Formates as hydrogen source

CAimHPFd Pd/C 150°C functionalised arenes as substrate ammonium formate as hydrogen source microwave-accelerated catalysis reduction of C=C, C C and N02. [53]... 

Hydrogenation of itaconic acid (14) with Rh(COD)Cl2 catalyst and commercially available triethylammonium formate as hydrogen source delivers (5)-(15) in good enantiomeric excess (equation 14) with hydrogen as reductant instead of ammonium formate a 94% ee is obtained. ... 

Selective dehalogenation. Aromatic halogen atoms can be removed catalyti-cally while using sodium formate as hydrogen source. For example, A,A-diacetyl-2,4,6-trichloroaniline gives the 2,6-dichloro compound on refluxing with Pd-C and HCOONa in MeCN. 

Berthold et al. (2002) reported use of formate salts such as ammonium formate and triethylammonium formate as hydrogen source with N-butyl-N -methylimidazolium hexaflourophosphate, ([bmim][PF ] for catal54 ic transfer hydrogenation of different homo- or heteronuclear organic compounds at 150°C with 92-98% yield. 

The MCR toward 2//-2-imidazolines (65) has found apphcation in the construction of A(-heterocyclic carbene (NHC) complexes (74). Alkylation of the sp Af-atom with an alkyl halide followed by abstraction of the proton at C2 with a strong base (NaH, KOtBu) resulted in the formation of the free carbene species, which could be trapped and isolated as the corresponding metal complexes (Ir or Rh) [160]. The corresponding Ru-complexes were shown to be active and selective catalysts for the transfer hydrogenaticm of furfural to furfurol using iPrOH as hydrogen source [161]. 

Ru/tppms and Ru/pta are also active catalysts for the hydrogenation of a series of substituted benzaldehydes to the corresponding benzyl alcohols with sodium formate as the source of hydrogen.247,261,491... 

Catalytic hydrogenolysis using Pd—C, Pd(OH)2 or Pd(OAc)2 is the most commonly employed method for the removal of benzyl ethers, and yields are often quantitative. Cyclohexene, cyclohexadiene, formic acid and ammonium formate can also be used as hydrogen sources rather than hydrogen. Benzyl ethers can also be removed by Birch reduction with lithium or sodium dissolved in liquid ammonia, but this procedure is not often applied in carbohydrate chemistry. 

An excellent method for cleaving benzylic ethers, esters, carbamates, and amines uses hydrogen In the presence of a transition metal catalyst such as Pd. Alternatively a process known as catalytic transfer hydrogenation can be employed which uses 1,4-cyclohexadiene, cyclohexene, formic acid or ammonium formate as the source of hydrogen24 The method Is exceptionally mild and compatible with most functional groups devoid of unsaturation. Hydrogenolysis of benzyloxycarbonyl (Z or Cbz) groups of amines was a major advance in the... 

Cyclohexane, cyclohexene and cyclohexadiene are used as hydrogen sources in the hydrogenation of alkenes to alkanes, when they are themselves oxidized to benzene. In these reactions, the driving force is the formation of the aromatic ring. 

Carbonylation of olefins in the presence of alcohols to give esters is called hydroesterification. Similarly, olefin carbonylation in the presence of carboxylic acids yields acid anhydrides. Both hydroesterification and acid anhydride formation by olefin carbonylation are covered in section 14.6.4. Other carbonylation variations, including the use of acetylenic substrates, thiols and amines as hydrogen sources and the carbonylation of allylic halides are not discussed. Several excellent reviews of hydrocarboxyiation and carbonylation of olefinshave appeared. 

The most effective catalyst precursors for ATH in water are half-sandwich chloro-complexes of Ru(II), Rh(III), or Ir(III) (I, Fig. 88). From the chlorides, the aqua solvates II are formed, which, in turn, could be deprotonated to the corresponding hydroxo complexes III. Taking into account that in these catalytic systems, sodium formate is generally used as hydrogen source, the equilibria of formation of formato- and hydrido complexes must be also considered (Fig. 88). Obviously, the pH value significantly affects these equilibria and hence the reaction rates in aqueous ATH reactions vary with solution pH values. In fact, for some catalytic systems, optimum pH windows have been determined for achieving the greatest rates (354). 

Hydrogenation of PBD with hydrazides (toluenesulfinyl hydrazide) as hydrogen source has also become common practice [53-55]. PBDs showed 30% hydrogenation after 2 h, and isomerization of the remaining double bonds with a preference for the trans configuration cisltmns= 1 2.5) [56]. The formation of p-tolylsulfinic acid can induce side reactions such as the addition of p-tolylsulfone to the double bonds and bond cleavage (Scheme 2) [57, 58]. Addition of amine base (tri-n-propyl amine) is useful to prevent this [59]. The cis-trans isomerization of... 

An excess of phosgene is used during the initial reaction of amine and phosgene to retard the formation of substituted ureas. Ureas are undesirable because they serve as a source for secondary product formation which adversely affects isocyanate stabiUty and performance. By-products, such as biurets (23) and triurets (24), are formed via the reaction of the labile hydrogens of the urea with excess isocyanate. Isocyanurates (25, R = phenyl, toluyl) may subsequendy be formed from the urea oligomers via ring closure. 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

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