Open-chain alkenes

Cycloalkenes are named similarly to open-chain alkenes but, because there is no chain end to begin from, we number the cycloalkene so that the double bond is between Cl and C2 and the first substituent has as low a number as possible. Note that it s not necessary to indicate the position of the double bond in the name because it is always between Cl and C2. As with open-chain alkenes, newer but not yet widely accepted naming rules place the locant immediately before the suffix in a diene. 

If the carbanion has even a short lifetime, 6 and 7 will assume the most favorable conformation before the attack of W. This is of course the same for both, and when W attacks, the same product will result from each. This will be one of two possible diastereomers, so the reaction will be stereoselective but since the cis and trans isomers do not give rise to different isomers, it will not be stereospecific. Unfortunately, this prediction has not been tested on open-chain alkenes. Except for Michael-type substrates, the stereochemistry of nucleophilic addition to double bonds has been studied only in cyclic systems, where only the cis isomer exists. In these cases, the reaction has been shown to be stereoselective with syn addition reported in some cases and anti addition in others." When the reaction is performed on a Michael-type substrate, C=C—Z, the hydrogen does not arrive at the carbon directly but only through a tautomeric equilibrium. The product naturally assumes the most thermodynamically stable configuration, without relation to the direction of original attack of Y. In one such case (the addition of EtOD and of Me3CSD to tra -MeCH=CHCOOEt) predominant anti addition was found there is evidence that the stereoselectivity here results from the final protonation of the enolate, and not from the initial attack. For obvious reasons, additions to triple bonds cannot be stereospecific. As with electrophilic additions, nucleophilic additions to triple bonds are usually stereoselective and anti, though syn addition and nonstereoselective addition have also been reported. 

Thus, the hydroboration of 1-phenylcyclopentene with (—)-IpcBH, (99% produces, after crystallization, the chiral organoborane 126 with 94% ee. The reaction of 126 with Et2BH replaces the isopinocamphenyl group with an ethyl substituent (50 C, 16 h) and provides, after the addition of i-Pr2Zn (25 °C, 5 h), the mixed diorganozinc 127. Its stereoselective allylation leads to the fraw5-disubstituted cyclopentane 128 in 44% yield (94% ee trans cis = 98 2) see Scheme 43 ° . This sequence can be extended to open-chain alkenes and Z-styrene derivative 129 is converted to the anf/ -zinc reagent 130, which provides after allylation the alkene 131 in 40% yield and 74% ee (dr = 8 92). 

Thermal decomposition of y-lactone tosylhydrazone sodium salts are reported to yield cyclobu-tanones, which can be accounted for by rearrangement of an intermediate oxycarbene. In this manner, the sodium salts of dihydrofuran-2(37/)-one tosylhydrazones 1 were decomposed as a loose powder, at 310 C in a bulb-to-bulb distillation apparatus at an initial pressure of 0.1 Torr, to give the corresponding cyclobutanones 2 in addition to enol ethers, cyclopropanes and open-chain alkenes. Condensable products (74-76%) were collected at — 78 °C, weighed and the ratio of components was determined from their relative GC peak areas.63... 

Cycloalkenes are named by the system used for the open-chain alkenes, except that the numbering always is started at one of the carbons of the double bond and continued around the ring through the double bond so as to keep the index numbers as small as possible ... 

The efficient azidobromination of cycloalkenes [Eq. (5.342)] and open-chain alkenes to give /3-bromoalkyl azides with iV-bromosuccinimide and azidotrimethylsi-lane is catalyzed by Nafion-H.919 Terminal alkenes and alkenes with bulky substituents do not react, whereas 2,3-trimethylbutene-2 reacts without catalysis. The stereochemistry of the process suggests the involvement of a bromonium ion intermediate. 

Fuchs, R. and Peacock, LA. (1979) Gaseous heat of hydrogenation of some cyclic and open chain alkenes. Journal of Physical Chemistry, 83 (15), 1975-8. 

Hydroxylation, the addition of two hydroxyl groups across double bonds, converts alkenes and cycloalkenes into vicinal dials. Stereochem-ically. the addition may occur in the syn or the anti mode. In open-chain alkenes (with the exception of terminal alkenes for which stereochemistry is irrelevant), syn hydroxylation transforms cis alkenes into erythro (or meso) diols and trans alkenes into threo (or dl) diols. anti Hydroxylation of cis alkenes gives threo (or dl) diols, whereas anti hydroxylation of trans alkenes yields erythro (or meso) diols. syn Hydroxylation of cycloalkenes gives cis diols, whereas anti hydroxylation furnishes trans diols (Table I). 

The Houk Rule for Steric Effects in Electrophilic Attack on Open-Chain Alkenes. As in... 

Stereochemical data support the occurence of these intermediates, as also shown by Doyle et al. (1984 b) They compared reactivities and stereoselectivities of cyclopropanations of phenyldiazomethane and eleven different open-chain alkenes containing a terminal double bond or a double bond in the chain, and a cyclic alkene (cyclopentene) catalyzed by the binuclear complex Rh2(OCOCH3)4 (8.127, see later in this section), with the reactivities and stereoselectivities of cyclopropanations of the same alkenes with (benzylidene)(pentacarbonyl)tungsten [(CO)5W(CHC6H5)], i.e., a stable metal-carbene. An almost perfect linear relationship of the cyclopropane derivatives of the eleven alkenes with the two carbene sources was obtained. On this basis, Doyle and his coworkers concluded that the reaction starts with an initial association of the alkene 71-bond with the electrophilic center of the metal-carbene complex, followed by o-bond formation with backside displacement... 

Open-chain alkenes undergo dimerizations in stereospecific [nl + tt ] paths in low yields on irradiation. For example, photodimerization of Z-2-butene 26 gives two products in which cw-geometry of methyl groups is retained. Similarly, the cycloadducts from irradiation of -2-butene 27 retains the trans-geovasXry of methyl groups [29]. Irradiation of Z-2-butene also gives cycloadduct with -stil-bene in a similar manner [29]. 

With prenylpotassium, carbon-carbon linking occurs at the inner end of the allylic entity exclusively, thus, giving rise to a quaternary carbon center. This unprecedented regioselectivity points at a competition of the solvent, the allylic backbone and the ethylene T -ligand for metal coordination. With both prenylpotassium and 3-ethyl-2-pentenylpotassium as reagents, the ethylated open-chain alkenes 3,3-dimethyl-... 

---