Tetra diborane

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. 

Distannyl derivatives have also been used as starting materials for the synthesis of fluorinated ort/io-phenylene diboranes. The reaction of l,2-bis(trimethylstannyl)tetra-fluorobenzene (18) with BCI3 affords 9,10-dichloro-9,10-dihydro-9,10-di-boraoctafluoroanthracene (15) (Scheme This compound can be further de-... 

Durch Natriumborandt erfolgt keine Ringaufspaltung284, jedoc n wird mit Diboran in Tetra-hydrofuran eine Reduktion der 4,5-C,N-Doppclbindung zu 4,5-Dihydro-l,2,4-oxadiazo-len beobachtet52 z. B. ... 

Reaction of sodium naphthalide with B2C14 at room temperature was reported to give a liquid product with a suggested structure, 17. The compound 17 reacts with four equivalents of (CH3)3N to form an adduct which decomposes above 100 °C to give the bis(trimethylamine) adduct of B2C14 and an unstable product that is thought to be 18 (Scheme 3). It was reported that the trimethylamine complex of tetra(methyl)diborane(4) was obtained from the reduction of bromodimethylborane with sodium or silver in trimethylamine.52... 

Several transamination reactions of B2(NMe2)4 20 with secondary amines have led to mixed tetra(amino)diborane(4) compounds B2(NMe2)4 ra(NR2)ra 24-26 (Scheme 5), and B2(NC5H10)4 23 has been characterized by an X-ray structure analysis which reveals the presence of a rather long B-B bond (Fig. 3). 

However, tetra(amino)diboranes(4) of type R2N(Me2N)B-B(NMe2)NR2 are more readily accessible from LiNR2 and B2(NMe2)2Cl2. Similarly,... 

A series of mixed tetra(amino)diborane(4) compounds bearing pyrrolyl, indolyl, and carbazolyl substituents besides dimethylamino groups has been prepared and subjected to reduction with elemental lithium in the... 

Tetra(dimethylamino)diborane(4) was treated with o-phenylene isothio-cyanato-boronate, to give an interesting reaction which involve not only disproportionation but also cleavage of the B-B bond giving compounds 51 and 52. In addition a third product, 2-(l,3,2-benzodioxaborolo)-l,3,2-benzodioxaborole 12, was isolated and compared with those previously reported elsewhere58 (Scheme 10). 

The conversion product tetra(carbahexa)borane 167 (molecular structure shown in Fig. 8) can be obtained from diborane 165 (Scheme 26) via a 2,5-diborabicyclo-[2,l,l]-hexane 166 and its structure have been reported by Enders et al.104 Synthesis of carbaboron compounds has also been reviewed previously.105,106... 

Interaction of tetra(fluoro)diborane with ethylene gives B-C-C-B compounds belonging to the bidentate Lewis acid.110 112 The reactions... 

Synthesis of arylboronates via the palladium(0)-catalyzed cross-coupling reaction of tetra(alkoxy)diboranes with aryl triflates has been reported.114 The cross-coupling reaction of (RO)2B-B(OR)2 (OR = methoxy and pinacolato) with aryl triflates to give arylboronates 178-189 was carried out at 80 °C in the presence of PdCl2(dppf) (3 mol%), and KOAc (three equivalents) in dioxane. The reaction was generalized to various functional groups such as nitro, cyano, ester, and carbonyl groups (Scheme 30). 

The first synthesis of arylboronic esters 209-215 via the coupling of bis(pinacolato)diborane(4) with easily prepared aryldiazonium tetra-fluoroborate salts was reported. The palladium-catalyzed borylation reaction proceeds efficiently under mild reaction conditions in the absence of a base to afford various functionalized arylboronic esters in moderate to high yields (Scheme 34).121... 

Nevertheless the correspondence of diborane with ethylene was found from extensive investigations on the infra-red absorption spectrum and the Raman spectrum to be greater than with ethane. There is also a correspondence in the Raman spectrum with A12C16 which is built up of two tetra-hedra with a common edge. 

At the other extreme, the most stable derivatives of the B2X4 type appear to be the tetrakis(dialkylamino) compounds. Thus, tetrakis(dimethylamino)-diborane(4) is stable at its boiling point of 206° C 17). The compound decomposes at 300° C to form bis(dimethylamino)borane and involatile residues containing B—C bonds. It is reported that tetra(A -methylanilino)-borane(4) is pyrolyzed at 300° C by a different mechanism to yield N-methylaniline 18). 

Tetra-alkylammonium A1H4 salts react with diborane in THF to give the B2H7 salt and A1(BH4)3 or a substituted A1(BH4)3.41... 

The reagent actually exists as the diborane derivative (1) rather than as the monomer.1 The Chemical Abstracts name for this reagent is tetra-3-pinanyldiborane. It has been referred to as Pn4B2H2. 

On treatment of the l-azido-2-iodo-l-methylcyclobutane (1) with diborane in tetra-hydrofuran, 2-iodo-l-methylcyclobutylamine (2) was effectively isolated in 50% yield, then lithium aluminum hydride reduction gave the same product of ring contraction 3. ... 

Diborane is not unique among the boron hydrides in its ability to form a carbonyl. High pressure reaction of either pentaborane-f / or tetra-borane-10 with carbon monoxide forms a substance which behaves like borane-carbonyl in its manner of decomposition. The formula of this polyborane-carbonyl has recently been established as OC B Hg (mp, — 114.5° bp, 59.6°) 41a). It reacts with trimethylamine without release of carbon monoxide. 

Diborane finds such wide use in steroid chemistry that a new and very simple procedure for its preparation is welcomed. Tetra-alkylammonium borohydrides, which are readily extracted from an aqueous solution of Na BH and R4N -HSOT into CH2CI2, are used in the dried CH2CI2 solution addition of an alkyl halide (e.g. Mel) generates diborane in situ, and allows all the usual reactions of reduction or hydroboronation to be carried out conveniently and in high yields. 

C-17, gave 3 -melhanesulphonyloxyandrost-5-en-17 -ol acetate (403). Diborane addition to the A -double bond and oxidation afforded the 6-keto-derivative (404). Elimination of the 3-mesylate, followed by iodine and silver acetate treatment furnished the 2-acetoxy-3-alcohol (405), which was successively acetylated at C-3, brominated at C-7, and dehydrobrominated to the triacetoxy-A -6-ketone (406). Enol acetylation of the conjugated carbonyl furnished the tetra-acetate (407), which was treated with peracid and then hydrolysed and isomerized at C-5 to yield the desired tetraol (408). ... 

The combination of crown ether and porphyrin has recently been extended by Hamilton et al. The macrotetracyclic cryptand 226 was prepared by condensation of the biphenyl-linked bis-crown ether 224 with the di-p-nitrophenyl ester porphyrin 225a, b in pyridine at 55 °C under high dilution conditions (45-54% after chromatography). Reduction to the tetra-amine 227 was effected by treating the zinc complex with diborane... 

In the presence of isopropyl bromide, aluminum chloride reacts with hexaethylditin to produce triethyltin bromide in addition to tetraethyltin (283). Boron triduoride, dimethylboron Ruoride, and diborane have also been shown to bring about the cleavage of hexamethylditin to form tetra-methyltin (284). 

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