2- Pentyne, rearrangement

The reaction of 77 with alkynes has further been elaborated for the synthesis of substituted phthalonitriles 81. An alternative for the synthesis of these compounds is the cycloaddition reaction of 77 with enamines followed by a retro-Diels-Alder loss of N2 and elimination of the amine (Scheme 16). Generally, more forcing reaction conditions are required and lower yields are obtained in reactions with alkynes than in reactions with enamines, for example, 4-ethyl-5-methylphthalonitrile is obtained in 51% yield from 2-pentyne (xylene, 150°C, 18 days) and in 73% yield from 4-(l-ethylprop-l-en-l-yl)morpholine (CHCI3, 70°C, 7 days) <1998T1809>. The mechanism of the reaction with enamines has been studied in detail. This revealed a [1,5] sigmatropic rearrangement in the cyclohexa-2,4-dien-1-amine intermediates formed after the loss of N2 <1998T10851>. 

Friedrichsen and co-workers (133) approached substituted benzotropolones from an aromatic substituted carbonyl ylide with a tethered alkyne as the intramolecular dipolarophUe (Scheme 4.67). Starting from an aromatic anhydride, Friedrichsen was able to make the tethered alkyne via addition of either pentyn-ol or hexyn-ol, then transform the recovered benzoic acid to the a-diazocarbonyl cycloaddition precursor. Addition of rhodium acetate resulted in the tandem formation of cyclic carbonyl ylide followed by cycloaddition of the tethered alkyne producing the tricyclic constrained ether 252. Addition of BF3 OEt2 opened the ether bridge, forming the benzotropylium ion, which subsequently rearranged to form the tricyclic benzotropolone (253). 

Jacobs [42] in 1951 showed that the rearrangement of 1-pentyne, 1,2-pentadiene, and 2-pentyne by means of 3.7 Nethanolic potassium hydroxide in a sealed tube at 175°C for about 3 hr gave the same equilibrium mixture. 

The reaction with 4-pentyn-l-ol gave only [Fe t/2-CH2=C(CH2)30) (CO)2(t/-C5H5)]+, and 3-hexyn-l-ol afforded (64, R = Et) (84) no evidence for the participation of the vinylidene tautomers was found. With ruthenium (45) and platinum (47) complexes, on the other hand, rearrangement to the vinylidene is faster than internal attack on the >/2-alkyne, and only the cyclic carbene complex is formed. 

The study of the addition of trifluoroacetic acid to several 5-substituted 1-pentyne derivatives 22 led Peterson and Duddey (1963, 1966) and Peterson and Bopp (1967) to the conclusion that for some of the compounds investigated the intermediate resulting from protonation of the triple bond may be a five-membered ring cation of type 23, rather than a linear vinyl cation. This is indicated by the observation of rearranged products deriving from 1-5 shift when Y = Cl, F, OMe, and OAc (see scheme 3). Kinetio data indicate that Y-participation is important in the transition state of the reaction. Anchimeric assistance effects, expressed in terms of the ratio of assisted to unassisted mechanisms, kjjkg (Heck and Winstein, 1957), have been estimated to be 3 4 for Y = Cl and 6 5 for Y = OMe. The entire kinetic picture almost matches... 

Any of several isomers of dibromopentane give mostly 2-pentyne on dehydrohalogenation with fused KOH at 200°C. In each case, the alkyne initially formed rearranges to the most stable isomer, 2-pentyne. 

Sol 10. (i) The l-alkenyl-4-pentyn-l-ol system (I) undergoes a microwave-assisted tandem oxyanionic 5-exo-dig addition reaction/Claisen rearrangement sequence to give cyclohept-4-enone derivative (II). 

The overall effect of this scheme would then result in a rapid forward proton transfer to 2-butyne to form 2 and an approximately equal reverse rate to give butadiene, rather than 2-butyne, for a pseudoequilibrium constant close to unity, as observed. The fact that the proposed reaction scheme mimics equilibijum behavior so closely for both 2-butyne and 1,2-butadi e is a result of a completely fortuitous coincidence of the proton affinity of 1,3-butadiene and the apparent proton affinity limit for protonation of 2-butyne and 1,2-butadiene with rearrangement to the allylic cation. Remarkably, 1-butyne, 1,1-dimethylallene, 3-methyl-1-butyne, and 2-pentyne all show a very similar pattern of behavior in proton-transfer experiments, with apparent heats of formation 25-50 kJ mol" too low for the vinyl cation products shown in Table 2 and 59-71 kJ mol" too high for the corresponding allylic ion products. Thus, the PA of 1-butyne is near to that of butadiene and the ultimate reaction product is apparently 1-methylallyl cation 2, as with 2-butyne and by the same sort of reversible proton exchanges within the ion-neutral reaction complexes. 

---