Cycloadditions with phosphines

Enediynes 38 undergo [2+2+2] cycloaddition reactions to afford polycyclic cyclohexadienes 39 in presence of a cobalt catalyst (Scheme 5.11) [14]- In this system, the presence of a NHC ligand improved the catalytic power of cobalt when compared with phosphine ligands. In addition to increased yields, lower ligand... 

The same reaction sequence performed in methanol affords a mixture of diastereo-mers of the phosphorylated phosphinic ester 48b, of which one pure isomer can be isolated32 . In the presence of piperidine, reductive elimination of nitrogen 28,29) from 45 to give bis(diphenylphosphoryI)methane competes with the prevailing formation of the phosphinic piperidide 48c32). Expected trapping of 47 by [2 + 2]-cycloaddition with benzaldehyde fails to occur in place of 1,2k5-oxaphosphetanes, products are obtained which arise mainly by way of the benzoyl radical32,33). 

Compared with the variety of existing carbon or nitrogen nucleophiles that were subjected to nucleophilic addition to there are few examples for phosphorus nucleophiles. Neutral trialkylphosphines turn out to be to less reactive for an effective addihon to Cjq even at elevated temperatures [114], Trialkylphosphine oxides show an increased reactivity. They form stable fullerene-substituted phosphine oxides [115] it is not yet clear if the reaction proceeds via a nucleophilic mechanism or a cycloaddition mechanism. Phosphine oxide addition takes place in refluxing toluene [115], At room temperature the charge-transfer complexes of with phosphine oxides such as tri-n-octylphosphine oxide or tri-n-butylphosphine oxide are verifiable and stable in soluhon [116],... 

Bis- and tris(trimethylsilyl-diazomethyl)phosphines undergo a double and triple regiospecific cycloaddition with a phosphaalkyne <88TL925>. 

Another type of chiral alkene applied in 1,3-dipolar cycloadditions are vinyl groups attached to chiral phosphine oxides or sulfoxides. Brandi et al. (150,151) used chiral vinyl phosphine oxide derivatives as alkenes in 1,3-dipolar cycloadditions with chiral nitrones. This group also studied reactions of achiral nitrones with chiral vinyl phosphine oxide derivatives. Using this type of substrate, fair endo/exo-selectivities were obtained. In reactions involving optically pure vinyl phosphine oxides, diastereofacial selectivities of up to 42% de were obtained. Chiral vinyl... 

Fluoro-substituted chiral vinyl sulfoxides such as 103 have been used in 1,3-dipolar cycloadditions with various benzonitrile oxides (Scheme 12.34) (158). The reaction proceeded slowly at room temperature, however, after 5-10 days the isoxazoline (104) was obtained with excellent de in good yield. In some cases, the product tends to eliminate the 5-methoxy substituent of the isoxazoline, thus, after loss of two chiral centers, an isoxazole is obtained (158,159). Other chiral suMnyl derivatives have also been used in 1,3-dipolar cycloadditions with nitrile oxides (160,161), and in one case a racemic vinyl phosphine was used in reactions with various nitrile oxides, but with moderate selectivities (151). 

Co2(CO)g has been used in numerous reactions in addition to the plethora of substitution reactions that it undergoes with phosphines, nitrosyls, alkynes, and so forth. Many of these substituted carbonyl complexes and their reactions have been discussed elsewhere in this report. One area in which Co2(CO)g has received much attention is catalysis two classes of catalytic reactions of which Co2(CO)g plays a major role are hydroformylation (see Hydroformylation) and Pauson Khand cycloaddition (see Pauson-Khand Reaction). 

The reactions with tertiary amines or phosphines that have no active hydrogen atoms result in platinacyclobutene cations, a rare species for late transition metal (Scheme 39). Substituted carbanions are added to the jj -aUenyl/propargyl platinum complex to yield the neutral substituted- ) -TMM derivatives that undergo huther [3 + 2] cycloaddition with good tt-acids as TCNE or maleic anhydride to produce highly substituted cyclopentanoids (Schemes 40, 41). 

Reactions with phosphine- or phosphite-modified catalysts, e.g. Ni(COD)2 + PPhs, show lower stereo-specificity and require higher reaction temperature. In the case of fumarate and maleate this catalyst system promotes only distal ring-opening cycloaddition (equation 85). ... 

The solution-phase synthesis and resolution of new phosphinopeptidic building blocks containing a triple bond and their involvement in 1,3-dipolar cycloaddition with a variety of in j// -prepared nitrile oxides allowed the diastereoselective preparation of a novel class of isoxazole-containing phosphinic peptides 619. Inhibition assays of some of these peptides revealed their behavior as very potent inhibitors of metalloproteases, outmatching previously reported phosphinic peptides in terms of potency <2003CEJ2079>. 

Keglevich et al. have reported a series of papers on the mechanistic aspects of what they term inverse Wittig reactions , i.e. the preparation of phosphoranes from the [2-f2] cycloadditions of phosphine oxides and acetylenedicar-boxylates, an example of which is given in Scheme 1. A raft of spectroscopic and structural evidence, coupled with theoretical calaculations, indicate that these reactions proceed via oxaphosphetane intermediates (16). ... 

Figure 5. Bioorthogonal reactions on sugars, A. Ketones react with hydrazides to give hydrazones. B. Thiols undergo Michael Reaction with maleimides. C, Azides undergo Staudinger ligation with phosphines or D. strain-promoted or copper catalyzed [3 2] cycloaddition with all nes. E. Alkynes undergo copper-catalyzed [3- 2] cycloaddition with azides.

Phosphines undergo cycloaddition with diones in the dark 120>. Recently, a report appeared 77> describing irradiation of phenanthrenequinone in wet benzene containing triphenylphosphine. The product was considered to be a hydrate of a species such as 27 intermediacy of 28 was also considered possible. No reaction was observed in the absence of water. 

Norbornadiene may undergo cycloaddition either once or twice the latter process is not usually a major complication 30-60 l o yields with simple acetylenes are typical and are further improved by adding a phosphine oxide [3, 99]. Under thermal conditions yields of cyclopentenones are limited by side-reactions of both diene and acetylene with the cobalt reagent. Cycloaddition with 4-pentyn-l-ol is such a case [Eq. (40)]. Much better yields, at the expense of some stereoselectivity, are obtained using an amine oxide to facilitate CO loss and alkene coordination [100]. Prior attachment of the alkynol to a Merrifield polymer is another method to suppress interfering processes and improve yields (Eq. (41)] (101). 

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