Imines basicity

This corresponds with MacDiarmid s observations which show that the second redox step is strongly pH-dependent. MacDiarmid further differentiated his redox model to take account of the fact that pure leucoemaraldine with its amine-N is already protonated at pH values 2, and that the totally oxidized pemigraniline with its less basic imine-N can also be protonated. This gives the following (simplified) reaction scheme ... 

The mercuration of ferrocenylimines with Hg(OAc)2 has been studied. - 6 Mercuration occurs selectively at the a-position relative to the imine group to afford compounds 86a-i (Scheme gy107,108 The regioselectivity of these reactions points to the directing role of the Lewis-basic imine functionality. Similar factors probably play a role in the formation of the ferrocenylketone and ferrocenylaldehyde derivatives 87a-f and 87g-j, respectively. These derivatives readily react with amines to afford the corresponding imines (Scheme 9). Presumably, the Lewis-acidic mercury center of the monomercurated ferrocenylketones and ferrocenylaldehydes activates the carbonyl functionality toward nucleophilic attack by the amine. 

The authors identify the new paracyclophane derivative as a catalyst lacking a hydrogen bond donor, and propose that addition is catalyzed by the Brpnsted basic imine moiety. Based on X-ray crystal data of the catalyst, it was hypothesized that... 

The amphoteric 6-hydroxyquinoline (6HQ) molecule has a weak acidic hydroxyl function (p A = 9.2) and a weak basic imine function (p A = 5.1) in the ground-state [1]. A cationic (quinolinium) form 6HQ(C), an anionic (quinolinate) form 6HQ(A), and a neutral form 6HQ(N) are thus predominantly present in acidic, alkaline, and neutral aqueous solutions, respectively, according to the following acid-base equilibria ... 

Alkyl- and aryl-l,3,4-oxadiazoles are neutral compounds and 2-amino derivatives are weak bases. Protonation is believed to occur at ring nitrogen in position 3 which facilitates ring cleavage in aqueous acid. The pKa (water) value of 2-amino-5-methyl-l,3,4-oxadiazole is 2.37 and values in the range 2.3-2.7 have been recorded for 2-amino-5-phenyl- and 2-7V-methyIamino-5-phenyI-l,3,4-oxadiazoIe in 50% aqueous ethanol. In the same solvent pK values of the more basic imines, 4-methyl-2-phenyl- and 4,AT-dimethyl-2-phenyl-A2-... 

Although various Lewis acid-catalyzed or -mediated reactions of aldehydes have been developed, less progress has been made in the reactions of imines using Lewis acids, probably because Lewis acids are often deactivated or decomposed by basic imines. Imines (37) have, however, been efficiently allylated by Zr(OTf)4 or Hf(OTf)4 catalyst in CH3CN to give homoallylamines (38) (Eq. 16) under similar conditions with using AICI3 or SnCU as catalyst low yields of allylated products were obtained [17]. 

JOC5347). When p-benzoquinone is used as a dipolarophile, the initially formed cycloadducts from 683 (R = MeO, R = H) can be treated with acetic anhydride to give compound 687, which contains the basic core dibenzo[a,d]cyclohepten-5,10-imine skeleton found in MK-801. However, it should be emphasized that cyclization of the Rh-carbenoids does not occur with weakly basic imines such as pyridines and the aromatic isoxazole system. For the synthesis of a nitrogen-bridged system, Padwa and coworkers have recently reported the successful employment of a domino transformation with displacement of a carbene center, followed by a tandem... 

It appears that the diastereofacial selectivity may be correlated with the nature of the enolate cation and the 0-protecting group. The sodium cation, known to be a weaker Lewis acid, cannot efflciently coordinate the silyloxy group of imine especially if it is large, so that facial selectivity follows, at least in part, a Felkin-Ahn model leading to preponderant formation of the anti isomer. However, the more basic imine nitrogen atom still coordinates the sodium cation so that the simple distereoselectivity still proceeds via a closed boat-like Zimmerman-Traxel transition state leading exclusively to the formation of (raru-azetidinones (Scheme IS). 

There are two major differences between the reduction of imines and aldehydes by 2-tol imines are reduced more rapidly than aldehydes and give amine complexes as stable products (Scheme 10) [57]. Substituents on the imine affect the rate of reduction and the stability of the amine complex product. PhCH=NPh reacts with 2-lol in THF at —40°C at a rate about 150 times faster than benzaldehyde. The more basic imine PhCH=NCH2Ph reacts with 2-lol in THF at —57°C at a rate about 15 times faster than PhCH=NPh. The stability of the amine complexes depends on the basicity and steric requirements of the complexed amine. Dialkylamine complex 10 is stable to 80°C, while the less basic aryl(alkyl)amine complex 9 is stable to only 50 C. The complex of the very bulky t-Bu(PhCH2)NH complex 11 decomposed above O C. In general, amine complexes with greater number of substituents are less stable. Beller has isolated the NH3 complex 12 from catalytic systems it is unusually stable and acts as a viable catalyst precursor [58]. 

In the first catalytic, enantioselective version of the Pictet-Spengler reaction, Jacobsen utilized the chiral thiourea derivative 171 as catalyst (Equation 12) [132], Such thioureas have emerged as a versatile class of chiral promoters and have proven to be of general utility in a number of enantioselective transformations [40], A particularly interesting feature of this study is the fact that in situ acetylation of the intermediary aldimine leads to an N-acyl iminium ion as the reactive species that subsequently undergoes cyclization. A priori, the more basic imine resulting from condensation of aldehyde 170 with tryptamine 169 would have been expected to be more likely than the N-acyl iminium ion to interact with catalyst 171. Nonetheless, the latter species was responsive to the H-donor catalyst, leading to N-acetylated tetrahydro-/ -carboline 172 in 81 % yield and 93 % ee [132],... 

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