Borane scavenger

Borane scavenger.2 Sodium borohydride in DMF can be used to reduce an acid chloride to an aldehyde in > 70% yield if a molar excess of pyridine is present as a borane scavenger. This methodology is now preferred to an earlier method from the same laboratory (10, 358) in which the reduction is limited by a quench with ethyl vinyl ether and propionic acid. 

By using sodium borohydride in N,N-dimethylformamide solution containing a molar excess of pyridine as a borane scavenger, direct conversion of both aliphatic and aromatic acid chlorides to the corresponding aldehydes can be achieved in >70% yield with 5-10% alcohol formation. 

Allylic alcohols. The tendency of NaBH4 to effect conjugate reduction of a,/3-unsaturated compounds is greatly reduced by C6F5OH. With borane scavenged by TMEDA or 1-hexene the selectivity is further improved. Only highly reactive ketones and acid chlorides are reduced by this reagent. ... 

This solvent is called tetrahydrofuran, or THF for short. Even though it somewhat stabilizes the empty p orbital on the boron atom in BH3, nevertheless the boron atom is very eager to look for any other sources of electron density that it can find. It is an electrophile—it is scavenging for sites of high electron density to fill its empty orbital. A pi bond is a site of high electron density, and therefore, a pi bond can attack borane. In fact, this is the hrst step of our mechanism. A pi bond attacks the empty p orbital of boron, which triggers a simultaneous hydride shift ... 

A disadvantage of borane and borate systems is that the alkylmetallocene cations are more instable and more sensitive to impurities and water. To overcome this higher sensitivity, a dialkyl species can be build by an in situ reaction with tri-isobutylaluminum (TIBA). TIBA acts as alkylation reagent and as a scavenger and stabilizes the dialkyl species in solution it is used as stock solution for the polymerization experiments (Fig. 12). 

Most of the spectroscopic investigations discussed above were carried out on well-defined metallocene systems, either isolated species or those generated from a well-defined metallocene alkyl precursor activated with one equivalent of a borane or borate activator. Most practical polymerisation catalysts, on the other hand, include a scavenger, usually an aluminum alkyl, and may contain ill-defined activators such as methylaluminoxane (MAO), usually at high MAO/Zr ratios. Such systems are less amenable to quantitative studies nevertheless, the identifications of species such as those depicted in Schemes 8.5-8.8 has enabled similar compounds to be identified in more complex mixtures. An idea of the possible mode of action... 

The Aloe group can be used for protection of the a-amino group instead of Boc or Fmoc as it is orthogonal to the semipermanent benzyl- or ferf-butyl-type side-chain and linker protections. This Aloe protection can be useful when some features of the peptide or pseudopeptide prevent the use of bases or acids in the deprotection step. In model studies, pseu-dometaUic hydrides (tributyltin hydride and borane-dimethylamine complex), which are neutral reagents, have been used as aUyl scavengers. 

The methylalumoxane may be replaced by a mixture of trialkylaluminum as an alkylating agent and dimethylaluminum difluoride as Lewis acid [31]. Dialkyl or dibenzyl metallocenes form active species when combined with the Lewis acidic tris(pentafluorophenyl)borane or organic salts of the noncoordinating tetrakis (pentafluorophenyl)borate, generating alkylmetallocenium ions [32-35]. With these co-catalysts a metallocene/co-catalysts ratio of 1 1 is used. Usually trialkylaluminum is added as a scavenger to prevent decomposition by impurities of the alkylmetallocenium ions generated in situ. 

The application of secondary amines as organocatalysts in different processes makes use of the inherent nucleophilicity of these compounds, but in this case they function as catalysts rather than reagents, and are thus covered in Chapter 4 of this book. Polymer-bound tertiary amines have so far only found use as depro-tecting agents for phosphorus-borane complexes (see above) [9]. They are widely used as scavengers and linkers, however. 

Reductions. iyn-Selective reduction of observed with borane-pyridine in the present predominate in the reduction with LiBHEt-.-C (Ph,P)4Pd catalyzes removal of the R SC the transformation of y,8-epoxy-o(,p-un.saiurj lers by the borane-dimethylamine complex Allyl group scavengers. In the solid-nitrogen by an (V-allyloxycarbonyl group can the presence of a borane-amine complex. 

OTHER COMMENTS used as a chemical intermediate in the preparation of higher bo-ranes, alkyl boranes, ethyl pentaborane, and various hydrides useful as jet and rocket fuels, catalysts, corrosion inhibitor fluxing agents, and oxygen scavengers. 

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. 

As for borane-activated catalyst systems, such as Cp TiMe3/B(QF5)3, the addition of TIBAis essential to increase the catalyst activity, syndioselectivity, and the molecular weight of the resultant sps 43-46,59 Herein, TIB A acts as a scavenger for impurities in the polymerization system. 

---