Solvent-free reactions phosphination

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. 

Several important improvements of this methodology have been reported since 1994. A broad range of organotins were used in the reactions with aryl-, heteroaryl-, or alkenyliodonium salts in aqueous organic solvents. This reaction (Equation (2)) takes place at room temperature in the presence of phosphine-free palladium... 

The simplest solvent-free method involves irradiation of neat reactants as interfacial reactions in an open container, either in a domestic oven or in a monomode reactor [34,55]. In the absence of reagents or supports, the scope for such processes appears to be limited to relatively straightforward condensations that can be conducted without added catalysts, to nucleophilic additions, Sn2 alkylations using neutral nucleophiles as amines or phosphines or to intramolecular thermolytic processes such as rearrangement or elimination. 

One problem is that coordinated oxygen oxidizes the tertiary phosphine ligands to form free tertiary phosphine oxide. Often the course of this reaction is solvent dependent in ethanol the phosphine oxide is more likely to be formed than in hydrocarbon solvents.945 The oxidation of ligands also occurs in the solid state reaction.942... 

Phosphine free catalysts and halogen-free reactions are known for the Heck reaction. Improvements on the palladium catalyst system are constantly being reported, including polymer-supported catalysts." °° The influence of the ligand has been examined." Efforts have been made to produce a homogeneous catalyst for the Heck reaction." The Heck reaction can be done in aq. media," ° in perfluori-nated solvents," in polyethylene glycol," ° in neat tricaprylmethylammonium... 

A better synthesis (89% yield) of Fe(CO)3(PPh3)2 is reported from [PPN]2[Fe4(CO)i3], where PPN+ = bis(triphenylphosphine)iminium. The C0X2 (X = Cl, Br, I) catalyzed substitution of CO in Fe(CO)j is reported to yield Fe(CO)4L species in 15 to 99% yield and Fe(CO)3(PPh3)2 was prepared (net 62% yield) from Fe(CO)s in a two-step procedure that requires a chromatographic separation. Strohmeier and Muller report that irradiation of Fe(CO)s in the presence of several phosphines produces Fe(CO)3L2 and Fe(CO)4L complexes in yields that range from 13% for the synthesis of Fe(CO)3[P(/i-Bu)3]2 to 35% for Fe(CO)3[P(c-C6Hn)3]2. For some of the compounds synthesized, vacuum sublimation is necessary to separate the Fe(CO)3L2 species from Fe(CO)4L. The one-step photochemical procedure we report here employs cyclohexane as a solvent. That enables unreacted phosphine, Fe(CO)s, and Fe(CO)4L to remain in solution while pure Fe(CO)3L2 precipitates. It is essential that the phosphines used in these reactions be free of phosphine oxides, which labilize CO and yield products other than Fe(CO)3(PR3)2 complexes. 

The main driving forces behind the development of new tertiary phosphine palladium complexes for C(sp )—C(sp) couplings have been (i) a reduction or elimination of side reactions, such as Glaser-type homocouplings (ii) the development of environmentally friendly reaction protocols, such as copper-free reactions in benign solvents (iii) the improvement of catalyst stabihty and activity [higher turnover number (TON) and turnover frequency (TOP)] and (iv) a cost reduction by using less-expensive aryl bromides, or even aryl chlorides under mild reaction conditions, for example, at ambient temperature. 

The presence of phase-transfer agents is, however, unnecessary. The addition of water to the organic solvent has often been noted to have a strong accelerating influence on Heck reactions. This influence is more strongly pronounced in phosphine-free reactions, though phosphine-assisted processes are also often notably facilitated. 

Reactions of Phosphines Palladium-catalyzed coupling of triarylphosphine can be accomplished with aryl bromides and triflates [222] (Scheme 20.66). A solvent-free approach has been demonstrated with improved yield [223]. The use of excess of triarylphosphine is crucial for second oxidative addition. The substrate scope can be extended to aryl chlorides employing Pd/C in the presence of Nal [224]. Scheme 20.66 shows the catalytic cycle. The oxidative addition of Pd(0) I with aryl halides/triflates can give the intermediate Pd(II) II that can lead to the formation... 

The other approach to P-ylides based on the nucleophilic addition of phosphorus(HI) reagents to the terminal carbon atom of 1,2-diaza-1,3-butadienes has continued to find application. The reaction with dialkyl-phosphonites or phosphorus(III) amides under solvent-free conditions (and in the presence of atmospheric moisture) was found to be a convenient approach to a-phosphanylidene hydrazones (42). The linear ylides (42) in THF solution undergo further intramolecular transformations to give 1,2,3X -diazaphospholes (43) in the case of phenylphosphinite as a starting substrate or 5-oxo-4-phosphoranyidene-4,5-dihydro-l/f-pyrazoles (44) using tris(dialkylamino)phosphine. 

Aminoalkyl acids and their derivatives. Montchamp and coworkers have synthesized new / ,A -heterocyclic frameworks [3-hydroxy-1,3-azaphospholane (121) and 3-hydroxy-l,3-azaphosphorinane-3-oxides (122)] from readily available oo-amino-Zf-phosphinates (123)-(124) and aldehydes or ketones via the intramolecular Kabachnik-Fields reaction under thermal or microwave heating. The heterocycles constitute novel analogues of a-amino acids, which might be useful in preparation of peptidomimetics. The Kabachnik-Fields reaction has been also employed in the one-pot and solvent-free synthesis of a-aminophosphonates using commercially available titanium oxide (Ti02) as an extremely efficient catalyst. " ... 

It has recently been shown that crown ether catalyzed Wittig reactions give product distributions typical of the so-called salt-free reaction [6]. Using potassium r-butoxide or potassium carbonate as base, the yields of olefin were high and the isomer ratio was quite solvent dependent. The condensation of ethylidenetriphenyl-phosphine with benzaldehyde, for example, produced over 90% of i3-methylstyrene in either tetrahydrofuran or dichloromethane, but the olefin was 85% cis in the former case and 78% trans in the latter [5]. 

---