Alkyne Substituents

The derivative 171 is of interest in this series of complexes since it undergoes isomerization of the alkyne substituent in position 1 to yield 172 and 173. The allyl complex 172 reacts with diphenylarsine to yield predominantly the chelate 174 (X = AsPh2) together with 175 and 176 (98IC1105). Complexes of the types 174 (X = PPh2) and 177 are known (93BSCF673 97BSCF471). 

The stoichiometric hydroamination of unsymmetrically disubstituted alkynes is highly regioselective, generating the azametaUacycle with the larger alkyne substituent a to the metal center [294, 295]. In others words, the enamine or imine formed results from an anti-Markovnikov addition. Unfortunately, this reaction could not be applied to less stericaUy hindered amines. 

This selectivity can only be achieved with the assistance of functional groups on the alkynes. Thus, if an alternative selective procedure that was independent of the nature of the alkyne substituents were to be available, it would be synthetically very useful. In order to develop such a new procedure for the formation of pyridine derivatives, azazirco-nacyclopentadienes, prepared from an alkyne and a nitrile, are important. As shown in Eq. 2.62, treatment of an azazirconacydopentadiene with an alkyne in the presence of NiCl2(PPh3)2 gives a pyridine derivative as a single product [8b]. Preparative methods for azazirconacydopentadiene derivatives will be discussed below. 

Some attack at the terminal carbon of the alkyne also takes place, and the regioselectivity of the reaction depends very much on the steric bulk of the alkyne substituents. 

A number of cis/trans 4,6-dialkyl-2,2-dimethyl-l,3-dioxanes were studied by C NMR spectroscopy (93JOC5251). The C NMR shifts of C -Me groups (Scheme 8) were found to be very sensitive to the 1,3-dioxane conformation [chair form Me(ax) ca. 19 ppm and Me(eq) ca. 30 ppm— pure 30.89 ppm in the twist-boat form both methyl carbons resonate at ca. 25 ppm (pure 24.70 ppm)]. With these values, AG° of the chair to twist-boat equilibrium was calculated (Table IV). For 13a (nitrile), 13b (alkyne), and 13e (methyl ester) (Scheme 8) in CH2CI2, the temperature dependence of the AG° values was determined. Depending on the substituent, small negative or positive entropy terms were found generally the enthalpy term dominates the -AG° value. In the tram isomers 13, the cyano and alkyne substituents favor the chair conformation, but CHO, ester, alkene, and alkyl substituents, respectively, clearly favor the twist-boat conforma-... 

Reaction outcomes are hardly affected by the electronic character of the alkyne substrates. On the other hand, large alkyne substituents favor (Z)-enyne formation, up to a certain threshold (entries 1-3). The most sterically encumbered alkynes are converted into (Z)-butatrienes (entries 4 and 5). 

An extensive series of stannoles results from trialkylboranes and dialkynylstannanes, notably if the alkyne substituent is H, Bu or Me3Si (Scheme 199) (78JOM( 148)137). However, if it is methyl the isomeric l-bora-4-stannacyclohexadiene is formed along with the stan-nacyclopent-3-ene (Scheme 200) (78JOM(153)153>. The lability of the tin-carbon bond makes the compounds useful synthetic intermediates, notably in the preparation of mWo-carboranes (Scheme 201) (77JOM(132)213). 

These reactions can be used to prepare a novel series of complexes where cyclic alkynes can be stabilized by coordination to platinum(O).831,832 The compounds are feasible because coordination of a triple bond to platinum causes a distortion of the alkyne from linearity by displacement of the alkynic substituents back away from the platinum. Also these methods can be used to prepare platinum(O) alkyne complexes with substituents other than triphenylphosphine.833-836... 

A tandem one-pot elimination-intramolecular Diels-Alder reaction occurs when the mesylate of 4-homoallylic azetidinone having a orc-alkene or alkyne substituent is heated in a sealed tube in the presence of an equimolecular quantity of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The method has been used to produce derivatives of oxace-pham. In a similar way, the 3,4-disubstituted azetidinone mesylate 473 afforded an 88% yield of 474. The method can be further elaborated through the introduction of a novel [3,3]-sigmatropic rearrangement of a-allenic mesylates thus, 475 yielded 476 on thermolysis C1999TL1015, 2000JOC3310, 2005EJ098>. 

Reports on ruthenium catalytic activity focus more on mechanistic consideration of the prototypical phenylacetylene dimerization than in establishing its synthetic applicability. It is not unusual that changing the alkyne substituents results in reversed selectivity (i.e. R = Ph or SiMe3 gave ( )- or (Z)- isomers, respectively) [27]. Competitive alkyne cyclotrimerization (R = COOMe) [27] or butatriene formation (R= CH2Ph, Bu) [10, 21] have occasionally been reported as possible drawbacks in enyne synthesis. The operating mechanism restricts the reaction to terminal alkynes. 

Palladium(ll) and cobalt-rhodium heterobimetallic nanoparticles can catalyze the reaction of twtfo-iodophenols with internal alkynes and carbon monoxide to furnish 3,4-disubstituted coumarins (Equation 281) <20000L3643, 2003JOC9423, 2004SL2541>. Unsymmetrical alkynes react to form two regioisomeric coumarins the major product usually features the more bulky alkyne substituent at G-3 (Equation 281) <20000L3643, 2003JOC9423, 2004SL2541>. 

Switching the alkyne substituent from branched alkyl to straight-chain alkyl groups leads to crystalline order that is dominated by interactions between the alkyl chains, which arrange as an insulating layer between the anthracene chro-... 

Fig. 14.37 Ethynylpentacenes (49-51), showing the increase in longitudinal slip as the size of the alkyne substituent is increased.

This observation is not related to traces of base or acid from the silver salts used since control experiments mled out this possibility. It was known from the literature that the 5-exo-dig versus 6-endo-dig cyclization mode could depend on the nature of the carbonyl group,56 57 of the alkyne substituent,58 59 and of the nature60 61 and oxidation state62 of the metallic source used. Also, work from Yamamoto25 demonstrated the importance of both a- and Jt-Lewis acidity properties of silver(I) complexes. Therefore, depending on the silver salt used, two mechanistic pathways were proposed (pathways A and B, Scheme 5.15). 

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