Boranes, alkyl

Sodium borobydride reacts with Lewis acids in nonprotic solvents to yield diborane [19287-45-7] 2 6 which can then be used to generate other useful organoboranes such as amine boranes, alkyl boranes, and boron hydride clusters. 

For example, pyrolysis of the previously mentioned nido-2,3-C2B4Hg gives the 3 c/o o-species shown above, whereas under the milder conditions of photolytic closure the less-stable isomer closo-, 2-C2 i is obtained. Pyrolysis of alkyl boranes at 500-600° is a related route which is particularly useful to monocarbaboranes though the yields are often low, e.g. ... 

Zahlreiche Alkyl-borane konnen zu selektiven Reduktionen eingesetzt werden. Sie besitzen in der Regel ein schwacheres Reduktionsvermogen als Diboran, und sind daher selektiver. 

Hydroboration is thermally reversible. B-H moieties are eliminated from alkyl-boranes at 160° C and above, but the equilibrium still favors of the addition products. 

Alkylmercury reagents can also be prepared from alkyl boranes. 

A series of potent, linear C2-symmetric HIV-1 protease inhibitors with K, values in the nanomolar range was prepared from a diaryl bromide precursor emanating from a carbohydrate scaffold, by application of Heck, Suzuki, Stille, and cyanation reactions. Included in this series was the first reported microwave-promoted Suzuki coupling with an alkyl borane [41]. A very high-yielding Suzuki coupling is presented... 

The carbanions take up 02 and these take up protons to give hydroperoxides in good yields. But because they are explosive in nature, they are not usually isolated and on reduction with sodium sulphite on trialkyl phosphite give alcohols. Alcohols can also be prepared via hydroperoxy molybdenum complexes and alkyl boranes. These reactions are summarized as follows ... 

Shmaryahu Hoz of Bar-Ilan University reports (J. Org. Chem. 68 4388, 2003) that alkyl boranes couple with dinitro aromatic rings such as 1 to give the alkylated aromatic, with loss of one of the nitro groups. This reaction shows remarkable regioselectivity, as illustrated by the formation of 2. Much more complex alkyl boranes participate also, as illustrated by the coupling of the 9-BBN derivative 3. The reaction proceeded to give 4 as a single diastereomer. 

Boron Hydride Derivatives. Alkyl boranes can be prepd by alkylation of key intermediates, of diborane or of higher boranes. Alkyiation of a borane will proceed more readily if a functional group, such as a halogen or an active merai arom, is attached to the borane. For example, a halo-borane may react with a metal aikyl to produce an alkyl borane, or a metal polyhydropolyborate may react with an alkyl halide to produce an alkyl borane(Ref 35). Boranes may be alkylated or ary-iated with an unsaturated hydrocarbon(Ref 13). Alkoxy derivs of boranes can be prepd by allowing a borane to react with an appropriate alccho (Ref... 

Alkyl Boranes. See under Boranes Alkyldichloroamines. See Dichloto alkyl-amines... 

This topological rule readily explained the reaction product 211 (>90% stereoselectivity) of open-chain nitroolefins 209 with open-chain enamines 210. Seebach and Golinski have further pointed out that several condensation reactions can also be rationalized by using this approach (a) cyclopropane formation from olefin and carbene, (b) Wittig reaction with aldehydes yielding cis olefins, (c) trans-dialkyl oxirane from alkylidene triphenylarsane and aldehydes, (d) ketenes and cyclopentadiene 2+2-addition, le) (E)-silyl-nitronate and aldehydes, (f) syn and anti-Li and B-enolates of ketones, esters, amides and aldehydes, (g) Z-allylboranes and aldehydes, (h) E-alkyl-borane or E-allylchromium derivatives and aldehydes, (i) enamine from cyclohexanone and cinnamic aldehyde, (j) E-enamines and E-nitroolefins and finally, (k) enamines from cycloalkanones and styryl sulfone. 

The majority of mono- and di-substituted alkenes undergo hydroboration to give trialkylboranes which are then available for a variety of synthetic procedures. With some alkenes, alkylation of borane does not go to completion and the resulting mono- or di-alkyl boranes are useful in synthesis as modified boranes. Thus, 2-methylbut-2-ene gives bis(3-methyl-2-butyl)borane (idisiamylborane). 

Preparation of alkyl boranes RB(C6F5)2 which do not contain (3-hydrogens can be accomplished by transmetallation between Cp2ZrR2 and ClB(C6F5)2.48b The methyl, benzyl and trimethylsilyl methyl bis-(pentafluorophenyl) boranes have been prepared in... 

Two dialkyl boranes arc in common use. The bicyclic 9-borabicyclo[3.3.1] nonane (9-BBN), introduced in Chapter 34 as a reagent for diastereoselcctive aldol reactions, is a stable crystalline solid. This is very unusual for an alkyl borane and makes it a popular reagent. It is made by hydroboration of cyclo-octa-1,5-diene. The second hydroboration is fast because it is intramolecular but the third would be very slow. The regioselectivity of the second hydroboration is under thermodynamic control. 

Carbon-carbon bonds can also be made with alkyl boranes. The requirement for a carbon nucleophile that bears a suitable leaving group is met by a-halo carbonyl compounds. The halogen makes enolization of the carbonyl compound easier and then departs in the rearrangement step. The product is a boron enolate with the boron bound to carbon. Under the basic conditions of the reaction, hydrolysis to the corresponding carbonyl compound is rapid. 

The oxidation of (secondary alkyl)boranes with chromic acid leads to ketones, " and in combination with hydroboration provides a high-yielding route from alkenes to ketones. The reaction gives ketones at... 

Pyridinium chiorochromate (PCC) is a very useful reagent for the oxidation of oiganoboranes to carbonyl compounds in mildly kaline and anhydrous conditions. As well as oxidizing (secondary alkyl)boranes to ketones, PCC oxidizes (primary alkyl)boranes to aldehydes in excellent yields (equation 47). The latter transformation cannot be accomplished with chromic acid. The reagent tolerates the presence of alkene, ester and acetal groups. ... 

An alternative indirect but efficient method for the bromination of all three groups of tri(primaiy alkyl)boranes involves initial reaction with mercury(II) acetate followed by in situ bromination. Alkenyldialkylboranes react with bromine to give bromoalkenes via an addition-elimination mechanism. The meftod of elimination controls the stereochemistry of the product bromoalkenes (Scheme 1). For reasons which are not clear, exactly opposite stereochemical results are obtained from (aryl-ethenyl)dialkylboranes as compared with (alkylethenyl)dialkylbotanes (Scheme 1). ... 

In die presence of sodium medioxide, tri(priniary alkyl)boranes react smoothly to yield 3 mol equiv. of iodoalkane. In die presence of sodium hydroxide, two C—B bonds of tri(primary alkyl)boianes are cleaved to die corresponding iodides.In die same conditions tri(secondary idkyl)boranes react signifi-candy more slowly and only one C—B bond is broken. For anti-Markovnikov addition of HI to terminal alkenes die process shown in equation (60) is therefore iqiplicable. ... 

The transition metal activates the C-X bond in the oxidative addition step and normally the substrates have sp or sp carbons at or immediately adjacent to an electrophilic centre. The reactivity of aliphatic C-X bond towards the oxidative addition with a transition metal is somewhat low. However, in 1992, Suzuki and co-workers discovered that Pd(PPh3)4 can catalyze couplings of alkyl iodides with alkyl boranes at 60°C in moderate yields (50-71%). These conditions tolerated a wide variety of functional groups such as esters, ketals and cyanides. 

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