Organoboranes radical reactions

The ability of organoboron compounds to participate in free radical reactions has been identified since the earliest investigation of their chemistry [1-3]. For instance, the autoxidation of organoboranes (Scheme 1) has been proven to involve radical intermediates [4,5]. This reaction has led recently to the use of triethylborane as a universal radical initiator functioning under a very wide range of reaction conditions (temperature and solvent) [6,7]. 

IV/V Synthetic aspects of free radical reactions of organoboranes 23... 

The reaction of dialkylchloramines with organoboranes gives tertiary amines, if the competitive free radical reaction leading to the formation of alkylchlorides is suppressed by adding of a radical scavenger such as galvinoxyl [72,73] (Scheme 26). 

There are two common types of radical reactions of organoboranes Inmolecular homolytic substitution (5h2 equation 4) and a-abstraction processes (equation 5). Both of these can lead to final products by chain processes, as illustrated for the a-abstraction reaction by the continuation shown in equation... 

There are two common types of radical reactions of organoboranes bimolecular homolytic substitution (5h2 equation 4) and a-abstraction processes (equation 5). Both of these can lead to final products by chain processes, as illustrated for the a-abstraction reaction by the continuation shown in equation (6). The radicals produced in these reactions are unlikely to retain complete stereochemical integrity except in special circumstances. The nature of the further reactions, such as equation (6), determines whether or not the products are such as to be included in this section. ... 

Organoboranes participate in either ionic or radical reactions [1]. However, there are a few reports on the apphcations of organoboranes as initiators for radical reactions [2, 3]. It is reported [4] that in the presence of a catalytic amount of 9-BBN or B-hexyl-9-BBN, or to some extent B-methoxy-9-BBN, initiates the radical addition of alkanethiols to alkenes under very mild conditions to provide the corresponding dialkylsulfides almost in quantitative yields (Scheme 14.1). On the other hand, the radical reactions of alkenes with thiols are generally initiated by thermal decomposition of peroxides or azo compounds, by UV radiations, or by radiolysis [5]. The reaction initiated by 9-BBN is completely inhibited in the presence of galvinoxyl, a radical trapping agent. This confirms that 9-BBN participates in the initiation of radical addition (Table 14.1) [4]. 

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). 

It is known that organoboranes readily undergo autoxidation in the presence of oxygen and this reaction has been used to prepare alcohols and alkyl hydroperoxides as well as to induce free radical reactions. Organoboranes can also be used to alkylate a,P-unsaturated carbonyl compounds through a free radical 1,4-addition reaction in the presence of oxygen. However, organoboranes do not normally react with saturated carbonyl compounds except for the reaction of formaldehyde with trialkylboranes in the presence of air. ... 

Alkylzirconocene derivatives can be converted to alcohols with H202/NaOH, /BuOOH, or m-chloroperbenzoic acid (m-CPBA) [98], These reactions appear to involve migratory insertion processes (Pattern 14) similar to those observed with organoboranes (Scheme 1.20). On the other hand, oxidation with 02 may be a radical process. 

Stannyl radicals are usually generated by homolytic substitution at hydrogen in a tin hydride, or at tin in a distannane, or, conjugatively, at the y-carbon atom in an allylstannane.453 The initiator is commonly AIBN at ca. 80 °C. In the presence of a trace of air, organoboranes are oxidized by a radical chain mechanism, and triethylborane is now commonly used as an initiator at temperatures down to —78°C,519 and it can be used in aqueous solution.520 9-Borabicyclo[3.3.1]nonane (9-BBN) has similarly been used to initiate the reaction of tin hydrides at 0 and —78°C,521 and diethylzinc works in the same way.522... 

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. 

Both chain and nonchain reactions involving an Sh2 attack are known for thiyl radicals. An example of the latter is the photolysis of Bu SSBu in the presence of an organoborane or Bu MgCl, when an alkyl radical is displaced by Bu S and can be detected by ESR. The former, known for B, Sb, and Bi, are illustrated by... 

Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Fast Exchange Reactions of Group I, II, and III Organometallic Compounds, 8, 167 Fluorocarbon Derivatives of Metals, I, 143 Free Radicals in Organometallic Chemistry, 14, 345 Heterocyclic Organoboranes, 2, 257... 

Oxidations of organoboranes involve numerous reagents and several different general mechanisms, most of which parallel those wluch occur for other types of oiganoborane reactions. The mechanisms fall under three broad headings (i) ionic, with a 1,2-shift fiom boron to a heteroatom (equations 1-3) (ii) radical and (iii) electrocyclic. 

There are no publications concerning the reaction of nitroarenes with organoboranes. The only known reaction, is the electrolysis of trialkylboranes in nitro methane in the presence of Rj NX salts, which leads to higher nitroalkanes.12 The reaction of RjB with NO, which proceeds by a radical mechanism, has been used for the preparation of hydroxylamine derivatives (RaNOH), heterocyclic boron derivatives or alkylamines (see ref.1, p. 283.)... 

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