Additions to Allenes and Alkynes

The reactivity of mercury salts is a fimction of both the solvent and the counterion in the mercury salt. Mercuric chloride, for example, is unreactive, and mercuric acetate is usually used. When higher reactivity is required, salts of electronegatively substituted carboxylic acids such as mercuric trifiuoroacetate can be used. Mercuric nitrate and mercuric perchlorate are also highly reactive. Soft anions reduce the reactivity of the Hg son by coordination, which reduces the electrophilicity of the cation. The harder oxygen anions leave the mercuric ion in a more reactive state. Organomercury compounds have a number of valuable synthetic applications, and these will be discussed in Chapter 8 of Part B. 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond  

The three basic mechanisms that have been considered to be involved in electrophilic additions to alkynes are shown below. The first involves a discrete vinyl cation. In general, it can lead to either of the two stereoisomeric addition products. The second mechanism is a termolecular process which would be expected to lead to stereospecific anti addition. The 

Schmid, The Chemistry of the Carbon-Carbon Triple Bond, Part 1, S. Patai, ed., John Wiley Sons, New Wirk, 1978, Chapter 3. 

Further details must be added for a complete description, but these outlines encompass most reactions of alkynes with simple electrophiles. 

The electrophile in oxymercuration reactions, HgX or Hg, is a soft acid and strongly polarizing. It polarizes the tt electrons of an alkene to the extent that a three-center, two-electron bond is formed between mercury and the two carbons of the double bond. A three-center, two-electron bond implies weaker bridging in the mercurinium ion than in the three-center, four-electron bonding of the bromonium ion. Oxymercuration of simple alkenes is usually a stereospecific anti addition. This result is consistent with the involvement of a mercurinium intermediate which is opened by nucleophilic attack. 

The basic mechanisms that are considered to be involved in electrophilic additions to alkynes are outlined below. The first involves a discrete vinyl cation. In general, this reaction will lead to a mixture of the two stereoisomeric addition products. Mechanisms B and C depict bridged intermediates formed without (B) or with (C) participation of a second electrophilic molecule. Mechanisms B and C should lead to anti addition. Mechanism D is a termolecular process that would be expected to be a stereospecific anti addition. Mechanisms A and B are of the Adg2 type, whereas C and D are classified as Ad S. Each of these mechanisms may involve a prior complex formation between the alkyne and an electrophile. 

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Organometallic complexes of the /-elements have been reported that will perform both intra-and intermolecular hydroamination reactions of alkenes and alkynes, although these lie outside of the scope of this review.149-155 Early transition metal catalysts are not very common, although a number of organometallic systems exist.156-158 In these and other cases, the intermediacy of a metal imido complex LnM=NR was proposed.159,160 Such a species has recently been isolated (53) and used as a direct catalyst precursor for N-H addition to alkynes and allenes (Scheme 35).161,162... 

Addition to alkynes and allenes. An ene-type reaction between alkynes and alkenes proceeds regioselectively at room temperature with [CpRufMeCNi lPFf,. In the presence of bromide salt(s) an atom-economical coupling of 1-alkynes and enones proceeds. ... 

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