Boranes radical reactions

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. 

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... 

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. 

Beilstein Handbook Reference) Borane, triethyl- Boron ethyl Boron triethyl BRN 1731462 EINECS 202-620-9 HSDB 897 Triethylborane Triethylborine Triethylboron. Used as an initiator of radical reactions. Liquid mp = -93° bp = 95° d = 0.700 ... 

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. 

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. 

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. ... 

A detailed and extensive review of the synthesis and reactivities of NHC - borane complexes has been published. Modification of the boron substituents of NHC - boranes, as well as their involvement in radical reactions (H-dot donors), ionic reductions, transition-metal-assisted group transfer, their catalytic applications, and their characterizations are thoroughly developed. 

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). 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

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. 

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]. 

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... 

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