Aryl pivalates

Atyl carbo lates and alkenyl acetates were also catalytically coupled with arylboroxines using NiCl2(PCy3)2 (Scheme 14.22). It was found that water can promote this transformation. The optimal amount of water is 0.88 equivalents (based on the acetate) for the atyl carbojgrlates and 1.0 equivalents for the alkenyl acetates.However, NiCl2(PCy3)2-catalysed coupling of aryl pivalates with aryl boronic acids does not require a water additive. ... 

Two recent computational studies have examined the mechanism of crosscouplings involving aryl acetates, earbamates, and sulfamates. In a study of synthetic advances with aryl pivalates, oxidative addition of Ni(0) to the aryl acetate was found to proceed by a three-centered mechanism via an T arene complex. Alternatively, in a separate study, oxidative addition involving aryl carbamates and sulfamates was found to proceed by a five-centered transition state, with coordination of the carbonyl oxygen to nickel being a key interaction in the oxidative addition. ... 

Utilizing similar conditions as Ackermann. Ackermann [153] also showed the C-H arylation of azine N-oxides with aryl tosylates and mesylates with their own protocol (but changed the base from K2CO3 to CsF). Very recently, Itami [154] discovered the first nickel-catalyzed C-H/C-O coupling between 1,3-azoles and various phenol derivatives such as aryl pivalates, carbamates, sulfamates, mesylates, and tosylates. Surprisingly, these reactions only proceeded when 1,2-bis(dicydohexylphosphino)ethane (108 dcype) was used as Hgand (Scheme 17.30). 

Aryl pivalates also serve as suitable substrates in this aminatirai reaction [Eq. (9)] [55]. The effective shielding of the carbonyl group by a tert-butyl moiety completely suppressed the undesired aminative cleavage of C(acyl)-0 bonds. A substantially wider range of substrates, when compared to the amination of aryl methyl ethers, were aminated under milder conditions. Aryl carbamates exhibit similar reactivity to the amination reaction under these nickel-catalyzed conditions [55, 56]. 

Itami s group developed a nickel-catalyzed cross-coupling of aryl pivalates, carbamates, and carbonates with azoles [Eq. (14)] [63]. The use of bidentate 1,2-bis(dicyclohexylphosphino)ethane is essential for the reaction to proceed. 

Mizoroki-Heck type reaction of aryl pivalates has been reported by Watson s group [Eq. (15)] [64], A bidentate phosphine serves as an effective ligand in this reaction as well. 

M Fe over the rate in the absence of Fe" ". The reaction was hindered by complexation of Fe " by CN , and produced pivalic acid as a by-product. Reduction was promoted more effectively by Cu than by Fe, and N-aryl-hydroxylamines added to the reaction mixture were oxidized to nitroso... 

Arylation of amines. Rapid transfer of an aryl group from ArjBifOAclj to amines is catalyzed by copper(II) pivalate (9 examples, 45-100%). 

Subtle alterations to sterics and electronics of the aryl group as in catalysts 63-65 (Figure 22) were generally well tolerated, but only led to minor differences in overall metathesis activity and Z-selectivity [43], Catalyst 66, which contains an V-DIPP substituent, was an obvious target, given the improved Z-selectivity observed in the case of the six-membered chelate (50). However, initial attempts to prepare 66 or other catalysts with significant variation of electronic and steric parameters were hampered by the instability of the products under the reaction conditions, which used silver pivalate. 

The intermolecular direct arylation of simple benzenes was also evaluated [12]. Under conditions established for the fluorinated substrates, none of the desired cross-coupled product was formed. With the goal of increasing the amount of soluble base in the reaction mixture, varying amounts of different carboxylic acids were added. Both the amount and the type of acid was found to influence significantly the reaction outcome, with larger acids used in substoichiometric amounts providing superior outcomes. For example, as the steric bulk is increased from acetic acid to pivalic acid, an increase in conversion is noted. Under optimal conditions, addition of 30 mol% pivalic acid results in 100% conversion of the aryl bromide and an 82% isolated yield of benzene direct arylation is obtained. To rationalize the reactivity, it was proposed that the potassium pivalate may behave as... 

Imidazole /V-oxide substrates may be used in a similar fashion. Initial investigations revealed that the use of palladium acetate in conjunction with an electron deficient 4-fluorophenylphosphine in acetonitrile at 70 °C provides C2 arylation in high yields. With the goal of achieving the same reactivity at or near room temperature it was determined that the use of palladium acetate in conjunction with a Buchwald ligand, catalytic copper bromide and 30 mol% pivalic acid in acetonitrile could also achieve high yields of C2 arylation at 25 °C. As was the case with thiazole V-oxides. if the C2 and C5 positions of the imidazole are blocked C4 arylation may also be achieved in synthetically useful yield (Scheme 15). 

O/t/20-arylation of benzoic acids is often preferable to ortho-arylation of benzamides if conversion of the amide moiety to other functional groups is desired. However, only a few reports have dealt with the orf/io-functionalization of free benzoic acids due to challenges that involve such transformations. The reactions can be complicated by decarboxylation of the product and the starting material. Despite those difficulties, several methods for direct o/t/io-arylation of benzoic acids have been developed. Yu has shown that arylboronates are effective in arylation of benzoic acids under palladium catalysis [59], The reactions require the presence of palladium acetate catalyst, silver carbonate oxidant, and benzoquinone. Even more interestingly, the procedure is applicable to the arylation of unactivated sp3 C-H bonds in tertiary carboxylic acids such as pivalic acid (Scheme 13) if aryl iodide coupling partner is used. Aryl trifluoroborates can also be used [60],... 

They were able to extend the scope of the direct arylation to simple, completely unactivated arenes, such as benzenes by development of a palladium-pivalic acid co-catalyst system (Scheme 28) [48]. 

Scheme 14 Vinylic to aryl to allylic palladium migration followed by pivalate displacement...

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