Complexes Containing Acyclic Alkenes

No other type of organic compound produces such a wide variety of multiplet types over such a large chemical shift range as the acyclic alkenes. Proton-proton coupling through four bonds is common. Many samples are found to contain both cis and trans isomers producing a spectrum more complex than might be expected from a proposed structure. 

This survey of the 2000 literature relating to rr-hydrocarbon complexes of the transition elements other than p-CsHs and r -arene complexes is similar in nature to previous reports. This chapter is sub-divided into the following sections dealing with reviews complexes containing allyls or monoalkenes unconjugated alkenes conjugated alkenes acyclic alkenes alkynes and polymetallic complexes. 

The report by Basset and co-workers on the metathesis of sulphur-containing alkenes using a tungsten alkylidene complex, mentioned previously for the acyclic cross-metathesis reaction (see Sect. 2.2), also contained early examples of ring-opening cross-metathesis of functionalised alkenes [20]. Allyl methyl sulphide was reacted with norbornene in the presence of the tungsten catalyst 5, to yield the desired ring-opened diene 35 (Eq. 29). 

Selective conversion of acyclic dienynes to fused bicyclic rings containing five-, six- and seven-membered rings is efficiently catalysed by Ru complex 22. The reaction of dienyne 160 having two terminal alkene chains gave the two products 162 and 164... 

Previously in Chapter 12 we have seen several examples of cydization reactions that have involved transition metal catalysis. In Chapter 11, we introduced Mo- and Ru-catalyzed RCM as a means of converting acyclic dienes, alkene-alkynes, and dialkynes into rings containing carbon-carbon double and triple bonds. Section 12-5 will cover a few cases where organotransition metal complexes effectively promote the construction of rings where two or more C-C bond connections occur during the same transformation. Some examples will be extensions of reactions already covered, whereas others will entail new chemistry. 

More complex products are obtained from cyclizations in which the oxidizable functionality and the alkene are present in the same molecule. y9-Keto esters have been used extensively for Mn(III)-based oxidative cyclizations and react with Mn(OAc)3 at room temperature or slightly above [4, 10, 11, 15], They may be cyclic or acyclic and may be a-unsubstituted or may contain an a-alkyl or chloro substituent. Cycloalkanones are formed if the unsaturated chain is attached to the ketone. y-Lactones are formed from allylic acetoacetates [10, 11]. Less acidic /3-keto amides have recently been used for the formation of lactams or cycloalkanones [37]. Malonic esters have also been widely used and form radicals at 60-80 °C. Cycloalkanes are formed if an unsaturated chain is attached to the a-position. y-Lactones are formed from allylic malonates [10, 11]. yff-Diketones have been used with some success for cyclizations to both alkenes and aromatic rings [10, 11]. Other acidic carbonyl compounds such as fi-keto acids, /3-keto sulfoxides, j8-keto sulfones, and P-nitro ketones have seen limited use [10, 11]. We have recently found that oxidative cyclizations of unsaturated ketones can be carried out in high yield in acetic acid at 80 °C if the ketone selectively enolizes to one side and the product cannot enolize... 

In addition to their thermodynamic propensity to dimerize [16], free acyclic car-benes containing alkyl groups are prone to decomposition via intramolecular C—H insertion reactions that lead to net elimination of an alkene (Scheme 16.3a) [35]. Loss of two alkene equivalents has been observed to occur from an Alder-type ADC bound to W or Mo tetracarbonyl fragments, resulting in conversion to an amidine ligand (Scheme 16.3b) [36]. This process appears to be limited to zerovalent metal complexes and may be facihtated by the unusual -(C,N) binding mode of the carbene. 

---