Metallacycles

Question 1

What are metallacycles and some of their features?

Answer 1

• defined as carbon rings that include a metal
• may be synthesized by a number of different routes but in this section we will concentrate
  on their formation by the linking of alkenes, alkynes and CO within the coordination
  sphere of the metal
• although the bonding is readily described in terms of simple s- and p-bonds the chemical
  properties of metallacycles often go beyond those of metal alkyl and vinyl complexes

Question 2

Draw and describe the mechanism of ethene and a Ni(0) centre

Answer 2

• in forming the metallacycle the oxidation state of the metal has increased by +2, so that
  this reaction is sometimes referred to as oxidative coupling (and the reverse reaction
  may be referred to as reductive extrusion)
• coupling of two alkenes with the metal affords a metallacyclopentane

Question 3

What are other products with the synthesis of metallacyclopentane?

Answer 3

• in general metallacyclopentanes are stabilized with respect to b-elimination because it is difficult for the four atoms
  M-C-C-H to become coplanar (compare with M–CH2CH3)
• nevertheless, under some conditions this reaction can take place to afford 1-butene
• other reactions of the metallacyclopentane are possible including the reductive extrusion of cyclobutane, or the reversal of the original reaction and the release of two equivalents of alkene

Question 4

Draw 2 examples of Cobalt cyclization reactions

Answer 4

Question 5

What else does metallacycle formation include?

Answer 5

carbonyl ligands may be incorporated in the metallacycle (note that only the carbon-atom is part of the ring), as may nitriles, isonitriles and other unsaturated ligands

Question 6

Draw the Pauson-Khand synthesis of cyclopentenones

Answer 6

Question 7

Draw the catalytic synthesis of pyridines from alkynes and nitriles

Answer 7

Question 8

Draw the mechanism for metallacycles in metathesis

Answer 8