Organotantalum chemistry explained

Organotantalum chemistry is the chemistry of chemical compounds containing a carbon-to-tantalum chemical bond. A wide variety of compound have been reported, initially with cyclopentadienyl and CO ligands. Oxidation states vary from Ta(V) to Ta(-I).

Classes of organotantalum compounds

Alkyl and aryl complexes

Pentamethyltantalum was reported by Richard Schrock in 1974.

Salts of [Ta(CH₃)₆]⁻ are prepared by alkylation of TaF₅ using methyl lithium:^[1]

TaF₅ + 6 LiCH₃ → Li[Ta(CH₃)₆] + 5 LiF

Alkylidene complexes

Tantalum alkylidene complexes arise by treating trialkyltantalum dichloride with alkyl lithium reagents. This reaction initially forms a thermally unstable tetraalkyl-monochloro-tantalum complex, which undergoes α-hydrogen elimination, followed by alkylation of the remaining chloride.^[2]

Tantalum alkylidene complexes are nucleophilic.^[2] They effect a number of reactions including: olefinations, olefin metathesis, hydroaminoalkylation of olefins, and conjugate allylation of enones. Ethylene, propylene, and styrene react with tantalum alkylidene complexes to yield olefin metathesis products.^[3]

Cyclopentadienyl complexes

Some of the first reported organotantalum complexes were cyclopentadienyl derivatives. These arise from the salt metathesis reactions of sodium cyclopentadienide and tantalum pentachloride. An example of this is the first transition metal trihydride, Cp₂TaH₃. More soluble and better developed are derivatives of pentamethylcyclopentadiene such as Cp*TaCl₄, Cp*₂TaCl₂, and Cp*₂TaH₃.^[4]

Tantalum carbonyls and isocyanides

Reduction of TaCl₅ under an atmosphere of CO gives the salts of [Ta(CO)₆]⁻.^[5] These same anions can be obtained by carbonylation of tantalum arene complexes.

A number of tantalum isocyanide complexes are also known.^[6]

Tantalum arenes and alkyne complexes

Treatment of tantalum pentachloride with hexamethylbenzene (C₆Me₆), aluminium, and aluminium trichloride gives [M(η⁶-C₆Me₆)AlCl₄]₂.^[7]

Tantalum-alkyne complexes^[8] catalyze cyclotrimerizations.^{[9] [10]} Some tantalum-alkyne complexes are precursors to allylic alcohols.^[11] Tantalacyclopropenes are invoked as intermediates.

Tantalum-amido complexes

Organotantalum compounds are invoked as intermediates in C-alkylation of secondary amines with 1-alkenes using Ta(NMe₂)₅.^[12] The chemistry developed by Maspero was later brought to fruition when Hartwig and Herzon reported the hydroaminoalkylation of olefins to form alkylamines:^[13]

The catalytic cycle may proceed by β-hydrogen abstraction of the bisamide, which forms the metallaaziridine. Subsequent olefin insertion, protonolysis of the tantalum-carbon bond, and β-hydrogen abstraction affords the alkylamine product.^{[14] [15] [16]}

Transmetalation

Organotantalum reagents arise via transmetalation of organotin compounds with tantalum(V) chloride.^[17] These organotantalum reagents promote the conjugate allylation of enones. Although the direct allylation of carbonyl groups is prevalent throughout the literature, little has been reported on the conjugate allylation of enones.^[18]

Applications

Organotantalum compounds are of academic interest, but few or no commercial applications have been described.

Notes and References

1. Kleinhenz, S. . Pfennig, V. . Seppelt, K. . Chem. Eur. J.. 1998. 4. 1687. 10.1002/(SICI)1521-3765(19980904)4:9<1687::AID-CHEM1687>3.0.CO;2-R. Preparation and Structures of [W(CH₃)₆], [Re(CH₃)₆], [Nb(CH₃)₆]⁻, and [Ta(CH₃)₆]⁻. 9.
2. Schrock. Richard R.. 1979-03-01. Alkylidene complexes of niobium and tantalum. Accounts of Chemical Research. 12. 3. 98–104. 10.1021/ar50135a004. 0001-4842.
3. McLain. S. J.. Wood. C. D.. Schrock. R. R.. 1977-05-01. Multiple metal-carbon bonds. 6. The reaction of niobium and tantalum neopentylidene complexes with simple olefins: a route to metallocyclopentanes. Journal of the American Chemical Society. 99. 10. 3519–3520. 10.1021/ja00452a064. 0002-7863.
4. Book: Inorg. Synth.. 36. Endy Y.-J. Min . John E. Bercaw . Bis(η ⁵ -Pentamethylcyclopentadienyl) Complexes of Niobium and Tantalum . Inorganic Syntheses: Volume 36. 52–57. 2014. 10.1002/9781118744994.ch11. 978-1-118-74499-4.
5. Book: Tris[Bis(2-Methoxyethyl)Ether]Potassium and Tetraphenylarsonium Hexacarbonylmetallates(1–) of Niobium and Tantalum. 16. J. E. Ellis . A. Davison . Tris[Bis(2-Methoxyethyl)Ether]Potassium and Tetraphenylarsonium Hexacarbonylmetallates(1-) of Niobium and Tantalum. 68–73. 10.1002/9780470132470.ch21. 1976. Inorganic Syntheses. 978-0-470-13247-0.
6. 10.1107/S2053229619000834. Tantalum isocyanide complexes: TaI(CNDipp)6 (Dipp is 2,6-diisopropylphenyl) and ionic [Ta(CNDipp)7][Ta(CNDipp)6], a formal disproportionation product of the 17-electron Ta0 metalloradical Ta(CNDipp)6. 2019. Brennessel. William W.. Romanenkov. Alexander. Young. Victor G.. Ellis. John E.. Acta Crystallographica Section C: Structural Chemistry. 75. 2. 135–140. 30720451. 73450348 .
7. Pampaloni, G. . Aromatic hydrocarbons as ligands. Recent advances in the synthesis, the reactivity and the applications of bis(η6-arene) complexes. Coordination Chemistry Reviews. 2010. 254. 5–6. 402–419. 10.1016/j.ccr.2009.05.014.
8. Labinger. Jay A.. Schwartz. Jeffrey. Townsend. John M.. 1974-06-01. Iodo- and hydridotantalum(III) complexes of dialkylacetylenes. Journal of the American Chemical Society. 96. 12. 4009–4011. 10.1021/ja00819a047. 0002-7863.
9. Cotton. F. Albert. Hall. William T.. 1979-08-01. Reactions of tantalum(III) with alkynes and nitriles. Journal of the American Chemical Society. 101. 17. 5094–5095. 10.1021/ja00511a064. 0002-7863.
10. Bruck. M. A.. Copenhaver. A. S.. Wigley. D. E.. 1987-10-01. Alkyne cyclizations at reduced tantalum centers: synthesis and molecular structure of (.eta.6-C6Me6)Ta(O-2,6-i-Pr2C6H3)2Cl. Journal of the American Chemical Society. 109. 21. 6525–6527. 10.1021/ja00255a056. 0002-7863.
11. Takai. Kazuhiko. Kataoka. Y.. Utimoto. K.. 1990-03-01. Tantalum-alkyne complexes as synthetic intermediates. Stereoselective preparation of trisubstituted allylic alcohols from acetylenes and aldehydes. The Journal of Organic Chemistry. 55. 6. 1707–1708. 10.1021/jo00293a008. 0022-3263.
12. Clerici. Mario G.. Maspero. Federico. 1980-01-01. Catalytic C-Alkylation of Secondary Amines with Alkenes. Synthesis. en. 1980. 4. 305–306. 10.1055/s-1980-29002. 94579838 . 0039-7881.
13. Herzon. Seth B.. Hartwig. John F.. 2007-05-01. Direct, Catalytic Hydroaminoalkylation of Unactivated Olefins with N-Alkyl Arylamines. Journal of the American Chemical Society. 129. 21. 6690–6691. 10.1021/ja0718366. 0002-7863. 2590937. 17474747.
14. Eisenberger. Patrick. Ayinla. Rashidat O.. Lauzon. Jean Michel P.. Schafer. Laurel L.. 2009-10-19. Tantalum–Amidate Complexes for the Hydroaminoalkylation of Secondary Amines: Enhanced Substrate Scope and Enantioselective Chiral Amine Synthesis. Angewandte Chemie International Edition. en. 48. 44. 8361–8365. 10.1002/anie.200903656. 19787670. 1521-3773.
15. Dörfler. Jaika. Doye. Sven. 2014-05-01. A Commercially Available Tantalum Catalyst for the Highly Regioselective Intermolecular Hydroaminoalkylation of Styrenes. European Journal of Organic Chemistry. en. 2014. 13. 2790–2797. 10.1002/ejoc.201400082. 1099-0690.
16. Payne. Philippa R.. Garcia. Pierre. Eisenberger. Patrick. Yim. Jacky C.-H.. Schafer. Laurel L.. 2013-05-03. Tantalum Catalyzed Hydroaminoalkylation for the Synthesis of α- and β-Substituted N-Heterocycles. Organic Letters. 15. 9. 2182–2185. 10.1021/ol400729v. 23600625. 1523-7060.
17. Shibata. Ikuya. Kano. Takeyoshi. Kanazawa. Nobuaki. Fukuoka. Shoji. Baba. Akio. 2002-04-15. Generation of Organotantalum Reagents and Conjugate Addition to Enones. Angewandte Chemie. en. 114. 8. 1447–1450. 10.1002/1521-3773(20020415)41:8<1389::AID-ANIE1389>3.0.CO;2-D. 19750774. 1521-3757.
18. Yamamoto. Yoshinori. Asao. Naoki. 1993-09-01. Selective reactions using allylic metals. Chemical Reviews. 93. 6. 2207–2293. 10.1021/cr00022a010. 0009-2665.