Hydride capture

Members of the retinoid natural product class, containing a quaternary benzylic stereocenter, were prepared by the ring closure of allyl-arylethers. The palladium catalysed process in the presence of sodium formate led to the formation of a partially reduced benzofurane derivative through an insertion-hydride capture reaction sequence (3.24.), The coupling, which was run in the presence of a silver-zeolite, showed acceptable enantioselectivity when... 

A strategy involving a palladium-catalyzed cyclization/hydride capture process has been used in the synthesis of chiral benzofuran derivatives (Scheme 8G.15) [36], Aryl iodides 15.1 and 15.3 were cyclized under cationic conditions in the presence of HC02Na to give benzofurans 15.2 and 15.4, respectfully, both with good enantioselection. Products resulting from a 6-endo-... 

The reversible formation of a N,N-dibenzyl iminium intermediate, which is reduced by hydride capture from the Hantzsch ester 1 was proposed. Subsequent hydrolysis regenerates catalyst 2 and releases the saturated aldehyde. The transition state A has been suggested for the hydride transfer. An example of the asymmetric version of this reaction was also realized, by using a chiral imidazolidi-none catalyst (the McMillan imidazolidinium salt 3 [13]) (see Scheme 11.4). 

Peroxide intermediates are not fhe only species that enable oxidation of secondary alcohols. Oppenauer oxidation of secondary alcohols is of practical value, because only catalytic amounts of aluminum species are required and without aid from transition metals, which are usually more toxic. A new type of Oppenauer oxidation was recently discovered by Ooi and Maruoka [167]. This mefhod includes the use of bidentate aluminum catalyst which is also effective for MPV reduction (Scheme 6.144). The Oppenauer oxidation is the reverse of MPV reduction when pivalaldehyde is used as hydride-capturing agent, however, fhe reaction is virtually irreversible, giving the ketone in high yield. 

Shortly afterwards, similar observations were made by Hu and Ruckenstein who noted that rapid reactions occur between LiH and gaseous NH3 such that effectively the hydride captures the gaseous ammonia [50]. This was demonstrated in an elegant but simple experiment in which two samples made up of layers of 1 2 molar ratio LiNH2 and LiH powders were prepared one with LiH as the initial layer, the other with LiNH2 so-arranged [51]. 

Carbopalladations may be succeeded by hydride capture the reaction of an alkene with an aryl halide under appropriate choice of reaction conditions can thus lead to an... 

Among the limited examples of thioheterocycles, unstable 3-methylene-2,3-dihydrobenzoMthiophene (74) was prepared in modest yield starting from propargyl o-iodophenyl sulfide 73 under classic hydride capture conditions with formic acid (Scheme 6.20) [64]. 

Comins et al. [70] showed that V-acetyl-2,3-dihydro-4-pyridone reacts in a regio- and stereoselective manner to give the 5-exo-cyclization products. Product 80 is obtained under Jeffrey conditions, whereas hydride capture delivers the formal 1,4-addition product 81 (Scheme 6.22). 

Scheme 12.22 Polyene cyclization and Mizoroki-Heck-hydride capture cascade reactions.

For nongroup-selective examples, one of the first reports of an enantioselective intramolecular Mizoroki-Heck reaction was a polyene cyclization (Scheme 12.22) [23b], The trienyl triflate 5 underwent two intramolecular cyclization reactions to give the tricycle 6 in high yield and 45% ee. A cascade intramolecular Mizoroki-Heck-hydride capture sequence was used in the synthesis of retinoid derivatives from aryl iodide 100 to give benzofuran 101 in 80-81% ee [49]. Poor enantioselectivity was observed when neutral reaction conditions were employed. 

Diaz, P., Gendre, R, Stella, L. and Charpentier, B. (1998) New synthetic retinoids obtained by palladium-catalyzed tandem cyclization-hydride capture process. Tetrahedron, 54, 4579-90. 

Regioselective ring opening of epoxides to the less-substituted alcohol, by hydride capture at the better potential carbenium ion centre, is achieved with a sodium cyanoborohydride-boron trifluoride combination anti ring opening is favoured. 

Addition begins m the usual way by protonation of the double bond to give m this case a secondary carbocation This carbocation can be captured by chloride to give 2 chloro 3 methylbutane (40%) or it can rearrange by way of a hydride shift to give a tertiary carbocation The tertiary carbocation reacts with chloride ion to give 2 chloro 2 methylbutane (60%)... 

One of the most promising appHcations of polyboron hydride chemistry is boron neutron capture therapy (BNCT) for the treatment of cancers (253). Boron-10 is unique among the light elements in that it possesses an unusually high neutron capture nuclear cross section (3.8 x 10 , 0.02—0.05 eV... 

To date, the most extensively studied polyboron hydride compounds in BNCT research have been the icosahedral mercaptoborane derivatives Na2[B22H22SH] and Na [(B22H22S)2], which have been used in human trials with some, albeit limited, success. New generations of tumor-localizing boronated compounds are being developed. The dose-selectivity problem of BNCT has been approached using boron hydride compounds in combination with a variety of deUvery vehicles including boronated polyclonal and monoclonal antibodies, porphyrins, amino acids, nucleotides, carbohydrates, and hposomes. Boron neutron capture therapy has been the subject of recent reviews (254). 

Polyhedral Boron Hydrides. These are used in neutron capture therapy of cancers (254), and as bum rate modifiers (accelerants) in gun and rocket propellant compositions. 

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

Photolysis in MeOD indicates that all three of the above products arise via initial protonation of the double bond to form the 2-phenyl-2-norbomyl cation, which then undergoes either nucleophilic capture by solvent, hydride... 

Benzyl Alcohols. Benzyl alcohols of nearly all kinds undergo reduction when treated with acid in the presence of organosilicon hydrides. The most obvious exception to this is the behavior of benzyl alcohol itself. It resists reduction by the action of trifluoroacetic acid and triethylsilane, even after extended reaction times.26 Reducing systems consisting of triethylsilane and sulfuric acid/acetic acid or p-toluenesullonic acid/acetic acid mixtures also fail to reduce benzyl alcohol to toluene.134 As previously mentioned, substitution of boron trifluoride for trifluoroacetic acid results in the formation of modest yields of toluene, but only when a very large excess of the silane is used in order to capture the benzyl cation intermediate and suppress Friedel-Crafts oligomerization processes.129,143... 

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