Monosubstituted ethylenes

Scheme 6.86 displays the generation of 417 from the cephalosporin triflate 416 and the formulas of the trapping products with ethylene, monosubstituted ethyl-enes, 1,1-disubstituted ethylenes and 1,1-dimethylallene [155], It was shown for two such reaction partners (styrene and phenyl vinyl thioether) that the exchange of the... 

Scheme 6.86 Generation of 4-methoxybenzyl (6R,7R)-8-oxo-7-(phenylacetamido)-l-aza-5-thiabicyclo[4.2.0]octa-2,3-diene-2-carboxylate (417) from the cephalosporin triflate 416 and products of the trapping of 417 by ethylene, monosubstituted ethylenes, 1,1-disubstituted ethylenes and 1,1-dimethylallene, according to Elliott and co-workers.

Monosubstituted alkenes (RCH=CH2) have a more stabilized double bond than ethylene (unsubstituted) but are less stable than disubsti tuted alkenes... 

Our discussion of stereoregularity in this chapter is primarily concerned with polymers of monosubstituted ethylene repeat units. We shall represent these by X... 

V-Alkylation can also be carried out with the appropriate alkyl haUde or alkyl sulfate. Reaction of aniline with ethylene, in the presence of metallic sodium supported on an inert carrier such as carbon or alumina, at high temperature and pressure yields V/-ethyl- or /V,/V-diethylaniline (11). At pressures below 10 MPa (100 atm), the monosubstituted product predominates. 

The parent compound in this series, that is, the agent substituted by a vinyl group, is obtained by direct vinylation. Reaction of the monosubstituted barbiturate, 129, with ethylene at elevated temperature and pressure in the presence of a zinc catalyst affords butylvinal (110). ... 

The intensities of the stretching vibrations of monosubstituted ethylenes are also related to Og° values Studies of this system yielded a value of 0.072 for cjg° of S02Me. 

There is still another situation that leads to second order spectra and this one usually cannot be anticipated. For example, take a look at the proton spectrum of 3,3,3-trifluoropropene in Fig. 2.9. This spectrum is not the simple one that one would expect for a monosubstituted ethylene. However, the second order nature of this spectrum can be understood after examining the fluorine-decoupled spectrum, which is given in Fig. 2.10. The decoupled spectrum displays the expected multiplets from the ABC system, each proton appearing as a doublet of doublets. The second order spectrum seen in Fig. 2.9 derives from the fact that the protons at 5.98 and 5.93 are seen from the 19F frequency as... 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

Negishi et al. reported the regioselective synthesis of diisoalkyl derivatives from monosubstituted alkenes in yields ranging from 58-95%, Scheme 8, from the in situ prepared ethylene complex Cp2Zr(C2H4).35 The zirconocene-ethylene complex presumably undergoes alkene insertion to furnish a zirconacyclopentane which further reacts with diethylzinc to yield the diisoalkylzinc compound. 

In contrast, polar and resonance effects must be separated in order to analyze the data for a-substituted arylolefins [ArC(R)=CHR with R H]. Their bromination involves open carbocation intermediates only. Resonance effects cannot be fully developed at the transition states, since the aromatic ring is not in the same plane as that of the developing carbocation, because of steric constraints. Accordingly, application of (33) gives pT < pn. Attenuation of resonance arises mainly from stereochemical factors, at least in the monosubstituted 1,1-diphenylethylene [20] and a-methylstilbene [21] series the pr/pn ratios can be related to the dihedral angle between the substituted phenyl ring and the plane of the ethylenic bond. 

This regioselectivity is practically not influenced by the nature of subsituent R. 3,5-Disubstituted isoxazolines are the sole or main products in [3 + 2] cycloaddition reactions of nitrile oxides with various monosubstituted ethylenes such as allylbenzene (99), methyl acrylate (105), acrylonitrile (105, 168), vinyl acetate (168) and diethyl vinylphosphonate (169). This is also the case for phenyl vinyl selenide (170), though subsequent oxidation—elimination leads to 3-substituted isoxazoles in a one-pot, two-step transformation. 1,1-Disubstituted ethylenes such as 2-methylene-1 -phenyl-1,3-butanedione, 2-methylene-1,3-diphenyl- 1,3-propa-nedione, 2-methylene-3-oxo-3-phenylpropanoates (171), 2-methylene-1,3-dichlo-ropropane, 2-methylenepropane-l,3-diol (172) and l,l-bis(diethoxyphosphoryl) ethylene (173) give the corresponding 3-R-5,5-disubstituted 4,5-dihydrooxazoles. 

Subsequent mechanistic studies suggested that the abovementioned effect of ethylene on reaction efficiency is connected to a mechanistic divergence that exists for reactions of terminal styrenyl ethers versus those of disubstituted styrene systems [13b]. Whereas with monosubstituted styrenyl substrates the initial site of reaction is the terminal alkene, with disubstituted styrene systems the cyclic ji-systems react first. This mechanistic scenario suggests two critical roles for ethylene in the catalytic reactions of disubstituted styrenes ... 

Transformations of the more highly substituted styrenyl ethers are notably more facile under an atmosphere of ethylene due to the presence of the more reactive LnRu=CH2 (formed by the reaction of la or lb with ethylene) [20]. Under an atmosphere of Ar and after the first turnover has transpired, LnRu= CHCH3 is likely the participating catalyst. When the reaction is performed under ethylene, LnRu=CHCH3 is immediately converted to LnRu=CH2. Reactions of monosubstituted styrenes do not require ethylene to proceed smoothly because, as illustrated in Scheme 10, with this class of starting materials, the more reactive LnRu=CH2 is formed following the first catalytic cycle. 

In many instances it is not necessary to isolate the acetonitrile complex or to carry out the reaction in acetonitrile. The use of amine oxide as a means of displacing carbonyl groups in metal carbonyls is well documented, and reaction proceeds smoothly with the carbonyl in the presence of a variety of ligands—e.g., ethylene or pyridine—to yield the monosubstituted derivatives. The advantage of the acetonitrile adducts is the stability of the compounds and the reactivity of the amine oxide toward acidic ligands. 

For the addition of ethylene, EtOAc as solvent was particularly advantageous and gave 418 in 60% yield (Scheme 6.86). The monosubstituted ethylenes 1-hexene, vinylcyclohexane, allyltrimethylsilane, allyl alcohol, ethyl vinyl ether, vinyl acetate and N-vinyl-2-pyrrolidone furnished [2 + 2]-cycloadducts of the type 419 in yields of 54—100%. Mixtures of [2 + 2]-cycloadducts of the types 419 and 420 were formed with vinylcyclopropane, styrene and derivatives substituted at the phenyl group, acrylonitrile, methyl acrylate and phenyl vinyl thioether (yields of 56-76%), in which the diastereomers 419 predominated up to a ratio of 2.5 1 except in the case of the styrenes, where this ratio was 1 1. The Hammett p value for the addition of the styrenes to 417 turned out to be -0.54, suggesting that there is little charge separation in the transition state [155]. In the case of 6, the p value was determined as +0.79 (see Section 6.3.1) and indicates a slight polarization in the opposite direction. This astounding variety of substrates for 417 is contrasted by only a few monosubstituted ethylenes whose addition products with 417 could not be observed or were formed in only small amounts phenyl vinyl ether, vinyl bromide, (perfluorobutyl)-ethylene, phenyl vinyl sulfoxide and sulfone, methyl vinyl ketone and the vinylpyri-dines. 

This manganese-copper-catalyzed conjugate addition reaction compares favorably with the classical copper-catalyzed reaction. The two reactions are easily and similarly carried out under mild conditions, but the first one gives higher yields. This difference, already observed in the case of p-monosubstituted o,p-ethylenic ketones, is especially noticeable with p,p-disubstituted or [Pg.70]

Isomerism is observed in the polymerization of alkenes when one of the carbon atoms of the double bond is monosubstituted. The polymerization of a monosubstituted ethylene, CH2=CHR (where R is any substituent other than H), leads to polymers in which every tertiary carbon atom in the polymer chain is a stereocenter (or stereogenic center). The... 

Fig. 8-1 Different polymer structures from a monosubstituted ethylene, —PCHjCHR—y,.

For disubstituted ethylenes, the presence and type of tacticity depends on the positions of substitution and the identity of the substituents. In the polymerization of a 1,1-disubstituted ethylene, CH2=CRR, stereoisomerism does not exist if the R and R groups are the same (e.g., isobutylene and vinylidene chloride). When R and R are different (e.g., —CH3 and —COOCH3 in methyl methacrylate), stereoisomerism occurs exactly as in the case of a monosubstituted ethylene. The methyl groups can be located all above or all below the plane of the polymer chain (isotactic), alternately above and below (syndiotactic), or randomly (atactic). The presence of the second substituent has no effect on the situation since steric placement of the first substituent automatically fixes that of the second. The second substituent is isotactic if the first is isotactic, syndiotactic if the first substituent is syndiotactic, and atactic if the first is atactic. 

Consider the description of the sequence distribution of isotactic and syndiotactic placements in the polymerization of a monosubstituted ethylene. The approach is general and can be applied with appropriate modification to the 1,4-polymerization of a 1,3-diene. Dyad tac-ticity is defined as the fractions of pairs of adjacent repeating units that are isotactic or syndiotactic to one another. The isotactic and syndiotactic dyads (XV) are usually referred to as meso and racemic dyads. The horizontal line in XV represents a segment of the polymer... 

Selective modification of polyols such as ethylene glycol, 1,3-propylene glycol, or glycerol with butadiene (1) has been studied [7-10]. The monosubstituted compounds are preferred due to their potential applications as surfactants, PVC plasticizers, or even in cosmetics. The telomerization of 1 with ethylene glycol yields a complex mixture including linear and branched mono- and ditelomers, as well as 1,3,7-octatriene and vinyl cyclohexene (Fig. 2) [11]. 

Ethylbenzene under Benzene and Monosubstituted Benzene Hydrocarbons Ethyl Chloride Chloroethane under Saturated Alkyl Halides Ethyl Lactate Hydroxypropanoic Acid, Ethyl Ester under Esters Ethylene under Alkenes, Cyclic Alkenes, and Dienes... 

The infrared spectra of some monosubstituted and the unsubstituted oxadiazoles show the vibration bands of the protons directly attached to the heterocycle in position 3 or 5- The wave length is 3.19—3.22 p, i.e. almost the same as for an ethylenic CH bond. 

According to the reaction scheme of the Schenck reaction one should expect four more products in the case of (+)-carvomenthene (19) and at least four more products in the case of (+)-limonene (16). The fact that with limonene no products were obtained which originate from an oxygen attack on the A8 double bond, is in agreement with the general rule that type II photosensitized oxygenation reactions occur much faster with tri- and tetrasubstituted ethylenes than with di- or monosubstituted ones (see p. 71). 

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