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Baldwin's rules in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
are a series of guidelines outlining the relative favorabilities of ring closure reactions in alicyclic compounds. They were first proposed by Jack Baldwin in 1976. Baldwin's rules discuss the relative rates of ring closures of these various types. These terms are not meant to describe the absolute probability that a reaction will or will not take place, rather they are used in a relative sense. A reaction that is disfavoured (slow) does not have a rate that is able to compete effectively with an alternative reaction that is favoured (fast). However, the disfavoured product may be observed, if no alternate reactions are more favoured. The rules classify ring closures in three ways: *the number of atoms in the newly formed ring *into ''exo'' and ''endo'' ring closures, depending whether the bond broken during the ring closure is inside (''endo'') or outside (''exo'') the ring that is being formed *into ''tet'', ''trig'' and ''dig'' geometry of the atom being attacked, depending on whether this
electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carr ...
carbon is ''tet''rahedral ( sp3 hybridised), ''trig''onal ( sp2 hybridised) or ''diag''onal ( sp hybridised). Thus, a ring closure reaction could be classified as, for example, a 5-''exo-trig''. Baldwin discovered that orbital overlap requirements for the formation of bonds favour only certain combinations of ring size and the ''exo/endo/dig/trig/tet'' parameters. There are sometimes exceptions to Baldwin's rules. For example, cations often disobey Baldwin's rules, as do reactions in which a third-row atom is included in the ring. An expanded and revised version of the rules is available: The rules apply when the nucleophile can attack the bond in question in an ideal angle. These angles are 180° (
Walden inversion Walden inversion is the inversion of a stereogenic center in a chiral molecule in a chemical reaction. Since a molecule can form two enantiomers around a stereogenic center, the Walden inversion converts the configuration of the molecule from ...
) for ''exo-tet'' reactions, 109° ( Bürgi–Dunitz angle) for ''exo-trig'' reaction and 120° for ''endo-dig'' reactions. Angles for nucleophilic attack on alkynes were reviewed and redefined recently. The "acute angle" of attack postulated by Baldwin was replaced with a trajectory similar to the Bürgi–Dunitz angle by Alabugin and coworkers. This change, fully consistent with the stereoelectronic rules for nucleophilic addition at a pi-bond, reversed Baldwin's predictions for alkyne cyclizations. The combined body of experimental and computational data, not available to Baldwin, strongly supports the notion that exo-cyclizations are more favorable than endo-cyclizations for both alkenes and alkynes.


Applications

In one study, seven-membered rings were constructed in a
tandem Tandem, or in tandem, is an arrangement in which two or more animals, machines, or people are lined up one behind another, all facing in the same direction. ''Tandem'' can also be used more generally to refer to any group of persons or objects w ...
5-''exo-dig''
addition reaction In organic chemistry, an addition reaction is an organic reaction in which two or more molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, ...
/
Claisen rearrangement The Claisen rearrangement is a powerful carbon–carbon chemical bond, bond-forming chemical reaction discovered by Rainer Ludwig Claisen. The heating of an allyl Vinyl group, vinyl ether will initiate a Sigmatropic reaction, ,3sigmatropic r ...
: : A 6-''endo-dig'' pattern was observed in an
allene In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms (, where R is hydrogen, H or some organyl group). Allenes are classified as diene#Classes, cumulated dienes ...
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alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
1,2-addition / Nazarov cyclization tandem catalysed by a
gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
compound: A 5-''endo-dig'' ring closing reaction was part of a synthesis of (+)-Preussin: :


Rules for enolates

Baldwin's rules also apply to aldol cyclizations involving
enolate In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the Organic synthesis, synthesis of organic compounds. Bonding and structure Enolate ...
s. Two new descriptors need to be defined: ''enolendo'' and ''enolexo'', which refer to whether both carbons of the enolate C-C fragment are incorporated into the ring formed or not, respectively. : The rules are the following:


Exceptions

These rules are based on empirical evidence and numerous "exceptions" are known.Finding the right path: Baldwin "Rules for Ring Closure" and stereoelectronic control of cyclizations. Alabugin, I. V.; Gilmore, K. Chem. Commun., 2013, 49, 11246 – 11250. http://pubs.rsc.org/en/content/articlehtml/2013/cc/c3cc43872d Examples include: * cyclisations of
cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s * reactions involving third-row atoms, such as
sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
* Transition metal catalysis


References

{{Reflist, 2 Eponymous chemical rules Physical organic chemistry Stereochemistry