Boron trialkyls

Some interesting observations have been reported recently by Japanese workers21 who found that boron trialkyls initiate polymerization. These investigators suggest that the polymerization is anionic in character however, work carried out by Fordham etalP... 

These rarely produce homoleptic species, but boron trialkyls can be obtained from diborane ... 

The first organic compound of boron was isolated by Frankland in 1862, by the interaction of zinc methyl and triethyl borate. This method and the use of zinc alkyls with boron trichloride was continued until the discovery of the Grignard reagent. In 1900 boron tri-methyl was prepared by the action of magnesium methyl iodide on trimethyl borate, and in 1921 a number of boron trialkyls were obtained by the interaction of magnesium alkyl halides and boron trifluoride. Of the derivatives of the type RgB, boron trimethyl is a gas, spontaneously inflammable in air boron triethyl, a spontaneously inflammable liquid whilst the higher members of the series are liquids, readily oxidised when exposed to air. The metl yL fpd ethyl derivatives combine with ammonia to foriii IliS type I BOH only... 

Many other compounds have been burnt in static-bomb calorimeters with some success, but the knowledge concerning the final state is frequently inadequate and the result therefore uncertain. Thus, for example, boron trialkyls and carboranes can be burnt to yield CO2, HgO, and H3BO3, with only traces of side reactions or of incomplete combustion, but it has to be assumed that a saturated aqueous solution of boric acid is produced... 

The general formula for boric acid esters is B(OR)2. The lower molecular weight esters such as methyl, ethyl, and phenyl are most commonly referred to as methyl borate [121 -43-7] ethyl borate [130-46-9J, and phenyl borate [1095-03-0] respectively. Some of the most common boric acid esters used in industrial appHcations are Hsted in Table 1. The nomenclature in the boric acid ester series can be confusing. The lUPAC committee on boron chemistry has suggested using trialkoxy- and triaryloxyboranes (5) for compounds usually referred to as boric acid esters, trialkyl (or aryl) borates, trialkyl (or aryl) orthoborates, alkyl (or aryl) borates, alkyl (or aryl) orthoborates, and in the older Hterature as boron alkoxides and aryloxides. CycHc boric acid esters, which are trimeric derivatives of metaboric acid (HBO2), are known as boroxines (1). 

BCl, BBr, and BI undergo exchange reactions to yield mixed boron hahdes. Exchange reactions also occur with trialkyl, triaryl, trialkoxy, or triaryloxy boranes and with diborane. Anhydrous metal bromides and iodides can be prepared by the exchange reaction of the metal chloride or oxide and BBr or BI (21)-... 

Boronic acids readily dehydrate at moderate temperatures (or over P4O10 at room temperature) to give trimeric cyclic anhydrides known as trialkyl(aryl)boroxines ... 

Boronic acids RB(OH)2 were first made over a century ago by the unlikely route of slow partial oxidation of the spontaneously flammable trialkyl boranes followed by hydrolysis of the ester so formed (E. Frankland, 1862) ... 

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

The stereoselectivity of Lewis acid promoted reactions between 2-butenylstannanes and aldehydes has been widely studied, and several very useful procedures for stereoselective synthesis have been developed. In particular syn-products are formed stereoselectively in reactions between trialkyl- and triaryl(2-butenyl)stannanes, and aldehydes induced by boron trifluoride-diethyl ether complex, irrespective of the stannane geometry66. 

Effective catalysts have recently been developed for the addition of trialkyl-aluminum reagents to alkenes (carboalumination). 6 -(Pentamethylcyclopentadienyl) zirconium dimethylide activated by fra-(pentafluorophenyl)boron promotes the addition of trimethylaluminum to terminal alkenes.221... 

In recent years, a variety of aryl boronic acids are commercially available, albeit in some cases they may be expensive for large scale purposes. During our work in the mid-1990 s boronic acid (II) was not commercially available and so two different protocols were used to prepare this acid. The first approach involved the transmetallation with n-butyl lithium of aryl bromide (I) and trapping the lithio species generated with trialkyl borate followed by an acid quench. Aryl bromide (I) is easily prepared by reaction of o-bromobenzenesulfonyl chloride with 2-propanol in the presence of pyridine as a base. The second approach was a directed metallation of isopropyl ester of benzene sulfonic acid (VII), to generate the same lithio species and reaction with trialkyl borate. The sulfonyl ester is prepared by reaction of 2-propanol with benzenesulfonyl chloride. From a long-term strategy the latter approach is... 

An alternative approach to reduce the levels of impurity (VII) would be to have a "transient" existence of the lithio species, so that it reacts instantaneously with trialkyl borate to form the aryl boronate, prior to being quenched by any extraneous proton source to form (VII). Thus, the preparation of boronic acid (II) was improved by changing the order of the reagents. The slow addition of n-butvl lithium also controls the exotherm of the reaction. There was no reaction observed between n-butyl lithium and triisopropyl borate (to form any butyl boronic acid), nor was there any formation of 2-butyl derivative of (VII) formed by reaction between butyl bromide and the lithio species. The reaction is veiy fast and as soon as the addition of n-butyl lithium is completed the reaction is finished. This indicates a rapid transmetallation and instantaneous boronation of the lithio species. The reaction is very much a... 

An essential requirement for such stabilisation is that the carbocation should be planar, for it is only in this configuration that effective delocalisation can occur. Quantum mechanical calculations for simple alkyl cations do indeed suggest that the planar (sp2) configuration is more stable than the pyramidal (sp3) by = 84 kJ (20 kcal) mol-1. As planarity is departed from, or its attainment inhibited, instability of the cation and consequent difficulty in its formation increases very rapidly. This has already been seen in the extreme inertness of 1-bromotriptycene (p. 87) to SN1 attack, due to inability to assume the planar configuration preventing formation of the carbocation. The expected planar structure of even simple cations has been confirmed by analysis of the n.m.r. and i.r. spectra of species such as Me3C SbF6e they thus parallel the trialkyl borons, R3B, with which they are isoelectronic. 

Preparation of 1-ethoxybenzylphosphonate — Reaction of a trialkyl phosphite with an acetal in the presence of boron trifluoride... 

Cragg and Weston (11) in their review of the mass spectra of boron compounds have shown the great amount of work being done with cyclic boron compounds. This trend has continued. Straight-chain trialkyl-borates fragment by C—0 cleavage reactions accompanied by hydrogen transfers. 

Boron trichloride undergoes several exchange reactions with dihorane, other boron halides and trialkyl-, triaryl-, or triaryloxy boranes (Lower, L.D. 1978. Boron compounds (halides). In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed. pp 129-135. New York Wiley Interscience). The reactions occur at ambient or lower temperatures ... 

Trialkyl derivatives of boron, and in fact many other molecules such as boroxines with carbon-boron bonds, react readily with oxygen. The initial products are peroxy derivatives with BOOR bonds, which tend to react further to form borate esters. The ease of the initial reaction is shown by the fact that reported examples of vinyl polymerization induced by trialkyl borons require oxygen and are actually radical processes induced by the boron oxygen reaction or intermediate peroxides (7). 

Reaction 17 resembles the displacements on phosphorus discussed earlier, except that since trialkyl borons are electron deficient with only six electrons around boron, no valence-shell expansion is required. It appears to be a general reaction for boron and has been suggested by Matteson (15) to explain the conversion of trimethyl boroxine by tert-buty hypo-... 

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