Cycloalkanes, formation

Fig. 10. Standard free energy change for cycloalkane formation at 550°K. alkane cycloalkane + H2 -X-, 1-alkene cycloalkane.

As an extension of this reaction the intramolecular cycloaddition of 5-propynyloxycycloalkanepyrimidines was studied. It was found that bi-and tricyclic annelated pyridine derivatives are formed by expulsion of either X-CH2-CN and/or HCN, respectively. A marked selectivity in the product formation was observed, depending on the size of the cycloalkane ring. With cyclohexapyrimidines a mixture of A and B is formed, while with the cycloheptapyrimidine derivative exclusive formation of the tricyclic compound B takes place (92T1643, 92T1657) (Scheme 39a). 

The photolytic cleavage of cyclic ketones 14 leads to formation of a diradical species, that can undergo analogously the various reactions outlined above. The decarbonylation followed by intramolecular recombination yields a ring-contracted cycloalkane 15 ... 

The original Demjanov reaction is the conversion of an aminomethyl-cycloalkane into a cycloalkanol consisting of a carbocyclic ring that is expanded by one carbon center e.g. the reaction of aminomethylcyclohexane 8 with nitrous acid leads to formation of cycloheptanol 9 ... 

Cycloalkanes, like alkanes, are saturated hydrocarbons containing only single bonds. Notice, though, that as a result of ring formation, each cycloalkane molecule contains two fewer hydrogen atoms than the corresponding alkane. 

Scheme 38 Formation of five-, six-, and seven-membered cycloalkane-annelated ethoxy-cyclopentadienes 175 from the phenylethynylcarbene complex 173 and cyclic enamines 174 [119,120]...

Anaerobic degradation of cycloalkanes has seldom been reported. The pathway used for the degradation of ethylcyclopentane by a sulfate-reducing enrichment is analogous to the fumarate pathway used for -alkanes (Part 1 of this chapter) with the formation of 3-ethylcyclopentanecar-boxylate followed by ring fission to 3-ethylpentan-l,5-dioate (Rios-Hamandez et al. 2003). 

Thiourea clathrates may also be applied for geochemical separations. They are similar to the molecnlar complexes of nrea bnt owing to the larger dimensions of the thiourea crystal channels, only branched alkanes, cycloalkanes, and their derivatives participate in clathrate formation. In contrast, the unbranched molecules are too small and hence cannot be held rigidly [77]. 

There is some increase in selectivity with functionally substituted carbenes, but it is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with i-propylbenzene, ethylbenzene, and toluene.212 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.213 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.214 Owing to low selectivity, intermolecular insertion reactions are seldom useful in syntheses. Intramolecular insertion reactions are of considerably more value. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene and good yields can frequently be achieved. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

Significant synthetic applications of the nickel-salen catalysts are the formation of cycloalkanes by reduction of <>, -a-dihaloalkanes255,256 and unsaturated halides,257,258 the conversion of benzal chloride (C6H5CHC12) into a variety of dimeric products 259 the synthesis of 1,4-butanediol from 2-bromo- and 2-iodoethanol260 or the reduction of acylhalides to aldehydes261 and carboxylic acids.262... 

In marked contrast to the n-alkanes, the cycloalkanes exhibit thermodynamic properties where such regularities are no longer present. Heats of formation (AH ) for a substantial number of cycloalkanes are available from heats of combustion. With the exception of cyclohexane, AH°f is always more positive than the quantity — 4.926n. The difference between the two quantities leads to a quantitative assessment of the important notion of ring strain. The AH -values and strain energy data listed in Table 1 were taken from Skinner and Pilcher (1963). Other references give different but usually comparable... 

Standard enthalpies of formation and strain-energies of cycloalkanes 1... 

The question of the choice of the appropriate symmetry number for con-formationally labile molecules is a subtle one and is at best an approximation. It is the opinion of this author that, if a = 18 for the n-alkanes is accepted as correct, then for the sake of consistency the a-values for all of the cycloalkanes have to be set at 2n, as shown by the following example. Consider the hypothetical reaction (36) for which a statistical correction is clearly... 

Fig. 10 EM-profiles for the formation of (O) lactones [4] in 99% Me2SO ( ) l,l-bis(ethoxycarbonyl) cycloalkanes [7] in Me2SO (A) N-tosylazacycloalkanes [9] in 99% Me2SO ( ) catechol polymethylene ethers [2] in 99% Me2SO...

Me2N(CH2)mNMe2 (Fig. 16) and that for the formation of N-tosylaza-cycloalkanes [9] (Fig. 10). We have also pointed out (p. 49) that the effect of chain length on the ease of closure of [11] by intramolecular acylation of the... 

Most recently, reductive Michael cyclization catalyzed by In(OAc)3 in the presence of PhSiH3 was reported.50 As demonstrated by the reductive cyclization of the homologous bis(enones) 25a and 26a, both five- and six-membered ring formation occurs in good yield to afford cycloalkanes 25d and 26b as single trails-Aiastereorncrs (Scheme 20). 

Cyclopolymerization of Nonconjugated Dienes. Cyclopolymerization is an addition polymerization that leads to introduction of cyclic structures into the main chain of the polymer. Nonconjugated dienes are the most deeply studied monomers for cyclopolymerization and for cyclocopolymerizations with alkene monomers 66 In general, (substituted and unsubstituted) dienes with double bonds that are linked by less than two or more than four atoms cannot undergo efficient cyclization and result in crosslinked materials.12 In fact, efficient cyclopolymerization processes have been described, for instance, for a,oo-dienes like 1,5-hexadiene, 2-methyl-l,5-hexadiene, 1,6-heptadiene, and 1,7-octadiene,67 73 which lead to formation of homopolymers and copolymers containing methylene-1,3-cycloalkane units. 

General Procedure for the Hydroformylation/N,N-acetal Formation. Synthesis of Biaza-cycloalkanes. The unsaturated diamine (leq), [Rh(OAc)2h (0.5 mol % Rh atoms) and BIPHEPHOS (2 mol %) were placed in an autoclave under N2 followed by deoxygenated benzene The vessel was flushed and evacuated three times with CO/H2 (1 1,13 bar) and then pressurized to 28 bar. The reaction was kept at 40-80 °C for 20h. The autoclave was cooled and the gases were released followed by selective extraction of the total product with light petroleum. Concentration of the solvent gave in most cases NMR pure material of the title compounds. 

This article will deal with metal-catalyzed cyclization reactions, with reference to oxide and dual-function catalysts. Product cycles may contain five or six carbon atoms. The respective prefixes C5 and will point to the resulting structure (5). The term dehydrocyclization will be applied to reactions that end up with aromatic products the formation of saturated (cycloalkane) rings will henceforth be called cyclization. ... 

The first mechanistic concepts of aromatization 16) originate from pregas-chromatography times. A direct alkane- cycloalkane reaction was proposed by Kazansky and co-workers 47). Several authors have interpreted the formation of six-membered rings over metal catalysts in terms of alkene-alkyl insertion (i.e., analogous to the Twigg mechanism) (7, 8, 14). 

Protonation of alkenes, cycloalkanes and alkanes lead to the formation of carbe-nium ions, carbonium ions and protonated cyclopropane rings (also called cyclo-... 

---