Tributylgermanium Hydride Research Articles

Reactions of [Os3(CO)10(μ-dppm)] and [HOs3(CO)8{μ3-Ph2PCH2P(Ph)C6H4}] with Bu3GeH: Ge–H and Ge–C bond cleavage at triosmium centers

The reactivity of the triosmium clusters [Os3(CO)10(μ-dppm)] [dppm = bis(diphenylphosphino)methane] and [HOs3(CO)8{μ3-Ph2PCH2P(Ph)C6H4}] with tributylgermanium hydride (Bu3GeH) has been investigated. Oxidative addition of Bu3GeH to [Os3(CO)10(μ-dppm)] at 110 °C affords [HOs3(CO)9(GeBu3)(μ-dppm)] (1) which undergoes further decarbonylation upon prolonged heating to yield [H2Os3(CO)7(GeBu3){μ3-Ph2PCH2P(Ph)C6H4}] (2) and [HOs3(CO)8(GeBu3)(μ-GeBu2)(μ-dppm)] (3). Control experiments confirm the bimodal formation of 2 and 3 from 1 during thermolysis. Cluster 2 reacts with additional Bu3GeH at 140 °C to give [Os3(CO)6(μ-GeBu2)2{μ3-PhP(C6H4)CH2P(Ph)C6H4}] (4), a thermolysis product also produced from 3. In contrast, oxidative addition of Bu3GeH to [HOs3(CO)8{μ3-Ph2PCH2P(Ph)C6H4}] occurs at room temperature to afford [H2Os3(CO)8(GeBu3){μ3-Ph2PCH2P(Ph)C6H4}] (5), which decarbonylates at 110 °C to yield 4. All new clusters have been characterized by elemental analyses and spectroscopic methods, and the molecular structures established by X-ray crystallography. Electronic structure calculations on the different hydride isomers based on 1 and 5 are reported.

Radical Reduction of Aromatic Azides to Amines with Tributylgermanium Hydride.

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Aromatic homolytic substitution using solid phase synthesis

Solid phase synthesis has been used to carry out intramolecular aromatic homolytic substitution with benzoimidazole precursors. The protocol attaches the radical precursors to the resins via the radical leaving groups (in the aromatic homolytic substitution). When the radical reactions are complete, the leaving group, unaltered starting material and reduced uncylised products remain attached to the resin, which facilitates easy separation of the cyclised products. Novel use of focussed microwave irradiation in solid phase radical reactions drastically shortens the reactions times. Tributylgermanium hydride has been used to replace the toxic and troublesome tributyltin hydride in the radical reactions.

Radical Reduction of Aromatic Azides to Amines with Tributylgermanium Hydride

[reaction: see text] Aromatic azides are inert toward tributylgermanium hydride under thermal conditions in the absence and in the presence of a radical initiator but in the presence of catalytic amounts of benzenethiol undergo fast reaction, yielding reduced anilines and 2-germylated derivatives in high overall yields.

Synthetic applications of aryl radical building blocks for cyclisation onto azoles

2-(2-Bromophenyl)ethyl groups have been used as building blocks in radical cyclisation reactions onto azoles to synthesise tri- and tetra-cyclic heterocycles. 2-(2-Bromophenyl)ethyl methanesulfonate was used to alkylate azoles (imidazoles, pyrroles, indoles and pyrazoles) for the synthesis of the radical precursors. Cyclisations of the intermediate aryl radicals yield new 6-membered rings attached to the azoles. The aryl radicals undergo intramolecular homolytic aromatic substitution onto the azole rings. Tributylgermanium hydride has been used with success to replace the toxic and troublesome tributyltin hydride. Initial studies show that the protocol can be used on solid phase resins. The molecular and crystal structures of methyl 5,6-dihydroimidazo[5,1- a]iso-quinoline-1-carboxylate and methyl 5,6-dihydroimidazo[2,1- a]isoquinoline-3-carboxylate were determined by X-ray crystallography.

Gas-phase reactivity of charged pi-type biradicals.

Four pi,pi-biradicals, 2,6-dimethylenepyridinium and the novel isomers N-(3-methylenephenyl)-3-methylenepyridinium, N-phenyl-3,5-dimethylenepyridinium, and N-(3,5-dimethylenephenyl)pyridinium ions, were generated and structurally characterized in a Fourier transform ion cyclotron resonance mass spectrometer. Their gas-phase reactivity toward various reagents was compared to that of the corresponding monoradicals, 2-methylenepyridinium, N-phenyl-3-methylenepyridinium, and N-(3-methylenephenyl)pyridinium ions. The biradicals reactivity was found to reflect their predicted multiplicity. The 2,6-dimethylenepyridinium ion, the only biradical in this study predicted to have a closed-shell singlet ground state, reacts significantly faster than the other biradicals, which are predicted to have triplet ground states. In fact, this biradical reacts at a higher rate than the analogous monoradical, which suggests that to avoid the costly uncoupling of its unpaired electrons, the biradical favors ionic mechanisms over barriered radical pathways. In contrast, the second-order reaction rate constants of the isomeric biradicals with triplet ground states are well approximated by those of the analogous monoradicals, although the final reaction products are sometimes different. This difference arises from rapid radical-radical recombination of the initial monoradical reaction products. The overall reactivity toward the hydrogen-atom donors benzeneselenol and tributylgermanium hydride is significantly greater for the radicals with the charged site in the same ring system as the radical site. This finding indicates that polar effects play an important role in controlling the reactivity of pi,pi-biradicals, just as has been demonstrated for sigma,sigma-biradicals.

Tributylgermanium hydride as a replacement for tributyltin hydride in radical reactions.

Tributylgermanium hydride (Bu(3)GeH) can be used as an alternative to tributyltin hydride (Bu(3)SnH) as a radical generating reagent with a wide range of radical substrates. Tributylgermanium hydride has several practical advantages over tributyltin hydride, e.g. low toxicity, good stability and much easier work-up of reactions. The reagent can be easily prepared in good yield and stored indefinitely. Suitable substrates include iodides, bromides, activated chlorides, phenyl selenides, tert-nitroalkanes, thiocarbonylimidazolides and Barton esters. Alkyl, vinyl and aryl radicals can be generated in radical reactions including reduction and cyclisation processes. Common radical initiators such as ACCN and triethylborane can be used. The slower rate of hydrogen abstraction by carbon-centred radicals from Bu(3)GeH as compared to Bu(3)SnH facilitates improved cyclisation yields. Polarity reversal catalysis (PRC) with phenylthiol can be used in reactions which generate stable radical intermediates which will not abstract hydrogen from Bu(3)GeH.

Stereospecific synthesis of 7-deoxy-6-hydroxy paclitaxel

The synthetic transposition of the 7-hydroxyl group of paclitaxel to the 6-position is described. The route presented prepares both the 6-α- and β-isomers stereospecifically. The key step of this transposition involves the stereoselective reduction of the 6,7-α-thiocarbonate using tributylgermanium hydride.

Reversibility in free-radical reactions of aryltellurides with tributylstannyl, tributylgermyl and tris(trimethylsilyl)silyl radicals

1H, 13C, 29Si, 77Se, 119Sn and 125Te NMR spectroscopies reveal that methyl, primary and secondary alkyl radicals, generated through the reaction of aryltelluroalkanes ( 4– 9) with tributyltin hydride, tributylgermanium hydride or tris(trimethylsilyl)silane) under standard radical conditions (benzene, AIBN) are capable of displacing tributylstannyl, tributylgermyl and tris(trimethylsilyl)silyl radicals from aryltellurotributylstannanes ( 1, 2), aryltellurotributylgermanes ( 10, 11) and aryltellurotris(trimethylsilyl)silanes ( 13, 14) respectively. These observations are in agreement with high-level ab initio molecular orbital studies. Calculations using a (valence) double- ζ pseudopotential basis set supplemented with polarization functions and with the inclusion of electron correlation (MP2/DZP) predict energy barriers for the displacement of stannyl (SnH 3), germyl (GeH 3) and trisilylsilyl ((H 3Si) 3)Si) radicals by methyl, ethyl and iso-propyl radicals to lie between 22 and 39 kJ mol −1, with reverse barriers of between 12 and 40 kJ mol −1. Consequently, the use of aryltellurides as alkyl radical precursors together with (standard) chain-carrying reagents such as tributyltin hydride, tributylgermanium hydride and tris(trimethylsilyl)silane may be complicated with equilibria which may result in diminished reaction yields.

Rate of Cyclization of Perfluoro-4-Oxa-5-hexenyl Radical. Use of Tributylgermanium Hydride as an Effective H-Transfer Agent for Perfluoro-n-alkenyl Radicals.

Rate of Cyclization of Perfluoro-4-Oxa-5-hexenyl Radical. Use of Tributylgermanium Hydride as an Effective H-Transfer Agent for Perfluoro-n-alkenyl Radicals.

ChemInform Abstract: Palladium‐Catalyzed Reduction of Acyl Chlorides with Tributylgermanium Hydride.

AbstractIn the presence of Pd(0) catalyst, acyl chlorides such as (I) are reduced in a mild and selective reaction to aldehydes (II).

Palladium-catalyzed reduction of acyl chlorides with tributylgermanium hydride

Tributylgermanium hydride in the presence of Pd(PPh 3 ) 4 , can reduce acyl chlorides selectively to aldehydes under mild conditions in good yields.

ChemInform Abstract: Evaluation of Tributylgermanium Hydride as a Reagent for the Reductive Alkylation of Active Olefins with Alkyl Halides.

AbstractThe reductive alkylation of acrylonitrile (II) or cyclohexenone (V) by alkyl halides such as (I) using Bu3GeH is systematically evaluated.

Evaluation of tributylgermanium hydride as a reagent for the reductive alkylation of active olefins with alkyl halides

The reductive alkylation of acrylonitrile and of 2-cyclohexen-1-one by alkyl halides using tributylgermanium hydride was systematically evaluated. The performance of the germanium reagent was compared to that of the more commonly employed tributyltin hydride. For certain applications the germanium reagent afforded improved yields without the use of large excess olefin concentrations. Disadvantages of the germanium reagent include a tendency to hydrogermylate active terminal olefins and a general low reactivity towards alkyl halide substrates. The following series of six other triorganotin and triorganogermanium hydrides were also briefly screened for possible application to reductive alkylations: trimesityltin hydride, trimesitylgermanium hydride, triphenyltin hydride, triphenylgermanium hydride, trineopentyltin hydride, and tricyclohexylgermanium hydride.

ChemInform Abstract: Alkyl Additions to Active Olefins by Tributylgermanium Hydride Reduction of Alkyl Halides.

ChemInform Abstract: Alkyl Additions to Active Olefins by Tributylgermanium Hydride Reduction of Alkyl Halides.

Alkyl additions to active olefins by tributylgermanium hydride reduction of alkyl halides

Tri( n -butyl)germanium hydride is a superior reagent for the reductive addition of alkyl halides to active olefins.