Cyclobutene Formation in PtCl₂‐Catalyzed Cycloisomerizations of Heteroatom‐Tethered 1,6‐Enynes

Abstract

Aza(oxa)bicyclo[3.2.0]heptenes are accessed through the PtCl2 -catalyzed cycloisomerizations of heteroatom-tethered 1,6-enynes featuring a terminal alkyne and amide as the solvent. It is shown that the weak coordinating properties of the solvent and alkyl substituent(s) at the propargylic carbon atom favor the formation of cyclobutenes instead of other possible cycloisomerization products such as 1,3-diene derivatives or cyclopropane-fused heterocycles.

Ruthenium‐Catalyzed [2+2+2] Cycloaddition of 1,6‐Enynes and Unactivated Alkynes: Access to Ring‐Fused Cyclohexadienes

“…Gratifyingly, tertiary propargyl alcohols 6 a – d afforded adducts 7 ba – bd as single regioisomers, although the reactions required 24 h for complete conversion. This trend was also observed with tertiary propargyl sulfonamide 6 e and alkyne 6 f , featuring the bulky TMS group, which afforded 7 be (64 %) and dienylsilane 7 bf (83 %). We wondered about the regioselectivity observed in these reactions, especially for cases involving tertiary propargyl alcohols, insofar as the coordination of the free hydroxyl group to ruthenium being invoked in a number of catalytic reactions involving this class of substrates .…”

“…On the basis of our previouso bservations, [18] we thought that increasing the steric bulk at the propargyl carbon atom of 6 could impacto nt he regioselectivity,e ven though the selectivity of the reaction( self-couplingv ersus cross-coupling) might be eroded. Gratifyingly,t ertiaryp ropargyl alcohols 6a-d afforded adducts 7ba-bd as single regioisomers,a lthough the reactionsr equired 24 hf or complete conversion.T his trend was also observed with tertiary propargyl sulfonamide 6e [21] and alkyne 6f,f eaturing the bulky TMS group, which afforded 7be (64%)a nd dienylsilane 7bf [18] (83 %). We wondered about the regioselectivityo bserved in theser eactions, especially for cases involving tertiary propargyl alcohols, insofar as the coordination of the free hydroxyl group to ruthenium being invoked in an umber of catalytic reactions involving this class of substrates.…”

Unveiling the uncatalyzed reaction of alkynes with 1,2-dipoles for the room temperature synthesis of cyclobutenes

“…Although metal-catalyzed strategies have merged recently, their widespread use in cyclobutene synthesis is precluded due to the narrow substrate scope and moderate selectivity, [10][11][12][13] or because of the use of environmentally unsafe or expensive transition-metal salts. [14][15][16][17] Alkynes are useful starting materials for the preparation of a variety of different compounds, 18,19 and when used as reactants in 1,3dipolar cycloaddition reactions they typically afford five-membered cycles. By analogy, the reaction of alkynes with 1,2-dipoles through a 1,2-dipolar cycloaddition reaction may be a possible solution to produce cyclobutenes with high reaction efficiency.…”

THF-enabled PtBr2-catalyzed desymmetric hydrogenative [3 + 2] cycloaddition of 2-alkynylbenzaldehyde-tethered cyclohexadienones

“…Incomplete deuterium incorporation might be attributed to Pt( ii )-catalyzed deuterium/hydrogen exchange due to the presence of residual water in the reaction media. 21…”