Methyl-1-pentene

Hydroborating agent 1-Hexene 2-Methyl-1 -butene 4-Methyl-2-pentene Styrene... 

Disubstituted Alkenes. Simple 1,2-disubstituted alkenes such as 2-octene or cyclohexene, which produce only secondary aliphatic carbocation reaction intermediates, do not undergo reduction upon treatment with a Brpnsted acid and an organosilicon hydride. Even when extreme conditions are employed, only traces of reduction products are detected.192 203 207-210,214 An exception is the report that 4-methyl-2-pentene forms 2-methylpentane in 70% yield when heated to 50° for 20 hours with a mixture of Et3SiH/TFA containing a catalytic amount of sulfuric acid. It is believed that 4-methyl-2-pentene is isomerized to 2-methyl-2-pentene prior to reduction.203... 

In favorable cases for some single olefmic species, thermodynamic equilibria with respect to double-bond position can be attained at low temperatures without significant oligomerization. For example, ris-2-bu-tene has been isomerized with the P(CH3)3-modified catalyst at -20°C to give the thermodynamic equilibrium mixture having the composition 78.8% trarts-2-butene, 19.5% m-2-butene, and 1.7% 1-butene. The isomerization of 4-methyl-1-pentene (or of 2-methyl-1-pentene) in the presence of the same catalyst at 0°C also leads to the thermodynamic equilibrium mixture with the composition shown in Table II. If the reaction is carried out at low temperature, the individual isomerization steps can be followed At -58°C, isomerization to ci i-4-methyl-2-pentene occurs and is followed by isomerization to the trans isomer this is then con-... 

In contrast to the examples of selectivity control discussed in the previous sections, the problem here is control of the regioselectivity of the individual reaction steps. This is evident from the Scheme 5. In the first reaction step the nickel-hydride species adds to propene forming a propyl- or isopropyl-nickel intermediate this step is reversible, and the ratio of the two species can be controlled both thermodynamically and kinetically. In the second step, a second molecule of propene reacts to give four alkylnickel intermediates from which, after j8-H elimination, 8 primary products are produced (Scheme 5). 2-Hexene and 4-methyl-2-pentene could be the products of either isomerization or the primary reaction. Isomerization leads to 3-hexene, 2-methyl-2-pentene (the common isomerization product of 2-methyl-1-pentene and 4-methyl-2-pen-tene), and 2.3-dimethyl-2-butene. It can be seen from the Scheme 5 that, if the isomerization to 2-methyl-2-pentene can be neglected, the distribution of the products enables an estimate to be made of the direction of... 

The preponderance of stereochemical data in the literature has been obtained from studies using 2-pentene, which now appears to have been a rather poor substrate for emphasizing steric aspects of the reaction. Recent experiments utilizing 4-methyl-2-pentene (76) have given much clearer indications of steric control in metathesis reactions (vide infra). 

Fortunately, steric control arising from interactions of alkyl moieties derived from reacting olefins can be enhanced and observed by selection of appropriate reactants. This effect was demonstrated in the work of Lawrence and Ofstead (76), who studied the metathesis of 4-methyl-2-pentene induced by a WCl6Et2OBu4Sn catalyst. This catalyst is not particularly unique, for the steric course of the metathesis of m-2-pen-tene with this system was found to be essentially equivalent to that previously observed (18) with a conventional catalyst employing an or-ganoaluminum cocatalyst. 

Methyl-2-pentene afforded the expected products, 2-butene and 2,5-dimethyl-3-hexene, a highly hindered olefin. (The equilibrium cis content of an equilibrated mixture of 2,5-dimethyl-3-hexene isomers was independently established to be 7% cis.)... 

Compositions of metathesis reaction mixtures obtained over a range of conversions starting with cis- and trans-4-methyl-2-pentene are shown in Figs. 1 and 2, respectively. Certain important differences are evident in comparisons with the course of reactions of 2-pentene isomers (18). 

Fig. 1. The metathesis of m-4-methyl-2-pentene. Effect of conversion on products structure (76).

A second observation was the fact that isomerization of the starting asymmetric olefin was much faster than the formation of new symmetric olefins. In fact, 40% of the initial cis olefin (Fig. 1) had isomerized to trans after only 4% conversion to new olefins. This result formally parallels the highly selective regenerative metathesis of a-olefins (60, 61), except that steric factors now prevail, because electronic effects should be minimal. Finally, the composition of the initially formed butene from r/j-4-methyl-2-pentene was essentially identical to that obtained when cA-2-pentene was used (18). When tra .v-4-methyl-2-pentene was metath-esized (Fig. 2), the composition of the initially formed butenes indicated a rather high trans specificity. 

This model represents an effort to deal with ligand effects, but it is difficult to believe that the wide variety of catalysts studied can universally be as coordinatively unsaturated as this model requires. In fact, even pentacoordinate forms of tungsten (e.g., W(CO)5) are generally held to be much less stable than hexacoordinate species. Their scheme would also seem to predict the highly selective the formation of cis-butene from c/s-4-methyl-2-pentene, whereas the observed stereospecificity is virtually the same as that obtained with m-2-pentene (18). 

This scheme now appears to be of very limited predictive value, because the results described previously with c / -4-methyl-2-pentene, which selectively gave pure trans-Ce as one of the products (76), would be entirely unexpected. Furthermore, if the commonly observed lack of stereospecificity in reactions of c/.v-2-pentene were to be attributed to an ionic process involving the metallocyclobutane, one would not expect uniformly similar steric results with diverse catalysts such as those summed up by Basset (81), as varying degrees of cationic isomerization would have been expected with these diverse catalysts. 

Unfortunately, this scheme does not anticipate the nonspecific formation of 50% rrani-3-hexene from c/s-2-pentene, nor the course of reaction for ci s-4-methyl-2-pentene mentioned earlier. 

With these guidelines, the results from the metathesis of 4-methyl-2-pentene now appear to be reasonably accountable for. Most importantly, the lack of formation of c7s-2,5-dimethyl-3-hexene indicates that cis- 1,2-diisopropyl-substitution on the ring is highly unfavored, and trans-1,2-substitution leading to adjacent equatorial substituents is acceptable ... 

The rapid isomerization of c/s-4-methyl-2-pentene relative to productive metathesis suggests further information regarding the isomerization process. If regenerative metathesis proceded selectively via an isopropyl carbene. and assuming that the isopropyl groups maintained equatorial... 

The moderate specificity for forming m-2-butene initially (see Fig. 1) is again consistent with equatorial orientation of isopropyl the rather low cis specificity indicates only a moderate preference for equatorial orientation of the a-methyl, probably because of the offsetting weak repulsions caused by cis- 1,2-dimethyl-substitution. This effect is absent in the metathesis of tra i-4-methyl-2-pentene, and trans specificity for... 

Methyl- 1-pentene 2- Methyl- 2-pentene cA-3-Methyl-2-pentene fra ,s-3-Methyl-2-pentene cA-4-Methyl-2-pentene... 

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