Phosphines alkylidene

The reaction of an alkylidene phosphorane 1 (i.e. a phosphorus ylide) with an aldehyde or ketone 2 to yield an alkene 3 (i.e. an olefin) and a phosphine oxide 4, is called the Wittig reaction or Wittig olefination reaction. ... 

N-Unsubstituted imines have been similarly converted into AT-alkylidene phosphinous amides, as 3, in reactions run in the presence of triethylamine [45] (Scheme 3). 

CM products from vinylhalides are highly desirable especially because of the possible use in metal catalysed coupling reactions. Johnson and co-workers, performed detailed studies of the possible deactivation pathways [161]. The Fischer-carbene complexes of the vinyl halides have an increased stabihty compared to their alkylidene counterparts and the Fischer carbenes may be deactivated either by migration of the phosphine or by elimination of HX leading to a carbide. 

More recently, a new metathesis catalyst involving a ruthenium-alkylidene complex with a sterically bulky and electron-rich phosphine ligand has been synthesized and applied to RCM in aqueous media (Figure 3.5).197 This catalyst has the benefit of being soluble in almost... 

The synthesis and olefin metathesis activity in protic solvents of a phosphine-free ruthenium alkylidene bound to a hydrophilic solid support have been reported. This heterogeneous catalyst promotes relatively efficient ring-closing and cross-metathesis reactions in both methanol and water.200 The catalyst-catalyzed cross-metathesis of allyl alcohol in D20 gave 80% HOCH2CH=CHCH2OH. 

In situ deprotonation combined with a substitution of a phosphine ligand was reported as a convenient way for the synthesis of ruthenium-alkylidene complexes (Scheme For imidazolidin-2-ylidenes, this is the only way... 

A breakthrough in catalytic metathesis applications was achieved with the second generation of ruthenium-NHC-alkylidene complexes In 54, 55, and 56 NHCs are combined with coordinatively more labile ligands such as phosphines... 

NHCs are certainly not the best choice for requirement (ii), but seem to be superior with respect to (i). This has been quantified by theoretical calculations The dissociation energies of NHCs and phosphines for ruthenium alkylidene model compounds by density functional (DFT) methods according to Eq. (41) demonstrate that the ligand dissociation energies ascend in the series PH3 < PMei < NHC (Table... 

Widenhoefer and co-workers have developed an effective protocol for the asymmetric cyclization/hydrosilylation of functionalized 1,6-enynes catalyzed by enantiomerically enriched cationic rhodium bis(phosphine) complexes. For example, treatment of dimethyl allyl(2-butynyl)malonate with triethylsilane (5 equiv.) and a catalytic 1 1 mixture of [Rh(GOD)2] SbF6 and (i )-BIPHEMP (5 mol%) at 70 °G for 90 min gave the silylated alkylidene cyclopentane 12 in 81% yield with 98% de and 92% ee (Table 4, entry 1). A number of tertiary silanes were effective for the rhodium-catalyzed asymmetric cyclization/hydrosilylation of dimethyl allyl(2-butynyl)malonate with yields ranging from 71% to 81% and with 77-92% ee (Table 4, entries 1-5). Although the scope of the protocol was limited, a small number of functionalized 1,6-enynes including A-allyl-A-(2-butynyl)-4-methylbenzenesulfonamide underwent reaction in moderate yield with >80% ee (Table 4, entries 6-8). 

Evidence for the dissociative mechanism was recently summarized. Key data include kinetics studies, tandem ESI-MS observations, computational studies, and interception of an intermediate in which a phosphine ligand of RuCl2(PCy3)2(=CHPh) is replaced by an olefin subtended by the alkylidene group. ... 

This finding is a significant improvement over aqueous ROMP systems using aqueous ROMP catalysts. The propagating species in these reactions is stable. The synthesis of water-soluble block copolymers can be achieved via sequential monomer addition. The polymerization is not of living type in the absence of acid. In addition to eliminating hydroxide ions, which would cause catalyst decomposition, the catalyst activity is also enhanced by the protonation of the phosphine ligands. Remarkably, the acids do not react with the ruthenium alkylidene bond. 

The most generally useful caibene complexes for transfer of methylene and other simple alkylidene groups are the cationic complexes of the general types shown in equations (12) and (13). In particular, the complexes containing the cyclopentadienyldicarbonyliron group, commonly written as Cp(CO)2Fe or simply Fp, have been the most useful to date. Also, derivatives of this system have been studied in which at least one of the carbonyl ligands has been replaced by another ligand such as a phosphine or a phosphite. 

Cyclic carbonates have also been synthesized from propargylic alcohol derivatives and C02 as the starting materials. This synthetic approach (Equation 7.22) is based on cyclization of the propargylic carbonate moiety (HC=CCH20C02-) into the corresponding a-alkylidene CC, in the presence of a suitable catalyst such as ruthenium [202], cobalt [203], palladium [204, 205], copper [206-211], or phosphine [212-214],... 

In this dissociative pathway (which is assumed to be the major one today) first a phosphine is displaced from the metal center to form an active 14-electron-intermediate 42. After alkene coordination cis to the alkylidene fragment the 16-electron-olefine-complex 43 undergoes [2 + 2]-cycloaddition to give a metallacylobutane 44. Compound 44 breaks down in a symmetric fashion to form carbene complex 45. The ethylene is replaced in the conversion to complex 46. In the next steps (they are not further discribed above), another intramolecular [2 + 2]-cycloaddition joins up the eight-membered ring 11 regenerating the catalyst 42. Each step of the reaction is thermodynamically controlled making the whole RCM reversible. With additional excess of phosphine added to the reaction mixture an associative mechanism is achieved, in which both phosphines remain bound. 

Weng, W., Yang, L., Foxman, B.M. and Ozerov, O.V. (2004) Chelate-enforced phosphine coordination enables a-abstractionto give zirconium alkylidenes. Organometattics, 23, 4700. 

In the synthesis of specially substituted methylene diphosphines, made from secondary phosphines and carbonyl derivatives (7), a carbenium ion adjacent to trivalent phosphorus as the transition state has been discussed. The transfer of this reaction principle to primary phosphines and suitable carbonyl compound revealed a further pathway to derivatives of dicoordinated phosphorus (8). Aromatic phosphines react with carboxylic acid amide acetals under elimination of alcohol giving dialkylamino-alkylidene phosphines (Scheme 5). A modification of the synthesis... 

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