Boranes, reaction with heterocycles

Bis(a-aminomethyl)phosphines having a mobile hydrogen atom could be expected to undergo a borylation reaction with borane, hydrogen being evolved. In fact, bis(Af-phenylaminomethyl)-phenylphosphine (199) and its sulfide (200) appeared to interact with borane under mild conditions, yielding a new type of phosphorus-boron-containing heterocycle—2-... 

The low yield observed in the isomerization of the 2,4,4-trimethylpent-2-ene (28) is because of the facile cyclization of the borane 30, with evolution of hydrogen, to produce a boron heterocycle 31. Oxidation of the reaction mixture provides... 

B-Heterocycles and reactions of boranes with heterocycles 93JOM(457)l, 93JOM(457)25. 

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. 

There is a lot of information on the mercuration of heterocycles in the old literature, but it is seldom used nowadays due to the major disadvantages of toxicity and associated waste management it can be, however, a very useful reaction. The advantages of mercuration are that it can be carried out in hydroxylic and acidic solvents and in the presence of air, and that mercury in the product is easily replaced by ipso-substitution with other electrophiles, such as halogens, and gives boronic acids by reaction with borane. 

In 2006, Stephan introduced the concept of frustrated Lewis pairs , systems in which steric congestion precludes the formation of a Lewis acid-base adduct [25,26]. These combinations offer novel reactivity involving the activation of a variety of small molecules including amines, alkenes, alkynes, and molecular hydrogen [94]. While initially phosphines were the Lewis base of choice for FLP reactions, Al-heterocyclic carbenes were soon proven to be viable candidates as well. For instance, an FLP of ItBu and tris(pentafluorophenyl)borane was shown to react with molecular hydrogen in the usual FLP fashion, forming an imidazo-lium borate salt. This FLP was also used to activate ammonia, aniline, and diphe-nylamine, by addition of the NHC to the amine-borane adduct as shown in Scheme 15.5 [95]. 

Aminomethylphosphines are convenient objects for a comparison of phosphorus and nitrogen donor ability in complexation reactions. Coordination compounds of heterocyclic aminomethylphosphines with metals are discussed in Section VII. In this section we present reactions of aminomethylphosphines with boranes (BH3). 

The reductive cleavage of phosphonio-substituents attached to other heterocyclic frameworks was first demonstrated in the reaction of the 3,5-bis-phosphonio-l,2,4-diazaphospholide 15 with NaBH4 or LiBEt3H to yield the zwitterionic borane-complexes 27. Liberation of the free zwitterion 28 was readily feasible by treatment of the complexes with an excess of NEt3 (Scheme 9) [34]. 

These results reveal that under isomerization conditions boranes with five carbon atoms or more undergo intramolecular cyclization to produce the boron heterocycle and hydrogen. The reaction is specially facile when the primary hydrogen is available at the 5 position [3]. 

The C-H coupling of aromatic heterocycles with bis(pinacolato)di-borane(4) was carried out in octane at 80-100 °C in the presence of a half equivalent [IrCl(COD)]2-(4,4 -di-tert-butyl-2,2 -bipyridine) catalyst (3 mol%). The reactions of five-membered substrates such as thiophene, furan, pyrrole, and their benzo-fused derivatives exclusively produced 2-borylated... 

Instead, it is the goal of this chapter to highlight some conceptually novel developments in the field of HAT reactions. We will concentrate on complexes of borane with N-heterocyclic carbenes (NHCs), on organometallic hydrogen atom donors in stoichiometric and catalytic reactions, and on the activation of water and alcohols for HAT. 

Organoboron heterocycles containing the boron-boron bond have also been prepared by the reaction of A -lithio derivatives of alkylbis(alkylamino)-boranes with l,2-dichloro-l,2-bis (dimethylamino)diborane(4) 74) ... 

Complexation of the latter at phosphorus with borane, followed by methanolysis of the siloxyether function, gives the related o-hydroxyaryl diazaphospholidines. The cage-like triaminophosphine (109) has been obtained from the reaction of tri(2-pyrrolyl)methane with phosphorus trichloride, in the presence of triethylamine. This compound is stable to methanolysis, hydrolysis, and aerial oxidation at room temperature. Phosphorylation of various 1,2,4-triazoles with halophosphines has given a route to the heterocyclic system (110), and routes to various [2,4,l]benzodiazaphosphinines, e.g., (Ill), have also been developed. ... 

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