Disubstituted Alkenes Cycloalkenes

With a few exceptions, 1,2-disubstituted alkenes are not polymerized because of steric hindrance. The exceptions include 1-deuteropropene (Sec. 8-4g) and cycloalkenes. Polymers are obtained from some 1,2-disubstituted alkenes, but the reactions involve isomerization of the monomer to a 1-alkene prior to polymerization, e.g., 2-butene yields poly( 1-butene) [Endo et al., 1979]. There is one report of polymerization of frans-2-butene to poly(frans-2-butene) using the a-diimine nickel initiators described in Sec. 8-8b [Leatherman and Brookhart, 2001]. 

Cycloalkenes undergo facile polymerization because ring strain is relieved on polymerization. Polymerization occurs using both traditional Ziegler Natta and metallocene initiators [Boor, 1979 Coates, 2000 DaU Asta et al., 1962 Ittel et al., 2000 Kaminsky, 2001 Natta 

Four different stereoisomers are possible for polymer XLII, poly (cyclobutane-1,2-diyl) (Sec. 8-lf). Cis and trans isomers are possible for pol3mier XLin, poly (but-1-ene-1,4-diyl). (XLni is the same polymer obtained by the l,4-pol3fmerization of 1,3-butadiene— Sec. 8.10). Traditional Ziegler-Natta initiators based on vanadium and metallocene initiators yield polymerizations almost exclusively through the double bond. Titanium, tungsten, and ruthenium initiators yield predominantly ROMP with varying amounts of cis and trans placements. 

Cyclopentene yields mixtures of ROMP and double-bond polymerization with some Ti and V initiators. ROMP occurs exclusively with molybdenum and tungsten initiators, as well as Re, Nb, and Ta initiators. The relative amounts of cis and trans structures vary with the initiator and temperature [DalTAsta et al., 1962 Pampus and Lehnert, 1974]. Metallocene initiators polymerize cyclopentene through the double bond, but the polymer structure consists of cis 1,3-placement (Coates, 2000 Kaminsky, 2001 Kelly et al., 1997]. 1,3-Placement occurs through an isomerization process similar to that responsible for 3,1-placement in propene polymerization (Sec. 8-5c-l). 1,3-Placement is also observed with nickel and palladium a-diimine initiators [Sacchi et al., 2001] (Sec. 8-8b). 1,3-Placement has not been reported for other cycloalkene polymerizations. 

Cyclohexene does not polymerize by either route except when it is part of a bicyclic structure as in norbornene. Stereochemistry in the ROMP of norbomene is complicated since the polymer, LXVI in Sec. 7-8, has possibilities of isomerism at both the ring and the double bond. Most polymerizations by the typical ROMP initiators yield cis stereochemistry at the cyclopentane ring with varying amounts of cis and trans placements at the double bond [Ivin, 1987]. Metallocene initiators yield predominantly double-bond polymerization with 1,2-placement [Janiak and Lassahn, 2001]. 

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Among disubstituted alkenes trans RCH=CHR is normally more stable than as RCH=CHR Exceptions are cycloalkenes cis cyclo alkenes being more stable than trans when the nng contains fewer than 11 carbons... 

The degree of enantionselectivity of (Ipc)2BH is not high for all simple alkenes. Z-Disubstituted alkenes give good enantioselectivity (75-90%), but /f-alkcncs and simple cycloalkenes give low enantioselectivity (5-30%). 

Substituted cycloalkenes usually react in the ring and not in the side chain. Internal alkenes with CH2 groups in both allylic positions yield a mixture of isomers, whereas terminal alkenes give primary alcohols as a result of allylic rearrangement. Later studies revealed, however, that the reactivity depends on both the structure of the alkene and reaction conditions.674 675 In alcoholic solutions, for example, racemic products are formed. Geminally disubstituted alkenes may exhibit a reactivity sequence CH > CH2 > CH3.675 676... 

A cascade Heck reaction with termination by nucleophiles is considered to start with an oxidative addition of a heteroatom-carbon bond (starter) onto a palladium(O) species (startup reaction), followed by carbopalladation of a nonaromatic carbon-carbon double or triple bond without subsequent dehydropalladation (relay), a second and possibly further carbopalladation of a carbon-carbon double or triple bond (second etc. relay). The terminating step is a displacement of the palladium residue by an appropriate nucleophile. It is crucial for a successful cascade carbopalladation that no premature dehydropalladation takes place, and that can be prevented by using alkynes and 1,1-disubstituted alkenes (or certain cycloalkenes) as relay stations since they give kinetically stable alkenyl- or neopentylpalladium intermediates, respectively. In addition, reaction of haloalkenes with alkenes in certain cases may form rr-allyl complexes, which are then trapped by various nucleophiles. 

For the cleavage of alkenes from a support by metathesis, several strategies can be envisaged. In most of the examples reported to date, ring-closing metathesis of resin-bound dienes has been used to release either a cycloalkene or an acyclic alkene into solution (Figure 3.38, Table 3.44). Further metathesis of the products in solution occurs only to a small extent when the initially released products are internal alkenes, because these normally react more slowly with the catalytically active carbene complex than terminal alkenes. If, however, terminal alkenes are to be prepared, selfmetathesis of the product (to yield ethene and a symmetrically disubstituted ethene) is likely to become a serious side reaction. This side reaction can be suppressed by conducting the metathesis reaction in the presence of ethene [782,783]. 

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