Tris borane formation

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

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). 

We have reported that the reaction of 2,4,6-tri(chloro)borazine CI3B3N3H3 with three equivalents of tris(isopropylamino)borane B(NHPr )3 led to the formation of 2,4,6-tri[bis(isopropylamino)boryl(isopropyl)amino]borazine 11 [Fig. 5(a)] in quantitative yield.26,27 The subsequent polycondensation carried out in vacuo up to 150°C, led to the formation of the polymer 12 containing B3N3 rings linked either through direct B—N interring bonds (minor pathway) or three-atoms —N—B—N— bridges... 

Although dimeric (t-butylmethyleneamino)dibutylborane can be prepared by the reaction of tri-t-butylborane with pivalonitrile at temperature above 150 °C by elimination of butene. preparation at lower temperatures involving triethylamine-borane according to Eq. (11), illustrates that the formation of this iminoborane proceeds via di-n-butylborane as an intermediate 6 ... 

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]. 

Abstract This paper proposes new ways of preparation of hybrid silicones, i.e. an alternated multiblock seqnence of silicone and alkyl spacers, via a polycondensation process catalyzed by the tris(pentaflnorophenyl)borane, a water-tolerant Lewis acid, between methoxy and hydrogeno fnnctionalized silanes and siloxanes at room temperature and in the open air. The protocol was first developed with model molecules which led to polydimethylsiloxane (PDMS) chains, in order to seize the best experimental conditions. Several factors were studied such as the contents of each reactants, the nature of the solvent or the rate of addition. The best conditions were then adapted to the synthesis of hybrid silicones, condensing alkylated oligo-carbosiloxanes with methoxy or hydrogeno chain-ends and complementary small molecules. A systematic limitation in final molar masses of hybrid silicones was observed and explained by the formation of macrocycles, which cannot redistribnte or condense further while formed. 

Tris(pentafluorophenyl)borane [B(CgFj)3], is a powerful and selective Lewis acid catalyst used in many reactions in organic chemistry [1 ]. Parks and Piers [5] found that B(CgFj)3 catalyzes the hydrosilylation of carbonyl compounds. The silylation of alcohols with the formation of Ft as the only by-product [6] and the cleavage of silyl ether and ether bonds catalyzed by B(C F5)3 [7, 8] provide an... 

Alcohols protected as methyl ethers can be retrieved by reaction with tribromo-borane. Activation of the methyl ether by co-ordination of the Lewis acidic tri-bromoborane followed by nucleophilic cleavage of the O—Me bond (with concomitant formation of bromomethane) is typical behaviour. However, the cleavage took a different course with 39.1 [Scheme L39] instead of the O—Me bond being cleaved, the alternative C—O bond cleaved owing to participation of the remote acetoxy group.72 The formation of bromide 39.4 with retention of configuration is circumstantial evidence implicating dioxonium ion intermediate 393. 

Finally, several years ago, Richman reported the formation of a boron inclusion compound (28) that alternatively could be considered to be a tris(amino)borane. Practical applications have been found for several aminoboranes. For example, an aminoborane reagent (29) has been reported to be useful in enantioselective addition of allyl to aldehydes. ... 

Oxidation of trialkyl borates and of trialkoxyboroxines. Trialkyl borates, B(0R)3, prepared by reaction of primary and secondary alcohols with borane-dimethyl sulfide, are oxidized by PCC to aldehydes and ketones in good yield. This indirect oxidation of alcohols does not involve formation of water, which could be detrimental in some cases. Of even greater interest, carboxylic acids can be converted into aldehydes by reaction with borane-dimethyl sulfide to form a tri-alkoxyboroxine followed by oxidation with PCC (equation I). 

SCHEME 18.30 Proposed mechanism of the n-BuLi catalyzed dehydrocoupling of tris(hydridosilyl-ethylene)boranes and ammonia. (1) deprotonation with formation of an amide, (2) substitution of silicon-bonded hydride with amide, (3) polymerization through condensation. 

An alternative access to boron-modified polysilazanes is a dehydrocoupling of ammonia or alkyl amines with tris(hydridosilylethyl)boranes, B(C2H4Si(R)H2)3 (R = H, CH3 C2H4 = CH2CH2, CHCH3 Scheme 2) [6]. This avoids the formation of solid by-products and allows for the synthesis... 

Tris(mono-n-alkylamine),B5H9 adducts decompose thermally by the following scheme, via p-monoalkylaminodiboranes(6), with the final formation of NN N -trialkylborazines, polymeric solid boranes, (BH) , and H2 284... 

Iriniethoxyborohydridc. Stannous bromide. Stannous chloride. Teltamethylammonium borohydride. Thioglycolic acid. Tin. Titanium tetrachloride. Tri-/-butylaluminum. Tri-n-butyltin hydride. Triethylene glycol (see Zinc). Triethyl phosphite. Trimethylamine borane. Trimethylammonium formate. Trimethylsilane (indirect). Triphenyltin hydride. Tris-(tripbenylphosphine)chlor(X hodium. Zinc. Zinc amalgam. Zinc-Copper couple. Zinc hydrosulfide. 

High temperatures lead to elimination of hydrocarbons from the thio-derivatives, and formation of borthiins, together with some loss of HjS, giving (R2B)2S the latter, however, tend to lose further H2S, giving borthiins and tri-organyl-boranes [reactions (23)—(25)]. ... 

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