Alkynes distortion

Coordination to alkynes distorts the triple bond character nearer to that of a double bond, decreasing the linearity. Utilization of the coordination effect makes it feasible to prepare cyclic alkynes whose synthesis is difficult to achieve. Highly strained cyclooctyne can be prepared by coordination. As an example, starting from (R)-pulegone, three of the four rings of the epoxydictymene skeleton 152 were constructed by the consecutive Lewis acid-promoted Nicholas reaction of allylic silane 149 to form 150, and the intramolecular Pauson-Khand reaction of 151. The total synthesis of (+)-epoxydictymene 153 from 152 has been achieved [38]. 

The bridged complex between two cobalt tricarbonyl units and all four r-electrons of the alkyne distorts the alkyne from being linear to being bent in a 150°C angle. For simplicity, all C02 (CO)6 complexes will be represented by a single bond from the Co2(CO)6 fragment to the alkyne (Scheme 189). 

We will illustrate the interplay between infra- and intermolecular effects in thermal (non-catalyzed) alkyne-azide cycloadditions. In this case, both infra- and intermolecular effects work in synergy and originate from the presence of the same acceptor group at the propargylic carbon. In this example, a single structural design element imbues more than one beneficial effect. The intramolecular effect involves stabilization of alkyne distortion and Jt-bond breaking, whereas the intermolecular effect involves assistance to bond formation (vide infra). 

The electronic structure of benzyne, shown in Figure 16.19, is that of a highly distorted alkyne. Although a typical alkyne triple bond uses sp-hybridized carbon atoms, the benzyne triple bond uses sp2-hybridized carbons. Furthermore, a typical alkyne triple bond has two mutually perpendicular it bonds formed bv p-p overlap, but the benzyne triple bond has one tt bond formed by p-p overlap and one tt bond formed by sp2 sp2 overlap. The latter tt bond is in the plane of the ring and is very weak. 

An X-ray structure analysis of 74 (R=C4Hg) revealed that the unsaturated portion of the molecule was planar, with the angles between adjacent acetylenic bonds deviating by 13 -15° from 180°, the value for a strain-free molecule. Since the connection of the alkyne moieties to the aromatic rings was only shifted slightly (2-3°), distortion of the acetylene linkages appears as the major source of instability in these macrocycles. 

More recently, tetranuclear complexes, related to those previously obtained for both ruthenium and osmium, have been obtained for iron, and their structures have been established by X-ray analysis 118). The reported adducts are [Fe4(CO)11(RC2R1)2] (R = H, R1 = Me, Et, n-Pr R = R1 = Me), and are obtained in very low yield (<1%). For R = H, R1 = C2H5, the structure is shown in Fig. 16. This involves a distorted tetrahedral metal system with the two alkyne groups bonding in a manner similar to that observed for the "butterfly molecule [Co4(CO)10(EtC2Et)] 119). 

These reactions can be used to prepare a novel series of complexes where cyclic alkynes can be stabilized by coordination to platinum(O).831,832 The compounds are feasible because coordination of a triple bond to platinum causes a distortion of the alkyne from linearity by displacement of the alkynic substituents back away from the platinum. Also these methods can be used to prepare platinum(O) alkyne complexes with substituents other than triphenylphosphine.833-836... 

It is possible to depict an RC = CR metal binding by several resonance structures including the metallocyclopropane form, — 111 form, and almost noncontributing form with two M = C bonds. It is noted that alkynes are usually nonlinear when coordinated [184e]. An interesting consequence of the distorted coordination geometry is the fact that small cyclic acetylenes, which are unstable as free molecules, are stabilized by coordination to a metal, for example CpTa(Me)2(benzene) or (cycloheptyne)Pt(PPh3)2 [21b]. 

Alkynes can be bound to metals using one or both sets of p-bonds, as 2, 3 or Ae donors. Acetylene bridges of several types are known. In acetylene there are two p-bonds at 90° to each other and both can be bound to a metal. The Co atoms and the acetylene carbon atoms forms a distorted tetrahedron and the C6H5 or other groups [Co2(CO)8(C2R2)] on the acetylene are bent away as shown... 

In the same way that terminal oxo or nitrene units create a pyramidal distortion in M(0)L5 131) and M(NR)L5 132) complexes, alkynes also tend to pull the metal out of the plane of the four adjacent ligands. In W(CO)(HC=COAlCl3)L3Cl the two PMe3 ligands cis to the alkyne form an L—W—L angle of 161° 130), and in the molybdenum structure the... 

It is perhaps premature to attempt to delineate the factors that determine whether symmetrical or distorted metallacyclobutadiene or t 3-cycopropenyl coordination is observed, given the comparitive sparsity of directly comparable examples, and the observation that tautomerism appears to operate between coordination modes in some cases. A similar situation arises for cyclobutadiene coordination vs. metallacyclopentadi-ene formation (Figures 6.38, 6.39, 7.19). In both cases the metallacycles are important intermediates in catalytic manifolds (alkyne metathesis and oligomerization, respectively) and in both cases the polyhapto variant represents a tangent to the productive catalytic cycle, formation of which may be reversible or in some cases may lead to termination. 

Benzyne has the electronic structure of a distorted alkyne and has one very weak n bond. 

---