Benzocyclobutadienes: An Unusual Mode of Access Reveals Unusual Modes of Reactivity

Abstract

The reaction of an aryne with an alkyne to generate a benzocyclobutadiene (BCB) intermediate is rare. We report here examples of this reaction, revealed by Diels-Alder trapping of the BCB by either pendant or external electron-deficient alkynes. Mechanistic delineation of the reaction course is supported by DFT calculations. A three-component process joining the benzyne first with an electron-rich and then with an electron-poor alkyne was uncovered. Reactions in which the BCB functions in a rarely observed role as a 4π diene component in Diels-Alder reactions are reported. The results also shed new light on aspects of the hexadehydro-Diels-Alder reaction used to generate the benzynes.

Direct observation ofo-benzyne formation in photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions

“…products were separated using ash chromatography. The structural characterization conrmed a product with a dimeric structure, i.e., a naphthalene derivative, 21,94 denoted Selftrapping-1 with its structure illustrated in Fig. 1 (path 2, green).…”

“…12 Instead of forming cyclohexene derivatives, the HDDA reaction [13][14][15][16] is believed to produce a highly reactive o-benzyne intermediate, and is of much current interest due to its unusual reactivity/mechanism [17][18][19][20][21][22][23][24][25][26][27] and numerous applications in synthetic chemistry. [28][29][30][31][32][33][34][35] A series of reports within the last three years, [36][37][38][39][40][41][42][43] examining the HDDA reaction from a more mechanistic viewpoint, demonstrate the immense versatility of HDDA-generated o-benzynes, including reaction with an alkyne forming a highly reactive benzocyclobutadiene 21 or reaction with electron-decient thioamides giving stabilized ammonium ylides in a new type of [3 + 2]-cycloaddition. 24 These and other examples emphasize the importance of HDDA-generated o-benzynes for the discovery of new reaction pathways.…”

CuI‐Mediated Bromoalkynylation and Hydroalkynylation Reactions of Unsymmetrical Benzynes: Complementary Modes of Addition

“…It is noteworthy that we have never observed a product indicating premature reaction of the HDDA polyyne precursor with any of the active organometallic species involved in this chemistry . Moreover, we observed no alkynylation product formation in the absence of the copper catalyst …”

Highly Conjugated π‐Systems Arising from Cannibalistic Hexadehydro‐Diels–Alder Couplings: Cleavage of C−C Single and Triple Bonds

“…Inspection of the molecular structure reveals that it is an aphthalene derivative (compound 8), at ype of reaction product recently reported by Hoye from multiple linked polyalkynes. [12] Naphthalene 8 arises from af ormal CCt riple bond cleavage process. The mechanism of this process will be discussed in detail below,b ut first we shall address the nature of additional intriguing self-trapping products.…”