Cyclopropyl Alkynes as Mechanistic Probes To Distinguish between Vinyl Radical and Ionic Intermediates

Abstract

[reaction: see text] The reactions of (trans-2-phenylcyclopropyl)ethyne, 1a, (trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, 1b, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1c, with either aqueous sulfuric acid or tris(trimethylsilyl)silane (or tributyltin hydride) and AIBN have been investigated. Protonation and addition of the silyl (or stannyl) radical occurred at the terminal position of the alkyne giving an alpha-cyclopropyl-substituted vinyl cation or radical, respectively. Under both reaction conditions, 1a yielded products derived from ring opening toward the phenyl substituent. Alkynes 1b and 1c, however, gave different products depending on whether radical or cationic conditions were used. When radical conditions were employed, products derived from regioselective ring opening toward the phenyl substituent were obtained. In contrast, when cationic conditions were employed, products derived from selective ring opening toward the methoxy substituent were isolated. The corresponding alpha-cyclopropyl-substituted vinyllithium derivatives were also synthesized and were found to be stable toward rearrangement. An estimate of the rate constants for ring opening of the alpha-cyclopropylvinyl cations was also made: values of 10(10)-10(12) s(-1) were found for the vinyl cations derived from protonation of the terminal carbon of alkynes 1a-c. Based on these results, cyclopropyl alkynes 1a-c can be classified as hypersensitive mechanistic probes for the detection of vinyl radical or cationic intermediates generated adjacent to the cyclopropyl ring and, in the case of 1b and 1c, the distinction between a radical or cationic intermediate is possible.

Addition of a cyclopropyl alkyne to tetramesityldigermene: evidence for a biradical intermediate

“…The data also do not support the formation of a cyclopropylvinyl anion since an anion is expected to be stable towards ring opening. 12 Thus, we conclude that the addition of alkyne probe 1 to tetramesityldigermene ( 2 There are several interesting features of this reaction. The formation of a second stereoisomer of the digermacyclobutene, 5b, was unexpected; in the analogous chemistry with disilenes, only one stereoisomer of the corresponding disilacyclobutenes was observed.…”

“…Based on these principles, our group developed an alkynic mechanistic probe, 1, for distinguishing between the formation of transient vinylic radicals, cations, and anions. 12,13 If an a-cyclopropylvinyl radical is generated along the reaction pathway, the cyclopropyl ring opens regioselectively towards the phenyl substituent; whereas, if an a-cyclopropylvinyl cation is generated, the ring opens regioselectively towards the methoxy substituent (Scheme 1). An a-cyclopropylvinyl anion does not rearrange.…”

“…An a-cyclopropylvinyl anion does not rearrange. Since the magnitude of the rate constant for rearrangement of the cation or the radical is large, 12,13 ring opening competes effectively with other processes that may take place during the reaction. Using alkyne 1, we have demonstrated that the addition of alkynes to disilenes 14 and Brook silenes 15 proceeds stepwise through a biradical intermediate.…”

Radical Cyclization of α‐Bromo Aluminum Acetals onto Alkenes and Alkynes (Radic[Al] Process): A Simple Access to γ‐Lactols and 4‐Methylene‐γ‐Lactols

“…Under the same reaction conditions, the cyclization of 9 r led to methylene-g-lactol 10 r in 78 % yield, without ring-opening of the cyclopropylvinyl radical intermediate ( Table 2, entry 13). [28] Mechanistic considerations: The following mechanistic rational can be proposed to account for the results obtained in both series (O-allyl-and O-propargyl esters). Aluminum acetal 12, obtained by reduction of the a-bromo ester with DIBAL-H, undergoes halogen abstraction by the tin-centered radical, generated from nBu 3 SnH in the presence of Et 3 B/O 2 as a radical initiator, to form the corresponding carbon-centered radical 13, which undergoes a 5-exo-trig or 5-exo-dig radical cyclization to give 14 (Scheme 8).…”

Synthesis of Polysubstituted γ‐Butenolides via a Radical Pathway: Cyclization of α‐Bromo Aluminium Acetals and Comparison with the Cyclization of α‐Bromoesters at High Temperature

“…As previously observed for the synthesisofmethylene-g-lactols, [17] butenolide 2f was obtained without ring-opening of the cyclopropyl unit. [21] Surprisingly,t he reduction of bromoester 1g required an excess of DIBAL-H( 3equiv) and the one-pot sequence gave butenolide 2g only in am odest 33 %y ield (Table 1, entry 7), togetherw ith the propargylic alcohol resultingf rom the thermal decomposition of the aluminium acetal intermediate (ca. 40 %y ield).…”