Alane reduction

A PEG-star supported triphenylphosphine analog (66) was synthesized and employed in Mitsunobu reactions. Four phenolethers were prepared within 3-18 h reaction time and 68-93% yield. Upon completion of the reactions, the formed polymer supported triphenylphosphine oxide was isolated by precipitation from diethyl ether in > 85% yield. The reagent could be recycled by means of alane reduction (73%). 

Alkali-metal hydrides, boranes or alanes react with alkoxysilanes or silanols to form Si—H bonds in reactions that often are preferred syntheses, depending on the availability of the silane reactant "Lithium hydride or alanate reductions used are ... 

In the synthesis of (—)-lupinine (926) by Santos et al., (lR,2S,5R)-8-phenyhnenthol was used as the chiral auxiliary in controlling the stereochemical outcome of the reaction of the piperidine-containing carbamate 1042 with the 2-silyloxyfuran 1043 (Scheme 130). With trimethylsilyl triflate as Lewis acid catalyst and butyhnethylimidazolinium tetrafluorobo-rate as additive, an 80% yield was obtained as a 9.7 1 mixture in favor of threo-product (—)-1044. After hydrogenation of the double bond, treatment of the intermediate (—)-1045 with sodium methoxide effected rearrangement to the (R,R)-(- -)-l-hydroxyquinolizidin-4-one (- -)-1046 in 93% overall yield. Mitsunobu inversion of the alcohol to the (lS)-epimer (—)-1047 followed by alane reduction of the lactam afforded the quinolizi-din-l-ol (—)-1048. A second inversion at C-1 with cyanide produced the... 

Temperature-Programmed Reduction for Solid Materials Characterization, Alan Jones and Brian McNichol... 

For the formation of oxepincarbaldehydes by oxidation of the corresponding alcohols, see Section A.3.1.1.1. and Houben-Weyl, Vol. 4/1 b, p 519 Vol.6/4, p468. For an experimental procedure for the reduction of oxepincarboxylates with lithium alanate to give the corresponding alcohols, see Houben-Weyl, Vol.6/4, p468f. 

The aziridine-2-carboxaldehyde 56 can also serve as synthon for the synthesis of sphingosines, which are important biomembrane constituents [64]. One possible route involves the addition of an alanate to the aldehyde. In a later stage of this synthetic plan the aziridine can be opened, either via the intermediacy of an oxazoline or directly with dilute acid. Unfortunately, the reaction of aldehyde 56 with a vinylalanate has a poor diastereoselectivity of 3 2. Therefore, an alternative approach was considered, namely one involving the addition of a vinylzinc reagent to the aldehyde thereby employing our N-tritylaziridinediphenyl-methanol 51 as the chiral catalyst. Gratifyingly, only one diastereomer was obtained. Reductive removal of the trityl function, acetylation of the hydroxy... 

T. H. Tsai, J. W. Lane, and C. S. Lin Temperature-Programmed Reduction for Solid Materials Characterization, Alan Jones and Brian McNichol Catalytic Cracking Catalysts, Chemistry, and Kinetics,... 

Closely related to, but distinct from, the anionic boron and aluminum hydrides are the neutral boron (borane, BH3) and aluminum (alane, A1H3) hydrides. These molecules also contain hydrogen that can be transferred as hydride. Borane and alane differ from the anionic hydrides in being electrophilic species by virtue of the vacant p orbital and are Lewis acids. Reduction by these molecules occurs by an intramolecular hydride transfer in a Lewis acid-base complex of the reactant and reductant. 

When Group VI carbonyl complexes were reacted with alane or Cp2NbH3 reduction of CO to ethylene was noted.- - Ethylene was the primary product of the Cp2NbH3 reduction although it subsequently was hydrogenated to ethane. Masters and co-workers suggested that ethylene was formed through an alkylidene dimerization as shown below. 

The most stable of the germanium hydrides, R GcI I4 (n = 0,1,2,3), are the tri-organogermanium hydrides, which are prepared by the reduction of the corresponding halides with lithium alanate, or with amalgamated zinc and hydrochloric acid5-7. 

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

Fig. 3.6b). It must be mentioned that both alanates NaAlH and LiAlH are extremely difficnlt to be effectively refined during ball milling. As can be seen the particle size redaction of LiAlH is only twofold after 20 h of milling. This can be compared with almost 40-fold particle size reduction of MgH milled for a mnch shorter time (Fig. 2.15 in Sect. 2.1.3). 

The mechanism of reduction by boranes and alanes differs somewhat from that of complex hydrides. The main difference is in the entirely different chemical nature of the two types. Whereas complex hydride anions are strong nucleophiles which attack the places of lowest electron density, boranes and alanes are electrophiles and combine with that part of the organic molecule which has a free electron pair [119]. By a hydride transfer alkoxyboranes or... 

---