Additions to Conjugated Enynes

Electronic and steric factors seem to determine both the site and the orientation of addition of diazoalkanes to conjugated enynes. Although rates of addition of DPD to ethene and ethyne bearing identical single substituents are approximately the same,48 addition to butenyne occurs almost exclusively at the double bond.46 

---

Not only electrophilic 1,4-addition, as shown above, but also radical 1,4-addition to conjugated enynes such as selenosulfonation is known to yield acceptor-substituted allenes [118]. Finally, monotitanation of conjugated diynes followed by treatment with benzaldehyde and aqueous workup leads to an ester of penta-2,3,4-tri-enoic acid, which is formally also a product of 1,4-addition [147]. 

Although not specified in the table, electrophilic addition to conjugated enynes (as in Scheme 6.70) appears to provide both 1,2- and 1,4-addition simultaneously. Thus, as shown in Scheme 6.71, the addition of hydrogen chloride across the conjugated enyne, but-3-en-l-yne (vinylacetylene, HC C-CH=CH2) produces both 2-chloro-l,3-butadiene (chloroprene [H2C=C(C1)CH=CH2]) and 4-chloro-l,2-butadiene (H20C=CHCH2C1). 

Although the path (a) has been verified by a stoichiometric reaction [23], the details of exact reaction mechanism remain unsettled. Triggered by this publication [and the Pd-catalyzed doublethiolation of alkynes described in Eq. (7.7) in Section 7-3], a number of transition metal-catalyzed additions of S-X or Se-X bonds to C-C unsaturated organic compounds started to be published. In 1994, BackvaU et al. applied the Pd(OAc)2-catalyzed hydrothiolation to conjugated enynes and obtained 17,... 

Scheme 6.85 Formation of l,2-dioxa-3,4-cyclohexadienes (415) by addition of singlet oxygen to conjugated enynes and retrograde dehydro Diels-Alder reaction of415.

Other conjugated terminal enynes reacted in a similar fashion but the more substituted the double bond, the less efficient the allylzincation reaction168,172,173. By comparison, organolithium reagents and allylmagnesium bromide underwent conjugate addition to terminal enynes with nucleophilic attack of the double bond168,171,172. 

Addition of the selenolate that is prepared from diphenyl diselenide by reduction with NaBH4 to conjugate enyne sulfones 20 occurs at the d-position of the sulfonyl group to give 4-phenylseleno-l-sulfonylbuta-1,3-dienes 21 regio- and stereoselectively (Scheme 28) [50]. On the other hand, addition of PhSeNa to the conjugated alkynyl sulfone 22 that has a -phenylseleno substituent affords the anti-Michael adduct 23 preferentially (Scheme 29) [51]. 

Scheme 8.21 Addition of in-situ-generated carbamates to conjugated enynes.

For a review of the addition of organometallic reagents to conjugated enynes see Miginiac, L. 

The addition of organolithiums to conjugated enynes occurs to the double bond - ... 

Note the presence of a product owing to further addition of the initial addition product to another butadiene, followed by cyclization this product of further reaction occurs even when, as in this example, a 1 1 stoichiometry is used. Additions are also observed to conjugated enynes . ... 

Additions to conjugated systems are further facilitated by some of the features discussed previously e.g., additions to dienes" " and enynes" in the presence of transition-metal catalysts and additions to enynes containing a suitably placed intramolecular function such as hydroxyl. Although uncatalyzed intermolecular additions to alkynylsilanes are not yet demonstrated, additions are seen in the presence of a transition-metal catalysts" and intramolecular additions to alkynylsilane functions by suitably placed internal organomagnesium-halide functions are facile. ... 

Benzenesulfenyl-1,3-dienes. These compounds are now available from Pd(II)-catalyzed addition of PhSH to conjugated enynes. They undergo oxidation to the sulfoxides, which are useful for the preparation of functionalized allylic alcohols. 

F. ADDITION OF SOFT CARBON NUCLEOPHILES TO CONJUGATED ENYNES... 

The Pd(OAc)2-catalyzed hydrothiolation procedure can be applied to the regioselective addition of thiols and selenols to conjugated enynes and aUenes. which provides a useful method for the synthesis of vinyUc sulfides and selenides (Scheme 13). 

Carbene Addition to Conjugated Olefins Table 2.6. Addition of dichlorocarbene to conjugated enynes... 

Scheme 3.16 Synthesis of 1,6-enynes 11-22 via 1,4-addition to conjugated enones...

Backvall JE, Ericsson A (1994) Palladium-catalyzed regioselective addition of thiophenol to conjugated enynes. Efficient synthesis of 2-(phenylsulfinyl) and 2-(phenylsulfonyl) 1,3-dienes. J Org Chem 59 5850-5851... 

The alkynyl iodide 150 undergoes the oxidative addition to form an alky-nylpalladium iodide, and subsequent insertion of an alkene gives the conjugated enyne 151 under phase-transfer conditions[120]. 

---