Aldehydes, reductive alkylation reagents

Sodium cyanoborohydride NaBIpCN in methanol is the reagent of choice for the reductive alkylation of ammonia, primary aliphatic and aromatic amines and secondary aliphatic amines with aldehydes and relatively unhindered ketones (equation 53). 

An excellent, broad review of the last 60 years of hydride reductions has been published,235 and the use of selectrides, Li and K tri-.v-butylborohydridcs or trisiamylborohydrides, has also been reviewed.236 A review of sodium borohydride-carboxylic acid as a reagent with novel selectivity in reductions has been written in particular, this reagent is useful for the A -alkylation of primary and secondary amines, through a sequence that is believed to involve sequential carboxylic acid to aldehyde reduction followed by reductive animation.237... 

Secondary amines can be prepared from the primary amine and carbonyl compounds by way of the reduction of the derived Schiff bases, with or without the isolation of these intermediates. This procedure represents one aspect of the general method of reductive alkylation discussed in Section 5.16.3, p. 776. With aromatic primary amines and aromatic aldehydes the Schiff bases are usually readily isolable in the crystalline state and can then be subsequently subjected to a suitable reduction procedure, often by hydrogenation over a Raney nickel catalyst at moderate temperatures and pressures. A convenient procedure, which is illustrated in Expt 6.58, uses sodium borohydride in methanol, a reagent which owing to its selective reducing properties (Section 5.4.1, p. 519) does not affect other reducible functional groups (particularly the nitro group) which may be present in the Schiff base contrast the use of sodium borohydride in the presence of palladium-on-carbon, p. 894. 

Optically active aliphatic propargylic alcohols are converted to corticoids (90% ee) via biomimetic polyene cyclization, and to 5-octyl-2(5ii)-furanone. The ee s of propargylic alcohols obtained by this method are comparable with those of the enantioselective reduction of alkynyl ketones with metal hydrides, catalytic enantioselective alkylation of alkynyl aldehydes with dialkyIzincs using a chiral catalyst ((S)-Diphenyl(l-methylpyrrolidin-2-yl)methanol) (DPMPM), and the enantioselective alkynylation of aldehydes with alkynylzinc reagents using A(A-dialkylnorephedrines. °... 

Although it is not possible to generate dianions from aldehyde tosylhydrazones (118), alkyllithium reagents add readily to the tosylhydrazone C=N linkage to give 119. The adduct 119 fragments to the organolithium species 120 and aqueous workup affords the reductive alkylation product 121 as shown in equation 68. Other examples for the conversion of aldehydes RCHO into reductive alkylation products RCH R1 via the tosylhydrazones are illustrated in equations 69-715 9. 

Poor purity was obtained for bulky R, groups such as phenyl or isopropyl however, excellent results were obtained with bromoacetic acid (Ri = H), 2-bromopropionic acid (Ri = Me), and 2-bromovaleric acid (Ri = Et). Following Fmoc removal with 20% piperidine in DMF, reductive alkylation of the free amine occurred in the presence of an aldehyde and sodium cyanoborohydrate (NaBHgCN). The formation of thiomorpholinone occurred via intramolecular amidation with HATU as the coupling reagent. 

RCHO —i- RCH R. This transformation can be accomplished by reductive alkylation of tosylhydrazones of aldehydes with alkyllithium reagents (equation... 

The tosylhydrazone of an aldehyde RCHO gives the reductive alkylation product RCH2R on reaction with organolithium reagent R Li. Although yields are mediocre, the procedure is simple and is particularly well suited for the introduction of branched alkyl groups.Treatment of a tosylhydrazone with n-butyl-lithium followed by MesMX (M = Si, Ge, or Sn) gives the vinyl-silane,... 

A solution of the divalent lanthanide derivative Sml2 may be prepared under mild conditions [equation (9)], and among a variety of reactions reported for this reagent are the reduction of aldehydes in the presence of ketones, and the reductive alkylation of ketones by organic halides or sulphonates [equation (10)]. The reagent solution can be easily stored under nitrogen, especially in the presence of small amounts of the metal (Sm), and Grignard -type apparatus is adequate for the reactions, which are usually clean. 

The synthesis of 112 was then modified to provide a single enantiomer. This called for an asymmetric synthesis of cyclization substrate 111. This was accomplished by Midland reduction of ketone 113 to provide 114 with excellent enantioselectivity (Steroids-21). Alkylation of 114 with the appropriate bromide (prepared from 2-methylfuran according to the procedures described on Steroids-18), followed by a few well-precedented reactions, gave 115, and thence 111 and 112. Application of the Midland reduction is notable. This is a relatively early application of a reagent-controlled asymmetric synthesis. It is also notable that the Midland method works extremely well on alkyl alkynyl ketones (because they look like aldehydes to the reagent) and thus, is well-suited to this application. ... 

Chiral oxazolines developed by Albert I. Meyers and coworkers have been employed as activating groups and/or chiral auxiliaries in nucleophilic addition and substitution reactions that lead to the asymmetric construction of carbon-carbon bonds. For example, metalation of chiral oxazoline 1 followed by alkylation and hydrolysis affords enantioenriched carboxylic acid 2. Enantioenriched dihydronaphthalenes are produced via addition of alkyllithium reagents to 1-naphthyloxazoline 3 followed by alkylation of the resulting anion with an alkyl halide to give 4, which is subjected to reductive cleavage of the oxazoline moiety to yield aldehyde 5. Chiral oxazolines have also found numerous applications as ligands in asymmetric catalysis these applications have been recently reviewed, and are not discussed in this chapter. ... 

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

---