Hydrogen-bonding activation compounds

Because a comprehensive discussion of the transition state of hydrogen-bond catalysis will be presented by Berkessel in Chapter 3, the hydrogen bond catalyzed hetero Diels-Alder reaction of butadiene with carbonyl compounds will be discussed briefly here. Huang and Rawal reported that the hetero Diels-Alder reaction of aminodiene with aldehyde exhibited significant solvent effects (Scheme 2.7) [15]. The reaction in CHCfi was accelerated 30 times in comparison with that in THF, while that in i-PrOH was accelerated 630 times. They proposed that the Diels-Alder reaction was promoted by the hydrogen-bond activation of aldehyde. This finding resulted in the development of TADDOL catalyst [3]. 

In addition to the activation of carbonyl compounds and imines, Schreiner studied on thiourea-catalyzed acetalization reaction, in which ortho esters were activated by hydrogen bond [19]. Jacobsen has utilized the hydrogen-bond catalysis in reactions with acyliminium ions, wherein hydrogen bond activates the acylim-inium salt through complexation with chloride [20]. 

Optically active 19a was previously obtained by inclusion complexation with N -benzylcinchon idi um chloride 21 [36], Compound 21 was also a very efficient resolving agent for rac-17 [37], Crystal structure analysis of a (1 1) complex of 21 and selectively included (+)-17 showed that the molecular aggregate was associated by formation of a Cl HO hydrogen bond. Racemic compound 20 could be efficiently resolved only by complexation with (R,R)-(—)-trans-2,3-bis(hydroxydiphenylmethyl)-l,4-dioxaspiro[4.4]nonane 3b. A crude inclusion complex of 1 1 stoichiometry of 3b was formed selectively with (+)-20 in a 2 1 mixture of dibutyl ether/hexane. One recrystallization from the above combination of solvents gave a 34 % yield of the pure complex. Optically active (+)-20 was obtained by dissolving the complex in 10% NaOH, followed by acidification with HC1 and then recrystallization. The optical purity determined by HPLC (Chiralpack As) was >99.9 %. As far as we know, this is the only report of the resolution of 4,4 -dihydroxybiphenyl derivatives. Conversely, an inclusion... 

The possibility of coordination of a two-electron ligand, in addition to arene, to the ruthenium or osmium atom provides a route to mixed metal or cluster compounds. Cocondensation of arene with ruthenium or osmium vapors has recently allowed access to new types of arene metal complexes and clusters. In addition, arene ruthenium and osmium appear to be useful and specific catalyst precursors, apart from classic hydrogenation, for carbon-hydrogen bond activation and activation of alkynes such compounds may become valuable reagents for organic syntheses. 

W. D. Jones, and F. J. Feher, Alkane Carbon-Hydrogen Bond Activation by Homogeneous Rhodium(I) Compounds, Organometallics 2, 562-563 (1983). 

For phenols, rotation around the C—O bond is clearly assumed. This rotation may be slowed down by intramolecular hydrogen bonding. For compounds such as 40, activation parameters for the rotation can be determined by dynamic NMR. A classic study is that of Koelle and Forsdn of aldehydes (5, R = H). The activation energy was determined as 37.9 kJ mol, = 35.6 kl moH and A5 = 43.9 I K moH. ... 

Jones WD, Feher FJ (1983) Alkane carbon-hydrogen bond activation by homogeneous rhodium compounds. OrganometaUics 2 562-563... 

Intense interest in how carbon-hydrogen bond activation may be engineered has led to detailed studies of several transition-metal compounds known to compass reactions of this type, which are sometimes initiated thermally, but more often photochemically. Two such compounds are the iron and ruthenium dihydrides M(dmpe)2H2 (M = Fe or Ru dmpe = Me2PCH2CH2PMe2), which react with alkanes RH under irradiation in accordance with Eq. (17) (61, 76) ... 

Recently, Gunnoe and co-workers showed in several detailed studies that TpRu (Me)(CO)(NCMe) is a very reactive compound with respect to C-H activation processes giving rise to several unusual transformations of organic substrates. For instance, TpRu(Me)(CO)(NCMe) was found to polymerize styrene and methyl methacrylate in benzene at 90 Furthermore, TpRu(Me)(CO)(NCMe) initiates carbon-hydrogen bond activation of benzene and at the 2-position of furan and thiophene to produce methane and TpRu(aryl)(CO)(NCMe) (aryl = phenyl, 2-furyl, 2-thienyl) (Scheme 24). The solid-state structure has been determined for TpRu(2-thienyl)(CO)(NCMe). The complexes TpRu(aryl)(CO)(NCMe) also serve as catalysts for the formation of ethylbenzene, 2-ethylfuran and 2-ethylthiophene from ethylene and benzene, furan, and thiophene, respectively (vide infra). DFT/B3LYP... 

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