Allylic alcohols reductive amination

BINAP (40a) was first reported as a ligand in an enantioselective hydrogenation in 1980 [172], and provides good selectivity for the reductions of dehydroamino acid derivatives [173], enamides, allylic alcohols and amines, and a,p-unsaturated acids [4, 9, 11, 12, 174, 175]. The fame of the ligand system really came with the reduction of carbonyl groups with ruthenium as the metal [11, 176]. The Rh-BINAP systems is best known for the enantioselective isomerizations... 

Fischli has used a related reagent, Cob(I)alamine,2 obtained by reduction of vitamin B,2 with zinc and acetic acid, as a catalyst for reduction with zinc and acetic acid of various unsaturated systems such as a,/ -unsaturated nitriles,3 esters,4 and carbonyl compounds,5 allylic alcohols and amines,6 and also isolated double bonds.6... 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

The features of Red-Al are the following It easily reduces halogenated derivatives even if acetylenic (Section 2.1) tertiary amides lead to aldehydes (Section 3.2.8) and propargylic alcohols and amines are reduced to corresponding allylic alcohols and amines (Section 4.1). Epoxides remain intact unless they carry an alcohol functional group at the a position The reduction is then regioselective (Section 2.3). Aromatic nitriles are reduced, but aliphatic nitriles are not affected (Section 4.3). 

As an extension of the 1,3-enyne coupling, acetylene was directly coupled to electrophiles (i.e., carboxaldehydes and imines) in the presence of cationic rhodium catalyst and Brpnsted acid as co-catalyst to provide (Z)-2,4-dienyl allylic alcohols or amines (Scheme 15). a-Ketoesters, activated aldehydes [31, 32], and A-arylsulfonylimine [33] were described as suitable electrophiles for the reductive coupling of acetylene delivering (Z)-dienyl allylic alcohols or amines in a stereocontrolled manner. 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

Allylic alcohols also may be arylated in / /-methylpyrrolidinone or DMF solution with sodium bicarbonate as the base.81 The use of this base improves the yields of aldehydes obtained compared with tertiary amine bases in the case of aryl bromides with electron-withdrawing substituents, where reduction to an arene is sometimes a problem,... 

Since 1,4 addition of a variety of appropriate aryl caibanions to 36 failed, an intramolecular version of the Heck reaction was examined. With this end in view, the ketone 36 was reduced stereospecifically to the corresponding allyl alcohol which was then protected as its p-methoxybenzyl ether 37. The amine, obtained on reduction of the azide 37, was condensed with 6-bromopiperonylic acid to furnish the amide 38. 

The cyclohexene 121, which was readily accessible from the Diels-Alder reaction of methyl hexa-3,5-dienoate and 3,4-methylenedioxy-(3-nitrostyrene (108), served as the starting point for another formal total synthesis of ( )-lycorine (1) (Scheme 11) (113). In the event dissolving metal reduction of 121 with zinc followed by reduction of the intermediate cyclic hydroxamic acid with lithium diethoxyaluminum hydride provided the secondary amine 122. Transformation of 122 to the tetracyclic lactam 123 was achieved by sequential treatment with ethyl chloroformate and Bischler-Napieralski cyclization of the resulting carbamate with phosphorus oxychloride. Since attempts to effect cleanly the direct allylic oxidation of 123 to provide an intermediate suitable for subsequent elaboration to ( )-lycorine (1) were unsuccessful, a stepwise protocol was devised. Namely, addition of phenylselenyl bromide to 123 in acetic acid followed by hydrolysis of the intermediate acetates gave a mixture of two hydroxy se-lenides. Oxidative elimination of phenylselenous acid from the minor product afforded the allylic alcohol 124, whereas the major hydroxy selenide was resistant to oxidation and elimination. When 124 was treated with a small amount of acetic anhydride and sulfuric acid in acetic acid, the main product was the rearranged acetate 67, which had been previously converted to ( )-lycorine (108). 

A regio- and stereoselective formation of homoallylic alcohols and amines has been realized by the reaction of easily available allylic acetates 213 and 217 with bispinacolatodiboron 149 in the presence of palladium catalysts followed by the addition of aldehydes/imines (Scheme 36) <2005EJO2539>. The mechanism involves the formation of an 7]3-allyl-Pd complex, which inserts between B-B bond, and subsequent reductive elimination to yield the allyl... 

Further functionality (i.e., 4-oxo) has been introduced by the cyclization of a-allyloxy-carbonylnitrones, which have traditionally been problematic to obtain. Tamura and co-workers have reported that simple alkoxycarbonylnitrones (e.g., (243)) undergo transesterification and cyclization with various allylic alcohols in the presence of titanium isopropoxide (Scheme 43) <95T107, 95T119). Alternatively, allyl a-oximinocarboxylates (246) are converted in situ to nitrones (247) and further cyclized to the bicyclic heterocycles (248) (Scheme 44) <91T4495>. Tetrahydro-1 //-pyrrolo[3,4-c]isoxazoles (250) have been prepared by the condensation of C-acylnitrones with allyl amines <78AJC2013> further reduction of the bicycles (250) with sodium borohydride affords the 3-oxa-2,7-diazabicyclo[3.3.0]octane (251) (Scheme 45). 

However, this intriguing catalyst system is capable of performing a subsequent reductive amination in the same pot and under the same conditions if secondary amines are added and if an atmosphere of hydrogen is introduced. Therefore, alkyne allyl alcohols 200 are readily transformed in moderate to good yields into /J-amino ethyl alkylidene tetrahydrofurans 203... 

Kawahata and Goodman utilized a chiral aziridine 166 as a simple precursor for the synthesis of / -aminoacids <1999TL2271>. The chiral aziridine is prepared in five steps from the corresponding allylic alcohol via a Sharpless asymmetric epoxidation. A one-electron reduction of aziridine 166 with SmG provided the ring-opened aziridine. Protection of the resulting amine as the BOC-derivative provided a 1.6 1 mixture of the BOC-amino ester diaster-eomers 167a and 167b in 66% yield (Equation 50). 

Chiral tertiary allylic amines 191 with /ra r-2-phenylsulfonyl-3-phenyloxaziridine 33 also gave rise to amine A -oxides 192, which underwent the [2,3]-Meisenheimer rearrangement to hydroxylamines 193 with a high level of stereoselectivity (Table 14) <1999J(P1)2327>. Reduction of 193 gave the corresponding allylic alcohols 194. 

The metal-alcohol reduction of imines usually produces the reduced amine in good yield however, the reduction of some allylic imines (81, R = Pr",Bu") with Na-alcohol proceeds with hydrogenolysis to give alkene (82). ... 

These reagents are strong Lewis acids that cleave THF and acetals (Section 2.4). Nevertheless, they leave bromo- and chloroderivatives intact (Section 2.1). The regioselectivity of the opening of epoxides is opposite to that observed for LAH in THF (Section 2.3). Diarylcarbinols can be reduced to hydrocarbons (Section 2.4), and a,p-unsaturated carbonyl compounds to allylic alcohols (Section 3.2.9). The reduction of amides to amines is easier than with LAH (Section 3.2.8), especially in the case of a,p-ethylenic amides or of -lactams. These reagents do not reduce NO2 groups. 

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