Cyclopentanes formation

Indeed, cyclopentane formation during all cyclic alkane hydrogenolysis and cyclopentane transformation into a cyclopentadienyl derivative, inert in hydrogenolysis, explains the rapid deactivation process of this catalyst in the presence of cyclic alkanes. 

The steric effects become clear on inspecting the Newman projections of the transition states. The conformation 43 a leads to cyclopentane formation, while the conformation 43 b would give / -H elimination. As the rhodium carbenoid becomes larger, conformation 43 b is increasingly favored. Thus, as the steric bulk of the ligands on the Rh carbenoid increases on going from acetate (entry 1) to the TPA catalyst (entry 4), there is a significant increase in the proportion of /9-hydride elimination. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

Formation of 5-Hexenyl Radicals Competing Cyclopentane Formation... 

Eq. 52 and 53 demonstrate remarkable characteristics of this [3 + 2]-cycloaddition starting with a pure diastereomer 130, two stereoisomeric cyclopentanes 131 are obtained. This stereorandom outcome is most simply rationalized assuming a stepwise mechanism with a 1,5-zwitterion as an intermediate in the cycloaddition. The vinylcyclopropane 132 only gives five-membered ring products 133 and no cyclo-heptene derivative, which would result from a conceivable [5 + 2]-cycloaddition. Less activated olefins or cyclopropanes do not undergo a similar [3 + 2]-cycloaddition. Due to the specific substitution pattern, the cyclopentane formation from these siloxycyclopropanes is of no preparative value. 

Rh-mediated C-H insertion is also useful for carhocyclic construction, as illustrated by the new asymmetric route to (+)-morphine (11) recently reported by White [6]. Cyclopentane formation is used to fashion a pentacyclic skeleton (10) from which the piperidine ring of 11 is evolved at a later stage. 

Cycloadditions. Oppolzer first used this chiral acrylate derivative as an auxiliary in the Diels-Alder reaction with cyclopen-tadiene. Promotion by Lewis acids such as TiCU SnCU, and Et2AlCl provides the adduct in greater than 90% de (eq 1). Lithium perchlorate-promoted [4 + 2] reaction between 1 and 1-acetoxybutadiene was similarly effective." More recently, an exo-selective Diels-Alder addition of 1 with 2-acylamino dienes provided a single diastereomer in 80% yield. Cyclopentane formation is possible through exposure of 1 to methylenecyclo-propane and Ni(0) (eq 2). An example of a higher-order cycloaddition with 1 gave only low diastereoselection (78 22) for the endo product. 

Larger ring size carbocyclic 1,2-diols can be synthesized via extension of the methods used for cyclopentane formation (equations 43-45). Thus, cyclizations of 1,6-dicarbonyls give the corresponding cis... 

A similar reaction pathway is discussed in other cases of palladium-mediated cyclopropane formation. Thus diastereoselective [2 + 1] cyclopropanation, instead of [3 + 2] cyclopentane formation, is observed if norbornene (3), norbornadiene (6), or dicyclopentadiene (9) is treated... 

The obtained catalysts showed similar activity in cyclopentadiene hydrogenation. The selectivity data in cyclopentadiene hydrogenation are given in Table I. The selectivity of consecutive reaction was determined as a rate ratio of cyclopentene/cyclopentane formation. On the contrary, the chitosan modification influenced essentially the selectivity of the catalyst on it basis. 

If the alkyl chain is shortened by one carbon, a substrate is generated that has the possibility for bicyclo[2.2.1]heptane generation (Eq. 81) [38]. After the initial alkyne insertion, the catalyst must choose between cyclobutane and cyclopentane formation. The five-membered ring is formed because of the lower... 

A1 MAS NMR experiments (see also ref. 10) indicated the absence of octahedrally coordinated A1 atoms, what suggests formation of bohemite-like structure inside the supercages (ref.17). Application of industrial y-alumina (AERO-1000, American Cyanamid Co.,99.99% pure) in cyclopentene hydrogenation resulted in cyclopentane formation with a yield of 50%. Bowman and Burwell (ref.18) found that y-alumina can catalyze propylene hydrogenation even below 425 K. 

At first glance and ignoring stereochemistry, one is tempted to predict cyclopentane formation, but in this event the cyclobutane 61 is mainly formed. 

In bisketal 362, the diradical formed on nitrogen expulsion may, owing to steric hindrance, react with the CH2 group of the double bond first (conformation ), generating a six-membered ring. This is then followed by the cyclopentane formation, leading to 364. 

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