Triple-bonded substituents

The effect of substitution of the cyano group into the aromatic ring has already been noted. Substitution into other systems has been studied by Evans and Walker [62], and Friswell and Gowenlock [70]. In these cases. 

Rate coefficients for reaction of cyanosubstituted chlorides with sodium 

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Monohydroboration of terminal alkynes with bulky hydroboranes (76) places the boron exclusively at the terminal position of the triple bond. Addition to unsymmetrically disubstituted alkynes is markedly affected by the triple bond substituent size. Protonolysis or oxidation with H2O2 converts... 

Kaszynski, R Pakhomov, S. Young, V. G., Jr. Investigations of electronic interactions between closo-boranes and triple-bonded substituents. Collect. Czech. Chem. Commun. 2002, 67,1061-1083. 

The second control element is space demand, leading preferentially to olefins of type 4 if R is the more bulky triple bond substituent. 

It is certainly noteworthy that this highly regioselective complex directed endo-mode cyclization was shown to be absolutely independent of the electronic properties of the triple bond substituent This means that one can switch from electron poor to electron rich substituents by employing the Nicholas complex. 

As portrayed m Figure 2 20 the two carbons of acetylene are connected to each other by a 2sp-2sp cr bond and each is attached to a hydrogen substituent by a 2sp-ls CT bond The unhybndized 2p orbitals on one carbon overlap with their counterparts on the other to form two rr bonds The carbon-carbon triple bond m acetylene is viewed as a multiple bond of the ct + rr + rr type... 

For purely alicyclic compounds, the selection process proceeds successively until a decision is reached (a) the maximum number of substituents corresponding to the characteristic group cited earliest in Table 1.7, (b) the maximum number of double and triple bonds considered together, (c) the maximum length of the chain, and (d) the maximum number of double bonds. Additional criteria, if needed for complicated compounds, are given in the lUPAC nomenclature rules. 

The hydration of 5-amino-3-cyano-l-(2,6-dichloro-4-trifluoromethylphenyl)-4-ethynylpyrazole was performed with p-toluenesulfonic acid monohydrate in acetonitrile (2 h, room temperature) to give the corresponding 4-acetyl derivative. An alkyl substituent at the triple bond decreases the rate of hydration the conversion of 5-amino-3-cyano-l-(2,6-dichloro-4-trifiuoromethylphenyl)-4-(prop-l-yn-l-yl) pyrazole to the 4-propanoylpyrazole was completed after 18 h (98INP9804530 99EUP933363). 

The attempted oxidation of pyrazole derivatives with eleetron-withdrawing substituents (R = Me R R = COOCH3 R" = CHO Seheme 91) failed (2001UP4). For the same reason the oxidation of the triple bond in 4-ehloro-5-(2-ehloro-5-nitrophenylethynyl)- ,3-dimethyl-1//-pyrazole was also unsueeessful. 

Since substituents at the triple bond also determine its polarization, the copper acetylides containing both electron-releasing and electron-withdrawing groups were introduced into the reaction of cyclocondensation (78IZV1175 81IZV902). 

The interaction between a 4-iodopyrazole-3-carboxylic acid and copper ace-tylides having both donor and acceptor substituents at the triple bond generated six- rather than five-membered lactones, as in the aromatic series (Scheme 117). 

Intramolecular addition of the amide group to the triple bond in pyrazoles is more difficult, and results in closure of the 5-lactam rather than the y-lactam ring. The reaction time of the 4-phenylethynylpyrazole-3-carboxylic acid amide under the same conditions is extended to 42 h (Scheme 129) (Table XXVII). The cyclization of l-methyl-4-phenylethynyl-l//-pyrazole-3-carboxylic acid amide, in which the acetylene substituent is located in the 7r-electron-rich position of the heterocycle, is the only one complete after 107 h (Scheme 130) (90IZV2089). 

In general, it is diffieult to prediet the outeome of eyelizations of alkynylpyrazole diazonium salts, even with elosely related arrangements of funetional groups, sinee reaetion ean oeeur at both the a- and /3-earbon atoms of the aeetylenie substituent. Moreover, it is known that the eleetrophilieity of the diazo group and the nueleophilieity of a triple bond markedly depend on their positions in the pyrazole ring and that this ean affeet both the eourse and ease of eyelization and even its viability (83IZV688). 

The Diels-Alder reaction,is a cycloaddition reaction of a conjugated diene with a double or triple bond (the dienophile) it is one of the most important reactions in organic chemistry. For instance an electron-rich diene 1 reacts with an electron-poor dienophile 2 (e.g. an alkene bearing an electron-withdrawing substituent Z) to yield the unsaturated six-membered ring product 3. An illustrative example is the reaction of butadiene 1 with maleic anhydride 4 ... 

The Pauson-Khand reaction was originally developed using strained cyclic alkenes, and gives good yields with such substrates. Alkenes with sterically demanding substituents and acyclic as well as unstrained cyclic alkenes often are less suitable substrates. An exception to this is ethylene, which reacts well. Acetylene as well as simple terminal alkynes and aryl acetylenes can be used as triple-bond component. 

In all the preceding cases, the double or triple bond of the dienophile is next to the positively polarized carbon of an electron-withdrawing substituent. Electrostatic potential maps show that the double-bond carbons are less negative in these substances than in ethylene (Figure 14.8). 

The bromo substituent in 10-bromodibenz[/>,/]oxepin can be replaced by nucleophiles. With copper(I) cyanide in the presence of pyridine, dibenz[fr,/]oxepin-10-carbonitrile (3) is obtained.161 The substitution of bromine by various TV-substituted piperazines to give dibenz[/>,/]oxepins 4 has been accomplished using potassium cm-butoxide.197 This latter reaction probably proceeds via an intermediate with a C-C triple bond.160... 

The reaction of propargylic chiral acetals with a catalytic copper reagent (RMgX/5% CuX) provides the expected alkoxy allenes in quantitative yield (Table 3)61. The diastereomeric excess is highly dependent on the size of the ring of the acetal and on the type of substituents it contains. The best diastereomeric excess is 85% with the acetal derived from cyclooctanediol. The use of lithium dimethylcuprate results in 1,2-addition lo the triple bond and the resulting lithium alkenyl cuprate bearing a cyclic acetal does not eliminate even at reflux temperature ( + 35°C). 

Much earlier information on the structure of diazonium ions than that derived from X-ray analyses (but still useful today) was obtained by infrared spectroscopy. The pioneers in the application of this technique to diazonium and diazo compounds were Le Fevre and his school, who provided the first IR evidence for the triple bonds by identifying the characteristic stretching vibration band at 2260 cm-1 (Aroney et al., 1955 see also Whetsel et al., 1956). Its frequency lies between the Raman frequency of dinitrogen (2330 cm-1, Schrotter, 1970) and the stretching vibration frequency of the C = N group in benzonitrile (2255 cm-1, Aroney et al., 1955). In substituted benzenediazonium salts the frequency of the NN stretching vibration follows Hammett op relationships. Electron donor substituents reduce the frequency, whereas acceptor substituents increase it. The 4-dimethylamino group, for example, shifts it by 103 cm-1 to 2177 cm-1 (Nuttall et al., 1961). This result supports the hypothesis that... 

There is one other substituent which is comparable with the diazonio group in the sense that it is cationic and that it has, in one of its mesomeric structures, a triple bond between the atom attached to the aromatic system and the second atom. It is the acylium group in 7.9. However, no substituent constants are known for this group, obviously because this cation is detectable in measurable concentrations only in superacidic media (see review by Olah et al., 1976). 

The second method is termed ad eosdem competition— to the same (products, plural) . Consider the solvolysis of 4-chlorobenzyne by methoxide ions and methanol. The nucleophile adds to one end or the other of the triple bond. Because of the 4-chloro substituent, both meta and para isomers are formed. Thus, there are four parallel reactions ... 

Double and triple bonds are counted as if they were split into two or three single bonds, respectively, as in the examples in Table 4.1 (note the treatment of the phenyl group). Note that in a C=C double bond, the two carbon atoms are each regarded as being connected to two carbon atoms and that one of the latter is counted as having three phantom substituents. 

We shall find it convenient to divide the data sets for substituent effects at carbon—carbon triple bonds into three categories acetylene, ethynylene, and reactions at carbon—carbon triple-bond sets. 

The data available are insufficient to provide a significant test of the validity of eqs. (21) and (22). The range of Oj for the constant substituent attached to the triple bond is only. 14 a unit, and the range of or is. 11 a unit. 

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