Cross-coupling, decarbonylative

Decarbonylation (s.a. Cross-coupling, decarbonylative Diene synthesis, intramolecular, -)... 

The synthetic potential of palladium-mediated cross-coupling reactions (Heck, Suzuki, Stille, Sonogashira, Buchwald-Hartwig) led to the search for a practical synthesis of p-[ F]fluoroiodo- and p-[ F]fluorobromobenzene. p-[ F]Fluoroio-dobenzene (G, X = iodine) can be obtained in poor yield from p F]fluoride and a trimethylammonium precursor (P7). p-p F]Fluorobromobenzene can be prepared in a more reproducible way from 5-bromo-2-nitrobenzaldehyde (radiochemical yields > 70%). The synthesis involves a two-step procedure radiofluorination (F for NO2 substitution), then a catalysed decarbonylation [190,191]. Also very efficient is the one-step reaction of p F]fluoride with a suitable diaryliodonium salt (P6) giving >70% radiochemical yield [192-194]. 

Decarbonylative cross coupling of acyl halides.8 This unusual reaction is observed on coupling aroyl chlorides with alkyl(phenyl)acetyl chlorides in the pres-... 

Terminal arylalkynes can be prepared by oxidation-decarbonylation of 3-arylpropargyl alcohols using manganese dioxide in the presence of alkali. The corresponding arylpiopargyl dcohols are available by palladium-catalysed cross-coupling of aryl halides with commercial propargyl alcohol. The yield of the second step can be improved by the addition of a phase-transfer catalyst (18-crown-6 Scheme 18). ... 

Various Pd-catalyzed carbonylation reactions have often been referred to as carbonyla-tive cross-coupling reactions (Scheme 4). However, these reactions involving the formation of two C—C bonds with incorporation of CO clearly display a pattern of chemical transformation that is different from Scheme 1. So, these reactions are discussed in Parts VI and VIII. On the other hand, Pd-catalyzed acylation with acyl halides and related derivatives are examples of the reaction represented by Scheme 1, where is acyl, and they are therefore discussed in this Part (Sect. III.2.12.1), even if CO may be used to prevent decarbonylation. 

Overall, the carbonylation of alkynes is rather complex, but it is possible to draw a general trend and to divide these processes into three classes depending on the alkyne (i) For most internal alkynes, the carbon-carbon bond-forming process can involve an acylpalladation step whether there is an isomerization or not. (ii) However, some of them may involve an electrophilic activation of the triple bond by the acylpalladium complex followed by nucleophilic attack and reductive elimination, (iii) On the other hand, terminal alkynes appear to undergo mostly cross-coupling for the first carbon-carbon bond formation. Aside from these mechanistic intricacies, it is important to point out that these processes usually involve incorporation of more than one molecule of CO and creation of two to three carbon-carbon bonds in one reaction, and they yield heterocycles in fair to good yields. Other multiple bond systems like alkenes, imines or dienes also provide nice entries to carbo- and heterocycles. The limitations are usually due to the necessary time balance between acylpalladation and the termination step to avoid polymeric or decarbonylation processes. 

SCHEME 22.4 (a) Nickel-mediated decarbonylative cross-coupling of phthalimides (b) Proposed reaction mechanism. 

Ni-CaUdyzed Reactions The nickel-mediated decarbonylative cross-coupling of dia-rylzinc reagents with phthaUmides, in the role of electrophilic coupling partner, has been reported, using reaction conditions similar to those described in Scheme 22.4 [9]. This reaction provides a highly efficient means to construct ort/io-substituted benzamides (Scheme 22.30). 

SCHEME 22.30 Ar-Ar bond formation via Ni-mediated decarbonylative cross-coupling of phthalimides with dioiganozinc reagents. 

Nakao, Y, Ebata, S., Yada, A., Hiyama, T., Ikawa, M., Ogoshi, S. (2008). Intramolecular arylcyanation of alkenes catalyzed by nickel/AlMe Q. Journal of the American Chemical Society, 130, 12874-12875. Havlik, S. E., Simmons, J. M., Winton, V. J., Johnson, J. B. (2011). Nickel-mediated decarbonylative cross-coupling of phthalimides with in situ generated diorganozinc reagents. The Journal of Organic Chemistry, 76, 3588-3593. 

The reaction of fluorinated vinylstannane 23 with n-BuLi in THF at -78 C for 1 h followed by the addition of carbonyl compounds afforded the corresponding allyl alcohols 26 in good yields (Scheme 5). On the other hand, the Pd(0)-catalyzed cross-coupling reaction of perfluorocyclopentenylstannane 23 with benzyl chloro-formate in THF at reflux for 2 h proceeded smoothly to form the decarbonylated coupling product 27 in high yield. 

In 2012, Yamaguchi, Itami, and coworkers discovered a nickel-catalyzed decarbonylative C-H coupling between 1,3-azoles and aryl esters and applied these catalytic conditions to the synthesis of muscoride A, a natural product that displays weak antibacterial activity (Scheme 16.28) [59]. Two azole esters, 137 and 138, were coupled under Ni/dcype catalysis to furnish the corresponding product 139 in 39% yield. As the conversion of 139 into 140 has been previously described [60], a formal synthesis of muscoride A has been completed. If the synthesis were planned and executed with typical cross-coupling substrates (aryl halides and organometalhc reagents) instead, it would have become much less efficient, with many added steps. 

Other sources for cross-coupling reactions are aldehydes and carboxylic acids after decarbonylation and decarboxylation, respectively, which can be reacted with aryl halides to form biaryls. The following Experimental Procedure illustrates the potential of this quite atom-economic reaction. In this case, a copper co-catalyst promotes both the decarboxylation and the cross-coupling. 

---