Michael Hagel

Synthesis and Coordination Properties of 6,6-Dimesityl-2,2-bipyridine

A synthetic route to 6,6′-dimesityl-2,2′-bipyridine is presented that involves a Suzuki coupling of 6,6′-dibromo-2,2′-bipyridine with mesityl boronic acid. The new sterically crowded ligand is investigated by X-ray analysis and its coordination behavior in the presence of copper(I) is examined.

Enantioselective organic syntheses using chiral transition metal complexes V. (2S,3S)-Bis(dibenzophospholyl)butane, a rigid (S,S)-CHIRAPHOS analog

The P–Ph cleavage of phenyldibenzophosphole ( 1 ) with lithium in THF gives lithium dibenzophospholide ( 2 ). Reaction of 2 with ethyleneglycol ditosylate produces the known chelate ligand 1,2-bis(dibenzophospholyl)ethane ( 3 ) in good yield. Similarly, 2 and (2 R ,3 R )-butanediol ditosylate give the new chiral chelate ligand (2 S ,3 S )-bis(dibenzophospholyl)butane ( 4 ). Ligand exchange of [CpRu(PPh 3 ) 2 Cl] with 3 or 4 yields the halfsandwich complexes [CpRu(C 12 H 8 PC 2 H 4 PC 12 H 8 )Cl] ( 5 ) and [CpRu(( S , S )-C 12 H 8 PCHMeCHMePC 12 H 8 )Cl] ( 6 ). Complex 6 was characterized crystallographically (monoclinic, space group P 2 1 (no. 4), a =820.6(4), b =1501.0(3), c =1172.8(6) pm, β =108.87(2)°, V =1.367(1)×10 9 pm 3 , Z =2). The most conspicuous feature of the structure of 6 is the perfect coplanarity of the two dibenzophosphole moieties imposed by their steric interaction with the Cp ligand. Complex 6 and the thiophene complex [CpRu(( S , S )-C 12 H 8 PCHMeCHMePC 12 H 8 )(SC 4 H 4 )]BF 4 ( 7 ) derived therefrom are remarkably unreactive with regard to ligand substitutions. A possible explanation is the lack of intramolecular M⋯H–C stabilization en route to the transition state of ligand substitution. The enantiomeric purity of 6 and 7 could nevertheless be demonstrated by conversion to diastereomerically pure [CpRu(( S , S )-C 12 H 8 PCHMeCHMePC 12 H 8 )(( S )-CNCHMePh)]BF 4 ( 8 ).

Benzo[b]fluorenes Formed in the Thermal Cyclization of 3-Ene-1,6-diynes

A three-step preparation of the benzofluorene core is presented. The last step involves thermal cyclization of 3 -ene-1,6 -diyne (7) leading to the formation of four benzofluorene derivatives, one of which has been investigated by X-ray analysis. The harsh thermal conditions indicate that the cyclization of 7 might not proceed via a biradical intermediate as would be anticipated by a mechanistic proposal from Ueda.

Chiral Ruthenium–Sulfene Complexes – Synthesis and C–C Coupling Reactions

Reaction of the chiral racemic complex [CpRu(mppe)(SO2)]PF6 (1, mppe = Me2PC2H4PPh2) with diazomethane or -ethane gave the sulfene complexes [CpRu(mppe)(RHC/SO2)]PF6 (R = H, 2a; R = Me, 2b). Treatment of 2a with prochiral enamines or deprotonated β-oxo esters yielded C–C coupling products with 32–60% de. An analog of 2a, [NmcpRu(mppe)(H2C/SO2)]PF6 (8, Nmcp = neomenthylcyclopentadienyl) was prepared in a four-step synthesis starting from LiNmcp and [RuCl2(PPh3)3]. Repeated crystallization of the intermediate [NmcpRu(mppe)Cl] (6) provided diastereomerically pure 6′ which added methylene stereospecifically to give diastereomerically pure 8′. Compound 8 turned out to be much less reactive towards nucleophiles than 2a, but still added deprotonated ethyl 2-methyl-3-oxobutanoate with 44% de. The chiral, enantiomerically pure sulfur dioxide complex [CpRu(chir)(SO2)]PF6 [10, chir = (S,S)-Ph2PCHMeCHMePPh2] was synthesized from [CpRu(chir)Cl] and SO2 and was characterized by X-ray crystallography. Reaction of 10 with diazomethane gave the enantiomerically pure sulfene complex [CpRu(chir)(H2C/SO2)]PF6 (11). Addition reactions of 11 with N-(1-cyclopentenyl)morpholine, as well as with various enolates derived from β-oxo esters or 1,3-diesters proceeded with high yields and 20–90% de. The structure of a diastereomerically pure addition product, [CpRu(chir)(SO2CH2C(Me){C(O)Me}{C(O)OtBu}] (13d′), was determined crystallographically and was shown to have (R) configuration at the quaternary carbon atom. After alkylation of one of the S/O functions, the sulfinate ligand was cleaved from the metal center by ligand substitution with acetonitrile, and the resulting acetonitrile complex 15 was converted back into 10 by treatment with SO2.

Tuning the Steric and Electronic Properties of Chiral Rhenium Thiolate Complexes

Abstract Reaction of the complex [CpRe(NO)(CO)2]BF4 with triisopropylphosphine gives the chiral CO substitution product [CpRe(NO){P(i-Pr)3}(CO)]BF4. The corresponding triphenylphosphite complex [CpRe(NO){P(OPh)3}(CO)]BF4 may be obtained by oxidative CO removal. Reduction of the remaining CO ligand with NaBH4 furnishes the corresponding methyl complexes [CpRe(NO)(L)(CH3)]. The structure of [CpRe(NO){P(i-Pr)3}(CH3)] was determined: triclinic space group P1̄ (No. 2), a = 8.442(4), b = 9.582(5), c -11.820(8) Å, α= 81.81 (4), β = 87.18(4), γ = 63.87(5)°, Z = 2. Reaction of the methyl complexes with HBF4 in the presence of thiols gives, after chromatographic workup, the thiolate derivatives [CpRe(NO)(L)(SR)] (L = CO, P(OPh)3, P(i-Pr)3, R = CH2Ph, CH2(4-C6H4Cl), CH2(4-C6H4OMe), CH3, C2H5). The structure of [CpRe(NO){P(i-Pr)3}(SCH3)] was determined: monoclinic space group P21 (No. 3), a = 7.0515(7), b = 17.3469(10), c = 7.9727(7) Å, β = 114.021(7)°, Z = 2. In both structures, a significant opening of the angle N-Re-X (X = C, S) suggests that antibonding interactions between orbitals of the ligand X and the second-highest MO of the [CpRe(NO)(L)] complex fragment are avoided.

Synthesis and One-Electron Oxidation Chemistry of Stable β,β-Dimesityl Enols with Heteroaryl Substituents

Four novel stable enols (one characterized by X-ray crystal structure analysis) were synthesized and investigated under oxidative conditions to yield benzofurans. Depending on the donor qualities of the heteroaryl substituent the reaction following the one-electron oxidation could be stopped on the stage of the cyclohexadienyl cation whose lifetime was measured. Oxidation potentials were determined for the enols, the enolates and the α-carbonyl radicals. Oxidation of benzofurans yielded dimeric species or intramolecular cyclization products.

OXYFUNCTIONALIZATION OF ALLYL THIOETHER RUTHENIUM COMPLEXES WITH DIMETHYLDIOXIRANE

Abstract Allyl thioether complexes [CpRu(P-P)(SRR′)]PF6 (P-P = Ph2PCH2PPh2 (dppm), Ph2PC2H4PP2 (dppe), R = Me, Et, Ph, R′ = 3-propenyl, 3-cyclohexenyl, 2-methyl-2-buten-4-yl) and [CpRu(chir)(SRR′)]PF6 (chir = (S,S)-Ph2PCHMeCHMePPh2, R = Me, CH2Ph, R′ = 2-methyl-2-buten-4-yl) are obtained from the corresponding thiolate complexes by reaction with the appropriate allyl bromide. Careful oxidation with dimethyldioxirane (DMD) gave the allyl sulfoxide complexes [CpRu(P-P)(MeS(O)CH2CH=CH2)]PF6 (P-P = dppm, dppe). Double oxidation to the corresponding sulfinylmethyl epoxide complexes can be readily achieved with an excess of DMD. Oxidation of the chir complexes proceeds with only moderate diastereoselectivity. The structure of the (R, R/S, S) diastereomer of [CpRu(dppm)(MeS(O)CH2CHCH2O)]PF6 was determined: monoclinic space group P21/c (No. 14), a = 11.21(2), b = 16.762(9), c = 18.45(4) Å, β = 94.4(1)°, Z = 4. For a representative example, the decomplexation of the sulfoxide-epoxide ligand by sodium iodide in acetone was demonstrated.