Aldehydes aryl bromides

An unusual palladium-catalyzed arylative fragmenation process of /3-hydroxy-substituted allenes was observed by Oh et al. [59]. Compounds such as 8 reacted with aryl bromides and iodides in the presence of Pd(PPh3)4 and K2C03 as base to give 1,3-dienes 120 and aldehydes 121 as second fragment (Scheme 14.28). The initially expected cyclization product, dihydropyran 122 (Scheme 14.29), was usually not formed. 

Aryl halides can be dehalogenated with triethylsilane in the presence of a palladium catalyst. The method is versatile and can also be used for the reduction of acyl chlorides to aldehydes, or benzylicbromides to the corresponding hydrocarbons. If different types of halides are present in the molecule, selective dehalogenation takes place. Thus, an aryl iodide can be reduced in the presence of a chloride, and benzylic bromide is reduced more easily than an aryl bromide. Finally, the method is even able to distinguish between two aryl bromides in the same molecule (Scheme 4.41)67. 

Allylic alcohols also may be arylated in / /-methylpyrrolidinone or DMF solution with sodium bicarbonate as the base.81 The use of this base improves the yields of aldehydes obtained compared with tertiary amine bases in the case of aryl bromides with electron-withdrawing substituents, where reduction to an arene is sometimes a problem,... 

Fig. 12. Versatile derivatization scheme of a tropane template, including palladium-mediated coupling, (a) TsOH, CH2C12 (b) aryl bromide, Pd(PPh3)4, THF, 66° (c) arylboronic acid, 2 N Na2C03, PPh3, THF, or anisole, 66° (d) formic acid, Et3N, PPh3, DMF, 66° (e) phenylacetylene, Cul, B114NCI, DMF, 66° (f) B114NF, THF, 50° (g) aldehyde, NaBH(OAc)3, DMF (h) HATU, Et3N, benzoic acid, HOAt, DMF (i) TFA/H2Q (20 1).

A procedure has been developed for the palladium-catalysed a-arylation of amides by aryl bromides using the zinc enolates of the amides. The reaction works well with bromoarenes carrying a variety of ring substituents and with bromopyridine. In addition, the reaction has been shown to be effective with morpholine amides to give products which are precursors for aldehydes and ketones.39 A new method has been reported for the allylation of aryl halides using homoallyl alcohols as the allyl source the palladium-catalysed reaction, which may be both stereo- and regio-sepecific, uses a retro-allylation reaction to form a a-allyl(aryl)palladium intermediate.40... 

The use of a relatively soluble base such as CS2CO3 allows good product yield. No products are formed via carbopalladation. Therefore the reaction is considered to occur on a dienolate anion generated from the enal to give an aryl(7r-allyl)palladium intermediate. The regioselectivity seems to be determined in the reductive elimination of the product. Treatment of aliphatic aldehydes with aryl bromides brings about aldol condensation followed by y-aryla-tion to afford 2 1 coupling products (Eq. 27). Note that y-arylation products are also produced in the arylation of a tin-masked dienolate [65,66]. 

DiMauro and co-workers [67] have developed a rapid and efficient synthesis of 3-amino-imidazopyridines 16 using a microwave-assisted one-pot cyclization. The intermediate 15 was further reacted with various aryl bromides in the presence of a catalytic amount of Pd(dppf)Cl2 to yield the target compounds 16. The reaction scope is quite broad with respect to the aldehyde and aryl bromide components which might be electron-rich, electron-poor, aromatic, aliphatic, or sterically encumbered (Scheme 15). 

What happens must be this a Grignard reagent is formed with the aryl bromide and adds to the aldehyde. 

On treatment with bromine, certain diarylcarbinols (I) are converted into a 50 50 mixture of aryl bromide (II) and aldehyde (III). 

Tt-Allylnickel halides. Billington has reviewed the preparation of these complexes from allylic halides using Ni(CO)4 or Ni(COD)2, and their use in synthesis, mainly of natural products (54 references). These complexes react with a wide range of both aliphatic and aryl bromides or iodides as well as aldehydes, ketones, epoxides, and quinones. One advantage is that both allyl ligands react. They do not react with acid chlorides, esters, ethers, nitriles, or acetals. 

Palladium-catalyzed phosphination of substituted aryl bromides, using triarylphosphines as the phosphinating agents, has been developed, as illustrated by the synthesis of (57) (Equation (18)).89 This method tolerates various functionalities, e.g., ketone, aldehyde, ester, nitrile, ether, and chloride. Under similar catalytic conditions, various P,/V-ligands, e.g., (58) (Scheme 4), were synthesized using triflate precursors instead.90,91 Arylphosphines can also be prepared by phosphination with triphenylphosphine catalyzed by palladium on charcoal.92... 

In a synthesis of decinine, a phenol was protected as a methanesulfonate that was stable during an Ullmann coupling reaction and during a condensation, catalyzed by calcium hydroxide, of an amine with an aldehyde. Aryl methanesulfonates are cleaved by warm sodium hydroxide solution, with LDA (THF, -78°C to rt, 57-95% yield) or with TMSOK/CH3CN. An aryl methanesulfonate was cleaved to a phenol by phenyllithium or phenylmagnesium bromide it was reduced to an aromatic hydrocarbon by sodium in liquid ammonia. ... 

A decade later, Corey introduced an effective aluminum-diamine controller for Diels-Alder and aldol additions. The C2-symmetric stilbenediamine (stien) ligands are available in good yield from substituted benzils, which are in turn derived from benzoic acids, aryl aldehydes, or aryl bromides [48]. Formation of the active catalyst 3 is achieved by treatment of the bis(sulfonamide) with tri-methylaluminum recovery of the ligand was essentially quantitative. Acryloyl and crotonyl imides 4 are particularly effective dienophiles for this system, as shown in Scheme 4. 

Although these conditions are quite general for a wide variety of aryl bromides, the use of a strong base (NaOr-Bu) can be incompatible with base sensitive, enolizable, substituents (R1). To address this limitation, Buchwald was able to replace NaOf-Bu with CS2CO3. This allowed for aryl bromides substituted with an ester, nitrile, nitro, ketone, or aldehyde to couple cleanly with primary amines and cyclic secondary amines. 

DiaryImethanols. Zinc in combination with the (dppe)NiBr2 complex mediates the reaction between aryl bromides and aromatic aldehydes. This process tolerates functional groups such as ketone, ester, amide, and nitrile. 

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