Radicals from boranes

Substituents are named as one of several classes. The most important ones are dealt with here. For monoatomic radicals from borane, methane, silane (and other Group IVA elements) they are named by replacing the ane ending by yl, i.e., ylene and ylidyne, to denote the loss of one, two, or three hydrogen atoms, respectively ... 

Alkyl radicals from boranes and oxygen 998 Allylic electrophiles are activated by palladium(O) 1088... 

The mechanism probably involves displacement of a tert-butyl radical from the borane by dioxygen, followed by addition of the tert-butyl radical to the vinyl group and ring cleavage. Much better yields of the same products were obtained when lithium dialkylcuprates replaced the trialkylboranes the nature of the intermediates is unknown in this case. 

The catalyst was synthesized from borane dimethyl sulfide and a chiral polymer 103 bearing N-sulfonylamino acid groups. The latter was prepared by co-polymerization of chiral sulfonamide 104 with styrene 58 and the cross-linking agent 105 in the presence of benzoyl peroxide as a radical initiator. 

However, H. C. Brown discovered that the reaction was completely inhibited by just 5% of the stable radical galvinoxyl (shown on p. 975), known to be an efficient scavenger for radicals. But where were the radicals coming from Further experiments showed that small amounts of oxygen were needed to make the reaction work. As you saw in Chapter 3, oxygen is a triplet diradical and displaces alkyl radicals from the trialkyl borane. This reaction looks at first like an 5 2 and is called an Sh2 (second order homolytic displacement), but in reality the oxygen adds to the empty p orbital of planar trigonal boron to release an alkyl radical and start the chain reaction. 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

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. 

Alkyl radicals can be generated from alkyl iodides in a chain process initiated by a trialkylborane and oxygen.204 The alkyl radicals are generated by breakdown of a borane-oxygen adduct. 

The best procedure to get the desired product is to generate the 1-alkene from the borane with 1-decene (Section 11-6C) and then add hydrogen bromide by a polar mechanism (Section 10-4). Incursion of radical-chain addition must... 

Controlled radical polymerization (CRP) is an attractive tool, because of the resultant controllability of polymerization, and because of it being a versatile method to synthesize of well-defined polymer hybrids. The three main radical polymerization techniques, ATRP, NMP, and RAFT polymerization, have thus been employed. Other techniques, such as the oxidation of borane groups, have also been studied. In general, using CRP techniques, block copolymers can be synthesized from terminally functionalized PO as PO macroinitiator, and block copolymers can be prepared from functionalized PO produced by the copolymerization of olefins with functional monomers. 

Radical Epoxide Reduction Borane-free Two-step Synthesis of anti-Markownikow Alcohols from Alkenes... 

The rate constant of HAT from 1 has been determined as (3.4 1.0) x 104 M-1 s-1 at 28 °C by using a cyclobutyl carbinyl radical as clock. Also, the log A term of the Arrhenius equation is normal for a second-order HAT and thus the entropic demand of the NHC boranes is similar to that of group 14 metal hydrides. From the rate constant a BDE of about 88 kcal mol 1 for 2 was estimated by applying an Evans-Polanyi relationship. This value is somewhat higher than the calculated value of 80 kcal mol-1. 

This substitutes very well for most tin hydrides in many reactions. Roberts, on the other hand, has demonstrated that thiols act as polarity reversal catalysts for hydrogen-atom transfer from organosilanes to alkyl radicals.185 The use of triethyl borane and oxygen as radical initiators166 has allowed the reaction to be carried out at much lower temperatures and hence mechanistic elucidations have been possible.187... 

The reaction of trialkylboranes with A-chloro- or A-(benzoyloxy)alkylamines afforded secondary amines via an anisotropic 1,2-shift of the alkyl group from boron atom to nitrogen in the B-N complex intermediate.519-521 Alkylation of A-chlorodimethylamine with primary trialkylboranes to give A,A-dimethylalkylamines was conducted in the presence of galvinoxyl to avoid the formation of alkyl chlorides via free radical process.522,523 A convenient approach to mixed secondary amines is alkylation of alkyl azides with relatively unhindered trialkylboranes in refluxing xylene followed by hydrolysis with water. The reaction smoothly took place at low temperature when trialkylboranes were replaced by alkyl(dichloro)boranes (Equation (108)).524 Intramolecular amination furnished cyclic amines (Equation (109)).400,525-528... 

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