Pinacol diborane

Borylation is the photochemically or thermally promoted conversion of substrates with C-H bonds into boronate esters using bis(pinacol)diborane(4) (B2pin2) or pinacolborane (HBpin) reagents (Scheme 1) [16-35]. The selectivity for methyl C-H bonds and the versatility of organoboron reagents [36, 37] give alkane borylation potential for use in synthetic contexts. 

Iridium-catalysed C-H-boronation can be carried out using either pinacol borane or pinacol diborane, both methods giving comparable results. Reaction occurs at a-positions of five-membered rings and is compatible with halogen substituents, as exemplified below. ... 

The preparation of the boronate ester utilizes pinacol diborane, which is expensive and available in limited quantities. 

The electronic nature of the coupling partners could be reversed by introducing a boronic acid fragment on the benzonitrile, such as boronic acid 22c, and affecting the coupling with an aryl triflate of pyridinium bromide. Because the aryl boronate could be prepared using the aryl lithium in the presence of triisopropyl borate, this would obviate the need for the expensive pinacol diborane. Reduction of the intermediate pyridine or tetrahydropyridines would provide the desired products (see Scheme 5.8). 

Alternative Names bis(pinacolato)diboron B2pin2t diborane pinacol ester pinacol diborane. 

Scheme 3.11 Synthesis of aryl boronates by palladium- catalyzed bo lation of a l halides with b/s(pinacol)diborane (37).

Fiirstner has described a synthesis of aryl boronates of type 42 bearing electron-withdrawing groups by cross-coupling of the respective para-substituted chloroarenes 43 with fois(pinacol)diborane (37) in the presence of a catalyst formed in situ from Pd(OAc)2 and the imidazolium chloride 44 [19]. A particularly noteworthy aspect of this reaction is the very significant rate acceleration, if the borylation is carried out with microwave heating. This allows the reduction of the overall reaction time from several hours to 10-20 min without affecting the yields (Scheme 3.13). 

Williams has reported the synthesis of biaryl moiety of proteasome inhibitors TMC-95/A/B by Suzuki cross-coupling of boronic ester 68 with indolyl iodide 69 to give precursor 70 in 90% yield [49]. The boronic ester 68 was obtained from the tyrosine derivative 71 with i)is(pinacol)diborane (37), Pd(dppf)Cl2 and KOAc via the Miyaura protocol (Scheme 3.31). 

A novel macrocyclization procedure involving ttvo distinct cross-coupling manifolds in a domino fashion has been reported by Zhu for the synthesis of bipheno-mycin model 86 [56]. Thus, treatment oflinear feis-iodide with fois(pinacol)diborane (37) in the presence of Pd(dppf)2Cl2 under defined conditions affords the biphenyl macrocyclic compound 86 in 45% yield through a Miyaura aryl boronic ester formation followed by its intramolecular Suzuki cross-coupling. The diiodide containing a free phenol function (R=H) gave the macrocycle (R=H) in only 22-25% yield under these conditions (Scheme 3.37). 

The rhodium-catalyzed borylation of methyl C-H bonds is compatible with several moieties containing oxygen, nitrogen, and fluorine.154 For example, the reaction of pinacol acetal of 2-hexanone with bis(pinacolato)diborane (B2pin2) in the presence of Cp Rh(774-C6Me6) catalyst gives the alkylboronate ester in 74% yield (Equation (111)). The rhodium-catalyzed C-H activation and borylation occur at the least hindered and least electron-rich methyl group. 

The cross-coupling reaction of bis(pinacolato)diborane(4) [(Me4C202) B-B(02C2Me4)] with allyl acetates provided the pinacol esters of allyl-boronic acids with common structure 74 with regio- and fi-stereoselectively in high yields as 75-84 have been reported (Scheme 15). The reaction was efficiently catalyzed by Pd(dba)2 in DMSO at 50 °C.66... 

Synthesis of pinacol arylboronates 190 200 via cross-coupling reaction of bis(pinacolato)diborane(4) with chloroarenes catalyzed by palladium(O)-tricyclohexyl-phosphine complexes has been demonstrated by Ishiyama et al. (Scheme 31).115... 

Compound (80), prepared from 2-methylcyclopentane-l,3-dione and methyl 2-chloroacrylate followed by the sequence (76)-> (77)-> (78)- (79) (resolved)— (80), combined with (71) (prepared from m-methoxyphenylstyrene and diborane) to form the seco-steroid (82)/ Acetic anhydride-toluene-p-sulphonic acid then cyclized this stereoselectively to furnish the triacetate (83) which on saponification gave the triol (84). Interestingly, this compound reacted with toluene-p-sulphonic acid in alcohol to produce, by dehydration and change of configuration at C-14, the compound (85), which served as a source of various 8a-oestrone compounds. Compound (84) on treatment with boron trifluoride etherate underwent pinacol transformation in preference to dehydration to yield the ketone (86) this ketone was correlated with the known compound (87). 

The photocatalytic generation of aryl radicals was also successfully applied to the formation of carbon heteroatom bonds. The aryl pinacolboronates 141 can be easily achieved by visible light irradiation of a solution of aryl diazonium salts 142 and diboron pinacol ester 143 containing 5mol% of eosin (Scheme 29.24) [89]. The proposed meehanism involves the addition of aryl radical 144 to the complex 145 that is generated by interaction of tetrafluoroborate anion and diboran pinacol ester 143. This process leads to the formation of the aryl pinacolboronates 141 and the radical anion intermediate 146. The oxidation of this intermediate by eosin radical cation completes the catalytic cycle. 

Scheme XXIII. Synthesis of dl-perhydrohistrionicotoxim (72). The 2-epimer was the major product if the free alcohol was used in reaction E. A i)Mg, TiCU (Formation of cyclopentanone pinacol), ii) H2SO4, heat. B i) n-ButylLi, ii) SOCI2, pyridine, iii) a. Diborane, b. H2O2, OH , iv) CINO. C i) hv, ii) Tosyl chloride, pyridine. D i) UAIH4, ii) (CH3)2t-BuSiCl, iii) N-Bromosuccinimide, iv) t-AmylOK. E i) n-AmylLi, ii) Tetrabutylammonium fluoride...

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