Alcohols, alkynyl, dehydration

There is no way in which dehydration of alcohols can be used to prepare triple bonds gem-diols and vinylic alcohols are not normally stable compounds and vic-diols give either conjugated dienes or lose only 1 mol of water to give an aldehyde or ketone. Dienes can be prepared, however, by heating alkynyl alcohols with triphenyl phosphine. ... 

Etherification using a metal vinylidene has also been combined with G-G bond formation through the reaction of an alkynyl tungsten complex with benzaldehyde (Scheme 14). The addition of an internal alcohol to the incipient /3,/Udialkylvinylidene that is generated leads to dehydration and the formation of a Fischer-type alkylidene complex. Further reactions of this carbene with a range of nucleophiles have provided access to various furan derivatives.374,375... 

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... 

The chemistry of iron vinylidene complexes is dominated by the electrophilicity of the carbon atom adjacent to the iron organometallic unit. While addition of water leads to an acyl complex (i.e., the reverse of the dehydration shown in equation 10), addition of an alcohol leads to a vinyl ether complex. Similarly, other iron vinyl complexes can be prepared by the addition of thiolate, hydride, or an organocuprate (Scheme 33). " The nucleophilic addition of imines gave enaminoiron intermediates that could be further elaborated into cyclic aminocarbenes. This methodology has been used to provide access to /3-lactams and ultimately penicillin analogs, and good diastereoselectivities were observed (6 1-15 1) (Scheme 34). 04 Iso, vinylidene complexes are intermediates in cyclizations of alkynyl irons with substituted ketenes, acid chlorides, and related electrophiles an example is shown (equation 11). These cyclizations led to the formation of a series of isolable and characterizable cyclic vinyl iron complexes. 

Several interesting derivatives of tpy have been prepared in which a ferrocene is linked to the 4 -position. The condensation of ferrocene carbaldehyde with 2-acetyl pyridine leads to the parent system (31a).34 Alternatively, l,5-bis(2-pyridyl)-3-(ferrocenyl)pentane-l,5-dione may be easily prepared in 77% yield, and subsequent treatment with ammonium acetate provides a 78% yield of (31a).35 If ferrocene l,l -dicarbaldehyde is used in the first procedure, the bisterpyridyl ferrocene can be synthesized.36 Addition of the anion derived from (25i) to ferrocene carbaldehyde affords the alcohol (31b), which can be dehydrated to the ethylene derivative (31c).37 Coupling of alkynyl ferrocene with the bromo-derivative (25e) leads to a tpy linked to ferrocene by a phenylacetylene bridge (31d).38... 

Using this remarkably effective catalyst alkynyl allylic alcohol (e.g. 265) are converted into furans 266 very efficiently. Similarly, allenic alcohols (e.g. 267) can be converted into 2,5-dihydrofurans (e.g. 268) by this relative of a 5-e rfo-dig process <93JOC7180 94JOC324>. At a higher oxidation level, conjugated allenic ketones 269 can be similarly cyclized, effectively by carbonyl hydration and a final dehydration to the furans 270 <92JOC3387 94JOC7169>. 

Liu et al. reported a TpRuPPh3 (CH3CN)2-PF6-catalyzed cycloisomerization of cis-3-en-l-ynes I or their precursor alcohols 210, which afforded cyclopentadiene 211 (Scheme 80) [43]. Mechanistically, 210 undergoes ruthenium-catalyzed dehydration to afford the real substrates cis-3-en-l-ynes 1, which is converted into mthenium-vinyhdene intermediate H via [1,2]-shift of alkynyl hydrogen. A [1,5]-... 

The pentatetraenylidene complex [lrCl(C=C=C=C=CPh2)(PPr 3)2] 603 was indeed prepared by dehydration of [Ir(H)Cl C C-C C-C(OH)Ph2 (PPr 3)2] 604 with triflic anhydride in the presence of NEts. The alkynyl(hy-dride) precursor 604 was in turn obtained via reaction of the dihydrido complex [lrH2Cl(PPr 3)2] with the unsaturated alcohol HC C-C C-C(OH)Ph2. Thermal or photochemical activation of 604 does not afford the pentatetraenylidene, but favors the isomerization to the (alkynyl)vinylidene [lrCl C=C(H)C=C-C(OH)Ph2 (PPr 3)2] 605 from which 603 cannot be obtained by dehydration methods (Scheme 87). 

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