Allylic chlorides, reactions with isoprene

Figure 8 shows an application of this reaction type in natural product synthesis. Under conditions of concentration control (rule A), the reaction of the cyclohexenyl chlorides 8 with isoprene terminates at the 1 1 product stage, since the terminally trialkylated allyl cations formed from 8 are better stabilized than the terminally dialkylated allyl cations, which arise from 9. Cyclization of 9 and successive treatment with KOtBu yields a mixture of 6- and Ya- muurolene. ... 

Addition of hydrogen chloride to isoprene gives prenyl chloride, together with some of the allylic isomer 2-chloro-2-methylbut-3-ene. The presence of this tertiary chloride is not deleterious since, whilst prenyl chloride reacts with acetone via an SN2 reaction, it reacts by an Sn2 mechanism to give the same product, methylheptenone (Scheme 4.8). Further elaboration to linalool, etc., is the same as in the processes... 

Silicon has also been used to control the acylation of 1,3-dienes in a manner analogous to the situation with alkenes. Isoprenylation with 2-trimethylsilylmethylbuta-1,3-diene follows the course expected of acylation of the allylic silane. In these very rapid reactions, titanium tetrachloride seems to be one of the more efficient catalysts, as is aluminum chloride. The method was used in synthetic approaches to the terpenes ipsenol (14) and ipsdienol (15 Scheme 16). Of particular interest is the comparison of this iso-prenylating agent with isoprene itself. The examples of Friedel-Crafts acylations cited show the regio-control that can be achieved by suitable choice of substrate. 

As an example of carbometallation, the 1,4-carbosilylation product 218 is obtained by the reaction of dienes, disilanes and acid chlorides of aromatic and a,/i-unsaturatcd acids at 80 °C. The phenylpalladium 216 is formed by the oxidative addition of benzoyl chloride, followed by facile decarbonylation at 80 °C, and reacts with butadiene to generate the benzyl-7i-allylic complex 217. Then, transmetallation with the disilane and reductive elimination afford 4-silyl-2-butenylbenzene 218 [92], Regioselective carbomagnesation of isoprene with allylic magnesium bromide 219 catalysed by Cp2TiCl2 gives 220, which is useful for terpene synthesis [93,94],... 

Cyclopentadienyl) (or related hgand). Treatment of (ry -allyl)dicarbonylnitrosyliron complexes (see Nitrosyl Complexes) with phosphite or phosphine gives five-coordinate ()] -allyl) complexes (22) of limited stability. The reaction of Fe(t-BuNC)5 with both aUyl bromide and chloride gives a product formulated as (23). Compounds of the general stracture (24) were prepared by reaction of [(CO)4Fe(SiR3)] and HFe(CO)4(SiR3) with allyl bromide and isoprene, respectively. ... 

Other allyl metal complexes can be used to generate carbon bonds, including allyl zinc complexes. Stille showed that 3-methyl-l-bromo-2-butene reacted with zinc chloride (ZnCl2) to generate the it-allyl zinc species. Coupling required the use of an organotin species such as a-trimethyltin isoprene (466). Reaction in refluxing THF led to a 94% yield of myrcene (467).303... 

---