Conversions tetrakis palladium

A series of palladium salts and complexes was further screened for the coupling of aiylbromides in C4C im][PF6], A stronger base such as tetrakis(dimethylamino)ethylene was required for electron-poor arenes and good conversion was observed independent from the catalyst precursor employed. However, bromoanisole remained unreactive under the conditions used. Catalyst recycling was possible after extraction of the product with diethyl ether but activity decreased considerably with each run. 

Reduction of acid chlorides to aldehydes One of the most useful synthetic transformations in organic synthesis is the conversion of an acid chloride to the corresponding aldehyde without over-reduction to the alcohol. Until recently, this type of selective reduction was difficult to accomplish and was most frequently effected by catalytic hydrogenation (the Rosenmund reduction section 6.4.1). However, in the past few years, several novel reducing agents have been developed to accomplish the desired transformation. Among the reagents that are available for the partial reduction of acyl chlorides to aldehydes are bis(triphenylphosphine)cuprous borohydride , sodium or lithium tri-terf-butoxyaluminium hydride, complex copper cyanotrihydridoborate salts °, anionic iron carbonyl complexes and tri-n-butyltin hydride in the presence of tetrakis(triphenylphosphine)palladium(0). ... 

An approach to enantiomerically pure (R)- and (S)-2-(alkenyl)tetrahydrofurans has been devised via a nucleophilic substitution on an allylic complex of palladium(O). This substitution proceeds with chirality transfer from a preexisting stereogenic center to the newly formed center. Thus, starting from (/ )-(,E)-7-methyI-4-octene-l,6-dioI (1), an efficient transfer is realized by treatment with a catalytic amount of tetrakis(triphenylphosphane)palladium in the presence of triethylamine in acetonitrile66. After 30-45 min at 35-37 °C, (5)-tetrahydro-2-f( )-3-methyl-l-butenyl]furan (2) is obtained in 95% yield and 80% ee. The absolute stereochemistry and the optical purity [a]D —10.7 (c = 2.279, CH2C12) are conveniently determined by conversion to the known (S)-tetrahydro-2-furanmethanol. 

Allyl protection of terminal a-carboxy groups has been used in convergent syntheses of N-linked glycopeptides on a solid support.PI For instance, the polymer-bound trisaccharide pentapeptide allyl ester 12 (Scheme 7) is built up by condensation of the polymer-linked trisaccharide with Z-Ala-Ile-Asp-Ile-Ser(Bzl)-OAl. Further conversion into the polymer-bound trisaccharide octapeptide derivative 14 is achieved by cleavage of the allyl ester with tetrakis(triphenylphosphine)palladium and A,A-dimethylbarbituric acid to give 13, followed by condensation with H-Asp(OMob)-Leu-Thr(Bzl)-OAl. The presence of the allyl ester group at the C-terminus of the octapeptide provides the opportunity for further chain elongation. 

This methodology has also been applied to the conversion of alkyl esters into vinyl ethers with high stereoselectivity favoring the (Z)-isomer (Scheme 30). While standard Li/amine reduction conditions were not applicable, the enol phosphates could be reduced using triethylaluminum and tetrakis(tri-phenylphosphine)palladium. 

Cyclopropane formation can compete with direct conversion of the homoallylmetal complex intermediate. Thus reaction of norbornene (9) with j3-bromostyrene in the presence of tetrakis(triphenylphosphane)palladium(0) and ammonium formate leads to 3-benzyl-la,2j6,3a,4, 5a-tricyclo[3.2.1.0 " ]octane (12 52%) and tra s -2-styrylnorbornane (13 ... 

Intramolecular [3-I-2]-cycloaddition reactions have also been performed using alkyne moieties as reactants. An early example of this methodology is the high-yield formation of 27 from alkyne 26. As in the case of the (Z)-3-benzyloxybut-2-enoate derivatives (vide supra), the presence of the 3-siloxy group is essential for the reaction to proceed. A diastereomeric mixture of 26 leads to the formation of the two diastereomeric bicyclic adducts 27 in a combined yield of 88%. ° Interestingly, albeit with only 60 /o conversion after 100 hours, the same products are also formed at room temperature under the influence of ultrasound and 6 mol /o of tetrakis(triphenylphosphane)palladium(0) as catalyst. In a purely thermal reaction in the absence of a metal catalyst, a totally different reaction, presumably involving a Diels —Alder type addition of the alkyne to a vinylbenzene moiety, takes place. ... 

According to earlier reports, palladium-catalyzed conversion of e <7o-5-vinylnorborncnc with hydrogen cyanide in the presence of tetrakis(triphenylphosphite)palladium(0) led to a tricyclic adduct with tram addition of cyanide and the vinyl group32,33. 

A general cyclization reaction of 6-lithiated (trimethylsilyl)alkynes is exemplified by the conversion of the iodo compound 499 into the methylenecyclopentane 500 by treatment with -butyllithium . Heating a mixture of the acetylenic silane 501 and 2-(benzyloxymethyl)allylzinc bromide 502 resulted in the diene 503, which cyclized to the methylenecyclopentene 504 in the presence of tetrakis(triphenylphosphine)palladium ... 

A variety of a-nitroepoxides undergo conversion to the corresponding 1,2-diketones or, in some cases, a-nitroketones with tetrakis(triphenylphosphine)palladium(0) (Scheme [io],[ii] Proposed reaction mechanism is shown in Scheme 16. 2,3-Epoxy alcohols also undergo the Pd(0)-catalyzed reaction to be isomerized to a- or /3-hydroxy ketones or both, depending on the nature of the substituents on the phenyl ring (Scheme... 

According to Scifmder, there are only few published records for the conversion of aryl bromides to silyl derivatives. The first reports in scientific journals - besides a patent claim from 1973 - appeared in 1975 by Matsumoto and co-workers showing that various disilanes can be cleaved with halobenzenes in the presence of tetrakis(triphenyl-phosphine)palladium(O) [Pd(PPh3)4]. ... 

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