Alkene addition reactions stereochemistry

Interactive to predict the products and stereochemistry of alkene addition reactions. 

Why does alkene hydroboration take place with non-Markovnikov regiochemistry, yielding the less highly substituted alcohol Hydroboration differs from many other alkene addition reactions in that it occurs in a single step without a carbocation intermediate. We can view the reaction as taking place through a four-center, cyclic transition state, as shown in Figure 7.6 p. 244). Since both C-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry is observed. 

From what you know about the stereochemistry of alkene addition reactions, predict the configurations of the products that would be obtained from the reaction of 2-butyne with the following ... 

CHAPTER 6 REACTIONS OF ALKENES ADDITION REACTIONS 124 CHAPTER 7 STEREOCHEMISTRY 156 CHAPTER 8 NUCLEOPHILIC SUBSTITUTION 184 CHAPTER 9 ALKYNES 209... 

An addition to an alkene can form up to two new chiral centers, and a reaction that occurs with only a syn or only an anti addition mechanism will give a product with predictable stereochemistry. Conversion of alkynes to alke-nes can also occur with either syn or anti stereoselectivity. When these alkyne reductions are taken in combination with alkene addition reactions, target molecules with a wide variety of stereochemical relationships can be prepared. 

This chapter and Chapter 10 continue our cataloging of the standard reactions of organic chemistry. To the SnI, Sn2, El, and E2 reactions we now add a variety of alkene addition reactions. Although there are several different mechanisms for additions, many take place through a three-step sequence of protonation, addition, and deprotonation. The following new problems allow you to practice the basics of addition reactions and to extend yourself to some more complex matters. Even simple additions become complicated when they occur in intramolecular fashion, for example. These problems also allow you to explore the influence of resonance and inductive effects, and to use the regiochemistry and stereochemistry of addition to help work out the probable mechanisms of reactions. 

Section 7 13 Addition reactions of alkenes may generate one (Section 7 9) or two (Sec tion 7 13) chirality centers When two chirality centers are produced then-relative stereochemistry depends on the configuration (E or Z) of the alkene and whether the addition is syn or anti... 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkable example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alkene electrophilic reactions. Subsequently, strong evidence supporting the mechanism came from the work of George Olah, who prepared and studied stable... 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

A summary of addition reactions of alkenes with 1-methylcyclopentene as the organic substrate. A bond designated means that the stereochemistry of the group is unspecified. For brevity the structure of only one enantiomer of the product is shown, even though racemic mixtures would be produced in all instances in which the product is chiral. 

The reaction of phenyl-substituted alkenes (2-phenylprop-l-ene, ( )-l-phenylprop-l-ene, 1,1-diphenylethene, 1,1-diphenylprop-l-ene) with F-Teda BF4 (6) in the presence of various alcohols results in the formation of vicinal fluoro alkoxy adducts with Markovnikov-type regioselec-tivity.89,94 The stereochemistry of the fluorination-methoxylation addition reaction is slightly syn predominant in the case of (Z)-stilbene, indene, and dibenzosuberenone, while equal amounts of both diastereoisomers are formed in the case of ( )-l-phenylprop-l-ene and acenaphthylene. 

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