Diboron , activation

Although a wealth of research has focussed on the use of platinum complexes in diboron activation chemistry, much less is known about the analogous palladium chemistry.Indeed, early theoretical studies have claimed that although the barrier for oxidative addition of B2(OH)4 to Pd(PH3)2 is smaller than for the analogous Pt complex (8.6 vs. 14.0 kcal/mol), the reaction is endothermic for Pd but exothermic for Pt. It has been postulated that [Pd(B(OH)2)2(PH3)2] complex resides higher... 

Within the last decade, another source of diboron activation has been promoted in basis to the efEcient application to catalysis. This is the use of nanoparticles which can interact with the diboron reagent and develop an enhanced performance toward diboration reaction. Fernandez observed in 2008 that the in situ formation of Au nanoparticles, from Au(I) complexes, could not only activate the B2cat2 but also deliver the boryl units on alkenes with total chemoselectivity. " The gold nanoparticles were estimated to have a mean crystaUite size of 10.5 0.3 nm. The gold nanoparticles were stabilized by 2,2 -bis-(diphenylphosphino)-l-l -binaphthyl (BINAP), diphenylphosphinoethane (DPPE), and L-glutathione. The core size and size distribution ofBINAP-Au nanoparticles were examined by transition electron microscopy (TEM), and the image shows disperse nanopartides 6.9 3.0 nm in diameter (Fig. 6). 

Diboron compounds with alkylamino or alkyloxy substituents are prepared from the appropriate halogenoborane derivative using active metals " ... 

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. 

Diboration of alkene is catalyzed by Pt(0),42,48-51 Rh(i),52-57 Au(i),52 and Ag(i)58 complexes. Phosphine-free platinum complexes such as Pt(dba)2 and Pt(cod)2 are efficient catalysts for diboration of alkene, whereas those with phosphine ligands show much lower catalytic activities (Equations (3) and (4)).48,49 A PtCl2(cod) complex, which may be readily reduced to Pt(0) species with diboron, also catalyzes the addition of bis(catecholato)diboron to alkenes.42 Platinum-catalyzed diboration has so far been limited to terminal alkenes and strained cyclic alkenes. 

In comparison with the platinum catalysts, rhodium catalysts are much more reactive to effect addition of bis(catecholato)diboron even to non-strained internal alkenes under mild reaction conditions (Equation (5)).53-55 This higher reactivity prompted trials on the asymmetric diboration of alkenes. Diastereoselective addition of optically active diboron derived from (li ,2i )-diphenylethanediol for />-methoxystyrene gives 60% de (Equation (6)).50 Furthermore, enantioselective diboration of alkenes with bis(catecolato)diboron has been achieved by using Rh(nbd)(acac)/(A)-QUINAP catalyst (Equation (7)).55,56 The reaction of internal (A)-alkenes with / //-butylethylene derivatives gives high enantioselectivities (up to 98% ee), whereas lower ee s are obtained in the reaction of internal (Z)-alkenes, styrene, and a-methylstyrene. 

Attempted reaction of 1,3-pentadiene with the optically active diboron derived from dialkyl tartrate in the presence of a phosphine-free platinum catalyst gave poor diastereoselectivity (20% de).63 Better selectivity has been attained with a modified platinum catalyst bearing a PCy3 ligand (Scheme 6).64 The reaction of allylborane thus obtained with an aldehyde followed by oxidation with basic hydrogen peroxide affords the corresponding diol derivative with moderate ee. 

Many other PF copolymers, which do not contain a particularly electron-active moiety, but nevertheless, can improve the performance of the material in PLED have been synthesized. The Huang group [364,365] at Institute of Materials Research and Engineering (IMRE, Singapore) synthesized deep-blue copolymer 272 by Suzuki copolymerization of fluorene-diboronic acid with dibromobenzene. The emission band of 272 has a peak at 420 nm and a well-defined vibronic feature at 448 nm with a fwhm of 69 nm, and virtually no green emission,... 

The direct borylation of arenes was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

Although not fitting exactly into the scope of this book, the iridium catalyzed borylation of five membered heterocycles through C-H bond activation also deserves mentioning. A recent report by Miyaura disclosed the reaction of bis(pinacolato)diboron with heteroaromatic systems, where thiophene, fiirane and pyrrole were converted to their 2-boryl derivatives with good selectivity (6.86.), The yields presented refer to the diboron compound since the heterocycles were used in excess in all cases. Indole, benzofurane and benzothiophene were monoborylated with similar efficiency.116... 

Cross-coupling reactions 5-alkenylboron boron compounds, 9, 208 with alkenylpalladium(II) complexes, 8, 280 5-alkylboron boron, 9, 206 in alkyne C-H activations, 10, 157 5-alkynylboron compounds, 9, 212 5-allylboron compounds, 9, 212 allystannanes, 3, 840 for aryl and alkenyl ethers via copper catalysts, 10, 650 via palladium catalysts, 10, 654 5-arylboron boron compounds, 9, 208 with bis(alkoxide)titanium alkyne complexes, 4, 276 carbonyls and imines, 11, 66 in catalytic C-F activation, 1, 737, 1, 748 for C-C bond formation Cadiot-Chodkiewicz reaction, 11, 19 Hiyama reaction, 11, 23 Kumada-Tamao-Corriu reaction, 11, 20 via Migita-Kosugi-Stille reaction, 11, 12 Negishi coupling, 11, 27 overview, 11, 1-37 via Suzuki-Miyaura reaction, 11, 2 terminal alkyne reactions, 11, 15 for C-H activation, 10, 116-117 for C-N bonds via amination, 10, 706 diborons, 9, 167... 

Addition of bis(pinacolate)diboron (206) to dienes is catalysed by Pt complexes. The 1 1 adduct 207 is obtained with Pt(Ph3P)4, whereas Pt(dba)2 without phosphine is a very active catalyst and the 1,8-adduct 208 is obtained at room temperature [90],... 

The starting material bis(pinacolato)diboron is a poor Lewis acid and 1 B-NMR of KOAc and B2bin2 in DMSO-d6 shows no evidence of the coordination of the acetoxy anion to a boron atom leading to a tetrahedral activated species. However, the formation of an (acetato)palladium(II) complex after the oxidative addition of the halide influences the reaction rate of the transmetalation step. The Pd-O bond, which consists of a hard Lewis base with a soft Lewis acid, is more reactive than a Pd-X (X=Br, I) bond. In addition, the high oxophilicity of boron has to be considered as a driving force for the transmetalation step, which involves an acetato ligand. 

Isotactic vinyl polymers often possess a helical conformation in the solid state however, without bulky substituents present (vide infra) in solution at room temperature, helix—helix reversal takes place fast and no optical activity is observed. Ortiz and Kahn reported a borderline case in which a non-bonded interaction between the monomers leads to the formation of isotactic 39 (Chart 7) by anionic polymerzation at —78 °C. Optically active polymers can be isolated, but in solution the proposed one-handed helicity is lost in less than 1 h.148 An intriguing class of polymers formed by polycondensation of diboronic acid and chiral tetraalcohols has been studied by Mikami and Shinkai and is exemplified by polymer 40 (Chart 7). In this D-mannitol-based polymer, the noncovalent intramolecular interaction between the amines and the boron atoms imposed a sp3-hybridization on boron, which, according to calculations, results in a helical conformation of the macromolecule.149... 

Dendrimers have been used as large compartmented hosts for drug delivery, as carriers for catalytically active sites in flow reactors, and for charge or energy transfer purposes. New types of such dendrimers were prepared starting from functionalized poly-p-phcnylcncs as polymeric cores whose functional groups were further used to anchor dendrons [165]. The polymers were synthesized from dibromide 117 and diboronic acid 118 via a Suzuki crosscoupling the dendrons were attached via a Williamson ether synthesis [165]. 

Fluoride-ion-promoted B—C bond activation has been employed for transition-metal-catalyzed C—C bond formation. For example, palladium-catalyzed homo coupling of vinyl bromide (18) is mediated by bis(pinacolato)diboron (19), which is activated by fluoride ion... 

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. 

Recently, rhodium-catalyzed direct borylation of alkane with diboron was reported [50], where Cp Rh(ri -CgMeg) (Cp =C5Me5) was used as a catalyst. This reaction has drawn considerable interest because the C-H o-bond activation and the introduction of some functional group into alkane were achieved in one pot. The similar reaction (Eq. 8) was theoretically investigated with the DFT method [51] ... 

The similar Ir-catalyzed direct borylation of benzene with diboron was recently reported experimentally [52] and its full catalytic cycle was theoretically investigated with the DFT(B3LYP) method [53]. In this reaction, the iridium(III) complex, Ir(bpy)(Beg)3 (bpy=2,2 -bipyridine eg=ethyleneglycolate),is an active... 

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