Diboration mechanisms

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 complexes [Cu(NHC)(MeCN)][BF ], NHC = IPr, SIPr, IMes, catalyse the diboration of styrene with (Bcat) in high conversions (5 mol%, THF, rt or reflux). The (BcaO /styrene ratio has also an important effect on chemoselectivity (mono-versus di-substituted borylated species). Use of equimolecular ratios or excess of BCcat) results in the diborylated product, while higher alkene B(cat)j ratios lead selectively to mono-borylated species. Alkynes (phenylacetylene, diphenylacety-lene) are converted selectively (90-95%) to the c/x-di-borylated products under the same conditions. The mechanism of the reaction possibly involves a-bond metathetical reactions, but no oxidative addition at the copper. This mechanistic model was supported by DFT calculations [68]. 

A stoichiometric reaction of tetrakis(triphenylphosphine)platinum(0) with bis(pinacolato)diboron gives cis-diborylplatinum(n) complex in high yield (Scheme 3).38 The diborylplatinum complex then reacts with an alkyne, giving m-diboration product.40,41 These results indicate that the diboration proceeds through the general mechanism shown in Scheme 1 (E1 = E2 = Bpin), which involves the formation of diborylplatinum(n), insertion of an alkyne into the B-Pt bond, and reductive elimination. 

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

For the diboration reactions of alkynes catalyzed by Pt(0) complexes, the reaction mechanism involves the oxidative addition of diborane to the Pt(0) center, followed by the insertion of alkyne into the Pt-B bond and reductive... 

Cui, Q., Musaev, D. G., Morokuma, K., 1998b, Density Functional Study on the Mechanism of Palladium(O)-Catalyzed Thioboration Reaction of Alkynes. Differences between Pd(0) and Pt(o) Catalysts and between Thioboration and Diboration , Organometallics, 17, 1383. 

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

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]. 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

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... 

Yang and Cheng observed an unusual variant of allene diborylation where an aryl or alkenyl halide can act as a co-catalyst with a phosphine-free palladium catalyst [51]. As shown with the formation of allylboronate 29, the addition is completely regiose-lective and highly stereoselective (Equation 17). Examples with aryloxy-substituted allenes were also successfully demonstrated. Notably, Pd(dba)2 does not promote the diboration of allenes without the co-catalyst, and a new mechanism involving oxidative addition of an I-B bond to the palladium center was proposed to rationalize these observations. 

Recently, we studied the detailed mechanisms of the diboration of acyclic a,p-unsaturated carbonyl compounds, acrolein, methylacrylate, and dimethyl-fumarate (DMFU), catalyzed by a Pt(0) complex, which were experimentally... 

Diboration of 1,3-Dienes, Allenes, Enones, and Other Unsaturated Molecules. As described previously in the original article, the Pt[0] catalyzed addition of 1,3-dienes with B2pin2 affords various allylboronic esters (eq 4). Reactions using phosphine-based catalysts such as Pt(PPh3)4 stereoselectively produce els -1,4-addition products with Z-configuration for aliphatic and cyclic dienes. The reaction mechanism suggests... 

Scheme 6 Proposed mechanism for the Pt(0)-diboration of alkynes with B2pin2.

The B NMR data for 10b provided a broad signal at 5 47.2 ppm, and reactivity studies confirmed that the stoichiometric addition of aUcynes led to the corresponding diborated products. Interestingly, mechanistic studies favored a mechanism where phosphine dissociation generated a three-coordinate intermediate that mediated the alkyne insertion. 

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... 

Cui Q, Musaev DG, Morokuma K. Molecular orbital smdy of the mechanism of platinum(0)-catalyzed aUtene and alkyne diboration reactions. Organometallics. 1997 16 1355-1365. 

Coombs JR, Haeffiier F, Kliman LT, Morken JP. Scope and mechanism of the Pt-catalyzed enantioselective diboration of monosubstituted alkenes. J z4m Chem Soc. 2013 135 11222-11231. 

Lin B, Gao M, Dang L, Zhao H, Marder TB, Lin Z. DFT stndies on the mechanisms of the platinnm-catalyzed diboration of acyclic a,P-unsaturated carbonyl compounds. Organometallics. 2012 31 3410-3425. 

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