A new synthesis of cyclobutanones: highly selective carbonylation of titanacyclobutane complexes prepared by free radical alkylation

The photodissociation dynamics and ultrafast electron diffraction image of cyclobutanone from the surface hopping dynamics simulation

The comprehension of nonadiabatic dynamics in polyatomic systems relies heavily on the simultaneous advancements in theoretical and experimental domains. The gas-phase ultrafast electron diffraction (UED) technique has attracted significant attention as a unique tool for monitoring photochemical and photophysical processes at the all-atomic level with high temporal and spatial resolutions. In this work, we simulate the UED spectra of cyclobutanone using the trajectory surface hopping method at the extended multi-state complete active space second order perturbation theory (XMS-CASPT2) level and thereby predict the results of the upcoming UED experiments in the Stanford Linear Accelerator Laboratory. The simulated results demonstrate that a few pathways, including the C2 and C3 dissociation channels, as well as the ring opening channel, play important roles in the nonadiabatic reactions of cyclobutanone. We demonstrate that the simulated UED signal can be directly interpreted in terms of atomic motions, which provides a unique way of monitoring the evolution of the molecular structure in real time. Our work not only provides numerical data that help to determine the accuracy of the well-known surface hopping dynamics at the high XMS-CASPT2 electronic-structure level but also facilitates the understanding of the microscopic mechanisms of the photoinduced reactions in cyclobutanone.

Palladium-catalyzed [2+1+1] annulation of norbornenes with (z)-bromostyrenes: synthesis of bismethylenecyclobutanes via twofold C(sp2)–H bond activation

A Pd(0)-catalyzed domino bismethylenecyclobutanation reaction was established. The [2+1+1] cycloaddition involves twofold C(sp²)–H bond activation and formation of three carbon–carbon bonds.

Stereocontrolled synthesis and functionalization of cyclobutanes and cyclobutanones

In the last decade a certain number of new cyclobutane and cyclobutanone synthesis and functionalization protocols have been published. Organo- and biocatalyzed eco-friendly approaches to cyclobutane-containing molecules have been developed with interesting results. Also, successful new total synthesis of bioactive compounds and drugs have been recently reported where a four membered ring represented the key intermediate. Therefore, the rising interest in this field represents a great point of discussion for the scientific community, disclosing the synthetic potential of strained four membered ring carbocyclic compounds. Herein we report a critical survey on the literature concerning the enantiocontrolled synthesis and functionalization of cyclobutane derivatives, with particular attention to metal-free, low impact methodologies, published during the period 2000-2013.

Applications of 1-alkenyl-1,1-heterobimetallics in the stereoselective synthesis of cyclopropylboronate esters, trisubstituted cyclopropanols and 2,3-disubstituted cyclobutanones

1-Alkenyl-1,1-heterobimetallics are potentially very useful in stereoselective organic synthesis but are relatively unexplored. Introduced herein is a practical application of 1-alkenyl-1,1-heterobimetallic intermediates in the synthesis of versatile cyclopropyl alcohol boronate esters, which are valuable building blocks. Thus, hydroboration of 1-alkynyl-1-boronate esters with dicyclohexylborane generates 1-alkenyl-1,1-diboro species. In situ transmetalation with dialkylzinc reagents furnishes 1-alkenyl-1,1-borozinc heterobimetallic intermediates. Addition of the more reactive ZnC bond to aldehydes generates the key B(pin) substituted allylic alkoxide intermediates. An in situ alkoxide directed cyclopropanation proceeds with the formation of two more CC bonds, affording cyclopropyl alcohol boronate esters with three new stereocenters in 58-89% isolated yields and excellent diastereoselectivities (>15:1 dr). Oxidation of the BC bond provides trisubstituted alpha-hydroxycyclopropyl carbinols as single diastereomers in good to excellent yields (75-93%). Facile pinacol-type rearrangement of the alpha-hydroxycyclopropyl carbinols provides access to both cis- and trans-2,3-disubstituted cyclobutanones with high stereoselectivity (>17:1 dr in most cases) from a common starting material. This methodology has been applied in the synthesis of quercus lactones A and B.

Structural and Spectroscopic Demonstration of Agostic C−C Interactions in Electron-Deficient Metallacyclobutanes and Related Cage Complexes: Possible Implications for Olefin Polymerizations and Metatheses

The reaction of the half-open titanocene, Ti(C5H5)(c-C8H11)(PMe3) (c-C8H11 = cyclooctadienyl), with two equivalents of PhC2SiMe3 leads to their incorporation and coupling to the dienyl fragment. One alkyne inserts into a C-H bond of the central CH2 group of the c-C8H11 ligand's edge-bridge, while the second undergoes a 5+2 coupling with the dienyl fragment, yielding coordinated sigma-allyl and olefin fragments, as demonstrated by X-ray diffraction. Together with the C5H5 and PMe3 coordinations, this leads to a 14-electron count. While the very electron-deficient titanium center passes up potential pi coordination of the allyl fragment, it instead engages in interactions with one or two C-C bonds, and perhaps a C-H bond, as revealed from the structural and spectroscopic data. Similar interactions have been found in electron-deficient metallacyclobutane complexes of titanium and zirconium, but not in the 18-electron molybdenum and tungsten analogues. These and other observations may have implications relating to metatheses and polymerizations of olefins.