Palladium catalysis isomerization

Synthesis of isomeric chiral protected (63 )-6-amino-hexahydro-2,7-dioxopyrazolo[l,2- ]pyrazole-l-carboxylic acid 280 is shown in Scheme 36. Crude vinyl phosphonate 275, obtained by treatment of diethyl allyloxycarbonylmethyl-phosphonate with acetic anhydride and tetramethyl diaminomethane as a formaldehyde equivalent, was used in the Michael addition to chiral 4-(f-butoxycarbonylamino)pyrazolidin-3-one 272. The Michael addition is run in dichloro-methane followed by addition of f-butyl oxalyl chloride and 2 equiv of Huning s base in the same pot to provide 276 in 58% yield. The allyl ester is deprotected using palladium catalysis to give the corresponding acid 277, which is... 

Direct allylation of rhodanine 49 (Scheme 13) under Pd(0)-catalysis with cinnamyl ethyl carbonate affords the /V-allylated compound 50. However, allylation with cinnamyl bromide and a base is not regioselective, producing a mixture of 50 and sulfide 51. Sulfide 51 isomerizes to 50 under palladium catalysis (N > S), thus indicating that Pd(0)-catalyzed allylation of 49 is thermodynamically controlled (93T1465). 

Metal carbon insertions of C02 occur in both main group and transition metal complexes.5,19 The reactions result in a strong M—O bond at the expense of a weak M—C bond (together with a C C a bond at the expense of a C O n bond). Insertion into t/weo-W(CO)5 CHDCHDPh gave the threo carboxylate, indicating retention of configuration at the o-carbon, as with CO insertion.29 Cis- RW(CO)4L (R = Me, Et or Ph) showed second-order kinetics toward C02 insertion, first order in anion and in C02. Replacement of CO by a phosphine or phosphite increased the insertion rate.30 Palladium catalysis of C02 insertion into unreactive Sn C bonds in allyl stannanes provided three isomeric tetra-carboxylates from tetraallyl tin. Attempted reactions of tetraalkyl, vinyl or aryl tin complexes did not proceed. Insertion into an intermediate rf -allyl palladium complex was suggested to lead to carboxylate products.31,32... 

The dimerization of chromium-coordinated carbene ligands typically requires temperatures above 120°C. Whereas the addition of catalytic amounts of Rh2(OAc)4 allows to only a minor decrease in temperature to 100°C, the dimerization occurs already at room temperature under palladium catalysis. [95a,96] When the reaction of chromium arylcarbene 16 was promoted by Pd(OAc)2 (10 mol-%) in the presence of NEt3 using THF as a solvent, a mixture of isomeric carbene dimers 95 was obtained with an E Z ratio of 2 1 (Scheme 42). The effects of the eatalyst load, phosphine additives, the reaction temperature, the solvent and a series of other palladium catalysts have been investigated systematically, but did not reveal significant changes. [96]... 

Palladium catalysis has been shown to be effective in inducing thermodynamically favorable and selective alkene and alkyne isomerization reactions. Aside from the formation of stereoisomeric mixtures, these reactions can be both selective and high-yielding and hence synthetically useful. 

Another typical example for the direct oxidative transformation of methyl arenes were developed by Wang and co-workers [118]. In this palladium catalyzed reaction, tert-butyl nitrite (TBN) was employed as both nitrogen source and oxidant N-hydroxyphthalimide (NHPI) was used as precursor of the active phthalimide N-oxyl (PINO) radical, which initiates the reaction to give benzylic radical A by grabbing hydrogen atom from the substrate. In the interaction between TBN and NHPI, TBN decomposes to NO radical and tert-butyl alcohol. NO radical would trap benzylic radical A to give nitrosomethyl benzene B, which isomerizes to aldoxime C [119]. Finally, nitrile product would be generated fitom C by palladium catalysis (Scheme 4.21). 

A related cyclization of 2-(alkynyl)phenylisocyanates with terminal alkynes to oxindoles was also reported by the same group (Equation (115)).472 (E)-exo-olefinic oxoindoles are selectively obtained. It was proposed that a palladium acetylide generated by the C-H activation of terminal alkynes regioselectively inserts to the alkyne moiety and the resulting vinylpalladium intermediate adds to the C=0 part of the isocyanate to give a (Z)-oxindole. This (Z)-isomer is isomerized to the ( )-isomer under the reaction conditions through catalysis of the phosphine. 

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

The palladium-catalyzed hydroesterification of oleic acid methyl esters was investigated by Frankel and was conducted at lower reaction temperatures in comparison to the nickel catalysis [61]. For example, the hydroesterification of oleic acid methyl ester and methanol with PdCl2 and PPh3 was achieved at a pressure of 270 bar. A 62% yield of the hydroesterification product was achieved after 12 h. The new C-C bond was formed mostly at positions 9 and 10 of the carbon chain. Approximately 10% of the hydroesterification products were observed at positions Cg and Cn to Ci3, which could be attributed to isomerization of the double bond during the reaction. 

The industrial synthesis of vinyl acetate [14] via palladium-catalyzed oxidative coupling of acetic acid and ethene using direct 02 reoxidation has already been mentioned (Scheme 3, d). Some NaOAc is required in the reaction medium, and catalysis by Pd clusters, as alternative to Pd(II) salts, was proposed to proceed with altered reaction characteristics [14]. Similarly, the alkenyl ester 37 (Table 5) containing an isolated vinyl group yields the expected enol acetate 38 [55] whereas allylphenol 39 cyclizes to benzofuran 40 with double bond isomerization [56]. 

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