Organometallic Allene [(μ-C)(Fe(CO)4)2]: Bridging Carbon Showing Transformation from Classical Electron-Sharing Bonding to Double σ-Donor and Double π-Acceptor Ligation

Allenes (R₂C=C=CR₂) have been traditionally perceived to feature localized orthogonal π-bonds between the carbon centres. We have carried out quantum-mechanical studies of the organometallic allenes envisioned by the isolobal replacement of the terminal CH₂ groups by the d⁸ Fe(CO)₄ fragment. Our studies have identified two organometallic allenes viz. D_2d symmetric [(μ-C)(Fe(CO)₄)₂] ( 2 ) and D₃ symmetric [(μ-C)(Fe(CO)₄)₂] ( 3 ) with trigonal bipyramidal coordination at the Fe atoms. Compound 2 features the bridging carbon atom in an equatorial position with respect to the ligands on the TM centre, while 3 features the central carbon atom in an axial position. The bis-pseudoallylic anionic delocalisation proposed in the C2-C1-C3 spine of organic allene is retained in the organometallic allene 2 , and is transformed to a typical three-centre bis-allylic anionic delocalisation in the organometallic allene 3 . The topological analysis of electron density also indicates a bis-allylic anionic type delocalisation in the organometallic allenes. The quantitative bonding analysis using the EDA-NOCV method suggests a transition from classical electron-sharing bonding between the central carbon atom and the terminal groups in 1 to donor-acceptor bonding in 3 . Meanwhile, both electron-sharing and donor-acceptor bonding models are found to be probable heuristic bonding representations in the organometallic allene 2 .     

Synthesis and Ligand Properties of a Persistent,All‐Carbon Four‐Membered‐Ring Allene

A single donor substituent at each terminus is sufficient to make the CCC skeleton of allenes very flexible and give carbon(0) character to the central carbon atom. This allows the synthesis of a four‐membered carbocyclic allene, which can be doubly protonated and behaves as a very strong η¹‐donor ligand for transition metals (see scheme).

     

Selenosulfonation of allenes

Se-Phenyl p-tolueneselenosulfonate (1a) undergoes highly regioselective, photoinitiated, free-radical addition to allenes (R₁CHCCR₂R₃) to afford the regioisomer R₁CH(SePh)C(SO₂Ar)CR₂R₃ (13) arising from addition of the p-tolylsulfonyl group to the central carbon of the allene and transfer of the phenylseleno group to the less highly substituted of the two terminal carbons. This regioselectivity, which contrasts with that observed in the majority of radical additions to allenes, can be explained by reference to concepts proposed by Heiba as being important in determining the orientation in different radical additions to allenes. Oxidation of the PhSe group in 13 to PhSe(O) gives allylic selenoxides that undergo a reaction sequence of facile, concerted, [2,3]-sigmatropic rearrangement followed by hydrolysis of the resulting selenenate to afford β-tolylsulfonyl-substituted allylic alcohols, R₁CHC(SO₂Ar)C(OH)R₂R₃ (14) in 70–98% yield. Photoaddition of 1a to allenes, followed by the conversion of 13 to 14 thus provides a simple, high-yield route to a wide variety of 14, a class of compounds that would seem to have a number of interesting possible uses in synthesis.     

Asymmetric Alkenyl C−H Functionalization by CpxRhIII forms 2H‐Pyrrol‐2‐ones through [4+1]‐Annulation of Acryl Amides and Allenes

An efficient Cp^xRh^III‐catalyzed enantioselective alkenyl C?H functionalization/[4+1] annulation of acryl amides and allenes is reported. The described transformation provides straightforward access to enantioenriched α,β‐unsaturated‐γ‐lactams bearing a quaternary stereocenter. The reaction operates under mild conditions, displays a broad functional‐group tolerance, and provides 2H‐pyrrol‐2‐ones with excellent selectivity of up to 97:3 er. Such scaffolds are frequently found in natural products and synthetic bioactive compounds and are of significant synthetic value. It is noteworthy that the allene serves as a one‐carbon unit in the [4+1]‐annulation.     

Substituent effects on rates of rhodium-catalyzed allene cyclopropanation

Rates of cyclopropanation for mono- and disubstituted allenes have been measured relative to standard substrates in reaction with aryldiazoacetate esters catalyzed by Rh₂(S-DOSP)₄. Phenylallene derivatives exhibited a linear correlation of rate with σ⁺ coefficients, indicating a resonance-based effect, though the magnitude of the effect for allenes is less than that reported for other cyclopropanations. Relative reaction rates for aliphatic allenes were found to be similar to those for aryl-substituted allenes, but silicon substitution was found to give a 5- to 14-fold rate increase. The rate enhancement effect for 1-silyl allenes can partially make up for loss of rate and regioselectivity, with 1-trimethylsilyl-1,2-butadiene exhibiting high levels of enantioselectivity and diastereoselectivity in reaction with the chiral catalyst.     

Phosphorylated Allenes in the Cycloaddition Reactions

Abstract

The reactions of phosphorus-containing allenes with the derivatives of phosphorus of low coordination are investigated. The interaction of the l -vinyl-l-phosphoryloxy-3,3-di-methylallene with (dimethylaminomethylidene)phenylphosphine was shown to occur over 1,3-diene system according to Diels-Alder reaction. The formation of the six-membered heterocycle was accompanied by changing the coordination of the phosphorus atom from P^II to P^III. The reaction of the phosphorus containing 1,3-dipole (diethylaminobenzylidene)-aminodimethoxyphosphine with 1-vinyl-1-phosphoryloxy-3,3-dimethylallene was found to give the seven-membered aza-phosphepin. The changing of methoxy-radicals by ethoxy-groups in benzylideneaminophosphine leads to azaphos-phorins as products. The formation of the seven-membered adduct in this case is small. The two-step mechanism of these reactions with the initial attack of atom P at central carbon atom of allene is in agreement with kinetic, thermochemical and ³¹P-NBR data.     

Transition-metal-catalyzed carbonylation of allenes with carbon monoxide and thiols

In the presence of a catalytic amount of tetrakis(triphenylphosphine)platinum(0), allenes undergo carbonylative thiolation with carbon monoxide and thiols to provide the corresponding α,β- and β,γ-unsaturated thioesters in good yields. In contrast, the use of rhodium(I) catalysts such as RhH(CO)(PPh₃)₃ in place of Pt(PPh₃)₄ leads to copolymerization of allenes and carbon monoxide without incorporation of thio groups.     

Carbopalladation of allenic hydrocarbons. : A new way to functionalized styrenes and 1,3-butadienes.

The palladium-catalyzed coupling reaction of allenes, vinyl or aryl halides and stabilized carbanions is described : π-allyl palladium complexes are formed by addition of a vinyl or an aryl-palladium complex to an allenic hydrocarbon and trapped by the sodium enolate of diethyl malonate giving rise with good yields to β-butadienyl or β-styryl malonates. With monoalkyi allenes, the reaction is regiospecific with attack of the nucleophile on the unsubstituted carbon of the Intermediate π-allyl complex and in many cases highly stereoselective with the predominant formation of the E configuration for the trisubstituted double bond of the diene. This configuration was demonstrated by ¹H NMR using NOE difference spectroscopy.     

Products and mechanism of some halogenation reactions of 1-sulfonyl-substituted tricyclo[4.1.0.0<Superscript>2,7</Superscript>]heptanes

Reactions of 1-methylsulfonyl- and 1-phenylsulfonyltricyclo[4.1.0.0^2,7]heptanes with iodine, dioxane dibromide, and dichloro-λ³-iodanylbenzene (under irradiation) gave products of stereoselective syn addition of halogen at the C¹–C⁷ central bicyclobutane bond. 7-Methyl-1-phenylsulfonyltricyclo[4.1.0.0^2,7]-heptane reacted with dioxane dibromide in carbon tetrachloride to produce a mixture of 2-bromo- and 2,3-dibromo-1-methyl-exo-7-phenylsulfonylnorcaranes at a ratio of 1: 4 as a result of cleavage of the C¹–C² bicyclobutane bond. 7-Bromo- and 7-methoxycarbonyl-1-phenylsulfonyltricyclo[4.1.0.0^2,7]heptanes take up bromine exclusively at the C¹–C⁷ central bond with strict syn stereoselectively. The regio- and stereoselectivity of the addition and their relations with the halogen nature were interpreted with account taken of structural specificities of intermediate 6-sulfonyl-substituted 6-norpinanyl radicals determined by ab initio quantumchemical calculations using 6-31G basis set.     

Regioselective radical bromoallylation of allenes leading to 2-bromo-substituted 1,5-dienes

The regioselective radical bromoallylation of allenes proceeded efficiently in the presence of AIBN as a radical initiator to give 2-bromo-substituted 1,5-dienes in excellent yields. The addition of a bromine radical took place regioselectively onto the central carbon of allenes generating a stable allyl radical, which underwent addition/β-fragmentation reactions with allylbromides. The products could be further functionalized by Pd-catalyzed coupling reactions.     

Bifunctionalized allenes. Part XXI. Electrophilic cyclization and addition reactions of 3-(α- or β-hydroxyalkyl)-allenylphosphonates and allenyl phosphine oxides

Abstract

The present paper discusses the electrophilic cyclization and addition reactions of 3-(α- or β-hydroxyalkyl)-allenylphosphonates and phosphine oxides. Treatment of 3-(α- or β-hydroxy-alkyl)-allenylphosphonates with electrophiles takes place with 5-endo-trig cyclization and gives 2-methoxy-2,5-dihydro-1,2-oxaphosphole 2-oxides as a result of the neighboring phosphonate group participation in the electrophilic cyclization. On the other hand, 3-(α- or β-hydroxyalkyl)-alk-(1E)-en-1-yl phosphine oxides were prepared by chemo-, regio-, and stereoselective electrophilic addition to the C²-C³-double bond in the 3-(α- or β-hydroxyalkyl)-alka-1,2-dienyl phosphine oxides and subsequent attack of the external nucleophile (halide anion). The paper proposes a possible mechanism that involves electrophilic cyclization and addition reactions of the phosphorylated (α- or β-hydroxyalkyl)allenes.     

取代基对有机钼化合物中α-氢转移反应势垒和产物稳定性的影响

使用B3LYP方法研究了发生在有机钼化合物R³R⁴Mo(≡CH)(CHR¹R²)和R³R⁴Mo(＝CH₂)(＝CR¹R²)之间的α-氢转移反应, 探讨了R¹, R², R³和R⁴位置上不同取代基对α-氢转移反应势垒和产物稳定性的影响. 研究发现, 金属钼有机化合物中, 发生α-氢转移的碳原子在过渡态中采用sp²杂化. R¹和R²位置上取代基对α-氢转移反应势垒的影响取决于取代基对过渡态中碳原子的未参与sp²杂化的p_z轨道上单电子的离域作用. 当R¹, R²位置是甲基时, 由于碳原子的p_z轨道与甲基的一个C—H键轨道间存在强的超共轭效应, 从而可以较大程度地降低α-氢转移反应的势垒. 研究结果还表明, 当R³, R⁴位置为SiH₃时的反应势垒较低. 所以当R¹和R² 位置为Me, R³和R⁴位置为SiH₃时, 反应势垒最低. 第一个甲基取代R¹或R²位置的H时, 反应势垒降低很大; 第二个甲基继续取代时, 反应势垒的降低约为第一个甲基的一半. 第一个SiH₃取代R³或R⁴位置的甲基时, 反应势垒降低较大; 第二个SiH₃继续取代时, 反应势垒的降低小于第一个SiH₃的一半. 对反应物和产物的相对稳定性的研究表明, 第一个甲基和第二个甲基对产物的相对能量的降低几乎相同; 第一个SiH₃降低产物的相对能量, 但是第二个SiH₃使产物的相对能量升高, 从而抵消了第一个SiH₃对产物的稳定作用.     

Some Reactions of gem‐Dibromocyclopropanes and Metal Carbonyls

A series of gem‐dibromocyclopropanes were treated with various metal complexes. Among the metal complexes, Ru(CO)₂(PPh₃)₃, Ru(CO)₃(PPh₃)₂, and Mo(CO)₆ were able to remove a bromine atom from 1,1‐dibromo‐2‐phenylcyclopropanes ( 1 ) to yield a series of corresponding of 1‐bromo‐2‐phenylcyclopropanes ( 2 ). Upon the treatment of 1 with Cr(CO)₆ in DMSO, a series of allenes were obtained in good yields. The correlation between the rate of formation of allenes and the substituents on the benzene gives a negative coefficient which suggests the dibromocyclopropanes possesses as an electrophile toward to Cr(CO)₆. In the presence of Cr(CO)₆, gem‐dibromobicyclo[n,1,0]alkanes ( 4 ) in DMF or DMSO solution underwent the cleavage of carbon‐bromine bond followed by ring‐expansion and coupling reaction to form bicycloalkenes 7 .     

半夹芯式16电子化合物CpCo(S2C2B10H10)与重氮乙酸乙酯及联烯三元反应体系

半夹芯式碳硼烷16e金属有机化合物CpCo（S₂C₂B₁₀H₁₀）与重氮乙酸乙酯及联烯在室温下反应,生成化合物1和2。在化合物1和2中,一分子重氮乙酸乙酯与一分子联烯以头-头方式插入Co-B键之间使碳硼烷B（3）/（6）位发生B-H键活化生成B-C键,另有一分子重氮乙酸乙酯使Co-S断裂形成硫叶立德。分别采用红外、核磁、元素分析、质谱和单晶X射线衍射等表征方法对化合物1和2进行了结构表征。     

半夹芯式16电子化合物CpCo(S_2C_2B_(10)H_(10))与重氮乙酸乙酯及联烯三元反应体系

半夹芯式碳硼烷16e金属有机化合物Cp Co(S2C2B10H10)与重氮乙酸乙酯及联烯在室温下反应,生成化合物1和2。在化合物1和2中,一分子重氮乙酸乙酯与一分子联烯以头-头方式插入Co-B键之间使碳硼烷B(3)/(6)位发生B-H键活化生成B-C键,另有一分子重氮乙酸乙酯使Co-S断裂形成硫叶立德。分别采用红外、核磁、元素分析、质谱和单晶X射线衍射等表征方法对化合物1和2进行了结构表征。     

Electrophilic additions to allenes—II : The reaction of benzeneselenenyl chloride with some 1,3-disubstituted allenes

Benzeneselenenyl chloride reacts with 1,3-dialkyl-substituted allenes in methylene chloride solution to give 1:1-adducts. Attack by selenium is found to occur exclusively at the central allenic carbon. In contrast to the analogous reaction of arenesulphenyl chlorides, the preferential formation of the Z-alkene is observed. A mechanism involving the preequilibrium formation of alkylideneseleniranium ions and/or alkylideneepiselenuranes which, in the product-determining step, collapse to products via an anti attack of chloride at the methine carbon of the ring is proposed to account for the preferential formation of the Z-isomers.     

Improving the rate and low-temperature performance of LiFePO<Subscript>4</Subscript> by tailoring the form of carbon coating from amorphous to graphene-like

A solid-state reaction process with poly(vinyl alcohol) as the carbon source is developed to synthesize LiFePO₄-based active powders with or without modification assistance of a small amount of Li₃V₂(PO₄)₃. The samples are analyzed by X-ray diffraction, scanning/transmission electron microscopy, and Raman spectroscopy. It is found that, in addition to the minor effect of a lattice doping in LiFePO₄ by substituting a tiny fraction of Fe²⁺ ions with V³⁺ ions, the change in the form of carbon coating on the surface of LiFePO₄ plays a more important role to improve the electrochemical properties. The carbon changes partially from sp³ to sp² hybridization and thus causes the significant rise in electronic conductivity in the Li₃V₂(PO₄)₃-modified LiFePO₄ samples. Compared with the carbon-coated baseline LiFePO₄, the composite material 0.9LiFePO₄·0.1Li₃V₂(PO₄)₃ shows totally different carbon morphology and much better electrochemical properties. It delivers specific capacities of 143.6 mAh g^?1 at 10 C rate and 119.2 mAh g^?1 at 20 C rate, respectively. Even at the low temperature of ?20 °C, it delivers a specific capacity of 118.4 mAh g^?1 at 0.2 C.     

1,1‐Organoboration of bis(trimethylstannyl)ethyne with trimethylsilyl‐ and dimethylsilyl‐dialkylboryl‐substituted alkenes: organometallic‐substituted allenes

The reactions of bis(trimethylstannyl)ethyne, Me₃Sn–C?C–SnMe₃ ( 4 ), with trimethylsilyl‐ or dimethylsilyl‐dialkylboryl‐substituted alkenes 1 – 3 afford organometallic‐substituted allenes 5 , 6 and 8 , 9 in high yield. In the case of (E)‐2‐trimethylsilyl‐3‐diethylboryl‐2‐pentene ( 1) , a butadiene derivative 7 could be detected as an intermediate prior to rearrangement into the allene. All reactions were monitored by ²⁹Si and ¹¹⁹Sn NMR, and the products were characterized by an extensive NMR data set (¹H, ¹¹B, ¹³C, ²⁹Si, ¹¹⁹Sn NMR). Copyright © 2003 John Wiley & Sons, Ltd.