Suggestion of a “Twist” Mechanism in the Oligomerisation of a Dimeric Phospha(III)zane: Insights into the Selection of Adamantoid and Macrocyclic Alternatives

Abstract

Snipping tool: Zn(OTf)2 is an efficient catalyst for selective cleavage of amides bearing a β‐hydroxyethyl group on the nitrogen atom. The mechanism involves an N,O‐acyl rearrangement and transesterification. This new catalytic system can be applied to sequence‐specific peptide bond scission at the amine side of a serine residue. Tf=trifluoromethanesulfonyl.

A growing family: New structures of coordination polymers containing adamantane-shaped phosphorus–nitrogen cage ligands

“…This report details the synthesis of a new member of the P 4 (NR) 6 family according to a two-step, one-pot preparation method shown in Scheme 2. This adaptation of synthetic routes used to build some phosphazane macrocycles minimizes the amount of amine required, and eliminates the presence of long-chain hydrochloride salts in the workup [12]. The first step of the process uses lithiated amine both as the nucleophile to form P-N bonds, and the base to neutralize liberated HCl.…”

The crystal structure of 2-chloro-1,3-bis(2,4,6-trimethylphenyl)-4,4-dimethyl-1,3,2λ3,4-diazaphosphasiletidine

“…However, additional steric factors should not be neglected: The mesityl substituents do not take a coplanar position with respect to the SiN 2 P ring, but form angles of 75.64(6)°and 77.16(6)°, respectively. For similar diazadiphosphetidines [ClPNR] 2 angles between 0°a nd 90°have been found depending on the steric requirements of the aryl substituents [24][25][26][27]. Considering the Si-N and P-N bond lengths as well as the absence of any residual electron density at the phosphorus atom opposite to the Cl atom position, there is no disorder model to account for the partial occupation site near Si1.…”

The First Synthesis of the Sterically Encumbered Adamantoid Phosphazane P4(NtBu)6: Enabled by Mechanochemistry

“…A classic illustration of strict steric control is the [XP(μ‐NR)] 2 dimer ( I ; Figure a) observed for sterically demanding groups (e.g., R= t Bu), whereas trimers [XP(μ‐NR)] 3 and larger structures are obtained with smaller substituents (e.g., R=Me, Et; Figure a, II – V ) . The steric demand of the nitrogen substituent is thought to control the stability and accessibility of different cyclophosph(III)azane isomers, which can interconvert under suitable conditions, for example by the twist mechanism found in the formation of the type IV phosphazane [{ClP(μ‐Npy) 2 } 2 {P 2 (μ‐Npy)}] (Figure b) . The twist mechanism was also proposed for the conversion of type V macrocycles, [{P(μ‐NR)} 2 (μ‐NR)] 2 , into the thermodynamically preferred type VI adamantoid isomer P 4 (NR) 6 (Figure c) .…”

“…The steric demand of the nitrogen substituent is thought to control the stability and accessibility of different cyclophosph(III)azane isomers, which can interconvert under suitable conditions, for example by the twist mechanism found in the formation of the type IV phosphazane [{ClP(μ‐Npy) 2 } 2 {P 2 (μ‐Npy)}] (Figure b) . The twist mechanism was also proposed for the conversion of type V macrocycles, [{P(μ‐NR)} 2 (μ‐NR)] 2 , into the thermodynamically preferred type VI adamantoid isomer P 4 (NR) 6 (Figure c) . For the isopropyl‐substituted macrocycle [{P(μ‐N i Pr) 2 } 2 (μ‐N i Pr)] 2 ( 1 ), conversion into the adamantoid isomer 2 is achieved by prolonged heating in a sealed tube (Scheme ) .…”

“…These results led to the conclusion that the type V macrocyclic backbone is the more stable one in the presence of bulky t Bu groups . Thus 4 has been classified as inaccessible on steric grounds and is usually cited as an example of steric control in PN compounds …”