Hydrocarbon substituents synthesis

As demonstrated in most of the other chapters of this book, there is a countless - and growing - number of applications of allenes in preparative organic chemistry. A need for an extra section on the use of allenic hydrocarbons in synthesis may therefore not be apparent. However, since this chapter concentrates on the construction and then preparation of novel jr-systems - neglecting all non-hydrocarbon substituents - some general remarks on the preparative usefulness of the parent systems seem to be in order. 

Furthermore, while some of the carboxylate-alumoxanes formed gels in THF only those with long chain substituents (e.g., dodecanoic acid) and hence low ceramic yield were melt-processable. It would thus be desirable not only to prepare alumoxane preceramics in a one-pot bench-top synthesis from readily available starting materials, but also to determine if lower hydrocarbon substituents could yield better processability. 

The balance of the compounds in Table VIII support the earlier statement that any compound containing fluorine is toxic to moths. These fluorinated hydrocarbons, phenols, acids, sulfonic acids, and sulfones probably act against moths as stomach poisons. The fluorosulfonic acid derivatives and the sulfone with a fluorinated substituent were key compounds whose toxicity to moths laid the groundwork for the deductions which led to the synthesis and testing of DDT as an insecticide. 

Ethylene and styrene derivatives react with various propargylic silyl ethers in the presence of zirconocene(II) to afford allenic products in high yield (Scheme 5.7). For example, substrate 67 was transformed into the trisubstituted allene hydrocarbon 68 in 93% yield under the reaction conditions [20]. In the synthesis of various tetraalkylated allenes, in which several of the alkyl substituents also contained triple bonds, allowing these substrates to be cydized intramolecularly into aromatic com-... 

A similar process may take place in the reduction of polar compounds with single bonds. A halogen, hydroxy, sulfhydryl or amino derivative by accepting an electron dissociates into a radical and an anion. In aprotic solvents the two radicals combine. In the case of halogen derivatives the result is Wurtz synthesis. In the presence of protons the anion is protonated and the radical accepts another electron to form an anion that after protonation gives a hydrocarbon or a product in which the substituent has been replaced by hydrogen. 

Positional Isomerization. A different type of isomerization, substituent migration, takes place when di- and polyalkylbenzenes (naphthalenes, etc.) are treated with acidic catalysts. Similar to the isomerization of alkanes, thermodynamic equilibria of neutral arylalkanes and the corresponding carbocations are different. This difference permits the synthesis of isomers in amounts exceeding thermodynamic equilibrium when appropriate reaction conditions (excess acid, fast hydride transfer) are applied. Most of these studies were carried out in connection with the alkylation of aromatic hydrocarbons, and further details are found in Section 5.1.4. 

Several factors contribute to the frequent use of (3 )-substituted allylic alcohols (13) for asymmetric epoxidation (a) The allylic alcohols are easily prepared (b) conversion to epoxy alcohol normally proceeds with good chemical yield and with better than 95% ee (c) a large variety of functionality in the (3E) position is tolerated by the epoxidation catalyst. Representative epoxy alcohols (14) are summarized in Table 6A.4 [2,4,18,41-53] and Figure 6A.3 (4,54-61], with results divided arbitrarily according to whether the (3E) substituent is a hydrocarbon (Table 6A.4) or otherwise (Fig. 6A.3). The versatility of these and other 3-substi-tuted epoxy alcohols for organic synthesis is illustrated with several examples in the following discussion. 

Most of the bulky ring substituents are simple hydrocarbon moities, either alkyl or aryl groups, and consequently the supra-Cp s can be divided into two classes peralkyl and peraryl systems. The use of bulky cyclopen-tadienes with functional group substituents causes problems in organometallic synthesis. Although there is a sizable number of Cp - metal complexes... 

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