Allylamines, isomerization

Bifunctional Cs-isoprenoid allylamines, isomerization, 10, 72 Bifunctional ligand hydrogenation catalysts, in organometallic synthesis, 1, 79... 

Prior chapters have covered the use of transition metals in asymmetric hydrogenations ( 6.2 and 7.1), hydroborations ( 7.3), hydrosilylations and hydro-cyanations ( 6.3, 6.4, 7.4 and 7.5), cyclopropanations ( 7.19), aldol reactions ( 6.11), allylations of carbanions ( 5.3.2), and some sigmatropic rearrangements ( 10.3). This chapter covers other reactions catalyzed by transition metal complexes including coupling of organometallic reagents with vinyl, aryl or allyl derivatives, Heck reactions allylamine isomerizations, some allylation reactions, car-bene insertions into C-H bonds and Pauson-Khand reactions. 

Allylamines have been used as nitrogen protective groups. They can be removed by isomerization to the enamine (t-BuOK, DMSO) or by rhodium-catalyzed isomerization. ... 

Treatment of allylamines with potassium amide on alumina causes their isomerization to enamines in good yields (124b). When allylamines are heated to about 55° the same type of isomerization takes place (I24c). 

Rhodium-catalyzed isomerization. Ru(cod)(cot) has been used to convert an allylamine into an enamine."... 

The thermal [1] or photochemical [5] isomerization of N-silylated allylamine in the presence of Fe(CO)5 provides the corresponding N-silylated enamines 7a and 7b. Z-enamine 7b does not react in any of the examined cycloadditions. The cyclopropanation of E-enamine 7a with methyl diazoacetate under copper(I) catalysis provides the donor-acceptor-substituted cyclopropane 9 [1], which can be converted in good yield into the interesting dipeptide 10 [6]. 

Allylic C/H insertion accompanied by an allylic rearrangement has been observed for carbenoid reactions of ethyl diazoacetate with allylamines (Scheme 23)1S1). Apparently, metal-catalyzed isomerization 117 118 proceeds the C/H insertion process. Although mechanistic details have not yet been unraveled, T)3-allyl complexes... 

In order to establish the industrial application, [Rh(BINAP)2]C104 was developed as a new catalyst, which has excellent thermal stability allowing multiple repetition of the catalyst recovery. A further improvement of the catalyst was accomplished by the use of [Rh(TolBINAP)2]Cl04, which possessed a better solubility in organic solvents and achieved even higher optical yields (>98% ee) for the isomerization of allylamines such as geranylamine and nerylamine.15... 

The enantioselective isomerization of bifunctional Cs-isoprenoid allylamines to optically active bifunctional aldehydes was developed by using Rh(l)+/( )-BIPHEMP as the catalyst (Scheme 2).17 These aldehydes are useful optically active bifunctional building blocks for isoprenoid homologation. 

The synthesis of a variety of chiral aliphatic aldehydes of high optical purity through the enantioselective isomerization of allylamines found many applications in organic synthesis. The enantioselective isomerization of diethylgeranylamine, which was prepared from myrcene, furnished (R, )-diethylenamine in >98% yield with >98% ee. This enamine is converted to (—(-menthol stereospecifically in high chemical yield (yield of each step >92%, Scheme 4).9 11... 

The rhodium-catalyzed isomerization of allylamine can be used for the deprotection of A-allyl protective groups (Scheme 7).19 20... 

Metal-catalyzed isomerization of allylamides is slower than that of allylamines. The isomerization was examined at higher temterature (>100°C) using [Rh((+)-BINAP) (cod)]+. Although the enantioselecdvity was high, the yield of the desired enamide was low due to the formation of dienamide (Equation (5)).9... 

The isomerization of multifunctionalized allylamines and allylamides is efficiently catalyzed by various ruthenium complexes in high yield (Scheme 8).23,24... 

In parallel to the asymmetric catalytic isomerization of allylamines, [Rh(BINAP) (solvent)2]C104 is a very efficient catalyst for the isomerization of allylic alcohols.9,11 By employing 0.5mol% of the catalyst, good to excellent conversions were achieved even in the case of substrates that are more difficult to isomerize, such as allylic alcohols having two alkyl groups in the terminal position (R1 = R2 = Me) and 2-cyclohexen-l-ol (Scheme 19). 

Although the asymmetric isomerization of allylamines has been successfully accomplished by the use of a cationic rhodium(l)/BINAP complex, the corresponding reaction starting from allylic alcohols has had a limited success. In principle, the enantioselective isomerization of allylic alcohols to optically active aldehydes is more advantageous because of its high atom economy, which can eliminate the hydrolysis step of the corresponding enamines obtained by the isomerization of allylamines (Scheme 26). 

Metal-catalyzed C-H bond formation through isomerization, especially asymmetric variant of that, is highly useful in organic synthesis. The most successful example is no doubt the enantioselective isomerization of allylamines catalyzed by Rh(i)/TolBINAP complex, which was applied to the industrial synthesis of (—)-menthol. A highly enantioselective isomerization of allylic alcohols was also developed using Rh(l)/phosphaferrocene complex. Despite these successful examples, an enantioselective isomerization of unfunctionalized alkenes and metal-catalyzed isomerization of acetylenic triple bonds has not been extensively studied. Future developments of new catalysts and ligands for these reactions will enhance the synthetic utility of the metal-catalyzed isomerization reaction. 

Isomerization is a frequent side-reaction of catalytic transformations of olefins, however, it can be a very useful synthetic method, as well. One of the best-known examples is the enantioselective allylamine enamine isomerization catalyzed by [Rh (jR)-or(S)-BINAP (COD)] which is the crucial step in the industrial synthesis of L-menthol by Takasago [42]... 

Alkaline earth metal oxides are active catalysts for double bond isomerization. For example, SrO exhibits high activity and selectivity for the isomerization of a-pinene to /1-pinene 110). MgO and CaO have excellent activities for isomerization of 1-butene and 1,4-pentadiene and, particularly, for isomerization of compounds containing heteroatoms, such as allylamine or 2-propenyl ethers 111-115). Recently... 

The enantioselective BINAP-Rh +-catalyzed isomerization of an achiral allylamine, such as diethylgeranylamine, to give an optically active enamine (e. g., 2) (for configurational assignment, see p 436)56. 

Another useful method involves the base-catalyzed isomerization of tertiary allylamines to cis- and rranj-propenylamines (Eq. 5). 

The mechanism involving simple nitrogen-coordinated complexes also accounts for reactivities of certain sterically constrained systems. For instance, 3-(diethyamino)cyclohexene undergoes facile isomerization by the action of the BINAP-Rh catalyst (Scheme 18). The atomic arrangement of the substrate is ideal for the mechanism to involve a three-centered transition state for the C—H oxidative addition to produce the cyclometalated intermediate. The high reactivity of this cyclic substrate does not permit any other mechanisms that start from Rh-allylamine chelate complexes in which both the nitrogen and olefinic bond interact with the metallic center. On the other hand, fro/tt-3-(diethylamino)-4-isopropyl-l-methylcyclohexene is inert to the catalysis, because substantial I strain develops during the transition state of the C—H oxidative addition to Rh. 

The diazo compound (181a), prepared from the nitrosamine, cyclized to a pyrrolopyrazoline in 80% yield.98 The diazo compound (181b), prepared from diethyl diazomalonate and allylamine, cyclized similarly but at a much more rapid rate. This is consistent with the lowered LUMO of the dipole of (181b), substituted with an ester group here the dipole LUMO-dipolarophile HOMO is the likely dominant interaction. The N—N double bond of the pyrrolopyrazoline products was readily isomerized to afford A2-isomers. 

Olefinic double-bond isomerization is probably one of the most commonly observed and well-studied reactions that uses transition metals as catalysts [1]. However, prior to our first achievement of asymmetric isomerization of allylamine by optically active Co(I) complex catalysts [2], there were only a few examples of catalytic asymmetric isomerization, and these were characterized by very low asymmetric induction (<4% ee) [3], In 1978 we reported that an enantioselective hydrogen migration of a prochiral allylamine such as AVV-diethylgerany-lamine, (1) or N V-diethylnerylamine (2) gave optically active citronellal ( )-enamine 3 with about 32% ee utilizing Co(I)-DIOP [DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane] complexes as the catalyst (eq 3.1). 

Although various transition-metal complexes have reportedly been active catalysts for the migration of inner double bonds to terminal ones in functionalized allylic systems (Eq. 3.2) [5], prochiral allylic compounds with a multisubstituted olefin (Rl, R2 H in eq 2) are not always susceptible to catalysis or they show only a low reactivity [Id]. Choosing allylamines 1 and 2 as the substrates for enantioselective isomerization has its merits (1) optically pure citronellal, which is an important starting material for optically active terpenoids such as (-)-menthol, cannot be obtained directly from natural sources [6], and (2) both ( )-allylamine 1 and (Z)-allylamine 2 can be prepared in reasonable yields from myrcene or isoprene, respectively, The ( )-allylamine 1 is obtained from the reaction of myrcene and diethylamine in the presence of lithium diethylamide under Ar in an almost quantitative yield (Eq. 3.3) [7], The (Z)-allylamine 2 can also be prepared with high selectivity (-90%) by Li-catalyzed telomerization of isoprene using diethylamine as a telomer (Eq. 3.4) [8], Thus, natural or petroleum resources can be selected. 

The first asymmetric isomerization of an allylamine was achieved with the optically active cobalt catalysts as mentioned above. The isomerization of 1 with Co(acac)2/(+)-DIOP/ Bu2AlH catalyst gave (3/ )-3 with 32% ee (39% chemical yield) accompanied by a considerable amount of undesired dienamine 4. Although with the same catalyst system, a higher enantioselectivity (57% ee) and chemical yield (60%) were obtained by using the secondary allylamine 5 as the... 

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