Wilson. Charles O.. Shapiro. I.. Infrared Spectra of Alkyldiboranes. III. 1,2-Dimethyl- and 1,2-Diethyldiboranes. The Journal of Chemical Physics. 1960. 33. 2. 590. 10.1063/1.1731190. 1960JChPh..33..590L.
  16. Sheldon. J. C.. Smith. B. C.. The borazoles. Quarterly Reviews, Chemical Society. 1960. 14. 2. 202. 10.1039/QR9601400200.
  17. Schlesinger. H. I.. Horvitz. Leo. Burg. A. B.. Hydrides of Boron. VI. The Action of Ammonia on the Methyl Diboranes. Journal of the American Chemical Society. March 1936. 58. 3. 409–414. 10.1021/ja01294a008.
  18. Book: Adams. Roy M. . Metal-Organic Compounds . September 1959. 23 . 92. 10.1021/ba-1959-0023.ch010. 17 August 2015. https://www.thevespiary.org/rhodium/Rhodium/Vespiary/talk/files/4262-AIC-023-METAL-ORGANIC-COMPOUNDS-13db1.pdf?topic=2297.0. Advances in Chemistry. 0-8412-0024-6. Organoboron Compounds .
  19. Mikhailov. B. M.. The Chemistry Of Diborane. Russian Chemical Reviews. April 1962. 31. 4. 209. 1962RuCRv..31..207M. 10.1070/RC1962v031n04ABEH001281. 250909492 .
  20. Deever. William R.. Ritter. David M.. Methyltetraboranes. I. 2-Methyl and 1,2-, 2,2-, and 2,4-dimethyl derivatives. Inorganic Chemistry. November 1969. 8. 11. 2461–2467. 10.1021/ic50081a043.
  21. Book: Addison. C. C.. Inorganic Chemistry of the Main-Group Elements. Davidson. G. . 1973. 1. 68. 10.1039/9781847556370-00053. http://pubs.rsc.org/en/content/chapter/bk9780851867526-00053/978-0-85186-752-6. 978-0-85186-752-6. Elements of Group III.
  22. Onak. Thomas. Friedman. Lawrence B.. Hartsuck. Jean A.. Lipscomb. William N.. Rearrangement of 1,2- to 2,3-Dimethylpentaborane(9). Journal of the American Chemical Society. July 1966. 88. 14. 3439–3440. 10.1021/ja00966a051.
  23. Book: Shore. S. G.. Muetterties. Earl L. Boron Hydride Chemistry. 1975. Academic Press. 0-12-509650-X. 150. https://books.google.com/books?id=hLUy6or0In8C&pg=PA150. Nido and Arachno Boron Hydrides.
  24. Dunstan. I.. Williams. R. L.. Blay. N. J.. 970. Boron hydride derivatives. Part V. Nucleophilic substitution in decaborane. Journal of the Chemical Society. 1960. IV. 5012–5015. 10.1039/JR9600005012. 19 August 2015.

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