Alkenyl organometallics

Halogenation of alkenyl organometallic compounds Addition of hydrogen halides to triple bonds Halogenation of alkynes or allenes Addition of alkyl halides to triple bonds Addition of acyl halides to triple bonds... 

This method is of value when the alcohol is readily available from natural sources, or when it can be prepared, for example, by the reaction of an alkenyl-organometallic reagent with an aldehyde. An example of the former is the oxidation of the terpenoid alcohol carveol to carvone (Expt 5.88) using pyridinium chlorochromate-on-alumina reagent. 

The cross-coupling of halopyridines with aryl and alkenyl organometallic reagents in the presence of palla-... 

However, their nonstatistical distribution as well as retention of stereochemistry if alkenyl organometallics were used (see Scheme 10.14),12 suggested that free radicals were not intermediates in such reactions. Such Wurtz-type couplings therefore probably proceeded by transmetallation and then recombination (Scheme 10.1). 

Athough transition metal alkylidene complexes are successfully used for the alkenation of carbonyl compounds, various 1,1-bimetalloalkanes, often prepared by the hydrometal-lation of alkenyl organometallics, are also useful reagents for the alkenation of carbonyl compounds. 

Substituted gem-dimetallic compounds, readily obtained via allylation of alkenyl organometallics, react with aldehydes in the presence of BF3-Et20 but do not react with ketones. When alkylidenemalonates are used instead of aldehydes, the Z-olefins see (E) (Z) Isomers) are obtained with a very high stereoselectivity (Scheme 35). A transmetallation reaction with copper cyanide significantly increases the reactivity of gm-dimetallic derivatives via formation of 1,1-zinca cyanocuprates. Indeed, when these compounds react with... 

In general, the trans-l-alkenyl organometallic reagents are prepared from the corresponding terminal acetylene (e.g., 77 and 79, Scheme 14). These intermediates are available by condensation of the appropriate aldehyde or ketone with ethynyl... 

For the reasons outlined last year (5,162), explorations of the reaction of various 2-alkenyl organometallic species with aldehydes and ketones to produce homoallylic alcohols, and its stereochemical outcome, have continued unabated. Two new approaches, shown in Scheme 20, are both analogous to Grignard reactions in one, allyl iodide is treated with cerium amalgam in the presence of a ketone, and in the other allylic bromides or iodides in the presence of metallic tin react with allylic inversion with a wide range of aldehydes and... 

Studies on the stereochemical outcome of the reaction of 2-alkenyl organometallic reagents with aldehydes have increased recently, largely because stereoselectively produced homoallylic alcohols are synthetically equivalent, by cleavage at the carbon-carbon double bond, to the type of aldol adduct stereoisomers required for macrolide antibiotic total synthesis. A new stereoselective synthesis of (Z)-2-alkenyltins (46a) or the corresponding silanes (46b) from allyl or vinyl 9-BBN derivatives has appeared (Scheme 20). The... 

The three schemes above all involve allyl transposition in the addition to aldehydes, and the general pattern observed (see also 4,149) is that (Z)-2-alkenyl organometallics favour ery//iro-addition whereas the ( )-2-alkenyl isomers give r/iren-selectivity. These results are rationalized by assuming 6-membered cyclic chair transition states as illustrated in (52) and (53), respectively. The expected preference for substituents to take up equatorial rather... 

Alkenyl halides, however, can react through the intermediate formation of alkenyl organometallics (see Exercise 11-6). These species allow access to a variety of spedficaUy substituted alkenes. 

---