Cyclization cocyclization

Under the conditions of the cobalt-mediated carbonylative A-oxide-promoted cocyclization (Pauson-Khand reaction) at room temperature, compound 547 provides exocyclic 1,3-diene 548 as the major product (>98%) together with only traces of the corresponding carbonylative product 549. Owing to the relative instability of the diene, it is more efficient to perform a one-pot cobalt cyclization/Diels-Alder process after A-oxide-promoted cyclization of the cobalt complexes. Compound 550 is obtained as a single diastereomer in 39% overall yield if MTAD is used as a dienophile (Scheme 90) <2003JOC2975>. 

Unlike nickel Catalysts, palladium complexes do not catalyze the homo-cyclization reaction to give CDT or COD. The difference seems to be due to a different degree of hydride shift and atomic volume. With palladium catalysts, the hydride shift is easier, and hence linear oligomers are formed. The characteristic reaction catalyzed by palladium is the cocyclization of two moles of butadiene with one-hetero atom double bonds such as C=N and C=0 bonds to give six-membered rings with two vinyl groups (19) ... 

Another molecule which takes part in the cocyclization [see Eq. (67)] is arylisocyanate. The C=N double bond, rather than the C=0 double bond in phenylisocyanate, reacts with butadiene to give 3-ethylidene-l-phenyl-6-vinyl-2-piperidone (114) in 75% yield. In this reaction, the double bond migration to the conjugated position took place (101). With isoprene, the selective head-to-head dimerization-cyclization took place at 100°C to give 3,6-diisopropenyl-l-phenyl-2-piperidone (115). 

Molybdenum-promoted cyclodimerization of cyclooctatetraene262 and its cocyclization with alkynes263,2633 also appear to proceed via three- and five-membered molybdenacycles followed by further cyclization reactions that may or may not involve promotion by Mo (Scheme 56). 

The reaction mechanism was considered to be oxidative cyclization, and pal-ladacyclopentene 32 was formed. Reductive elimination then occurs to give cyclobutene 33, whose bond isomerization occurs to give diene 28. The insertion of alkyne (DMAD) into the carbon palladium bond of 32 followed by reductive elimination occurs to give [2+2+2] cocyclization product 27. Although the results of the reactions of E- and Z-isomers of 29 with palladium catalyst 26a were accommodated by this pathway, Trost considered the possibility of migration of substituents. Therefore, 13C-labeled substrate 25 13C was used for this reaction. 

Porphyrazines (pz), or tetraazaporphyrins, are compounds that can be viewed as porphyrin variants in which the meso carbon atoms are replaced with nitrogen atoms, as Fig. 1 shows (1). This difference intrinsically gives porphyrazines discrete physiochemical properties from the porphyrins. In addition, despite their similar molecular architecture, porphyrazines are prepared by an entirely different synthetic route than porphyrins—by template cyclization of maleonitrile derivatives, as in Fig. 2, where the open circle with the A in it represents the peripheral substituent of the pz—rather than by the condensation of pyrrole and aldehyde derivatives (1). The pz synthetic route allows for the preparation of macrocycles with chemical and physical properties not readily accessible to porphyrins. In particular, procedures have been developed for the synthesis of porphyrazines with S, N, or O heteroatom peripheral functionalization of the macrocycle core (2-11). It is difficult to impossible to attach the equivalent heteroatoms to the periphery of porphyrins (12). In addition, the preparation and purification of porphyrazines that bear two different kinds of substituents is readily achievable through the directed cocyclization of two different dinitriles, Fig. 3 (4, 5, 13). 

An important development in pz chemistry was the successful preparation of unsymmetrical porphyrazines achieved by the cocyclization of two different dinitriles, Fig. 4. This synthetic advance was part of an effort to prepare sulfur appended porphyrazines designed to bind metal ions at the pz periphery as well as in the central cavity (4-6, 21, 22). To avoid the statistical mix of six different isomers from such cyclizations, Hoffman and co-workers (13) developed techniques to selectively prepare specific isomers. Mixed cyclizations are described in all sections of this review. 

Aminoporphyrazines can be prepared by cyclization of tetralkylated diami-nomaleonitrile derivatives, either by themselves or in cocyclizations, using Lin-stead s magnesium propoxide method (7). Although DAMN 98 is commercially available, tetraalkylation is necessary because the unprotected amines do not withstand cyclization. The electron-rich amino pzs function as electron donors, bind metal ions, and undergo a variety of other interesting chemical reactions. 

The mechanism of these new enyne with diyne cyclizations has been studied using the deuterated enyne 163 [69]. D1 in dideuterated 163 migrates, but D2 does not. The deuterated (fT)-enyne 164 cocyclizes with the diyne 165 to give 166 with migration of D. However, no migration of D was observed in the cyclized product 168 by the reaction of (Z)-enyne 167 with 165. 

A partially intermolecular verison is possible, but less efficient. Thus the bromocnync 4 and 3-hexync cyclize to 5 under the same conditions in 40% isolated yield (equation 1). However, cocyclization of (E)-/f-bromostyrcne and the diync 6 results in a fulvene derivative 7 (equation II). 

The reaction was first reported by Khand and Pauson et al. in 1973d It is the dicobalt octacarbonyl [Co2(CO)8l mediated or promoted one-step synthesis of a,p-unsaturated cyclopentenone from the [2+2+1] cycloaddition of alkyne, alkene and carbon monoxide, through an intermediate of alkynedicobalt hexacarbonyl complex. Therefore, this reaction is generally known as the Pauson-Khand reaction, Pauson-Khand cyclization, or Pauson-Khand cycloaddition. Occasionally, this reaction is also referred to as the Pauson-Khand annulation, Pauson-Khand multicomponent cycloaddition, Pauson-Khand carbonylative cocyclization, Pauson-Khand bicyclization, Khand annulation, Khand cycloaddition, Khand cyclization (cyclisation ), or Khand reaction.Among these names, the Pauson-Khand reaction is the one used most often. 

Two groups have employed allylalcohol as a substrate in cyclization or cocyclization with other olefins. Whereas for one group the reaction worked... 

Another type of cyclization reaction, known as the Pauson-Khand reaction, involves the catalytic cocyclization of alkynes with alkenes and carbon monoxide... 

---