3.7- dimethyl-l-octene

Early attempts by Pino and Giacomelli to resolve racemic 3,7-dimethyl-l-octene (37) by treatment with 0.3 equiv. of triisobutylaluminum in the presence of bis[(S)-seobutylsaHcyhdeneiminejnickel Ni[(S)-seobusal]2 and subsequent hydrolysis gave (S)-2,6-dimethyloctane (38) with an enantiomerical excess of 1.2% along with the unreacted starting material (S)-37 with 1.8% ee, as judged by optical rotation (Schemes 2-17 and 2-18) [28]. 

Studies on stereoselective polymerization of racemic olefins also support this view.338 Polymerization of 3,7-dimethyl-l-octene (the chiral center is in a position to the double bond) took place with 90% stereoselectivity yielding an equimolar mixture of homopolymers of the two enantiomers. No stereoselectivity was observed in the polymerization of 5-methyl-1-heptene (the chiral center is in y position to the double bond). The conclusion is that the ability of a catalytic center to distinguish between the two enantiomers of a monomer required for stereoselective polymerization must arise from its intrinsic asymmetry. The first-ever chiral polypropylene synthesized using a chiral zirconium complex with aluminox-ane cocatalyst is the latest evidence to testify the role of the catalyst center in isotactic polymerization.339... 

Table 4. Physical properties of different fractions of polymers of 3-methyl-1-pmtene and 3.7-dimethyl-l-octene obtained by polymerisation of the racemic monomers with (+ )-bis-[(S -2-methyl-butyl]-zinc and TiClt or TiCl3 "ARA ...

Monomer 3-methyl-l-pentene (704) 3.7-dimethyl-1 -octene (704) 3.7-dimethyl-l-octene (107) ... 

No optical activity has been found both in the polymer and in the recovered monomer polymerizing 3.7-dimethyl-l-octene in the presence of catalysts obtained from TiCl4 and the reaction product of LiAlH4 with (R)-3.7-dimethyl-l-octene (43). 

XXI n = 1) were also polymerized by Murahashi and co-workers 96) and the properties of poly-(R)-3.7-dimethyl-l-octene (XXII) were also investigated by Goodman and co-workers 43). 

In the case of the polymerization of (S)-3-methyl-l-pentene 115) and of (R)-3.7-dimethyl-l-octene 101) the non polymerized and non isomerized monomer has about the same optical purity as the starting monomer this indicates that the catalytic systems used do not racemize the monomer. 

As shown in Table 22 in most examples, the prevailing absolute configuration of the asymmetric carbon atoms of the lateral chains of the first eluted fractions is opposite to the one of the support this indicates that the polymer having the same structure as the support is more strongly adsorbed. However, this is not a general phenomenon, as it is shown by the chromatography of poly-3.7-dimethyl-l-octene obtained from the racemic monomer using poly-(S)-3-methyl-l-pentene as support (118). 

Acetone ins., ethyl acetate sol. Poly-(R)-3.7-dimethyl-l-octene 9.1... 

Poly-3.7-dimethyl-l-octene Poly-3-methyl-1-pentene (S) (S) (118)... 

Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9], Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the acdve site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of R - and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate M5. 

The stereoselectivity for the rearrangements of tertiary acetates providing trisubstituted allylic acetates is moderate, as the energy difference between the intermediates is smaller. For example, rearrangement of 3-acetoxy-3,7-dimethyl-l-octene gave a 78 22 mixture of ( )- and (Z)-l-ace-toxy-3,7-dimethyl-2-octcnc (8)12. 

Figure 5.1a (1) cis and trans linalool oxide (5-ethenyltetrahydro-a,a,5-trirnethyl-2-furanmethanol) (furanic form) (2) linalool (3,7-dimethyl-l,6-octadien-3-ol) (3) a-terpineol (a,a,4-trimethyl-3-cycloexene-l-methanol) (4) (Z,E) ocimenol (2,6-dimethyl-5,7-octen-2-ol) (5) cis and trans linalool oxide (pyranic form) (6) citronellol (3,7-dimethyl-6-octen-l-ol) (7) nerol (Z), geraniol (E) (3,7-di-methyl-2,6-octadien-l-ol) (8) Ho-diendiol I (3,7-dimethyl-l,5-octadiene-3,7-diol) (9) endiol (3,7-dimethyl-l-octene-3,7-diol) (10) Ho-diendiol II (3,7-dimethyl-l,7-... 

Under similar conditions, but with the addition of racemic 3,7-dimethyl-l-octene in a two-step version after homopolymerization of (5)-3-methyl-l-pentene has started, a polymer consistent with block copolymer formation of the 5-enantiomers is obtained73 ... 

We must remember that branched alkenes may contain centres of optical activity an example is (—)3,7-dimethyl-l-octene, which when hydrogenated on platinum shows little racemlsation, but on palladium this happens extensively, to an extent depending upon the form of catalyst and reaction conditions. - Isomerisation of the double bond to the 2-position negates the optical activity, so that it may return to the terminal position in either the (-1-)- or (—) form. When tetra-substituted alkenes of the type RR C=CCRR are hydrogenated, two centres of optical activity are created the fi-form gives the meso product, while the Z-form gives a racemic mixture. ... 

In these isotactic polymers, the optical purity of the monomer affected the optical activity via the relationship to the excess helical sense of the polymer (Figure 1). ° In the case of isotactic poly((S)-4-methyl-l-hexene) (2) and poly((i )-3,7-dimethyl-l-octene) (3), an increase in the optical purity of the monomers resulted in an increase in the optical activity of the polymers in a nonlinear fashion the optical activity of the polymers leveled off when the optical purity of the monomer reached -80%. By contrast, in the case of isotactic poly((S)-5-methyl-l-heptene) (4), the relation was linear. These findings imply that the side-chain chiral centers of 4, which are separated from the main chain by three covalent bonds, may be too far from the main chain to affect the helical conformation. 

Figure 4-21. Molar optical rotation per monomeric unit [< )] as a function of optical purity % of the monomer for poly[(S)-4-methyl-l-hexene] (top curve) poly[(S)-5-methyl-l-heptene], and poly[(R)-3,7-dimethyl-l-octene] (bottom curve) (after P. Pino, F. Ciardelli, G. Montagnoli, and O. Pieroni).

The stereoselectivity and then the enantiomorphic structure of the active sites in the supported catalyst has been susequently and simply confirmed33 by the copolymerization of a racemic a-olefin, (R)(5)-3,7-dimethyl-l-octene [(/f)(5)DMO] with optically active... 

Another way to get citronellol is by the reductive dimerization of isoprene with formic acid and triethylamine using a 1% pcdladium phosphine catalyst [32]. The two head-to-tail dimers are formed in up to 79% yields, which can easily be separated from the head-to-head and tail-to-tail dimers by conversion with aqueous hydrochloric acid, yielding 7-chloro-3,7-dimethyl-l-octene. Hydroboration and pyrolysis of this chloro derivative produces a 1 3-mixture of a- and jS-citronellol. The mono-chloro compound can also be oxidized with tert- mty peracetate and a cuprous bromide catalyst to the chloroacetate, which is reduced with LiAJH4 and pyrolyzed to linalool in 64% overall yield. 

Isotactic polymers of racemic 3,7-dimethyl-l-octene can be separated in fractions having optical activity of opposite sign by elution on an optically active crystalline support consisting of poly-[(S)-3-methyl-l penteneJ (2l). The separation degree F even for highly isotactic fractions is not higher... 

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