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2.12.16 Organometallic Complexes of Scandium, Yttrium, and the Lanthanides (Update 2013)
J. Hannedouche 2.12.16.1 Rare Earth Metal Catalyzed Hydroamination Reactions
This section deals with the syntheses and catalytic applications of rare-earth complexes with oxidation state +2 or +3 in the direct addition of an amine onto an unactivated carbon–carbon triple or double bond, the so-called hydroamination reaction. The term “rare earth” refers to the group 3 metal elements including scandium, yttrium, and the lanthanide series from lanthanum to lutetium, and is abbreviated Ln. This section is not intended to comprehensively review all title complexes of the product subclasses but only those which find applications as catalysts in the hydroamination reaction. Typical complexes of the product subclass contain at least one kinetically labile, s-bonded bis(trimethylsilyl)amido, bis(dimethylsilyl)amido, diisopropylamido, bis(trimethylsilyl)methyl, trimethylsilyl, or methyl ligand which, under the hydroamination reaction conditions, is promptly protonated by the amino substrate to generate a new metal amide species. This species will further react with the alkyne or the alkene functionality through a s-insertive or noninsertive mechanism to deliver the hydroamination product.[1–6] With few exceptions, the relative reactivity of rare-earth complexes in hydroamination is poorly influenced by the nature of the s-bonded ligand [except for the less basic bis(dimethylsilyl)amido ligand][7,8] and is mainly governed by the ionic size of the metal and the steric/electronic properties of the ancillary ligand(s). The metal ionic radius increases going from scandium (the smallest) to lanthanum (the largest), and from lutetium to cerium.[9] Due to the higher reactivity and electron density of alkynes relative to alkenes, the hydroamination of alkynes is more readily achieved than that of alkenes. As a general trend, the rate of cyclohydroamination reaction is consistent with classical, stereoelectronically controlled cyclization processes; strictly speaking, the rate of formation of five-membered rings is higher than that of six- and, to a higher extent, seven-membered rings. Almost all of the complexes described in this product class should be synthesized, handled, and stored under an inert atmosphere using Schlenk or glovebox techniques. All solvents should be dried and degassed prior to use. With some exceptions, most of the catalytic applications are conducted in an NMR tube under inert atmosphere, and the reported yields are determined by NMR spectroscopy or gas chromatography using an internal standard. The hydroamination reactions are performed in noncoordinative aliphatic or aromatic solvents. The relevant literature up until mid-2012 has been covered. 2.12.16.1.1 Rare-Earth(II) Complexes
Rare-earth complexes in oxidation state +2 are much less widely explored for catalytic hydroamination than those in the +3 oxidation state despite there being convenient routes to synthesize such lower-oxidation-state complexes. The most successful approach to these divalent complexes is salt metathesis of tetrahydrofuran-solvated ytterbium(II), europium(II), and samarium(II) iodide with potassium reagents. Although poorly investigated, ytterbium(II) and samarium(II) complexes nevertheless demonstrate the ability to catalyze the intramolecular hydroamination of alkynes and alkenes. Under the catalytic conditions, oxidation of the divalent lanthanide complexes to trivalent species is postulated. 2.12.16.1.1.1 Synthesis of Rare-Earth(II) Complexes 2.12.16.1.1.1.1 Method 1: Salt Metathesis Tetrahydrofuran-solvated europium(II) and ytterbium(II) iodide are reacted with potassium complex 1 and potassium hexamethyldisilazanide in a 1:1:1 molar ratio (? Scheme 1).[10] After removal of potassium iodide by filtration and crystallization, bis(tetrahydrofuran)-solvated {[7-(isopropylimino)cyclohepta-1,3,5-trienyl]amido}ytterbium(II) 2 (Ln = Yb) and its europium(II) analogue are obtained as tiny brown (36% yield) and red crystals (17% yield), respectively. The use of iodide and potassium reagents in the course of the syntheses avoids coordination of lighter alkali halides such as lithium chloride. An analogous procedure is applied for the preparation of bis(phosphorimidoyl)-methanide–ytterbium(II) iodide complex 4 as red crystals from potassium salt 3.[11] Complexes 2 and 4 are characterized by standard analytical and spectroscopic techniques, and their solid-state structures have been established by single-crystal X-ray diffraction. Solid-state analysis of complex 4 reveals that the ytterbium center is six coordinated with a long methanide carbon—metal bond. Bis(?5-pentamethylcyclopentadienyl)bis(tetrahydrofuran)samarium(II) (5) is prepared by a metathetic reaction between diiodobis(tetrahydrofuran)samarium(II) and potassium pentamethylcyclopentadienide.[12] Recrystallization from a tetrahydrofuran solution affords large purple crystals of a disolvate. X-ray crystallographic analysis of complex 5 confirms the structure typical of bent metallocenes. ? Scheme 1 Syntheses of Ytterbium(II), Europium(II), and Organosamarium(II) Complexes[10–12] Ln Yield (%) Ref Eu 17 [10] Yb 36 [10] [Bis(trimethylsilyl)amido]{isopropyl[7-(isopropylimino)cyclohepta-1,3,5-trienyl]amido}bis(tetrahydrofuran)ytterbium(II)(2, Ln = Yb); Typical Procedure:[10] THF was condensed at -196 °C onto a mixture of YbI2(THF)2 (0.5 mmol), complex 1 (0.121 g, 0.5 mmol), and KHMDS (0.100 g, 0.5 mmol). The mixture was then stirred for 36 h at rt. The red soln was filtered to remove KI, and then the solvent was removed under reduced pressure. Finally, the remaining powder was washed with pentane and crystallized (THF/pentane 1:3) to give tiny brown crystals; yield: 0.110 g (36%). Bis(?5-pentamethylcyclopentadienyl)bis(tetrahydrofuran)samarium(II) (5):[12] K(Cp*) (5.43 g, 31.2 mmol) was added to a stirred soln of SmI2(THF)2 (7.78 g, 14.2 mmol) in THF (75 mL) in a 125-mL Erlenmeyer flask. The color of the soln rapidly changed from blue-green to purple as off-white solids (KI) were formed. After 4 h at rt, the THF was removed by rotary evaporation and toluene (100 mL) was added. The resulting soln of product 5 with suspended potassium salts was stirred vigorously for 10 h and then filtered. The solvent was removed from the filtrate by rotary evaporation, leaving solid (Cp*)2Sm(THF)n (1 = n = 2). The degree of solvation was conveniently monitored by integration of the absorptions in the NMR spectrum in benzene-d6. Dissolving this solid in THF and then removing the solvent by rotary evaporation gave product 5; yield: 5.95 g (74%). Recrystallization (sat. THF soln at 30 °C, cooled to -25 °C overnight) gave large purple crystals; yield: 5.52 g in two crops (69%). 2.12.16.1.1.2 Applications of Rare-Earth(II) Complexes in Organic Synthesis 2.12.16.1.1.2.1 Method 1: Catalytic Intramolecular Hydroamination Reaction of Alkynes Well-defined [bis(phosphorimidoyl)methanido]ytterbium(II) iodide complex 4 (see ? Section 2.12.16.1.1.1.1) is applied as catalyst for the cyclohydroamination of pent-4-yn-1-amine and 5-phenylpent-4-yn-1-amine under mild to harsh conditions. In the presence of catalytic amounts of complex 4, pent-4-yn-1-amine and 5-phenylpent-4-yn-1-amine undergo highly regioselective cyclization to give 3,4-dihydro-2H-pyrrole derivatives 6 as the sole product (? Scheme 2). The best turnover frequency is reached with 2.8 mol% of complex 4 for the reaction of pent-4-yn-1-amine at 120 °C. Sluggish activity is observed at room temperature. A color change of the reaction mixture from red to yellow at the initial stage of the catalysis indicates in situ oxidation of ytterbium(II) to ytterbium(III). ? Scheme 2 Catalytic Cyclohydroamination of Pent-4-yn-1-amine and 5-Phenylpent-4-yn-1-amine[11] R1 mol% of 4 Conditions TOFa (h-1) Yieldb (%) Ref H 2.8 120 °C, 3 h 11.9 >95 [11] H 1.4 60 °C, 192 h 0.22 60 [11] Ph 5.3 120 °C, 6 h 3.15 >95 [11] Ph 5.3 60 °C, 108 h 0.14 80 [11] a TOF = turnover frequency. b Determined by 1H NMR spectroscopy. 3,4-Dihydro-2H-pyrroles 6; General Procedure:[11] Tetrahydrofuran-solvated...
