Reactions of Conjugated Enynes

The reactivity of other related enynes such as 1-alkylenyne and dialkylenyne is much lower. Attempts at homodimerization did not 

The synthesis of derivatives of phenol and aniline via Pd-catalyzed benzannulation of substituted enynes and diynes has been described. For example, phenol derivatives 2.224, 2.226, and 2.229 were prepared by the benzannulation with alkoxyenynes 2.222 and 2.225 or 

2 Acceleration of Pd-catalyzed [4+2]-Benzannulation in the Reaction of Enynes and Diynes in the Presence of Lewis Acids and Bronsted Bases 

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Scheme 6.3S Dehydro Diels-Alder reactions of conjugated enynes with alkenes. ...

A completely new benzannulation reaction of conjugated enynes and alkynes catalysed by Pd complexes to give substituted benzenes has been developed recently. When the 1,3-enyne 150 is treated with a Pd(0) catalyst, homocyclodimerization of the enyne takes place to give the 2,6-disubstituted styrenes 151 as a single product in good yield [64], An interesting application of this reaction is the preparation of paracyclophane 153 by the intramolecular reaction of the bis-enyne 152 [65],... 

Saito S, Yamamoto Y (2002) Palladium-Catalyzed Benzannulation Reactions of Conjugated Enynes and Diynes. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 1635... 

Inter- and intramolecular palladium-catalyzed [4 - - 2] homocycloaddition reactions (i.e., benzannulation reaction) of conjugated enynes and bis-(enynes) have been reported by Yamamoto and co-workers (10, 507-509). Reaction of two-substituted conjugated enynes in the presence of Pd(PPh3)4 (2mol%) smoothly undergoes [44-2] intermolecular homocycloaddition to afford 1,4-disubstituted benzenes in good yields (10) (Scheme 73). 

IV.10.2 Palladium-Catalyzed Benzannulation Reactions of Conjugated Enynes and Diynes... 

Intramolecular [4+2] cycloaddition reactions of conjugated enynes, namely, dehydro-Diels-Alder reactions, afford benzo-fused CPDNs [254]. Thus, 2-propynyldiarylacetylenes 413 undergo thermal intramolecular [4+2] cycloadditions to give benzo[fc]fluorene derivatives 414 in good yields (Scheme 6.108a). The reactivity of the reactions increases in the order hydrocarbon... 

SCHEME 14.2 Transition-metal-catalyzed benzannulation reactions of conjugated enynes with alkynes. 

Saito, S. and Yamamoto, Y. (2002) Palladium-catalyzed benzannulation reactions of conjugated enynes and diynes, in Handbook of Organopalladium Chemistry for Organic Synthesis (eds E. Negishi and de A. Meijere), John Wiley Sons, New York, p. 124. 

Gevorgyan, V., Takeda, A., Homma, M., Sadayori, N., Radhakrishnan, U. and Yamamoto, Y. (1999) Palladium-catalyzed [4-1-2] cross-benzannulation reaction of conjugated enynes with diynes and triynes. Journal of the American Chemical Society, 121(27), 6391-6402. 

It was described that the use of the combination of Pd(OAc)2/dppp /THE resulted in the highest yield. It is not clear whether such a combination of catalyst, ligand and solvent is restricted to the hydrothiocarboxylation of conjugated enynes or also may be applied to other alkynes. Furthermore, it remains to be explored whether, in the Pt-catalyzed reaction performed in CHjCN [see Eq. (7.18)], conjugated enynes can be used. 

In 1993, Hayashi and co-workers reported a catalytic asymmetric synthesis of alle-nylboranes 256 by palladium-catalyzed hydroboration of conjugated enynes 253 (Scheme 4.66) [105]. Reaction of but-l-en-3-ynes 253 with catecholborane 254 in the presence of a catalyst, prepared from Pd2(dba)3 CHC13 (1 mol%) and a chiral mono-dentate phosphine ligand (S)-MeO-MOP 255 (1 mol%), gave an allenylborane 256. The ee of 256 was determined by the reaction with benzaldehyde affording the corresponding optically active homopropargyl alcohols 257 with up to 61% ee (syn anti= 1 1—3 1). 

A variety of palladium-catalyzed dimerizations of conjugated enynes and their additions to diynes and triynes gave rise to styrene and phenylacetylene derivatives, respectively. Inter alia, 1,2,4-cyclohexatrienes have been invoked as intermediates in these reactions [134], 5,6-Diphenyl-l,2,4-cyclohexatriene has been proposed as an intermediate in the rearrangement of 4,4-diphenylcyclohexa-2,5-dienylidene to o-ter-phenyl and its possible existence was supported by quantum-chemical calculations [135],... 

Metathesis of conjugated enyne-enes has been carried out using bispyridine-substituted ruthenium benzylidene catalyst li. An intramolecular version with conjugated enynes affords novel butadienyl cycloalkenes (Equation (8)). The reaction does not proceed with Ic or Ig. 

The reagent employed in the first reaction, with structure 44. is from a method developed by Gtrcx for the synthesis of conjugated enynes.1 Addition of TBA1 leads to quantitative formation of target structure 1... 

Asymmetric epoxidation of conjugated dienes and enynes catalysed by the chiral man-ganese(EI) complex 232 give monoepoxides exclusively reactions of cw-enynes give /raw.v-alkynyl epoxides as the major products with a high level of asymmetric induction362. 

Another cascade reaction using conjugated enynals as starting materials with a palladium catalyst was reported. Good to high yields of 2,3,4-trisubstituted furans were afforded in the reaction. An example is shown in the following scheme 070L1191>. 

Doucet H, Hierso JC (2007) Palladium-based catalytic systems for the synthesis of conjugated enynes by sonogashira reactions and related alkynylations. Angew Chem Int Ed 46 834-871... 

Cross-coupling reactions between 1-alkynyl halides and 1-alkenylboranes, which are readily available via hydroboration of alkynes, can also be catalyzed by Pd-phosphine complexes. Stereoselective synthesis of conjugated ( )-enynes 143 was achieved by the coupling of the alkynyl bromide with an alkenylborane [Eq. (48)] [69]. 

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