Cyclic dialkynes

Cyclobutadiene complexes are formed preferentially if cyclic dialkynes are treated with CpCo(CO)2. Depending... 

Products arising from propargylic C—C cleavage are not usually formed in solution, and the dialkyne 5 is photochemically inert to radiation of wavelength 254 nm However, cyclo-octa-l,5-diyne does cleave to give butatriene (equation 10), and the extra driving force here comes from the strain in the cyclic compound . 

The majority of the known heterocyclic dienes are pentadienes, largely as a result of the discovery that alkynes react with alkali metals and form dianions via dimer addition of the acetylene anion radical. A method that has become useful for the generation of larger cyclic dienes is the addition of an organometallic dihydride to a suitable dialkyne. Synthetic approaches to six-membered dienes tend to be unique for each system, and no general method has been developed. Some of the six-membered heterocyclic dienes have been converted to trienes for the study of systems that may be aromatic. 

Iron complexes favor the codimerization of BD with alkynes in a 1 1 ratio to (substituted) cyclohexadienes [39]. Two BD molecules and one alkyne give cyclodecatrienes with zerovalent nickel catalysts and good electron-donating ligands such as Ph3P [7, 40]. Ten-membered rings are in fact the principal products of such a reaction the variety of dienes seems to be limited to BD, iso-prene and 1,3-pentadiene, whereas numerous alkynes - simple alkyl-substituted alkynes, alkynes with aprotic functional groups, dialkynes, and cyclic alkynes... 

One of the oldest and still one of the most powerful methods of making cyclic alkynes uses the reaction of metal acetylides with halogen compounds [4]. If the halogen (or a similar leaving group) is situated at an sp -hybridized carbon atom, the reaction can usually be carried out without the help of a catalyst in moderately polar solvents such as THE Recently, it has been found that lithium acetylides give better yields than sodium acetylides in the preparation of cyclic dialkynes (Scheme 8-1) [5]. 

The high reactivity of propargylic halides toward nucleophiles allows the preparation of some rather strained cyclic alkynes and especially dialkynes, in surprisingly simple ways. Thus, the ten-membered ring 22 can be prepared in one step from methylamine and 1,4-dibromo-2-butyne (20), [12] (Scheme 8-2). 

Particularly useful in cyclic dialkyne chemistry is sulfide as a nucleophile. This is due to a discovery made by Regen and co-workers, who found that the yield of dialkyl sulfides is strongly enhanced when the sulfide reagent is first adsorbed onto AI2O3 thus, the substitution reaction takes place on the surface of the aluminum oxide. As the preparation of cyclic sulfides from dibromides is a stepwise process, these conditions were expected to increase the yield of cyclic sulfides dramatically as one end of the chain is immobilized after displacement of the first bromine [14]. Nicolaou first used this principle in the preparation of a series of cyclic thiadiynes [Eq. (6)] [15]. The cyclic thiadiynes prepared in this way are very useful as they can be converted to cyclic enediyne systems via the Ramberg-Backlund ring-contraction reaction (see below). 

Another possibility is to use the high reactivity of cyclic dialkyne systems toward electrophiles in order to add the additional carbon atom to the ring. When treated with dichloro-carbene at low temperature, cyclic dialkynes give mainly cyclic diynones [Eq. (18)]. In the same fashion, cyclononyn-3-one (91), the smallest conjugated cycloalkynone yet, has recently been made [Eq. (19)] [35]. 

Later, Dale used the extensive triple-bond migration, which takes place easily in macrocyclic dialkynes (C12-C20) when they are treated with KO-t-Bu in DMSO, to confirm his predictions for the relative stability of cyclic diyne isomers [4 c, 62]. These predictions were based on conformational considerations. Thus, 1,7-cyclotetradecadiyne (119) is almost completely rearranged to the 1,8-isomer 120 because in the latter compound both angle and terminal strain are absent, i.e., an ideal conformation is possible (Scheme 8-11). Dale s predictions, which were made long before the routine use of X-ray crystallography, were summarized in a seminal paper [63] and later confirmed by various X-ray investigations. The triple bond migration in cyclic diynes proceeds via allenic intermediates, which, however, are found only in small quantities in the reaction mixture. 

The 9-membered cyclic ethers 116 have been prepared via oxy-Cope rearrangement of 115, ultimately derived from D-glucose. The 10-membered bis-ether 118 was prepared via Bergman cyclization of the bis-acetylene 117 (R=2,3,4,6-tetra-0-benzyl-p-D-glucosyl). Solution conformational analysis was reported from NMR experiments. Dialkyne 117 was prepared by Pd-catalysed coupling of the P-D-glucosyl-l-alkyne and 1,2-diiodobenzene, and a mechanism was proposed for... 

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