Alkenes intramolecular hydrogen bonding

Recent research on the Cambridge structural data base shows139 11 alkenes (and 2 alkynes) which are involved in interactions with the —OH group, with distance H- C of olefin <2.4 A. 66 is an instance of intramolecular hydrogen bonding interactions140. Intermolecular hydrogen bonds can be observed in 67141 and 68142. 

Peracids tend to adopt an intramolecularly hydrogen-bonded conformation in solution, and the high degree of polarisation results in an electrophilic oxygen atom that is able to add to alkenes. 

Although they are not considered carbenoids, peracids are mechanistically similar in their additions to alkenes. The intramolecular hydrogen bond (shown by two resonance forms) partially breaks the O-H bond, making it a better nucleophile. 

Peroxy acids possess strong intramolecular hydrogen bonds. The concerted progress results in a stereo-specific reaction. (Z)-alkenes yield cw-oxiranes, (jE)-alkenes ra -oxiranes. 

Ishihara, K. et al. reported an enantioselective [2+2] cycloaddition of unactivated alkenes (e.g., 161) with a-acyloxyacroleins 162, catalyzed by chiral organoammo-nium salts, catalyst 163, Scheme 3.52 [68], A possible stepwise mechanism was proposed by authors to account for the stereoselectivity, which includes initial Michael addition of alkene to (Z)-iminium enal intermediate and intramolecular cyclization to afford the cycloadducts. The proposed transition states were stabilized by aromatic n-n stacking and intramolecular hydrogen-bonding interaction. 

The enol tautomers of 1-(2, 4, 6 -trialkylphenyl)-2-methyl-1,3-diketones form a range of alkene, epoxide, ether, and hydroperoxide products on reaction with singlet oxygen. The product distribution is substantially affected by the solvent, apparently owing to the disruption of intramolecular hydrogen bonding of the enols in polar solvents. 

The epoxidation of simple alkenes with peroxycarboxylic acids involves an electrophilic attack of peracid with the formation of a bicyclic or a mono-cyclic transition state (Scheme 6.1), as first proposed by Bartlett [7] or recently suggested by theoretical studies [8] respectively. The reaction takes place readily in nonpolar solvents such as dichloromethane (DCM) and benzene and one can argue that hydrogen-bonding solvents, in particular, should retard the rate by interfering with intramolecular hydrogen bonding... 

Very recently, the cyclization of aromatic nitriles with alkenes was also explored. A variety of 3-methyleneisoindolin-l-ones were formed in high Z-stereoselective manner via in situ generated cationic Ru(II) catalyst (Eq. (7.51)) [61]. The high Z-stereoselectivity was attributed to the intramolecular hydrogen bonding. 

In instanees where a functionalized aromatic moiely is desired in the target structure, the Heck reaction of an atyl/vinyl halide/triflate with an alkene in the presenee of a palladium catalyst and base is an obvious choice and, indeed, has found application in the macrocycle arena. The first demonstration of a Pd(0) process on the solid phase actually involved Heck chemistry and was directed toward the synthesis of small libraries containing 20- to 24-membered macrocycles (94, Scheme 11.10). Analogous procedures were employed for the assembly of macrocyclic RGD mimet-ics °° and p-turn mimics (97, Figure 11.12, site of reaction and palladium species employed indicated) on the solid phase. For these conversions, it was postulated that an intramolecular hydrogen bond in the precursor assisted in the formation of the cycle. 

Another important reaction principle in modem organic synthesis is carbon-hydrogen bond activation [159]. Bergman, Ellman, and coworkers have introduced a protocol that allows otherwise extremely sluggish inter- and intramolecular rhodium-catalyzed C-H bond activation to occur efficiently under microwave heating conditions. In their investigations, these authors found that heating of alkene-tethered benzimidazoles in a mixture of 1,2-dichlorobenzene and acetone in the presence of di-//-... 

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