Pinacolborane halides

In addition, the reactions of dialkoxyboranes, such as pinacolborane, with aryl halides and base have been reported.182,183 The yields are lower for reactions of this reagent in some cases than they are for reactions of the diboron reagent, but good yields are obtained from aryl bromides in most cases. Dialkoxyboranes are less expensive than the analogous diboron compounds, but the diboron compound is very stable thermally and is stable in air. 

Pinacolborane is extensively used in the borylation of aryl halides 114 in the presence of a base (typically pyridine or Et3N or KOAc) and catalytic amount of PdCl2(DPPF) to furnish aryl boronates 115 (DPPF = l,l -bis(diphenyl-phosphino)ferrocene Equation 7) <1997JOC6458, 2000JOC164>. Pinacolborane is compatible with esters, ketones,... 

Important examples of this type of compounds include pinacolborane, catecholborane, etc. These compounds are readily available by the reaction of pinacol 331 or catechol with either borane or boron halides. Transesterification also provides an attractive alternative for the synthesis of these compounds (Scheme 55). 

Murata, M., Watanabe, S., Masuda, Y. Regio- and stereoselective synthesis of allylboranes via platinum(0)-catalyzed borylation of allyl halides with pinacolborane. Tetrahedron Lett. 2000, 41, 5877-5880. 

Suzuki and Stille couplings. For coupling of aryl chlorides with arylboronic acids the catalytic system contains (CyjPjjPdClj, CsF in NMP. co-Arylalkanoic acids are prepared by coupling of aryl halides with carboxylic esters bearing a 9-BBN substituent at the other terminus, and saponification. A biaryl synthesis from two different aryl halides is accomplished with in situ boronate formation which depends on the (dppfjPdClj catalyst. For access to aryl boronates either the coupling of aryl triflates with bis(pinacolato)diboron" or that of aryl iodides with pinacolborane may be employed. 

Allylboranes Displacement of allyl halides with pinacolborane is promoted by (dba)2Pt-Ph3As-Et3N. 

Aryl chlorides typically do not react under these conditions. However, Miyaura has been able to extend the scope of this reaction to include aryl chlorides by changing the catalyst system to Pd(dba)2 and PCys and replacing pinacolborane with bis(pinacolato)diboron. Vinyl iodides and trifiates undergo borylation with pinacolborane under similar conditions in the presence of triphenylarsine (AsPEb) (eq 17). Benzylic halides react with pinacolborane in the presence of PdCl2, PPhs, and N,N-diisopropylethyl amine (eq 18). Borylation of allylic halides in the presence of Pt(dba)2, AsPhs, and EtsN leads to highly regio-and stereoselective allylboronates (eq 19). 

Palladium-catalyzed substitution of an allylic acetate such as 37 with diboronyl reagent 24 gives good yields of allylboronates in DMSO as solvent (Equation 21) [57]. This reaction is a steieoconvergent process the boron tends to add to the least substituted end of unsymmetrical allylic units, and both E and Z isomers of acetate 37 yield the E-isomer of 38. Another drawback to this process is that the product is often formed with variable yields of the allylic dimer (e.g., 39). Allylic halides were also employed as substrates with pinacolborane as the borylating agent and a platinum catalyst [58]. 

Clary, J. W., Rettemnaier, T. J., Snelling, R., Bryks, W., Banwell, J., Wipke, W. T., and Singaram, B. (2011). Hydride as a leaving group in the reaction of pinacolborane with halides under ambient Grignard and Barbier conditions, one-pot s5Tithesis of alkyl, aryl, heteroaryl, vinyl, and aUyl pinacol-boronic esters. J. Org. Chem., 76, 9602-9610. 

Murata, M. Oyama, T. Watanabe, S. Masuda, Y. 2001. Synthesis of benzylboronates via palladium catalyzed borylation of benzyl halides with pinacolborane. Synthetic Commun. 32 2513-2517. 

Suzuki-Miyaura Cross Coupling. Arylboronates are valuable reagents in organic synthesis owing to their widespread use in Suzuki-Miyaura cross coupling reactions. Pinacolborane is extensively used in the horylation of aryl halides in the presence of a base (typically pyridine, EtsN, or KOAc) and a catalytic amount of PdCl2(dppf) affording arylboronates (eq 16). 

If the substrate does not c ntam electrophiles, try a one-pot magnesiation/borylation of aryl halides using pinacolborane [2]... 

A modification to the classic preparation of arylboronates using a Grignard-type approach entailed the use of elemental magnesium to generate the organometallic reagent in solution from the aryl halide in the presence of pinacolborane (Scheme 6.22) [2]. This... 

If the substrate does not contain electrophiles, try a one-pot magnesiation/boiylation of aryl halides nsing pinacolborane [2]. If the snbstrates do contain sensitive functional groups, try adding a reagent that will modify the reactivity of the Grignard reagent [41,42]... 

SCHEME 637 Palladium-catalyzed coupling of vinyl halides with pinacolborane [51]. 

Alternatively, the borylation can be conducted with pinacolborane (HBpin) that takes place smoothly for aryl and 1-alkenyl halides or triflates (Scheme 5-166). For this reaction, sometimes named the Matsuda reaction, both carbocyclic and heterocyclic systems can be used. 

Recently, a novel coupling reaction, based on the used of pinacolborane as an efficient reagent for the electrochemical functionalization of aryl halides in the presence of an Mg anode afforded arylboronic pinacol esters [2]. However, this reaction presents some limitations low faradic efficiencies with aryl chlorides,... 

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