Addition of X-Y Reagents

A regioselective iodoperfluoroalkylation of terminal alkynes (R—C = CH) has been reported, and is based on photolysis ofthe C—I bond in perfluoroalkyl iodides (Rp-I). Addition of the thus-formed RF radical onto the alkyne afforded a vinyl radical that in turn abstracts an iodine atom from the starting Rp—I to form the end olefin R-C(I)= CH-Rf. A xenon lamp through Pyrex (hv 300 nm) was used for the reaction, where aliphatic alkynes gave a better alkylation yield with respect to phenylacetylene [81], 

In this chapter we have highlighted a diverse range of mild and efficient photochemical methods for the intermolecular addition of different molecules onto carbon-carbon multiple bonds. The examples reported demonstrate that, in several cases, C— X bonds were formed through highly selective and efficient photochemical processes. In many cases, the procedures described compared favorably with thermal analogues, allowing a controlled synthetic approach to a wide variety of organic products. 

In addition, the above-discussed photochemical methods can be thought of as offering a dean (or green ) alternative method [84,85], which represents an active area of current research. 

The obvious practical relevance and potential economic benefits of using light as a cheap, readily available, nontoxic reagent for bond-forming reactions should suffice to drive research towards all aspects of the photochemistry. Accordingly, this chapter may hopefully lead readers to embark on new research projects that further enhance such utility, and to promote the more widespread use of photochemical processes in organic chemistry. 

1 Semmelhack, M.F.(vol. ed.) (1991) Additions to and substitutions at C—C Jt-bonds, in Comprehensive Organic Synthesis, Selectivity, Strategy el Efficiency in Modem Organic Chemistry, Vol. 4 

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E. Block, A. L. Schwan, Electrophilic Addition of X-Y Reagents to Alkenes and Alkynes, in Comprehensive Organic Synthesis (B. M. Trost, I. Fleming, Eds.), Vol. 4, 329, Pergamon Press, Oxford, 1991. 

Addition of various organometalic reagents to chiral nitrones, derived from L-erythrulose, proceeds with variable diastereoselectivity, depending on Lewis acids as additives. ZnBr2 facilitates the attack at the Si face of the C=N bond, whereas Et2AlCl makes the attack at the Re face more preferable. The obtained adducts can be transformed into derivatives of /V - h y d r o x y - u. u - d i s u b s t i t u t e d -a-amino acids, with their further conversion into a,a-disubstituted a-amino acids (193, 202). 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

Addition and substitution reactions of heteroatom-centered radicals with multiple bonds have been extensively studied and are sometimes preparatively useful.11 This section will briefly consider the addition reactions of H—Y and X—Y reagents (Kharasch reactions) and substitution reactions (Scheme S6).245... 

If the addition of an unsymmetrical reagent X-Y to an unsymmetrical olefin proceeds without skeletal rearrangement to give exclusively one of two or more possible isomers it is termed regiospecific. If, however, there is a significant preponderance of one isomer the reaction is said to be regioselective (Sec Ref. 358). 

The last entry of Thble 1-1 is detailed in (1-8) a two-step nucleophilic aliphatic substitution in which the loss of the leaving group Y is concerted with cyclization involving the group Z in position 2, the latter moving in the second step to the position 1, a process that is again concerted with addition of the nucleophilic reagent X. 

For general reviews on aUylations, see (a) S. E. Denmark, J. Fu, Chem. Rev. 2003, 103, 2763-2793. Catalytic enantioselective addition of aUylic organometaUic reagents to aldehydes and ketones, (b) C. E. Masse, J. S. Panek, Chem. Rev. 1995, 95, 1293-1316. Diastereoselective reactions of chiral allyl-and aUenylsilanes with activated C=X n bonds, (c) Y. Yamamoto, N. Asao, Chem. Rev. 1993, 93, 2207-2293. Selective reactions using aUylic metals. 

Addition (Section 6 1) Reaction in which a reagent X—Y adds to a multiple bond so that X becomes attached to one of the carbons of the multiple bond and Y to the other 1 2 Addition (Section 10 10) Addition of reagents of the type X—Y to conjugated dienes in which X and Y add to adja cent doubly bonded carbons... 

Addition (Section 10 10) Addition of reagents of the type X—Y to conjugated dienes in which X and Y add to the ter mini of the diene system... 

Anti addition (Section 6 3) Addition reaction m which the two portions of the attacking reagent X—Y add to opposite faces of the double bond... 

Entry X Y n Step 1 (Michael addition) reagents and conditions Yield of 492 (%) Step 2 IMDAF) solvent and conditions Yield of 65+66 (%) 65 66... 

The more activated the ester, the less stable is the compound. All the esters mentioned above can be used as shelf-stable reagents except benzotriazolyl esters, which decompose too readily. In addition to their use as activated forms of the A - a I ko x y ca r bo n y I am i n o acids, the esters derived from hydroxamic acids are implicated as intermediates in coupling reactions in which the A-hydroxy compounds have been added to promote efficient coupling between an acid and a primary or secondary amine (see Section 2.10). It is pertinent to mention that the O-acylisourea generated from carbodiimides (see Section 2.02) is an activated ester but one of nature different than those alluded to above. 

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