Tris borane complexes

Reduction of 3,5,5-tris-aryl-2(5// )-furanones 115 (R, R, R = aryl) with dimethyl sulfide-borane led to the formation of the 2,5-dihydrofurans 116 in high yields. However, in the case of 3,4-diaryl-2(5//)-furanones 115 (R, R = aryl R = H or r = H R, R = aryl), the reduction led to a complicated mixture of products of which only the diarylfurans 117 could be characterized (Scheme 36) (88S68). It was concluded that the smooth conversion of the tris-aryl-2(5//)-furanones to the corresponding furan derivatives with the dimethylsulfide-borane complex in high yields could be due to the presence of bulky aryl substituents which prevent addition reaction across the double bond (88S68). 

Tris(trimethylsilyl)silane reacts with phosphine sulfides and phosphine selen-ides under free radical conditions to give the corresponding phosphines or, after treatment with BH3-THF, the corresponding phosphine-borane complex in good to excellent yields (Reaction 4.45) [82]. Stereochemical studies on P-chiral phosphine sulphides showed that these reductions proceed with retention of configuration. An example is given in Reaction (4.46). 

Borane complexes, borane-THF and borane dimethylsulfide, are generally the appropriate reducing agents in this reaction. Borane thioxane has been used, but it suffers from the inconvenience that thioxane is quite expensive. Diborane gas has been tried by Callery and has proved to be very efficient. The use of this reagent could open new industrial developments for this technology. Catecholborane has shown some advantages at low temperature when selectivity is needed.40... 

Intensive studies also showed that many transition metal complexes are able to catalyze aromatic C-H borylation of various arenes (Scheme 7), e.g., Cp Ir(H)(Bpin)(PMe3) [64,65], Cp Rh(Ti4-C6Me6) [65,66], ( 75-Ind)Ir(COD) [67], (776-mesitylene)Ir(Bpin)3 [67], [IrX(COD)]2/bpy (X = Cl, OH, OMe, OPh) [68-70]. A very recent study by Marder and his coworkers showed that [Ir(OMe)(COD)]2 can also catalyze borylation of C-H bonds in N-containing heterocycles [71]. For the Ir-catalyzed borylation reactions, it is now believed that tris(boryl)iridium(III) complexes [67,69], 40c, [72] are likely the reactive intermediates and a mechanism involving an Ir(III)-Ir(V) catalytic cycle is operative [67,69]. A recent theoretical study [73] provided further support for this hypothesis. A mechanism, shown in Scheme 8, was proposed. Interestingly, there are no cr-borane complexes involved in the Ir-catalyzed reactions. The very electropositive boryl and hydride ligands may play important roles in stabilizing the iridium(V) intermediates. 

A novel trifluorophosphine-tris(difluoroboryl)borane complex B4F3 PF3 (m.p. 55°C, b.p. 74°C) is formed by reacting the high-tempera-ture species boron monofluoride with PF3 on a cold surface (316). The crystal structure has recently been determined by X-ray diffraction and is shown in Fig. 7 (74). 

Mechanistic investigations by Hartwig and coworkers into Cp2TiMe2-catalyzed hydroboration led to the isolation of the first tr-complex of a tri-coordinate borane.Thus, the reaction of dimethyltitanocene with three equivalents of HBcat yields the thermally fragile bis-borane complex Cp2Ti(K -HBcat)2 (25) together with methane and MeBCat (Scheme 6). 

The synthesis of phosphorous-functionalized click ligands also requires special conditions. Phosphine building blocks need be protected as either the corresponding borane complex (l-2a and 4-5a) or oxide (l-2b and 4—5b) in order to prevent an undesirable Staudinger reaction with the azide synthons (Scheme 1). A range of bi-and tridentate P,N click ligands have been synthesized, and the free clickphines (3 and 6) are liberated by a final deprotection of the phosphine group with either DABCO in the case of the borane complexes [86-90] or reduction with tri-chlorosilane [88-92] in the case of the phosphine oxides. 

Dinuclear palladium complexes catalyze m-hydroarylation of alkynes with arenes.56 The reaction of 3-hexyne with benzene in the presence of a dinulear palladium complex Pd2R2(M-OH)(//-dpfam) [dpfam = j/V,Ar -bis[2-(diphenyl-phosphino)phenyl]formamidinate, R=/>-Tol] and tri(/z-butyl)borane at 100 °C for 4h affords ( )-3-phenyl-3-hexene quantitatively (Equation (53)). The hydroarylation of 3-hexyne with monosubstituted benzenes ( )-3-aryl-3-hexenes with a 2 1 ratio of the meta- and ra -isomers. This regioselectivity is different from that of the hydroarylation of diphenylacetylene catalyzed by Rh4(GO)12.57... 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

Bis[copper(U) tris-pyrazolyl borane] dioxygen complex, 42 107-108, 110 Bis crown ethers, see Ferrocene, crown and bis crown ethers... 

The following amine boranes are manufd by the Callery Chemical Co a)Dimethylamine-borane, (CH,)aNH BHS, wh solid b)Tri-methylamine-borane, (CHS)SN BH, wh solid e)Pyridine-borane, CBHSN BHb, col liquid These amine-boranes are relatively stable complexes and are of interest because they act as selective reducing agents, polymerization catalysts, anti-oxidants and stabilizing agents. They may also be used for the prepn of diborane and as petroleum additives. Further information may be obtained from Tech Bull C-200(Ref 2)... 

Methylaluminoxane-free catalysts, such as cationic complexes derived from TiBz4 and tris(pentafluorophenyl)borane [B(C6F5)3], appear to be much less active, poorly stereospecific catalysts for the polymerisation of styrene. Under analogous conditions, the use of N, yV-dimethylanilinium tetrakis(pentafluor-ophenyl)borate [Me2N(Ph)H]+[B(C6F5)4]- instead of B(C6F5)3 does not yield active catalysts only traces of syndiotactic polystyrene were obtained [70]. 

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