Nonheteroatom stabilized chromium alkynylcarbene complexes 3,6, generated in situ from Fischer cabene complexes 1, smoothly react with furan-2-ylmethanimines 4 to provide substituted 7-aminobenzofurans 5 in moderate to good yields. The procedure involves the construction of the arene ring by a regioselective [3+3] carbocyclization reaction.

Heads or tails? Chemo‐, regio‐, and stereoselective access to cross‐ and linear conjugated poly‐ynene scaffolds from alkynyl‐derived group six metal carbenes is reported. The metal carbenes shown (M=Cr, W; TIPS=triisopropylsilyl) readily undergo low‐temperature, tail‐to‐tail dimerization in the presence of potassium tert‐butoxide by reaction at either of the Cβ atoms to provide E‐polyalkynylethenes.

Hawaii Five‐Oxazine: The sequential one‐pot reaction of a methoxy‐stabilized carbene with an acetylide and an imine provides stable, metal–carbene‐containing 2‐azetines. Subsequent mild treatment with an alkyne affords a regioselective, three‐component cyclization route to fully substituted fused 1,3‐oxazines.

We highlight the versatility of non-heteroatom-stabilized tungsten-carbene complexes 3 synthesized in situ, which have been used in a modular approach to access 2-benzazepinium isolable intermediates 5. By employing very mild conditions, benzazepinium derivatives 5 have been obtained in high yield from simple compounds, such as acetylides 2, Fischer-type alkoxycarbenes 1, and phenylimines 4. The process, involving a formal [4+3] heterocycloaddition, occurs in a totally regioselective manner, which differs from the approach previously observed in similar procedures for other carbene analogues. This work, which involves three components, reveals a control of the reactivity of non-heteroatom-stabilized carbene complexes 3 ([4+3] vs. [2+2]-heterocycloaddition reactions) depending on the acetylide substitution pattern. The influence of the substitution pattern in the behavior of the complexes has been computationally analyzed and rationalized. Finally, elaboration of the 2-benzazepinium intermediates allows access to 3H-benzo[c]azepines 6 and 3H-1,2-dihydrobenzo[c]azepines 7-9 with high control of the substitution of the nine positions of the heterocycle.

Dedicated to Professor Julius Rebek, Jr. on the occasion of his 65th birthday Small-ring heterocycles are of prominent importance because of their potential as bioactive compounds and synthetic building blocks. Whilst the chemistry of three-membered nitrogen heterocycles has been widely reported, studies on their four-membered counterparts have focused primarily on 2-azetidinones and, to a much lesser extent, azetidine rings. [1] The 2-azetine system 1,2-dihydroazete, which has a strained cyclic enamine ring, is particularly elusive. [2] Most 2-azetine compounds undergo spontaneous electrocyclic ring opening to afford their 1-azadiene analogues; as such, there are few examples of stable 2-azetines in the literature and those reported require electron-withdrawing substituents, for example, carbonyl, carboxyl, sulfonyl, or nitro groups, attached to the nitrogen atom. [3,4] As a consequence, the chemistry of 2azetines remains largely unexplored, and has been predominantly focused on their potential as a precursor to azadienes. Moreover, there are no general synthetic routes to 2azetines. [5] Although, the [2+2] cycloaddition of alkynes and imines appears to be a convenient route to this type of heterocyclic framework, only a few specific examples have been reported; in these cases, electron-rich alkynes, as ynamines, [6] alkynyl selenides, [7] or alkynyl sulphides, [8] are able to form the expected 2-azetine skeleton, which is not isolated but rapidly opens to the azadiene system. The [2+2] cycloaddition of electron-poor alkynes and imines has only previously been suggested as an intermediate in the cyclization reaction between an alkynyl(ethoxy)carbene of tungsten and imine fluorenones to afford pyrroline derivatives. [9] As part of our interest in Fischer-type metal carbenes, we have recently considered more-electrophilic metal carbenes, the so-called non-heteroatom-stabilized carbenes. [10] These compounds are particularly useful in organic synthesis, and our preliminary results have revealed significant differences in the reactivity of both heteroatom- [11] and non-heteroatomstabilized systems.Herein, we report the [2+2] cycloaddition reaction of alkynyl-substituted (pentacarbonyl)chromium or -tungsten carbene complexes with imines as a suitable procedure for accessing stable 2-azetine derivatives. Moreover, the conjugation of the resulting azetine unit with the metal carbene allows for a facile synthesis of novel cyclopenta[e]-[1,3]oxazines involving treatment with alkynes. This threecomponent process (azetine, alkyne, and CO ligand) features cleavage of the azetine C3 À C4 bond, rather than the expected N À C4 bond, and the formation of three C À C bonds and one CÀO bond (Scheme 1).Stable, easy-to-handle methoxycarbenes 1 are used as in situ precursors of alkynylcarbenes 3 using the methoxyacetylide 2 exchange reaction ( Table 1). The resultant solution of 3 in tetrahydrofuran was treated at À80 8C with 1.3-5.0 equivalents of imine 4 (see the Supporting Information) and the mixture was allowed to warm to 0 8C....