Alkynes substituent effects

In the study of substituent effect on the thermal electrocyclic ring-closing reactions of vinylallenes, two stereo-isomeric boryl-substituted vinylallenes 13 and 14 are synthesized by means of the palladium-catalyzed stannaboration of alkynes (Schemes 59 and 60).248... 

An illustrative example of how rehybridization can be used to control the Bergman cyclization is provided by substituent effects at the alkyne termini of enediynes. This effect in cycloaromatization chemistry was first studied by Schreiner and coworkers, who found dramatic acceleration of the Bergman cyclization upon... 

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

Alkynes and ynamines have also been developed as useful participants in RCM approaches to pyrrolidine derivatives (Scheme 13). Mori and co-workers reported the formation of a cyclic dienamide via ene-ynamide RCM <020L803>. They also studied substituent effects on the course of RCM of enynes and reported the formation of both pyrrolidines and piperidines <02MI678>. The syntheses of 5- and 6-ring carbocycles and O-containing heterocycles from appropriate precursors were also reported in that work. Synthetically-useful dienylboronic acids have been prepared from acetylenic boronates <00AG(E)3101>. ... 

Elsewhere in this chapter we have used oxygen nucleophilic processes to illustrate stereoselectivity (Sections II.B and II.C), coelectrophiles (Section I.A), etc. and shall not repeat these here. The literature of oxygen nucleophile attacks on alkynes includes trends in substituent effects but the data are often qualitative and usually scattered. Numerous examples indicate that the reactions of alkoxides with C2H2 are relatively slow and that most substituents facilitate the addition (Tables 13 and 14) ... 

Since in every instance the reactivity of an alkyne, RC CR, toward Bu 2AIH can be related to electron release by R and R, substituent effects are perfectly consistent with the mechanism depicted in Scheme 11, where R2 AIH attacks the rr-cloud of the alkyne electrophilically in the slow step. 

The Bergman cyclization is quite sensitive to substituent effects, and substituents at the alkyne termini are generally the most effective. It has been demonstrated that electron acceptor substituents, and more in particular cr-acceptors and/or Tr-donors. can facilitate the thermal cyclization, but apart from the work reported by Sander for the tetrafluoro-substituted enediyne 110 (which undergoes an exothermic Bergman cyclization), so far this approach has not produced particularly promising results (Scheme 19.30). 

A comprehensive treatment of the benzannulation of Fischer carbene complexes with alkynes is not possible in this review, and thus instead the material presented here will hopefully serve to give the reader an overview of its scope and limitations. The first report of this reaction was in 1975 by Dotz in which he describes the formation of the naphthol chromium tricarbonyl complex (236) from the reaction of the phenyl chromium complex (la) with diphenylacetylene. In the intervening years over 100 papers have been published describing various aspects of this reaction.The reaction of the generic cartene complex (233 Scheme 34) with alkynes will serve to focus the organization of the scope and limitations of the benzaimulation reaction. The issues to be considered are (i) the regioselectivity with unsymmetri-cal alkynes (ii) possible mechanisms (iii) applications in natural product syntheses (iv) the effect of substitution on the aryl or alkenyl substituent of the carbene carbon (v) functionality on the alkyne (vi) effects of the solvent and the concentration of the alkyne (vii) tandem applications with other reactions of carbene complexes (viii) reactions where aromatization is blocked (cyclohexadienone annulation) (ix) annulation of aryl versus alkenyl carbene complexes (x) the effect of the ligands L on the metal (xi) the effect of the ancilliary substituent RX and (xii) reactions with —C X functionality. 

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