Diboration of alkenes and alkynes

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. 

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 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... 

The insertion of alkenes and alkynes into M-E (E = SiRa, SnR3, BR2) is not substantially different from the insertion into M-H or M-C bonds. The same orbital considerations hold in these cases (see Section 6.2.1) and theoretical methods have been applied to analyze these processes, generally in the context of mechanistic studies of catalytic reactions where this may be an important step (hydrosilation, hydroboration, diboration, silylstannation, etc.). Table 6.11 contains some calculated activation energies for insertion of ethylene and alkynes into M-ER bonds of comparable systems. The balance of bond breaking/bond making is less favorable for M-Si than for M-C or M-H bonds. Moreover, the directionality of the 0(M-Si) bonds is detrimental for insertion as discussed when... 

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)). ° ... 

Diboration of terminal and internal alkynes by (Bcat) [B(cat)2 alkene = 1 1] was also achieved by using complexes 78 and 79 at room temperature in THF. Best activity was observed with 78 containing the less electron donating triazolyhdene carbene hgand. Terminal alkenes can also be diborylated under the same conditions, however the selectivity for the diborylated product was much lower (up to 65%) [64]. 

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... 

Proposed mechanisms of transition metal catalysed hydroboraton of alkenes and diboration of alkynes... 

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. 

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 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. A number of useful C-heteroatom coupling reactions have also been reported recently, including alkene hydrophosphination and alkyne silylation. ... 

NHC-Cu-catalyzed diborations of styrene and phenylacetylene were first studied by the groups of Perez and Fernandez on the basis of initial studies on the borocupration of unactivated alkenes made by the group of Sadighi. Since these first developments, numerous methods have been developed. It has been shown that carbon n-systems such as alkenes, alkynes and allenes could be variously functionalized by trapping the organocopper species intermediately formed with an external electrophile. 

To prepare alcohols by the oxidation of alkyl boranes, borane, dibor-ane, alkyl and dialkyl boranes, and dialkoxy boranes are added to alkenes or alkynes. The addition takes place in the anti-Markovnikov mode because boron is more electrophilic than hydrogen. 

Several synthetically very useful processes involve the 1,2 insertion of unsaturated substrates into the M-SiRs bond (hydrosilation [193], dehydrogenative silation [193a,194], and disilation [176]) or the M-BR2 bond (hydroboration [195], dehydrogenative boration [196], and diboration [191,195,197]) Catalytic reactions have been developed lately that comprise the 1,2 addition of two different ER groups to an alkene or alkyne (silylstannation [198], borylsilation... 

The mechanism of the processes where the alkene or alkyne is functionalized by two ER groups necessarily involves the insertion of the substrate into one M-ER bond. The actual mechanism depends on the specific reaction, type of substrate, and catalyst. Eor diboration or disilation of alkenes, theoretical studies have found this step rate determining when the catalyst is a Pt complex [176,179]. The insertion of acetylene into Pt-BR2 bonds is faster than the insertion of ethylene, and this step is not rate determiiung for diboration of alkynes [178,179]. As was pointed out before (see Section 6.4.1 (a)), insertion of an alkyne into the Pd-SnRs bond is preferred over insertion into the Pd-SiRa in the silylstannation of alkynes [177]. 

Diboration of Unsaturated Hydrocarbons. B2pin2 adds to unsaturated hydrocarbons in the presence of a catalytic amount of a Pt complex to afford diborated products in high yield with excellent regio- and stereoselectivity. The reaction is recognized to proceed through a catalytic cycle, which involves (a) oxidative addition of the B-B bond to Pt , (b) insertion of the unsaturated hydrocarbon into the B-Pt bond, and (c) reductive elimination of the product to regenerate Pt (eq 1). The diboration of alkynes, allenes, conjugated dienes, methylenecyclopropanes, anda, -unsaturated carbonyl compounds is efficiently catal) ed by phosphine-based Pt complexes (eqs 2-6), whereas phosphine-free Pt complexes are favorable for the reaction of simple alkenes because of the low coordination ability of the alkene over phos-... 

A theoretical study has been carried out by Morokuma for the mechanism of Pt(0)-catalyzed alkyne and alkene diboration reactions with the B3LYP density functional method. The complexation energy between (0H)2B-B(0H)2 and Pt(PH3)2 has been calculated to be 3.7 kcal/mol, where the B-B and P-Pt-P axes are perpendicular to each other. The next step is the activation of the B—B bond, and the activation barrier has been calculated to be 12.5 kcal/mol, relative to the molecular complex (OH)2B— B(OH)2-Pt(PH3)2 or 8.8 kcal/mol relative to the reactants.In parallel, Sakaki theoretically investigated the insertion of Pt(PH3)2 into X2B—BX2 (X = H or OH) with the ab initio MO/MP4SDQ, SD-CI, and coupled cluster with double substitution methods. They found that this reaction proceeds with a moderate of 15 kcal/mol and a considerable of 20 kcal/mol for (OH)2B—B(OH)2. They noted that the B—B bond undergoes the insertion reaction of Pt(PH3)2 much more easily than does the C—C bond. Nowadays, the Pt-catalyzed diboration of terminal... 

Ishiyama T, Matsuda N, Murata M, Ozawa P, Suzuki A, Miyaura N. Platinum(O)-catalyzed diboration of alkynes with tetrakis(alkoxo)diborons an efficient and convenient approach to a s-bis(boryl)alkenes. Organometallks. 1996 15 713-720. 

Chelated and CNC pincer NHC complexes of Pd catalyzed the diboration of monosubstituted alkenes, with [B(cat)]2 in the presence of NaOAc in THF, in high yields. In contrast, Pd complexes were found to be inactive. From this observation and DFT calculations the authors hypothesized that heterofytic B-B cleavage is likely, rather than oxidative addition of B-B bond to the metal. Fernandez and co vorkers reported that with [(NHC)Pt] catalysts 205 instead, branched alcohols were the major products (Scheme 13.4). Alkynes were also used as substrates with 205. Although good activities were obtained, the selectiw ity was only moderate and styrene derivatives were formed in all cases. 

Figure 9. The reaction mechanism of Pt(0)-catalyzed alkyne and alkene diboration reactions calculated by Morokuma and co-workers.

The Pd(0)-catalyzed addition of the B-S bond to alkynes (thioboration) regio- and stereoselectively produces (Z)-2-(organothio)-l-alkenylboron reagents (Scheme 2-25) [63]. Tlie addition of (tetraalkoxy)diboron to alkynes to give cw-bis(boryl)alkenes (diboration) is catalyzed by a platinum(O) catalyst [34]. 

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