Diboronate esters

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

The Ag complex 74 catalyses the diboration of internal and terminal aUcenes to 1,2-bis-diboronate esters by bis(catecholato)diboron, (Bcat), in THF at room temperature. Variable conversions (30-90%) were obtained at 5 mol% loading after 60 h. 

This route involves the conversion of a 3,4-diiodopyrrole (139) to the corresponding 3,4-diboronate ester (140) followed by a bis Suzuki cross-coupling reaction with a bromoquinoline, which generates the halitulin core (141). This pyrrole (141) is then alkylated with a tosylated cycloazadecane to generate a pentasubstituted pyrrole (143), which is converted to halitulin by debenzylation under hydrogenolysis conditions. 

Although the [Ir(Ti -C5Me5)(NHC)] complex was initially tested in the diboration of styrene, this did not lead to the formation of any diboronate esters when bis(catecholato)diboron (B2cat2) was added to a solution of the catalyst and styrene in tetrahydrofuran (THF) under argon (Scheme 7.23). However, the addition of... 

An alternate approach to the formation of pyridylboronic acids is the cross-coupling of a halopyridine with a diboronate ester (usually bis(pinacolato)diboron, 7.7.)9 The analogous reaction of 2-chloropyridine led to pyridine formation through protodeboronation. The product of the reaction, either after hydrolysis to the boronic acid or in the ester form, can be further reacted with another aryl halide to give a biaryl. In certain cases the reaction might also be carried out in a one-pot manner.10... 

The coupling procedure appears to be well suited to the formation of dialkylamino- and mixed alkyl-dialkylaminodiboron derivatives. It is less satisfactory for the formation of diboron esters of low molecular weight alcohols. 

B.iii.c. ttsns-ArPd(OAc)L2 Complexes (L = PPh ). Irani-ArPd(OAc)(PPh3)2 complexes are reagents for the transmetallation step in Suzuki cross-coupling when acetate ions are added as a base. Indeed, Ishiyama and co-workers have reported that trans-ArPd(OAc)(PPh3)2 complexes, formed by substitution of bromide by acetate in trans-ArPdBr(PPh3)2 (see Sect. undergo transmetallation reactions with diboronic esters... 

Several years later, Reynolds and co-workers [179] extended this work to prepare another sulfonated poly(p-phenylene) using the same approach by replacing 1,4-diboronic acid with 4,4 -biphenyl diboronic ester ... 

MHz H NMR spectrum of a benzene-1,4-diboronic ester to illustrate the level of purity considered sufficient. The signal of the NMR solvent (CDCI3) and impurities contained in it ( ) are marked ( ). The signals of residual solvent (diethylether) from which the monomer was recrystallized are also marked ( ). The insets show strongly... 

The synthesis of poly(p-phenylene) via the homocoupling of p-hro-mophenylboronic acids was first reported by the Max Planck Institut. After this discovery, various new poly(p-phenylenes) were synthesized. A rigid-rod poIy(p-phenylene) was prepared by cross-coupling a biphenyl diboronic ester and a dibromide in aqueous DMF in the presence of a water soluble palladium catalyst (Eq. 78). The... 

The Suzuki polymerization is the AA-BB type, and the monomers are usually diarylbromide compounds and diarylboronate esters. It is possible to use diboronic acids as monomers, but the diboronate esters are generally used because they are more easily purified than diboronic acids. 

An easy route to boron-substituted carbanions via the deprotonation of pinacol(phenylthio)methaneboronate by lithium di-isopropylamide (LDA) has been published (reaction (9)]. The preparation of several l-alkene-1,1-diboronic esters (16) and their subsequent conversion into l,l-bis(chloromercuri)-1-alkenes (17) has appeared [reaction (10)] as has a report concerning the oxidative rearrangement of B-methylated l-alkene-l,l-diboronic esters to methyl ketones. ... 

SCHEME 5.11 The preparation of organoboronic acids using diboron esters. 

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