Diboron reagents

We thank the NSF and NIH for support of these projects, Johnson-Matthey for metal salts, and Frontier Science and Allychem for donations of diboron reagents. 

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. 

Palladium-catalyzed reaction of aryl halides with diboron reagent to produce aryl-boronates. Also known as Hosomi-Miyaura borylation. 

Rhodium-catalyzed 1,4-addition reactions of bis(pinacolato)diborane(4) and bis(neopentyl glycolato)diboron to a,(3-unsaturated ketones give the corresponding boron derivatives 222, 223, and 224-230 (Scheme 40).131 This is the first reported example in which a rhodium catalyst was used in addition reaction of diboron reagents to ot,(3-unsaturated electron deficient alkenes. 

Catalyzed Addition of Diboron Reagents to Unsaturated Organics... 

Important work by Chen et al. [123b] has shown how borylation of alkanes can be achieved both photochemically and thermally from diboron reagents to give alkylboranes (Eq. 2.43). The best catalysts, [CpRh(ethylene)2] and [Cp Rh(r/ -CeMce)], are active at 150°C. The B-B bond oxidatively adds to the metal probably followed by CH oxidative addition. Reductive eMmination gives rise to a new B-C bond being formed. Functionalization occurs at the terminal position of a linear alkane as in the alkane chemistry described above. Since C-B bonds are in principle precursors to a wide variety of functional groups, this reaction has great promise for future development. 

The combination of NiBr2 diglyme and 7 (R = r Pr) catalyses the crosscoupling of unactivated primary, secondary and tertiary alkyl halides with diboron reagent, pinB-Bpin, affording alkylboronates in good yields (Scheme 14.12). Compared with related nickel-catalysed carbon-carbon bond-forming reactions, this reaction exhibits an inverse reactivity order of all l halides. Thus, tertiary all l halides are more reaetive than seeondaty or primary all l halides in this C-B bond formation. ... 

Af-Aiyl amides can be catalytically activated in Ca yi-N or Cat-N cleavage manners depending on catalysts and reaction conditions. Ni(COD)2/IMes catalyses botylation of N-atyl amides with diboron reagents via Cat-N cleavage in the presence of NaOtBu. Ni(COD)2/PCy3 catalyses the reduction of iV-aiyl amides or iV-atyl carbamates with HBpin to afford Ar-H (Scheme 14.37). ... 

Usually, boronic acids are synthesized by reaction of an aryl magnesium or aryl lithium compound with a borate ester [6]. However, in the case of 4-bromo-benzonitrile, a metal-organic intermediate would react with the nitrile group. Aryl boronic acids could be also synthesized by palladium-catalyzed reaction using diboron reagents [7]. This method would certainly tolerate the cyano func-tionahty, but the diboron reagents are very expensive. 

The unsymmetrical diisopropanolaminato diboron has been developed as a novel sp sp diboron reagent for the j8-boration of a,)8-unsaturated conjugated compounds... 

Most metal-boryl complexes have been synthesized by salt elimination or oxidative addition. Examples of these reactions are shown below. Ottier less common routes to metal-boryl complexes include transmetaUation of a boryl group from one transition metal to another and reactions of boranes with metal-olefin complexes. Boryl complexes have also been generated as intermediates in catalytic processes by "transmetaUation" of a metal halide with a diboron reagent. ... 

The scope of enantioselective, copper-catalyzed allylic substitution reactions is not limited to so-called hard carbon nucleophiles and achiral acyclic linear electrophiles. A recent report from Ito, Sawamura, and co-workers showed that a diboron reagent can serve as a pronucleophile for enantioselective, copper-catalyzed boronation of (Z)-aUylic carbonates (Equation 20.85). The corresponding chiral allylboronates were isolated in good yields with high enantioselectivities. 

Conjugate Addition with Diboron Reagents and Diboration... 

---