Metalation alkynes

Melikyan GG, Nicholas KM (1995) The Chemistry of Metal-Alkyne Complexes. In Stang... 

Finally, the reaction of (L)AuX complexes with metallated alkynes in an inert solvent and at low temperature is a particularly safe pathway to complexes with more delicate components (X = halogen M = alkali metal Equation (21)).74 75... 

Schore, N. E. Transition Metal Alkyne Complexes Pauson-Khand Reaction. In Comprehensive Organometallic Chemistry II Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds. Elsevier Oxford, 1995 Vol. 12, pp 703-739. 

The vinyl metal intermediate arising from intermolecular nucleophilic addition of an oxygen nucleophile to a metal-alkyne complex has been harnessed for further transformations prior to protonation. An example is the ruthenium-catalyzed benzannulation of 1,5-enedyines that occurs through a tandem sequence involving hydroalk-oxylation, carbometallation, and protonation (Equation (82)).293... 

The first complexes to be described were obtained from reactions between MCI2CP2 and metallated alkynes and initially formulated as the dimers M(/r-C=CR)Cp 2. Further studies have clarified the nature of these complexes together with those formed from RC CC CR (R = Me, Bu, Ph, SiMe3) and MCp2. The several structural types obtained contain 1 /1,1 /2, 2/1, and 2/2 ratios of MCp2 to diyne, as shown in Scheme 48. The nature of the complexes formed depends on diyne substituent and metal. 

Schore NE (1991) Org React 40 1 Schore NE (1991) The Pauson-Khand reaction. In Trost BM (ed) Comprehensive organic synthesis, vol 5. Pergamon Press, Oxford, p 1037 Pauson PL (1985) Tetrahedron 41 5855 Schore NE (1995) Transition metal alkyne complexes Pauson-Khand reaction. In Abel EW, Stone EGA, Wilkinson G (eds) Comprehensive organometallic chemistry II, vol 12. Pergamon Press, Oxford, p 703 Iwasawa N (1992) Chem Lett 473... 

The nucleophilic displacement of the acetoxy group of 4-acetoxy-l,3-dioxanes was also effected by metalated alkynes. Organoaluminium and organotin compounds have been employed <1998TL3103>. The stereochemical outcome is similar to that of the analogous reaction with a high preference for the 7 //-product (Equation 45). 

Anodic oxidations of the Mo(II) bis-cyclopentadienyl alkyne compound Mo(jj -C5H5)2( i -C2Ph2), and of metal-alkene analogs such as Mo(j -C5H5)2(j7 -C2H4) involve a reversible one-electron oxidation at very facile potentials. The v(CC) alkyne stretch in the IR spectrum of 17-electron Mo-alkyne cation (1824 cm ) was shifted by 50cm to a higher wave number to that of its parent, consistent with a decrease in the metal-alkyne interaction in the Mo(III) cation. 

In fact, this approach constituted one of the earliest trials. One is introduction of a chiral ligand like glyphos on cobalt by replacing one of the carbon monoxides to obtain an enantiomerically enriched new metal-alkyne complex. The second approach is use of the propargyl alcohol bearing a chiral auxiliary. Replacement of only one carbon monoxide would lead to a mixture of diastereomers. In both cases, a I I mixture of products was obtained, and each diastereomer was separated before the... 

Reactivity of allylic organozinc reagents towards metallated alkynes 936... 

It is also possible to achieve the formation of five-membered rings by coupling the allylzincation of a metallated alkyne with an intramolecular nucleophilic substitution. Thus, allylzincation of the ethylzinc alkynylide generated from 5-iodo-l-pentyne (66)45, or direct treatment of the latter with excess crotylzinc chloride177, led to the alkenyl... 

Nevertheless, chiral propargylic amines remain interesting substrates for achieving the diastereoselective addition of substituted allylic organozinc compounds to metallated alkynes. Besides crotylzincation, one example of diastereoselective addition of zincated allyl ethyl ether to 328 has also been reported183. 

Although the cyclization of allenylzincs to alkynylsilanes proceeded quite efficiently, it was demonstrated that a metallated alkyne was an even much higher reactive enophilic partner. Treatment of the diyne 360 with s-BuLi in ether at room temperature resulted in... 

Bromoalkynes, which are important intermediates in coupling reactions,241 can be synthesized through the corresponding metalated alkynes 241,242... 

In contrast to many stable transition metal-alkyne complexes, cationic cyclopentadienyliron-alkyne complexes are reactive toward a range of nucleophiles. However, since most of the nucleophiles studied were carbanions, discussion of this chemistry is deferred to Volume 4, Chapter 3.2, Section 3.2.3. 

Once an alkyne moiety is introduced into a peptide, e.g., as propargyl glycine (Pgl) in place of a simple glycine in Enk, it can of course be used directly for complexation to a metal complex. Along those lines, we have reacted Co2(CO)g with the peptide Ac-Tyr-Gly-Pgl-Phe-Leu-NH2, to yield the Co2(CO)6 complexed to the Pgl alkyne side chain [32]. In a real SPPS scheme with a pre-formed metal-alkyne complex, the amino acid building block [Tp WI(CO)(ri2-Fmoc-Pgl-OH)] was incorporated to yield the side-chain substituted Enk derivative H-Tyr-Gly-[Tp W(I)(CO)(r 2-Pgl)]-Phe-Leu-OH (Tp tris(3,5-dimethylpyrazolyl)-borate) [44]. 

The term four-electron donor is sometimes used to describe the alkyne ligand in circumstances where alkyne ir donation supplements classic metal-olefin bonding. The utility of this scheme lies in its simplicity, and with some reluctance we shall rely on the four-electron donor terminology to suggest global properties of metal alkyne monomers. The general implications and specific hazards characterizing these descriptors... 

The metal-alkyne bonding parameters of molecules of the type CpML-(RC=CR)X are similar to those of other classes of four-electron donor alkynes in Table I. Both cationic and neutral derivatives with cyclopen-tadienyl (Cp = t75-C5H5) or indenyl (tj5-C9H7) illustrate the structural regularities of these molecules. The preponderance of molybdenum derivatives relative to tungsten reflects more extensive and diverse synthetic chemistry with molybdenum to date. 

Two potentially informative infrared probes in alkyne complexes with ancillary carbonyl ligands are (1) v(C=C) absorptions and (2) v(CO) absorptions. The first provides direct information about the carbon-carbon multiple bond of the bound alkyne ligand, but it is usually weak and has proven particularly difficult to detect in M(CO)(RC=CR)(S2CNEt2)2 complexes (57). The second provides only indirect information about the metal-alkyne moiety, but its intensity and unambiguous assignment as y(CO) have stimulated an extensive literature data bank to assist interpretation. 

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