Aryl- and 1-alkenylboronic acids

Various rhodium(i) complexes catalyze the addition reaction of aryl- and 1-alkenylboronic acids to cr,/3-unsaturated ketones 489,935 936 4 93,941 and 495,937 aldehydes 490,938 esters 491 939 and 493 940 lactones 494,941 and amides 492942 in an aqueous solvent (Scheme 36). 

The reaction is catalyzed by transition metal complexes coordinated with phosphine ligands. Since chiral phosphine ligands are the chiral auxiliaries most extensively studied for transition metal catalyzed asymmetric reactions, one can use the accumulated knowledge of the chiral phosphine ligands for the asymmetric reaction. The asymmetric 1,4-addition of aryl- and 1-alkenylboronic acids to enones proceeded with high enantioselectivity in the presence of a chiral phosphine-rhodium catalyst (Table 2). 

The synthesis of alkyl-, aryl-, and 1-alkenylboronic acids or their esters from Grignard or lithium reagents and trialkylborates is a classical and efficient method for making relatively simple boron compounds in large quantities (Scheme 2-6) [25]. The stereocontrolled synthesis of alkenylboronic acids and esters involves the reaction of a (Z)- or ( )-2-buten-2-ylmagnesium bromide with trimethyl borate [26]. 

The efficiency of transmetalation from boron to palladium in the catalytic 1,4-addition of aryl or 1-alkenylboronic acids to enones encouraged us to extend the protocol to the addition of aryl- and 1-alkenylboronic acids to alddiydes in an aqueous solution (Eq. 4). The insertion of carbonyl groups into transition metal-carbon bonds has not received much attention, but the catalytic use of transition metals may allow such addition of various organometallics which are inert without a catalyst, the asymmetric addition using a chiral phosphine complex, or the reaction in an aqueous phase. 

Aqueous TlOH solution, which produces highly insoluble salts similar to the barium base, completes the coupling reaction of mesitylboronic acid with iodobenzene within 30 min at room temperature (Table 2 and the procedure in the text)." However, this excellent base for mesitylboronic acid, unfortunately, does not work well for other less substituted arylboronic acids. Aryl- and 1-alkenylboronic acids transmetallate to thallium salts. 

The rhodium-catalyzed addition of aryl- and 1-alkenylboronic acids tooc, unsaiurated ketones, aldehydes, esters, and amides gave the conjugate 1,4-addition products in high yidds. The ifaodium(I) complexes also catalyzed the 1,2-addition of organoboronic acids to aldehydes or N-sulfonyl aldimines. The dficiency of protocol was demonstrated in the asymmetric addition reactions of organoboronic acids in the presoice of a rhodium(acacV BINAP complex. 

Alkenyl, aryl, and heteroaryl boronic acids reacted with 1,2-diamines and glyoxylic acid giving directly piperazi-nones (Equation (123)).571 2-Hydroxymorpholines were synthesized from glyoxal, 2-aminoethanols, and aryl- or 1-alkenylboronic acids (Equation (124)).572... 

Miyaura and coworkers reported the Rh(l)-catalyzed conjugate addition of aryl- or 1-alkenylboronic acids, RB(OH)2,to enones in high yields at 50°C in an aqueous solvent. A... 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

However, the application of these classical procedures for 1-alkenylboronic acid or ester synthesis may suffer from the formation of small amounts of the opposite stereoisomers, or from bis-alkenylation leading to the boronic acid derivatives. Also, formation of trialkylboranes may occur. A recent useful variant utilizes organolithium reagents and triisopropyl borate, followed by acidification with HCl to give directly alkyl-, aryl-, 1-alkynyl-, and 1-alkenylboronic esters in high yields, often over 90% (Scheme 2-6) [27]. Triisopropyl borate was shown to be the best of the available alkyl borates to avoid multiple alkylation of the borates. 

In 1997, Miyaura and co-workers reported the nonasymmetric version of 1,4-addition of aryl- and alkenylboronic acids to a,/ -unsaturated ketones using rhodium-phosphine complex as the catalyst.97 Later, Hayashi and Miyaura realized the asymmetric 1,4-addition with high catalytic activity and enantioselectivity.98 In the presence of ( y)-BINAP, the reaction of 2-cyclohexenone with 2.5 equiv. of phenylboronic acid gave (A)-3-phenylcyclohexanone with 97% ee (BINAP = 2,2 -bis (diphenylphosphino)-l,l -binaphthyl Scheme 29).99... 

A broad substrate scope for the rhodium-catalyzed asymmetric 1,4-addition has been observed.98 Both arylboronic acids with either electron-donating or electron-withdrawing aryl substituents and alkenylboronic acids can be introduced into acyclic or cyclic enones with high enantioselectivities (Scheme 30). 

Addition of Organometallic Reagents to Enones in Aqueous Media Rhodium-catalyzed 1,4-addition of organometallic reagents to a,p-unsaturated compounds was first developed by Miyaura in 1997. Thus, Rh(acac)(CO)2/dppb was found to catalyze the 1,4-addition of aryl- and alkenylboronic acids to several ot,(3-unsaturated ketones in water-containing solvents at 50°C. The reaction conditions were successfully modified for the development of an asymmetric variant of this process by Hayashi and Miyaura in 1998. The important points of modification are (1) the use of Rh(acac)(C2H4)2/(5)-binap as a catalyst and... 

The reaction was efficiently catalyzed by a rhodium complex generated in situ by mixing Rh(acac)(CH2=CH2)2 with 1 equiv of (S)-binap in an aqueous solvent at 100 1C. It was interesting that the enantioselectivity was kept constant at the reaction temperature ranging between 40-120 1C in the addition to 2-cyclohexenone. High enantioselectivity exceeding 90%ee were readily achieved for both cyclic and acyclic a,3-unsaturated ketones with variety of aryl- and alkenylboronic acids. 

Alkenylboron derivatives are readily available by stereo-defined hydroboration of alkynes and used for the preparation of styrene derivatives by alkenyl-aryl coupling. Vinylboronic acid (81), the most simple alkenylboronic acid, is difficult to handle, because it undergoes uncontrollable polymerization. Its anhydride, 2,4,6-trivinylcyclotriboroxane (82)-pyridine complex, is stable, and can be used conveniently for the coupling. Vinylation of 2-bromoanisole with 82 proceeds in refluxing DME using K2CO3 as a base fo provide 2-vinylanisole (83) [80]. 

De la Herran, G., Murcia, C. and Csdky, A.G. (2005) Rhodium-catalyzed reaction of aryl- and alkenylboronic acids with 2,4-dienoate esters conjugate addition and Heck reaction products. Org. Lett., 1, 5629-32. 

In combination with catalytic amounts of Pd, CuTC has given rise to novel cross-coupling reactions under mild and neutral con-ditions that have furnished a number of very useful organic building blocks. Thus, an unprecedented cross-coupling of thiol esters with aryl and alkenylboronic acids in the presence of a catalytic amount of Pd(0) and 1.6 equiv of CuTC furnished a wide variety of aryl ketones under baseless conditions (eq 7). ... 

Compared to aryl- and alkenylboronic adds, alkylboronic acids and esters have found limited use as synthetic intermediates aside for their oxidation into alcohols (Section 1.5.2.1). This is due in part to their inferior shelf-stability. In addition, their trans-metallation with transition metal catalysts such as palladium is presumed to be more difficult than that of the unsaturated and aromatic boronic acid derivatives [296]. For example, alkylboronic adds have long been known to be reluctant substrates in the Suzuki-cross-couphng reaction, and they have become eflfident in this apphcation only very recently with the use of special bases and the advent of new and highly active catalyst systems (Section 1.5.3.1). Perhaps the most synthetically useful class of alkylboronic adds are the a-haloalkyl derivatives popularized by Matteson (Section 1.3.8.4), and their elegant chemistry is described in Chapter 8. 

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