Platinum boryls

Phosphine dissociation from platinum boryl intermediates (PPh3)2Pt(BCat)2 allows the coordination of an alkyne prior to the boryl transfer step. The diborylation of alkenes requires phosphine-free catalysts.45,46 Boron—boron bonds react with a,j8-unsaturated ketones with 1,4-addition.47... 

The reaction of the T-shaped three coordinate platinum boryl complex trans-[ Cy3P)2Pt B(Br)Fc ] [BAr 4] (21) (Fc = ferrocenyl) with 4-methylpyri-dine leads to the formation of trans-[(Cy3P)2Pt(Br) B(Fc)(NC5H4Me-4) ]... 

Table 12 Selected data for structurally characterized platinum boryl complexes...

Let us take acrolein as an example to illustrate the findings from the DFT calculations. For a platinum boryl complex (37), the boryl ligand can act as a nucleophile if the Pt-boryl o-bonding electron pair plays the role to initiate the reaction and as an electrophile if the empty p orbital to initiate the reaction. Scheme 9 shows three possible insertion pathways for the reaction of the bis-boryl diimine platinum(II) complex with acrolein whose a -electron density distribution is also illustrated in 37. The free energy barriers for the pathways of the boryl attacks at C4 and C2 were calculated to be 35.7 and 53.3 kcal/mol, respectively [106]. Interestingly, the free energy barrier for the pathway of the boryl attack at O was calculated to be only 18.6 kcal/mol [106]. These results indicate that the boryl ligand, in this case, behaves as an electrophile. 

Diboration of a,/ -unsaturated ketones is promoted by platinum(O) complexes. Reaction of 4-phenyl-3-buten-2-one with bis(pinacolato)diboron in the presence of a platinum catalyst affords a boryl-substituted (Z)-boron enolate, that is, a 1,4-diboration product, in high yield with high stereoselectivity (Scheme 8). The isolated boron enolate is easily hydrolyzed by exposure to water, giving / -boryl ketones in high yields.66 Similar diboration of a,/ -unsaturated ketones has also been achieved with Pt(bian)(dmfu) (bian = bis(phenylimino)acenaphthene, dmfu = dimethyl fumarate).67 Although the... 

In relation to the mechanistic proposal, an interesting reactivity of (boryl)(silyl)platinum(n) complex has been reported.223 The complex is prepared by the reaction of silylborane with Pt(cod)2 complex via oxidative addition (Scheme 46). The (boryl)(silyl)platinum complex undergoes insertion of alkynes at the B-Pt bond to give (/3-borylalkenyl)(silyl)platinum(n) complex in high yield. Importantly, the insertion takes place regioselectively, with Pt-G bond formation at the internal. -carbon atom. This result may indicate that the boron-transition metal bond is more prone to undergo insertion of unsaturated molecules. 

Addition of silylpinacolborane to alkenes has been achieved with platinum catalysts (Equation (83)).225 The reaction proceeds regioselectively to provide products in which the silyl groups are attached to the terminal carbon atoms contrary to the silaboration of alkynes. Although no regioisomers are detected, 1-boryl-l-silylalkenes are formed as major byproducts. 

For each case we will also present catalytic analogues, namely (1) the activation of methane to form methanol with platinum, the reaction of certain aromatics with palladium to give alkene-substituted aromatics, and (2) the alkylation of aromatics with ruthenium catalysts, and the borylation of alkanes and arenes with a variety of metal complexes. 

In the realm of C-H bond transformations applied toward the synthesis of fine chemicals, iridium has not achieved the prominence attained in recent years by the second-row platinum group metals, particularly palladium [10]. A notable exception, however, has been the leading role of iridium in the valuable chemistry of arene borylation [11]. 

Heteroalkenes, with iron, 6, 132 Heteroannulation, allylic benzylamines, 10, 156 Heteroarene chromium carbonyls, preparation and characteristics, 5, 260 Heteroarenes borylation, 10, 242 C—H functionalizations, 10, 127 as metal vapor synthesis milestone, 1, 237 with titanium, 4, 246 vanadium complexes, 5, 48 7]6-Heteroarenes, with platinum, 8, 664 Heteroaromatic compounds... 

In 1993, Ishiyama et al.60 reported the synthesis of isomerically pure cis-1,2-bis(boryl)alkenes 53a-57a from their corresponding alkynes 53-57 (Scheme 12)60,61 via platinum complexes. The solvents did not play an important role in the reaction, but a comparison of the reaction rates at 50 °C revealed that the addition was apparently accelerated in polar... 

Bis(pinacolato)diborane(4) selectively adds to terminal aikenes and cyclic aikenes having internal strain to provide bis(boryl)alkanes in 76-86% yields 85-89 in the presence of a catalytic amount of Pt(dba)2 at 50 °C67 (Scheme 16). It is interesting to mention that Pt(dba)2 directed 1,2-addition to certain conjugated dienes, whereas 1,4-addition through a 7i-allyl-platinum(II) intermediate is an energetically more favorable process. The 1,4-addition to penta-1,3-diene at 80 °C with Pt(PPh3)4 gives 90, but the same reaction with Pt(dba)2 selectively produced the 1,2-addition product 91 at room temperature (Scheme 16). 

Benzyl alcohol, dehydrogenation of, 279 Bifunctional dicarboxylic acids, 85 Biphasic olefin dimerization, 274-276 Bis(boryl) platinum compounds, 228 Bis(catecholato)diborane(4) compounds, 195, 200-201... 

Silyl(pinacol)borane added to terminal alkenes in the presence of Pt(GH2=CH2)(PPh3)2 to give l-boryl-2-silylalk-anes 247 Platinum-catalyzed silylboration of methylene cyclopropanes provided alkenylboron derivatives via a proximal G-G bond cleavage (Equation (41))250 Nickel(0)-catalyzed silylboration of vinyl cyclopropanes and cyclobutanes provided allylsilane derivatives via analogous G-G bond cleavage (Equation (42)).251... 

Borylation of 1-alkenyl iodides and triflates with HBpin was catalyzed by palladium catalyst in the presence of Et3N (Equation (61)).330 Platinum(0)-AsPPh3 complexes catalyzed the borylation of allyl chlorides (Equation (62)).331,332... 

Many different routes are available for the synthesis of vinylboranes and several of them are shown in Scheme 7. Hydroboration and diboration reactions of alkynes and borylated alkynes provide access to the frill series of mono-, di-, tri-, and tetraborylated olefins. 1,2-Diborylated olefins (33) are obtained via diboration of alkynes and 1,1-diborylated olefins (34) are accessible through hydroboration of borylalkynes. An alternative route to 1,1-disubstituted products involves the diboration of carbenoids formed in situ from vinylhalides and butyl hthium. In certain cases, metal-catalyzed dehydrogenative borylation of olefins may be used. Borylalkynes serve as precursors to triborylated (35) and tetraborylated (36) olefins. Thus, the sparingly soluble tetraborylethylene derivative (36) forms in good yield through platinum-catalyzed diboration of diborylacetylene in toluene at 40 °C if the base-free catalyst [Pt(cod)2] is used. If the reaction, however, is performed at higher temperature, ftnther diboration of (36) leads directly to the hexaborylated ethane (23) shown above. Intramolecular B-O interactions were postulated for (36) based on HF-SCF calculations. ... 

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]. 

Murata, M., Watanabe, S., Masuda, Y. Regio- and stereoselective synthesis of allylboranes via platinum(0)-catalyzed borylation of allyl halides with pinacolborane. Tetrahedron Lett. 2000, 41, 5877-5880. 

As with rhodium and platinum, iridium boryl systems have proved to be a valuable structural proving ground on which to establish fundamental properties of the BX2 ligand (Table 10). In a manner also mimicking rhodium, a significant body of work has emerged more recently due to the implication of iridium boryls in hydrocarbon activation [10,126,129,136-148,150,151, 155-159]... 

Fig. 34 Crystallographically characterized square planar platinum(II) mono(boryl) sys-terns 10.8-10.13...

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