Unsymmetric reagents

Markovnikovs rule (Section 6 5) An unsymmetrical reagent adds to an unsymmetncal double bond in the direction that places the positive part of the reagent on the carbon of the double bond that has the greater number of hydrogens... 

When an unsymmetrical reagent is added to an unsymmetrical substrate, the question arises Which side of the reagent goes to which side of the double or triple bond The terms side and face are arbitrary, and a simple guide is shown to help understand the arguments used here. For electrophilic attack, the answer is given by... 

In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate. 

D.R. McMillin, Purdue University You have suggested that certain oxidants are more sensitive to steric effects when reacting with the tin alkyls. Is there evidence for preferential attack of the less bulky alkyls when unsymmetrical reagents such as SnR2R2 or SnR3R are used ... 

Prototem 6.23 Unsymmetrical reagents like HX add to unsymmetrical alkenes such as propene according to Markovnikov s rule the positive portion, e.g., H of HX, adds to the C that has more H s ( the rich get richer ). Explain by stability of the intermediate cation. ... 

When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

Thus the problem of predicting which of the two possible products will be favored in the addition of unsymmetrical reagents to alkenes under kinetic control reduces to predicting which of two possible carbocation intermediates will be formed most readily. With simple alkenes, we shall expect the preference of formation of the carbocations to be in the order tertiary > secondary > primary. 

Like alkenes, the double bonds of ,/3-unsaturated acids can be brominated, hydroxylated, hydrated, and hydrobrominated, although the reactions often are relatively slow. In the addition of unsymmetrical reagents the direction of addition is opposite to that observed for alkenes (anti-Markownikoff). Thus propenoic (acrylic) acid adds hydrogen bromide and water to form 3-bromo-and 3-hydroxypropanoic acids ... 

Finally, we will mention a paper171 in English that deals with historical aspects of the Markovnikov Rule for the orientation of addition of an unsymmetrical reagent to C=C. The origins of this rule can hardly be deemed to be part of physical organic chemistry, but the rule has of course been of great significance in the development of various aspects of the subject. 

IB Diels-Alder Reactions Using Unsymmetrical Reagents... 

Q Predict the products of Diels-Alder reactions, including the orientation of cycloaddition with unsymmetrical reagents and the stereochemistry of the products. 

The nitrogen atom in A3-piperideines would be expected to influence the addition of unsymmetrical reagents to the double bond. Although this effect has not been extensively studied, the results on the hydroboration-oxidation of a number of N- substituted A3-piperideines show a preference for the piperidin-3-ol (Scheme 16) (70JOC802). Initial coordination of borane with the nitrogen atom was proposed and the observed regiochemistry is believed to be the result of electronic factors (70TL1133). 

With substituted unsymmetrical benzene-1,2-diamines and unsymmetrical reagents this type of reaction leads to isomeric products. Regioselective synthesis of quinoxaline derivatives and the effect of substituents at C3 in the aromatic ring on annulation has been studied for unsymmetrically substituted starting benzene-1,2-diamine derivatives. ... 

If the addition of an unsymmetrical reagent X-Y to an unsymmetrical olefin proceeds without skeletal rearrangement to give exclusively one of two or more possible isomers it is termed regiospecific. If, however, there is a significant preponderance of one isomer the reaction is said to be regioselective (Sec Ref. 358). 

Addition of an unsymmetrical reagent to a double bond may in general yield two different products. In our discussion of alkenes (Secs. 6.11 and 6.17), we found that usually one of the products predominates, and that we can predict which it will be in a fairly simple way in either electrophilic or free-radical addition, the first step takes place in the way that yields the more stable particle, carbonium ion in one kind of reaction, free radical in the other kind. Does this rule apply to reactions of alkenylbenzenes ... 

In general, it is observed that addition of an unsymmetrical reagent to an a,)3-unsaturated carbonyl compound takes place in such a way that hydrogen becomes attached to the a-carbon and the negative group becomes attached to the -carbon. For example ... 

The other approach being made at the University of Texas has to do with the specificity of ozone attack which occurs with unsymmetrical olefins. This is analogous to the specificity observed in the additions of unsymmetrical reagents such as hydrogen chloride to unsymmetrical olefins. 

When an unsymmetrical reagent adds to an unsymmetrical alkene, two addition products are possible. When the electrophile bonds, it can bond to either carbon of the carbon-carbon double bond to form two different carbocations. The more stable carbocation is favored and the addition product resulting from the more stable carbocation intermediate is the predominant product. 

The mechanism of electrophilic addition to alkynes is the same as with alkenes. Orientation of addition of unsymmetrical reagents to unsymmetrical alkynes is determined by the stability of the intermediate carbocation. 

In reality, however, only 2-bromopropane is formed. This phenomenon was observed in all reactions between unsymmetrical reagents and alkenes. In 1871, Vladimir Markovnikov postnlated a generalization that enables ns to predict the ontcome of snch an addition reaction. This generalization, now known as Markovnikov s rule, states that in the addition of nnsymmetrical (that is, polar) reagents to alkenes, the positive portion of the reagent (nsnally hydrogen) adds to the carbon atom that already has the most hydrogen atoms. 

As expected, 3,4-dihydro-2//-pyrans show reactions of electron-rich double-bonded systems. For instance, electrophilic addition of HX and HOX (X = halogen) as well as hydroboration occur regioselectively with unsymmetrical reagents ... 

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