research summaries f o r teachers WILLIAM R. DOLBIER, Jr
organic chemistry
Vniverrity of Florida
Electrophilic Additions to Alkenes
The major goal in any course of organic chemistry must hc to impart to one's students a basic understanding of t,he nat,ure of molecular reactivity and the effects of structure and reaction conditions on this reactivity. One obstacle t,o t,he conveyance of such understanding is a superficial classroom treatment of those topics which when treat,ed in some depth are capable of imparting real insight to one's students. While it may appear a t times more practical t o teach for example that aliphatic nucleophilic displacement reactions proceed by eit,her SN1or SN2mechanisms, one surely must realize that the desired level of perception will bc stimulated only by a treatment of this subject in greater dept.h. Thus, most modern organic chemistry courses introduce the concept of borderline mechanisms and discuss the importance of ion-pairs in nucleophilic displacement. reactions, including discussions of the effects of stmct.ure and reaction conditions on mechanism. However, in these same courses, one is generally content. t,o discuss electrophilic halogen and hydrogen halide additions to allm1es in terms of halonium and open carboilium ions, respectively. These oversimplifications are generally rationalized on the basis that the total picture is too little understood or too complicated t,o present a t this level. Happily, enough research has been done in recent an e n l i ~ h t e n i ndiscussion ~ of vears to uresent~lv " nermit . electrophilic additions t,o alBenes even a t the undergraduate level. Just as it can be demonstrated that SN1and SN2mechanisms represent merely the two extremes of a whole spectrum of available pathways for nuclcophilic displacement,, so also may it he shown that there is a broad spectrum of operative mechanisms for electrophilic additions to akenes where the onium (halonium or protonium) ion and the open carbonium ion . . represent only the t,wo extremes. u
Stereochemistry of Addition
I n examining the available literature one can find examules both of stereosuecific trans and stereoselective cis electrophilic additions to alkenes (1-4). CH3*.-'
-
+ C1, nest, -99:
exclusive trans addition (1)
-% H a
H 342
,H
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Journal of Chemical Education
H..
.=,,*H
+
1
+
DBr
CH,CI,. -0°C
75% ris addition
(3)
Ph
H..
,.H .=I
CH,CI, -09:
A
83%cis addition
(4)
Ph
I t is possible, nevertheless, t o encompass all electrophilic additions t o alkenes with one unifginy mechanistic picture.' To he able to understand these reactions in this way is quite an exciting feat and is the kind of thing which will he greatly appreciated by any budding scientists in a n introductory organic chemistry course. The stereochemical course of electrophilic addition reactions is determined largely by the relative stahilities of the onium ion and the open carhonium ion. To the extent that the onium ion is favored, trans products will be favored, and as the open carbonium ion becomes more stable, cis addition becomes favored. The relative stabilities of the two types of intermediate ion are determined largely by the nature of the adding species and/or the structure of the alkene. The Adding Species
Two factors are believed to contribute to bonding in onium species (5). To some extent the intermediate can be formulated as a s-complex in which the filled aorhital of the alkene is used to form a dative hond hy interaction with the empty valence orbital of the acceptor X + . On the other hand, if the acceptorX+ has unshared p- or d-electrons, these can he used to form a re' verse dative hond t o the alkene hv interaction with the empty anti-bonding a-molecular "orbital of the alkene. From Table 1, one can see that I and ArS are most capable of forming stabilized onium ions, if one, by utilizing a common alkene, can take trans-stereoselectivity as a measure of relative onium ion stabilitv. The ability of X + to form a stable onium ion seems to decrease in the order: S, I > Br > C1 > F > H. Thus, thelarThere is, however, no textbook, iutroductary or advanced, which makes the attempt to present s w h s. unified picture.
Table 1 .
b
The Effect of the Adding Species on the Stereochemistry of Electrophilic Additions to Alkenes
Adding Species
Conditions
Alkene
% trans Addition
DBr Fz Clz Brn INCO ArSCl
C&Cb, O°C CCIJF, -126°C CH2Cb, O T CCI,, 2 3 ' C EbO CCL, 25°C
I-Phenylpropene 1-Phenylpropene 1-Phenylpropene 1-Phenylpropene Qdeuteriostyrene 1-Phenylpropene
12 22*, 2 P 25*, 33 X3s, 8Sb 100 100
Ref.
(4 (6) (3) (7)
(8) (7)
From cis alkene. From trnns alkene
ger and more eleetro-positive atoms form the most stable onium species. Independent and concurring evidence for the relative stabilities of the various halonium ions comes from studies of halogen as a neighboring group in solvolysis reactions (9). While neighboring sulfur, iodine, and bromine are clearly able to assist in ionization, there is no evidence for participation by neighboring chlorine or fluorine in solvolyses reactions. trans-2-halocyclohexyl brosylates (10)
Table 2. Effect of Structure of the Alkene and Polarity of the Solvent on Stereochemistry of Chlorination
Alkene cis-2-butene cis-2-hntene
Table 3.
9 9 9 OBs
OBs
Structure of the Alkene
The structure of the alkene appears to be unimportant in cases where extremely stable onium ions can be formed, such as in the ease of iodonium or episulfonium ions. I n these cases the onium ion takes precedence even where open benzylie earbonium ions could he formed. I n the ease of bromine addition, only with potential benzylie carbonium ions does the open ion play a role, albeit generally a minor one. A maximum of 37% cis adduet has been detected for bromine addition to trans-anethole (7). I n the ease of onium ions less stable than bromonium, however, the structure of the alkene is of major significance in determining the stereoehemical outcome of the reaction. As we can see from Table 2, chlorination runs the gamut from essentially totally trans to 78% cis addition. The borderline situation of ehloronium ion is indicated nicely by some more of Olah's low temperature nmr work. While, as shown earlier, the tetramethyl
Ref.
100 100
(1
Dichloride Products From Chlorination of Methyl Cinnamotes in HOAc a t 20°C
p-suhst. NOz CFs C1 CI H H Me Me0 Me0
Rate Enhancement; trans/&
X=Cl,Br,I Y =F,CI,Br
D/, trans Product
-%
OBs
Such rate enhancements coupled with the retention of configuration in the solvolysis products leads the authors to invoke bromonium (11) and iodonium ions as intermediates in these reactions. Direct observation of halonium ions by low temperature nmr has also been aeeomplished (It). Bridged halonium ions were observed to arise on ionization of 2,3-dihalo-2,3-dimethylbutanesas shown below. 2,3Difluoro-2,3-dimethylbutane, on the other hand, gave or-fluoroisopropyldimethylcarbonium ion in which the fluorine atom is rapidly exchanging intramoleeularly between the two equivalent sites.
Conditions neat, - 9 T HOAc. 2 5 T
Cinnamate isomer used
threo adduct
trans trans trans cis tfans
13 16
C18
trans tfans CIS
39
11 40 11 49 23 5
Yields.---erythro adduct trace trace 1.5 52 12 43 28 77 90
In all cases the major other product w m the acetoxychloride which would be obtained from trans addition of acetat,e ion to the intermediate ehloronium ion. a
ethylene ehlorinium ion was more stable that the open carbonium ion isomer, in ionization of l-ehloro-2fluoro-2-methylpropane the open-chain ion, ehloro-tbutyl catsionwas formed, rather than the 1,l-dimet,hylethylene ehloronium ion (14).
For I and Br the onium ion is still shown to be more stable than the open ion. Addition of H X also varies tremendously, with addition of HBr to 1,2-dimethylcyelopenteneproceeding exclusively trans (15), while addition of DBr onto l-phenylpropenes proceeds 88% cis (4). Addition of fluorine has had only limited study with additions generally proceeding cis. However, additions to simple aliphatic alkenes have not beeninvestigated, and it is probable that such studies would turn up a system where trans addition would become important. There is some indication from recent studies on chlorination of methyl cis- and trans-cinnamates (16) that as the open carbonium ion becomes more and more stable, these reactions become largely non-stereoselective and the products become relatively independent of the geometry of the alkene (see Table 3). Thus, in such eases onc has apparently reached the other end of the spectrum from the onium ion, that is the totally "free" open carbonium ion. Volume 46, Number 6, June 1969
/ 343
Polarity of Solvent
It is also possible that the polarity of the solvent may play a small role in bowlerline situations. For example, the amount of cis adduct formed from hromination of cis-st,ilbene has been found to he larger in polar than in nonpolar solvenk (17). Generally, though, as seen from Table 2, solvent polarity does not play a major role in determining stereochemistry, although the scope of polarities studied is yet quite limited. A Unified Mechanistic Piclure
The above results can be explained by the formulation of the following detailed mechanistic scheme.
the carhonium ion becomes yet more stahle and thus more long-lived, such as where X = C1, R' = p-anisyl, and R = C02CH,, the reaction will proceed on through intermediates V and VI which will give rise t o a nonstereospecific, thermodynamically-controlled product mixture. Thus with a basic understanding of the importance of alkene structure and nature of adding species, a student can realistically he expected t o he ahle to predict trends in series of electrophilic addition reactions and to rationalize these trends mechanistically. With some care of presentation, one should be ahle to impart a real understanding of electrophilic additions to allcenes to one's students in little more time than it takes to make one's current presentation. Literature Cited ( 1 ) POUTSMA, M. L., J . Am. Chem. Soe., 87, 2172 (1965). ( 2 ) MUELLER, W. H., A N D BUTLER, P. E., J . Am. Chem. Soe., 88, 2866 (1966). ( 3 ) FAHEY,I t . C., AND ~CHUBERT, C., J . Am. Chem. Soe., 87. 5172 119661. , ~, ( 4 ) DRWAR, M. J. S., AND FAHEY,R.. C., J. Am. Chem. Soc., 85, 3645 (1963). R., c.,Angew. Chem. internal. ( 5 ) DINAR, M. J. S., AND F ~ H E Y Edil., 3 , 246 (1964). (6) MERRIT, R. F., J . Am. Chem. Soc., 89, 609 (1967). (7) FAHEY, R. C., AND SCHNEIDER, H. J., J . Am. Chem. Soc., 90. 4429 (19681. ( 8 ) H A S S M IA,, ~ , AND HT:ATHCOCK, C. C., Tetrahedron Letters, 1125 (1964). (9) CAPONE, B., Quart. Reu. (London), 18,45 (1964). (10) BROWN,H. C., Chem. Eng. News, 45.87 (Feb. 13,1967). X (11) A complete discussion of the
&
"HR' Vl
I
v
cis adduct
cis adduct
trans adduct
I n systems where the onium ion is more stahle such as whcrc X = I or ArS, the initially formed ?r-complex, I, will collapse primarily to I1 which will lead to predominant trans product. As the carbonium ion becomes more stable, such as where X = H , CI, or F and R and/or R' = Ph, I1 will he less important, and species I11 and/or I V will he formed, which if quenched to product lead to predominant cis stereochemistry. As
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Journal of Chemical Educofion
cis or trans adduct
evidence for the bromonium ion may be found in: 3. E. TRAYNHAM. J.CHEM. EDUC.. 40, 392 (1963). (12) OLAH,G. A,, AND BOLLINGER, J. M., J . Am. Chem. Soe., 89, 4744 (1967). BLY, R . S., J. Am. Chem. Soc., 82,
(13) CRISTOL, S. J., AND 142 (1960). (14) OMH, G. A,, AND BOLLINRER, J. M., J. Am. Chem. Soc. 90. 947 11968). , ~ - , (15) HAMMOND, G. S., A N D COLLINS, C. H., J . Am. Chem. Soe., 82, 4323 (1960). (16) JOHNSON, M. D., AND TRACEITENBERG, E. N., J. Chem. Soe. ( B ) , 1018 (1968). (17) HEUBLEIN, J., J . Prakt. Chem., 31, 84 (1966).
