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(javascript required). 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) 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. :
A 5-''endo-dig'' ring closing reaction was part of a synthesis of (+)-Preussin:
:
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 materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
are a series of guidelines outlining the relative favorabilities of ring closure reactions in alicyclic
In organic chemistry, an alicyclic compound contains one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character. Alicyclic compounds may have one or more aliphatic side chains attached.
The ...
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 car ...
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
Orbital may refer to:
Sciences Chemistry and physics
* Atomic orbital
* Molecular orbital
* Hybrid orbital Astronomy and space flight
* Orbit
** Earth orbit
Medicine and physiology
* Orbit (anatomy), also known as the ''orbital bone''
* Orbito ...
overlap requirements for the formation of bonds favour only certain combinations of ring size and the ''exo/endo/dig/trig/tet'' parameters. Interactive 3D models of several of these transition states can be seehere
(javascript required). 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) 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. :
Applications
In one study, seven-membered rings were constructed in atandem
Tandem, or in tandem, is an arrangement in which a team of machines, animals or people are lined up one behind another, all facing in the same direction.
The original use of the term in English was in ''tandem harness'', which is used for two ...
5-''exo-dig'' addition reaction
In organic chemistry, an addition reaction is, in simplest terms, an organic reaction where two or more molecules combine to form a larger one (the adduct)..
Addition reactions are limited to chemical compounds that have multiple bonds, such as ...
/ Claisen rearrangement
The Claisen rearrangement is a powerful carbon–carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen. The heating of an allyl vinyl ether will initiate a ,3sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl, ...
:
:
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 centres (). Allenes are classified as cumulated dienes. The parent compound of this class is propadiene, which i ...
- alkyne
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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 no ...
1,2-addition / Nazarov cyclization
The Nazarov cyclization reaction (often referred to as simply the Nazarov cyclization) is a chemical reaction used in organic chemistry for the synthesis of cyclopentenones. The reaction is typically divided into ''classical'' and ''modern'' vari ...
tandem catalysed by a gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
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 involvingenolate
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.
Bonding and structure
Enolate anions are electr ...
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 ofcation
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 conve ...
s
* reactions involving third-row atoms, such as sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
References
{{Reflist, 2 Physical organic chemistry Stereochemistry