CuI‐Catalyzed Alkyne–Azide “Click” Cycloadditions from a Mechanistic and Synthetic Perspective

Publisher Summary This chapter explores that the term “four-electron donor,” which is used to describe the alkyne ligand in circumstances where alkyne π ⊥ donation supplements classic metal–olefin bonding. The utility of this scheme lies in its simplicity, and with some reluctance is relied on the “four-electron donor” terminology to suggest global properties of metal alkyne monomers. The general implications and specific hazards characterizing these descriptors are typical of broad classification schemes in chemistry—they are often conceptually helpful but seldom specifically correct. Criteria for recognizing four-electron alkyne donation encompass stoichiometry, structure, spectra, and reactivity. The chapter reviews that the chemistry that has been developed for molybdenum (II) and tungsten (II) alkyne monomers encompasses syntheses, structures, spectra, molecular orbital descriptions, and reactions. The Mo (II) and W (II) complexes addressed in the chapter are not unique in terms of alkyne π ⊥ donation. Related alkyne chemistry is appearing for d4 metals other than molybdenum and tungsten, as well as for d 2 complexes in general. The chapter also examines that chromium alkyne chemistry and reflects the importance of π ⊥ donation, but the stoichiometries differ from those of heavier Group VI monomers.

Four-electron alkyne ligands in molybdenum(II) and tungsten(II) complexes

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Metallaaromatic Chemistry: History and Development.

This section will address transition metal mediated cycloaddition processes that fall into the [2 + 2 + 2] category: reactions involving three components, each of which formally contributes two electrons to-wards the final product of cycloaddition Since the designation (in square brackets) refers to the electrons and not the atoms contributed by each cycloaddition component, reactions involving cycloaddition participants such as CO, isonitriles and heteroatoms are included Products formed will include four-membered rings from 2 + 1 + 1 cycloaddition (atom, not electron designation), five-membered rings from 2 + 2 + 1 cycloaddition, and, of course, six-membered rings from 2 + 2 + 2 processes Several reactions which fall into the formal [2 + 2 + 2] category are covered elsewhere: the Pauson-Khand (2 + 2 + 1) reaction (Volume 5, Chapter 91), 2 + 1 + 1 and 2 + 2 + 1 cycloadditions of metal carbenes (Volume 5, Chapter 92), and zirconium-promoted 2 + 2 + 1 bicyclization of enynes (Volume 5, Chapter 95)

[2 + 2 + 2] Cycloadditions

Organic reactions with rhodium carbonyl cluster catalysts

Reaction of [MR(CO)3(η5-C5H5)](M = Mo, W; R = Me, CH2Ph, COCF3) with but-2-yne affords either the vinylketone complexes [[graphic omitted])R(CO)2(η5-C5H5)], which on treatment with ligands L form lactone complexes [M(CO)L(η3-{[graphic omitted]Me})(η5C5H5)], or the eight-membered ring complex [[graphic omitted](CO)(η5-C5H5)]; the identity of the products is established by X-ray crystallography.

Reactions of acyl- and alkyl-(tricarbonyl)(η5-cyclopentadienyl)-molybdenum and -tungsten complexes with but-2-yne; molecular structures of [Mo(CO)(ButNC)(η3-{O·C(O)·CMe·CMe·CMe})(η5-C5H5)] and [Mo·C(O)·C(Me){C(Me)}2C(Me)·C(CF3)·O(CO)(η5-C5H5)]

Formation of ?3-bonded lactone complexes and eight-membered ring metallacycles with ketene groups by the insertion of acetylenes into carbon?molybdenum and ?tungsten ? bonds; molecular and crystal structures of carbonyl(?-cyclopentadienyl)(3?5-?-2,3-dihydro-3,4,5-trimethyl-2-oxo-3-furyl)(t-butyl isocyanide)molybdenum and carbonyl-(?-cyclopentadienyl)(3?4:5?6-?-2,3,4,5-tetramethyl-6-oxo-1-trifluoromethylhexa-1,3,5-trienyloxo)molybdenum

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Reactions of acyl- and alkyl-(tricarbonyl)(η5-cyclopentadienyl)-molybdenum and -tungsten complexes with but-2-yne; molecular structures of [Mo(CO)(ButNC)(η3-{O·C(O)·CMe·CMe·CMe})(η5-C5H5)] and [Mo·C(O)·C(Me){C(Me)}2C(Me)·C(CF3)·O(CO)(η5-C5H5)]