Homopropargylic system

Formation of rearranged products in the solvolysis of homopropargyl systems need not involve triple-bond participation and vinyl cations in all instances. Ward and Sherman investigated the formolysis of 4-phenyl-1-butyn-l-yl brosylate, 57 (80). At 80°C in the presence of one equivalent of pyridine, they observed formation of phenyl cyclopropyl ketone, 58, and... 

Interestingly, the [RuCl2(p-cymene)]2 catalyst used for selective synthesis of (Z)-vinylsilanes produces instead the a-vinylsilanes with appropriately positioned hydroxyl groups.57 For the homopropargylic system shown (Scheme 12), the selectivity is 98 2. For propargylic or bishomopropargylic systems, only small amounts (2-13%) of the a-product... 

Carhoalumination of propargyl and homopropargyl systems. The high syn-slereoselcctivity of (he carhoalumination of alkynes with /kl(C ll,)3 C UZrCp, is... 

A somewhat special case is represented by the cation involved in the homopropargyl participation. Calculations based on the modified CNDO method for the system C4HB+, reported by Fischer et at., (1969)... 

Homopropargylic alcohols ot-allenic alcohols. The synthesis of alkylallenes (5, 397) by the reaction of trialkylboranes and lithium chioropropargylide, ClCH2C=CLi (1), has been extended to these two systems. Thus addition of acrolein to the organoborane a can lead to either 2 or 3, depending on the temperature at which a is kept before reaction with the aldehyde (equation I). If the aldehyde is... 

Homopropargylic alcohols as well as propargylic epoxides and pentynols readily form cyclic ruthenium alkoxycarbenes upon intramolecular nucleophilic addition of the OH group to the electrophilic a-carbon of ruthenium-vinylidene species. Their oxidation in the presence of N-hydroxysuccinimide leads to the formation of penta-lactones. The best catalytic system reported until now for this transformation of but-3-ynols is based on RuCl(C5H5)(cod), tris(2-furyl)phosphine, NaHCOs as a base, in the presence of nBu4NBr or nBu4NPp6, and N-hydroxysuccinimide as the oxidant in DMF-water at 95 °C (Scheme 8.11) [22]. 

SnClj leads to allyl ki (from the bromohydri system in water-dich Homopropargyl depending on reageni... 

Allenyltitanium systems are formed from propargyl halides or carbonates and a low-valent diisopropoxo-772-propene Ti(n) species. They are excellent reagents for the synthesis of homopropargyl alcohols by reaction with carbonyl compounds, as examples of carbon-carbon bond-forming processes.36,37... 

Interestingly, the presence of a bis-homopropargylic secondary amide does not lead to insertion into the N-H (as happens with a tungsten catalyst [8c]) competing with formation of the dihydropyran as shown in Equation 1.4 [10]. This reaction extends to the insertion into a phenolic OH to form benzofurans (Equation 1.5). In this case, the presence of an amine such as n-butylamine or pyridine appears to be required [11]. It should be noted that insertion into the benzylic OH to form the pyran system does not compete. 

The work of Harding and Hanack on this system has confirmed the view that the reaction is observed only in solvents of low nucleophilicity in highly nucleophilic solvents, a bimolecular reaction proceeds without rearrangement. A series of isotope effect measurements confirms that in solvents of low nucleophilicity and good ionizing power, the homopropargylic assistance is of importance in the solvolysis reaction. 

Other Unsaturated Alcohols. The review mentioned above of carbometallation of alkynes, includes a survey of additions of alkenyl-metal derivatives to epoxides to give homoallylic alcohols. The zirconium-catalysed carboalumination of propargylic systems (Scheme 16) is also applicable to the homopropargylic to homoallylic alcohol transformation. ... 

Unlike the situation for homoallylic coupling in most acyclic alkenes, homopropargylic coupling is almost always observed in the H NMR spectra of internal alkynes. As we saw above, essentially all conformations of the C—H trbond on the carbon a to the triple bond allow for partial overlap with the /r system of the alkyne, resulting in coupling constants significantly larger than those observed for... 

Starting from pent-4-yn-l-ols (bis-homopropargylic alcohols), the catalytic system [L] based on RuCl(Cp)(tris(p-fluorophenyl)phosphine)2 (5 mol%), tris... 

Scheme 26 ndo-cyclization of bis-homopropargylic alcohols with catalytic systems L and M... 

An intramolecular version of alkyne hydration was reported in 2006 by Belting and Krause [127] providing an efficient route to tetrahydrofuranyl ethers 32. This transformation consists in a tandem cycloisomerization-hydroalkoxylation of homopropargylic alcohols 31 in the presence of an alcohol in a dual catalyst system (a gold precatalyst and a Bronsted acid) under mild conditions (Scheme 13). The reaction proceeds satisfactorily with terminal and internal alkynes, with bis-homopropargylic alcohols and alkynyl phenols to provide cyclic acetal skeletons that occur in a variety of natural products. Substituted furanones can be obtained by gold(III)-catalyzed activation of alkynes by heterocyclization and subsequent 1,2-alkyl shift [128]. 

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