Arylboron dichloride

The cyclization can be carried out with halogenated amines, and substitution products of the new ring systems can also be obtained in the conventional way, by nitration, etc. Similar compounds can be prepared directly by using arylboron dichlorides in place of boron trichloride in the procedure indicated above. The parent borazarene derivatives, with hydrogen attached to boron, can be made from the B-hydroxy compounds with lithium aluminum hydride in the presence of aluminum chloride. N-Alkyl derivatives can be made either by using N-alkyl derivatives of the aminobiphenyls as starting materials, or by N-alkylation of the unsubstituted compounds via their N-lithio derivatives. Apart from their inherent interest, compounds of this type can serve as intermediates in various syntheses. Thus benzocinnolines and 2,2 -dihydroxybiphenyls can be obtained from derivatives of 2-aminobiphenyl. ... 

Since our group (22) and Hehnchen s (23) independently announced a new class of chiral acyloxyboranes derive from iV-sulfonylamino acids and borane THF, chiral 1,3 -oxazaborolidines, their utility as chiral Lewis acid catalysts in enantioselective synthesis has been convincingly demonstrated (2(5). In particular, Corey s tryptophan-derived chiral oxazaborolidines 10a and 10b are highly effective for not only Mukaiyama aldol reactions (24) but also Diels-Alder reactions (25). More than 20 mol% of 10b is required for the former reaction, however. Actually, the reaction of the trimethylsilyl enol ether derived from cyclopentanone with benzaldehyde afforded the aldoI products in only 71% yield even in the presence of 40 mol%of 10b (24). We recently succeed in renewing 10b as a new and extremely active catalyst lOd using arylboron dichlorides as Lewis acid components (2(5). 

An enantioselective Diels-Alder reaction induced by an axial chiral arylboron dichloride bearing the binaphthyl skeleton has also been reported. Although the reaction of cyclopentadiene and methyl acrylate in the presence of 10% of this catalyst gives the endo adduct with >99% diastereoselectivity, only moderate enantiomeric excess is obtained. 

A similar approach was taken for the synthesis of 45 by Miyaura. " Shaughnessy and Booth synthesized the water-soluble alkylphosphine 46, and found it to provide very active palladium catalysts for the reaction of aryl bromides or chlorides with boronic acids. The more sterically demanding ligand 47 was shown to promote the reactions of aryl chlorides with better results than 46. Najera and co-workers recently reported on the synthesis of di(2-pyridyl)-methylamine-palladium dichloride complexes 48a and 48b, and their use in the coupling of a variety of electrophiles (aryl bromides or chlorides, allyl chlorides, acetates or carbonates) with alkyl- or arylboronic acids very low catalyst loadings at Palladium-oxime catalysts 8a and 8b) have also been developed. In conjunction with... 

Examples for o-phenylene scaffolds for bis-carbene ligands come from the research groups of Peris [344,345] and Herrmann [346]. Synthesis of the bis-imidazolium salt is achieved by reaction of a,a -xylene dichloride and the N-substituted imidazole. The rhodium(l) and iridinm(I) complexes can then be made by addition of the imidazolium salt to a solution of [M(cod)Cl]2 (M = Rh, Ir) in ethanol or acetonitrile (with NEtj as auxiliary base) (see Figure 3.108). The rhodium complexes were used successfully in the hydrosi-lylation of styrene [344] whereas both the rhodium and iridium complexes were used for the direct borylation of arenes making functionalised arylboronic acid esters accessible by a simple one-pot reaction [346]. 

The Pd-catalyzed Suzuki coupling of dichloropyrazines 58 with arylboronic acid has been reported. The reaction proceeded smoothly with bis(triphenylphosphine)palla-dium(II) dichloride under anaerobic conditions to give the corresponding diarylpiper-azine 59 in moderate to good yields [39]. 

The monoarylthallium halides are obtained by boiling the corresponding arylbor-onic acid with xs Tl(III) halide in HjO for 20-30 min. The diarylthallium halides are produced by boiling equimolar Tl(III) halide and arylboronic acid for 5-6 h. The mono-and dichlorides can be separated owing to their differing solubility in HjO. The dibromide is also readily separable, but the monobromide decomposes to Ar2TlBr and TlBt3. Substituted phenyls are synthesized in this manner " , with yields of 30-85%. 

Apart from arylboronic acids, tetraarylborates, under photochemical conditions [24] or in the presence of a stoichiometric amount of diphenylsilicon dichloride, under an oxygen atmosphere, give biaryls in good to high yields [25], Namely, tetramethyl ammonium tetra(4-tolyl)borate (441) was coupled to 4,4 -dimethylbiphenyl (442) in 96% yield, respectively, Scheme 10. 

Aryliodoninm salts, Aral X [50], and some other hypervalent aryliodonium compounds take a part in further important alternative cross-coupling reactions with arylboronic acids [51], aryltrifluoroborates [52], triarylbismuth(V) compounds [53], and diaryltellurium dichlorides [54] to afford biaryls in good to excellent yields. For example, hypervalent iodine compoimd 466, readily produced from 2-iodobenzoic acid, reacts with arylboronic acids in palladium-catalysed process in Suzuki-Miyaura-fashion to give the 2-arylbenzoic acids in high yields [51], Table 12. 

Catalyst lOd was prepared sinply by treatment of iV-(/ -toluenesuIfonyl)-(5)-tryptophan with 3,5-bis(trifluoromethyl)phenylboron dichloride and subsequent removal of the produced HCl in vacuo (eq 15) (26). The 5-Aryloxazaborolidine could not be prepared from the arylboronic acid. 

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