Radical borane-based

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. 

Among all the radical processes previously described, more specific techniques can lead to the synthesis of macromonomers. For instance, the use of borans in radical polymerization, or the radical polymerization based on unimolecular terminations, may allow macromonomers to be obtained. These specific techniques will be briefly summarized as only a few workers have investigated the use of such techniques, aiming at the synthesis of macromonomers. 

The unexpected good control in the incorporation of borane groups to polyolefin by metallocene catalysis and the subsequent radical chain extension by the incorporated borane groups prompted us to examine this free radical polymerization mechanism in greater details. Several relatively stable borane-based radical initiators were discovered, which exhibited living radical polymerization characteristics, with a linear relationship between polymer molecular weight and monomer conversion [27] and producing block copolymers by sequential monomer addition [28]. This stable radical... 

The perception by R. Schaeffer that nido-BeHjo (stmcture 10, pp. 154, 159) could act as a Lewis base towards reactive (vacant orbital) borance radicals has led to several new conjucto-boranes, e.g. ... 

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. 

A modified version of the Brown-Negishi reaction using B-alkylcatechol-boranes was reported (Scheme 32). This novel method is based on a simple one-pot procedure involving the hydroboration of various substituted alkenes with catecholborane, followed by treatment with catalytic amount of oxygen/DMPU/water and a radical trap. Efficient radical additions to a,ft-unsaturated ketones and aldehydes have been reported. Primary alkyl radicals are efficiently generated by this procedure and the reaction has been applied to a 300 mmol scale synthesis of the y-side chain of (-)-perturasinic... 

Organoboranes react with bromine to form brominated boranes,l44 but they also react with brominated substrates. a-Bromo esters, for example, react with organoboranes to give coupling products if a base is added to the bromination reaction.Adding a base suppresses radical bromination, allowing an ionic reaction to... 

Purpose. The oxidation of an alkene to an alcohol is investigated via the in situ formation of the corresponding trialkylborane, followed by the oxidation of the carbon-boron bond with hydrogen peroxide. The conditions required for hydroboration (a reduction) of unsaturated hydrocarbons are explored. Alkylboranes are particularly useful synthetic intermediates for the preparation of alcohols. The example used in this experiment is the conversion of 1-octene to 1-octanol in which an anti-Markovrukov addition to the double bond is required to yield the intermediate, trioctylborane. Since it is this alkyl borane that subsequently undergoes oxidation to the alcohol, hydroboration offers a synthetic pathway for introducing substituents at centers of unsaturation that are not normally available to the anti-Markovnikov addition reactions that are based on radical intermediates. 

The addition of borane-protected secondary phosphines to terminal alkynes proceeds under microwave-assisted conditions without the addition of a transition metal, base, or radical initiator (Scheme 4.287) [450]. Furthermore, the authors discovered that the addition reaction did not need to be carried out under an atmosphere of nitrogen. The chemistry predominately generated the Z-isomer, although the formation of the fi-isomer was never completely prevented. It was noteworthy that a terminal alkyne containing a primary alcohol was successfully functionalized. Clearly, the... 

A synthetic method for the introduction of an ethynyl group in cyclic structures is based on an intramolecular silicon-tethered radical cyclization with iodine transfer (Scheme 25.8). For example, starting from alkyne 10, a triethyl borane-initiated process allows a radical formation and cyclization to give the alkenyl radical 11. An iodine atom is abstracted from the starting material to give the alkenyl iodide 12, thus, propagating the chain. Treatment of 12 with tetra-/j-butylammonium fluoride (TBAF) results in elimination to furnish 13 in good yields. 

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