Alkenes stereospecific

In an attempt to synthesize (Z)-l,2-dihalo-l-alkenes from bromoboration adducts of 1-alkynes with B-X-9-BBN, all efforts have been unsuccessful. Fortunately, it has found that the haloboration of 1-alkynes with tribromoborane, followed by the reaction with iodine or bromine chloride in the presence of sodium acetate gives the expected (Z)-l,2-dihalo- 1-alkenes stereospecifically (>98%) in good yields (Eq. 122)187>. 

W-C4H9) [106, 107]. Ab initio MO calculations have shown that the [2 - - 2] cycloaddition between cis- or Jraw-alkenes and isocyanates to yield y9-lactams takes place by a concerted mechanism in the gas phase, with retention of the alkene configuration in the product. However, increasing solvent polarity contributes to the asynchronicity of the reaction to such an extent that the mechanism changes from a concerted to a two-step process involving a zwitterionic intermediate, with a consequent loss of alkene stereospecificity, as observed in the reaction between chlorosulfonyl isocyanates and vinyl ethers [794]. 

The spin state of the reacting acylcarbene has a decisive influence on the reactivity of this short-lived species (Houben-Weyl, Vol.4/5b, pp 1158-1257 Vol. E19b, pp 1052 and 1231 refs 67,100-103). As far as cyclopropanation is concerned, carbene addition to a (Z)- or ( )-alkene can occur with retention or with loss of the stereochemical relationship present in the alkene (stereospecific and nonstereospecific cyclopropanation, respectively). Singlet carbenes add to C-C double bonds in a concerted manner, and therefore, stereospecifically. In contrast, a triplet carbene undergoes stepwise addition via an intermediate triplet 1,3-diradieal. Sinee spin-in-version must occur before ring closure of the latter species, the extent to which stereospecificity... 

The alcohol was protected as its TMS ether, and the C-15,16 alkene stereospecifically dihydroxylated to give compound 50. The diol was then converted to its cyclic sulfate derivative according to the Sharpless protocol.29 Attempted base-catalyzed elimination of the sulfate to introduce the C-14,15 alkene was plagued by side-reactions involving epoxide formation by displacement of the sulfate by the adjacent TMS ether, perhaps aided by enolization of the methyl ketone. Instead, displacement of the sulfate by iodide ion occurred uneventfully to provide 51 as its tetrabutylammonium salt. 

In some instances the vinyl azide does not undergo the conversion to the 2/f-azirine and instead yields a nitrene, which adds to alkenes and dienes. This is seen with the azide 653 which, on irradiation through Pyrex, affords the corresponding nitrene that adds to alkenes stereospecifically to yield aziridines. Stereo specific addition to dienes is exemplified by the irradiation of the quinone azide 654 in the presence of trans,trans-hQX3,-2,4-diene. Aziridines do not result from this process and instead the stereoisomeric indole derivatives 655 and 656 are obtained. This process has been developed as a route to... 

Until the last decade, product studies formed the main evidence for carbene formation singlet carbenes formed cyclopropanes from alkenes stereospecifically, while triplet carbenes formed cyclopropanes non-stereospecifically. Formation of a cyclopropane (though not by addition to an alkene) via a carbocation route was demonstrated and, more recently, it has been shown that p values for insertion-addition selectivity and for cyclopropanation stereoselectivity vary as to photochemical or thermal generation of the carbene. The authors of this latter study suggest that a ground state diazo compound could be masquerading as a carbene in its thermal reaction with olefins, possibly by electrocyclic... 

A new and convenient method of generating fluorocarbene involves photolysis of fluorodi-iodomethane at 350 nm. In the presence of an alkene, stereospecific fluoro-cyclopropane syntheses can be effected in modest (15—45%) yields. The carbene... 

Another useful synthetic approach to alkenes involves starting with an alkyne. This is particularly useful if we need to produce a cis- or traws-alkene stereospecifically we have available to us the reactions of Figure 19.10, which accomplish this. The process is more commonly used to produce cw-isomers, as they are more difficult to prepare in other ways, being thermodynamically less stable. The alkyne may then be discoimected to give the dianion of ethyne and two alkyl halides or other appropriate electrophile. Practical examples are provided in Figure 19.11. 

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