A’-Phosphane

Some cases are known in which Diels-Alder reactions of electron-deficient allenes and dienes compete with [2 + 2]-cycloadditions (see also Section 7.3.7) [12, 151, 335, 336]. Recently, a phosphane-catalyzed [4 + 2]-annulation starting from allenic ester 337 and N-tosylaldimines 338 was published [337]. However, the formation of the tetrahydropyridines 339 isolated in excellent yields is explained by a multi-step mechanism and only resembles a Diels-Alder reaction. 

The intramolecular hydroamination of substrates 170 is catalyzed by a Pd(0) catalyst which is generated in situ from a Pd(II) precursor and a phosphane. One equivalent of acetic acid has to be added for efficient catalysis this is a hint of a hydropallada-tion mechanism. Meguro and Yamamoto obtained good yields of the vinyltetrahy-dropyrroles or the vinylhexahydropyridines 171 in that way (Scheme 15.53) [109]. 

More attention has been devoted to aromatic and heteroaromatic substrates since first reported in 1983 [40]. The results are shown in Table 2 [25, 41-51]. All these reactions were run with nickel complexes associated with a phosphane or bpy ligand. Depending on the experimental conditions, the polymers were either precipitated during the electrolysis or deposited as films at the surface of the electrode. The method is also convenient to prepare copolymers from a mixture of two aryl dihalides. A mechanistic investigation on the nickel-bpy catalyzed polymerisation has been reported very recently [52]. 

Following the success with cobalt and rhodium, Shibata reported Ir(i)-based enantioselective catalytic reaction. Right after their observation that the efficiency of [IrCl(COD)]2-catalyzed PKR substantially increased by addition of a phosphane co-ligand, they moved directly to use chiral phosphanes and examined the enantioselectivity. " TON and TOE of the reaction were low and the number of examples was limited. Typically, the reaction required a fair amount of Ir(i) catalyst [IrCl(COD)]2 (0.1-0.15 equiv.) and (reaction time. However, this has remained as the best in terms of enantioselectivity to date. Moreover, this catalytic system provided the first asymmetric intermolecular reaction as well. 

Trifluorodi(phenyl)-A >-phosphane and difluorotri(phenyl)-/L -phosphane are suitable fluorinating agents for the direct substitution of hydroxy groups in alcohols.10,12... 

In IUPAC nomenclature, a "phosphane" c normally a derivative of what wc ordinarily cal "phosphine. PH . PentavaJcnl derivatives of the liypethetkal PHj (phosphoranc) are Libeled X. Similarly, derivatives of H S would be ailfanes. and (he tctravalcnt and hexavakru derivatives would be labeled X4 and X6. respecuvdy. 

Reaction of enol ether 1 a with sodium methoxidc or ethoxide. or 1 b with sodium methoxide, gives ketene acetals, which rearrange in the presence of triethylamine to give esters 3.3 The phosphorus ylide trimethoxy[2,2,2-trilluoro-l-(trifluoromethyl)cthylidene]-A ,-phosphane f(CF3)2C = P(OMc),] rearranges readily to dimethyl f2.2,2-trifluoro-1-mcthyl-l-(trifluoro-mcthyi)ethyl]phosphonate [(CF,)2C(Me)PO(OMe)2] in a similar 1.3-shift.5-0... 

The procedure reported here involves a one-pot reaction of the inexpensive reagent K[FeH(CO)4], which is readily available from Fe(CO)s, with a stoichiometric quantity of a phosphane in a protic medium. It affords the complexes Fe(CO)2(PR3)3 or Fe(CO)2[P(OR)3]3 in fair to high yields using tributylphosphine, dimethylphenylphosphine, and trimethyl, triethyl, and... 

In fact, P3RhCl has been shown to produce more than 3 mol of carbonate per Rh, thus demonstrating that the Rh(C03) species can indeed act as a catalyst. Moreover, an oxophile such as a phosphane ligand can extract an O-atom from the coordinated carbonate (Equation 7.17c), thus regenerating Rh(I) and... 

Figure 52 Lipase-catalysed kinetic resolution of a phosphane-borane complex 107.

Giirtler, C. Buchwald, S. L. A phosphane-free catalyst system for the Heck arylation of disub-stituted alkenes application to the synthesis of trisubstituted olefins. Chem. Eur. J. 1999, 5, 3107-3122. 

Transition metal compounds having a phosphoranide ligand remained unknown. The approach we chose to accede to this new class of compounds was to provoke the intramolecular addition of an ionic fifth substituent on the phosphorus atom of a phosphane previously coordinated to the transition metal. The anionic site was created by abstraction of a proton from a secondary amine. 

The status for homogeneous catalysis by gold in solution was dramatically transformed in 1998 by the results of Teles et al.11 This BASF group described the use of cationic Au1 complexes of the type [L-Au+] (where L is a phosphane, phosphite or arsine) for the addition of alcohols to alkynes in the presence of a Lewis or Brpnsted acid as a co-catalyst in methanol at 313 K. An example of such a reaction is the conversion of 3-hexyne to 3,3-dimethoxyhexane ... 

The kind of bond with respect to type B may be interpreted analogous to the ethylene complexes as a dative x-alternating effect (Fig. 24). As opposed to type A complexes, the properties of type B complexes are determined in a characteristic way by the back donation into the unoccupied n molecular orbital (LUMO) of the double bond (Fig. 24). This strengthens the metal-ligand bond, as proved by a shorter interatomic distance, and weakens the double bond (occupying anti bonding molecular orbitals), as is evident by a widened PC distance. The phosphaalkene P atom has, so to speak, more of a phosphane... 

On treatment with CsF, 74j yields the salt 75a 119). The same anion together with a cation containing a dioxaphospholane ring 75b has been synthesized from thermolysis of the aminoiminodiphosphorane 74n 229). Compound 74j forms a stable donor-acceptor complex with trimethylphosphane 119). Reduction of 74j with trimethylsilyldiphenylphosphane affords the 2 -dioxaphospholane 73a, which serves as a phosphane ligand in the molybdenum complex 76 229). The phosphane ligands are in cis-positions. 

Figure 8. A phosphane modified PEO-PPO-PEO block copolymer as thermoresponsive catalyst in the hydrogenation of allylic alcohols.

Note The second generation Grubbs catalyst [Ru(PCyJ(IMes)(=CHPh)ClJ [117] employs a phosphane and a NHC and is activated by loss of the PCy ligand. The Herrmann variant [Ru(IMes)J=CHPh)ClJ [116] is inferior simply because it would have to lose one of the two NHC ligands, a process that takes longer because NHC bind stronger than phosphanes. 

A similar situation prevails in other C-C and C-N coupling reactions since they also contain a PdL key intermediate. It is therefore no surprise that mixed NHC/phosphane ligand systems have been employed for the Mizoroki-Heck, Suzuki-Miyaura and StiUe reactions [238,255-258]. In all these cases, the incorporation of a phosphane ligand instead of the second NHC ligand improves the activity of the catalytic reaction. Similar results are reported for the allylic alkylation of dimethylmalonate using mixed NHC/phosphane palladium catalysts [252]. 

Liao et al. used a carboxylic acid amide functionalised carbene and a phosphane in a mixed NHC/phosphane palladium(II) catalyst [261]. The system shows the usual ligand exchange behaviour meaning that the PPhj ligand can be substituted by PCyj. This made it possible to study the influence of the phosphane ligand on the performance of the catalyst. In the Suzuki-Miyaura reaction between phenylboronic acid and p-chloroacetophenone, the yield changes dramatically. When PCyj is chosen as the phosphane ligand, then quantitative yield is observed (for both the saturated and unsaturated NHC), but in the case of PPhj the yield drops to 8% (unsaturated NHC) or even 4% (saturated NHC). When... 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

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