Cyclopentanes Methylenecyclopentanes

The second termination reaction is alkyl chain end transfer from the active species to aluminium [155]. This termination becomes major one at lower temperatures in the catalyst systems activated by MAO. XH and 13CNMR analysis of the polymer obtained by the cyclopolymerization of 1,5-hexadiene, catalyzed by Cp ZrCl2/MAO, afforded signals due to methylenecyclopentane, cyclopentane, and methylcyclopentane end groups upon acidic hydrolysis, indicating that chain transfer occurs both by /Miydrogen elimination and chain transfer to aluminium in the ratio of 2 8, and the latter process is predominant when the polymerization is carried out at — 25°C [156]. The values of rate constants for Cp2ZrCl2/MAO at 70°C are reported to be kp = 168-1670 (Ms) 1, kfr = 0.021 - 0.81 s 1, and kfr = 0.28 s-1 [155]. 

This is by analogy to cyclopentane, cyclopentene and methylenecyclopentane, all from References 17 by Greenberg and Liebman. 

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

Versatile [3 + 2]-cycloaddition pathways to five-membered carbocycles involve the trimethylenemethane (= 2-methylene-propanediyl) synthon (B.M. Trost, 1986). PaIladium(0)-induced 1,3-elimination at suitable reagents generates a reactive n - -methylene-1,3-propa-nediyl complex which reacts highly diastereoselectively with electron-deficient olefins. The resulting methylenecyclopentanes are easily modified, e.g., by ozonolysis, hydroboration etc., and thus a large variety of interesting cyclopentane derivatives is accessible. 

Other 1,3-dienes, such as 1,3-butadiene, isoprene and methyl-2,4-pentadienoate, either do not react with methylenecyclopropanes or yield only 3-vinylmethylene-cyclopentane derivatives exclusively (Table 10 and Eq. 114). Quite unexpectedly, methyl-2,4-pentadienoate reacts only at the terminal C=C bond, giving a vinyl-methylenecyclopentane in poor yield 224) (Eq. 114). 

The two Pd(0) or Ni(0) catalyzed [3+2]-cycloadditions starting with the readily accessible trimethylenemethane -precursors [2-(acetoxymethyl)-3-allyl]trimethyl-silan, methylenecyclopropane, and their substituted derivatives are important new methods for the synthesis of methylenecyclopentanes. Because of the simplicity with which many problems of cyclopentane-syntheses can be solved in a convenient one pot reaction this new methodology may be compared with the synthesis of six-membered rings by the powerful 4+2]-cycloaddition of the Diels-Alder reaction. 

With Ni(0) catalysts they can react by two different reaction mechanism which give the possibility of synthesizing methylene-cyclopentanes with different substituent patterns (see Eqs. 81, 86 and 87). In Table 11 some of the most interesting results obtained in the preparation of methylenecyclopentanes from [(2-acetoxymethyl)-3-allyl]trimethylsilan or methylenecyclopropanes and electron deficient olefins by this new [3+2]-cycloaddition methodology are summarized. 

Methylenecyclopentanes and alkylidencydopentanes are easily converted by oxidation into the corresponding cyclopentanone derivatives in high yield (90-95%). The oxidations of (diphenylmethylene)cyclopentanes are more difficult these reactions are often imcomplete or do not work at all. 

These reactions proceed smoothly at 20-40 °C with high regio- and stereoselectivity. For example, dimethyl maleate forms largely the cw-fused cyclopentane. This process has been used to excellent effect in the synthesis of optically active, methylenecyclopentanes by employing chirally modified acrylate acceptors (sec Section 1.6.1.2.3.3). 

Singleton has developed an intermolecular [3+2] addition strategy for the synthesis of functionalized cyclopentane rings using strained methylenecyclopropanes as radical traps (Scheme 22) [53]. The success of methylenecyclopropane 27 with electron-rich and unactivated alkenes arises from the ready formation of the highly stable electrophilic radicals 28b. Thus, this reaction works well with equimolar amounts of unactivated and electron-rich alkenes but does not work with electron-poor alkenes. The reagent 29 and 30 are prepared by the structural modification of 27. Furthermore, [3+2] methylenecyclopentane annulations of electron-poor alkenes can be carried out with unactivated methylenecyclopropane 31 and 32 [54]. 

Methylenecyclopentane Annulation. The acylated derivative 2-acetyloxymethyl-3-trimethylsilylpropene is known to be an effective three-carbon component in cyclopentane annulations. It has been demonstrated that 2-acetoxymethylallyltrimethylsilane adds to a variety of electron-deficient alkenes in the presence of a catalytic amount of tetrakis(triphenylphosphine) paUadium(0) and l,2-bis(diphenylphosphino)ethane to produce methylenecyclopentanes (eqs 6-8). 

Transition-metal-assisted cyclopentane ring-forming reactions again feature prominently this year. Wilkinson s catalyst brings about the cyclization of 1,6-dienes to methylenecyclopentanes e.g. (37) - (38), whereas cyclopentanones are obtained from the rhodium(i)-catalysed intramolecular hydroacylation of y,5-unsaturated aldehydes e.g. (39) - (40). ... 

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