Diboration of alkene

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. 

Fig. 2.12 Silver, gold and platinum complexes with monodentate NHC ligands as catalysts for the diboration of alkenes and alkynes...

The stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

The phosphine-based platinum(O) catalysts do not catalyze the diboration of alkenes because of the high coordination ability of phosphine over the alkene double bond, but platinum(O) complexes without a phosphine ligand such as Pt(dba)2 [128] and Pt(cod)2 [129] are an excellent catalyst allowing the alkene insertion into the B-Pt bond under mild conditions (Scheme 1-30). The diboration of aliphatic and aromatic terminal alkenes takes place smoothly at 50°C or even at room temperature. The reaction is significantly slow for disubstituted alkenes and cyclic alkenes, but cyclic alkenes having an internal strain afford ds-diboration products in high... 

Ishiyama, T., Yamamoto, M., and Miyaura, N., Diboration of alkenes with bis(pinacolato)diboron catalyzed by a platinum(O) complex, Chem. Commun., 689, 1997. 

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. 

Commercially available Pt(cod)Cl2 catalyzes the diboration of alkenes, alkynes, and aldimines using bis(catecholato)diborane(4) (cod =1,5-cyclooctadiene). Catalyzed aldimine diboration provides the first direct... 

Potential difference in reactivity between two G-B bonds allowed the transformation of l,2-bis(boryl)-l-alkenes to 1-alkenylboranes via a cross-coupling with the aryl, 1-alkenyl, benzyl, and cinnamyl halides (Equation (23)).211-213 This tandem procedure synthetically equivalent to a yy/z-carboboration of alkynes was used for synthesizing Tamoxifen derivatives via stepwise double coupling with two of the G-B bonds.212,213 Hydrogenation of the resulting bisborylalk-enes with a chiral rhodium catalyst is synthetically equivalent to an asymmetric diboration of alkenes (Equation (24)).214... 

Pt(cod)2, Pt(dba)2, and Rh(acac)(dpm) were found to be excellent catalysts allowing the diboration of terminal alkenes and cyclic alkenes having an internal strain such as norbornene (Equation (25)). In attempts for the asymmetric diboration of alkenes, the reaction between 4-methoxystyrene and chiral diboron BZ(R,R-... 

This interest in catalytic hydroboration led to the development of the transition-metal-catalyzed diboration of alkenes and alk5mes. The diboration of alkenes and alkynes generates bifunctional products, and additions to alkenes have now been conducted with high enantioselectivity. The following sections describe the t es of catalysts used for catalytic hydroboration and diboration of alkenes, alkynes, and dienes, as well as catalytic cycles that accoimt for selectivities and side products formed during these processes. 

The diboration of alkenes has been more challenging to develop. Table 16.5, which is also adapted from the Miyaura review, summarizes a series of diborations of alkenes. These examples show that complexes of group 10 metals lacking phosphine or isonitrile ligands have been most active for diese additions. Gold and rhodium complexes have also been shown to catalyze the diboration of vinylarenes. 

The diborations of alkenes and alkynes are thought to occur by the general pathway shown in Scheme 16.15 and analogous silylborations and stannylborations would occur by related mechanisms. By this mechanism, the oxidative addition of the diborane(4), silylborane, or stannylborane reagent leads to a bisboryl, a silylboryl, or a stannylboryl complex. These oxidative addition processes are described in Chapter 6. In brief, Miyaura and Ishiyama, Marder, and Smith all published examples of the oxidative addition of diboron compounds to and Marder has published examples of the oxidative... 

Scheme 3.11 Diboration of alkenes, (Adapted with permission from ref. 75 2011 Wiley-VCH Verlag GmbH.)...

Details on oxidation and reduction reactions mediated by [(NHC)Pd] complexes can be found in Chapters 12 and 13, respectively. Further reports of interest disclosed in recent years include diboration of alkenes catalysed by a pincer complex, deuteration of C-H bonds with an N,0-functionalised NHC complex, and an intriguing Suzuki-type reaction of [FeI(Cp)(CO)2] with arylboronic acids. 

Recently, Fernandez and Mata reported the use of mixed Pt -siloxane/NHC complexes as active catalytic species in 1,2-diboration of alkenes or alkynes (Equation (10.35)). ° ... 

The first report of a NHC Ag-catalyzed reaction appeared only in 2005 when Peris and Fernandez disclosed the diboration of alkenes in the presence of a bis-NHC silver complex (Scheme 11.3). The menthol-derived NHC ligand 7 showed only limited scope in this reaction but provided a proof-of-concept for NHC-Ag-based catalysis. Subsequent studies by the same groups examined chiral NHCs 8-10 (Figure 11.3) in this reaction, but enantiomeric excess remained very low (9% ee at best). ... 

Figure 11.3 Structures of chiral NHCs used in diboration of alkenes.

In general, the diboration of alkenes with [Pt(dba)2] required 50 while the use of [Pt(NBE)2] or [Pt(COD)2] (NBE = norbomene, COD = 1,5-cyclooctadiene) as precursors could diborate alkenes at room temperature." The reactions proceed smoothly to give 1,2-diborylaIkanes in high yield, and the catalysts were compatible with common functional groups. Further developments in the area by Baker showed that [Pt(COD)Cl2] could efficiently activate B2cat2 and promote the diboration of terminal alkenes, vinylarenes, alkynes, and aldimines (Scheme 9). The last example was a great advance because it represented the first direct approach toward a-aminoboronate esters. 

The first example of copper-catalyzed diboration of alkenes with Cu complexes involved NHC Hgands. Fernandez and Perez studied the nature of the interaction between the unsaturated complex [Cu(NHC)(NCMe)] and B2cat2 through a theoretical DFT study with the B3LYP functional in... 

Ishiyama T, Yamamoto M, Miyama N. Diboration of alkenes with bis(pinacolato) diboron catalysed by a platinum(0) complex. Chem Commun. 1997 689—690. 

---