Propargylic alcohols dehydration

A 1,2,3-triazole-promoted iron(III)-catalysed propargyl alcohol dehydration has been developed for the synthesis of conjugated enynes (Scheme 3) The products were obtained in good to excellent yields (up to 95%) with a large substrate scope and excellent Z stereoselectivity. Addition of readily available 1,2,3-triazole to FeCl3 provided a practical and efficient catalyst system, which gave the critical chemoselectivity for the carbon-oxygen bond activation. 

Scheme 2 Reaction pathways for the dehydration of terminal propargylic alcohols...

The reactions are those of functionalized 1-alkynes, the first of which were described for alkynes bearing substituted hydroxymethyl groups, such as substituted propargyl alcohols, HC=CCRR (OH), and often proceed further to form metal allenylidene complexes, by spontaneous dehydration of a (usually unobserved) hydroxy-vinylidene complex (Equation 1.22) ... 

In order to obtain some information on the reaction mechanism, the reaction of propargylic alcohol with acetone in the presence of a catalytic amount of 5a was monitored. The result indicated that the catalytic formation of the hexadienone proceeded via the initial isomerization of propargylic alcohol to dnnamaldehyde followed by aldol condensation between the produced aldehyde and acetone, and then dehydration. In fad, heating of propargylic alcohol in the presence of a catalytic amount of 5a gave only dnnamaldehyde (Scheme 7.41), and the separate reaction ofcinna-... 

Since the first discovery of transition metal allenylidene complexes (M=G=C=C<) in 1976, " these complexes have attracted a great deal of attention as a new type of organometallic intermediates. Among a variety of such complexes, cationic ruthenium allenylidene complexes Ru =C=C=GR R, readily available by dehydration of propargylic alcohols coordinated to an unsaturated metal center, can be regarded as stabilized propargylic cation equivalents because of the extensive contribution of the ruthenium-alkynyl resonance form... 

A propargylic alcohol is selectively oxidized with active M11O2 in the presence of a primary aliphatic amine. The resulting alcohol condenses with the amine, thanks to the action of the added molecular sieves as dehydrating agent, leading to the formation of an imine in a very... 

A similar reaction applied to propargyl alcohols in place of terminal alkynes led to the one-step catalytic head-to-head cyclodimerization of propargyl alcohols and to the formation of alkylidenecyclobutene derivatives [90] (Eq. 69). It was shown that the reaction occurs via cyclobutadieneruthenium and cy-clobutenylruthenium intermediates, dehydration and carboxylate addition. 

CpRu(CH3CN)3PF6-catalyzed linear dimerization of propargyl alcohols was also carried out in the presence of water and produced conjugated dienones in good yields via a head-to-head oxidative coupling, followed by successive dehydration and addition of water [91] (Eq. 70). 

The dehydration of alcohols under mild conditions is affected by copper(II) Lewis acids. Copper sulfate has long been utilized as a dehydrating agent. An example of its effectiveness for alcohol dehydration is demonstrated in the conversion of the sensitive propargylic alcohol 1 to enyne 2 (Sch. 1) [4]. A carbocation mechanism is suggested by the formation of bis ethers in these reactions [5]. The addition of pyridine... 

Mononuclear allenylidene complexes have been prepared by either (i) reaction of a deprotonated propargyl alcohol with a binary metal carbonyl or (ii) dehydration of the acetylenic alcohol HC=CCR20H with an appro-... 

Trienes A formal 1,4-dehydration of propargylic alcohols to afford cumulenes is realized on C-stannylation and subsequent treatment with MsCl-EtjN. 

Dehydration. Propargylic alcohols are readily dehydrated after complexation with a dicobalt hexacarbonyl residue. Interestingly, renewed treatment of the diols (formed by dihydroxylation of the double bond) with BF,-OEt2 leads to the deconjugated... 

The facile acid-promoted dehydration of cobalt-complexed propargyl alcohols [18], via P-pro-ton loss from the derived cations (Scheme 4-50) offers improved chemo-, regio- and stereoselectivity vis a vis the free propargylic alcohols, with a strong preference for the more substituted ( )-ene-yne complex [151,152]. Such Co-mediated dehydration has afforded routes to enantiomerically pure manicone and normanicone (4,6-dimethyl-4-octen-3-one,... 

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