Alkynes n complexation

Although alkyne n complexes of group 10 (VIII) and 11 (IB) metals have been known for many years, recent work in this field has revealed that the... 

Scheme 26. Exo-endo dig cyclization modes involving nucleophilic attack of the arene upon the metal-alkyne n complex.

The more complex [2 -f 2 + 2] cycloisomerization reaction of acetylene units is also catalyzed by transition metal-alkyne n complexation and can be readily utilized for the synthesis of a variety of polysubstimted benzene derivatives in a straightforward manner (10, 352, 353). Recently, this methodology has been applied to the cyclization of 15-membered, nitrogen-containing di- and triacetylenic macrocycles. Upon coordination with Pd(0) to the triacetylenic macrocycle at ambient temperature, the ti-coordinated Pd(0) complex results. Subsequent refluxing of this species in toluene promotes cycloisomerization to the hexasubstituted arene (354) (Scheme 28). The Rh(I) [e.g., RhCl(CO)(PPh3)2] complex also catalyzes these same transformations in high (>80%) yields. 

Synthetic access to in situ generated metallocenes, especially of the titanium triad but also its neighbors, has given a growing number of reactive alkene and alkyne n-complexes that readily transform according to Eq. (b)i5-2o,23,96-98 chemical... 

The magnitude of coordinative distortions can be taken as an indication of the importance of the above-mentioned bonding components [5]. We can illustrate three extreme situations with complexes 12-14 (Scheme 4-4). Thus, the alkyne unit in 12, the first lan-thanide-alkyne n-complex [6], shows very little distortion from the free ligand (a = 177.4°, / = 1.15 A vs. 1.21 A in free butyne), consistent with nearly exclusive contribution from n (alkyne) M and minimal backbonding (the metal is d°) the rare main group-alkyne... 

A stoichiometric reaction of tetrakis(triphenylphosphine)platinum(0) with bis(pinacolato)diboron gives cis-diborylplatinum(n) complex in high yield (Scheme 3).38 The diborylplatinum complex then reacts with an alkyne, giving m-diboration product.40,41 These results indicate that the diboration proceeds through the general mechanism shown in Scheme 1 (E1 = E2 = Bpin), which involves the formation of diborylplatinum(n), insertion of an alkyne into the B-Pt bond, and reductive elimination. 

In relation to the mechanistic proposal, an interesting reactivity of (boryl)(silyl)platinum(n) complex has been reported.223 The complex is prepared by the reaction of silylborane with Pt(cod)2 complex via oxidative addition (Scheme 46). The (boryl)(silyl)platinum complex undergoes insertion of alkynes at the B-Pt bond to give (/3-borylalkenyl)(silyl)platinum(n) complex in high yield. Importantly, the insertion takes place regioselectively, with Pt-G bond formation at the internal. -carbon atom. This result may indicate that the boron-transition metal bond is more prone to undergo insertion of unsaturated molecules. 

A thermodynamically stable (silyl)(stannyl)palladium(n) complex is synthesized by an oxidative addition of the Si-Sn linkage to palladium(O) (Scheme 63).267 The complex has the square-planar geometry with a m-arrangement of the silicon and tin atoms. An alkyne reacts with the complex to afford a silastannated product as a mixture of cisjtrans stereoisomers (10 1). 

Coordination of Ni(0) to the alkyne gives a n complex, which can be written in its Ni(II) resonance form. Coordination and insertion of another alkyne forms the new C6-C7 bond and gives a nickelacyclopenta-diene. Maleimide may react with the metallacycle by coordination, insertion, and reductive elimination to give a cyclohexadiene. Alternatively, [4+2] cycloaddition to the metallacycle followed by retro [4+1] cycloaddtion to expel Ni(0) gives the same cyclohexadiene. The cyclohexadiene can undergo Diels-Alder reaction with another equivalent of maleimide to give the observed product. 

On the basis of the above-mentioned calculations it seems that coordination chemistry is a viable alternative to stabilize this heterocumulene. However, the experimental access to metal complexes containing the tricarbon monoxide ligand remains a challenge. Thus, to date, the coordination chemistry of C3O is confined to [Cr(=C=C=C=0)(C0)s] (89), obtained by treatment of [n-Bu4N] [CrI(CO)5] with the silver acetylide derived of sodium propiolate in the presence of Ag" (Scheme 28) [105]. Reaction of the presumed Tt-alkyne intermediate complex 88 with thiophosgene generates the heterocumulene 89. Neither structural nor reactivity studies were undertaken with this complex. 

The reaction using 11a as a substrate in the presence of several oxides as additives revealed that addition of tributylphosphine oxide, hexamethylphos-phoric triamide, and dimethyl sulfoxide all accelerate the reaction considerably. Furthermore, when about 10 molar amounts of N-methylmorpholine M-oxide (NMO) is added to the alkyne-cobalt complex 12b in THF,the reaction proceeds even at room temperature and cyclopentenone 13 b is obtained in 37% yield accompanied by another rearranged product, the methylenecyclobutanone 35, obtained in 23% yield as a mixture of ( )-and (Z)-isomers (Scheme 14). These facts indicate that dissociation of the carbonyl ligand of the alkyne-cobalt complex 12 is the rate-determining step in this rearrangement. This is also supported by the fact that under a CO atmosphere in refluxing THF the reaction is completely suppressed. 

H, Me, r-Bu, or Ph or R = H and R = Me, r-Bu, or Ph), was performed. Two possible reactions were investigated (a) the reactions suitable for the gas-phase interactions, which start from a 1 1 Br2-alkyne r-complex and do not enter into a 2 1 Br2-alkyne jt-complex and (b) the processes passing through a 2 1 Br2-alkyne 7r-complex, which appear more realistic for the reactions in solutions. The structures of the reactants and (g) the final products and also the possible stable intermediates have been optimized and the transition states for the predicted process have been found. Both trans- and cw-dibromoalkenes may ensue without the formation of ionic intermediates from a n-complex of two bromine molecules with the alkyne (solution reactions). The geometry around the double bond formed in dibromoalkenes strongly depends on the nature of the substituents at the triple bond. The cluster model was used for the prediction of the solvent influence on the value of the activation barrier for the bromination of the but-2-yne.35... 

Mononuclear complexes [U(C5Me5)2(NHR)2] (R = 2,6-dimethylphenyl, Et, or Bu) have been synthesized and structurally characterized. It was shown that in the presence of terminal alkynes and amines these complexes catalyze the intermolecular hydroamination of terminal alkynes [453]. Complex formation reactions of U(VI) with neutral N-donors in DMSO were reported [454]. 

Scheme 5. Proposed reaction pathway leading to C=X bond cleavage (X = NAr and O) based on analogous reactions employing alkynes. The essential features of the reaction sequence are (i) 1 1 adduct (n complex) formation (ii) C—C bond formation between coordinated C=X and u-CSiMe3 (iii) OR group transference between tungsten atoms and C—X bond cleavage resulting in (iv) the W=X and

To our knowledge there is no example in the literature of a stable alkene (or alkyne) complex with magnesium as central atom, n Complexes have, however, been postulated as intermediates in the rearrangement reactions of some alkenyl Grignard reagents (86). 

Bisalkyne derivatives have been crucial to the development of a comprehensive model for n donation in d4 monomers. The presence of two equivalent alkynes in the coordination sphere allows unambiguous interpretation of certain bonding properties, and in particular a formal donor number of three applies for each alkyne (N = 3). Bisalkyne complexes have been exploited to prepare monoalkyne monomers as well as for alkyne coupling reactions and ligand based transformations. 

Both combinations of alkyne n orbitals find filled dir orbital symmetry matches in these d4 complexes. Extended Huckel calculations on Mo(HC=CH)2(S2CNH2)2 indicate a large HOMO-LUMO gap of 1.62 eV. These octahedral complexes have proved to be quite robust and resist exchange and substitution reactions in accord with a substantial frontier orbital energy gap (153). 

The Pauson-Khand reaction starts with the replacement of two CO molecules, one from each Co atom, with the alkyne to form a double a complex with two C-Co a bonds, again one to each Co atom. One CO molecule is then replaced by the alkene and this n complex in its turn gives a a complex with one C-Co a bond and one new C-C a bond, and a C-Co bond is sacrificed in a ligand coupling reaction. Then a carbonyl insertion follows and reductive elimination gives the product, initially as a cobalt complex. 

As mentioned above, in derivatives containing n-bonded metal-ligand fragments, changes to the geometries of the carbon chains also parallel those found in analogous alkyne-MLra complexes.383... 

The cluster valence electron (c.v.e.) count usually corresponds to 12 + 22 electrons. Bonding of the C2 unit involves stabilization of a, a, and ji orbitals by interaction with radial metal MOs of the same symmetry, together with overlap of ji orbitals with filled metal MOs, i.e., a similar synergic interaction to the familiar bonding mode found in alkyne-metal complexes. For the model [Co8(C2)(n-L)(L)8]4 based on two trigonal... 

Numerous W complexes of the type W(CO)3 (L) (X)2 (jj -alkyne) (n =, 2) are known, most of which have been prepared from reactions of appropriate or... 

The 7c-complexing ability of the carbon groups can affect the thermodynamics of Eq. (a). While 7t-complexation by alkenes and alkynes with main-group metals is weak, that with transition metals leads to stable n-complexes. With such transition-metal n-complexes as products, transmetallation can be favorable, e.g., in the reaction of cyclopentadienyldiethylalane with FeClj the cyclopentadienyl rather than the Et group is transferred ... 

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