Diboryl complexes

Platinum diboryl complexes (PPh3)2Pt(BCat)2 react with alkynes to give cis-diborylation products, via a Pt-vinyl intermediate.199 Alkynes are also known to insert into M—Si, M—N, and M—Cl bonds.200... 

Scheme 1 Synthesis of diboryl complex Cp2NbH(Bcat)2 (5.10)...

Insertion into metal boryl bonds is represented by two examples described for a late and an early transition metal complexes. Platinum diboryl complexes react with alkynes to give the diborylated alkene (Scheme 6.61, a) [191]. A titanium(II) metallocene with a coordinated ethylene reacts with borane to give, via insertion of the alkene into a M-B bond and subsequent j6-H ehmination, a complex with a coordinated vinylborane (Scheme 6.61, b) [192]. Both processes can be made catalytic. 

Iverson CN, Smith III MR.. Reactivity of organoplatinum complexes with C6H4O2B-BO2C6H4 synthesis of a platinnm diboryl complex with, and without, metathesis of boron-boron and metal-carbon bonds. Jz4m Chem Soc. 1995 117 4403-4404. 

Iverson CN, Smith III MR. Mechanistic investigation of stoichiometric alkyne insertion into Pt-B bonds and related chemistry bearing on the catalytic diborylation of alkynes mediated by platinnmill) diboryl complexes. Organomelallics. 1996 15 5155-5165. 

Diboration of terminal alkenes has also been studied with other d " metals (Fig. 2.12) including the Ag and Au complexes 75-77 and the Pt" complexes 78-79. Styrene is diborylated with 100% selectivity and good conversions in THF (46% for 75 and 94% for 77 at 5 mol%, 60 h) using equimolecular amounts of (Bcat)j. The difference in activity between the Ag and Au complexes has been ascribed to the increased lability of the Ag-NHC bond, which may lead to catalyst decomposition under the reaction conditions, hi both catalytic systems it is believed that the active species involves only one coordinated NHC ligand. Complex 77 is less active than 74 and 75, possibly due to steric reasons. The enantioselectivity of 77 in the diboration of prochiral alkenes is very low [63]. 

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]. 

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]. 

Part of the intense interest in M-B compounds derives from the participation of these species in various catalytic processes.1,2 Scheme 6 depicts proposed catalytic cycles for metal-catalysed alkene hydroboration and alkyne diborylation. Key steps involve the insertion of unsaturated organic substrates into M-B bonds and a key intermediate involved in the formation of product is molecule A in which there are ad jacent M-C and M-B bonds. The ruthenium and osmium boryl complexes described in this section provide models for these steps and intermediates. 

The key step to the tricyclic compound 17 seems to be the stereoselective aza Diels-Alder reaction (20) of the intermediate enamine dienophile 18 with azadiene adduct 19 to produce the imine complex 21. Allylboration of C=N bond in the latter proceeds also stereoselectively (21, arrow) furnishing the diborylated compound 22. 

Late-metal complexes of Pd, Pt, and Rh can also catalyze hydrosilylation, hy-drostannylation, hydroboration, and diborylation reactions of 7r bonds. Both C=C and C=0 bonds may be hydrosilylated or hydroborated, whereas hydrostannyla-tion is usually carried out only on C=C bonds. (Some boranes add to C=0 and C=C bonds in the absence of catalyst, but less reactive ones, such as catechol-borane ((C6H402)BH), require a catalyst. Moreover, the metal-catalyzed reactions sometimes display different selectivities from the uncatalyzed variants.) The mechanisms of all these reactions are the same as hydrogenation, except that oxidative addition of H-H is replaced by oxidative addition of a R3Si-H (R3Sn-H, R2B-H, R2B-BR2) bond. 

Styrylboronic ester 24 was subjected to the catalytic hydroboration with cat-echolborane in the presence of rhodium complexes coordinated with chiral bis-phosphine ligands. Oxidation of the resulting 1,2-diboryl product 25 gave optically active 1-phenyl-1,2-ethanediol (26) (Scheme 6) [26]. The reaction with BINAP (7) at -60 °C gave (S)-diol 26 of over 70% ee. 

The double hydroboration of propargyl bromide with 9-borabicyclo[3.3.1]nonane (9-BBN-H) proceeds smoothly to afford the corresponding l,l-diboryl-3-bromo-propane adduct which is quantitatively converted to the B-hydroxy-B-cyclopropyl-9-BBN and / /3-hydroxy-9B BN complexes in equal molar amounts. Addition of this mixture to aryl, alkynyl and alkenyl bromides containing a catalytic (3%) amount of Pd[PPh3]4, after 8-16 hours at reflux temperature, provides the corresponding cyclopropylated arenes, alkynes and alkenes in good to excellent yields.111... 

The diborylated cobaltocenium species [(CpBR2)2Co] + (R = t-Pr) is an interesting example that illustrates the different binding modes encountered for bifunctional Lewis acids.With hydroxy counterions, an oxygen-bridged complex (178), which represents an inverse chelate structure, was confirmed in the solid state and in solution (R = t-Pr, F). Salt-like structures on the other hand were observed with PFe as the counterion and a zwitterionic 1 1 complex (179) formed upon reaction of the diborylated cobaltocene with hexachloroethane. Low-temperature NMR studies show that the chloride rapidly exchanges position between the two Lewis-acidic boron centers. 

The five-coordinate Ru-boryl complex, Ru(Bcat)Cl(CO)(PPh3)2, inserts ethyne into the Ru-B bond forming the borylvinyl-compound (13) (Scheme 8). This reaction and product models the acetylene insertion step to give intermediate C in the metal-catalysed diboryl-ation reaction shown in L... 

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