Synthesis, Structure, and Reactivity of Novel 3,4-Diphosphacyclobutenes Bearing Silyl Groups

Abstract

[reaction: see text] Copper-mediated homocoupling of sterically hindered 2-(2,4,6-tri-tert-butylphenyl)-1-trialkylsilyl-2-phosphaethenyllithiums afforded 1,2-bis(trialkylsilyl)-3,4-diphosphacyclobutenes (1,2-dihydrodiphosphetenes) through a formal electrocyclic [2+2] cyclization in the P=C-C=P skeleton as well as 2-trimethylsilyl-1,4-diphosphabuta-1,3-diene. Reduction of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes followed by quenching with electrophiles afforded ring-opened products, (E)-1,2-bis(phosphino)-1,2-bis(trimethylsilyl)ethene and (Z)-2,3-bis(trimethylsilyl)-1,4-diphosphabut-1-ene. The structures of the ring-opened products indicated E/Z isomerization around the C=C bond after P-P bond cleavage of 5, and the isomerization of the P-C=C skeleton. Ring opening of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes affording (E,E)- and (Z,Z)-1,4-diphosphabuta-1,3-dienes was observed upon desilylation.

Incomplete Electrocyclization of a Sterically Hindered 1,4‐Diphosphabutadiene

“…Figure a shows an ORTEP drawing (50 % probability level) of the cyclized form at 6.82(5)% occupancy, and the heterobutadiene structure of the remaining 93 % occupancy is shown in Figure b. The P1′‐P2′ distance is 2.302(4) Å, which is slightly longer than that of 1 (average 2.260 Å) and 7 (2.233 Å, Figure c) and comparable to the DFT‐optimized structure (2.3373 Å, M06‐2X/6‐31G(d)). The skeletal P−C distances in the cyclized structure of 4 are slightly shorter than those of the DFT‐optimized structure (1.8045, 1.8034 Å), 1 (average 1.817 Å), and 7 (1.846, 1.847 Å; Figure c) .…”

“…The P1′‐P2′ distance is 2.302(4) Å, which is slightly longer than that of 1 (average 2.260 Å) and 7 (2.233 Å, Figure c) and comparable to the DFT‐optimized structure (2.3373 Å, M06‐2X/6‐31G(d)). The skeletal P−C distances in the cyclized structure of 4 are slightly shorter than those of the DFT‐optimized structure (1.8045, 1.8034 Å), 1 (average 1.817 Å), and 7 (1.846, 1.847 Å; Figure c) . On the other hand, the C1′‐C2′ distance of 1.495(17) Å is not reasonable for a C=C double bond, and is considerably longer than that of 1 (average 1.374 Å), 7 (1.386 Å), and the DFT‐optimized structure (1.3788 Å).…”

“…The unusual structural aspects of 4 in solution were probed further by studying its photo‐absorption and electrochemical properties. The UV‐Vis spectrum of 4 showed a small absorption band around 343 nm in dichloromethane ( ϵ =4100, Figure S2a), similar to 1 and different from 6 (Figure S2c). As shown in Figure S2b, electrochemical analysis of 4 showed a quasi‐reversible oxidation potential, and differential pulse voltammetry (DPV) measurements provided an E ox value of +0.46 V vs. Fc/Fc +.…”

“…The use of appropriate kinetic stabilization substituents such as the supermesityl (Mes*; 2,4,6‐tri‐ t ‐butylphenyl) group is a promising approach for isolating A by retarding the thermodynamically preferred electrocyclization process . However, previous studies revealed that the oxidative homocoupling of the silyl‐substituted phosphavinyllithium afforded the corresponding 1,2‐dihydrodiphosphete (3,4‐diphosphacyclobutene) 1 exclusively: the corresponding heavier butadiene species was not observed at all (Figure ) . The exclusive formation of the silyl‐substituted 1,2‐dihydrodiphosphete correlates with the isolation of a silyl‐substituted cyclobutadiene derivative, indicating that the silyl group promotes the strained cyclic structures by increasing the s character of the skeletal carbon atoms .…”

Preparation and X-ray analyses of siloxane-bridged acyclic and 8-membered cyclic phosphaethenes

“…As for phosphaethenes bearing trialkylsilyl group at the sp 2 carbon atom, we reported that copper-mediated homocoupling reaction of 2-trialkylsilyl-2-phosphaethenyllithiums [Mes * P@C(SiMe 2 R)Li; R = Me, t-Bu] afford 3,4-diphosphacyclobutenes, indicating that the silyl group prompts [2 + 2] cyclization of the P@CAC@P skeleton or isomerization of phosphaethenyllithium into the phosphinocarbene structure [15]. Additionally, structure of a 2-trimethylsilyl-1,3-diphosphapropene [Mes * P@C(SiMe 3 )PPh 2 ] suggests the effects of silyl group: presumably the group stabilizes inherently predominant C 1 symmetric P@CAP < conformation [16].…”