Boranes radical reactions with

The adducts derived from the reaction of a variety of alkyl or alkoxyl substituted silyl radicals with aminoboranes have also been recorded [82]. The silyl radical addition takes place at the boron site to give the aminyl-borane radical (Reaction 5.43) and structural information for this class of radicals has been obtained. 

A soln. of borane in tetrahydrofuran added dropwise at 0° to cyclohexene and dry tetrahydrofuran, warmed 3 hrs. at 50°, cooled to -78°, stirred and treated with oxygen until Og-absorption ceases after 2 moles have been consumed, then aq. 30%-HgOg added dropwise, and stirred 0.5 hr. at 0° cyclohexyl hydroperoxide. Y 80-95%. - The low-temp, autoxidation of organoboranes provides a rapid and convenient synthesis of alkyl hydroperoxides. F. e. s. H. C. Brown and M. M. Midland, Am. Soc. 93, 4078 (1971) synthetic applications of organoboranes, review, s. Chem. Britain 7, 458 (1971) radical reactions with boranes, review, s. Ang. Ch. 84, 702 (1972). 

Similarly, reaction of B6H10 with Fe2(CO)9 (p. 1 104) at room temperature results in the smooth elimination of Fe(CO)s to form [Fe(r -B6Hio)(CO)4] as a. stable, volatile yellow solid. Use of these electron-donor propcities of B(,H o towards reactive (vacant orbital) borane radicals resulted in the preparation of several new coti/uncm-boranes. e.g. BnHiy, BuHji and B15H23 (p. 162),... 

Synthesis of 12- and 13-membered sulfur-containing lactones by homolytic macrocyclization of mercaptoacetic esters and alkynes has been investigated [95S307]. Reaction of the mercaptoester 245 with alkynes using triethyl borane radical initiation gave the macrolactones 247 and 248 in low yield [95TL2293], Remote asymmetric induction is observed during the cyclization. 

Borane, methanol, and water, which have B-H and O-H bonds too strong to allow participation in radical reactions, are activated towards HAT by complexa-tion with NHCs, boranes, or titanocene(III) complexes. This novel concept has resulted in exciting applications in both radical chain reactions and transition metal mediated and catalyzed transformations. 

Radical reactions of substrate 27 were initially conducted at low temperature in toluene with triethyl borane as initiator and tributylstannane as reducing agent. Under these conditions no conversion was detected. Increasing the temperature to ambient temperature led to 99% yield and a diastereomeric ratio (d.r.) of 47/53 in favour of the cis compound cis-29. Reactions in the presence of the chiral complexing agent... 

Boranes are generally less electron rich than corresponding aluminium analogues and electron transfer mechanisms are usually not considered for reduction reactions. However, an increase of the hydridic character in complex hydrides such as.LiBEt3H ( super hydride ) [68] or LiAlH4 [66] allows for the formation of well-characterized radical products in reactions with unsaturated acceptor heterocycles such as (3). Electron transfer mechanisms for the reduction by complex hydrides should be quite intricate because the coordinatively saturated donor moiety (MHn ) and the a acceptor part (e.g. Li" ) can now well separately interact with the coordinating n acceptor substrate. 

Similar migration of the other two R groups and hydrolysis of the B-0 bonds leads to the alcohol and boric acid. Retention of configuration is observed in R. Boranes can also be oxidized to borates in good yields with oxygen," with sodium perborate NaBOs," " and with trimethylamine oxide, either anhydrous" or in the form of the dibydrate." The reaction with oxygen is free radical in nature." ... 

The alkoxides induce radical reactions having induction periods. Some loss of configuration is found with norbornylboranes e.g., from 99% pure tris(exo-norbornyl)borane, only 80-85 % exo-norbornylmercury alkoxides are obtained. 

Electron-rich catalysts such as 38 are rather better than electron-poor ones. In contrast, those containing electron withdrawing groups, i.e., 39-41, are poor catalysts. A radical redesign of the catalyst structure led Wills and his coworkers to synthesize catalyst 45 derived from (S)-diphenyl prolinol [45]. In this case, upon reaction with borane, a borate ester would be formed and act as the Lewis acid site. 

If boron of an alkylborane could be replaced with a halogen, the product would be an alkyl halide. However, reaction of alkylboranes (neat) with chlorine, bromine, or iodine is very difficult. a when halogenation is done with bromine or iodine dissolved in dichloromethane, however, the reaction is fast and is synthetically useful.A simple example is the reaction of alkenes with boranes followed by addition of bromine, which leads to the alkyl bromide. An example is taken from the synthesis of 2-bromobutane (70) from 2-butene in 88% yield. 0 jhe bromination occurs by a free radical mechanism. Initial reaction with bromine generates a... 

Although benzeneselenol adds across the triple bond of phenylacetylene in the presence of molecular oxygen to give the corresponding vinyl selenide (equation 15), inactivated acetylenes proceed very slowly under similar conditions". However, the addition of PhSeH to inactivated acetylenes has been achieved in the presence of a catalytic amount of diselenide upon irradiation (equation 16)". Evidence that these reactions proceed via a radical mechanism is also given. Tris(phenylseleno)borane and tris(methylseleno)borane react also with terminal acetylenes to give (Z)-vinyl selenides". ... 

If you want to get the other regioisomer, with the heteroatom at the end, you can use hydro-boration (Chapter 19) or the radical reactions described in the next section. Here is a brief reminder of hydroboration. Reaction between a borane having at least one B-H bond with an alkene gives an alkyl borane in which all the hydrogens are replaced by alkyl groups. Oxidation gives the terminal alcohol. 

Group III.—Boron. It has only recently been conclusively shown that the stoicheiometry of the boric acid-tartaric acid complex is 1 1. Kinetic results from T-jump experiments indicate attack by an alcohol-oxygen lone-pair at the electron-deficient boron. Other references to reaction mechanisms of three-co-ordinate boron compounds deal with hydrolysis and methanolysis of boron halides, with disproportionation of boranes, and with Sh2 radical reactions of alkylboranes. ... 

The conjugated cisoid ketones react satisfactorily with B-alkenyls-9-BBN, whereas transoid conjugated ketones, like cyclopentenone and cyclohexenone, afford a complex mixture of products. The reaction proceeds thermally and does not appear to involve radicals. The alkenyl group of borane is transformed with strict retention of configuration. As the reaction occurs only with cisoid enones, and thus suggesting a cyclic transition state (Chart 7.8) [10]. 

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