Intramolecular reactions hydrogenation

Intramolecular reaction can be used for polycyclization reaction[275]. In the so-called Pd-catalyzed cascade carbopalladation of the polyalkenyne 392, the first step is the oxidative addition to alkenyl iodide. Then the intramolecular alkyne insertion takes place twice, followed by the alkene insertion twice. The last step is the elimination of/3-hydrogen. In this way, the steroid skeleton 393 is constructed from the linear diynetriene 392(276]. 

The reverse reaction (formation of metal alkyls by addition of alkenes to M-H) is the basis of several important catalytic reactions such as alkene hydrogenation, hydroformylation, hydroboration, and isomerization. A good example of decomposition by y3-elimination is the first-order intramolecular reaction ... 

Carboxylic esters 1 that have an a-hydrogen can undergo a condensation reaction upon treatment with a strong base to yield a /3-keto ester 2. This reaction is called the Claisen ester condensation or acetoacetic ester condensation, the corresponding intramolecular reaction is called the Dieckmann condensation ... 

A new process developed by Institut Francais du Petrole produces butene-1 (1-butene) by dimerizing ethylene.A homogeneous catalyst system based on a titanium complex is used. The reaction is a concerted coupling of two molecules on a titanium atom, affording a titanium (IV) cyclic compound, which then decomposes to butene-1 by an intramolecular (3-hydrogen transfer reaction. ... 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

Scheme 20 Hydrogen-mediated intramolecular reaction of keto-enones...

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

In the direct transfer mechanism, the metal ion coordinates both reactants enabling an intramolecular reaction, and activates them via polarization. Consequently, strong Lewis acids including Alln and the Lnln ions are the most suitable catalysts in this type of reactions. In the hydride mechanism, a hydride is transferred from a donor molecule to the metal of the catalyst, hence forming a metal hydride. Subsequently, the hydride is transferred from the metal to the acceptor molecule. Metals that have a high affinity for hydrides, such as Ru, Rh and Ir, are therefore the catalysts of choice. The Lewis acidity of these metals is too weak to catalyze a direct hydride transfer and, vice versa, the affinity of Alm and Lnm to hydride-ions is too low to catalyze the indirect hydrogen transfer. Two distinct pathways are possible for the hydride mechanism one in which the catalyst takes up two hydrides from the donor molecule and another in which the catalyst facilitates the transfer of a single hydride. 

Intramolecular reactions often differ from their intermolecular counterparts in the exceptionally high rates that are observed and some reactions can occur intramolecularly that are impossible between separate molecules. Because of the importance of intramolecular catalysis, the subject has been reviewed frequently, particularly with reference to its connection with enzymic catalysis (Page, 1973, 1984 Fife, 1975 Jencks, 1975 Kirby, 1980 Fersht, 1985 Menger, 1985). The present coverage is limited to examples of intramolecular catalysis that owe some of their efficiency to intramolecular hydrogen bonding. The role that hydrogen bonds play in enzymic reactions is discussed in Section 5. 

The proposed mechanism for the photochemical cleavage of nBn ethers (see Scheme 3) involves a n tt transition of the nitrogroup and an intramolecular, benzylic hydrogen abstraction by the excited nitro group. Rearrangement leads to a hemiacetal that decomposes to a free alcohol and to 2-nitrosobenzaldehyde that undergoes further thermal and photochemical reactions. 

Intramolecular carbon-hydrogen insertion reactions have well known to be elTectively promoted by dirhodium(ll) catalysts [19-23]. Insertion into the y-position to form five-membered ring compounds is virtually exclusive, and in competitive experiments the expected reactivity for electrophilic carbene insertion (3°>2° 1°) is observed [49], as is heteroatom activation [50]. A recent theoretical treatment [51] confirmed the mechanistic proposal (Scheme 15.4) that C-C and C-H bond formation with the carbene carbon proceeds in a concerted fashion as the ligated metal dissociates [52]. Chemoselectivity is dependent on the catalyst ligands [53]. 

This study indicates that the oxidation of dihydroanthracene in a basic medium involves the formation of a monocarbanion, which is then converted to a free radical by a one-electron transfer step. It is postulated that the free radical reacts with oxygen to form a peroxy free radical, which then attacks a hydrogen atom at the 10-position by an intramolecular reaction. The reaction then proceeds by a free-radical chain mechanism. This mechanism has been used as a basis for optimizing the yield of anthraquinone and minimizing the formation of anthracene. 

We have demonstrated that intermolecularly, amidyl radicals preferentially abstract an allylic hydrogen rather than add to a TT bond of olefins such as cyclohexene and 1,3-pentadiene (33). This reactivity pattern is completely reversed in intramolecular reactions as shown in the following examples of alkenyl mitro-samide photolysis. In every case, the amidyl radicals generated from photolysis preferentially attack the ir bonds intramolecu-... 

Many of the limitations of C—C bond formation by C —H insertion outlined for intermolecular reactions (Section 1.2.1.) can be overcome by making the reaction intramolecular. Thus, hydrogen atom abstraction followed by intramolecular radical-radical coupling or radical addition to an alkene are increasingly popular processes. Two-electron carbene insertions, either thermal or transition metal catalyzed, have also been used extensively. In either case, ring construction involves net C—C bond formation at a previously unactivated C-H site. 

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