Cyclohexyne insertion

A computational study of the mechanism of cyclohexyne insertion into a C(0)-Ca bond of cyclic ketones has been reported to proceed through a stepwise 2 + 2-cycloaddition of cyclohexyne to the enolate, followed by thermally allowed conrotatory electrocyclic ring opening, thermally forbidden disrotatory electrocyclic ring opening, or nonpericyclic C-C bond cleavage (Scheme 34)... 

Generally, cyclohexyne is an unstable molecule because of its ring strain. However, it can be stabilized by coordination to transition metals.35 The reduction of 1,2-dibromocyclohexene by sodium/mercury in the presence of a nickel-bromide complex afforded the Ni-alkyne complex 66 as a thermally stable and isolable compound (Scheme 22).36 Complex 66 smoothly reacted with C02 under atmospheric pressure to give nickelacycle 67 in good yield. Dimethyl acetylenedicarboxylate was inserted into the vinyl-nickel bond in 67 to give the seven-membered oxanickelacycle 68. 

The complex [(77 -cyclohexyne)Pt(PPh3)2] inserts electrophilic or strained alkenes (CH2=CHR R = COMe, CHO, COOMe, CN) into the platinum-cycloalkyne bond to yield platinacyclopentenes 310 <1997CB1029, 2000JOM(600)37>. 

Similar to the benzynezirconocene, cyclohexyne, cyclopen-tyne, alkyne, alkene, cycloaUcene zirconocenes, and related species insert various substrates such as alkynes, alkenes, aldehydes, ketones, nitriles or phosphaalkynes. They lead in general five-membered zirconacycles, which can be converted by transmetalation or exchange reactions into fused-ring aromatic or heterocyclic compounds. The extension of this chemistry to heterobenzyne complexes can be realized, for instance, in phosphinine compounds. Consequently, under mild conditions, ) -phosphabenzyne-zirconocene complexes are formed and can be isolated either as PMes adducts or as dimers when the elimination reaction is carried out without added phosphane (Scheme 28). 

Alternative indirect methods have been used in the synthesis of (3,3-dimethylcyclopen-tyne)Cp2Zr(PMe3> [52] and (cyclohexyne)[CpMo(CO)2]2 [53]. The smallest cycloalkyne stabilized by complexation is cyclobutyne, incorporated by Adams into tri- and tetrametallic ruthenium and osmium clusters as a P3-ligand 30 using 1-Br and -SPh substituted cyclobutene precursors (Scheme 4-11) [54]. Liberation of these strained alkynes from their metallic bondage has not been reported but some have been shown to be reactive towards insertion of unsaturated substrates, as illustrated in Scheme 4-12 with the Zr-cyclohexyne derivative 31 [55]. 

The insertion of CS2 into a Ni-C coordination bond is proposed in the reaction of the cyclohexyne nickel(O) complex 45, as in the case of the reaction with CO2, although the product 46 could not be separated from the byproduct (Scheme 12) [95]. 

Under the same reaction conditions, the Sn—Sn o-bond of a distarmane also readily adds to the triple bond of arynes, yielding diverse 1,2-distannylarenes which have the potential to act as bis-anion equivalents (Equation 12.73) [141]. Moreover, highly strained cyclohexynes undergo this Sn-Sn a-bond insertion smoothly under the reaction conditions in yields up to 90%. 

Platinacyclopentanes with heteroatoms or functional groups in the five-membered chelate ring may also be obtained as stable complexes. Single and double insertion of isonitrile into the Pt-C bond of a cyclohexyne-Pt(O)... 

---